Typically, the highest temperatures occur in those places where the soil is dry and the atmosphere contains little water - desert areas. High temperatures result because nearly all of the net radiation received is available for heating the air and the soil. The drier the soil, the less the evaporation, and thus, the higher the maximum temperature and the lower the humidity. For this reason, tropical rain forests are too humid to be sites for global hot spots. From previous research it appears that the maximum temperature for a location that is freely-evaporating moisture only rarely exceeds 90 F.

The global daily maximum temperature will most likely occur in desert regions of whichever hemisphere is experiencing summer. From December through February, the highest temperatures are usually recorded in the Southern Hemisphere. In March and April, the pre-monsoon, parched areas of India and Pakistan often have the world's highest temperatures, and by late summer the deserts of North America, the Middle East and northern Africa are usually the hottest places on Earth.

Though the summer solstice occurs about December 21 in the Southern Hemisphere, the highest temperatures are usually recorded in January and February. In the Northern Hemisphere, the highest temperatures occur between July and September. Even thought the Sun's ray are most direct and closest to being overhead in the sub tropics at noon near the time of the solstice, it takes a while for the land, oceans and atmosphere to heat up. The Earth is about 3% closer to the Sun in December than in June, but since there's so much more water in the Southern Hemisphere, things don't usually heat up as quickly and to the same degree as in the Northern Hemisphere.

Will the warming trends that have been witnessed the past decade contribute to higher daily global maximum temperatures in the future? Possibly, but keep in mind that according to climate change models, the places projected to warm up the most are not the desert areas. The last decade has been witness to several of the warmest years in the past millennium. However, the record highest temperature ever measured in the U.S (134 degrees F at Death Valley, CA in July of 1913), and the hottest temperature ever recorded anywhere on the globe (136 F at Al' Aziziyah, Libya in September of 1922) occurred well before global warming was a buzzword, and despite the recent series of warm years, globally, these records have not seriously threatened.

Can the temperature ever exceed 136 F? In all likelihood it already has, somewhere. It would be truly amazing if the thermometer in Al' Aziziyah was placed at the exact spot on the Earth's surface that happened to have the hottest temperature ever. On that sizzling September day in 1922, the vicinity near Al Aziziyah was certainly extremely hot, but since there were few meteorological stations nearby, it's highly improbable that this particular point was indeed the hottest spot.

For example, although Pakistan has been extremely hot this past week, the world's hottest temperatures have shifted between several different locations during this period. On May 18 the temperature was 118 F in Sibi, and the next day it was 123 F in Pad Idan, and on the 20th it was 120 F again in Pad Idan, and on the 21st it was 118 F in Nawabshah, and yesterday it was 119 in Sibi. All of these readings were the highest daily temperatures for anywhere in the world. Nawabshah and Sibi are separated by approximately 300 miles.

You might think that Al Aziziyah, the site of the world's highest ever temperature, would be located somewhere in the middle of the Sahara Desert. Surprisingly, it's just a few miles south of the Mediterranean Sea, near the Libya's capitol city of Tripoli. Rarely, if ever, during the past several years has this town recorded the world's highest daily temperature. In September 1922, it would not have been out of the question that somewhere in northwestern Libya, other than Al Aziziyah, the temperature exceeded 136 degrees.

The place that has the world's highest average temperature is Dakol, Ethiopia, in the Danakil Depression. The mean temperature is an almost unbelievable 94 degrees F! Located in northeastern Ethiopia, at about 13 degrees N latitude, and less than 100 miles from the Red Sea, this place must be insufferable. One key reason why it's so hot there is because it's in a depression. Just as air cools when it rises, a parcel of air will warm (dry adiabatic warming) as it descends. The rate of heating (or cooling) for air that's dry is about 5.5 degrees F per 1000 ft (about 9.8 degrees C per km). In most cases, the further it descends, the warmer it'll become.

This is why Death Valley is so hot, and also so appropriately named. Death Valley is the lowest point in the Western Hemisphere. At 267 feet (86 m) below sea level, air compresses as it moves down the lee slopes of the Sierra Nevada mountains, substantially warming and drying it by the time it reaches the valley floor. At the bottom of this valley, daytime temperatures are routinely above 100 degrees F between about mid May through early October, and it's not unusual for temperatures during some summer days to crack 125 degrees. In July of 1913, when the official temperature was 134 F, a point somewhere in the valley may have surpassed 135 F.

It should be kept in mind that it's not the surface temperature that's being measured, but rather the air temperature at about 5 feet (1.6 m) above the surface in an enclosed shelter. Of course, it's important that the temperature sensor is not exposed to direct sunlight - the shelter is louvered to permit air flow across the sensor.

The ideal circumstances to achieve world record warmth include not only a dry location and a low location but a location in the mid or low latitudes as well. At locations above about 45 degrees north or south latitude, the Suns ray are never direct enough, even in summer, to result in extremely high temperatures. Another important ingredient is lack of wind. Wind causes turbulence, which in general, prevents temperatures from rising as high, or falling as low, as they would in its absence. In addition, the sensor should be over the desert floor and not over a meadow, for instance - air will heat up faster over desert sand then over a grassy field. If all the above conditions are met, the temperature at Death Valley or some other forsaken spot on the Earth's surface, could climb to the 140 degree mark. It's unlikely, though, that the temperature of a reliable sensor would ever approach 150 degrees F. As it turns out, zero degrees on the F scale is about equidistant between the hottest and the coldest temperatures ever measured on the Earth's surface.

For more about this see the Science Question for January 17, 2002 and February 10, 2000.