Air masses, weather, and fronts

1. Mr. Lanik, Ms. Cooley, Mr. Liebowitz Air masses, weather, and fronts

2. Vocabulary • Meteorology • Air Mass • Front • Cold Front • Warm Front • Occluded Front • Stationary Front

3. Meterology • The study of the process that makes up the Earth’s atmosphere and what helps make weather predictions.

4. Air Mass • A large body of air that has similar characteristics throughout it.

5. Front • The boundary (line) that separates different air masses.

6. Cold Front • The line between an advancing cold air mass and the warmer air mass it is moving.

7. Warm Front • The line between the advancing warm air and the cold air mass it is moving.

8. Occluded Front • When a cold front “catches up” to a warm front.

9. Stationary Front • If a front (cold or warm) is not moving forward.

10. Guided Notes • An air mass is a large body of air in the lower troposphere that has similar characteristics throughout. • An air mass can be several thousand kilometers in diameter and several kilometers high. • Two or three air masses can cover the entire U.S.!

11. Guided Notes • Throughout an air mass, • temperature and • Humidity are nearly uniform. • The temperature and humidity of an air mass depends on where the mass originates. • In polar regions the lack of sunlight causes the ground to be very cold. • If an air mass stays in a polar region for a long time, the air mass becomes cold. • The opposite happens in tropical regions.

12. Guided Notes • The moisture of an air mass depends on the surface where it came from. • If an air mass stays over land for a long period of time, it becomes dry, where an air mass over the ocean absorbs water vapor and becomes moist.

13. Guided Notes • When an air mass moves from one place to another, it takes the temperature and humidity of its place of origin with it. • However, some of its characteristics may change as it moves across the country. • When cold polar air moves south, it affects the weather of the area it enters, as well as slowly heats up as it moves across a warmer surface. • Earth’s topography also helps change the temperature and humidity of an air mass as it travels.

14. Guided Notes • There are five main air masses in North America: • Continental Arctic • Continental Polar • Maritime Polar • Maritime Tropical • Continental Tropical

15. Guided Notes • Continental Arctic (cA) air masses originate in the arctic regions where the air is extremely cold. • Because cold air is incapable of containing much moisture, cA air masses are very dry.

16. Guided Notes • Continental Polar (cP) air masses start over Alaska and Canada and are somewhat warmer than the cA air masses. • The differences in temperature and humidity between cA and cP air masses are usually slight. • When cP air passes over the Great Lakes region in the late fall when the water is still warm, the cold, dry air picks up moisture from the lakes, then dumps the precipitation as snow, causing lake effect snow.

17. Guided Notes • Maritime Polar (mP) air masses form over the ocean in high latitudes. These air masses are both cold and damp. • mP air is not as cold as cP air because of the difference in temperatures between the land and the oceans. • Occasionally mP air rom the North Atlantic brings heavy snowstorms called nor’easters to the East Coast in the winter and cool, clear weather to the area in the summer.

18. Guided Notes • Maritime Tropical (mT) air masses form over warm tropical ocean and gain both warmth and moisture. • In the summertime, mT air from the Bahamas and the Gulf of Mexico moves counterclockwise around the low pressure over the Atlantic Ocean, bringing heat and humidity to the Midwest and Eastern U.S. • Since the air mass contains a lot of moisture, thunderstorms often develop. • As the sun heats the ground, the moist mT air is heated from below and rises, forming thunderclouds. • When the sun sets, the clouds and thunderstorms leave as the surface cools.

19. Guided Notes • Continental Tropical (cT) air masses originate over the deserts and are hot and dry. • Most of the time a cT air mass originates as a maritime air mass but becomes dry as it passes over mountains. • cT air masses produce massive heat waves in most of the United States. • mTair usually produces temperatures no higher than 100 degrees Fahrenheit, but cT air is much hotter, with temperatures exceeding 100 degrees Fahrenheit. • cT air is dry and does not bring clouds of thunderstorms, however if this type of air mass comes into a crop region it can cause serious damage to them.

20. Guided Notes • Air masses of different types do not mix easily. • The boundary that separates different air masses are called fronts. • The width of a front can range from 200 meters to 200 kilometers and can effect weather patterns in areas hundreds of kilometers wide.

21. Guided Notes • Fronts are most common at mid-latitudes, where southward moving polar air masses and northern moving tropical air masses often meet. • At the front, the less-dense air mass is forced to rise over the dense air mass. • Because of this the front is usually wedge shaped.

22. Guided Notes • Fronts usually bring precipitation. • At the surface of the front the less-dense air rises into the troposphere where the air cools, and if humid enough, can create clouds and bring precipitation.

23. Guided Notes • The weather associated with a particular front depends on two things: • Type of air mass • Speed of the air mass

24. Guided Notes • A cold front is the boundary between an advancing cold air mass and a warmer air mass. • Cold air is denser than warm air, so the cold air slides underneath the warm air, forcing it upward. • Cold air can create thunderstorms, or just a change in wind. • In the summer, cold fronts can cause a decrease in humidity • In the winter, a cold front can cause • Rain • Snow

25. Guided Notes • If warm air moves cold air, the boundary between the air masses is known as a warm front. • The advancing warm air rises above the denser cold air, which moves away slowly. • The first signs of a warm front are high cirrus clouds. • Cirrus clouds may warn of approaching precipitation.

26. Guided Notes • Cold fronts usually move twice as fast as warm fronts. • If a cold front “catches up” to a warm front, the result is an occluded front. • The warm air caught between the two colder air masses is forced upward. • Once the warm air rises, it cools, often resulting in • Cloudiness • Precipitation

27. Guided Notes • If a front is not moving forward, it is a called a stationary front. • The warmer air rises over the denser, colder air, and clouds and precipitation may form.