The 2012 Drought in America series so far has looked at the immense impacts of the drought on farmers and taxpayers, impacts to the energy sector, how the drought may reshape American agriculture, strategies for building resilience, and finally the longer-term climate implications for drought. I thought it may be useful to have a post that looks at some of the basic underlying mechanisms of the drought-heat dance that can be applied not only to our current drought, but those of past and future as well. Or as one paper states it, as soil moisture decreases, “hot days tend to get hotter to a greater degree than cool days get warmer.”
It’s all about energy flow
The simplest climate model is only one equation and gives the temperature a body (like Earth) must be at to give off as much energy as it’s receiving from a source (like the sun). It is a simple balancing of energy flow in and energy flow out. Things get complicated in a hurry when you add greenhouse gases, ice sheets, and oceans, but the story never stops being one of balancing energy flows. Indeed, climate change arises when this balance of energy flow is disrupted and much work goes into trying to measure these flows from the deep seas all the way up to low-Earth orbit.
And where the energy goes
The energy flows of an entire planet are important to know, but it’s also just as important for on-the-ground conditions to know how incoming energy is divvied up. One only has to step outside on a hot summer day after a rain shower to see or feel this. The energy in would be from the sun. When it’s dry out that energy is primarily absorbed by the ground and heats up — sensible heat is the technical word. In this case the sun’s energy ends up heating the air just above the surface making it feel hot for us. Once the ground heats up it also can dissipate that heat by re-emitting it back to the air as infrared heat. This heat can be absorbed by heat-trapping gases in the air, which in turn re-radiates that energy in all directions. Infrared energy is a longer wavelength than sunlight and may be more familiar as those “night goggle” images of people and pets outside in the dark.
Now comes a nice rainstorm that soaks the ground and puddles abound. You’ll now notice that even after the clouds have parted and the sun is back out it stays cool for a while. That’s because now the sun’s energy can take a path that doesn’t involve heating the air just above the ground. That’s through latent heat.
Basically, the sun’s energy is going into heating and evaporating the various puddles and soggy surfaces. Energy is being carried away by this evaporation that may have otherwise gone into heating the ground and air near the surface. This energy is released somewhere else as the moisture eventually condenses out as clouds up in the atmosphere. It’s only after the surface moisture has evaporated (e.g. no more puddles and dry parched soil) that the temperatures begin to climb again.
But there’s more to it than just puddles after rainstorms and simple physics
There is a growing body of scientific literature looking at the connections between drought and high temperatures. The latest IPCC Assessment report included a section looking at these relationships and reported that often higher temperatures occur with lower precipitation and vice versa. There has been other work that discusses how low soil moisture (dry conditions) “primes the pump” for subsequent heat.
One study found that the distribution of daily maximum temperatures shifts with soil moisture content and that the hotter end of the distribution feels greater effects than the cooler end. They also found that in some regions elevated temperatures can hang around for weeks after the low soil moisture conditions. Also, reduced soil moisture was found to have played a role in elevating temperatures during the killer European heat wave of 2003.
Finally there was a recent study that found for many parts of the world, including North America, that the occurrence or risk of experiencing above average numbers of hot days increases 60-70 percent after periods of reduced precipitation. This study even zoomed in on Texas, which has been faced with a crippling drought over the past year. Dry years for Texas led to increases in the number of hot days experienced during those years. This probably won’t come as a surprise for people living there.
So, there are some pretty simple explanations linking precipitation (or lack of), surface moisture, evaporation, and temperature. It is a more detailed and intricate dance than presented here, but the short message is if there’s a drought there’s likely a heat wave lurking around the corner. And as we’ve seen in this series, so far, the impacts can be massive.