What is A Drought? How Does Summer 2024 Compare to Others?

Rainfall over the last 24 hours as of this writing (August 3) has been the most widespread in quite some time across Ohio and surrounding states. We needed it!

Yet each summer, it seems as if we go through the same process:  Rain at the beginning in May and early June with a gradual drying out with limited rain across northern Ohio by late June/July and into August. The grass gets brown. We mow every two weeks or longer versus every other day in early May. Most of us will mix in the word "drought" in  everyday conversation describing the conditions as we exclaim "this happens every summer."  At least that's how we perceive it. But is this really the case?  What does the data show?

First what is the definition of a "drought"?

Drought is generally defined as "a deficiency of precipitation over an extended period of time" resulting in a water shortage. This is per the National Integrated Drought Information System on drought.gov (a great resource). However drought can mean different things to different regions. If you are a farmer, a drought means below normal rainfall resulting in damage to crops. So not enough water means low crop yield for the season. 

Scientifically, drought can be measured through comparing precipitation departures over a period of time along with soil moisture, evaporation, temperatures among others.  A great way to compare year to year is to use the US Drought Monitor.  This is a blend of around a dozen different variables including the ones listed above. This is combined along with potential drought impacts and other expert insight. 

The US Drought Monitor index is on a scale of zero to four. One being low on the drought scale up to four meaning highly extreme drought conditions as defined above.

Just looking at precipitation changes since April you can see how the conditions in parts of Ohio/southern Ohio, the Ohio Valley and into the Carolinas/SE US have dried out significantly.

Interestingly, the corn belt and most of the Great Lakes, conditions have not been dry at all. In fact, soil moisture has been well above normal.

Back to the original question. What about the comment that we are always in a drought every summer here in northern Ohio.  Is this true?

Look at the Drought Monitor history for Ohio as a whole. Here are the years since 2000 (data only goes back that far) with portions of the state in at least a D2 drought with areal percentages.  The last time ANY part of Ohio was under a D2 drought was 2016.

Can we go back further?  Absolutely, but we need to use a different set of data.  It's called the Palmer Drought Severity Index. This data goes back more than 100 years. The PDSI is good at determining long term drought using temperature and precipitation data to estimate dryness.  So a +4 shows extremely wet conditions. A -4 is extremely dry. 

Here are the PDSI conditions across the US as of late July:

Notice that the conditions across Ohio are not as severe when looking at the LONG TERM drought conditions.

Let's go back in time just looking at the summer months of June, July and August combined. To do this, we can examine specific climate divisions versus state wide conditions. Here in northern Ohio, we have four climate divisions that encompasses our viewing area. Two, three, six and seven.

I highlighted some of the memorable drought summers for Division two. The dust bowl years of the 1930s, the early 1950s and 1960s were long periods with summer drought. Of course 1988, 1991 and the early 2000s also featured very dry summers.

Here is climate division 3:

Climate division 6:

Climate division 7:

As you can see, there are much drier periods over the last 100 years across northern Ohio.  There are also (green bars on these charts) with above normal summer wet conditions.  In fact in the last 30 years, the summers with above normal wet conditions have outnumbered the abnormally dry ones. 

Number of Abnormally Dry summers (JJA) Since 1995

Climate Division 2:  9 of 30 summers

Climate Division 3:  12 of 30 summers

Climate Division 6:  9 of 30 summers

Climate Division 7:  9 of 30 summers

While our lawns dry out each summer to some degree, is it true that every summer or even most summers in northern Ohio we end up with drought conditions?  Not even close

2016 Drought Conditions vs 2012

The dry conditions in northern Ohio are getting worse as the daytime temperatures reach the lower 90s coupled with higher humidity levels--heat index between 100 and 105--this weekend.  Many people are comparing summer to 2012 both in temperature and in degree of dryness. Is it a valid comparison? How do our dry conditions compare to other years in mid July?  When was the last time we had conditions this dry in mid July in northern Ohio?

The index that's used in determining the degree of dryness or wetness historically is the Palmer Drought Severity Index. It gives us a good barometer in comparing drought events of the past.

According to the National Drought Mitigation Center:  "The PDSI is calculated based on precipitation and temperature data, as well as the local available water content (AWC) of the soil. (Link here) From the inputs, all the basic terms of the water balance equation can be determined, including evapotranspiration, soil recharge, runoff, and moisture loss from the surface layer." 

The more negative the number, the more severe the drought.  Whereas higher numbers indicate wetter conditions. Each week the Climate Prediction Center updates the drought conditions nationwide for each climate division. I circled Ohio for reference.

Based on the PDSI, this summer shows little similarities to the summer of 2012 nationally. The PDSI is falling to 2012 levels in some spots in northern Ohio but the overall degree of dryness in 2012 was far more severe. You could argue that the summer of 2007 is a closer match to 2016. I threw in the Super El Nino summer of 1998 at the end since we are at the end of the Super El Nino of 2015-16.

2016

2015

2012

2007

1998