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TOPIC 7 WEATHER
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Weather Defn: The condition of the atmosphere in a given place and time. Weather Variables
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Temperature Amount of heat energy in the atmosphere.
Instrument: Thermometer Thermometers work due to expansion.
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Air Pressure: The weight of the overlying air Instrument: Barometer
Millibars and Inches of Mercury ( see ref. tables page 13)
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Pressure Conversions Convert to inches 996.0 mb = 1033.0 mb =
Convert to millibars 29.50 in = 30.06 in = 28.79 in =
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Factors Affecting Air Pressure:
Altitude As altitude increases, air pressure decreases. Temperature As temperature increases, air pressure decreases. Humidity (water vapor) As the humidity increases, air pressure decreases. All 3 are indirect relationships!!!
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Moisture: As the air temperature increases, it’s ability to hold water vapor increases. OR Warm air can hold more water vapor than cold air.
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Relative Humidity%- The amount of water vapor the air is holding compared to what it can hold. Dew Point Temperature- The temperature at which the air will become saturated (rel. humidity = 100%) Condensation occurs when the air becomes saturated. Examples:
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Instrument- Sling Psychrometer (wet bulb/dry bulb)
See page 12 in Ref. Tables
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Dew Point Chart
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Relative Humidity
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As the temperature and the dew point get closer…
The relative humidity increases. The chance of clouds and/or precipitation increases.
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Which city has the greatest chance of clouds and precip.?
Which city has the lowest relative humidity? City Temperature Dew Point Syracuse 25o 20o Rochester 19o 10o Utica Albany 24o 22o
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Wind Defn: the horizontal movement of air
Wind is caused by a difference in air pressure (caused by the uneven heating of the Earth) Air moves from regions of high pressure towards regions of low pressure
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CIRCULATION H L
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Instrument: Vane (direction)
Anemometer (speed) Wind speed is determined by the pressure gradient. As the gradient increases, the wind speed increases. The closer the isobars, the higher the wind speeds.
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Wind Vane Anemometer
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Station Model Map
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Station Model 27 040 3 = +12/ 24 .75 Wind Speed (25 knots) Temp. (oF)
Wind Direction (NE) 27 040 Visibility (3 miles) Present Weather (rain) Air Pressure ( mb) 3 = +12/ Dew Point (o F) Barometric trend (1.2 mb increase) 24 .75 Precip. (.75 inches in past 6 Hours) Cloud Cover (50%)
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Practice Station Model
Temp = 22o F Dew Point= 18o F Winds 8 mph Cloudy Snow showers Air Pressure = mb Visibility 4 miles
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974 32 7 * 30
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Water Cycle
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Evaporation Liquid to gas (water vapor) Requires the addition of heat
Transpiration- water vapor entering the atmosphere through plants
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Factors Affecting Evaporation
Temperature As temp increases, evap rates increase. Wind As wind increases, evap rates increase. Humidity As humidity increases, evap rates decrease. Surface area As surface area increases, evap rates increase.
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Condensation Gas to liquid Heat is released to the atmosphere.
Condensation occurs when… The air becomes saturated with water vapor and Condensation nuclei are present.
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Cloud Formation Clouds usually form in air that is rising! AIR RISES
EXPANDS COOLS CONDENSES
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Precipitation: When water returns to the Earth’s surface.
Occurs when the water droplets or ice crystals become heavy enough to fall. A method by which the atmosphere cleanses itself. Ice crystals will form when the temp = the dew point and they are at or below 32o F
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Air Masses A “blob” of air with similar moisture and temperature characteristics. Air masses get their characteristics from the surface over which they form. (source region)
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cP Continental Polar – Dry and cold
Temperature Polar – Cold (P) Tropical – Warm (T) Moisture Continental – Dry (c) Maritime – Moist (m) cP Continental Polar – Dry and cold cT Continental Tropical – Dry and Warm mP Maritime Polar – Moist and cold mT – Moist and warm cA Continental Arctic- Dry and Very Cold
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cP mP cT mT mT
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L H WEATHER SYSTEMS Cool/ Dry air Clear/ Fair weather
Air is sinking and diverging Warm/ Moist air Clouds & Precip. Air is converging and rising. L H
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More Pressure Less Pressure
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The air inside this balloon is heated to make it less dense so it rises
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Why is dry air heavier than moist air?
PINGPONG BALL = WATER VAPOR GOLF BALL = AIR MOLECULES “MOIST AIR” “DRY AIR”
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H L Weather Systems High Pressure Low Pressure Cold/ Dry Warm/ Moist
Clear/ Fair Sinking & Diverging Low Pressure Warm/ Moist Clouds/ Storms Rising & Converging H L
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Cold Front – cold air pushes into warm air.
Conditions: Temp decreases Precip is heavy and short (t-storms). Vertical cloud development Cumulonimbus clouds
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Warm Front - warm air displacing cold air.
Conditions Temp increases. Precip. is steady light and long. Horizontal cloud development Stratus clouds
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Stationary Front - the boundary between warm and cold air remains in the same place.
Conditions: Weather similar to a warm front.
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Occluded Front: Occurs when a cold front catches up to a warm front in a wave cyclone. Wave Cyclone L L
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Winds blow parallel to a front ahead of it and perpendicular to the front behind it !
When a front passes… Temperature changes Air pressure changes Wind direction changes Usually clouds and/or precip. occurs
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Localized Weather Factors
Lake Effect Snow Heavy snowfall that occurs on the leeward side of large lakes. Warm lake water, cold air & wind
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Nearness to Large Body of Water
Cooler summers and warmer in winters. Water heats and cools slower than land. Increase in precipitation
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Orographic Effect Windward Leeward
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Land and Sea Breezes
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Hazardous Weather Thunderstorms
Severe storms associated with warm air. Usually occur along cold fronts. Conditions Heavy rain Winds Lightning Hail(?) Thunder
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Precautions (Lightning)
Don’t be the highest object in the area. Don’t stand under a tree or near a window. Stay off the phone. Don’t touch plumbing or electrical devices. Stay inside a car or crouch down.
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Tornadoes Small scale, short lived violent storms associated with severe t-storms. “Twisters” Fujita Scale 0 – 5 Weak Strong
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lat
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Precautions (Tornadoes)
Get to the interior lowest level of the building. Cover your head. Get out of your car and lay in a ditch.
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Hurricanes Large scale, severe low pressure systems.
Stages of Development
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Hurricane Season Late summer/ Early fall Conditions: High winds Flooding Heavy precip Storm Surge T-storms Storm surge- wall of ocean water that invades low coastal areas caused by the high winds and low pressure.
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Bonus: What is the name given to the “wall of ocean water that invades low coastal areas caused by the high winds and low pressure.”?
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Atlantic Hurricane Track
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Hurricanes form and intensify over warm ocean water!
Precautions Early evacuation Move to higher ground Stock up on non-perishable food, water radio & batteries Board up windows.
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Blizzards Storm with winds greater than 35 mph and considerable amounts of falling and/or blowing snow. shut down transportation Disrupt telephone and electrical service Prevent people from obtaining fuel, food and needed services.
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Precautions Have food, fuel and other supplies on hand. Stay indoors.
Do not travel.
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BONUS: The air pressure is currently inches of mercury. How would this air pressure be shown on a station model?
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