What is a heat index, and why does it differ from temperature?

The heat index combines temperature and humidity to estimate how hot it feels and how hard it is for sweat to cool you. At 90 degrees with 70 percent humidity, the heat index is around 105 (Excessive Heat Warning territory) because sweat evaporates poorly in humid air.

Are fans safe during a heat wave?

Fans help below about 95 degrees of ambient air, when they move cool air across your skin. Above 95, fans circulate hot air and can accelerate dehydration without cooling you. The CDC and NWS guidance specifically warns against relying on fans during excessive heat warnings.

How fast can frostbite happen?

At wind chill of minus 20 degrees, frostbite on exposed skin can occur in roughly 30 minutes; at minus 40, in about 10 minutes; at minus 60, in about 5 minutes. Wind speeds the process. Cover face, ears, and hands first.

Where are the city cooling centers and warming centers?

The City of Philadelphia activates cooling centers during heat events at most public libraries, rec centers, and senior centers. Warming centers activate during cold events at similar locations and at additional shelter sites. The 311 line and the Office of Emergency Management website list current locations.

Should I cancel an outdoor workout during a heat advisory?

Yes for high-intensity sessions; lower-intensity early morning or evening sessions are usually fine for healthy adults with good hydration. Above an excessive heat warning, move all sessions indoors.

Is thunderstorm asthma a real thing?

Yes. The combination of pollen rupture (storms break grass and tree pollen into smaller, more inhalable fragments) and rapid pressure shifts can trigger asthma flares hours before or during a storm. The major documented event was in Melbourne in 2016 (10 deaths, thousands of presentations), but milder versions happen here.

What should I do if my power goes out during severe weather?

Stay in your home if it remains safely habitable. Run generators outside only, never in a garage. Keep refrigerators and freezers closed (4 hours and 24 to 48 hours, respectively). Check on neighbors. If your home becomes unsafe (extreme cold without heat, extreme heat without AC), leave for a public warming or cooling center or for family or friends with power.

Is it safe to drive through standing water?

No. Two feet of moving water can carry a car. Most flood deaths happen in vehicles. The NWS slogan "turn around, don't drown" exists because the lesson keeps being relearned.

Why is the urban heat island effect significant in Philadelphia?

Dense brick and asphalt absorb daytime heat and release it slowly overnight, so nighttime cooling is reduced and the cardiovascular recovery window is shorter. Center City and dense rowhome blocks (parts of South Philly, parts of West Philly) can run 5 to 15 degrees hotter than parkland or suburbs at peak. The combination is harder on older adults in walk-up apartments without AC.

What is the dose-response for cold exposure and cardiovascular events?

Cold drives sympathetic activation, peripheral vasoconstriction, increased blood pressure, and increased platelet activation, all of which raise the risk of myocardial infarction and stroke in vulnerable patients. Shoveling snow during the first 30 to 60 minutes of cold exposure is a particularly high-risk event, especially for sedentary adults with undiagnosed coronary disease.

How does cold-air bronchospasm work in asthma and COPD?

Cold air increases airway resistance through reflex vasoconstriction and through direct cooling and drying of the airway mucosa. The result is bronchoconstriction, mucus thickening, and exacerbation. Pre-treatment with a short-acting beta agonist 15 to 30 minutes before exposure, plus warming and humidifying inhaled air with a scarf or buff, reduces flares meaningfully.

What is the public-health significance of CO poisoning during storms?

Generators, charcoal grills, and gas appliances used indoors or near windows during power outages produce carbon monoxide at lethal rates. CO poisoning is one of the leading post-storm causes of death in the US. A working CO detector with a battery backup is one of the highest-leverage 30-dollar purchases in any home.

How does flash flooding cause delayed harm?

Beyond the immediate drowning and trauma risk, floodwater is contaminated with sewage, fuel, agricultural runoff, and debris. Wound infections, gastrointestinal illness, and post-flood mold-related respiratory illness are common. Mold remediation that starts within 24 to 48 hours of dry-out prevents most chronic respiratory consequences.

When should I worry about thunderstorm asthma even without a Severe Thunderstorm Warning?

On high-pollen days, when a strong storm front is approaching, and especially in spring grass pollen season. Patients with hay fever and a known asthma component are most vulnerable. Have rescue inhaler accessible; consider pre-treatment if a strong front is forecast and you have a flare-prone pattern.

How does pregnancy change the calculus across weather extremes?

Pregnancy reduces heat tolerance (higher baseline core temperature, lower exercise tolerance), increases venous stasis (relevant during prolonged sheltering), and changes the medication options for symptom relief. Pregnant patients during severe weather should have a lower threshold for getting indoor cooling or warming, for hydration, and for getting evaluated for new symptoms.

What is a written household plan, and what should it include?

A one-page document on the fridge: who you check on (names, addresses, phone numbers), where you go if power fails or your home becomes unsafe, who you call for medical questions during an event (us, your specialist, the city's nurse line, 911), a 7-day medication list with doses, and where the emergency supplies live in the home. Update it once a year.

How do severe weather events stack on each other in Philadelphia?

Heat plus ozone, heat plus power loss, winter storm plus extreme cold, and storms plus flooding are the most common combinations. The clinical lesson is that the second alert often does the harm, not the first. A heat advisory followed by a power outage is a different event than a heat advisory alone.

What is a heat index, and why does it differ from temperature?

The heat index combines temperature and humidity to estimate how hot it feels and how hard it is for sweat to cool you. At 90 degrees with 70 percent humidity, the heat index is around 105 (Excessive Heat Warning territory) because sweat evaporates poorly in humid air.

Are fans safe during a heat wave?

Fans help below about 95 degrees of ambient air, when they move cool air across your skin. Above 95, fans circulate hot air and can accelerate dehydration without cooling you. The CDC and NWS guidance specifically warns against relying on fans during excessive heat warnings.

How fast can frostbite happen?

At wind chill of minus 20 degrees, frostbite on exposed skin can occur in roughly 30 minutes; at minus 40, in about 10 minutes; at minus 60, in about 5 minutes. Wind speeds the process. Cover face, ears, and hands first.

Where are the city cooling centers and warming centers?

The City of Philadelphia activates cooling centers during heat events at most public libraries, rec centers, and senior centers. Warming centers activate during cold events at similar locations and at additional shelter sites. The 311 line and the Office of Emergency Management website list current locations.

Should I cancel an outdoor workout during a heat advisory?

Yes for high-intensity sessions; lower-intensity early morning or evening sessions are usually fine for healthy adults with good hydration. Above an excessive heat warning, move all sessions indoors.

Is thunderstorm asthma a real thing?

Yes. The combination of pollen rupture (storms break grass and tree pollen into smaller, more inhalable fragments) and rapid pressure shifts can trigger asthma flares hours before or during a storm. The major documented event was in Melbourne in 2016 (10 deaths, thousands of presentations), but milder versions happen here.

What should I do if my power goes out during severe weather?

Stay in your home if it remains safely habitable. Run generators outside only, never in a garage. Keep refrigerators and freezers closed (4 hours and 24 to 48 hours, respectively). Check on neighbors. If your home becomes unsafe (extreme cold without heat, extreme heat without AC), leave for a public warming or cooling center or for family or friends with power.

Is it safe to drive through standing water?

No. Two feet of moving water can carry a car. Most flood deaths happen in vehicles. The NWS slogan "turn around, don't drown" exists because the lesson keeps being relearned.

Why is the urban heat island effect significant in Philadelphia?

Dense brick and asphalt absorb daytime heat and release it slowly overnight, so nighttime cooling is reduced and the cardiovascular recovery window is shorter. Center City and dense rowhome blocks (parts of South Philly, parts of West Philly) can run 5 to 15 degrees hotter than parkland or suburbs at peak. The combination is harder on older adults in walk-up apartments without AC.

What is the dose-response for cold exposure and cardiovascular events?

Cold drives sympathetic activation, peripheral vasoconstriction, increased blood pressure, and increased platelet activation, all of which raise the risk of myocardial infarction and stroke in vulnerable patients. Shoveling snow during the first 30 to 60 minutes of cold exposure is a particularly high-risk event, especially for sedentary adults with undiagnosed coronary disease.

How does cold-air bronchospasm work in asthma and COPD?

Cold air increases airway resistance through reflex vasoconstriction and through direct cooling and drying of the airway mucosa. The result is bronchoconstriction, mucus thickening, and exacerbation. Pre-treatment with a short-acting beta agonist 15 to 30 minutes before exposure, plus warming and humidifying inhaled air with a scarf or buff, reduces flares meaningfully.

What is the public-health significance of CO poisoning during storms?

Generators, charcoal grills, and gas appliances used indoors or near windows during power outages produce carbon monoxide at lethal rates. CO poisoning is one of the leading post-storm causes of death in the US. A working CO detector with a battery backup is one of the highest-leverage 30-dollar purchases in any home.

How does flash flooding cause delayed harm?

Beyond the immediate drowning and trauma risk, floodwater is contaminated with sewage, fuel, agricultural runoff, and debris. Wound infections, gastrointestinal illness, and post-flood mold-related respiratory illness are common. Mold remediation that starts within 24 to 48 hours of dry-out prevents most chronic respiratory consequences.

When should I worry about thunderstorm asthma even without a Severe Thunderstorm Warning?

On high-pollen days, when a strong storm front is approaching, and especially in spring grass pollen season. Patients with hay fever and a known asthma component are most vulnerable. Have rescue inhaler accessible; consider pre-treatment if a strong front is forecast and you have a flare-prone pattern.

How does pregnancy change the calculus across weather extremes?

Pregnancy reduces heat tolerance (higher baseline core temperature, lower exercise tolerance), increases venous stasis (relevant during prolonged sheltering), and changes the medication options for symptom relief. Pregnant patients during severe weather should have a lower threshold for getting indoor cooling or warming, for hydration, and for getting evaluated for new symptoms.

What is a written household plan, and what should it include?

A one-page document on the fridge: who you check on (names, addresses, phone numbers), where you go if power fails or your home becomes unsafe, who you call for medical questions during an event (us, your specialist, the city's nurse line, 911), a 7-day medication list with doses, and where the emergency supplies live in the home. Update it once a year.

How do severe weather events stack on each other in Philadelphia?

Heat plus ozone, heat plus power loss, winter storm plus extreme cold, and storms plus flooding are the most common combinations. The clinical lesson is that the second alert often does the harm, not the first. A heat advisory followed by a power outage is a different event than a heat advisory alone.

TL;DR · 30-second take

Severe weather is mostly a question of preparation, not heroics. The National Weather Service issues a small set of alerts that materially change health risk: heat advisories and excessive heat warnings, wind chill / cold weather advisories and extreme cold warnings, severe thunderstorm and tornado warnings, flash flood warnings, and winter storm warnings. The medications that change the math most are diuretics, beta blockers, anticholinergics, GLP-1s, lithium, and any drug with a sedating or hypotensive effect. The conditions that change it most are kidney disease, heart failure, COPD, pregnancy, and older age. The single highest-leverage move is to have a written plan that exists before the alert: who you check on, where you go, what you take with you. The sections below tell you exactly what to do for each alert.

Severe Weather in Philadelphia: Heat, Cold, and Storms - What to Actually Do

Philadelphia gets a wider weather spectrum than most outsiders expect. Heat waves stack up in July and August across an urban heat island of asphalt and row-home brick. Ida-scale flooding hit the Schuylkill and Manayunk floodplain in 2021 and will again. Wind chill warnings can hit in January and February. Severe thunderstorms cross every summer, with tornado warnings occasional but not theoretical. The clinical pattern across all of it is the same. The patients who do well had a plan written down. The patients who end up in the emergency department often had every piece of the plan in their head but had not assembled it before the alert hit.

For elevated AQI events that sometimes overlap with heat, see our Philadelphia AQI guide.

Who is at higher risk across weather extremes

Some groups feel weather more than others. The list largely matches the air-quality risk groups, with a few additions specific to temperature and storms.

Adults over 65. Lower physiologic reserve, often more medications, and reduced sensation of thirst and temperature.
Infants and young children. Higher surface area to body mass ratio; they heat and cool faster than adults.
People with cardiovascular disease, heart failure, or recent stents. Heat raises cardiac demand; cold triggers vasoconstriction and platelet activation.
People with chronic kidney disease. Heat and dehydration are common triggers for acute-on-chronic kidney injury.
Asthma and COPD. Cold air and storms (thunderstorm asthma) trigger flares.
People on diuretics, beta blockers, anticholinergics, GLP-1 medications, lithium, antipsychotics, or any sedating drug. All of these change how the body handles heat, hydration, or fall risk.
People who are pregnant. Heat tolerance is reduced; flash flooding adds mobility considerations.
Outdoor workers and athletes. Construction, landscaping, delivery, food service patios; long runs along the Schuylkill in August.
People without reliable air conditioning, heating, or transportation. Cooling centers and warming centers exist citywide for a reason.

If you are in any of these groups, the action step at any tier starts one level earlier than for everyone else.

What these admissions actually look like

A composite from years of hospital medicine, because the pattern repeats every heat event. Someone in their 60s or 70s is on a diuretic for blood pressure or heart failure. A heat-index day arrives. They keep taking the medication, which is correct, but the hydration that normally balances it does not go up with the heat. Two or three days later they are in the emergency department - lightheaded, confused, kidney numbers double their baseline. The heart, the kidneys, and the brain are where it shows up: new arrhythmias and chest pain, acute kidney injury layered on chronic kidney disease, and the quiet neurological changes families describe as "she just seemed off." None of it started as an emergency. All of it started as a forecast. The part worth sitting with: the people in those beds did not think of themselves as vulnerable that week. Vulnerability is not a fixed trait. It is a moving line, and medications, age, training load, and the day's heat index all move it. If you are athletic, the move on advisory days is cutting the load, not proving a point. If you take any of the medications listed above, the move is "keep taking it, raise the water, and check in if unsure" - never silently skipping doses. Heat is one lens of the Environment pillar. The same thinking applies to air, pollen, and UV.

Heat Advisory

NWS issues a Heat Advisory when the heat index (apparent temperature combining heat and humidity) is forecast to reach roughly 100 to 104 degrees for two or more hours. In Philadelphia, this often coincides with the urban heat island making downtown row-home blocks feel 10 to 15 degrees hotter than the regional forecast.

Everyone: Move heavy outdoor exertion to early morning or evening. Hydrate proactively, not reactively (water plus electrolytes if sweating heavily). Light-colored, loose-fitting clothing.
Sensitive groups: Stay in air conditioning during the heat of the day. If your home does not have AC, plan ahead to a city cooling center (free, at most libraries and rec centers during heat events).
Medication review: Anyone on diuretics, beta blockers, anticholinergics, GLP-1 medications, lithium, or antipsychotics should not silently adjust their own dose during heat. Check in if you are concerned, especially if you are losing weight on a GLP-1 and adding heat-related fluid loss.
Athletes: Reduce session length, increase rest intervals, and weigh in before and after long efforts. A drop of more than 2 percent body weight is meaningful dehydration.
Check on neighbors: Especially older adults living alone. This is the single highest-leverage move in a heat event.

Excessive Heat Warning

Issued for forecast heat index of 105 degrees or higher for two or more hours, or 110-plus for any duration. This is the dangerous tier.

Everyone: Avoid outdoor exertion entirely during the heat of the day. If you must be outside, plan it for sunrise hours or after dark.
Run the air conditioning even if you usually do not. Fans alone are not protective above 95 degrees of ambient air; they can actually accelerate fluid loss without cooling you.
Recognize heat illness early. Heat exhaustion: heavy sweating, weakness, headache, lightheadedness, nausea, muscle cramps. Move to AC, rehydrate, cool the skin with damp cloths. Heat stroke: confusion, altered consciousness, hot dry skin, body temperature above 104. This is a medical emergency. Call 911. Begin active cooling (ice packs to neck, groin, axillae) while waiting.
Sensitive groups: Cooling centers and family members. Do not power through. Heat stroke mortality in older adults is significant.
At home: Close blinds, cluster activity in one cool room, take cool showers. If you lose power, leave for a cooling center; sheltering in a sealed home without AC is more dangerous than the trip.
Cardiovascular caution: Heat events increase emergency department visits for heart attacks, strokes, and arrhythmias. New chest pain, breathlessness, palpitations, or syncope during a heat event are not "just the heat." Get evaluated.

Cold Weather Advisory

NWS issues a Cold Weather Advisory (the modern replacement for the older Wind Chill Advisory) when wind chill values are forecast in the 0 to minus 14 degree range. In Philadelphia, January and February cold snaps and early-morning windy days are the usual triggers.

Everyone: Layer in three stages (wicking base, insulating middle, wind-blocking outer). Cover face, hands, and ears. Limit time outdoors.
Asthma and COPD: Cold dry air is a powerful airway trigger. Pre-treat with rescue medication 15 to 30 minutes before extended outdoor time. Cover the mouth and nose with a scarf or buff to warm and humidify the air.
Cardiovascular disease: Shoveling, brisk walking in cold, or hurrying outdoors raises cardiac demand sharply. Take it slowly, pause if you feel chest tightness, and warm up gradually.
Ice falls: Most cold-weather emergency visits in Philadelphia are not from cold itself but from falls on ice. Microspikes or YakTrax for boots cost 20 dollars and are worth it.
People experiencing homelessness: Project HOME and the city run cold-weather outreach; if you encounter someone in distress, the city's homeless services line is the right call.

Extreme Cold Warning

Issued when wind chill values are forecast at minus 15 degrees or colder. Less common in Philadelphia than further north, but happens during major Arctic blasts.

Everyone: Limit outdoor exposure to brief, essential trips. Hypothermia and frostbite times shorten dramatically; at wind chill of minus 20, frostbite on exposed skin can occur in roughly 30 minutes, and faster with wind.
Pets, pipes, and people: Bring pets in, drip faucets to prevent pipe bursts, and check on older neighbors. Hypothermia in older adults can develop quietly indoors when heating fails.
Recognize hypothermia and frostbite. Hypothermia: shivering (early), confusion, drowsiness, slurred speech. Frostbite: numbness, white or grayish skin, hard or waxy texture. Warm gradually in lukewarm water, not direct heat; do not rub frostbitten skin. Severe hypothermia (no shivering, altered consciousness) is a 911 call.
Heart failure and COPD patients: Cold drives both. Have rescue medications and a plan; thresholds for getting evaluated should be lower during an extreme cold warning.

Severe Thunderstorm Warning

Issued when a thunderstorm with damaging winds (58 mph-plus) or large hail (1 inch-plus) is occurring or imminent. The summer pattern in Philadelphia is afternoon cells crossing from the west.

Indoor shelter: Get inside immediately. Avoid windows. Lightning kills 20 to 30 Americans a year; severe wind takes more. The "30/30 rule" still applies: if thunder follows lightning within 30 seconds, you are within strike range, and stay indoors 30 minutes after the last thunder.
Outdoor activities (Eagles tailgates, Phillies games, marathons, kids' sports): Move inside, into a vehicle, or into a substantial building. Open shelters and dugouts do not count.
Asthma: Thunderstorm asthma is a real phenomenon. The combination of pollen rupture, rapid pressure shifts, and dust can trigger flares hours before or during a storm. Pre-treat if you have a flare-prone pattern.
Power outages: Charge devices, have flashlights, do not run generators or grills indoors or in attached garages (CO poisoning is a leading cause of post-storm death). Refrigerator and freezer guidance: a closed fridge keeps food safe for about 4 hours, a closed freezer 24 to 48 hours.

Tornado Warning

A tornado has been sighted or indicated by radar. Uncommon in Philadelphia but not theoretical; small tornadoes have touched down in the metro area.

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Shelter immediately. Lowest interior room, away from windows. Basements, interior bathrooms, and closets. Cover with mattresses or heavy blankets if possible.
Mobile homes and cars: Leave for sturdier shelter if there is any safe option. If trapped in a vehicle and a sturdy building is not reachable, lie flat in a low area away from trees and power lines.
After: Watch for downed power lines, gas leaks, structural instability. Photograph damage for insurance.
Asthma and respiratory patients: Dust and debris in the air after a tornado are a serious flare trigger. Mask up before going outside to assess.

Flash Flood Warning

Issued when flooding is imminent or occurring. Philadelphia's vulnerable zones include the Vine Street Expressway tunnel, the Schuylkill banks (Manayunk, Conshohocken corridor), low-lying neighborhoods near the rivers, and many basement-finished row homes citywide. Ida in September 2021 was the recent reference point.

Turn around, do not drown. Two feet of moving water can carry a car. Do not drive through standing water of unknown depth; flooded streets are the leading cause of flood deaths.
At home: Move people, pets, and valuables to higher floors. Unplug electronics in the basement. If water rises rapidly, get out before the basement stairs become a hazard.
After: Floodwater is contaminated (sewage, gasoline, debris). Avoid skin contact when possible. Tetanus boosters should be current. Mold remediation starts within 24 to 48 hours of dry-out.
Medications and medical equipment: If you rely on home dialysis, oxygen, CPAP, or infusion pumps, have a 7-day backup plan and know your nearest dialysis center, oxygen vendor, or hospital before the alert hits.

Winter Storm Warning

Issued for significant snow accumulation (typically 6 inches or more in 24 hours in this area), or for an ice storm with at least a quarter inch of ice. Often combined with cold weather alerts.

Pre-storm: Stock the basics (water, shelf-stable food, flashlights, batteries, medications for 7 days, baby formula and pet food if applicable). Charge devices and battery banks. Park strategically to avoid being blocked in.
During: Stay off the roads if possible. Salt and ice on the Schuylkill Trail and sidewalks make falls common; wait 24 to 48 hours after city treatment for any non-essential walk.
Shoveling: Snow shoveling is a classic heart attack trigger. Push, do not lift; take frequent breaks; warm up first. If you have known coronary disease, delegate or use a snow blower.
Power outages and generators: Generators belong outside, away from windows. CO poisoning during winter storms is a recurring tragedy. CO detectors save lives in this scenario.
Vulnerable neighbors: Heat, food, and a check-in are the high-leverage things during a winter storm with power loss. If older neighbors live alone, this is the moment to knock.

Air Quality Alert

NWS sometimes issues Air Quality Alerts during heat events, ozone-action days, or wildfire smoke episodes. The clinical actions overlap heavily with the heat or smoke guidance.

The combined risk is more than the sum. Heat plus ozone, or heat plus PM2.5, hits cardiovascular and respiratory systems together.
Run indoor air filtration during smoke days. A HEPA in the bedroom, MERV 13 in the HVAC, windows closed, HVAC on recirculate.
See the Philadelphia AQI guide for the tier-by-tier action steps for the air-quality side.

Guidance from the clinic

"The single biggest determinant of how a weather event goes for a patient is whether the plan existed before the alert hit. Who do you check on. Where do you go if your power fails. What medications change the math. When do you call us versus go directly to the emergency department. Write it down once, keep it on the fridge, and update it once a year. The drama is almost always avoidable; the preparation is almost always boring."

Actionable Steps

Set this up before the next major event.

Build the household plan. Where do you go if you lose power for more than 24 hours. Who do you check on, and how. What is your nearest cooling center and warming center.
Stock the basics. 7 days of medications, water (one gallon per person per day for 3 days), shelf-stable food, flashlights, battery banks, a working CO detector, and a tetanus booster within the last 10 years if you might encounter floodwater.
Know your highest-risk medications. Diuretics, beta blockers, anticholinergics, GLP-1 medications, lithium, and antipsychotics all change the calculus in heat. Ask us at your next visit how yours specifically change during severe weather.
Subscribe to NWS alerts and the city's notification system. ReadyPhiladelphia for citywide alerts; the NWS Philadelphia/Mount Holly office for forecast and alert updates.

Key Takeaways

The alerts that matter most for health in Philadelphia are Heat Advisory and Excessive Heat Warning, Cold Weather Advisory and Extreme Cold Warning, Severe Thunderstorm and Tornado Warnings, Flash Flood Warning, and Winter Storm Warning.
Medications that change the math most: diuretics, beta blockers, anticholinergics, GLP-1 medications, lithium, antipsychotics, and any sedating drug.
Conditions that change it most: cardiovascular disease, heart failure, kidney disease, asthma and COPD, pregnancy, and older age.
The single highest-leverage move is a written household plan that exists before the alert hits.
During combined alerts (heat plus air quality, cold plus power loss), the risk is more than the sum of either alone.

Scientific References

Bouchama, A., & Knochel, J. P. (2002). Heat stroke. New England Journal of Medicine, 346(25), 1978-1988.
Basu, R. (2009). High ambient temperature and mortality: a review of epidemiologic studies from 2001 to 2008. Environmental Health, 8, 40.
Anderson, B. G., & Bell, M. L. (2009). Weather-related mortality: how heat, cold, and heat waves affect mortality in the United States. Epidemiology, 20(2), 205-213.
D'Amato, G., Vitale, C., Rosario, N., et al. (2019). Climate change, allergy and asthma, and the role of tropical forests. World Allergy Organization Journal, 12(2), 100011.
Watts, N., Amann, M., Arnell, N., et al. (2021). The 2020 report of The Lancet Countdown on health and climate change. The Lancet, 397(10269), 129-170.
National Weather Service. (2024). NWS Watch, Warning, Advisory Display - definitions and thresholds. National Oceanic and Atmospheric Administration.

Philadelphia AQI: what to do - the practical playbook for air quality days in Philly
Pollen in Philadelphia - the tree/grass/weed calendar and when to pre-treat
Microplastics in Philadelphia - the five highest-leverage moves to lower your exposure
UV Index in Philadelphia - the daily UV calculus for skin and eye protection
Philadelphia Environmental Health (overview) - the city-wide environmental health framework

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2418 E York St, Philadelphia, PA 19125(267) 360-7927 hello@fishtownmedicine.comHSA/FSA Eligible

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Frequently Asked Questions

Common Questions

Deep-Dive Questions

Heat raises cardiac demand by increasing heart rate, decreasing peripheral resistance, and reducing effective circulating volume through sweating and vasodilation. In people with coronary disease, heart failure, or recent stents, this combination raises the risk of myocardial infarction, decompensation, and arrhythmia. Heat events show measurable spikes in cardiovascular emergency department visits across multiple US cities, including Philadelphia.

Diuretics reduce effective circulating volume; beta blockers blunt the heart-rate response and reduce sweating; anticholinergics (older tricyclics, some bladder medications, some psychiatric drugs) reduce sweating directly; GLP-1 medications can cause volume contraction and contribute to dehydration; lithium has a narrow therapeutic window that shifts with hydration; antipsychotics impair central thermoregulation. None of these mean stopping the medication during heat; they mean adjusting hydration, monitoring, and sometimes timing.

Shelter in place is the default for tornado warnings (no time to evacuate safely), severe thunderstorms (short duration), and most winter storms. Evacuate (or relocate to a public cooling/warming center) when your home becomes unsafe: extended heat without AC, extended cold without heat, flooding rising past safe egress, or post-event structural damage. The decision is easier when the criteria are written down before the event.

Still have a question?

He answers personally. Usually within a few hours.

TL;DR · 30-second take

Severe Weather in Philadelphia: Heat, Cold, and Storms - What to Actually Do

For elevated AQI events that sometimes overlap with heat, see our Philadelphia AQI guide.

Who is at higher risk across weather extremes

Some groups feel weather more than others. The list largely matches the air-quality risk groups, with a few additions specific to temperature and storms.

Adults over 65. Lower physiologic reserve, often more medications, and reduced sensation of thirst and temperature.
Infants and young children. Higher surface area to body mass ratio; they heat and cool faster than adults.
People with cardiovascular disease, heart failure, or recent stents. Heat raises cardiac demand; cold triggers vasoconstriction and platelet activation.
People with chronic kidney disease. Heat and dehydration are common triggers for acute-on-chronic kidney injury.
Asthma and COPD. Cold air and storms (thunderstorm asthma) trigger flares.
People on diuretics, beta blockers, anticholinergics, GLP-1 medications, lithium, antipsychotics, or any sedating drug. All of these change how the body handles heat, hydration, or fall risk.
People who are pregnant. Heat tolerance is reduced; flash flooding adds mobility considerations.
Outdoor workers and athletes. Construction, landscaping, delivery, food service patios; long runs along the Schuylkill in August.
People without reliable air conditioning, heating, or transportation. Cooling centers and warming centers exist citywide for a reason.

If you are in any of these groups, the action step at any tier starts one level earlier than for everyone else.

What these admissions actually look like

Heat Advisory

Everyone: Move heavy outdoor exertion to early morning or evening. Hydrate proactively, not reactively (water plus electrolytes if sweating heavily). Light-colored, loose-fitting clothing.
Sensitive groups: Stay in air conditioning during the heat of the day. If your home does not have AC, plan ahead to a city cooling center (free, at most libraries and rec centers during heat events).
Medication review: Anyone on diuretics, beta blockers, anticholinergics, GLP-1 medications, lithium, or antipsychotics should not silently adjust their own dose during heat. Check in if you are concerned, especially if you are losing weight on a GLP-1 and adding heat-related fluid loss.
Athletes: Reduce session length, increase rest intervals, and weigh in before and after long efforts. A drop of more than 2 percent body weight is meaningful dehydration.
Check on neighbors: Especially older adults living alone. This is the single highest-leverage move in a heat event.

Excessive Heat Warning

Issued for forecast heat index of 105 degrees or higher for two or more hours, or 110-plus for any duration. This is the dangerous tier.

Everyone: Avoid outdoor exertion entirely during the heat of the day. If you must be outside, plan it for sunrise hours or after dark.
Run the air conditioning even if you usually do not. Fans alone are not protective above 95 degrees of ambient air; they can actually accelerate fluid loss without cooling you.
Recognize heat illness early. Heat exhaustion: heavy sweating, weakness, headache, lightheadedness, nausea, muscle cramps. Move to AC, rehydrate, cool the skin with damp cloths. Heat stroke: confusion, altered consciousness, hot dry skin, body temperature above 104. This is a medical emergency. Call 911. Begin active cooling (ice packs to neck, groin, axillae) while waiting.
Sensitive groups: Cooling centers and family members. Do not power through. Heat stroke mortality in older adults is significant.
At home: Close blinds, cluster activity in one cool room, take cool showers. If you lose power, leave for a cooling center; sheltering in a sealed home without AC is more dangerous than the trip.
Cardiovascular caution: Heat events increase emergency department visits for heart attacks, strokes, and arrhythmias. New chest pain, breathlessness, palpitations, or syncope during a heat event are not "just the heat." Get evaluated.

Cold Weather Advisory

Everyone: Layer in three stages (wicking base, insulating middle, wind-blocking outer). Cover face, hands, and ears. Limit time outdoors.
Asthma and COPD: Cold dry air is a powerful airway trigger. Pre-treat with rescue medication 15 to 30 minutes before extended outdoor time. Cover the mouth and nose with a scarf or buff to warm and humidify the air.
Cardiovascular disease: Shoveling, brisk walking in cold, or hurrying outdoors raises cardiac demand sharply. Take it slowly, pause if you feel chest tightness, and warm up gradually.
Ice falls: Most cold-weather emergency visits in Philadelphia are not from cold itself but from falls on ice. Microspikes or YakTrax for boots cost 20 dollars and are worth it.
People experiencing homelessness: Project HOME and the city run cold-weather outreach; if you encounter someone in distress, the city's homeless services line is the right call.

Extreme Cold Warning

Issued when wind chill values are forecast at minus 15 degrees or colder. Less common in Philadelphia than further north, but happens during major Arctic blasts.

Everyone: Limit outdoor exposure to brief, essential trips. Hypothermia and frostbite times shorten dramatically; at wind chill of minus 20, frostbite on exposed skin can occur in roughly 30 minutes, and faster with wind.
Pets, pipes, and people: Bring pets in, drip faucets to prevent pipe bursts, and check on older neighbors. Hypothermia in older adults can develop quietly indoors when heating fails.
Recognize hypothermia and frostbite. Hypothermia: shivering (early), confusion, drowsiness, slurred speech. Frostbite: numbness, white or grayish skin, hard or waxy texture. Warm gradually in lukewarm water, not direct heat; do not rub frostbitten skin. Severe hypothermia (no shivering, altered consciousness) is a 911 call.
Heart failure and COPD patients: Cold drives both. Have rescue medications and a plan; thresholds for getting evaluated should be lower during an extreme cold warning.

Severe Thunderstorm Warning

Issued when a thunderstorm with damaging winds (58 mph-plus) or large hail (1 inch-plus) is occurring or imminent. The summer pattern in Philadelphia is afternoon cells crossing from the west.

Indoor shelter: Get inside immediately. Avoid windows. Lightning kills 20 to 30 Americans a year; severe wind takes more. The "30/30 rule" still applies: if thunder follows lightning within 30 seconds, you are within strike range, and stay indoors 30 minutes after the last thunder.
Outdoor activities (Eagles tailgates, Phillies games, marathons, kids' sports): Move inside, into a vehicle, or into a substantial building. Open shelters and dugouts do not count.
Asthma: Thunderstorm asthma is a real phenomenon. The combination of pollen rupture, rapid pressure shifts, and dust can trigger flares hours before or during a storm. Pre-treat if you have a flare-prone pattern.
Power outages: Charge devices, have flashlights, do not run generators or grills indoors or in attached garages (CO poisoning is a leading cause of post-storm death). Refrigerator and freezer guidance: a closed fridge keeps food safe for about 4 hours, a closed freezer 24 to 48 hours.

Tornado Warning

A tornado has been sighted or indicated by radar. Uncommon in Philadelphia but not theoretical; small tornadoes have touched down in the metro area.

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Shelter immediately. Lowest interior room, away from windows. Basements, interior bathrooms, and closets. Cover with mattresses or heavy blankets if possible.
Mobile homes and cars: Leave for sturdier shelter if there is any safe option. If trapped in a vehicle and a sturdy building is not reachable, lie flat in a low area away from trees and power lines.
After: Watch for downed power lines, gas leaks, structural instability. Photograph damage for insurance.
Asthma and respiratory patients: Dust and debris in the air after a tornado are a serious flare trigger. Mask up before going outside to assess.

Flash Flood Warning

Turn around, do not drown. Two feet of moving water can carry a car. Do not drive through standing water of unknown depth; flooded streets are the leading cause of flood deaths.
At home: Move people, pets, and valuables to higher floors. Unplug electronics in the basement. If water rises rapidly, get out before the basement stairs become a hazard.
After: Floodwater is contaminated (sewage, gasoline, debris). Avoid skin contact when possible. Tetanus boosters should be current. Mold remediation starts within 24 to 48 hours of dry-out.
Medications and medical equipment: If you rely on home dialysis, oxygen, CPAP, or infusion pumps, have a 7-day backup plan and know your nearest dialysis center, oxygen vendor, or hospital before the alert hits.

Winter Storm Warning

Issued for significant snow accumulation (typically 6 inches or more in 24 hours in this area), or for an ice storm with at least a quarter inch of ice. Often combined with cold weather alerts.

Pre-storm: Stock the basics (water, shelf-stable food, flashlights, batteries, medications for 7 days, baby formula and pet food if applicable). Charge devices and battery banks. Park strategically to avoid being blocked in.
During: Stay off the roads if possible. Salt and ice on the Schuylkill Trail and sidewalks make falls common; wait 24 to 48 hours after city treatment for any non-essential walk.
Shoveling: Snow shoveling is a classic heart attack trigger. Push, do not lift; take frequent breaks; warm up first. If you have known coronary disease, delegate or use a snow blower.
Power outages and generators: Generators belong outside, away from windows. CO poisoning during winter storms is a recurring tragedy. CO detectors save lives in this scenario.
Vulnerable neighbors: Heat, food, and a check-in are the high-leverage things during a winter storm with power loss. If older neighbors live alone, this is the moment to knock.

Air Quality Alert

NWS sometimes issues Air Quality Alerts during heat events, ozone-action days, or wildfire smoke episodes. The clinical actions overlap heavily with the heat or smoke guidance.

The combined risk is more than the sum. Heat plus ozone, or heat plus PM2.5, hits cardiovascular and respiratory systems together.
Run indoor air filtration during smoke days. A HEPA in the bedroom, MERV 13 in the HVAC, windows closed, HVAC on recirculate.
See the Philadelphia AQI guide for the tier-by-tier action steps for the air-quality side.

Guidance from the clinic

Actionable Steps

Set this up before the next major event.

Build the household plan. Where do you go if you lose power for more than 24 hours. Who do you check on, and how. What is your nearest cooling center and warming center.
Stock the basics. 7 days of medications, water (one gallon per person per day for 3 days), shelf-stable food, flashlights, battery banks, a working CO detector, and a tetanus booster within the last 10 years if you might encounter floodwater.
Know your highest-risk medications. Diuretics, beta blockers, anticholinergics, GLP-1 medications, lithium, and antipsychotics all change the calculus in heat. Ask us at your next visit how yours specifically change during severe weather.
Subscribe to NWS alerts and the city's notification system. ReadyPhiladelphia for citywide alerts; the NWS Philadelphia/Mount Holly office for forecast and alert updates.

Key Takeaways

The alerts that matter most for health in Philadelphia are Heat Advisory and Excessive Heat Warning, Cold Weather Advisory and Extreme Cold Warning, Severe Thunderstorm and Tornado Warnings, Flash Flood Warning, and Winter Storm Warning.
Medications that change the math most: diuretics, beta blockers, anticholinergics, GLP-1 medications, lithium, antipsychotics, and any sedating drug.
Conditions that change it most: cardiovascular disease, heart failure, kidney disease, asthma and COPD, pregnancy, and older age.
The single highest-leverage move is a written household plan that exists before the alert hits.
During combined alerts (heat plus air quality, cold plus power loss), the risk is more than the sum of either alone.

Scientific References

Bouchama, A., & Knochel, J. P. (2002). Heat stroke. New England Journal of Medicine, 346(25), 1978-1988.
Basu, R. (2009). High ambient temperature and mortality: a review of epidemiologic studies from 2001 to 2008. Environmental Health, 8, 40.
Anderson, B. G., & Bell, M. L. (2009). Weather-related mortality: how heat, cold, and heat waves affect mortality in the United States. Epidemiology, 20(2), 205-213.
D'Amato, G., Vitale, C., Rosario, N., et al. (2019). Climate change, allergy and asthma, and the role of tropical forests. World Allergy Organization Journal, 12(2), 100011.
Watts, N., Amann, M., Arnell, N., et al. (2021). The 2020 report of The Lancet Countdown on health and climate change. The Lancet, 397(10269), 129-170.
National Weather Service. (2024). NWS Watch, Warning, Advisory Display - definitions and thresholds. National Oceanic and Atmospheric Administration.

Philadelphia AQI: what to do - the practical playbook for air quality days in Philly
Pollen in Philadelphia - the tree/grass/weed calendar and when to pre-treat
Microplastics in Philadelphia - the five highest-leverage moves to lower your exposure
UV Index in Philadelphia - the daily UV calculus for skin and eye protection
Philadelphia Environmental Health (overview) - the city-wide environmental health framework

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Severe Weather in Philadelphia: What to Do When the Alert Hits

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Heat Advisory

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Extreme Cold Warning

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Tornado Warning

Flash Flood Warning

Winter Storm Warning

Air Quality Alert

Guidance from the clinic

Actionable Steps

Key Takeaways

Scientific References

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Severe Weather in Philadelphia: What to Do When the Alert Hits

Severe Weather in Philadelphia: Heat, Cold, and Storms - What to Actually Do

Who is at higher risk across weather extremes

What these admissions actually look like

Heat Advisory

Excessive Heat Warning

Cold Weather Advisory

Extreme Cold Warning

Severe Thunderstorm Warning

Tornado Warning

Flash Flood Warning

Winter Storm Warning

Air Quality Alert

Guidance from the clinic

Actionable Steps

Key Takeaways

Scientific References

Related at Fishtown Medicine

Fishtown Medicine | Longevity

Frequently Asked Questions

Common Questions

Deep-Dive Questions

Related Intelligence

Longevity Strategies | Fishtown Medicine

Air Quality in Philadelphia: How to Read AQI and What to Actually Do

Metabolic Health

Talk it through with Dr. Ash.