100-Degree Days Are More Common Than You Think

Tom Moore
Published: July 15,2016
We know extreme heat occurs frequently in the Desert Southwest and occasionally in the South during the warmer months, but there are other areas of the nation where triple-digit heat occurs more often than you might think.
This is the time of the year when many of us have to prepare for that scorching summer heat. High temperatures of 90 degrees or above are not uncommon for many parts of the nation, but we generally think of triple-digit heat as rare for most areas.
The map below confirms the Desert Southwest has the most 100-degree days by far, and it's not surprising that this trend extends southward into Mexico. Across the South, there are also some triple-digit heat days, which probably isn't shocking.
More surprising is the number of 100-degree days up through the Plains states and over parts of the Northeast.
(MORE: Where Next Week's Heat Wave Could Be Worst)

Average number of days with high temperatures of 100 degrees or more from 1981 to 2015.
(Brian Brettschneider)

Geographical location, elevation and humidity are factors that can contribute to triple-digit heat in the U.S.
At higher latitudes, the sun angle is lower and there is less solar radiation. As a result, temperatures are generally lower, unless influenced by other factors.
Since temperatures generally drop off as you move higher in the atmosphere, it's more likely to see higher temperatures at the lower elevations than in the mountains.
Atmospheric moisture (relative humidity levels) also come into play. Dry air heats more readily than moist air because more solar radiation is absorbed by the Earth rather than being reflected by clouds. Therefore, if the humidity is higher, it could feel warmer but the actual temperature will be somewhat lower.

Triple-Digit Heat By Region

Southwest
It's no surprise that the lion's share of triple-digit heat per year is in the Southwest. Their latitude is farther south than most other regions, and the air, especially across the desert, is dry most of the time. Several locations in Southern California and Arizona experience 100-degree or higher temperatures well over 100 days per year.
Examples of this excessive heat can be seen in the chart below, compiled by Brian Brettschneider, who studied yearly averages dating back to 1981.

Location	Average number of 100-degree days per year
Death Valley, California	134
Imperial, California	122
Yuma, Arizona	119
Phoenix, Arizona	110
Las Vegas, Nevada	75

There is often a stark contrast in temperature across Southern California; a great example is in the Los Angeles area. Riverside, just east of Los Angeles, records about 20 days above 100 degrees per year, while that type of heat hardly ever occurs at Los Angeles International Airport, located beside the cool Pacific Ocean.
Northwest
As expected, triple-digit heat occurs rarely, if at all, near the Pacific Coast. The number of 100-degree days in eastern Washington and Oregon is somewhat surprising, at first glance. This area, east of the Cascades, is much drier than locations west of the mountain range. Also, westerly winds "downslope" off the Cascade mountains.
Under these conditions, sinking air warms and allows temperatures to rise quickly.
As a result, Walla Walla, Washington, averages eight 100-degree days per year, while Seattle averages zero.
Northeast
Because of the higher latitude, lack of overly dry air and no large mountain ranges to the west, triple-digit heat is rare in the Northeast, but it does occur. Boston averages about one triple-digit high temperature every decade, while New York City experiences about one every three years. The Baltimore-Washington D.C. area records between one and two per year, on average.
Southeast
It would make sense to think that the lower latitude would make this region ripe for triple-digit heat. Although high temperatures in the 90s in the summer are common, it's more difficult to experience triple-digit heat due to relatively high humidity levels. Humid conditions and proximity to bodies of water make much of the Florida Peninsula immune to 100-degree heat. The heat index, however, provides more than enough compensation to keep things uncomfortable.
Midwest
It's also possible for triple-digit heat to creep into the Upper Midwest and Corn Belt. Chicago records triple-digit temperatures about every other year, while Minneapolis averages one every three years. St. Louis averages just above three 100-degree days per year.
The Plains and Texas
Texas can be very hot, but triple-digit heat is less likely near the Gulf of Mexico than inland. What could be considered surprising is that triple-digit temperatures are actually quite common across much of the Great Plains, even up to the Dakotas.
The Great Plains are situated east of the Rocky Mountains, and the area doesn't receive as much Gulf of Mexico moisture as areas farther east. As a result, the air can be dry much of the time. Also, high-pressure ridges aloft (bulges in the jet stream) frequently set up across this region during the summer months.
Below are some locations that record triple-digit heat each year.

Location	Average number of 100-degree days per year
Austin, Texas	22
Dallas, Texas	20
Dodge City, Kansas	15
Oklahoma City, Oklahoma	12
Topeka, Kansas	10
Rapid City, South Dakota	5
Bismarck, North Dakota	2

More Heat In The Future?

Strong ridges of high pressure ridge aloft are becoming more common and could lead to more triple-digit heat.

Heat waves are more common when we have certain conditions present in the upper atmosphere.
The most common feature associated with extreme heat is a large ridge of high pressure (bulge in the jet stream).
Since the 1980s, strong and widespread high-pressure ridges are occurring more often.
Meteorologists, climatologists and other scientists are keeping a close eye on this from year to year, and if this trend continues, the number of days with triple-digit heat will become more prevalent across much of the nation.
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