Why This Summer Feels Hotter Than the Thermometer Says
If it feels like this summer has been brutal, the numbers back you up. Since late June 2026, a powerful heat dome has parked itself over the central and eastern United States, breaking or tying dozens of daily temperature records. Washington, D.C. hit 102°F on back-to-back days — a record that had stood since 1872. Philadelphia touched 103°F, tying a mark last set in 1901. New York’s Central Park hit 100°F for the first time since 2012.
But here’s the detail that matters most for your safety: the air temperature on the news isn’t actually what your body experiences. That number is the heat index — and in many of the hardest-hit cities this summer, it’s climbed 10–15 degrees above the actual thermometer reading.
What a Heat Dome Actually Does
A heat dome forms when a large area of high pressure parks itself over a region and traps hot air underneath, like a lid on a pot. The trapped air compresses and warms further as it sinks, while the high pressure also blocks clouds and storms from moving in to cool things down. The result is day after day of intensifying heat with little relief, even overnight.
That’s exactly what’s played out this year: overnight lows in many affected cities haven’t dropped below 75–80°F, robbing people of the recovery period their bodies normally get after a hot day. Combined with unusually high humidity, this is what’s pushed heat index values into the 105–115°F range across the I-95 corridor and beyond.
Heat Index vs. Air Temperature: What’s the Difference?
The air temperature is just that — a straightforward physical measurement. The heat index (sometimes called “feels like” temperature) factors in humidity, because humidity directly affects how efficiently your body can cool itself.
Here’s why that matters: your body cools itself primarily through sweat evaporation. When humidity is high, the air is already saturated with moisture, so sweat evaporates more slowly — and your body’s main cooling system becomes far less effective. That’s why 90°F with 70% humidity can feel more dangerous than 95°F in dry air.
Use our Heat Index Calculator to check the real “feels like” temperature for your exact conditions, not just the number on your weather app.
How to Read Heat Index Risk Levels
- 80–90°F: Caution — fatigue possible with prolonged exposure or activity
- 91–103°F: Extreme Caution — heat cramps and heat exhaustion possible
- 104–125°F: Danger — heat exhaustion likely, heatstroke possible with continued exposure
- 126°F+: Extreme Danger — heatstroke highly likely
Several of the cities affected by this summer’s heat dome have spent multiple consecutive days in the Danger category. That’s a big part of why officials in Philadelphia and Washington, D.C. canceled outdoor Fourth of July parades and events entirely this year — the risk to marching bands, performers, and crowds standing outside for hours was simply too high.
Who’s Most at Risk
Heat is the deadliest weather hazard in the U.S., outpacing floods, tornadoes, and hurricanes in average annual fatalities. The people most vulnerable during events like this include:
- Older adults, especially those without reliable air conditioning
- Young children, who overheat faster and sweat less efficiently than adults
- Outdoor workers (construction, landscaping, delivery)
- People with heart, lung, or kidney conditions
- Anyone exercising or engaging in strenuous activity outdoors
How to Protect Yourself During a Heat Dome
- Check the heat index, not just the temperature, before heading outside — run your local numbers through our Heat Index Calculator
- Front-load outdoor activity to early morning hours, before the heat index peaks in early-to-mid afternoon
- Hydrate before you’re thirsty — thirst is a lagging indicator of dehydration
- Know the warning signs of heat exhaustion (heavy sweating, weakness, nausea, cool/clammy skin) versus heatstroke (hot/dry skin, confusion, no sweating) — the second is a medical emergency
- Never leave children or pets in parked cars, even briefly — interior temperatures can climb 20°F in ten minutes
- Check on elderly neighbors or relatives, especially those living alone without A/C
Is This the New Normal?
Climate researchers who study event attribution have found that heat waves of this intensity and duration are becoming significantly more likely as global temperatures rise. Analysis of this summer’s heat dome specifically found that heat and humidity of this magnitude would have been extremely unlikely without the influence of fossil-fuel-driven climate change. That doesn’t make any single heat wave “caused” by climate change in isolation, but it does mean these extreme events are becoming both more frequent and more intense over time — which is exactly why understanding your real heat index risk (not just the raw temperature) matters more each summer.
Planning ahead for the next heat wave? Try our Heat Wave Calculator to see how upcoming conditions in your area stack up, and bookmark our Heat Index Calculator for quick daily checks all season long.