2 WeatherWeather is the local, short term atmospheric condition resulting from the interaction of certain variables.All weather occurs in the troposphereThe weather in the United States generally moves from the southwest to the northeast.
4 Atmospheric Variables There are five (5) constantly changing factors that cause weather:TemperatureAir PressureWindMoistureAtmospheric Transparency
5 1. TemperatureThe Earth’s surface heats up at different rates due to duration and intensity of insolation, land and water differences and altitude.Temperature is a measure of how cold or hot something is.
6 2. Air PressureCaused by the weight of the air, which varies due to the temperature, altitude and moisture content.Warm air is less dense, therefore, it’s lighter.The higher the altitude, the less air above, therefore, less pressure.Water vapor is lighter than dry air.Air pressure is measured with a barometer.At sea level, air pressure is 14.7 lbs/in2 (one atmosphere) or inches of mercury, or millibars.
7 3. WindMovement of air horizontal to the Earth’s surface caused by differences in air pressure.Wind blows from high pressure areas to low pressure areas.The greater the difference between the air pressures of two areas, the higher the wind speed.
8 4. MoistureWater vapor enters the atmosphere by a process called evapotranspiration.Evapo ~ comes from evaporation (liquid into gas)Transpiration ~ when plants release water vapor into the atmosphere.
9 5. Atmospheric Transparency If the atmosphere was made out of clear, colorless, gases, it would be transparent. However, there are aerosols in the air.The more aerosols, the less transparent; resulting in an increase in insolation absorbed or reflected by the atmosphere and the more chance of clouds and rain.
10 Atmospheric Temperature Isotherms ~ Temperature data shown on a map by the use of isolines.
11 6 Factors determine temperature: Amount of insolationGenerally, the more insolation at a location, the warmer the temperature.Amount of radiationTerrestrial radiation is reradiated back into the atmosphere and heats it.CondensationChange of water vapor into liquid water. Releases a large amount of latent heat.
12 Sublimation 5. Frictional Drag: Wind Change of water vapor into ice. Releases a large amount of latent heat.5. Frictional Drag:Due to the Coriolis Effect, the interface between the atmosphere and the Earth’s surface produces frictional heat.WindHeat is moved around by the wind.
13 Adiabatic Heating and Cooling Due to molecular movement:When gas expands, it’s temperature decreases.When gas is compressed, the temperature increases.This automatic change in temperature of a gas due to expansion or compression is Adiabatic temperature change.
14 Atmospheric Moisture/ Energy of Evaporation It takes 540 calories/gram to change liquid water to water vapor (esrt)When evapotranspiration occurs, the more energetic water molecules leave first.The molecules that are left behind are not as energetic.The result is a low temperature of an evaporating liquid.
15 Factors that effect evaporation rate: The higher the temperature, the faster the rate of evaporation.The greater the surface area of the water, the faster the rate of evaporation.The stronger the wind speed, the faster the rate of evaporation.If the air has a high moisture content (saturated), it cannot absorb any more.
16 Saturation Vapor Pressure Saturation: When air is holding the maximum amount of water vapor it can.Vapor Pressure: The pressure that water vapor exerts.Saturation Vapor Pressure: The pressure that is exerted when the air is saturated.Dynamic equilibrium is reached when saturated vapor pressure is reached (condensation equals evaporation)
17 Measuring Relative Humidity and Dew Point Temperature Sling Psychrometer: Contains an ordinary thermometer (dry-bulb) and a thermometer with a cloth around the bulb (wet-bulb).When the cloth is moistened and the psychrometer is whirled around, the wet-bulb temp drops due to evaporation.The amount of cooling depends on the rate of evaporation.
18 Humidity Humidity: The water vapor content in the atmosphere. Absolute Humidity: The weight of water vapor in a given volume of air.Directly related to vapor pressure.Air can hold more water vapor at higher temperatures.Warmer temps = more humidity
19 Relative HumidityRelative Humidity: The ratio of the amount of water in the air to the maximum amount that could be present at that temperature.High temperatures can hold more water.RH is expressed as a percent.When RH reaches 100%, the air is saturated.
20 Dew PointDew Point Temperature: The temperatured at which the air is saturated with moisture (relative humidity is 100%)A further drop in temperature would result in condensation (rain) or sublimation (snow).The reason is that colder air can not hold as much water as warmer air.Dew Point does not depend on relative humidity; only on the amount of water vapor in the air.
21 Air Masses and FrontsWeather is caused by the interaction of Air Masses.Air Mass ~ Large body of air with similar characteristics (pressure, moisture, and temperature)Air masses form when a large air mass sits over a part of the Earth’s surface for a long time.The part of the Earth that an air mass forms over is called a source region.
22 Air masses combine two of five characteristics: Temperature:Arctic (A)Polar (P)Tropical (T)Moisture:Continental (c) – dryMaritime (m) – wet
23 An air mass is described by its source region . . . Cold:Arctic (A)Polar (P)
27 Everything comes down to Everything comes down to The interaction of weather variables: temperature, air pressure, moisture content (relative humidity and dew point), precipitation (rain, snow, hail, sleet, etc.) wind speed/direction and cloud cover.Air will either: ·Rise, expand, cool and condense in a low-pressure, convergent, counterclockwise system.Or . . .Sink, compress, heat and evaporate in a high-pressure, divergent, clockwise system