Context:

High humidity combined with extreme heat is making outdoor conditions feel more oppressive across much of the United States, particularly in urban areas. The heat index, which accounts for both temperature and humidity, can make temperatures feel significantly hotter, posing health risks such as heat exhaustion and heat stroke. Urban heat island effects exacerbate these conditions, with cities retaining heat overnight, further preventing relief. Dew points are also contributing, with values above 65 degrees Fahrenheit making conditions feel muggy and uncomfortable. A persistent heat dome is forecasted to maintain these high temperatures and humidity levels, especially in cities on the East Coast, with limited overnight cooling relief expected.

Dive Deeper:

• Humidity significantly affects how hot weather feels, as it slows down the body’s natural cooling process of perspiration, leading to increased discomfort and health risks. This is measured by the heat index, which can be much higher than the actual temperature, especially in areas with high moisture content in the air.

• Urban areas experience a phenomenon called the urban heat island effect, where heat is absorbed by dark surfaces and released slowly, keeping city temperatures warmer through the night. This effect is compounded by tall buildings blocking breezes and structures retaining moisture, making cities feel hotter and more humid.

• Meteorologists use the dew point to measure humidity comfort levels, with values over 65 degrees Fahrenheit indicating muggy and oppressive conditions. This factor, combined with high temperatures, has led to significant discomfort in areas like Washington D.C., where dew points have reached 72 degrees Fahrenheit.

• A heat dome has settled over the Eastern United States, causing widespread high temperatures and maintaining a lid of hot air. This weather pattern is responsible for the prolonged heat wave, causing temperatures to soar into the 90s and 100s, with heat index values nearing 110 degrees in some locations.

• Excessive heat warnings have been issued in cities like Philadelphia and New York City, prompting public health responses such as opening cooling centers and advising residents to check on vulnerable populations. These measures are crucial as the overnight temperatures remain high, offering little respite from the daytime heat.