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Nighttime Heat Waves, Downpours and Planetary Warming

by: Paul Kando

We had more rain fall last week than is normal for all of September and parts of Colorado were under flood waters over the weekend. Locally, during the last two weeks we went from chilly days and nights to midsummer heat at night. What's going on?

photo credit: NOAA

Our climate is ultimately the product of the interaction of heat, air and moisture. As solar radiation impacts matter on Earth, it is converted to heat. Some of that heat is reflected back into space, some of it is trapped by the atmosphere. The natural atmospheric carbon balance results in conditions favorable to life on the planet. Our burning fossil fuels upsets this natural balance by releasing excess carbon safely stored away for millions of years. A more carbon-rich atmosphere traps more heat and warms the planet.

When ice absorbs heat, it melts. When liquid water absorbs heat, it evaporates. The air absorbs more and more water vapor as it warms - hence the term "relative humidity". When air absorbs heat, it expands, becomes lighter by volume, and rises. It takes with it the moisture it absorbed - and the heat energy the air and water have absorbed as they warmed. The rising warm air is replaced by cooler air rushing in, creating wind. When the risen warm air eventually cools, it can no longer hold the water vapor it has absorbed. The moisture condenses into rain, hail or snow. As it does, it also releases all the energy it absorbed when it evaporated. Each pound of water absorbs 1 Btu of energy per degree of warming, but 970 Btus to vaporize. Therefore a single gallon of water, weighing over 8.3 pounds, releases over 8050 Btus of energy when it condenses into rain. This is the energy that powers powerful storms, (whose circular wind patterns are created by the Earth revolving on its axis), heat waves and other extreme weather.

We can observe the same phenomena, small scale, in our heated houses. As we warm indoor air, it absorbs more and more moisture. If this warm, moist air can escape into the cold attic or into a cold wall cavity, the vapor condenses. The result is mold, mildew, perhaps ice dams on the roof, not to mention structural damage.

Heat waves, increasing in number and intensity as the planet warms, are also produced by the interaction of heat, air and moisture. Daytime temperatures may rise into the 100s over several days. Researchers at Oregon State University define a heat wave as three consecutive days of temperatures at the warmest 1 percentile over the past century. Using that metric, they documented 13 examples of regional daytime heat waves between 1901 and 2009, only two of them in the last 20 years. In contrast, nighttime heat waves have been clustered over the past two decades, and they appear to increase in frequency. Of 15 nighttime heat waves from 1901 through 2009, ten have occurred since 1990, five between 2006 and 2009. Since this study was accepted for publication in the Journal of Applied Meteorology and Climatology, another nighttime heat wave took place at the end of June 2013.  Nighttime heat waves seem to be triggered by high humidity. The minimum temperature remains abnormally high overnight because elevated low-level moisture at night tends to trap heat.

Sometimes daytime and nighttime heat waves coincide. This happened in 2009 when temperatures in the Pacific Northwest set all-time records in Washington state, including 103ºF at Seattle-Tacoma Airport, and over 105ºF in Portland, Eugene, Corvallis and Medford, Oregon. It was the second most-intense daytime heat wave in a century, but lasted only three days by the above 1 percentile definition.  However, that same stretch of hot weather resulted in a nighttime heat wave lasting eight days, by far the longest stretch since records were kept beginning in 1901. The latest nighttime heat wave began in late June of this year, and continued into early July. 

Nighttime heat waves tend to occur when a large high-pressure ridge settles in at the same time that monsoonal moisture comes up from the South. Moisture is trapped behind the high pressure ridge, elevates relative humidity and traps the warm air at night. Here in Maine so far we have been lucky. Our hottest, most humid days are normally followed by the relief of a cooler night. Our hot summer nights have been moderate and few. Studies suggest a warming climate is likely to change this. The global temperature has increased. There is now 4% more water vapor in Earth's atmosphere than 30 years ago. As a result, every extreme weather event has a global warming contribution in it. Just before last October's super-storm Sandy, for example, the ocean off New Jersey was 9ºF warmer than average - providing a huge supply of energy and water vapor for that storm.

Of course, hurricanes, floods and droughts would occur without man-made global warming. But nighttime heat waves have been rare and all weather events are now more pronounced than they used to be. Extreme events are more extreme. Heat waves are warmer and last longer. The hurricane scale traditionally featured categories 1 to 5. Now it is adding a 6th. The fingerprint of man-made global warming is all over our recent storms, floods, and extreme weather. Including heat waves, daytime or night.