10-1 CHAPTER 10. AIR MASSES, FRONTS, AND THE WAVE CYCLONE MODEL
10.1 Air Masses. An air mass is a large body of air with generally uniform temperature and humidity. The area from which an air mass originates is called a source region.
Air mass source regions range from extensive snow-covered polar areas to deserts to tropical oceans. The United States is not a favorable source region because of the relatively frequent passage of weather disturbances that disrupt any opportunity for an air mass to stagnate and take on the properties of the underlying region. The longer the air mass stays over its source region, the more likely it will acquire the properties of the surface below.
10.1.1 Air Mass Classification. Air masses are classified according to the temperature and moisture properties of their source regions (see Figure 10-1).
10.1.1.1 Temperature Properties:
• Arctic (A)—An extremely deep cold air mass which develops mostly in winter over arctic surfaces of ice and snow.
• Polar (P)—A relatively shallow cool to cold air mass which develops over high latitudes.
• Tropical (T)—A warm to hot air mass which develops over low latitudes.
10.1.1.2 Moisture Properties:
• Continental (c)—A dry air mass which develops over land.
• Maritime (m)—A moist air mass which develops over water.
10.1.1.3 Five Air Masses. When this classification scheme is applied, the following five air masses may be identified:
• Continental Arctic (cA)—Cold, dry.
• Continental Polar (cP)—Cold, dry.
• Continental Tropical (cT)—Hot, dry.
• Maritime Polar (mP)—Cool, moist.
• Maritime Tropical (mT)—Warm, moist.
Note: Maritime Arctic (mA) is not listed since it seldom, if ever, forms.
10-2 Figure 10-1. Air Mass Classification
10.1.2 Air Mass Modification. As these air masses move around the Earth, they can begin to acquire different attributes. For example, in winter an arctic air mass (very cold and dry air) can move over the ocean, picking up some warmth and moisture from the warmer ocean and becoming a maritime polar (mP) air mass—one that is still fairly cold but contains moisture. If that same polar air mass moves south from Canada into the southern United States, it will pick up some of the warmth of the ground, but due to lack of moisture it remains very dry. This is called a continental polar (cP) air mass.
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 which has moved north over cooler water.
A warm, moist air mass moving over a cold surface (see Figure 10-2) produces stable air associated with stratiform clouds, fog, and drizzle.
Figure 10-2. Air Mass Modification—Warm, Moist Air Mass Moving Over a Cold Surface
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10.1.2.1 Lake Effect. 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 of the Great Lakes. This phenomenon is termed lake effect snow.
In fall and winter, cumuliform clouds and showers often develop in bands over, and to the lee of, large, ice-free lakes (see Figure 10-3). 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.
Figure 10-3. Lake Effect
10.2 Fronts. 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 10-4).
10-4 Figure 10-4. Fronts
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. Cold fronts have a steep slope, and the warm air is forced upward abruptly (see Figure 10-5). 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. Warm fronts typically have a gentle slope, so the warm air rising along the frontal surface is gradual (see Figure 10-6). 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. Stationary frontal slope can vary, but clouds and precipitation would still form in the warm rising air along the front (see Figure 10-7).
10-5 Figure 10-5. Cold Front
Figure 10-6. Warm Front
10-6 Figure 10-7. Stationary Front
Cold fronts typically move faster than warm fronts, so in time they catch up to warm fronts. As the two fronts merge, an occluded front forms (see Figure 10-8). At the occluded front, the cold air undercuts the retreating cooler air mass associated with the warm front, further lifting the already rising warm air. Clouds and precipitation can occur in the areas of frontal lift along, ahead of, and behind the surface position of an occluded front.
Figure 10-8. Occluded Front
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10.3 The Wave Cyclone Model. A wave cyclone3 is a low pressure circulation that forms and moves along a front. The circulation about the cyclone center tends to produce a wavelike kink along the front. Wave cyclones are the primary weather producers in the mid-latitudes. They are large lows that generally travel from west to east along a front.
They last from a few days to more than a week.
A wave cyclone typically follows a predictable evolution. Initially, there is a stationary front separating warm air from cold air (see Figure 10-9).
Figure 10-9. Wave Cyclone Model—Stage 1
A low pressure wave forms on the front (see Figure 10-10). The front develops a kink where the wave develops. Precipitation develops with the heaviest intensity (dark green) located in the zone of lift along the front.
Figure 10-10. Wave Cyclone Model—Stage 2
As the wave intensifies, both the cold and warm fronts become better organized (see Figure 10-11).
3 A wave cyclone should not be confused with the alternative name for a tornado. They are quite different.
10-8 Figure 10-11. Wave Cyclone Model—Stage 3
In the fourth stage, the wave becomes a mature low (see Figure 10-12). The occluded front forms as the cold front overtakes the warm front.
Figure 10-12. Wave Cyclone Model—Stage 4
As the cold front continues advancing on the warm front, the occlusion increases and eventually cuts off the supply of warm moist air (see Figure 10-13). This causes the low to gradually dissipate.
Figure 10-13. Wave Cyclone Model—Stage 5
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10.4 Dryline. A dryline is a low-level boundary hundreds of miles long separating moist and dry air masses. In the United States, it typically lies north-south across the southern and central High Plains during the spring and early summer, where it separates moist (mT) air from the Gulf of Mexico to the east and dry desert (cT) air from the southwestern states to the west (see Figure 10-14).
Figure 10-14. Dryline Example
The dryline typically advances eastward during the afternoon and retreats westward at night. However, a strong wave cyclone can sweep the dryline eastward into the Mississippi Valley, or even further east, regardless of the time of day. Low-level clouds and early morning fog often prevail in the moist air, while generally clear skies mark the dry side. Severe and sometimes tornadic thunderstorms often develop along a dryline or in the moist air just to the east of it, especially when it begins moving eastward.
A typical dryline passage results in a sharp drop in humidity (hence the name), clearing skies, and a wind shift from south or southeasterly to west or southwesterly. Blowing dust and rising temperatures also may follow, especially if the dryline passes during the daytime. These changes occur in reverse order when the dryline retreats westward.
11-1 CHAPTER 11. VERTICAL MOTION AND CLOUD FORMATION
11.1 Introduction. A cloud is a visible aggregate of minute water droplets and/or ice particles in the atmosphere above the Earth’s surface. Fog differs from cloud only in that the base of fog is at the Earth’s surface while clouds are above the surface.
Clouds form in the atmosphere as a result of condensation of water vapor in rising currents of air, or by the evaporation of the lowest layer of fog. Rising currents of air are necessary for the formation of vertically deep clouds capable of producing precipitation heavier than light intensity.
11.2 Vertical Motion Effects on an Unsaturated Air Parcel. As a bubble or parcel of air ascends (rises), it moves into an area of lower pressure (pressure decreases with height).
As this occurs, the parcel expands. This requires energy, or work, which takes heat away from the parcel, so the air cools as it rises (see Figure 11-1). This is called an adiabatic process. The term adiabatic means that no heat transfer occurs into, or out of, the parcel.
Air has low thermal conductivity (see Table 2-3), so transfer of heat by conduction is negligibly small.
The rate at which the parcel cools as it is lifted is called the lapse rate. The lapse rate of a rising, unsaturated parcel (air with relative humidity less than 100 percent) is approximately 3 °C per 1,000 feet (9.8 °C per kilometer). This is called the dry adiabatic lapse rate. This means for each 1,000-foot increase in elevation, the parcel’s temperature decreases by 3 °C. Concurrently, the dewpoint decreases approximately
0.5 °C per 1,000 feet (1.8 °C per kilometer). The parcel’s temperature-dewpoint spread decreases, while its relative humidity increases.
This process is reversible if the parcel remains unsaturated and, thus, does not lose any water vapor. A descending (subsiding) air parcel compresses as it moves into an area of higher pressure. The atmosphere surrounding the parcel does work on the parcel, and energy is added to the compressed parcel, which warms it. Thus, the temperature of a descending air parcel increases approximately 3 °C per 1,000 feet
(9.8 °C per kilometer). Concurrently, the dewpoint increases approximately
0.5 °C per 1,000 feet (1.8 °C per kilometer). The parcel’s temperature-dewpoint spread increases, while its relative humidity decreases.
11-2 Figure 11-1. Unsaturated Ascending/Descending Air Parcel Example
At the surface, the air parcel has a temperature of 18 °C and a dewpoint of 3 °C, which indicates that it is unsaturated. As the parcel ascends, its temperature decreases at the dry adiabatic lapse rate of 3 °C per 1,000 feet, while the dewpoint decreases at
0.5 °C per 1,000 feet. The temperature-dewpoint spread decreases while relative humidity increases. The process is reversible if the parcel remains unsaturated.
11.3 Vertical Motion Effects on a Saturated Air Parcel. The Lifting Condensation Level (LCL) is the level at which a parcel of moist air lifted dry adiabatically becomes saturated. At this altitude, the temperature-dewpoint spread is zero and relative humidity is 100 percent.
Further lifting of the saturated parcel results in condensation, cloud formation, and latent heat release. Because the heat added during condensation offsets some of the cooling due to expansion, the parcel now cools at the moist adiabatic lapse rate, which varies between approximately 1.2 °C per 1,000 feet (4 °C per kilometer) for very warm saturated parcels to 3 °C per 1,000 feet (9.8 °C per kilometer) for very cold saturated parcels.
Concurrently, the parcel’s dewpoint decreases at an identical rate. For simplicity, examples shown in this AC use a moist adiabatic lapse rate of 2 °C per 1,000 feet.
Regardless of temperature, the relative humidity remains constant at about 100 percent.
11-3 As the saturated air parcel (see Figure 11-2) expands and cools, however, its water vapor content decreases. This occurs because some of the water vapor is condensed to water droplets or deposited into ice crystals to form a cloud. This process is triggered by the presence of microscopic cloud condensation (and ice) nuclei, such as dust, clay, soot, sulfate, and sea salt particles. The cloud grows vertically deeper as the parcel continues to rise.
Figure 11-2. Ascending Air Parcel that Becomes Saturated Example
At the surface, the air parcel has a temperature of 18 °C and a dewpoint of 13 °C, which indicates that it is unsaturated. As the parcel ascends, its temperature decreases at the dry adiabatic lapse rate of 3 °C per 1,000 feet, while the dewpoint decreases at 0.5 °C per 1,000 feet. The temperature-dewpoint spread decreases while relative humidity increases until the parcel achieves saturation at its LCL of 2,000 feet. As the parcel continues to ascend, condensation produces cloud formation. Because the heat added during condensation offsets some of the cooling due to expansion, the parcel now cools at the moist adiabatic lapse rate of 2 °C per 1,000 feet. The parcel’s dewpoint decreases at an identical rate as the lost water vapor condenses to form the cloud.
The relative humidity of the ascending saturated (i.e., cloudy) parcel remains constant at about 100 percent.
A descending saturated air parcel (see Figure 11-3) quickly becomes unsaturated.
Its temperature increases 3 °C per 1,000 feet, while its dewpoint increases at 0.5 °C per 1,000 feet (see Table 11-1). The temperature-dewpoint spread increases while relative humidity decreases.