Weather 2013

Part 1. Station Model Barometric Pressure – millibars (mb) If # > 500, move decimal 1 place to left and put a “9” in front. Ex. 998.6 mb If # < 500, move decimal 1 place to left and put “10” in front. Ex. 232 = 1023.2 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)

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

Part 1. Station Model

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

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

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

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

Part 4. Fronts Cold Front

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

Part 4. Fronts Warm Front

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)

Part 4. Fronts Stationary Front

Part 4. Fronts Occluded Front Clouds and precipitation

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

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

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

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)

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

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

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