Brief rains have brought some relief to the parched southwestern states, but as of now, these rains have not been sufficient to end one of the worst droughts this century in parts of the Texas and Oklahoma panhandles. Not only has it been dry, but it's been hot, too. For the past several weeks, daytime temperatures over 90 degrees F (about 32 C) have routinely occurred across the southern tier of the U.S. The soil is so baked that when it does rain, much of the water is running off rather than soaking in. If the drought persists much longer, agriculture officials fear that thousands of farms could fail in Texas and Oklahoma alone.

More than 1/3 of the Earth's land surface lacks the moisture necessary to support a continuous cover of vegetation. However, just because an area is dry doesn't mean that it's experiencing drought. Droughts are temporary departures from the normal or expected precipitation pattern, which results in a deficiency in the soil moisture reserves. Droughts and heatwaves have occurred at one time or another in most all parts of the U.S., but when they occur in the semi-arid, prairie, steppe, or savannah lands, the damage can be devastating, since it's here where much of the world's produce is grown

This year's weather patterns in the southwest are not too dissimilar to the conditions experienced during the Dust Bowl days in the 1930s, when blowing dust and severe soil erosion were responsible for disastrous agricultural losses. The meandering jet stream, which moves cyclonic storms across the country, has generally been positioned over the central U.S., largely ignoring the fields and ranges of the southwest for a number of months. As a result, high pressure systems have made themselves at home on the range (sorry), and the air has remained dry. Farmers in parts of the midwest and Great Plains have had to deal with a different problem. Much like 1993, when record rains caused unprecedented flooding, the jet stream has been stubborn about wanting to move elsewhere, and thus a series of rainstorms has drenched much of this area.

Most all droughts happen when the prevailing motion of the air aloft is downward (subsidence). When air is subsiding, precipitation potential is reduced because as air subsides and compresses, not only does it warm, but its relative humidity decreases as well. The lowered humidity of the air accelerates the evaporation of moisture from the soil, allowing the ground to dry out and become dusty, thus setting the stage for soil erosion by gusty winds (duststorms).

While droughts can occur at any time of year and persist through several seasons, they generally are associated with hot weather. Heatwaves are almost always a part of the summer season. There can be drought without heat, and there can also be extreme heat without drought, but drought and heatwaves are often linked. Dry conditions make the air hotter, since the Sun does not have to expend energy (heat) to evaporate moisture, such as fog or dew. Also, dry soil does not readily conduct heat into the ground. In addition, the dry ground is brighter than wet ground, so it reflects more of the Sun's heat into the layer of air just above the surface.

In the U.S. and in most all areas of the Northern Hemisphere, the hottest temperatures occur in July and August. These are the so-called "dog days" of summer. It's thought that this term came about because at this time of year the Dog star, Sirius, rises with the Sun. Sirius is the the brightest star in the night sky and can be found in the constellation of Canis Major (the big dog). This constellation is visible at night during the winter months, but as the stars change position with the seasons, during the summer, Canis Major is in the same part of the sky as the Sun.

The descending motion of the air aloft in drought and heatwave scenarios creates a kind of cap that prevents pollution from dispersing or mixing in with cleaner air higher in the atmosphere. A yellowish pall is noticeable over many urban areas, and sometimes the haze and pollution is thick enough to blot out the setting Sun. As the discomfort index (a measure of the thermal stress produced by the actual air temperature and the humidity) reaches over about 110 degrees F, the shade offers no relief, and the mere act of breathing seems to cause profusive sweating (are you still looking forward to summer?).