Heat Wave: Where Does It Come From? Where Does It Go When It’s Over?


Home / Heat Wave: Where Does It Come From? Where Does It Go When It’s Over?

The forecast for Thursday shows that the jet stream will erode the high pressure that provided the warm cap for the western heat wave. Forecast courtesy of NOAA.

The current heat wave in the northwest United States, like all heat waves, will eventually end.

When that happens, will someone else inherit the wind and have a heat wave? Or will the air just somehow cool down?

Let’s take a scientific look at heat waves.

What Is A Heat Wave?

First of all, let’s define a heat wave: Unusually hot weather for the location and time of year, lasting for at least several days. So temperatures over 100 in July in Las Vegas are hot — but a week of them doesn’t constitute a heat wave because the normal high temperature in Las Vegas in July is 105.

The National Weather Service’s definition of heat wave also includes high humidity. This is wrong. First of all, there are several measures of humidity. If they mean the relative humidity, which is the term most people are familiar with (it isn’t a good measure of humidity, but that’s a matter for another time), then there can never be a heat wave. The relative humidity is always low when the temperature is 100 — never above 50%, even when the moisture content is as high as it ever gets (see why it’s not a good measure?).

How Do We Get A Heat Wave?

There are only two ways that a heat wave can arise: Hot air can arrive from somewhere else, or the air in place can be heated.

It’s much easier to get a heat wave by importing the hot air from somewhere else, typically a lower latitude where it gets hotter because of the higher angle of the sun.

In the case of the current heat wave in the American northwest, the air has come from the desert southwest.

Normally the jet stream cuts off the northward flow and shunts the hot air to the east. But first we had Weather Pattern Hotzilla, with the jet stream taking a northern route and allowing warm air to move north from the desert into parts of the intermountain west. Then Hotzilla bent itself into Westzilla, as the already displaced jet stream formed a bulge over the northwest US and southwest Canada.

The result has been an intrusion of desert-heated air well north of its usual haunts.

Capping Off A Heat Wave

As the jet stream contorts to an amplified wave with its maximum in the west, the pressure builds at all levels of the atmosphere. The inflowing air is warm and has created a ‘warm cap.’ This warm cap is the reverse of a cyclone. In a cyclone, which is a low pressure center, air is flowing in at the bottom and out at the top. The process produces precipitation. In a warm cap, air flows in at the top, out at the bottom, and there is gentle, dry subsidence in the broad center.

When air descends, it warms at the dry-adiabatic rate of 5 degrees per thousand feet (dry-adiabatic means no heat is added in any way besides that caused by the increasing pressure as the air descends).

Normally the lapse rate (the rate of temperature decrease with height in the atmosphere) is less than the dry-adiabatic rate. Thus air under a warm cap will be somewhat warmer than normal.

How Can a Heat Wave Dissipate?

The simplest way to end a heat wave is to have cooler air replace the warm. In the case of the Westzilla heat wave, the jet stream will move south, cooler air from northern Canada will flow in underneath; or the flow will turn more westerly, so that air from the cold Pacific Ocean will enter the northwestern states.

But Where Does The Hot Air Go?

The first thing you would think of is that it will be pushed eastward, but that’s not likely to be the case. Since cool air is denser than warm, advancing cooler air can lift the warm air. Most heat waves end when the warm air is forced aloft by advancing cooler air. In fact, this is the mechanism for reducing the continually-produced potential energy of air masses side-by-side caused by the differential heating of the sun between pole and equator.

This Summer’s Asian Heat Waves

The May heat wave in India, and last week’s heat wave in Pakistan, ended in a different way. The jet stream is too far north to affect the region in summer, so the temperature is controlled by the monsoon.

Monsoon clouds and rain over India and Pakistan have ended the heat waves. Satellite photo courtesy of US Navy.

May was characterized by hot winds from the northwest (locally known as Loo) over India.

The heat wave ended when the monsoon took over with winds off the cooler ocean. When rain began, temperatures fell even further. The monsoon was later coming to Pakistan, so its maximum heat came in June. The winds have now shifted in Pakistan, and the rains should start this week.

Will There Be More Heat Waves This Summer?

Once the monsoon has started on the Indian subcontinent, the heat is over. Not so in the United States, where the jet stream plays a crucial role in summer weather.

Westzilla will last for another week before the jet stream begins to flatten out again. Another round of Hotzilla heat in the southeast is possible in a couple of weeks, but indications are that Westzilla will return after that. If you live in the northwest, take solace in the fact that fall eventually comes.

Global Warming Is Not Helping

With land temperatures now about two degrees higher than the 20th century average, it is natural to have more — and more intense — heat waves.

But besides the simple arithmetic of higher temperatures is the stagnant nature of weather patterns recently: Unending snow in Boston all winter; Unending rain in Texas and Oklahoma in the month of May. And now we have long spells of hot weather, first in the southeast, and now in the northwest.

So expect more heat waves as the battle of the Zillas goes on.

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