Weather Weather Variables and Variables and ForecastingForecasting

Modified from National Weather Service Modified from National Weather Service www.srh.noaa.gov/shvwww.srh.noaa.gov/shv

Air Air MassesMasses

4 Principal Air Mass Types that affect North America:

P = Polar - Located poleward of 60° Lat. c = Continental – From over large land masses m = Maritime – From over oceans T = Tropical - Located between 25° N & S

Air Masses are separated by FRONTS

A large body of air with uniform temperature and humidity conditions inherited from a "source region.”

Source regions = areas where air stagnates, taking on the properties of the underlying region.

Weather Forecasting requires that we examine multiple variables over space and time.

Meteorology – primarily concerned with Large-scale weather systems, such as cyclonesMajor frontsMovement of the jet streams

Pressure and WindPressure and Wind

• High Pressure: air moves High Pressure: air moves downward, clockwise (N-downward, clockwise (N-hemisphere) and awayhemisphere) and away

• Low Pressure: air moves Low Pressure: air moves upward, counter clockwise upward, counter clockwise (N-hemisphere) and towards(N-hemisphere) and towards

• Isobars: Lines of equal Isobars: Lines of equal pressurepressure

• Pressure Gradient: the Pressure Gradient: the difference in the pressuresdifference in the pressures– where the isobars are close where the isobars are close

together the gradient is strongtogether the gradient is strong– Strong pressure gradient force Strong pressure gradient force

= High wind velocities= High wind velocities

Origin of WindOrigin of Wind

How Do Clouds Form?How Do Clouds Form? Air must be forced upwardAir must be forced upward

Orographic LiftOrographic Lift Frontal WedgingFrontal Wedging ConvergenceConvergence Convective LiftConvective Lift

If upward moving air contains moisture, If upward moving air contains moisture, that moisture will cool as it is lifted that moisture will cool as it is lifted upward in the atmosphere.upward in the atmosphere.

When air cools to its dew point, the When air cools to its dew point, the moisture will condense and form clouds. moisture will condense and form clouds.

Cloud Cloud FormationFormation

Convective Lift: Convective Lift: • Warm air rises because it Warm air rises because it

is less denseis less dense• As air rises it gradually As air rises it gradually

cools cools • Moisture in the air will Moisture in the air will

condenses as the condenses as the temperature approaches temperature approaches dewpoint dewpoint

• Water droplets bond Water droplets bond onto condensation onto condensation nuclei. nuclei.

• These nuclei will collide These nuclei will collide with other nuclei, with other nuclei, eventually forming a eventually forming a cloud. cloud.

Cloud FormationCloud Formation

Orographic LiftOrographic Lift

Types of CloudsTypes of Clouds Low clouds: Consist of stratoform (flat) clouds, or Low clouds: Consist of stratoform (flat) clouds, or

cumuloform (puffy) clouds, lying on or just above the cumuloform (puffy) clouds, lying on or just above the surface (up to ~10,000 ft).surface (up to ~10,000 ft).

Stratus: Stratus: Cumulus:Cumulus:

Types of Clouds (ContTypes of Clouds (Cont’’d)d) Middle Clouds: Consist of altoform (meaning a Middle Clouds: Consist of altoform (meaning a

middle cloud) type clouds, extending from 10,000 middle cloud) type clouds, extending from 10,000 – 20,000 feet above the surface.– 20,000 feet above the surface.

Altostratus: Altocumulus:Altostratus: Altocumulus:

Types of Clouds (ContTypes of Clouds (Cont’’d)d) High Clouds: Consists of mainly ice crystals High Clouds: Consists of mainly ice crystals

suspended above 20,000 feet from the surface. suspended above 20,000 feet from the surface. These clouds are wispy/crisp/featherlike in These clouds are wispy/crisp/featherlike in appearance, and do not produce precipitation.appearance, and do not produce precipitation.

Cirrus: Cirrostratus: Cirrus: Cirrostratus: Cirrocumulus:Cirrocumulus:

Altocumulus-LenticularisAltocumulus-Lenticularis

Formation of PrecipitationFormation of Precipitation Collision & Coalescence processCollision & Coalescence process

water droplets continue to grow, water droplets continue to grow, forming clouds. forming clouds.

If enough lift is present in the If enough lift is present in the atmosphere updrafts are created, atmosphere updrafts are created, forcing the cloud to grow taller. forcing the cloud to grow taller.

The water droplets continue to grow The water droplets continue to grow inside of clouds, until they become too inside of clouds, until they become too heavy to be suspended by the updraft.heavy to be suspended by the updraft.

It is then that these droplets fall down to It is then that these droplets fall down to the earth as rain. the earth as rain.

Types of FrontsTypes of FrontsCold Front: A transition zone where cold/dry, and Cold Front: A transition zone where cold/dry, and stable air, replaces warm/moist, and unstable air. stable air, replaces warm/moist, and unstable air. It is depicted by a blue line with triangles It is depicted by a blue line with triangles pointing towards the direction of movement.pointing towards the direction of movement.

WarmerTemperatures

ColderTemperatures

27

33

62

54

Cold Front (Vertical Profile) Cold Front (Vertical Profile)

Rising Warm

air

AdvancingCold air

Warm FrontWarm Front

55

62

WarmFront

WarmerTemperatur

es

ColderTemperatures

45

38

Warm Front (Vertical Profile)Warm Front (Vertical Profile)

Cold airWarm air

Stationary FrontStationary Front

WarmAir

ColdAir

A boundary that has essentially no movement, but separates warm, moist air from cold, dry air. It is depicted by an alternating red and blue line containing blue triangles and red semicircles. The triangles point towards the warmer air, while the semicircles point towards the cooler air.

Mapping ExerciseMapping Exercise

Draw and analyze a surface weather Draw and analyze a surface weather map for pressure and temperature, map for pressure and temperature, and draw the fronts that are and draw the fronts that are indicated by these two variables.indicated by these two variables.

Discuss our results.Discuss our results.

Pressure/Front MapPressure/Front Map

Temperature/Front MapTemperature/Front Map