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July-August 2014

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Weather Queries

I have a degree in meteorology, but I don't remember any equations that relate storm intensity to the release of latent heat when water vapor condenses. It seems reasonable that the recent EF5 tornadoes in Oklahoma would have been less intense if atmospheric water vapor had been at a pre-global warming level, all other conditions remaining equal. Is this a possibility?

Lee Sims

Tucson, Arizona

Global warming accelerated beginning in the 1970s. It has slowed in recent years, but it is still true that all 10 of the warmest years, averaged over the globe, have occurred since 1997. This is for a record stretching back well over 100 years. Since the 1970s, a rise in atmospheric moisture has also been observed both at the surface and in values integrated through a vertical column. The latter measure is often expressed as total precipitable water—the depth of liquid that would result if all the moisture in a vertical column of air condensed and fell to the ground as rain. Almost all ocean areas have experienced an increase in moisture, but not all land areas, particularly those far removed from any large body of water. The simplest explanation for the increase in vapor content of the air is that its maximum value depends strongly on temperature. The warmer the air, the more water can evaporate into it. Because the oceans are slowly warming as well as the air, the potential for evaporation rises, too.

I have not found any statistics to support the idea that the average dew point of air entering thunderstorms is any higher now than, say, 30 or 40 years ago. This would be consistent with global warming and higher moisture content, but, even if this is true, the amount of latent heat realized through the condensation of all the vapor that goes into cloud particles and precipitation does not alone determine whether a thunderstorm will be severe or produce a tornado. For violent tornadoes (EF4 or EF5 on the Enhanced Fujita Scale), two ingredients are necessary (but not always sufficient): marked atmospheric instability and strong vertical shear of the horizontal wind.

Strong atmospheric instability involves copious low-level moisture and a rapid decrease of temperature with altitude. As low-level moisture is drawn into the base of a thunderstorm, it condenses, releasing latent heat. This warms rising volumes of air in the thunderstorm. If the temperature of the air outside the cloud decreases rapidly with altitude, air in the updraft, warmed by latent heating, can easily become warmer than air outside the cloud at the same altitude. When this happens, the air in the updraft becomes buoyant and will rise of its own accord, like a hot-air balloon. The difference in temperature between the updraft and air outside the cloud at the same level, measured at all altitudes for which the updraft air is buoyant, is a cumulative indicator of maximum updraft speed and storm intensity. A thunderstorm forming in highly unstable air will produce heavy rain, hail, and strong downdraft winds, but not necessarily a violent tornado. For that, one needs, in addition, strong vertical shear.

Strong vertical shear of the horizontal wind means that wind speeds increase rapidly from the ground up. It also includes directional shear, usually a clockwise turning of the wind with increasing altitude, for example, SE wind at the surface, S wind at 3,000 feet, SW wind at 8,000 feet, and W wind at 20,000 feet. A shear profile like this imparts rotation to a thunderstorm updraft forming in an unstable air mass. Rotating updrafts, which are a feature of supercell thunderstorms, are responsible for violent tornadoes but do not always produce them.

The short answer to your question is that the amount of moisture entering a thunderstorm is only weakly correlated with storm intensity, if at all. Other atmospheric conditions have to be just right to create a severe or tornadic storm.

What is the theoretical maximum surface temperature that could be reached by a heat burst?

Karen Saraf

Houston, Texas

Heat bursts were discussed in this column in two previous issues of Weatherwise: September/October 1995 and September/October 2012, but your question has not been addressed until now. I must caution readers that my answer is based on a “back-of-the-envelope” calculation rather than a carefully developed theory.

As context for the answer I will repeat information from the 2012 issue. Figure 1 diagrams what is thought to occur in a heat burst. Read the caption for details.

The necessary conditions for a heat burst are as follows:

  • Thunderstorms recently in the vicinity, often in the dissipating stage

  • Region of stratiform (gentle as opposed to showery) rain toward the rear of the thunderstorm complex

  • Sloping back side of the anvil extending upwind with respect to the air flow around the cloud

  • Dry air and a steep lapse rate in the air immediately beneath the anvil

  • Moderate to strong air flow in mid-troposphere along the sloping back side of the anvil

  • Precipitation evaporating into drier air beneath the anvil

  • Shallow, stable layer of rain-cooled air near the ground

Some thunderstorm complexes generate an area of low pressure in the mid-troposphere. This “low” may become visible as a cyclonically curved, mid-level cloud formation after the thunderstorms dissipate. While the storms are still active, this low pressure can accelerate mid-tropospheric air toward the rear of the complex. That provides some of the air that participates in the heat burst.

Air pumped to the upper troposphere by strong thunderstorm updrafts spreads laterally, forming the familiar anvil shape at the top of the thunderstorm. Air flow in the back side of the anvil is against the prevailing wind, and so the anvil may be viewed as a barrier to the mid-tropospheric flow being accelerated toward the rear of the storm complex. This latter flow is forced downward. If it penetrates the shallow layer of rain-cooled air and reaches the ground, we call it a heat burst.

Now for the back-of-the-envelope calculation. Suppose air that eventually reaches the ground in a heat burst starts its descent near the base of the anvil at a pressure of 400 millibars (mb). In a typical mid-latitude summertime atmosphere, it would have a temperature of about −25°C. But suppose this air is unusually warm, and its actual temperature is −20°C. Suppose a dry adiabatic lapse rate exists in the clear air below the anvil, that is, the temperature decreases with altitude at the rate of about 1°C per 100 meters of altitude. This is a highly unusual situation but not impossible. Finally, suppose that the air exits the anvil in a downdraft containing ice crystals and perhaps some supercooled water. The precipitation slowly evaporates as it falls into drier air below the anvil. The descending air warms at the moist adiabatic rate, that is, it warms by compression, but the rate of warming is reduced because of the energy extracted from the air as the precipitation evaporates. If all the precipitation has evaporated just as the descending air reaches 500 mb, the air will have a temperature of –9°C, but its surroundings will be 5°C warmer. That means the sinking air is negatively buoyant and will continue to sink of its own accord. Negative buoyancy is abetted by air drawn into the rear of the storm and forced downward by the narrowing wedge of the anvil (refer again to the figure). From 500 mb to the ground, assumed to be at 1000 mb, the air warms at the dry adiabatic rate because it is no longer saturated (humidity less than 100%). If its temperature is −9° at 500 mb, it will be about 50°C at 1000 mb, or 122°F.

Stories are told of heat bursts producing even higher temperatures and scorched vegetation, but hard documentation of such events does not exist, as far as I know. This is not to say it couldn't happen, but the peculiar combination of conditions required for such an extreme event is exceedingly rare, the area affected is invariably small, the duration of high temperatures is only a few minutes, and the odds of well-calibrated instruments recording the event are slim indeed.

The temperature calculations above were made with what is called a Skew-T/Log-p diagram, which is drawn for the metric system. That is why I did not use English units except at the very end.

Why can one usually obtain a clear view of thunderheads in the Midwest but not so often in the East?

Kenneth Mack

York Springs, Pennsylvania

If you'll permit me, I'll contrast viewing conditions in the Great Plains with those in the eastern states because the contrast is greater. Views of approaching thunderstorms in the East are frequently obscured by stratus or stratocumulus clouds, often coming from the south, the direction of the moisture source: the Gulf of Mexico. This is much less common in the Great Plains because moisture from the Gulf becomes more scarce the farther west one travels. The reason is that southeast Texas marks the western limit of the moisture source. Consequently, the moisture content of air in the lowest 5,000 feet over the Great Plains is often considerably less than in the eastern third of the country, and so low cloud decks occur less frequently.

Air pollution is another reason why advancing thunderheads are more difficult to see in the East than in the Great Plains. When flying over the eastern United States in summer, I often marvel at how hazy the sky is, sometimes all the way up to flight altitude. The heavy concentration of industry, population, and vehicular traffic is responsible for the pollution. If surface visibility is less than, say, 10 miles, it's impossible to see a line of approaching storms until it is fairly close.

Finally, severe thunderstorms may induce sinking air in the mid-troposphere within the nearby environment, say, within a few tens of miles, compensating for the very strong updrafts within the thunderstorm. I noticed this when I lived in Saint Louis: a tendency for stratocumulus clouds to dissipate just before a strong thunderstorm arrived. Thus I could sometimes view an oncoming storm 30 minutes before it arrived. Sinking air behind strong storms promotes clearing, affording a good view of the receding cloud towers.

Why is there such a difference in humidity between the Southwest and the Gulf?

Rick Sleeper

Covington, Washington

The Gulf of Mexico is the major source of low-level moisture for parts of the Great Plains, the Mississippi Valley, and points east. The moist air from the Gulf flows northward on southerly winds. The western boundary of this southerly flow fluctuates from west Texas to Louisiana, and it often shuts off with a wind shift and the passage of a front from the north. From a climatological standpoint, however, the supply of low-level moisture, cloudiness, and annual precipitation all increase from west to east, from New Mexico to the central Gulf Coast because of the geography of the Gulf Coast. Gulf moisture seldom penetrates west of the Continental Divide. Moisture from the Pacific Ocean is deposited on the higher terrain of the western United States. Pacific air is usually well depleted of moisture by the time it reaches east of the Continental Divide. Much of the desert Southwest is far from a moisture source and/or mostly surrounded by topographical barriers that capture moisture that would otherwise reach that area. The exception is from mid-July to early September, when low-level moisture penetrates Arizona, New Mexico, and Utah from western Mexico and the Gulf of Baja.

Weatherwise Contributing Editor THOMAS SCHLATTER is a retired meteorologist and volunteer at NOAA's Earth System Research Laboratory in Boulder, Colorado. Submit queries to the author at weatherqueries@gmail.com, or by mail in care of Weatherwise, Taylor & Francis, 530 Walnut Street, Suite 850, Philadelphia, PA 19106.


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