Air Masses & Weather September 13, 2007

More Adiabatic Processes Air masses are one of the following…. –Unstable – describes an air mass that will just keep rising because its warmer than the surrounding air Occurs when the ELR is higher than the MAR and DAR Cloudy –Stable – describes an air mass that will resist rising because it is cooler or the same temperature as the surrounding air Occurs when the ELR is lower than the MAR and DAR Clear weather

–Conditionally unstable – describes an air mass that is stable unless it is forced to begin cooling at the MAR Occurs when the ELR is between the DAR and the MAR Very common over mountain ranges where air is forced to rise

Example An unsaturated air parcel at the ground is 10ºC, the ELR is 8ºC/1000m, and the dewpt temperature is 2ºC. The air parcel is going over a mountain that is 2000m tall. What happens to the air parcel? 10ºC/1000m * x = 8ºC –X = 800m, height of dewpt What is the temperature of the air outside of the air parcel? ELR = 8ºC/1000m

8ºC/1000m * 800m = 6.4ºC 10ºC – 6.4ºC = 3.6ºC, temp of surrounding air at 800m Temperature inside the air parcel at 800m is 2ºC. What will happen to the air parcel?

Example What happens on the leeward side of a mountain? As an air mass moves down the leeward (dry) side of the mountain, it warms by the DAR (10ºC/1000m). 12ºC 22ºC 1000m Outside air temp: 20ºC

Cloud Formation Processes involved in cloud formation: –Condensation involves… Adiabatic cooling Dewpt temperature is reached 100% relative humidity Moisture droplet – clouds or fog Cloud – condensation nuclei – involves particles in the atmosphere

Air Masses Air masses are distinctive bodies of air that reflect the characteristics of its source region –Categorized based on moisture content and temperature –Moisture categories: m = maritime (wet) and c = continental (dry) –Temperature categories: A (Arctic; coldest), P (polar; cold), T (Tropical; warm), and E (Equatorial; hot)

Air Mass Modification As air masses move away from their source region, they can take on characteristics more like the region they are moving over –An mT air mass traveling to Chicago would cool as it moves from the Gulf Coast –A cP air mass traveling to Texas would warm as it moves from Canada –Lake-Effect Snow

Fronts A front is the contact between 2 air masses of differing characteristics

Cold Front

Weather conditions associated with a cold front. Weather Phenomenon Prior to the Passing of the Front Contact with the Front After the Passing of the Front TemperatureWarmCooling suddenlyCold and getting colder Atmospheric PressureDecreasing steadilyLeveling off then increasingIncreasing steadily WindsSouth to southeastVariable and gustyWest to northwest PrecipitationShowers Heavy rain or snow, hail sometimes Showers then clearing Clouds Cirrus and cirrostratus changing later to cumulus and cumulonimbus Cumulus and cumulonimbusCumulus

Warm Front

Weather conditions associated with a warm front. Weather Phenomenon Prior to the Passing of the Front Contact with the Front After the Passing of the Front TemperatureCoolWarming suddenlyWarmer then leveling off Atmospheric PressureDecreasing steadilyLeveling off Slight rise followed by a decrease WindsSouth to southeastVariableSouth to southwest Precipitation Showers, snow, sleet or drizzle Light drizzleNone Clouds Cirrus, cirrostratus, altostratus, nimbostratus, and then stratus Stratus, sometimes cumulonimbus Clearing with scattered stratus, sometimes scattered cumulonimbus

Occluded Front

Stationary Front An almost motionless collision of air masses Common in summer Produces wide range of weather, depending on strength of air masses Eventually dissipates, or becomes warm or cold front