1. Weather
Earth Science Chapter 16

2. Chapter 16-1
Objectives:
Explain how solar heating and water vapor in the atmosphere affect weather.
Discuss how clouds form and how they are classified.
Describe how rain, hail, sleet and snow develop.

3. Meteorology Meteorologists are people who study the weather.
A> Weather is the specific daily condition of the Earth’s atmosphere.
The atmospheric factors that interact to cause weather are air pressure, wind, temperature, and moisture.

4. Heating the atmosphere
The Sun’s heat evaporates water and it also heats the air directly and indirectly.
Temperature is the measure of air molecule movement.
a. The Sun energizes molecules to move more rapidly…temperatures are high and it feels warm.
When less energy reaches air molecules = cold

5. Wind = air mass moving in a specific direction
The temperature of air affects air pressure. As the Sun heats air, the air expands, becomes less dense, rises, and has low air pressure.
Cooler air is denser and sinks, causing high atmospheric pressure.
Wind is air that is moving from an area of high pressure to an area of low pressure.
Wind speed can be measured with an anemometer.

6. Humidity
Water vapor, or moisture, in the air is called humidity. The amount of moisture varies from place to place and time to time. Warmer air can hold more water vapor.
Relative humidity is the percentage of moisture the air holds relative to the amount it could hold at a specific temperature.
When the relative humidity is 100 percent, the air is saturated.

7. Dew
When air cools, it can’t hold as much water vapor, so the vapor condenses into a liquid or forms ice crystals.
The dew point is the temperature at which air is saturated and condensation forms.

8. Clouds
B> Clouds form as warm air is forced upward and cools. Then the water vapor condenses into tiny droplets that stay suspended in the air.
The shape and height of clouds vary with temperature, pressure and water vapor in air.
Three shapes…

9. Types of Clouds
Stratus
Cumulus
Cirrus
Cloud Types

10. Cloud height prefixes
Cirro = high
Alto = middle
Strato = low

11. Nimbus clouds are very dark and so full of water that sunlight can’t penetrate them.
Cumulonimbus clouds are thunderstorm clouds.
High winds can flatten the top of the cloud into an anvil-like shape.
When we see cumulonimbus clouds, we may have heavy rain, snow, hail, lightning and even tornadoes.

12. Stratus Clouds
Smooth, gray clouds that cover the whole sky and block out the sun are called stratus clouds. They form at an altitude of about 2.5 kilometers.
Light rain and drizzle are associated with stratus clouds. Nimbostratus clouds bring rain and snow.
When stratus clouds form near the ground, the result is fog.

13. Cirrus Clouds
Feathery or fibrous clouds are called cirrus clouds. Cirrus clouds form at very high altitudes.
Cirrus clouds are usually made of ice crystals.

14. Precipitation
Water vapor that condenses and forms clouds can fall to the Earth as rain, sleet, snow or hail, as determined by air temperature. Water that falls from the atmosphere to the Earth is called precipitation.

15. Sleet
When falling snow or rain passes through an extremely cold layer of air, they sometimes freeze into small ice pellets called sleet. Sleet reaches the Earth only in winter.

16. Snow
Snow forms when water vapor changes directly into a solid. Snowflakes are flat six-sided ice crystals that have beautiful shapes. Because they sometimes clump together, it is hard to see the separate crystals. No two snowflakes are exactly alike.

17. Hail
Hail is one of the most damaging forms of precipitation forms in cumulonimbus clouds. Hailstones are small balls of ice ranging from 5-75 mm in diameter.
Hailstones are formed when water droplets hit ice pellets in a cloud and freeze.

18. Chapter 16-2 Weather Patterns
Objectives:
Describe how weather is associated with fronts and high and low pressure systems.
Explain how tornadoes develop from thunderstorms.
Discuss the dangers of severe weather.

19. A. Because air and moisture move in the atmosphere, weather constantly changes.
Air masses are large bodies of air that form over the same area and take on the properties of that part of Earth’s surface.
Stormy weather is associated with low pressure.
Fair weather is associated with high pressure.
Air pressure is measured with a barometer.

20. B. Fronts
When two air masses that have different properties meet, they do not mix easily. A boundary forms between the two air masses. This boundary is called a front.
The weather at a front is usually unsettled and produces clouds, precipitation and storms.
There are four types of fronts:

21. Cold Front
A cold front forms when a mass of colder air meets and pushes under a mass of warm air. The cold air mass forces its way underneath the warm air mass and pushes it upward. Violent storms are associated with a cold front. Fair, cool weather usually follows.

22. Warm Front
A warm front forms when a mass of warmer air overtakes a colder air mass and moves over it. Rain and showers usually accompany a warm front. Hot, humid weather usually follows.

23. Occluded Front
A cold front travels faster than a warm front. When a cold front overtakes a warm front, an occluded front forms. As the warm air is pushed upward, the cold air meets cool air. An occluded front may also occur when cool air overtakes a cold front and warm air is pushed upward. An occluded front produces less extreme weather than a cold or warm front.

24. Stationary Front
When a warm air mass meets a cold air mass and no movement occurs, a stationary front forms. Rain may fall in an area for many days when a stationary front is in place.

25. C. Severe Weather
A storm is a violent disturbance in the atmosphere. It is marked by sudden changes in air pressure and rapid air movements. Some storms may cover a huge area, whereas others cover only a small area.

26. 1. Thunderstorms Occur inside warm, moist air masses and at fronts.
Warm, moist air is forced rapidly upwards where it cools and condenses.
Strong updrafts of warm air and sinking, rain-cooled air cause strong winds.

27. 2. Lightning
Movement of air inside storm clouds can cause parts of cloud to have be oppositely charged.
Current flows between the oppositely charged regions, forming a lightning bolt.
Lightning, an electrical discharge, striking the ground is the leading cause of forest fires.
Lightning

28. 3. Thunder
When lightning superheats the air, it expands rapidly and then contracts. This sudden expansion and contraction of the air results in sound waves, or thunder.

29. 4. Tornado
A tornado is whirling, funnel-shaped cloud. Tornadoes often form from a type of cumulonimbus cloud called a wall cloud. They are usually formed from severe thunderstorms.
The air pressure at the bottom of the funnel of swirling air is extremely low. When this low-pressure area touches the ground, it acts like a giant vacuum cleaner.
A tornado over water is called a water spout. They can travel 95 km per hour.
Tornadoes 101

30. Fujita Scale
The Fujita scale ranks tornadoes according to how much damage they cause.
Only 1% is classified as an F5.

31. Cyclones and Anticyclones
Air pressure has a great effect on the weather. An area of low pressure that contains rising warm air is called a cyclone. In a cyclone, cooler air moves in to take the place of the warm air and spiral around and into the center of the cyclone.
As the winds blow into a low pressure area, they swirl counterclockwise direction in the Northern Hemisphere. Cyclones usually cause rainy, stormy weather.
The weather caused by high pressure systems, called anticyclones, is usually clear, dry and fair.

32. 5. Hurricane
A hurricane is a powerful cyclone that forms over tropical oceans. Hurricanes that form over the western Pacific Ocean are called typhoons. In the Indian Ocean they are called cyclones.
Hurricanes that affect the United States move from the Eastern Atlantic near Africa to the north west striking the East Coast, usually between Florida and North Carolina.
Hurricanes 101

33. Formation of Hurricanes
Warm, moist air begins to rise rapidly. Cooler air moves in and the air begins to spin.
As the air pressure in the center drops more air is drawn into the spinning system. The system begins to spin faster.

34. Damage of a Hurricane
The high waves and strong winds of a hurricane often cause great damage, especially in coastal areas. Heavy rain may also cause serious flooding.
Meteorologists can track the path of a hurricane and issue watches or warnings. A typical hurricane lasts for about 9 days. Some may last 3 or 4 weeks.

35. 6. Blizzard
A blizzard is a winter storm with strong winds, cold temps, and low visibility that lasts more than three hours.

36. Severe Weather Safety
When severe weather threatens, the National Weather Service will issue a watch or a warning.
During a watch conditions are favorable for severe weather to develop. Stay tuned to a radio or television for updates/instructions.
During a warning, severe conditions already exist; you should immediate action.

37. Chapter 16-3
Objectives:
Explain how data are collected for weather maps and forecasts.
Identify the symbols used in a weather map.

38. Weather Forecasts Meteorologists study and predict weather.
Weather data from 300 local stations are used by the National Weather Service to prepare daily weather maps. The information is recorded in the form of numbers and symbols.
Symbols are used to show wind speed and direction, cloud cover, precipitation, position and direction of fronts and areas of high and low pressure.
How to Read a Weather Map

39. 1. Station Models show weather conditions at a specific location.
Meteorologists interpret weather information from local weather observers, balloons, satellites, and weather stations around the world.
When meteorologists gather data it is recorded on a map using a combination of symbols forming a station model.

40. Isotherms
Isotherms are lines that connect locations with the same temperature. The number on the end of an isotherm indicates the temperature at all point along the isotherm.

41. Isobars
Isobars are lines that join places on a weather map that have the same air pressure.
The number at the end of an isobar represents the air pressure recorded at each observation.
The air pressure can be given in millibars, inches of mercury or both.

42. Frontal Movement
In North America, weather fronts usually moved from west to east. Why?