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May-June 2017

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Weather Queries

Are there any physical reasons, such as the present make-up of the atmosphere, that constrain maximum and minimum temperatures at the surface to approximately 130°F and –130°F?

Jim Henry and Kevin O'Toole

This is one of those simple questions that is perhaps not so simple to answer.

Just to provide a little background, the standard meteorological “surface” temperature measurement is of the air temperature two meters (just shy of 79 inches) above the ground, over a natural surface well away from artificial sources of heat (or cold), such as buildings, paved surfaces, and other disturbances to the natural state made by humankind. Routine, reliable, and standardized measurements of surface air temperature are only available for the past 100–150 years or so; therefore, this discussion does not attempt to delve into what extremes might have been present at earlier points in the earth's history.

We will proceed by recalling the coldest and warmest air temperatures recorded near the earth's surface. Then we will look at some general constraints on heat and cold unique (at least in our solar system) to the earth and how these constraints are minimized but still present at the locations where these records were set.

The coldest temperatures on earth are observed during the austral winter on the elevated plateau of East Antarctica in the Southern Hemisphere. The warmest are observed near (or below) sea level within a few weeks of the summer solstice over the desert areas of the Middle East and North Africa, and Death Valley in California, all in the Northern Hemisphere. Specifically, the coldest temperature recorded is –128.6°F at Vostok, east Antarctica (78.5°S, 106.9°E, 11,440 feet elevation), July 21, 1983 (see Figure 1). There seems to be no serious argument about this measurement or about this location being the coldest or very near the coldest place on earth. However, in the case of the warmest temperatures ever recorded, there is some dispute. Recently, Christopher C. Burt, who is a meteorologist and blogs for the Weather Underground, has looked into this matter in some detail and concludes that the highest temperatures recorded are near 129.2°F, at Furnace Creek, Death Valley, California, on June 30, 2013, and at Mitribah, Kuwait, on July 21, 2016 (see Figure 2). This is not quite as hot as some earlier-quoted records. From my personal experience as an observer and in monitoring data for input into forecast models, questionable daytime measurements of air temperature under sunny conditions tend to most often be on the high side due to exposure problems (restricted ventilation in light winds, in particular), so I am inclined to trust Mr. Burt's judgment that these cooler temperatures are the true records.

So, what are the constraints on these numbers? Why aren't there more extreme conditions? I point to two primary constraints: (1) the presence of a great deal of liquid H2O on the surface of our planet and water vapor in our atmosphere, and (2) that our planet has a relatively fast rotation rate (the average time between sunrise on successive days is 24 hours, not close to four months as on Venus).

The surface of our planet is about 71% covered by water. Another few percent of the surface is covered by permanent snow and ice. The earth is thought to contain about 1.386 × 109 cubic kilometers of water, of which 96.5% is in the oceans. This is a lot of water: if the earth were completely covered by water, the average depth of the water would be almost 9,000 feet. As you know from heating water for coffee or tea, it takes a lot of energy to heat not very much water; in fact it takes over four times as much energy to increase the temperature of a kilogram of water by one degree as it does to increase the temperature of a kilogram of air by one degree. (And, at sea level, the kilogram of air occupies about 800 times the one-liter volume of that kilogram of water.) So, the very large volume of water on the earth's surface has a great deal of “thermal inertia.”

For example, the temperature of the water at the surface of the ocean warms much less than 15°C from winter to summer over the great majority of the world's ocean. Stated another way, the ocean surface temperature warms by less than 5% from winter to summer over most of the world's oceans. And, below the surface, the temperature changes are even smaller, becoming negligible in the deep ocean. Since the exchange of heat between the ocean and the atmosphere is an important source and sink of heat for the atmosphere, the sheer volume of water in the oceans and their large areal coverage act as a restraint on large excursions of temperature in the air.

In addition to this, the composition of our atmosphere does have importance. Water vapor is a “greenhouse gas,” the most important greenhouse gas in the atmosphere. (We hear about greenhouse gases frequently because of their centrality in the theory of climate change.) The greenhouse gases [next to water vapor, carbon dioxide (CO2) and methane (CH4) are the most important ones] exert a further modifying effect on the temperature at the ground surface during the night. After sunset, the earth's surface cools down, as it is no longer receiving any radiation from the sun, and it itself is radiating energy upward. Some of this radiation is trapped by the greenhouse gases in the atmosphere (as well as by clouds, if there are any), and is re-radiated back to the earth's surface, partially canceling the cooling that would otherwise occur if these greenhouse gases were not present. The greenhouse gases, particularly water vapor, thus prevent the temperature near the earth's surface from cooling so rapidly at night, and also overall cause the earth's surface and the atmosphere in contact with it to be somewhat warmer than if these greenhouse gases were not present.

The fact that the earth has a relatively fast rotation rate is also highly relevant. Except in the polar regions, the duration of time that locations on the earth's surface are exposed to sunlight or shielded from sunlight (other than by cloud cover) is never as much as 24 hours. This means that there is a limit in the amount of solar energy a location at the earth's surface can experience without a break. If the earth rotated more slowly (that is, if days were longer), there would be opportunity for a given location to heat up more (or cool down more at night) than is the case now. Although the polar regions poleward of the Arctic or the Antarctic circle can in principle receive sunlight for more than 24 hours at a stretch, in fact over 180 days at the North and South Poles, the sun remains close to the horizon so that the amount of direct solar radiation at the ground is small compared to closer to the Equator. Besides this, the prevalent snow and ice cover in the polar regions reflect much of the solar energy received at the surface there. On the other hand, the absence of solar radiation during the polar night does allow the buildup of cold air near the ground, but here the cooling is constrained by the presence of the greenhouse gases in the atmosphere as well as by the presence of clouds. Further, it so happens that the fast rotation rate of the earth also exerts a profound constraint on the motion of the atmosphere through the Coriolis effect, which, together with the smaller heating and greater cooling of the polar regions relative to the equatorial latitudes, leads to the formation of cold and warm fronts and low and high pressure areas (cyclones and anticyclones), the jet stream and it's meanders, and other features of the global circulation. These aspects of the global circulation transport air from one region to the other, and in particular, tend to carry cold air equatorward and warm air poleward, such that the effect of the smaller amount of solar energy available to heat up the earth's surface in polar regions compared to closer to the Equator is further muted.

The hottest places on earth, as noted above, are at low elevations in areas of desert. Why low elevations, particularly when one considers that at higher elevations the intensity of sunlight reaching the ground is typically greater, as there is less atmosphere to absorb or scatter the solar radiant energy from the sun? This is because, particularly during the late spring and early summer, when the days are longest and when the sun is highest in the sky, the primary source of heat to warm the atmosphere comes from the ground surface heated by the sun, particularly on sunny days. This warm air tends to mix with the air above through convection currents of rising and sinking air. If you fly during the summer on a sunny day, the first few minutes after takeoff and before landing are often bumpy as the aircraft encounters these currents of rising and sinking air. Because pressure decreases with height in the atmosphere, the air in these rising currents cools by expansion as its pressure becomes less, and the air in sinking currents experiences warming as pressure increases. So it is no accident that the warmest places on earth are at low elevations.

Why are the hottest temperatures generally observed in deserts? With their sparse vegetation and dry soil, solar energy reaching the surface and not reflected back upward from the earth's surface goes mainly to heating up the ground and the atmosphere next to the ground rather than to photosynthesis in plants or to evaporating water from the soil. But not all deserts are created equal. Those that are close to large bodies of water are often protected from extreme heat by cooling sea breezes that flow onshore during the daytime, or by occasional or frequent penetrations of more moderate marine air inland. However, in a situation like Death Valley, which is a closed drainage basin below sea level far from the Pacific Ocean and Gulf of California and which is surrounded by mountainous terrain that is well above sea level, the access to such marine air is blocked. As a result, moderating oceanic influences are largely kept out.

For a location to experience extreme cold, moderating oceanic influences must also be kept to a minimum. The weather pattern leading to the extreme cold at Vostok has been described in a study published in 2009 in the Journal of Geophysical Research by Turner et al. They describe how the extreme cold, extreme even for that location with its high elevation and high latitude, resulted from a combination of factors. Very cold and dry air aloft within an upper-level polar vortex moved over the area and remained for several days, creating an unusually long cloud-free period that also minimized moderation by the greenhouse effect because of the very dry air. The polar vortex also insulated East Antarctica from oceanic influences from the adjacent Indian Ocean for an extended period of time.

In summary, we see how a favorable combination of circumstances act together to prevent the extremes of temperature seen on other planets in our solar system.

JOHN M. BROWN was born and raised in San Diego CA, where he became fascinated by clouds, wind and rain from his earliest years. Since completing his education at UCLA and MIT, he has worked as a meteorologist in various capacities. He is currently a research meteorologist at NOAA's Earth System Research Lab in Boulder, Colorado.       

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