It's summer and the heat is on. The summer solstice in the Northern Hemisphere, or top half of Earth, is around June 21. That's when the sun reaches its most northern point over our planet. The days around June 20 are the longest of the year, and the rays from the sun beat almost directly down on use and warm us up.

Longest, Not Hottest

You might think that the longest days of the year would be the hottest. But they are not. The hottest weather, on average, comes about a month after the summer solstice.  This is because the amount of heat from the sun continues to accumulate during the long hot days, and the short nights don't allow as much heat to leave. The days start to cool down only after the days grow short enough to allow more heat to leave Earth's surface than arrives.


It is not the heat, they say, but the humidity. Humidity is the amount of moisture in the air.  When the air has so much moisture in it, our bodies don’t do a good job of cooling us down because sweat doesn’t evaporate as quickly from our skin. That is why we feel so hot on a humid day. In fact, the temperature we feel may be warmer than the actual air temperature.   This is called the heat index. Look at the figure below. If the temperature is 100 degrees and the relative humidity is 50 percent, the heat index, or the temperature we actually feel, is 120. 
Hot humid days can be dangerous for humans and animals, so stay out of the sun and drink lots of water when the humidity is high.


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