Archive for March, 2012

It still rains a lot in Atlanta

March 20, 2012

Here at ECOVIE, we hear a lot of comments about how it never rains in the summer here in Atlanta.  What’s the point of having a rainwater collection system if it ‘never’ rains?  Well, lets take a look at our actual rainfall results over the last few years and compare that with the recently updated 30 year averages.  You will see that while in fact the amount of rainfall has dropped in recent years and this has dropped the 30 year average slightly, it still rains a lot here.  What’s even better is that we do not have a distinct wet and dry season as is found in other part of the US and the world.  Our rainfall tends to come in heavy bursts which fill our rainwater tanks and then come again just before they are empty.  Indeed, even with our diminished rainfall over the last few years, metro Atlanta continues to be a near ideal location for rainwater collection.

Let’s dive into the numbers.  Over the lat 5 years, we have had below average rainfall in four of them.  In 2007, we had the least rainfall in 50 years at 32 inches. Image The one heavy rainfall year was 2009 with 69 inches, the most in 50 years.  2008 and last year, 2011, we had moderate droughts with 41 and 39 inches respectively.  2010 was near normal at 48 inches.  The point of throwing all these numbers at you is to show that even during our droughts, we still enjoy a lot of rainfall.

You may say, “that’s nice, but it hardly rains in the summer”.  OK, let’s take a look at June, July, and August.  First of all, the absolute heaviest rain month on average is now July for metro Atlanta (measured 1980-2010 at Hartsfield-Jackson Airport) with 5.3 inches!  June and August are now averaging 4.0 and 3.9 inches respectively.  Just as point of reference, 4″ of rain is enough to collect up to 4,400 gallons of water from a 2,000 square foot rooftop (i.e. a lot).  Well, there’s one hypothesis out the window.

You may say, “Not so fast!  It hardly rained at all last summer and what about the dreaded summer of 2007?”  Fair enough. Let’s drill down even deeper.  Sure enough, we had less rain last summer.  


Here are the actual numbers for Jun -August for the last 3 years and for the drought year of 2007.  Last year especially, we were below average all summer with even less rain than during those 3 months in 2007.  Even so, we had a few nice night time showers that filled our rainwater tanks.  The 1.5 inches that fell in August was enough to capture up to 1,700 gallons of rainwater from our 2,000 square foot rooftop.  

Here’s where I will say it could be worse.  Let’s take our supposed rain blessed Pacific Northwest.  You may not be aware that Portland and Seattle have very distinct wet and dry seasons.  It rains all winter and doesn’t rain much in the summer.  Guess what the 30-year average rainfall is in those two cities in the summer?  Yes, it’s less than even our low rainfall of last summer.  



Check out this table.  While, we did suffer through a dry summer last year, it is relative to what we are used to and not relative to what happens elsewhere.  Based on historical data, we can expect abundant rain during the summer even though some years may have less and some years may have more rainfall than others.

In case you may want to accuse me of cherry picking data, I encourage you to go to to find average rainfall data from just about anywhere in the US.  You will see that the Southeast US tends to have more rainfall than just about anywhere else.

Let me wrap up with a look at our average rainfall through the year.  The 1970-2000 data showed and average of 50.3″ which dropped to 49.7″ in the years 1980-2010.  The newly updated 30 year average show surprisingly (at least to me ) that the least rainfall now falls in April with a mere 4.0 inches. 


What happened to our April showers?  October used to be the driest month with those beautiful fall football days.  One the other end, July has overtaken March as the wettest month.  This may be due to a larger number of big storms in the summer.

The main things to point out in our average data is the relatively low spread (1.9″) between the wettest and driest months.  It can rain anytime, and does.  

What does all this mean for rainwater collection as a viable water supply in the Atlanta area?  First, it means that we can count on copious rainfall even when it rains less than usual.  Second, we can design systems with smaller storage tanks than in other areas, because of the pattern of rainfall.  There are other design advantage given by our rainfall patter as well.  Thirdly, rainwater collection arguably does more to prevent runoff and flooding here than elsewhere due to our clay soils covering granite which given little ability for water to infiltrate.

Please respond to the post with any questions you may have.  I am looking for interaction with fellow rainwater geeks!


Bob Drew