1. Weather 2013

2. Part 1. Station Model
Barometric Pressure – millibars (mb)
If # > 500, move decimal 1 place to left and put a “9” in front. Ex mb
If # < 500, move decimal 1 place to left and put “10” in front. Ex. 232 = mb
Air Temp (°F)
15 / .5
986 21
1.5 24 *
Visibility (miles)
Barometric Trend (change) during last 3 hours (move one decimal to left) = 1.5 mb
Dew Point (°F)
Amount of precipitation in last 6 hours (in)

3. Part 1. Station Model
Wind Direction – like an arrow coming in from where the wind originated (ex. East)
15 / .5
986 21
1.5 24 *
Wind Speed
= 10 knots
---- = 5 knots
Present Weather
= 50 knots

4. Part 1. Station Model

5. Part 2. Air Masses Characteristics of air masses Naming Air Masses
Determined by where they originate
Air over land = dry, water = humid
Air near poles = cold, equator = warm
Naming Air Masses
Continental – form over land; dry air
Maritime – form over water; wet air
Polar – form near poles; colder air
Tropical – form near equator; warmer air
Arctic – form over poles; Very cold air

7. Part 3. Air Pressure Systems
LOW Pressure
Low density air (lighter), rising air
Usually warm, moist air
Has precipitation
Counter clockwise, into the center

8. Part 3. Air Pressure Systems
HIGH Pressure
High density air (heavier), sinking air
Usually cooler, drier air
Fair, clear weather
Wind clockwise
AWAY from center

10. Part 3. Air Pressure Systems
Measurement of Air Pressure
Barometer – measures air pressure
Units
Inches of mercury
Millibars (mb)
Barometric Trends
Rising – clearing weather likely
Falling – increasing chance of precipitation
Steady - unchanging

11. Part 4. Fronts
Cold Front

12. Part 4. Fronts Cold fronts move QUICKLY (less friction)
Weather conditions
Cooler temps
Short period of precipitation
As front passes, air becomes cooler and drier (clearing skies)
Wind shifts to north (cold air from north)
Air pressure rises since cooler, drier air is heavier than warm, moist air

13. Part 4. Fronts
Warm Front

14. Part 4. Fronts Warm fronts move SLOWLY (more friction)
Weather conditions
Gradually warmer temps
Increased clouds
Steady precipitation for a longer time
Winds shift to a south direction
Air pressure decreases (warm, moist air weighs less)

15. Part 4. Fronts
Stationary Front

16. Part 4. Fronts
Occluded Front
Clouds and precipitation

17. Part 4. Fronts Finding Fronts on Weather Maps
Look for sudden changes in
Temperature
Air Pressure
Wind direction

18. Part 4. Fronts Development of Low Pressure Centers (Cyclones)
Form at interface between warm and cold air masses cP mT

19. Part 5. Forecasting Weather
Weather patterns in US move from West to East (or SW to NE)
Low pressure = cloudy and precipitation
Rising, unstable air
Falling air pressure usually means the approach of stormy weather
High pressure = fair weather (cool, dry air)
Rising air pressure usually means fair weather is approaching

20. Part 5. Forecasting Weather
Fronts cause unsettled weather, rising air causes precipitation
Warm fronts have longer periods of precipitation since they move slower than cold fronts (cold fronts have short and heavy periods of precip)

21. Part 5. Forecasting Weather
Large pressure differences cause strong winds
Isobars are close together = greater wind speed

23. Part 6. Violent Weather
When very cool, dry air collides with warm, humid air violent weather can occur (thunderstorms or tornadoes)

24. Tornadoes Hurricanes Formation Area Effected Size Duration Wind Speed
Cloud Shape
Over land from collision of very different air masses (temp and moisture); spring - summer
Over tropical water (energy from warm water); summer – early fall; lose strength when go over land
Most common in mid-west states
Coastal areas of SE and Eastern US
100 meters (up to a mile wide)
650 km (up to 400 miles)
Minutes
Days
200+ miles/hr
75 – 150+ miles/hr
Funnel
Huge spiraling low pressure area