Weather Variables and Forecasting Modified from National Weather Service

Air Masses 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 cyclones Major fronts Movement of the jet streams

Pressure and Wind High Pressure: air moves downward, clockwise (N- hemisphere) and away High Pressure: air moves downward, clockwise (N- hemisphere) and away Low Pressure: air moves upward, counter clockwise (N- hemisphere) and towards Low Pressure: air moves upward, counter clockwise (N- hemisphere) and towards Isobars: Lines of equal pressure Isobars: Lines of equal pressure Pressure Gradient: the difference in the pressures Pressure Gradient: the difference in the pressures –where the isobars are close together the gradient is strong –Strong pressure gradient force = High wind velocities

Origin of Wind

How Do Clouds Form?  Air must be forced upward  Orographic Lift  Frontal Wedging  Convergence  Convective Lift  If upward moving air contains moisture, that moisture will cool as it is lifted upward in the atmosphere.  When air cools to its dew point, the moisture will condense and form clouds.

Cloud Formation Convective Lift: Warm air rises because it is less dense Warm air rises because it is less dense As air rises it gradually cools As air rises it gradually cools Moisture in the air will condenses as the temperature approaches dewpoint Moisture in the air will condenses as the temperature approaches dewpoint Water droplets bond onto condensation nuclei. Water droplets bond onto condensation nuclei. These nuclei will collide with other nuclei, eventually forming a cloud. These nuclei will collide with other nuclei, eventually forming a cloud.

Cloud Formation

Orographic Lift

Types of Clouds Low clouds: Consist of stratoform (flat) clouds, or cumuloform (puffy) clouds, lying on or just above the surface (up to ~10,000 ft). Low clouds: Consist of stratoform (flat) clouds, or cumuloform (puffy) clouds, lying on or just above the surface (up to ~10,000 ft). Stratus: Cumulus: Stratus: Cumulus:

Types of Clouds (Cont ’ d) Middle Clouds: Consist of altoform (meaning a middle cloud) type clouds, extending from 10,000 – 20,000 feet above the surface. Middle Clouds: Consist of altoform (meaning a middle cloud) type clouds, extending from 10,000 – 20,000 feet above the surface. Altostratus: Altocumulus: Altostratus: Altocumulus:

Types of Clouds (Cont ’ d) High Clouds: Consists of mainly ice crystals suspended above 20,000 feet from the surface. These clouds are wispy/crisp/featherlike in appearance, and do not produce precipitation. High Clouds: Consists of mainly ice crystals suspended above 20,000 feet from the surface. These clouds are wispy/crisp/featherlike in appearance, and do not produce precipitation. Cirrus: Cirrostratus: Cirrocumulus: Cirrus: Cirrostratus: Cirrocumulus:

Altocumulus-Lenticularis

Formation of Precipitation  Collision & Coalescence process  water droplets continue to grow, forming clouds.  If enough lift is present in the atmosphere updrafts are created, forcing the cloud to grow taller.  The water droplets continue to grow inside of clouds, until they become too heavy to be suspended by the updraft.  It is then that these droplets fall down to the earth as rain.

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

Cold Front (Vertical Profile) Rising Warm air Advancing Cold air

Warm Front Warm Front Warmer Temperatures Colder Temperatures 45 38

Warm Front (Vertical Profile) Cold air Warm air

Stationary Front Warm Air Cold Air 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 Exercise  Draw and analyze a surface weather map for pressure and temperature, and draw the fronts that are indicated by these two variables.  Discuss our results.

Pressure/Front Map

Temperature/Front Map