wind lecture #5

Midlatitude Cyclones

Commonly referred to as "low pressure centers" or "frontal systems," these major weather makers occur in the midlatitudes (35° - 60°) and are larger than hurricanes but have less intense winds. They can occur approximately every 4-7 days and can have thunderstorms and tornadoes associated with them. The ingredients to midlatitude cyclones are fronts.

Cyclones

Associated with clouds, precipitation, and winds, cyclones are characterized by low pressure centers and exhibit a characteristic movement from west to east. The first sign of a cyclone approaching from the west is the appearance of cirrus clouds roughly 12 to 24 hours in advance of the warm front.

Climatology of the Midlatitudes

The polar front region, located between the Polar and Ferrel cells, is the battleground between cold front and polar jet stream.

Air Masses

Midlatitudes are the battle ground between continental polar (cP) and maritime tropical (mT) air masses. cP air is stable, while mT air is unstable.

Fronts

Occur at the division/collision center between air masses.

Polar Front Theory

The polar front is a semi-continuous boundary separating cold polar air from more moderate mid-latitude air. Midlatitude cyclones (wave cyclones) form and move along the polar front in a wavelike manner, with low pressure or cyclones being the principle weather makers at midlatitudes.

Great Lake Storms

Major weather events that impact lake levels, cause erosion and sediment transport, and result in ship and property damage. Approximately 20% of midlatitude cyclones have their origins at the Great Lakes.

Hurricanes vs Midlatitude Cyclone

Hurricanes: energy source is latent heat release, have warm cores, weaken with height, have a central eye with sinking air, winds are strongest at surface, isobars are more circular, have no fronts, the centre pressure drops, and occur near the ocean.

Mid lat cyclonesL energy source is contrast of air masses, have cold cores, intensify with height, air rises in the centre, winds are stronger aloft, isobars are less circular, have fronts, centre pressure drops, and occur aloft.

Perfect Storm

A storm that occurred in October 1991, also known as the Halloween Storm, which was formed by the combination of cold air from the US West meeting a warmer air mass from the Atlantic. It involved the remnants of Hurricane Grace and later transformed into an unnamed hurricane.

Storm of the Century

A storm that occurred in 1993, starting as a frontal wave off the Texas Coast and intensifying into a deep open wave cyclone over Florida. It caused significant damages, including a deep snow blanket, damage to homes, and the loss of lives. It occurred because within a couple hours many low pressure centres formed, and cold and warm fronts were developed as well as pivot points.

1998 Ice Storm

A midlatitude cyclone that resulted in freezing rain and caused significant damage in Quebec. It was the worst natural disaster in Canada, causing loss of lives, power outages, and economic losses.

1999 Blizzard of Toronto

A series of four storms that passed to the south of Toronto and resulted in larger than normal snowfall. It was an unusual event that does not appear to be part of a trend. The winds swept across Lake Ontario gathering moisture and energy which led to large r than normal snowfall.

Freezing Rain

A type of storm that can develop into freezing rain, starting as a midlatitude cyclone. It occurs when mT air mass overrides cP air, leading to precipitation that forms aloft and freezes upon contact with the cold ground.

anti-cyclonic

characterized by clear skies and light winds and are caused by sinking air that can cause very high temperatures in the summer

Stationary Front

Stationary fronts are stable due to a lack of latent heat. Low pressures, horizontal wind shear, and are identified by alternating blue triangles and red semi circles

Cold Front

cold fronts occur when cold air pushes into a warm air mass. Designated by blue line with triangles facing warm air. Heavy precipitation when the cold air pushes the warm mT air up. There is a strong temperature gradient, shift in wind direction, and change in pressure

Warm front

Warm fronts occur when warm air pushes into a cold air mass. Gentle precipitation. Designated by red line with semi circles pointing toward cold air

Occluded fronts occur when the cold front catches up with the warm front.

Occluded fronts occur when the cold front catches up with the warm front. Most intense part of the storm. Warm air is forced above the surface. Associated with alternating blue triangles and red semi circles on one side, and the colour purple.

Step One Polar Front Theory

stationary front with a strong horizontal wind shear. cP on the tip of the stationary front coming and meeting mT air. As soon as they come close to the stationary front it causes a disturbance

wind shear

horizontal gradient of wind direction and can be unstable

step two polar front theory

low pressure centre develops. Cold anticyclonic fronts, and warm cyclonic fronts develop from opposite sides. Cold air pushes, and warm air rises. There is a collision between cP and mT. Precipitation starts

pivot point

lowest local pressure and low pressure centre

step three polar front theory

warm front and cold front are more intense. Pivot point develops and so does a circular isobar. A wave fully develops and moves East or Northwest. The centre pressure continues to drop and large bands of precipitation form. A warm sector forms in the region between fronts, which becomes a fuel centre for storms as the warm air brings latent heat.

step four polar front theory

pivot point is even narrower, isobars are more closely spaced, centre pressure continues to drop. Precipitation band continues to grow longer, the faster moving cold front catches up with the warm front which reduces the size of the warm sector.

step five polar front theory

the storm intensifies. an occluded front develops because the cold from catches up with the warm front. the pressure continues to drop. It is the most intense part of the storm with widespread precipitation and comma shaped clouds.

step six polar front theory

storm begins to dissipate after occlusion. The source of the energy mT air has been cut off because there is no more latent heating.