No naps in the sun today. A Western Lowland gorilla finds some afternoon shade at the National Zoo. Angela N via Flickr.

*Air Quality Alert. Avoid strenuous activity or exercise outdoors* 

Happy Fathers Day! Conditions were not very conducive to spending a lot of time outdoors today. However, we’ve had some beneficial cloud cover for much of the day, moderating temperatures. Unfortunately, there won’t be much in the way of moderation Monday. The humidity will build tonight, and the heat will join the party tomorrow, making it a very uncomfortable day to be outside.

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Through tonight: Some isolated showers have been popping up to the west and south of the District, but I don’t expect any of these showers to affect our area. An air-quality alert remains in place, so make sure to avoid exercising outside. Cloud cover will slowly begin to break up as we approach sunset. It will remain quite warm well into the evening hours, with temperatures likely staying in the 80s until about 10 p.m. We won’t cool off very much overnight, with lows expected to be in the low to mid 70s. Dew-point values will be on the rise as well, likely reaching the low 70s by tomorrow morning. That means some areas of patchy fog are likely to develop. Winds will remain light and variable, generally from the south.

View the current weather at The Washington Post.

Tomorrow (Monday): Any low clouds or fog in the early morning will quickly give way to mostly sunny skies. After that, we are off to the races in the temperature department. Pretty much everyone should see temperatures reach the low 90s in the early afternoon, with metro locations likely to hit the mid 90s under a light southwest wind. Dew-point values will be in the low 70s, so the midday heat index will vary between 100 and 105 degrees. Technically, this would keep us below the heat advisory threshold (see below), but that won’t make it feel any less unpleasant. Another air quality alert is likely to be issued, so please limit strenuous outdoor activity. Quite warm and muggy tomorrow night, with lows in the mid to upper 70s.

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What is a heat advisory? We throw the phrase “heat advisory” around a lot this time of year, but what are the environmental conditions that prompt such advisories?

The heat index is often referred to as the “apparent temperature” because it represents the temperature that our bodies feel when we incorporate relative humidity.

If the body gets too hot, we sweat or perspire as a means to cool ourselves and regulate temperature. However, sweat and perspiration needs to evaporate off our skin to complete the cooling process. When the relative humidity of the atmosphere is high, there is essentially no more room in the surrounding environment for liquid, including sweat. Combine high temperatures with high relative humidity and you get a human body that is going to warm up very quickly without a proper cooling mechanism to regulate overheating.


Heat index advisory thresholds for the eastern U.S. via NOAA.

You may be surprised to learn that the threshold values for when a heat index advisory is issued vary by region. Take a look at the official threshold values in the image above, and you will notice that in general, it takes “more” to have a heat advisory issued the farther south one goes. Climatology is primarily considered when developing these thresholds, and not just atmospheric climatology. To a certain degree, the local populations’ exposure frequency to high heat and humidity is considered as well. Simply put, the D.C. region’s  population is more acclimated to high heat and humidity than most regions on the map.

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