Name Teacher Period Date hapter22 Weather Maps hapter 22 Skill Sheet 1: Fronts Air masses are bodies of air that are relatively uniform in temperature and humidity. These bodies of air take on the characteristic of their geographic place of origin. For example, in summer, an air mass moving into New York from the South Atlantic Ocean is likely to be moist and warm. This would be a maritime tropical air mass. Most air masses are classified according to their humidity as maritime (moist) or continental (dry). The temperature is indicated by tropical, polar, or arctic. Arctic air masses move into in New York State only in the winter. They bring our coldest winter weather. Figure 22-1 is a satellite photograph of a low-pressure system. Low-pressure systems, also known as cyclones, are zones of air convergence. At the center of a cyclone, relatively warm, moist air rises. It rises because warm air and moist air are lower in density than cool air and dry air. As the lighter air at the center of the cyclone rises, different air masses move into (converge on) the low-pressure center. opyright 2007 AMSO Publications, Inc. FIGURE 22-1. This counterclockwise and inward circulation of winds is a characteristic of all Northern Hemisphere mid-latitude cyclones. SKILL SHEET 1: FRONTS 245
High-pressure systems, also known as anticyclones, are zones of air divergence. They form where descending cool and/or dry air spreads out at Earth s surface like pancake batter poured on a griddle. The boundaries (interfaces) between different bodies of air are known as fronts. Most weather changes are associated with the passage of fronts. 1. What are the temperature and humidity characteristics of a maritime polar air mass? Moist and cool 2. What is the most likely source region for a continental polar air mass blowing into New York State? anada (Northern anada) 3. How is a continental tropical air mass abbreviated in the Earth Science Reference Tables? ct 4. What four-word name for a cyclonic weather system tells you that air masses are drawn in toward the center? Zone of air convergence 5. What air motion causes cloud formation, and causes the winds to converge into cyclones? Air rising at the center of the low pressure system 6. Define weather front. A boundary or interface between different air masses. 7. When water vapor is added to the atmosphere, how does this affect the air s density? Water vapor makes the atmosphere less dense. There are four types of frontal boundaries. Each brings its own characteristic weather. The intensity of these fronts is determined by the difference in atmospheric pressures within the cyclone. The atmospheric pressure depends on the temperature and humidity of the air masses. Figure 22-2 illustrates the development of a cyclone. Warm fronts are usually preceded over a period of several days by thickening clouds. Wispy cirrus clouds may move lower and change to layered stratus clouds. As the front moves closer, steady rain and fog are common. The passage of the front is marked by a change old Air Warm Air ool Air FIGURE 22-2. This developing cyclone has an advancing cold front, shown by the triangles, and an advancing warm front, shown by the half circles.
Name Teacher Period Date to warmer but usually hazy weather. Warm fronts often bring maritime, tropical air masses in the Northeast. old fronts generally bring cool, crisp, and dry weather. They pass quickly, sometimes within an hour or two. A few, puffy cumulus clouds rapidly build into massive clouds of great height as a cold front approaches. Precipitation can be intense, but brief. Summer cold fronts often bring thunderstorms. As the front passes, the temperature drops, while air pressure suddenly increases, because the cool dry air following the cold front is relatively dense. Although fronts may pass quickly, they are not the sharp interfaces you might conclude from looking at a weather map. Most fronts are zones in which temperature and other atmospheric variables change rapidly. The severity of frontal weather depends on the difference in temperature and humidity between the air masses. Thunderstorms and tornadoes are often associated with cold fronts. However, a weak front may just cause cloudy weather without precipitation. 8. omplete the table below based upon the text you just read. Warm Front old Front Relative speed Slow Fast Temperature change Increase Decrease Precipitation Long and steady Brief, but intense Barometric pressure Usually decreases Usually increases Air mass that follows Often maritime tropical (mt) Often continental polar (cp) a strong front 9. Why do warm fronts advance more slowly than cold fronts? Warm air is less dense so it cannot push very hard. opyright 2007 AMSO Publications, Inc. 10. On what does the severity of frontal weather depend? The strength of the front or the magnitude of change in air pressure Some fronts do not move. These fronts are called stationary fronts. There is very little mixing of air masses along a stationary front. The polar front that separates the polar zone of easterly winds from the mid-latitude westerlies is often a stationary front. Winds generally blow in opposite directions along a stationary front. Nevertheless, as the name implies, the frontal boundary of a stationary front does not move. SKILL SHEET 1: FRONTS 247
An occluded front develops when a fastmoving cold front overtakes a warm front. When an occluded front develops, cold air pushes the warm air aloft. As shown in Figure 22-3, the denser, drier, cooler air pushes up the warm air. The line with the alternating half circles and triangles is the occluded front. Of these four weather fronts, this is the only one in which the frontal boundary does not touch the ground. The weather near an occluded front is usually cloudy or rainy for a relatively long period. old Occluded Front Warm Air Aloft ool FIGURE 22-3. Occluded stage. 11. When warm air is pushed up by cooler air closing in from both sides, what kind of front results? Occluded front 12. Name the front that generally stays in the same place. A stationary front 13. What front separates the cold polar easterly winds from the mid-latitudes westerlies? The polar front 14. How does an occluded front form? A cold front overtakes a warm front. Figure 22-4 shows how an eddy (swirl) develops in the polar front. The eddy grows into a mid-latitude cyclone as a zone of air convergence forms and draws in different air masses. The boundaries between these air masses are weather fronts. Polar front A B D WARM AIR OLD AIR OLD FRONT WARM FRONT FIGURE 22-4. Mid-latitude cyclones start and end as an eddy in the polar front.
What happens to the air that is drawn into a low-pressure system? It rises and spills out in the upper atmosphere. Therefore a zone of convergence at the surface sits below a zone of divergence (a high) in the upper atmosphere. onversely, a high-pressure area at the surface sits below a low-pressure center high in the atmosphere. 15. Write the type of front shown by each letter A through D in Figure 22-4. A. Stationary front B. old front. Warm front D. Occluded front Figure 22-5 illustrates a mature cyclone. The first diagram is a map view of the low-pressure system (cyclone). The three cross-sectional views show the three types of fronts found in a mature cyclone. Low cold warm E cool 16. Name the three kinds of fronts in Figure 22-5. E. Occluded F. old G. Warm ca cp mt THE WHOLE SYSTEM MOVES TO THE EAST Figure 22-6 shows the generalized circulation pattern of Earth. Notice that the polar front has been labeled. In winter, the polar front moves south, bringing the cold, crisp conditions of winter weather. Winter thaws occur when the polar front temporarily moves back north. In the summer, the polar front moves well to the north, and it is seldom a factor in our summer weather. 17. Why do occluded fronts bring a relatively small change in temperature? The warm air is isolated high in the atmosphere. cold F warm G fog cool FIGURE 22-5. A map view of a mature cyclone with vertical cross sections through the fronts. Westerlies Northeast trade winds LOW Southeast trade winds Westerlies LOW Polar easterlies POLAR FR O NT POLA R FRONT onvection cell onvection cell LOW Polar easterlies FIGURE 22-6. Global wind belts.
Answer the following by matching each item with the correct letter from the map in Figure 22-7. Note that three letters label frontal lines and the other three do not. 18. The cold front. E 19. The center of the warm air mass. D A 20. Location at which it will soon to get much warmer. B 21. Front with the whole warm air mass pushed aloft. A New York State B 22. Location at which the weather is cool and crisp. F 23. The warm front. F E D 24. The occluded front. A 25. Parts of the polar front. E and 26. The warmest ocean location. D FIGURE 22-7. A weather system in the Middle Atlantic states. 27. The front that is slowly pushing back the cold air. 28. What movement of an important front brings us our cold winter weather? The polar front moving south 29. What is an interface between different air masses called? A front 30. Low-pressure systems (cyclones) make fronts. Why do fronts not form in anticyclones? An anticyclone is a single spreading air mass. 31. Meteorologists often make weather maps at different elevations above Earth s surface. Where there is a zone of air divergence at Earth s surface, what would you expect to see directly above the zone, and why? A zone of air convergence (a low) to supply downdraft to feed the surface s zone of air divergence