Why June seemed cooler than it was | Weather

A heat advisory is in effect for Wednesday from Richmond to Hampton Roads. The combination of heat and humidity, known as the heat index, will approach 105 during the hottest part of the day, making it especially difficult for the body to cool itself.

Every day through Friday will bring a chance of a shower or thunderstorm, with the chance each day just a little bit better than a coin flip. Weak disturbances will move in from the northwest to help trigger the occasional shower or thunderstorm, but a real perceptible break in the heat and humidity will have to wait until the weekend.

For Richmond, the recent surge in high humidity comes a little bit later than in an average year and may be why last month did not feel especially hot, even though it was warmer than normal for June.

Last month was the least-humid June in a decade in Richmond. Not coincidentally, it had the coolest nights of any June since then.

The average low temperature last month in Richmond was 64.3 degrees, which was the lowest since 2012 (62.3 degrees). And 2012 was the only June so far this century that had nights, on average, that were cooler than the full 20th century average.

The cooler nights can directly be tied to the lower humidity last month, as drier air cools more easily than humid air. Humidity can be measured in two ways. The relative humidity is relative to the temperature. If the air is saturated, the relative humidity is 100%, whether the actual air temperature is 32 or 82 degrees. When it comes to summer humidity, the percentage is not easily perceived by the body.

A more direct way to measure humidity is the dew point temperature, which is the temperature at which saturation occurs in the atmosphere. When that number is higher, more water molecules are physically present in the air, and that is what our bodies notice when heading outside.

During summer in Virginia, dew points in the lower-to-middle 60s are quite nice. Occasionally, they will edge downward into the 50s, which gives the air a spring or fall feel. But most of the time, the summer dew point in Richmond is between 65 and 75. Once that value gets above 70, the air has a truly muggy feeling to it. And above 75, it is genuinely oppressive.

It is rare for the value to climb as high as 80, which would be stifling. It also serves as a reminder that “95 degrees with 95% humidity” doesn’t really happen in Virginia. If that were the case, the dew point would be 93 degrees, and that wouldprobably make it legitimately dangerous to be outside.

Over the last 70 years, the average June dew point in Richmond is 64.4. But since 2010, only three Junes have been below that average: 2012, 2016 and 2022. The value for this past June was 63.6.

But the long term trend is upward. As the planet warms, more water can evaporate into the atmosphere. Examining the first 20 years of the record compared to the last 20 years illustrates the slow but steady upward tick.

The average for 1951 through1970 was 63.4. The average from 2001 through 2020 was 65.1. There will still be occasional ups and downs; but in the long term, heat and humidity are both on the rise as the climate warms.