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Heat Season in Pennsylvania

Philadelphia typically sees its first 90 F day around May 29 and its last around September 6. Here is the full window, the rest of the state, the overnight lows, and any active National Weather Service heat product.

Typical 90 F season at Philadelphia: May 29 to September 6
Days 90 F or hotter (Philadelphia)
30
Typical first 90 F day
May 29
Nights 75 F or warmer
10
Cooling degree days
1358

Pennsylvania's 90 degree season

1991-2020, Philadelphia
Today
The bar spans the median first 90 F day to the median last one at Philadelphia. Half of years open earlier than the left edge and half close later than the right, so treat it as the middle of the distribution rather than a boundary.
May 3
Early years
first 90 F by here, 1 year in 10
May 29
Median first 90 F
the 50/50 date
September 6
Median last 90 F
the other bookend
102 days
Median span
first 90 F day to last

Philadelphia's heat is the Northeast corridor version: asphalt, humidity, and nights that do not let go. Pittsburgh sits higher in valleys that drain at night, and Erie keeps the lake's brake through most of early summer.

City by city

Heat across Pennsylvania, station by station

A single state number would hide the thing that actually matters: elevation, latitude and distance from water move these figures further inside Pennsylvania than a whole decade of warming does. Each row is one NOAA reference station. The day counts are quoted from NOAA's published 1991 to 2020 normals; the dates and the warm night counts are our own derivation from the same agency's daily observations, because NOAA publishes no date shaped heat normal.

StationDays 90 F or hotterDays 100 F or hotterFirst 90 F dayLast 90 F dayNights 75 F or warmer
Philadelphia300.5May 29September 610
Pittsburgh100June 19September 10.3
Erie4.70June 18August 121.4

Hottest days

30

days a year at or above 90 F at Philadelphia, the highest of this page's Pennsylvania stations.

Quoted NOAA normal

Warmest nights

10

nights a year at Philadelphia whose low never falls below 75 F.

Our derivation

Hottest on record

104 F

at Philadelphia on July 3, 1966, in a station record running 1940 to 2026.

Our derivation
Two National Park Service rangers beside the Furnace Creek visitor center sign in Death Valley reading 129 degrees Fahrenheit
Rangers at Furnace Creek in Death Valley, 190 feet below sea level, beside the visitor center display. Official climate records come from a separate instrument in a standard ventilated shelter, which is the kind of reading every number on this page is built from, and it is why an official high so often looks low next to a sign or a car dashboard. “Furnace Creek Visitor Center park rangers digital thermometer sign, 20 July 2020” — E. Deldin / National Park Service — Public domain
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The number nobody publishes

The nights are the story, not the afternoons

Almost every heat page on the internet counts hot days. Very few count hot nights, and the overnight low is the half of the day that decides what a hot spell actually does. A 98 degree afternoon followed by a 68 degree night is a completely different event from a 94 degree afternoon followed by a night that never leaves the 80s: buildings shed their stored heat overnight, air conditioning gets its only break overnight, and the body's recovery happens overnight.

In Pennsylvania, Philadelphia averages 10 nights a year whose low never falls below 75 F, and its warmest night on record bottomed out at 83 F. NOAA's published normals stop at 70 F, so the 75 F count is our own arithmetic on NOAA's daily minimum temperatures over the same 30 years as every quoted normal on this page.

Nights that stay above 75 F

10

a year at Philadelphia. Derived from NOAA daily minimum temperatures, 1991 to 2020.

Our derivation

Nights that stay above 70 F

44

a year at Philadelphia, on the same derivation. The gap between this and the 75 F figure is the band where a night is warm but still recovering.

Our derivation

Cross check against NOAA

30 vs 30

This site's count of 90 F days at Philadelphia against NOAA's published normal for the same station and years. The check runs on every station in this dataset; the warm night counts use the same method against a threshold NOAA does not publish.

Derived vs NOAA's
People still out at a lit riverfront fountain in Cincinnati at dusk, with the Roebling Bridge and downtown skyline behind them
Cincinnati's riverfront after sunset, still busy. Pavement, brick and asphalt release stored heat for hours once the sun goes down, which is why a city's overnight low can sit several degrees above the countryside a few miles out on the very same night. “Interactive fountain, Cincinnati, Ohio” by LouisvilleUSACE / Carol Labashosky, U.S. Army Corps of Engineers, CC BY 2.0
Right now, from the agency

What the National Weather Service has out in Pennsylvania

Everything below is read live from api.weather.gov when this page loads and is shown exactly as the issuing forecast office wrote it, with the office named.

Live from the National Weather Service
Active heat products in Pennsylvania

Checking api.weather.gov for active heat products.

Product text, issuing office, area and times are passed through exactly as the National Weather Service issued them. As of 2025 the NWS uses three heat products: Extreme Heat Warning, Extreme Heat Watch and Heat Advisory. Read . Source: api.weather.gov

Live from the National Weather Service
The next seven days at Philadelphia

Loading the National Weather Service gridpoint forecast.

The cooling season

When Pennsylvania actually needs cooling

Cooling degree days are the measure utilities and mechanical engineers plan against, and NOAA publishes them directly, so every figure in this section is quoted rather than calculated by us. One cooling degree day is one degree of daily mean temperature above 65 F, added up. At Philadelphia the peak month is July, and the months carrying real cooling demand run May through September.

Cooling degree days by month at Philadelphia, quoted from NOAA's published 1991 to 2020 normals. One cooling degree day is one degree of daily mean temperature above 65 F, so the bars show how much cooling the calendar actually asks for rather than how hot any single afternoon got.

NOAA's 1991 to 2020 normals publish four things these pages quote directly: annual and monthly cooling degree days, the mean number of days a year at or above 90 F, the same at 100 F, and the mean number of days whose low stays under 70 F. NOAA publishes nothing date shaped for heat and nothing about nights warmer than 70 F, so the dates and the 75 F counts on this page are this site's derivation, labelled as such.

Our summary of information published by NOAA NCEI 1991-2020 U.S. Climate Normals

Rooftop air conditioning condensers on brick buildings in Washington, D.C.
Rooftop condensers over Washington, D.C. Cooling degree days are ultimately a building question: the same summer costs very different amounts depending on what the building is made of and whether the night gives it a chance to dump its heat. “Air conditioning systems on rooftops in Washington, D.C.” by Ralf Roletschek, CC BY-SA 3.0
Heat index and heat illness

What the agencies say, in their own words

The heat index is a National Weather Service product. Where a heat index appears on this page it is the value the NWS itself published for that gridpoint. Here is the NWS defining it, and here is the caveat that gets left out most often:

“The Heat Index is a measure of how hot it really feels when relative humidity is factored in with the actual air temperature.”

National Weather Service, Heat Forecast Tools

“Since heat index values were devised for shady, light wind conditions, exposure to full sunshine can increase heat index values by up to 15°F. Also, strong winds, particularly with very hot, dry air, can be extremely hazardous.”

National Weather Service, Heat Forecast Tools

That second quote is the one worth rereading. A heat index is a shade figure. The NWS publishes the full heat index chart, which pairs air temperature against relative humidity, at weather.gov/safety/heat-index, and that chart is the thing to read rather than any version of it we could redraw.

Heat and health: the agencies, in their own words

Health guidance on heat comes from the National Weather Service and the CDC. Their words are below, and their full guidance is linked.

The National Weather Service on who heat affects: “Heat can be very taxing on the body and can lead to heat related illnesses or make existing health conditions worse. Everyone can be vulnerable to heat, but some more so than others.” (National Weather Service, Heat Safety Tips and Resources)

The CDC on knowing when something is wrong: “If your body gets too hot, you can get sick. Know the symptoms of your body overheating and know when to seek medical care.” (CDC, About Heat and Your Health)

Full guidance: NWS heat safety and CDC Heat and Health. The CDC also runs a CDC HeatRisk Dashboard for forecast heat by ZIP code.

On the warning signs of heat illness, the National Weather Service is quoted directly below. We have nothing to add to it and no business rephrasing it.

“Symptoms: Heavy sweating; weakness or tiredness; cool, pale, clammy skin; fast, weak pulse; muscle cramps; dizziness, nausea or vomiting; headache; fainting.”

National Weather Service, Heat Cramps, Exhaustion, Stroke

“Symptoms: Throbbing headache; confusion; slurred speech; nausea; dizziness; body temperature above 103°F; hot, red, dry or damp skin; rapid and strong pulse; fainting; loss of consciousness.”

National Weather Service, Heat Cramps, Exhaustion, Stroke

“Call 911 or get the victim to a hospital immediately. Heat stroke is a severe medical emergency. Delay can be fatal.”

National Weather Service, Heat Cramps, Exhaustion, Stroke

“Use fans, but only if indoor temperatures are less than 90°F. In temperatures above 90°F, a fan can increase body temperature.”

CDC, About Heat and Your Health

“It is NEVER safe to leave a child, disabled person or pet locked in a car, even in the winter.”

National Weather Service, Heat Safety Tips and Resources
Children playing in a public plaza fountain beside the Atlantic in San Juan, Puerto Rico
A plaza fountain in San Juan, Puerto Rico. Shade, water and somewhere cooler to be are what heat guidance from every agency comes back to. Many places also open cooling centers during declared heat events, and that list is published by county or city emergency management rather than by the weather service. “Fountain with children playing and Atlantic Ocean, San Juan, Puerto Rico” by MusikAnimal, CC BY-SA 4.0
Where these numbers come from

How the Pennsylvania figures are calculated

Two registers, kept apart. Quoted: the day counts and the cooling degree days are NOAA NCEI's published 1991 to 2020 U.S. Climate Normals, printed here as NOAA publishes them. Derived: the first and last 90 F dates, the season span and the warm night counts are ours, computed from NOAA NCEI GHCN-Daily station observations over the same 30 years, because the normals product publishes no date shaped heat variable and stops at 70 F for nights.

The rules are the same at every station. A 90 F day is a day whose high reached 90 F or more with a clean NOAA quality flag. A warm night is a day whose low never fell below 75 F. A year only counts if at least 85 percent of its days reported; incomplete years are excluded rather than filled in. Percentile dates are the 10th, 50th and 90th over the years that qualified, and where a percentile lands on a year that never reached 90 F at all, no date is printed. Records use each station's full period of record, which runs decades further back than the normals period.

The cross check: NOAA publishes a 90 F day count, and this site computes its own from the daily records. Across all 150 stations in this dataset the two agree within two days a year at 149 of them, and the single exception is a station missing eight of its thirty years. The warm night counts use exactly the same counting rule against a threshold NOAA does not publish, which is the best evidence available that they are right. One detail matters if you try to reproduce this: a station reports whole degrees Fahrenheit, and NOAA's day count normals include the threshold itself, so a night recorded at exactly 75 F counts as a 75 F night here, as it does for NOAA.

The page's shape follows those numbers. Where a state's primary station clears 90 F on 30 or more days a year, a first 90 F date carries almost no information and the page leads with season length and nights instead. Where the station reaches 90 F in fewer than half of years, there is no median and the page leads with cooling degree days, records and warm nights. Pennsylvania is on the countable heat shape.

Live conditions come from api.weather.gov, read in your browser at page load. Sources: NOAA NCEI 1991-2020 U.S. Climate Normals, NOAA NCEI GHCN-Daily. Both are US Government works in the public domain.

Heat shimmer distorting the far end of a straight two lane highway through Mississippi farmland
Heat shimmer on a Mississippi highway: air near the road surface hot enough to bend light. Thirty years of daily observations is enough to describe a normal summer and nowhere near enough to predict next week. Once you are inside the season, the forecast takes over from the climatology. “Truck approaching in Mississippi” by Jeroen Komen from Utrecht, Netherlands, CC BY-SA 2.0
Common questions

Pennsylvania heat season FAQ

Keep planning

The rest of Pennsylvania's summer

Pennsylvania UV indexThe other thing the same sun is doing. Peak UV runs on a different calendar than peak heat.Pennsylvania air qualityHeat drives ground level ozone, so the worst air days and the hottest days tend to be the same days.Pennsylvania mold seasonWarm nights that never dry out are what indoor mold runs on. The dewpoint side of the same summer.Pennsylvania heating seasonThe cold half of the same year on the same stations: when the heating season opens, how long it runs, and how far the nights fall.

Heat season in nearby states

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Day counts and cooling degree days are quoted from NOAA NCEI's 1991-2020 U.S. Climate Normals. Dates, season spans and warm night counts are our derivation from NOAA NCEI GHCN-Daily observations over the same period. Live products come from the National Weather Service and are shown verbatim. National picture: heat season in every state