Thermal Inversion Cause
In stable air, the temperature decreases at a fairly uniform rate as altitude increases. This holds true for several kilometers upward. Frontal systems upset the uniformity, and this is partly responsible for the formation of rain clouds.
In the vicinity of a frontal system, the temperature drops at first with increasing altitude, but when the boundary between air masses is reached, the temperature abruptly increases. As the altitude increases further, the normal pattern resumes.
Fig. 4-2 is a vertical-slice diagram of a warm front. The thermal boundary is shown by the heavy curve. The warm air, because it is lighter than the cold air, tends to rise. As the front advances, the warm air flows over the top of the cold air mass, resulting in a thermal inversion at an altitude that depends on the horizontal distance from the front itself (the point at which the boundary between the air masses meets the surface).
- Fig. 4-2. Vertical-slice diagram of a warm front associated with temperate-zone low-pressure systems of spring and summer. The vertical scale is exaggerated.
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Horizontal distance, kilometers
Fig. 4-3. Vertical-slice diagram of a cold front associated with temperate-zone low-pressure systems of spring and summer. The vertical scale is exaggerated.
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Horizontal distance, kilometers
Fig. 4-3. Vertical-slice diagram of a cold front associated with temperate-zone low-pressure systems of spring and summer. The vertical scale is exaggerated.
The inversion in advance of a warm front can sometimes extend for hundreds of kilometers. Because a warm front usually moves at a sluggish pace—a few kilometers per hour—the cloudiness and rain can last for two or three days in some places. Warm fronts in the spring and summer sometimes produce severe weather, but usually they cause only moderate showers and thundershowers.
A cold front is more likely to give rise to severe thunderstorms. Figure 4-3 is a vertical-slice diagram of a cold front. Because cold air is more dense than warm air, the cold air mass pushes underneath the warm air mass. The leading edge of the front is well defined, and it advances rapidly along the surface. The temperature difference can be more than 10° C (18° F) at ground level at points separated by only a few kilometers.
If a cold front encounters irregular terrain, or if the front is moving fast, the leading edge of the cold air mass may "tumble over itself." This is because the air moves a little faster at an altitude of a few thousand meters than it does at the surface. If this happens, warm air pockets are trapped under the leading part of the cold air mass, creating powerful updrafts and turbulence as the warm air rises. Such conditions can produce large hail.
If an occluded front (Fig. 4-4) develops in a low-pressure system, severe weather can occur in the vicinity of the occlusion. The advancing cold air mass behind the front has a lower temperature than the air ahead of it, but the difference is not as large as it is when a cold front pushes rapidly into a mass of warm, moist air.
Warm air mass
Warm air mass
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Horizontal distance, kilometers
Fig. 4-4. Vertical-slice diagram of an occluded front associated with temperate-zone low-pressure systems of spring and summer. The vertical scale is exaggerated.
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Horizontal distance, kilometers
Fig. 4-4. Vertical-slice diagram of an occluded front associated with temperate-zone low-pressure systems of spring and summer. The vertical scale is exaggerated.
PROBLEM 4-1
What happens to a low-pressure system after an occlusion has occurred?
SOLUTION 4-1
A new warm front and cold front can form, as the winds around the center continue to pull warm air from the tropics and cold air from the poleward side of the system.
PROBLEM 4-2
What happens to the fronts in a low-pressure system as it moves away from land and over the ocean?
SOLUTION 4-2
Warm, cold, and occluded fronts can still form, and can still exist, in low-pressure systems over the ocean. The temperature difference between the warm and cold air masses in an oceanic low-pressure system is usually smaller than the temperature difference between the warm and cold air masses in a low-pressure system over land.
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