WeatherUnit 6 Climate & Weather

Weather•Weather is the day to day

state of the atmosphere.

•Depends on temperature, precipitation, wind, pressure, clouds and visibility

•Driven by air masses

Air Masses•Air Mass- large body of air

•Defined by temperature and dew point (moisture parameters)

•Air masses are classified based on the source region of the air mass itself.

•Air masses are classified by its source and thermal type.

Air Mass ClassificationSource Regions Thermal Types

• Maritime (m) – originates over oceans or large bodies of water

• Continental (c) – originates over land

• m or c indicate influence of surface on air mass characteristics (water and land)

• Tropical (T) – from low latitudes

• Polar (P) – from mid-high latitudes

• Arctic (A) – from high latitudes (> 65°N)

• P and T suggest importance of latitude of source regions

Air Mass Properties-Continental Polar (cP) Summer Winter

• Source – central Canada

• Ample warming of surface through insulation

• Melts snow and permafrost

• Cool, dry and sometimes unstable due to insulation heating of lower layers

• Source – Central Canada and Siberia

• Frozen surface – ice and snow

• Intense radiation cooling, lack of insulation heating

• Extremely cold, stable, and dry

• Clouds are non-existent

Air Mass Properties •Continental Tropical

(cT)

• Summer Only

• Source – Northern Mexico and extreme SW deserts of US

• Hot, dry and unstable

Air Mass PropertiesMaritime Polar (mP)Summer Winter

• Source – open oceans in high latitudes – Gulf of Alaska and North Atlantic

• Cool and moist in lower layers and cool and dry aloft

• Overall temperature higher than in winter

• Instability in lower layers

• Source – open oceans of high latitudes – Gulf of Alaska and North Atlantic

• cP air mass from Syberia becomes mP air mass as it moves over ocean▫ Lower layers modified by

warmer water surface• Cool, moist, unstable in

surface layers

• Cool, dry aloft

Air Mass PropertiesMaritime Tropical (mT)

Summer Winter

• Source – semi permanent high centers near 15N

• Very warm, moist and unstable

• Source – over open ocean near 30N

• Great semi permanent subtropical high pressure centers and in SW Caribbean

• Warm, moist and very unstable

Fronts•Front- a narrow transition zone between two

or more air masses with different densities.

•Depends on temperature and humidity

•4 types of fronts:▫Cold▫Warm▫Stationary ▫Occluded

Front Properties• Cold Front- cold air

advancing replacing warm air.

• Leading edge of cold air▫ cP replacing mT▫ cP due south, mT due north

• Shown by solid blue triangles oriented towards the direction in which front is moving

• Rapid movement – up to 30 mph

Front Properties• Cold dense air wedges under warm air forcing it upwards

• The faster the front the steeper the slope of the front

• Cold fronts are associated with dramatic changes in weather

Front Properties •Warm front- Transition zone

between a retreating cold air mass and advancing warm air mass

•Shown by red half-circles pointing in the direction of where it’s going

•Warm and moist mT replacing dry cold mP

Front Properties• Frontal changes are less abrupt than cold air

frontal passages▫ Long spells of cold weather do not come to a

rapid end• Cold air lies as a wedge under warm air

▫ As warm front approaches depth of cold air decreases

• Warm fronts are slow and depending on the moisture of the air can bring storms

Front Properties •Stationary front- an air

mass with no movement

•Shown by alternating red half circles and blue triangles

•Obstacles prevent front from progressing▫Mountain ranges

Front Properties•Occluded fronts- a frontal

system that forms when a cold front over takes a warm front

•Designated by a purple line with alternating purple triangles and purple semi-circles, all pointing in the direction of the frontal movement.

•2 types cool-type & cold-type

Fronts on Weather Maps

Weather map symbols:1. cold front;2. warm front;3. stationary front;4. occluded front;5. surface trough;6. squall/shear line;7. dry line;8. tropical wave

Troughs, Squalls, Dry-Lines, Tropical Waves

•Trough: an elongated region of relatively low atmospheric pressure, usually coming before a cold front

•Squall: a sudden, sharp increase in wind speed which is usually associated with active weather, such as rain showers, thunderstorms, or heavy snow▫A squall line is a line of severe thunderstorms

that can form along or ahead of a cold front

Squall Line

Dry Line•A boundary separating moist and dry air

masses

Dry Line

Tropical Wave•a type of atmospheric trough, an extended

area of moderately low air pressure, leaning north to south, which move from east to west across the tropics creating areas of cloudiness and thunderstorms▫Approximately 60% of Atlantic tropical cyclones

originate from tropical waves, while approximately 85% of intense Atlantic hurricanes (Category 3 and greater) develop from tropical waves

Tropical Wave

Global Wind Systems: Polar Easterlies, Prevailing Westerlies, & Trade Winds

•Polar easterlies: wind zones between 60◦ N latitude and the north pole, and 60◦ S latitude and the south pole▫Dense, polar air ▫As earth spins, it is deflected in an easterly direction away from each pole.▫Cold, weak, sporadic

Global Wind Systems: Polar Easterlies, Prevailing Westerlies, & Trade Winds

•Prevailing Westerlies: wind systems located between latitudes 30▫ N and 60▫ N, and 30▫ S and 60▫ S▫Originate from the west▫Steady winds

Global Wind Systems: Polar Easterlies, Prevailing Westerlies, & Trade Winds

• Trade Winds: 2 circulation belts of wind between latitudes 30▫ N and 30▫ S ▫Air sinks, warms, and moves toward the equator in

an easterly direction, then rises and moves back towards 30▫ N and 30▫ S, where it sinks

and the process repeats.

▫ the sinking air associated with trade winds creates an area of high pressure, resulting in a belt of weak surface winds called horse latitudes

Jet Streams

•A large temperature gradient in upper-level air combined with the Coriolis effect results in strong westerly winds called jet streams.▫A narrow band of fast wind

Hadley, Ferrel, and Polar Cells•Hadley cell: the strongest of the three cells of

circulation▫formed as warm air rises above the Equator and

starts to flow northward.

Hadley, Ferrel, and Polar Cells•Ferrel Cell: The mid-latitude circulation cell

between the Polar cell and the Hadley cell

Hadley, Ferrel, and Polar Cells•Polar Cell: Cold dense air

descends over the poles, which creates high pressure▫this cold air moves along the

surface to lower latitudes.▫At around 60°N & 60°S, this air

has been warmed up and rises upwards, creating a zone of low pressure.

Weather Tools

Hygrometer

•an instrument used for measuring the moisture content in the environmental air, or humidity.

Psychrometer•two thermometers, one which is dry and

one which is kept moist with distilled water on a sock or wick. The two thermometers are thus called the dry-bulb and the wet-bulb.

•Used to find relative humidity.

Anemometer

•a device for measuring wind speed, and is a common weather station instrument.

Barometer•a scientific instrument used in

meteorology to measure atmospheric pressure.

Thermometer

•a device that measures temperature or temperature gradient using a variety of different principles.

Rain Gauge•a type of instrument used by

meteorologists and hydrologists to gather and measure the amount of liquid precipitation over a set period of time.

•also known as a udometer or a pluviometer or an ombrometer or a cup

Wind Vane

•an instrument for showing the direction of the wind. They are typically used as an architectural ornament to the highest point of a building.