1. What is Weather? Weather Patterns Weather Forecasts

2. Weather Factors
Weather : state of the atmosphere at a specific time and place
Weather describes conditions such as air pressure, wind, temperature and amount of moisture in air
Sun provides almost all of earth’s energy
It encourages evaporation, which condenses and water returns to earth’s surface
The land absorbs the heat and warms the atmosphere and causes uneven heating of atmosphere
Air and water currents are caused by sun’s uneven heating

3. Air Temperature
Temperature is a measurement of the motion of molecules
The more movement, the greater the temperature
Less movement of molecules, the cooler the temperature

4. Wind Air moving in a specific direction is called wind
Air near the surface is warmed by conduction
Air expands, becomes less dense, rises
(warm rising air forms low pressure at surface)
Cool dense air tends to sink that yields to high pressure areas
Wind results because air moves from areas of high to low pressure

5. Wind Wind direction is measured by a wind vane
(points in the direction from which wind is blowing)
Wind is measured by an anemometers which has cups that spin with wind strength

6. Humidity When water evaporates into the atmosphere it fits into spaces
The amount of water vapor present in an air mass is called humidity
Warmer air can hold more water vapor
Cooler air holds less water
If temperature is cool enough water vapor condenses and the air mass is considered saturated

7. Relative Humidity
Relative humidity is a measure of the amount of water vapor present in the air compared to amount needed for saturation at a specific temperature
Relative humidity is dependent on the specific temperature and changes as temperature changes (warmer air holds more water than cooler air)
If a cubic meter of air at 25˚C is saturated with 22 g of H2O it is said to have a RH of 100%
If it has 11 g of H20 it has RH of 50%

8. Dew Point
When temperature drops, less water vapor can be held and it condenses to a liquid or forms ice (depending of the temperature)
The temperature at which condensation occurs is called Dew Point

9. Forming Clouds Warm air rises As air rises it cools
Cooler air doesn’t hold as much water and if it cools to saturation point, its dew point is met
Water droplets form around nuclei of dust, smoke, smog to form clouds after saturation point is met

10. Classifying Clouds Classified by shape and height
Shape and height of clouds vary with temperature, pressure, and amount of water vapor present

11. Cloud Shape Stratus (form layers or smooth even sheets)
3 Main types
Stratus (form layers or smooth even sheets)
Found at lower altitudes and may be associated with fair weather or rain or snow
Fog is low stratus clouds formed by saturation near ground level

12. Stratus Clouds : low and flat

13. Cloud Shapes
Cumulus clouds: masses of puffy white clouds often with flat bases
Sometimes they tower to great heights and can be associated with fair weather or thunderstorms

14. Cumulus clouds
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15. Cloud Shapes Cirrus clouds: appears fibrous curly, wispy
They are high, thin, white feathery made of ice crystals
Associated with fair weather, or can indicate approaching storms

16. Cirrus clouds
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17. Cloud Heights Prefixes help describes the height of clouds
Cirro (high clouds)
Alto (middle clouds)
Strato (low level clouds)
Some combine the altitude with stratus or cumulus
Cirrostratus (high clouds-fair weather or warning of approaching storm)
Altostratus: not thick, middle layers, sunlight filters through

18. Rain or Snow-producing clouds
Nimbus (dark rain cloud) refers to clouds that produce rain or snow
Because they are thick, you can’t see through them
Cumulus clouds grows into thunderstorms called cumulonimbus (12 miles up)
Nimbostratus clouds are layered clouds that can bring long steady rain or snowfall

19. Cumulus nimbus

21. Nimbus stratus
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22. Precipitation Water falling from clouds is known as precipitation
Water grows around nuclei (dust, smoke) and grow larger until they fall to ground
Raindrop size depends on strength of updrafts in cloud (strong updrafts let condensed water move up and down, growing in size)
If the temperature is warm as droplet falls it can evaporate before reaching surface

23. Precipitation
Air temperature determines whether water forms rain, snow, sleet, or hail
Above freezing falls as rain
Snow forms when temperatures is so cold that water vapor changes directly to a solid
Sleet forms when raindrops pass through layers of freezing air near surface forming ice pellets

24. Hail Hail is precipitation in the form of lumps of ice
Forms in cumulonimbus clouds when thunderstorms develop
Hailstones grow as they are tossed up and down in cloud
Can grow from 2.5 cm (1inch) to softball size

25. Role of water vapor and weather
Relative humidity helps determine whether you will have a dry or wet day
The temperature of atmosphere determines the form of precipitation
Winds are dependent on rising air

26. Weather Patterns
Air Mass: large bodies of air that have properties similar to the part of the earth’s surface over which they develop
Continental polar: develops over continent (dry) and cold
Continental tropics: develops over continent (dry) and warm
Maritime polar: develop over body of water (moisture) and is cold
Maritime tropics: develop over water and warm

28. Low Pressure Winds blow from high pressure to low pressure
Pressure is measured by amount of air above the surface
As winds blow into low pressure systems, the rotation of earth causes low pressure systems to move counter-clockwise called cyclone
Low pressure are areas of rising air (temperature decreases, clouds form, precipitation probable)

29. High Pressure
Winds that blow from High pressure spiral clockwise in northern hemisphere (anticyclone)
High pressure is associated with fair weather
Barometers measure the pressure above the surface
Variations of pressure in atmosphere affects weather
Sinking air in high pressure systems indicates warming air mass, less clouds (or no) usually is fair weather

31. Fronts
Boundary between two air masses of different density, moisture, or temperature is called a front
Cloudiness, precipitation and storms sometimes occur

32. Cold front
Cold front is shown on map with a blue line with triangles pointing in direction of air mass movement ( when colder air advances toward warm air)
Cold air wedges and lifts warm air upward
Warm air cools, clouds form (if enough moisture)
Thunderstorms form when temperature differences are large

33. Warm Front
Warm front forms when warm air advances over heavier colder air
It is written with red lines and semi-circles pointing in the direction of the warm air advances

34. Occluded front This involves 3 air masses of different temperature
Cold air moves toward cool air with warm air between the two
Colder air is forced up, closing off the warm air from the surface
Shown on maps with purple lines with triangles and semicircles

35. Stationary Front When boundary between air mass stop advancing
Remain in same place for several days, producing light wind and precipitation
Shown as alternating warm and cold front symbols

38. Severe Weather
Thunderstorms : occurs when warm moist air rises along fronts
Cumulonimbus clouds form that can reach heights of 12 miles up
In cloud smaller droplets of water collide to form bigger ones
As droplets fall, the air is cooler, and brings about updrafts of warmer air causing strong winds
Hail forms in clouds if temperature is below freezing in the clouds and droplets freeze in up and down drafts

41. Thunderstorm Damage
Flash flooding occurs when streams can’t contain all the water
Strong winds originating from the thunderstorm causes damage
Hail damages property
Destroy crops

42. Thunderstorm Damage
Lightning: warm air is lifted faster in clouds when clouds form as cooler air sinks
Movement of air can cause different parts of the cloud to be oppositely charged
Currents then flow in opposite direction causing lightning
Lightning can occur cloud to cloud, cloud to ground and within a cloud

43. Thunderstorm Damage
Lightning can reach temperatures of ,000 °C (sun’s photosphere is 6000°C)
This extreme heat causes air around the lightning to expand rapidly
Then it cools quickly and contracts
That rapid movement of the molecules forms sound waves heard as thunder

44. Tornados
Tornado’s are violent rotating columns of air in contact with the ground
In thunderstorms, wind at different heights blow at different speeds
These differences are called wind shear and rotates the column parallel to ground
It can create a funnel cloud as it rotates upward
If it touches the ground it is called a tornado

45. Hurricanes
Hurricanes are large swirling low pressure system that originates over warm oceans
It moves warm air away from the ocean to the atmosphere as wind
Typhoons and cyclones are another name
In the Atlantic Ocean hurricanes develop near Africa and winds direct them to U.S.
Gain strength as move over warm oceans
When hurricanes reach land they cause destruction and damage, tornados spin off but land cuts off their energy source, so they are de-energized and loose power

48. Blizzards Blizzards in the northwest are common
Temperatures must be low as well as visibility (400m) caused by blowing winds (56 mph)

49. Weather Forecasts Meteorologist: person who studies weather
Take data measurements like temperature, air pressure, winds, humidity, and precipitation
Balloons weather satellites, doppler radar and computers help meteorologists
They produce weather maps to help predict weather

50. Forecasting Weather
Station models: weather data is recorded in combination of weather symbols at station models
Temperature: lines of equal temperature on a weather map are called isotherms
Pressure: lines on map with equal pressure is called an isobar
If the isobars are close together, usually high winds are present
Large pressure differences over small area causes strong winds
Isobars also tell you areas of high and low pressure

51. Weather Maps Shows high and low pressure
Shows movements of types of fronts
Can get a big picture of what is going on weather-wise