Saturday, August 21, 2010

Why has it been so hot? And when is it going to end?

The Mid-South summer of 2010 will likely be remembered (after the flooding rains of early May) for the intense and prolonged heat and humidity. The vast majority of the comments and questions I get from folks run along the lines of "when is this heat/humidity going away?" This post will hopefully answer two questions for you: when and why.

First the why. While the ultimate "why" may not truly be understood yet from a scientific viewpoint, the reason for this summer's unrelenting heat across much of the eastern and central U.S. can be found by looking up - into the middle reaches of the atmosphere. A recent blog post by The Weather Channel's well-known Severe Weather Expert, Dr. Greg Forbes, helps to explain. While the main focus of the post attempted to explain the abnormal heat and lack of summertime thunderstorms across central FL, it's pertinent for this region of the country as well.

At about 20,000 feet up (meteorologists refer to this layer as the 500mb pressure level), a large and very strong ridge of high pressure has set up this summer over the southeastern portion of the U.S. Commonly referred to as the "Bermuda High," this summer it ended up being stronger and more well-entrenched than usual (see the image below showing the departure from normal of the pressures at the 500mb layer, courtesy The Weather Channel). This high pressure cell of warm air aloft kept frontal systems at bay well to our north for much of the summer, while also causing sinking air underneath it, which squashed the rising columns of air required to produce widespread thunderstorms.

The strength and blocking nature of the high is directly responsible for the generally uncomfortable conditions we experienced 20,000 feet lower - at the surface of the earth. The image below shows the departure from normal for surface temperatures, which correlates well to the same areas experiencing the strongest high pressure in the first image (also courtesy The Weather Channel).

So the next logical, or perhaps more pressing, question is, when does it end?? Well, I have good news! The frontal system passing through the Mid-South this weekend is actually the start of a transition we will be seeing over the coming days as the massive ridge of high pressure breaks down some and high pressure at the surface originates from the cooler northern climes rather than the tropics.

The image below (click for larger image) is one model's forecast of the temperature, dewpoint, and humidity trends for the coming week. Notice in particular the downward trend in temps and dewpoints (a measure of humidity in the air) for the next several days, as well as the reduction in relative humidity. While our dewpoints have predominantly been in the 70s this summer (with a few brief spells of drier air), the long-term trend shows dewpoints dropping into the 50s next week. While it will still be warm, the air will be drier, so there will be less talk of heat index and definitely some cooler mornings to look forward to! Hang in there! Fall is also just around the corner!

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Anonymous said...

Memphis weather for the last 2 yrs.have been some of the Hottest weather I have ever lived in.I have lived mostly in East Tn. most of my life Zone 7a.And Memphis is also zone 7a,I don't think so! If anything it is zone 9 or 10.I have lived in Tx. La. Ga.,went to many places in summer time,and memphis heat is like a desert!Maybe someone needs to look into climate zone again.I am a gardener and try to buy plants that needs this zone! Thanks

Meteorologist Erik Proseus said...

This summer in particular was much above normal temperature wise. The climate zones are not set based on one summer, but climatological averages over decades. It doesn't make it any easier to plant the appropriate vegetation necessarily, but climate zones will not be updated based on 1 or 2 hot summers. Sorry it didn't work out better for you this year!