Wednesday, July 2, 2014

What's the hottest day of the year?

The answer to the question of "what's the hottest day of the year?" depends on where you are.  And the differences may surprise you.

The National Climatic Data Center (NCDC) created a neat map that shows the average date of the hottest day of the year across the lower 48 states.  Now some people might guess that it would be the same no matter where you are located.  Others might think that latitude comes into play.  But the answer is much more complicated than that.

The first thing you notice is that there is quite a bit of variance.  For instance, if you live in El Paso (western Texas), your hottest day is usually in the latter half of June.  But over in Houston (eastern Texas), your hottest day isn't until mid August.  That's a big difference.  Which leads to the question:  what causes these variations?  They're actually rather explainable.  Let's look at a few.

The western U.S has the most drastic variability.  Much of the West has the warmest date around the 1st of August, give or take a week.  But there's a ribbon of purple colors along the west coast from Newport WA down to San Diego.  Those are late August and September.  What's up with that?  Well, most of the west coast of the U.S is blanketed by fog and low clouds for much of the summer.  

MODIS Visible Satellite image 20 July 2013

The cold Pacific waters are responsible for making the fog, and then the hot inland temperatures act to pull the fog onshore.  We all know about how cold it can be in San Francisco in summer.  

But as summer wanes, that "onshore flow" lessens.  So the fog isn't as extensive along the coast.  Also, we often see high pressure move into the western US behind a cold front.  This reverses the winds, and causes off-shore flow (winds blowing from east to west).  This not only pushes the fog away from the coast, but brings the warm air from the interior to the coastal towns. So some of the warmest weather on the west coast is actually in September.  Here's the normal temperatures for San Francisco.  

The peak of the brown shading (normal temperatures) is in early September.  And the hottest day ever at San Francisco Airport was 103F on Sept 14th, 1971.

Cliff Mass has a great blog and some of his entries do an excellent job of describing and explaining the effect of off-shore flow.

So what about the desert Southwest?  Why is late June and early July the hottest time of the year for Arizona and New Mexico?  The answer is what we call the "Southwest Monsoon".  During May and June, the desert Southwest is generally sunny, hot and very dry.  But by mid-July, moisture from the south (i.e. Mexico) routinely moves into the Southwest US, resulting in frequent clouds, showers, and thunderstorms.  The satellite image below shows an example of all the thunderstorm activity over the desert SW on 1 Aug 2013.

MODIS Visible Satellite image 1 Aug 2013

All of this cloudiness tends to keep temperatures a little cooler.   Here's the average temperatures for Tucson.

The brown shading shows the "Normal" temperatures.  And you can see that they do indeed peak in late June.  It's still hot in July through September, just not quite as hot as late June. 

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