Chapter 38 34.1

1. Presented by April Senger Chapter 38 & 34.1 Weather and the Hydrologic Cycle

2. Presented by April Senger Hydrologic Cycle The natural circulation of water from the oceans to the air, to the ground, then to the oceans and back to the atmosphere See page 601 for illustration of global flow of water Water is consistently flowing and carries a tremendous amount of energy

3. Presented by April Senger Humidity & Relative Humidity Measure of the water vapor in the air (mass of water per volume) Relative humidity is the ratio of the amount of water contained in the air to the amount that the air could hold 50% means the air is holding half of what it could at that temperature

4. Presented by April Senger Saturation Water holding its maximum capacity is at saturation Warm air can hold more water than cold air can Air rises as it expands due to lower pressure Air cools as it expands so it tends to form condensation

5. Presented by April Senger Concept Check What is the major difference between fog and a cloud? Altitude

6. Presented by April Senger Condensation Water has to condense around another particle or ion sometimes called a condensation nuclei such as dust This creates clouds, rain, dew, frost, fog etc If an air mass reaches its dew point, it has to release its water because cooler water can�t hold as much as when it was warm Freezing dew gives us frost

7. Presented by April Senger What Controls Weather? Air pressure, temperature, and density help us to understand and predict weather Faster moving molecules (warm ones) have more pressure or collisions = high pressure The denser the air the more collisions that result in greater air pressure All three are interrelated

8. Presented by April Senger Adiabatic Processes In simplest terms�When the heat transfer is zero or nearly zero Example: if air is suddenly compressed, the temperature changes, and therefore the pressure changes The air became warmed even though no heat exchange took place

9. Presented by April Senger Atmospheric Stability Stable air resists upward movement Air forced to rise on a mountain is denser than the surroundings because of its temperature and tends to sink Both rising and sinking cause the air to spread out horizontally and if they have moisture, it will form horizontal clouds

10. Presented by April Senger Chinooks A dry wind that blows down from the Rocky Mountains across the Great Plains is often a Chinook The cold air coming off the mountain is compressed as it moves to lower elevations due to higher air pressure This compression warms it This is adiabatic warming

11. Presented by April Senger Temperature Inversion For the most part, warm air rises and cool sinks However, if a large parcel of cold air sinks and is trapped below a warm parcel a temperature inversion occurs Example: The forest fires and Missoula Valley of 2006

12. Presented by April Senger Concept Check What happens to the air temperature in a valley when dry, cold air blowing across the mountains descends into the valley? The air is adiabatically compressed and temp rises The Rocky Mountains are known for this and enjoy banana belt weather in winter What would happen to the temperature of a dry cleaner bag full of air that we have at 6 km if we pull a string rapidly to the surface? It will dramatically increase. Dry Abiabatic rate is 10 degrees C for every km so it would actually jump to a temperature of from -10 to 50 degrees C

13. Presented by April Senger Clouds A mixture of suspended water droplets and ice crystals are clouds They form from rising warm, moist air Higher altitudes cool the air and cold air holds less water than warm air reaching its dew point Clouds have 4 classifications based on altitude

14. Presented by April Senger High Clouds Altitudes of above 6000 meters Prefix is Cirro- Air is cold and dry so most are ice crystals Wispy clouds are cirrus clouds from the high winds, look like a mare�s tail, & indicate fair weather Cirrocumulus clouds are white puffs found in patches and cover only a small portion of the sky. They have small wavy ripples that look like fish scales and are often called a mackerel sky Cirrostratus are thin, sheet like and cover the whole sky. They refract light and halo the sun/moon. When they thicken it indicates rain or snow

15. Presented by April Senger Middle Clouds They form between 2000-6000 meters, are noted Alto- & can have water or ice crystals Altostratus are gray to blue-gray, cover the sky, are so thick you don�t see shadows & are noted before storms Altocumulus are gray, puffy, masses in parallel wave or bands & are much bigger/darker than cirrocumulus. They are seen on warm, humid mornings before afternoon thunderstorms

16. Presented by April Senger Low Clouds They range from the surface to 2000 meters and are called Stratus Mostly made of water droplets but can have ice and snow Uniformly gray and mask whole sky Common in winter with hazy, gray look They look like fog that isn�t on the ground and are associated with mists or dizzles

17. Presented by April Senger Low Cloud Kinds Stratocumulus are either low, lumpy layers that grow horizontally in rows or patches OR look like round masses if there is a weak rising condition Generally are light to dark gray and don�t carry rain or snow If you hold your hand to a stratocumulus cloud it would be the size of your fist where a altocumulus cloud would be the size of your thumbnail Nimbostratus clouds are dark and foreboding and carry light to moderate rain or snow

18. Presented by April Senger Vertical Clouds Cumulus clouds floating cottony clouds with sharp outlines and flat base They can be white to light gray and are found at about 1000 meters Their tops look like rising towers and the tops can flatten Cumulonimbus clouds turn dark and have precipitation and often become thunderheads Updraft is about 1 m/s This is faster than gravity pulling down Cloud droplets would evaporate before they hit the ground but rain drops are bigger, thus falling faster to beat the updraft and make it to the surface

19. Presented by April Senger Air Mass An air mass is a volume of air that covers large portions of the Earth�s surface with its own characteristics They are classified as Polar (P), Arctic (A) or Tropical (T) A second classification is Continental (c) or Maritime (m) Examples: mT Caribbean or cP Canada When two or more air masses meet a variety of weather conditions develop

20. Presented by April Senger Convectional & Orographic Lifting Convectional Lifting is a result of the Earth heating unequally If an air mass touches an area that has absorbed solar radiation it warms, rises and expands Cool sinks and warm rises Orographic Lifting is an air mass that is pushed upward over an obstacle such as a mountain range

21. Presented by April Senger Leeward Sides & Rain Shadows Air forced up the side of mountain is cooled and clouds can form Moisture is depleted as it climbs When the air mass gets to the leeward side of a mountain it is dry and warms adiabatically There is no moisture for rain so we call it a rain shadow

22. Presented by April Senger Fronts Fronts are a contact zone between two different air masses If a cold air mass moves into a warm air mass the zone of contact is a cold front If a warm air mass moves into a cold air mass the zone of contact is a warm front If neither are moving it is a stationary front Fronts often have wind, clouds, rain & storms

23. Presented by April Senger More About Cold Fronts Cold fronts have high cirrus clouds, a shift in wind direction, a drop in temperature, and a drop in air pressure The warm air is forced upward It cools and water vapor condenses to form cumulonimbus clouds or thunderheads with heavy showers & gusty winds After it passes, the air cools, sinks, pressure rises, rain ceases, skies clear, and we have calm

24. Presented by April Senger More About Warm Fronts The less-dense warmer air gradually rides up and over the cold, dense air It may have cirrus clouds and is more gradual They can thicken into altocummulus and altostratus clouds making it gray and overcast Light to moderate rain/snow can develop and winds are brisk At the front the air warms and rain/snow turns to drizzle and the air behind is warm with scattered clouds

25. Presented by April Senger Thunderstorms They begin with humid air rising, cooling, and condensing into a single cumulus cloud The clouds build and grow upward as long as they are being fed by a updraft of warm air Rising updrafts and sinking chilled downdrafts make a storm cell They can tower 12 km in height Winds flatten them into an anvil shape

26. Presented by April Senger Thunder & Lightning There are 1800 thunderstorms present at any time Water bumping and rubbing causes electrons to be released building up to thunder and lightning Lightning heats the air expanding it to form thunder Lightning strikes the Earth some 100 times per second Bolts can carry 100 million volts Lightning killed 200 victims last year

27. Presented by April Senger Tornadoes Winds rotating over a large area speed up when the radius of rotation decreases producing a tornado It is a funneled shaped cloud extending downward from a large cumulonimbus cloud It is only called a tornado if it touches down Winds travel at 800 mi/hr counter clockwise in the NH They can move horizontally across land at 45-95 km/hr Texas, Kansas, Oklahoma & Missouri experience some 300 tornadoes a year and is called Tornado Alley Most areas have ground shelters

28. Presented by April Senger Hurricanes The high humidity of the tropics favors the development of cumulus clouds and thunderstorms Wind speed and collisions can cause the moist air to tilt inward and spiral Hurricanes are winds with 300 km/hr winds They gain energy from the warm water source The low pressure zone in the center is relatively calm and is called the eye of the storm

29. Presented by April Senger Concept Check Would storms occur if all parts of the Earth�s surface were heated evenly? No�Storms need contact between warm and cool air

30. Presented by April Senger Weather Prediction Data used to be collected and analyzed by hand This took hours Now we use computers to analyze statistics and make predictions based on the probability of weathers conditions from the past