Unit 4: Weather Dynamics Supplemental Notes

Types of Precipitation 1. drizzle fairly steady, light precipitation drop size: smaller than rain (0.5 mm diameter)

2. Rain separate drops of water fall to the Earth's surface from clouds

3. freezing rain begins as snow, falling from a cloud towards earth and melts completely on its way down through a layer of relatively warm (above freezing) air. As it falls toward the earth's surface, it encounters a layer at lower level of colder air and becomes supercooled This water will freeze on impact with any object it then encounters. The ice can accumulate to a thickness of several centimetres

4. Snow Crystallized water (ice) Consists of snowflakes that fall from clouds

5. Hail consists of balls or irregular lumps of ice (hailstones) 5 mm–50 mm in diameter on average much larger hailstones result from severe thunderstorms

6. Dew In the evening, surfaces will cool by radiating its heat Atmospheric moisture will condense at a rate greater than that of which it can evaporate Water droplets then form on surfaces

7. Frost Ice crystals that grow on a cold surface Results from water vapour in the air coming in contact with that cold surface

Weather Systems What is a weather system? a set of temperature, wind, pressure and moisture conditions for a certain region, that moves as a unit over a period of days

North American Weather Systems (p. 546 – 549) In the mid-latitude regions of North America weather is difficult to forecast because it changes so often. It’s easier to forecast near the equator (usually hot and humid) and near the North Pole (usually cool and dry).

Air Masses The basis of weather systems in N.A. Large bodies of air in which temperature and moisture content at a specific altitude are fairly uniform Vary in size, from 100 km across to 1000 km across

Most form where air above surface is fairly still for days or weeks The air takes on the moisture and temperature properties of the surface Help maintain Earth’s energy balance i.e. Convection and prevailing winds move warm, tropical air northward and cold, polar air southward.

North American Air Masses There are 6 air masses in North America, defined by their: Temperature Moisture Content Where they form Direction they move

Polar air masses originate over cold regions. Tropical air masses originate over the warm tropics. Continental air masses originate over land Maritime air masses originate over water

Low-Pressure Systems These systems tend to bring cloudy skies and stormy weather

Fronts boundaries between two air masses with different temperatures or densities. Temperature can change abruptly when fronts pass overhead.

A cold front is the boundary between warm and cool air, when the cool air is advancing to replace the warm air. At a cold front cold air following warm air undercuts the warm air This heaves it upwards with a more violent thrust, compared to the steady rise of air at a warm front.

Warm Front A warm front exists when warm air is rising over cold air. The boundary takes the form of a gradual slope and lifting is slow but persistent.

As the air lifts into regions of lower pressure, it expands, cools and condenses Condensed water vapour forms a flat sheet cloud (altostratus), from which rain can start to fall once cloud has thickened to about 2,500 metres from the ground

Formation of a Low-Pressure System (Storm) A front forms between a cold air mass and a warm air mass  Fast-flowing air in the jet stream pulls air up out of both air masses, creating a low-pressure system near the ground. The low-pressure area pulls in air near the surface The rising air swirls in a counter-clockwise direction (Coriolis effect)

The warm front rises over the cold air mass, carrying moisture with it and the cold front pushes under the warm air mass, causing warm, moist air to rise steeply.  A region of precipitation forms in front of the warm front as the jet stream continues to pull air away. Cumulonimbus clouds form and bring precipitation An occluded front forms as the warm front is caught by the cold front, cutting it off from the cooler air below (in the low-pressure system) Storm ends as upper air flow no longer pulls air away from the low-pressure area and a stationary front forms

Occluded Fronts Occluded Fronts are usually associated with warm and cold fronts separating warm and cold sectors of air. The lighter warm air rises above the heavier cold air, more gently at a warm front but more vigorously at the cold front following behind. Cold fronts usually travel faster than warm fronts, and therefore at some stage of depression development, the cold front catches up with the warm front.

Stationary Fronts Stationary Fronts occurs when two unlike air masses face one another, and there is little movement of air, the surface which separates them. The frontal slope is like that of a warm front. The type of weather that exists at a stationary front is similar to warm front weather. Eventually, either one or both of the air masses begin to move resulting in a warm front or cold front.

Cyclogenesis the process of forming a cyclone Cyclone Anticyclone a low-pressure system that rotates counterclockwise (in the Northern Hemisphere) and usually brings cloudy, stormy weather Anticyclone a high-pressure system that rotates clockwise (in the Northern Hemisphere) and usually brings clear skies

Warm high-pressure systems: form at 30° latitude, when air in the equatorial convection currents that are heading northward becomes colder and more dense and move downward, toward the surface. Cold high-pressure systems: occur in central Canada and the Arctic regions, especially in winter, from the cold, dry, descending Arctic air