WSET.com - ABC13Extreme Summer Heat by Matt Ferguson

So far the summer of 2010 has been a very hot one for our region, because of a persistent upper level high pressure system.  With us now in the middle of our third heat wave of the season, I thought it would be interesting to discuss summer extremes.  This week's scorching heat is likely to tie or exceed the high temperatures earlier this month.  As of Wednesday 7/21, the hottest weather this summer has been on Wednesday 7/7...  Danville 104, Roanoke 100, Lynchburg 98 and Blacksburg 94  You may be wondering..  What's the hottest temperatures ever recorded across our area???  Well it ranges from 99 in Blacksburg to 107 in Danville.   Check out the graphic below!

Typically our hottest weather occurs during the months of July and August.  Usually one particular weather setup will lead to the mercury hitting the century mark across Central and Southside Virginia.  Here is what happens...  A large ridge of high pressure develops aloft due not only to surface heating, but from above average temperatures high in the atmosphere.  This large upper level high helps to develop a surface high pressure system beneath it due to sinking air at all levels of the atmosphere.  When you have sinking air it's tough for clouds to develop much less rain.  Therefore you have lots of sunshine that leads to late day temperatures easily reaching the 90s.  Now we add in the downslope factor.

Air around a high pressure system flows clockwise.  If the high is located to our southwest, usually we receive a west to northwest wind.  This is called a downsloping wind.  As that 90 plus degree air is forced down the east side of the Appalachian Mountains (where we live), the air is compressed.  When this happens it automatically heats, that's just physics.  Also, with an upper level ridge and surface high nearby you usually have mostly sunny skies further promoting extreme heat.  This is the perfect setup to easily record high temperatures above 100.