Cause of the Southwest Heat Wave: Too Little Water Vapor
Yesterday we read the following headlines:
"Dangerous heat wave forecast in Southwest"
– USA TODAY
"Scorching Southwest heat wave could challenge all-time records"
– Washington Post
"Weekend heat wave to bake western US; temps in southwestern cities to near 120"
- Star Tribune
As of now, the Southwestern United States is threatened with a heat wave and for good reason: there is not enough water vapor present in the air to keep the temperature down. Take a look at this snapshot of the distribution of water vapor over North America taken June 27th.
As you can see there is a dearth of water vapor in the Southwestern United States at this time and without the presence of water vapor to keep the lower atmosphere refrigerated the temperature predictably goes up, just like what happens when you don't put water in your swamp cooler. Humidity low = temperature high; Humidity high = temperature low. Thankfully for the people living in Kentucky and Tennessee they have plenty of water vapor present in the air to keep their June temperatures moderate this year. For example yesterday in Bowling Green, Kentucky the temperature averaged 84 °F, which is 20 °F cooler than its record high of 104 °F in 1914; in Nashville, Tennessee the temperature also averaged 84 °F, which is 18 °F cooler than its record high of 104 °F in 1952 and yesterday in Little Rock, Arkansas the temperature averaged 88 °F, which may seem high but was still 10 °F cooler than its record high of 98 °F in 2011.
Speaking of Little Rock, there is no mystery why its higher humidity keeps its temperature lower than the temperature in arid Las Vegas. It is explained in detail in Climatology textbooks and is demonstrated in the following graph of data gleaned from weather balloon soundings over each location during the past four days:
In this graph the red line is the temperature profile above Las Vegas at 700 m, 6 km and 12 km in altitude and the blue line is the temperature profile above Little Rock at the same altitudes measured by daytime weather balloons over the past four days. The "mixing ratio" (g/kg of water vapor in the air) was three time greater in Little Rock and as you can see that extra humidity decreased the temperature lapse rate (the rate at which the air warms as one descends in altitude) in the lower atmosphere above Little Rock down to 6.3 °C/km while the temperature lapse rate in the lower atmosphere above Las Vegas was 7.9 °C/km over the past four days. You will also notice that above 6 km the air temperature and lapse rates above both locations are virtually identical as the air cools above that altitude at a rate of 8°C/km.
It is well understood that as the daytime sun heats the ground, which, in turn, heats ground level air that this high-energy air expands, becomes less dense and ascends skyward. This, in turn, draws cooler, more-dense air from aloft down to the ground to replace the ascending air. As we see in the above graph, the air temperature at 6 km above both Little Rock and Las Vegas is nearly identical at -6 °C. So, as this -6 °C air descends into the arid Nevada climate it warms at a rate of 7.9 °C/km, while at the same time the air from the same altitude at the same temperature only warms at a rate of 6.3 °C/km as it descends into the humid Arkansas. Therefore a nearly 9 °C temperature differential has emerged by the time the air from 6 km in altitude above each location has descended to ~700m in altitude (the altitude of Las Vegas) with the arid air above Las Vegas being the warmer air.
Climatology textbooks also explain why. "The most common atmospheric adiabatic phenomena are those involving the change of air temperature due to change of pressure. If an air mass has its pressure decreased, it will expand and do mechanical work on the surrounding air . . . the energy required to do work is taken from the heat energy of the air mass, resulting in a temperature decrease. When pressure is increased, the work done on the air mass appears as heat, causing its temperature to rise. The rates of adiabatic heating and cooling in the atmosphere are described as lapse rates and are expressed as the change of temperature with height. The adiabatic lapse rate for dry air is very nearly 1 °C per 100 m.
"Large-scale atmospheric motions are approximately adiabatic."
—Fairbridge, Rhodes, w., Columbia University, The Encyclopedia of Climatology, Van Nostrand Reinhold Co, New York 1987
As the air descends in both locations, "the work done on the air mass appears as heat", but in the more humid climate a portion of that work energy is diverted to keeping the water vaporized, i.e., it turns into "latent heat" and "latent heat" does not raise the air's temperature.
Thus standard Climatology explains why heat waves only occur during times of low humidity, as is the current situation in the Southwestern United States. I understand that this runs counter to the "greenhouse effect" hypothesis, which asserts that water vapor "traps" heat in the air and therefore the more humid climate should be the warmer climate, but science is not about what might be a popular belief at any particular point in time, rather it is about what is seen to happen in the real world.