Back on July 6 and 7, when we suffered through the hottest days of the summer thus far, you may recall it wasn't all that humid. Though Reagan National sizzled to 102 both of those days and BWI touched a jaw-dropping 105 on 7/6, afternoon relative humidity values averaged just over 20% and dew points were in the relatively comfortable mid-to-upper 50s.
Compare those conditions to what we're predicting for this Saturday - when we think highs will reach the upper 90s to near 100 but relative humidity values will likely be closer to 40% with dew points around 70. Due to the added humidity, the coming heat may feel more oppressive than the record-breaking heat of just two weeks ago.
So what's the reason for the difference in the character of these two hot spells?
It all has to do with the origin of the two air masses.
The air mass responsible for the record heat in early July originated in the Upper Midwest and Great Lakes as a giant ridge of high pressure developed and then intensified as it headed towards the East Coast. Because the hot air mass was born over the interior of the North American continent and away from the ocean - it was relatively dry.
Such an air mass is classified as "Dry Tropical" as it was produced by rapidly descending air via strong subsidence (sinking) which tends to dry out the air. The intensity of the heat over our region and lack of humidity may have been amplified by a lack of soil moisture resulting from the rainfall deficit at the time.
Fast forward to this week and we're dealing with an air mass with a totally different origin. This week's air mass is originating from high pressure over the western Atlantic ocean, also known as the classic Bermuda High. The clockwise circulation around the Bermuda High brings warm and humid air mass from the south and southwest over the mid-Atlantic and is typically classified as "Moist Tropical" (MT). According to its description the MT air mass "is typically found in warm sectors of mid-latitude cyclones [low pressure] or in a return flow on the western side of an anticyclone [high pressure]." On any given day, you can determine what the air mass type is by visiting the Spatial Synoptic Classification Web site.
This coming Saturday, the potential for extreme heat and humidity from this MT air mass is elevated as the Bermuda high pumps in juicy warm air while we're simultaneously in the warm sector of an approaching area of low pressure system to the north. We could enter MT+ territory, reserved for the statistically most oppressive days.
The bottom line: this Saturday has the potential to be the most uncomfortably (and dangerously?) hot day of 2010. It may also break records. Saturday's record highs are: 96 at DCA, and 97 at IAD and BWI, all set in 1987.