Air Masses and Weather Fronts

Thunderstorm- cause and lifecycle

The sun heats the earth's surface during the day. The heat on the surface and warms the air around it. Since warm air is lighter than cool air, it starts to rise (known as an updraft). If the air is moist, then the warm air condenses into a cumulus cloud. The cloud will continue to grow as long as warm air below it continues to rise. When the cumulus cloud becomes very large, the water in it become large and heavy. Raindrops start to fall through the cloud when the rising air can no longer hold them up. Meanwhile, cool dry air starts to enter the cloud. Because cool air is heavier than warm air, it starts to descend in the cloud (known as a downdraft). The downdraft pulls the heavy water downward, making rain. This cloud has become a cumulonimbus cloud because it has an updraft, a downdraft, and rain. Lightning and thunder start to occur, as well as heavy rain. The cumulonimbus is now a thunderstorm cell. After about 30 minutes, the thunderstorm begins to dissipate. This occurs when the downdrafts in the cloud begins to dominate over the updraft. Since warm moist air can no longer rise, cloud droplets can no longer form. The storm dies out with light rain as the cloud disappears from bottom to top.

Hurricane (tropical cyclone)

surface air spirals inward toward a hurricane, rises through the towering wall of clouds, and then flows outward above the storm. Falling air near the storm's center creates the eerie calm in the eye of the hurricane

Pressure gradients - influence on wind direction and speed

wind blows in response to differences in pressure; pressure gradient- wind speed is determined by the magnitude of the pressure difference over distance; high pressure→ low pressure

Source regions of major air masses in North America

wind direction is clockwise around the high pressure system and counter-clockwise around the low pressure system

Fronts - cause and characteristics

The boundary between a warmer air mass and a cooler one is called a front

Anticyclone

a high pressure system with accompanying winds

Air mass

a large body of air with approximately uniform temperature and humidity at any given attitude.

Cyclone

a low pressure region with an accompanying surface wind

Seasonal (monsoon)

a seasonal wind and weather system caused by uneven heating and cooling of continents and oceans

Tornado

a small, short-lived funnel-shaped storm that protrudes form the base of a cumulonimbus cloud

Rain Shadow

desert forms where moist air rises over a mountain range and precipitates most of its moisture on the windward side and crest of the range. The dry, descending air on the leeside absorbs moisture, forming a desert

diurnal

each day

Occluded front

forms when a faster moving cold mass traps a warm air mass against a second mass of cold air. Thus the warm air mass becomes trapped between two cold air masses. The faster moving cold air mass then slides beneath the warm air, lifting it completely off the ground. Precipitation occurs along both frontal boundaries, combining the narrow band of heavy precipitation of a cold front with the wider band of lighter precipitation of a warm front.

Cold front

forms when moving cold air collides with stationary or slower moving warm air. The dense, cold air distorts into a blunt wedge and pushes under the warmer air. Thus the leading edge of the cold front is much steeper than that of a warm front. The steep contact between the air masses causes the warm air to rise rapidly; creating a narrow band of severe weather commonly accompanied by cumulus and cumulonimbus clouds.

Warm front

forms when warm air collides with a stationary cold mass. The moving warm air rises over the cooler dense air as the two air masses collide. The rising warm air-cools adiabatically and the cooling generates cloud and precipitation. Precipitation is generally light because the air rises slowly along the gently sloping frontal boundary.

Wind (advection)

horizontal air flow

Ascending and descending air masses - development of high and low pressure systems

o Rising low-pressure air creates clouds and precipitation. Air flows inward toward the low pressure zone, creating surface winds o Sinking high-pressure air creates clear skies. Air flows outward from the high-pressure zone and also creates surface winds

Stationary Front

occurs along the boundary between two stationary air masses. Under these conditions, the front can remain in an area for several days. Warm air rises, forming conditions similar to that of a warm front

Orographic lifting

occurs when an air mass is forced from a low elevation to a higher elevation as it moves over rising terrain. As the air mass gains altitude it quickly cools down adiabatically, which can raise the relative humidity to 100% and create clouds and, under the right conditions, precipitation.