Forecast Heat Index Values: Courtesy Weather Channel

Much of the eastern two-thirds of the United States has been in the grip of an oppressively hot and extended humid spell of weather this summer, with over 27 states in some sort of heat advisory. In some locations, the heat has been intense, with triple-digit temperatures running into record periods combined with a lack significant precipitation (rainfall). As of August 7th, Dallas has had 37 days in a row of high temperatures 100 degrees or higher.

So just what is the cause for this incredible siege of hot weather?

Inflating like a balloon

Air pressure is the force exerted on us by the weight of tiny particles of air (air molecules). Although air molecules are invisible, they still have weight and take up space. When it's compressed (pressed together), air is said to be "under high pressure". Weather forecasters measure air pressure with a barometer. Barometers measure the current air pressure at a particular location in "inches of mercury" or in "millibars" (mb).

Coinciding with Independence Day weekend, a ridge of high pressure both in the lower and upper levels of the atmosphere began to inflate almost like a balloon over the middle of the country. Because high pressure is associated with sinking air (think of it as pressing down), air from the upper levels of the atmosphere descends and rotates outward. The large swell of hot air has been expanding and moving the bubble of heat with it eastward, almost like the crest of a wave.  

The outward flow also prevents other weather systems from moving in. Because high pressure is also associated with clear skies, the intense sunshine can become punishing as the air mass continues to maintain its heat content.  

We saw here that the record floods experienced this spring in the Midwest have saturated the ground. The rivers and lakes are out of their banks. The excess water is resulting in the water turning to vapor and being released into the atmosphere, resulting in high dew points. When dew point is lower (low humidity of about 40 to 50 degrees), the weather is comfortable as the sweat from our bodies can evaporate allowing our it to cool. However, the high dew points are oppressive especially when they are over 75 degrees Fahrenheit.

Nearly three weeks after it began to evolve, the high pressure system has enveloped much of the nation's midsection and east coast. Anytime such a weather feature becomes established, weather forecasters know that there will be only very slow changes in the weather, because there are no upper winds to steer the high pressure system away.

Illustration of how an Urban Heat Island occurs

Urban Heat Island

Another phenomenon occurring in urban centers is that buildings, concrete, asphalt, and the human and industrial activity have caused cities to maintain higher temperatures than their surrounding countryside. This increased heat is known as an Urban Heat Islands (UHI). 

The combination of the high pressure build up and the UHIs will likely cause the excessive heat to persist until a significant pattern change in the atmosphere occurs.