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Unit 4: Earth’s Fluid Spheres
Section 2: Severe Weather
Thunderstorms, tornadoes, or floods can happen almost anywhere. In some regions, these events occur very often, whereas in others they are unusual. However, at some time in your life you will probably experience some form of severe weather.
In this chapter, you will learn what conditions create thunderstorms.
How and Why Warm Air Rises (F 70)
As air warms, it becomes less dense than its cooler surroundings and rises. Warm air is less dense than cold air because molecules in warm air are more active than the molecules in cold air. As molecules move around with increasing speed, the distance between them gets larger and their density decreases.
Air masses can also be forced to rise when they flow over mountains or collide with other air masses. There are four main mechanisms that cause air to rise on a regional scale.
1. Convective Uplift – Convection refers to upward and downward motions of air caused by
differences in air temperature (and therefore differences in density). As the Sun warms
the ground, the ground warms the air immediately above it. Cooler, denser air from
above sinks and forces the warmer air upward. The ascending warm air expands and
cools and eventually sinks back to ground, completing the convective circulation.
2. Orographic Uplift – (orographic refers to mountains) When the wind encounters a
mountain range, the mountain range acts as a barrier, and forces the air upward.
3. Frontal Wedging – When a cold air mass meets a warm air mass, the more dense cold
air wedges or forces its way beneath the less dense warm air along a cold front.
4. Convergence – When winds blowing from different directions meet head-to-head, or
converge, they have nowhere to go but up.
What is a Front?
A front is a narrow zone of transition between air masses that contrast in temperature and/or humidity. The map on the next page shows the “source regions” for the air masses that regularly move over North America. As indicated by the arrows, cold air usually flow southeastward and warm air masses flow northeastward. As air masses move out of their original regions, they change depending on the route they travel.
Example: Cold air traveling southward will warm up before it reaches Florida so that is why Florida never gets as cold in the winter.
Air masses are classified by their temperature and humidity (or moisture content), as follows…
Ø Continental: relatively dry air masses that form over land.
Ø Maritime: relatively humid air masses that form over the ocean.
Ø Polar: cold air masses that form at high latitudes. (Example: northern Canada)
Ø Tropical: warm air masses that form at low latitudes. (Example: Gulf of Mexico)
General Types of air masses include:
1. continental polar – cold and dry
2. continental tropical – warm and dry
3. maritime polar – cold and humid
4. maritime tropical – warm and humid
5. artic air – exceptionally cold and dry
When a front develops, whether it is a cold or warm front is determined by which air mass retreats or gives way to the other. If the colder (or drier) air advances while the warmer (or more humid) air retreats, the transition zone is considered a cold front. On the other hand, if the warm air advances while the cold air gives way, it is considered a warm front. Example of a cold front.
Development of a Thunderstorm
Thunderstorms can be caused by air rising along either warm fronts or cold fronts. However, the most intense thunderstorms develop from cold fronts. As a general rule, the greater the difference in temperature between the air masses, and the more rapidly the cold-air mass wedges under the warm air mass, the greater the chance a thunderstorm will form. Another way thunderstorms can form is through local convection of a warm and humid air mass.
A thunderstorm is a relatively small, short-lived weather system. They usually affect an area the size of a city and for the duration of about an hour. A thunderstorm completes its “lifecycle” in three stages: cumulus, mature, and dissipating. The most severe weather occurs during the mature stage. At any given moment about 1800 thunderstorms are happening on Earth. This equates to 16 million thunderstorms per year worldwide.
During the first stage, cumulus clouds build upward and laterally with a flat bases and towering tops. Cumulus clouds develop where air ascends as an updraft. As the air reaches the higher levels, it expands because the pressure is lower and that makes the air cool. The water vapor in the air condenses because of the colder temperatures producing clouds. No precipitation occurs during this stage and sometimes the cumulus clouds do not produce a thunderstorm.
A thunderstorm that produces severe weather requires a very strong updraft that builds the developing cumulonimbus (nimbus means rain) cloud to great altitudes. Usually convection alone is not enough to produce a thunderstorm. Frontal wedging, orographic uplifting, or converging surface winds can strengthen an updraft and cause the cumulus clouds to build vertically.
When a cumulus cloud gets tall enough, it will produce rain, hail, or even snow. Once the precipitation begins, the thunderstorm has reached its mature stage. Falling precipitation causes a downdraft next to the continuing updraft. During the mature stage, precipitation is the heaviest, lightening is most frequent, and hail or even tornadoes may develop. The higher the thunderstorm builds, the more severe the weather.
Precipitation and the downdraft eventually spread throughout the thunderstorm as the system enters its dissipating stage. Precipitation tapers off and ends, clouds dissipate, and the chances for severe weather decrease dramatically.
Review Questions
1. What happens as air warms? ______
2. Why is warm air less dense than cold air? ______
______
3. What are two things that can force air masses to rise?
a. ______
b. ______
4. Describe what happens during convection. ______
5. What happens when a cold air mass meets a warm air mass? ______
6. In what direction do warm and cold air masses usually flow?
______
7. Why are winter temperatures warmer in Florida then they are in the Great Lakes region?
______
8. What are air masses classified by?
______and ______
9. Describe each one of the five air masses.
a. ______
b. ______
c. ______
d. ______
e. ______
10. How does a cold front form? ______
11. How does a warm front form? ______
12. What causes a thunderstorm? ______
13. With what type of fronts do the most intense thunderstorms form? ______
14. What are the three stages in the lifecycle of a thunderstorm?
a. ______b. ______c. ______
15. During which stage is the weather the most severe? ______
16. How do cumulus clouds form and what do they look like? ______
17. What are thunderstorm clouds called once they produce rain? ______
18. What causes cumulus clouds to build into high altitude thunderstorm clouds? ______
19. What event signals the beginning of the mature stage?______
20. What is the relationship between the height of a thunderstorm and the weather it produces? ______
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How Clouds Form (F 80)
All air contains water vapor, which is water that exists in the atmosphere as an invisible gas. Water vapor is not the same as clouds, fog, or steam, which is composed of tiny, visible droplets of water suspended in the air. There is a limit to how much water vapor can be concentrated in the air and when that limit is reached, the air is considered saturated.
Warm air can hold more water vapor than cold air. When air is saturated with water vapor, some water vapor condenses into tiny droplets of water (or, if the temperature is really low, tiny ice crystals) that are visible in the form of clouds. The most common way clouds develop is by cooling of the air as it rises in the atmosphere.
Ascending air cools because of expansion. A gas cools when it expands. Air coming out of a tire is cool because it is expanding as it escapes. The pressure is much less on the outside than on the inside the tire.
Air Pressure
Air pressure is the cumulative force of a multitude of air molecules colliding with a unit surface area of any object in contact with air. Think about a column of air pushing down on a specific surface area at the base of the column. Air pressure decreases with altitude, because the mass of air above you is getting smaller.
Air that rises, like in an updraft, expands as it rises and loses heat. With that cooling, the air becomes saturated and excess water vapor condenses into droplets, which form clouds. This happens in the updraft of a thunderstorm. Conversely, if air is descending it is encountering growing air pressure and it warms and becomes compressed. During the dissipating stage of a thunderstorm the downdraft spreads through the thunderstorm cloud, and with compressional warming, the cloud vaporizes.
Condensation Nuclei
In addition to saturated air, cloud formation requires surfaces on which the water vapor can condense. Earth’s atmosphere contains an abundance of tiny solid and liquid particles that water vapor can condense on. These particles are called condensation nuclei. Nuclei are products of many different natural and human-related activities.
Forest fires, volcanic eruptions, wind erosion of soil, saltwater spray, motor vehicle exhaust, and various industrial emissions are all sources of nuclei. Nuclei that promotes condensation is called condensation nuclei and nuclei that promotes the formation of ice crystals are called ice-forming nuclei.
Types of Clouds
Meteorologists classify clouds into three broad categories based upon shape: cirrus, stratus, and cumulus. Cirrus clouds are wispy, stratus clouds are layered, and cumulus clouds are puffy (like cottonballs).
Clouds are further classified by their altitude: high, middle, low, or clouds with vertical development. And finally, clouds are classified by their composition: water droplets or ice crystals. Photographs of these clouds can be found on page F 82-83 of your text.
The Mature and Dissipating Stages of a Thunderstorm (F 84)
In the previous lesson, you learned that a thunderstorm begins when a cumulus cloud develops in an updraft of air. The more humid the air, the better chance a thunderstorm will form. In general, the more vigorous the updraft, the greater the altitude to which a thunderstorm cloud builds, and the more likely that the thundercloud can build. Consider the four different layers of the Earth’s atmosphere (listed from lowest to highest): troposphere, stratosphere, mesosphere and thermosphere. The boundaries between these layers are defined by air temperature.
We live in the troposphere. On average, air temperature drops with increasing altitude up to the top boundary of the troposphere which is called the tropopause.
The next layer up is the stratosphere, in which the air temperature is at first constant with altitude and then increases with altitude. A thunderstorm cloud that pushes above the tropopause and into the lower part of the stratosphere will be colder and denser than the surrounding air and will sink back down into the troposphere. Because of this reason, even a very intense thunderstorm cannot build much higher than the tropopause. This is also why cumulonimbus clouds develop their characteristic flat top.
Thunderstorm precipitation falls through the updraft, weakening it and eventually dragging air downward, producing a downdraft alongside the updraft. The downdraft leaves the base of the cloud and flows along the ground ahead of the shaft of precipitation. The leading edge of this rain-cooled gusty air is like a miniature cold front and is known as a gust front.
In an intense thunderstorm, a downdraft may strike the ground with wind speeds in excess of 100 km/hr (60 mph). An intense downdraft is known as a downburst. A downburst pushes ahead of the thunderstorm and can be strong enough to uproot trees and damage buildings.
As precipitation spreads throughout the thunderstorm cloud, so does the downdraft. Eventually the downdraft overpowers the updraft. The updraft dies, precipitation comes to an end, and the cloud slowly vaporizes. This is the final, or dissipating, stage of the thunderstorm.
Review Questions
21. What is water vapor? ______
22. What does the upper limit of water vapor in the atmosphere depend on?
______
23. Compare the amount of water vapor in warm and cool air. ______
24. What happens when air is saturated with water vapor? ______
25. How do clouds develop in the atmosphere? ______
26. What happens to ascending air? ______
27. What happens to air pressure as altitude increases? ______
28. What happens to descending air?______
29. What are the sources of the nuclei? ______
30. What are the three things that meteorologist use to classify clouds?
a. ______
b. ______
c. ______
31. Describe each one of the three categories of clouds.
a. ______
b. ______
c. ______
32. What are the four layers of the Earth’s atmosphere from lowest to highest?
a. ______c. ______
b. ______d. ______
33. What defines the boundaries of these four layers?______
34. What is a downburst?______
35. Describe the dissipating stage of a thunderstorm.______
How Radar Works (F 90)
Although most thunderstorms last for less than an hour, some can produce severe weather and damage for several hours such as flash flooding, destructive hail, and tornadoes. In this chapter, you will learn about radar and how it is used to track storm systems.
36. What is radar an acronym for? ______
37. Describe how radar works. ______
______
38. What type of energy does radar use? ______
39. What is radar echo? ______
______
40. How is echo strength calibrated? ______
______
41. Why does the curve of the Earth limit the range of radar? ______
______
42. What is radar’s maximum range? ______