The Gulf Coast states and the eastern third of the country commonly experience the tropical air mass in the summer. Continental tropical (cT) air is dry air pumped north off of the Mexican Plateau. If it becomes stagnant over the Midwest, a drought may result. Maritime tropical (mT) air is air from the tropics that has moved north over cooler water.

A warm, moist air mass moving over a cold surface (see Figure 11-2) produces stable air associated with stratiform clouds, fog, and drizzle.

Lake effect is the effect of any lake in modifying the weather near its shore and for some distance downwind. In the United States, the term is applied specifically to the region around the Great Lakes, and sometimes the Great Salt Lake in Utah. A lake effect can sometimes generate spectacular snowfall amounts to the lee side of the Great Lakes. This phenomenon is termed “lake effect snow.”

In autumn and winter, cumuliform clouds and showers often develop in bands over, and to the lee of, large, ice-free lakes (see Figure 11-3). As initially cold, dry, stable polar air over land flows over the relatively warm water, the air is heated and moistened, and stability decreases. Shallow cumuliform clouds develop with low tops. The strength of the convection increases with increasing temperature differences between warm water and cold air, increasing wind speeds, and decreasing relative humidity within the cold, dry air.

Air masses can control the weather for a relatively long time period ranging from days to months. Most weather occurs along the periphery of these air masses at boundaries called fronts. A front is a boundary or transition zone between two air masses. Fronts are classified by which type of air mass (cold or warm) is replacing the other (see Figure 11-4).

Fronts are usually detectable at the surface in a number of ways: significant temperature gradients, or differences, exist along fronts (especially on the cold air side); winds usually converge, or come together, at fronts; and pressure typically decreases as a front approaches and increases after it passes.

Fronts do not exist only at the surface of the Earth; they have a vertical structure in which the front slopes over the colder (denser) air mass.

A warm front occurs when a warm mass of air advances and replaces a body of colder air. Warm fronts move slowly, typically 10 to 25 mph. The slope of the advancing front slides over the top of the cooler air and gradually pushes it out of the area. Warm fronts typically have a gentle slope, so the warm air rising along the frontal surface is gradual (see Figure 11-5). This favors the development of widespread layered or stratiform cloudiness and precipitation along, and ahead of, the front if the warm rising air is stable.

Generally, prior to the passage of a warm front, cirriform or stratiform clouds, along with fog, can be expected to form along the frontal boundary. In the summer months, cumulonimbus clouds (thunderstorms) are likely to develop.

Light to moderate precipitation is probable, usually in the form of rain, sleet, snow, or drizzle, accentuated by poor visibility. The wind blows from the south-southeast, and the outside temperature is cool or cold with an increasing dewpoint. Finally, as the warm front approaches, the barometric pressure continues to fall until the front passes completely.

During the passage of a warm front, stratiform clouds are visible, and drizzle may be falling. The visibility is generally poor but improves with variable winds. The temperature rises steadily from the inflow of relatively warmer air. For the most part, the dewpoint remains steady and the pressure levels off. After the passage of a warm front, stratocumulus clouds predominate, and rain showers are possible. The visibility eventually improves, but hazy conditions may exist for a short period after passage. The wind blows from the south-southwest. With warming temperatures, the dewpoint rises and then levels off. There is generally a slight rise in barometric pressure, followed by a decrease of barometric pressure.

A cold front occurs when a mass of cold, dense, and stable air advances and replaces a body of warmer air. It is so dense, it stays close to the ground and acts like a snowplow, sliding under the warmer air and forcing the less dense air aloft. Cold fronts have a steep slope, and the warm air is forced upward abruptly (see Figure 11-6). This often leads to a narrow band of showers and thunderstorms along, or just ahead of, the front if the warm rising air is unstable. Cold fronts move more rapidly than warm fronts, progressing at a rate of 25 to 30 mph. However, extreme cold fronts have been recorded moving at speeds of up to 60 mph.