We have another student question out of Aplington-Parkersburg this week! A student asked, "How can a drought happen in Iowa when it's mostly humid?". Well with the "right" parameters in place, most locations can experience a drought. Let's take a look at the mechanisms that cause droughts to occur.
Blocking patterns can cause specific weather conditions to persist for days or even weeks. The blocking pattern pictured above is the Omega Block. The shape of it resembles the Greek letter Omega. You have high pressure in the middle of the blocking pattern and low pressure on either side. Those under high pressure experience warm and dry conditions, while those under low pressure will experience cool, cloudy, and rainy weather. Blocking patterns can be tough to move, since air has a difficult time traveling around the high pressure system.
So now that we have the basic knowledge of a blocking pattern down, it's time to look at how a drought can occur in Iowa. The conditions we need are warm temperatures and no rain for an extended period of time.
During non-drought conditions, saturated soils will have water to evaporate into the atmosphere in the form of water vapor. Plants also undergo the process of transpiration, where water vapor is released into the atmosphere from the plant. All of the moisture in the atmosphere helps create clouds, through the process of condensation. Eventually, water droplets become heavy enough and precipitation will fall from the clouds. This process will continue under "normal" circumstances.
When you have a drought, these processes cannot occur. After many hot days with little to no rain, soils will dry up. That means no water can be evaporated from the soil into the atmosphere. Also, plants will shrivel up due to no soil moisture. That's two moisture sources not releasing water vapor into the atmosphere. Couple no evapotranspiration with no rain falling and you get continued dry weather. This promotes a positive feedback loop. That is, the process keeps feeding itself, until a big weather change happens.
A pretty significant drought occurred over Iowa last year. Below normal rainfall during the summer, led to dry conditions persisting through much of the state. Around July 28, 2020, the Drought Monitor shows severe drought over the western and central portions of the Hawkeye state.
By late August, drought conditions worsened, with extreme drought developing over those same areas as the month prior.
At the beginning of October, the central portion of the state improved slightly, while the northwest corner continued to dry up. Below is a comparison of monthly rainfall totals for Des Moines and Sioux City. You can see that conditions did improve in Des Moines starting in September with above normal rainfall reported, while Sioux City didn't have the same luck.
|Des Moines||Rain Total||Above/Below Normal|
|Sioux City||Rain Total||Above/Below Normal|