Weather

1. Weather

2. Coriolis effect • The tendency of a moving object to follow a curved path rather than a straight path because of the rotation of the Earth. • This also works with air (wind) and water (ocean currents). • Northern Hemisphere- The curve is to the right. • Southern Hemisphere- The curve is to the left.

3. Global winds • Winds that blow from a specific direction and travel a long distance. • The direction of the wind is the direction it comes from. • Example a westerly wind comes from the West.

4. Global winds • Trade Winds- Air move out of the east. Trading ships used these to sail west. 0˚ - 30˚ • Prevailing Westerlies- Air moves from the west. This moves the weather pattern from west to east across the United States. 30˚ - 60˚ • Polar Easterlies- East winds. 60˚ - 90˚ • Doldrums- Light surface winds at the equator. Sailing ships may be stuck here for days. 0˚ • Horse latitudes- Sailing ships were stuck here and threw horses overboard when they were running out of food and water. 30˚

5. Jet stream • A narrow band of strong winds that blow at an altitude of about 7 miles. • The jet stream moves faster in the winter than in the summer. • Speeds may reach up to 300 mph.

6. Jet stream

7. Local wind • Breeze- A gentle wind that extends over a distance of less than 60 miles.

8. Local wind • Sea Breeze- During the day air over land is warmer because the land heats up faster that the water. The warm air rises and the cooler air over the water moves to replace it. Air moves from the sea to the land.

9. Local wind • Land Breeze- At night the land cools faster than the water, so air over the sea is warmer. The air over the water rises and cooler air from the land moves in to replace it. The air moves from the land to the sea.

10. Water phases • Melting-Solid to liquid • Evaporation- Liquid to gas • Condensation- Gas to liquid • Freezing- Liquid to solid • Sublimation- Solid to gas • Deposition- Gas to solid

11. Water in the air • Humidity- Water vapor in the atmosphere. • Dew Point- The temperature to which air must be cooled to be saturated. Condensation occurs when air is saturated. • Absolute Humidity- The actual amount of water vapor in the air. • Relative Humidity- The ratio of water vapor in the air to the amount of water vapor air can hold at a given temperature. Warm air holds more water vapor than cold air.

12. Water in the air • Psychrometer- An instrument that measures relative humidity.

13. Clouds • Cloud- A collection of small water droplets or ice crystals suspended in the air, which forms when the air is cooled and condensation occurs. • Condensation nuclei- Solid particle that provides the surface for condensation. Dust, dirt, salt. • There must be three things present for clouds to form: • Water vapor • Condensation nuclei • Temperature must be at dew point

14. Types of clouds • Clouds are classified by their shape and altitude. • Stratus clouds- Gray clouds that are flat, thin, and are at low altitudes. • Cumulus clouds- Low altitude, fluffy clouds with a flat base. • Cirrus clouds- High altitude, feathery clouds.

15. Types of clouds • Cloud names can be combined to list the type of cloud at a different altitude. • Prefixes and suffixes are also used: • Alto- means at a middle altitude. • Nimbo- or –nimbus means rain. • Altostratus • Altocumulus • Stratocumulus • Cirrostratus • Cirrocumulus • Nimbostratus • Cumulonimbus (Thunderheads)

16. Types of clouds • Fog- A cloud at ground level. • (Fog in San Francisco)

17. San Francisco fog

18. Liquid precipitation • Precipitation- Any form of water that falls to Earth from the clouds. • Rain-Liquid precipation. • Mist- Smallest • Drizzle- Next smallest • Raindrop- Largest

19. Frozen precipitation • Snow- Forms by deposition onto frozen condensation nuclei. The most common frozen precipitation. • Sleet- Frozen rain after leaving the cloud.

20. Frozen precipitation • Hail- Solid precipitation in the form of lumps of ice. Usually forms in cumulonimbus clouds. The more times the hail is carried upward, the larger it becomes.

21. hail

22. Air masses • Air mass- A large body of air that has the same temperature and moisture throughout. 2-4 miles deep and covers thousands of square miles. • Continental- Forms over land, low humidity • Maritime- Forms over water, high humidity • Tropical- Forms in the tropics, warm air • Polar- Forms near the poles, cold air

23. North American Air masses • Continental Tropical- Warm, dry air. U.S. Southwest and Mexico • Continental Polar- Cold, dry air. Canada • Maritime Tropical- Warm, moist air. Tropical Pacific, Tropical Atlantic, Gulf of Mexico • Maritime Polar- Cold, moist air. Polar Pacific, Polar Atlantic

24. fronts • Front- The boundary between two air masses. The average front is several hundred miles long. Changes in weather usually take place along fronts. Clouds form along fronts because air is rising.

25. Fronts • Cold Front- When a cold air mass overtakes a warm air mass. • Cumulus clouds precede • Cumulonimbus clouds if enough moisture • Heavy rain for a short time • Possible hail, lightning, thunder, tornadoes

26. fronts • Warm Front- When a warm air mass overtakes a cold air mass. • Cirrus clouds precede • Nimbostratus if enough moisture • Light rain for a long time

27. fronts • Occluded Front- When a fast moving cold front overtakes a warm front and lifts the warm air completely off the ground. • Cold fronts move faster than warm fronts.

28. fronts • Stationary Front- A front in which air masses move very slowly or not at all. Usually held in place by a high pressure area.

29. Cyclone and anticyclone • Cyclone- An area of low pressure. • North- counterclockwise • South- clockwise • Anticyclone- An area of high pressure. • North- clockwise • South- counterclockwise

30. Weather instruments • Thermometer- Measures temperature. • Barometer- Measures air pressure. • Anemometer- Measures wind speed. • Wind Vane- Measures wind direction. • Psychrometer- Measures relative humidity. • Hygrometer- Measures humidity. • Rain Gauge- Measures amount of rain.

31. Weather forecast • Daily Forecast- Predicts weather for a 48 hour period. • Extended Forecast- Predicts weather for 3 to 7 days. • Medium-Range Forecast- Predicts weather for 8 to 14 days. • Long-Range Forecast- Monthly and seasonal periods.