In Georgia and Alabama at the south end of the Appalachians, winds from the south are most likely to cause clouds and rain as the air rises over the southern end of the mountains, and the mountainous areas of these states have the highest annual rainfall because of orographically-induced rain.
As mentioned in previous topics, fronts on weather maps represent the leading edge of either warm or cold air. Both kinds of fronts can provide lift for rain showers and thunderstorms, but they do not have the same characteristics as discussed below. Cold air is more dense than warm air, and as a result, it undercuts and pushes the warm air vertically ahead of it as it moves.
The slope of the cold air with a cold front is very steep, so the air is rapidly pushed up and can sometimes result in strong to severe thunderstorms if ample moisture and instability are available.
Figure B. Cold front advancing. At first, it may seem counter-intuitive that warm fronts can provide lift; however, warm fronts can produce widespread precipitation. Unlike a cold front that undercuts the air at the surface, the warm air of a warm front will rise over the cooler air at the surface due to its lower density.
This provides lift for clouds and showers to form along and ahead of the warm front. As a result, thunderstorms are more likely to form with cold fronts rather than warm fronts.
Figure C. Warm front advancing. Figure D. Converging of air at the surface. In the center of low pressure areas, winds converge toward the center of the low due to the effects of friction. You may wish to review this document on dynamical lifting of air , which was originally used in a previous reading page. When air moving along the surface of the Earth is confronted by a mountain, it is forced up and over the mountain, cooling as it rises. If the air cools to its saturation point, the water vapor condenses and a cloud forms.
Heating of the mountain slopes by the Sun also causes air to rise upward through the process of surface heating and free convection described above. These types of clouds are called "orographic clouds", which develop in response to lifting forced by the topography of the earth. While air on the windward side of a mountain is forced to rise, often resulting in clouds and precipitation, the air on the leeward side of a mountain is forced to sink.
Thus on the leeward side of a mountain, we often see clear skies and warm, dry conditions. The leeward side of a mountain range is often called a "rain shadow" region because clouds and precipitation do not form where air is sinking. The great basin area of the United States is a rain shadow region. A front is defined as the transition zone between two air masses of different density.
The warmer air mass is less dense than the colder air mass. Fronts extend not only in the horizontal direction, but in the vertical as well. The best example of a location where low-level convergence is important at continental scales is the Intertropical Convergence Zone ITCZ , which encircles the Earth, more or less centered on the equator.
In the satellite image above, the ITCZ appears as the belt of moist air and clouds shown in red for its position during the northern summer and blue for its position during the southern summer. Along the ITCZ, the northeast trade winds, coming from north of the equator, converge with the southeast trade winds coming from south of the equator. The two sets of trade winds converge at the ITCZ, causing cloud cover as moist air is uplifted to a height at which it cools to its dew-point temperature.
Low-level convergence also occurs by a wind that is slowed down, such as by friction with the surface. The slowed air causes faster moving air behind it to begin to pile up, forcing some air to rise.
A common scenario for this involves a sea breeze, where winds blowing across a large water body slow down as they encounter the more irregular land surface. Mountainous areas represent obstructions to low-level winds. As air encounters topography, it slows down, piles up, and rises, called an orographic effect.
As the air rises , it cools by adiabatic expansion, forming a cloud if it cools to the dew-point temperature. This orographic effect is why thunderstorms are usually more common over mountain peaks than over the adjacent valleys. During daytime, the Sun heats the surface more efficiently than the air above it. As the surface of a mountain warms, the adjacent air warms, which then flows upslope.
When winds flowing up opposite sides of the mountain converge at the summit, they rise more. Large scale convergence can lift a layer of air hundreds of kilometers across. Surface low pressure regions marked by L's on surface weather maps , are areas where surface convergence takes place.
Divergence is an atmospheric condition that exists when there is a horizontal net outflow of air from a region. When air diverges just below the top of the troposphere, air from below is forced to rise up and take its place. You can think of the tropopause as a "lid" or boundary that does not allow air to move up or down through it. Weather reporters often use the terms "upper-level disturbance" or "upper-level energy" to indicate regions where the rising air motion is forced by the atmospheric conditions km above the Earth's surface.
Soon we are going to see how we can use the mb height pattern to indicate regions of upper level divergence and forced rising motion. These types of forced rising air motion are often called "dynamical lifting" because the rising air is forced due to the dynamics or movement or pattern of the horizontal air flow. You may want to review these Notes on dynamical lifting of air that was refered to when we covered the topic on winds. When air is confronted by a mountain, it is lifted up and over the mountain, cooling as it rises.
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