AOS 100: Weather and Climate Instructor: Nick Bassill Class TA: Courtney Obergfell

Miscellaneous Exams & Homeworks

Review of September 29 th : Intro to Cyclones and Fronts Cyclones are areas of low pressure that have winds which rotate counterclockwise around the center (in the northern hemisphere) Extratropical cyclones affect areas roughly north of 30º N These cyclones exist because of the different amounts of heating across the planet, so they act to move warm air poleward and cold air equatorward

Review Continued Extratropical cyclones often have fronts Fronts are areas that separate two airmasses, often warm vs. cold, and/or dry vs. moist Cold fronts are locations where warm air is being replaced by cold air Warm fronts are locations where cold air is being replaced by warm air Cold fronts are often sharper than warm fronts, partially because it is easier for more dense air to replace less dense air

Fronts The intersection of different airmasses are called fronts The most common type are Warm Fronts and Cold Fronts Fronts typically move in the direction the triangles or semicircles are pointing

Fronts Continued Fronts are commonly located near regions of precipitation This is due to the fact that fronts are often areas where warm and moist air is forced to rise There are four types of fronts: - Cold Fronts - Warm Fronts - Occluded Fronts - Stationary Fronts

Cold Fronts This often acts to focus upward vertical motion, leading to a narrow band of precipitation Severe weather is commonly observed in advance of the cold front

Warm Fronts Continued A warm front has a much shallower slope compared with cold fronts Precipitation associated with warm fronts is often lighter and has a greater coverage than with cold fronts

A Comparison

Occluded Fronts Due to the different densities of cold and warm air, cold fronts typically move faster than warm fronts Therefore, sometimes the cold front “catches up” to the warm front, especially with strong, long- lived cyclones When this occurs, an occluded front is formed (often depicted in purple)

Occluded Fronts Continued Recall that precipitation is produced in regions where warm moist air is forced to rise Once an occluded front has formed, there is no source of warm air, so precipitation often weakens in intensity From:

Stationary Fronts As the name implies, stationary fronts are very slow moving (if they move at all) Stationary fronts separate cold air from warm air, and can be associated with precipitation Stationary fronts frequently turn into cold fronts or warm fronts From: mac/1400/FIG09_008.jpg

Cyclone Life-cycle and Front Evolution

Finding Fronts Fronts will often be associated with all or most of: –A locally strong temperature gradient –Relatively lower pressure –Clouds and precipitation –Converging (coming together) winds –Positive vorticity (i.e. a sharp turning of the winds cyclonicly) –Locally strong moisture gradients It is often helpful to first locate the location of lowest pressure, since fronts normally originate from it

L

L

L

L ?

L

Fronts in Other Images Besides looking at surface observations, fronts can often be seen in other ways If precipitation is associated with the front, they can be seen on radar images The clouds associated with fronts can be seen on satellite images also

L

L L

L

How Do Low Pressures Form? Keep in mind that pressure is basically just a measure of the mass of air above the surface (or some other level) So a surface pressure of 990 mb means that the weight of the atmosphere above that location is less than a spot with a pressure of 1000 mb This means that to decrease surface pressure, the mass of the atmosphere above that location needs to decrease

Low Pressure Formation Continued In order to decrease the mass of the column of air above the surface (i.e. to form a low pressure system), you need a net divergence of air in that column Conversely, to strengthen a high pressure system, you need a net convergence of air in that column This is where features in the upper levels of the atmosphere become important But first, we need an understanding of atmospheric forces

Why Are Forces Important? When we speak of “forces,” we’re really describing why the air in the atmosphere moves the way it does This describes both how the air moves horizontally (which direction it moves), and why the air moves vertically For example, precipitation occurs because something forces air upward until it condenses

Horizontal Movement When a studying the horizontal movement of air, three primary forces are often discussed: -The Pressure-gradient force (or PGF) -The Coriolis force -The Frictional force