Does Snow = Drought Relief?

The counties in red on this map all
declared drought disasters in 2012!

In the past several years, states in the southern portion of the Great Plains have suffered from extreme drought conditions.  Last year, these conditions spread to include a large portion of the nation and had a major impact on crops nationwide.  However, little gets said about the drought during the winter months.  Many people wonder why that is.  The answer can be summed up in one word: snow.

During the winter months, much of the nation sees precipitation fall in form of snow or other frozen precipitation.  This frozen precipitation falling on a cold ground does not get absorbed into the soil until it begins melting AND any frost has left the ground.  In other words, even though it is snowing like crazy outside, it will not improve the drought conditions until it melts.  And how quickly it melts is important as well.  If we get a fast melt of the snow pack, there will be plenty of runoff with the water flowing right into area river ways (even more so if the frost has not left the ground yet)!  Thus, not much of the water is absorbed by the drought stricken ground.

Another reason why snow has little impact on drought conditions is the fact that there is actually very little water in most snow.  Most snow that falls in the Upper Midwest has a snow to rain ratio of 12: 1, meaning that 12 inches of snow is equivalent to just one inch of rainfall!  And sometimes this ration can be as high as 30:1!!!  Talk about dry snow.  The truth is that there typically is just not enough water in snow to offset our drought conditions.

U.S. Seasonal Drought Outlook valid Feb. 21 - May 31, 2013.
Released Feb. 21, 2013.
Courtesy of the Climate Prediction Center.

So what does that mean for our drought conditions now?  Well, the drought has improved very little over the winter months.  And the drought is expected to persist for a good chunk of the nation (especially over the Great Plains) into the next several months.

On another note, some may be wondering what caused the drought in the first place.  Well, scientists of the Potsdam Institute for Climate Impact Research (PIK) have proposed that a common physical cause is to blame.  They suggest due to man-made climate change, the atmospheric flow pattern (known as the jet stream) is repeatedly disturbed through a subtle resonance mechanism.



Meridional windfield over four different timespans.
(Credit: Image courtesy of Potsdam
Institute for Climate Impact Research (PIK))

The jet stream, that fast current of air circulating the globe high up in the atmosphere, oscillates between the tropical and arctic regions of the globe, bringing cold, dry air from the arctic toward the tropics and warm, moist air in the other direction.  However, lead author Vladimir Petoukhov stated “What we found is that during several recent extreme weather events these planetary waves almost freeze in their tracks for weeks. So instead of bringing in cool air after having brought warm air in before, the heat just stays. In fact, we observe a strong amplification of the usually weak, slowly moving component of these waves.”  And when the heat stays, the heat keeps building and building which leads to a heat wave like what the United States and Russia have seen in the past few years.

Example of a heat dome on a weather map.

In other words, the droughts are caused by heat waves?  In a way.  Like stated above, the heat is brought in from the south repeated, and forms what is called a heat dome under a stationary ridge of high pressure.  As the heat keeps building, the ridge remains quite strong and prevents storm systems from passing through.  And if one or two do happen to slide through the ridge, they are so weak that they produce little in the way of precipitation!

Example of the jet streams moving air around the Earth.

But how does climate change cause the jet stream to freeze in its track?  Man-made climate change does not mean uniform global warming.  The relative increase in temperatures in the Arctic is higher than on average because it is amplified by the loss of snow and ice.  This in turn reduces the temperature difference between the Arctic and say Europe (Temperature difference is the main driver of air flow around the globe!).  In addition, continents generally warm and cool more easily and quickly than the oceans.  These two factors result in an unnatural pattern of the mid-latitude jet stream so that for extended periods slow synoptic waves (ridges and troughs, i.e. highs and lows on the weather map) get trapped in place.

So will we see this pattern over the United States again this year?  Only time will tell.  But for now, let’s hope, and pray that our drought stricken areas get much needed rainfall this spring to help relieve the drought!