How Hot was July 2007?

While there is no doubt that July of 2007 was unusually hot, there is still the question of just how hot was it. In other words, how does it compare to other hot summers?

First, a quick overview of what caused the abnormally warm temperatures. The figure below shows the average pressure field at about a 20,000 foot elevation for the month of July. Meteorolgists tend to look at this level for several reasons, but mainly because it's far enough above the earth's surface to dimish the influence of the ground. Another way of putting it is that we look higher up in the atmosphere to see what was causing the heat. If we look at lower levels, then we see more of the effect of the heat rather than it's cause.

In the figure below you can see that the values should be between 575 and 580 for Washington and north Idaho. Also the packing of these contours implies that the jet stream should be approximately along the US/Canadian border. This is an average of all 31 days of July over about a 30 year period. So hot and cold spells are going to be averaged out. But in general, this figure shows that the jet stream should typically be in our vicinity.

The next image shows the average pressure for the month of July 2007 (thin lines). Compare this to the image above and you'll notice quite a difference. In fact, the difference between normal July and July 2007 is the shaded part of the image below. As you can see, the largest positive difference (higher pressure than normal) is centered over northern Montana.

Looking down at the surface, the map below shows the average temperatures (average of both daily high and low temperature) for July 2007. The number beneath the average temperature is the number of degrees above normal for that location.

All across the West, temperatures were much warmer than normal for July. The greatest anomalies were in western Montana, where temperatures were more than 10 degrees warmer than normal. Meanwhile on the west coast, temperatures were actually quite close to normal. Some of the more remarkable facts of this heat wave are:

• Missoula, Boise, Pocatello, and Salt Lake City all had their warmest month ever. Not just their warmest July, but their warmest month for any month of the year.
• Boise's max temperature reached 90F or better every day except on the 19th (89F). They reached 100F or better on 14 days. Their low temperature never dropped below 60F the entire month, and failed to drop below 70F nine times.
• Missoula set an all-time record high of 107F. They too only had one day where it failed to reach 90F.
• On July 15th, Salt Lake City had an average temperature of 92F (high of 105F and a low of only 78F). On 16 nights Salt Lake City remained at 70F or warmer.

More local to the Inland Northwest, temperatures easily exceeded their normal values, and in some cases were record-breaking. The table below for selected sites shows a comparison of July 2007 with the all-time records.

One obvious feature from the above table is the other hot July's: 1906, 1960, 1985, and 1998. It's interesting to note that only 1998 had above normal temperatures, while 1960 and 1985 were markedly cooler than normal. Thus, a hot July doesn't mean an equally hot August, and in fact can mean just the opposite.

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