OVERVIEW:

We will discuss the types and characteristics of surface pressure systems and airmasses.

OUTLINE:

1. Terminology.

1.1. Cyclone -- Low pressure center - Circulation is a balance between Pressure Gradient Force (PGF) and Horizontal Deflection Force (HDF) - Counter-clockwise rotation in northern hemisphere. (Friction turns wind into the low near the Earth's surface.)

1.2. Anticyclone -- High pressure center - Circulation is a balance between Pressure Gradient Force (PGF) and Horizontal Deflection Force (HDF) - Clockwise rotation in northern hemisphere. (Friction turns wind out of the high near the Earth's surface.)

1.3. Trough (trof) -- Area of open cyclonic circulation and low pressure. (Isobars do not close off.)

1.4. Ridge -- Area of open anticyclonic circulation and high pressure. (Isobars do not close off.)

1.5. Isobars -- lines of equal pressure.

1.6. Isotherms -- lines of equal temperature.

1.7. Warm core -- Warmest air is in the center of the pressure system.

1.8. Cold core -- Coldest air is in the center of the pressure system.

1.9. Barotropic -- Isobars and isotherms do not cross. System circulation alone does not change the temperature at a point on the Earth's surface.

1.10. Baroclinic -- Isobars and isotherms do cross. System circulation alone does change the temperature at a point on the Earth's surface.

2.1. Warm core -- Isobars and isotherms do not cross, therefore wind does not result in movement of warmer air into colder regions (barotropic) (illustrations here and here):

2.1.1. High - The Subtropical Ridges (illustration here). Deep tropospheric features caused by three-cell circulation. Centered over ocean at 30 degrees north and south, although positions shift poleward in the summer hemisphere and equatorward in the winter hemisphere. Semipermanent; strongest in summer, weakest in winter. Strong surface expression.

2.1.2. High - "Cut-Off" High (illustration here). Deep tropospheric features associated with "omega-shaped" ridges in the polar jet stream and deep, warm columns of air displaced far to the north of their point of origin. May become quasi-stationary, remaining over a fixed location for weeks. Often blocks the normal eastward progression of migratory weather systems, resulting in stagnant airmasses and heat waves. Weak surface expression.

2.1.3. Low - The Equatorial Trough (a.k.a. The Intertropical Convergence Zone, or ITCZ) (illustration here). Deep tropospheric feature caused by a combination of surface-based heating and three-cell circulation. Migrates seasonally between 15 degrees north and 5 degrees south. Permanent feature; strong all year round. Strong, complex surface expression.

2.1.4. Low - Thermal Lows (e.g. The "Taco" Low) (illustration here). Vertically-shallow feature caused by surface-based heating and the resulting reduction in air density in the lower troposphere. Often capped by high pressure cell in the upper troposphere. Occurs only over land; strongest at tropical latitudes. Often completely disappears in the winter. Strong surface expression.

2.2. Cold core -- Isobars and isotherms do not cross, therefore wind does not result in movement of colder air into warmer regions (barotropic) (illustrations here and here):

2.2.1. High - The Polar Highs (illustration here). Relatively deep tropospheric feature caused by a combination of three-cell circulation and intense cooling. Positions shift somewhat, but the systems are essentially centered over both poles. The southern polar high is much stronger, because the air over Antarctica is much colder than the air over the Arctic Ocean. Semipermanent; strongest in winter, weakest in summer. Strong surface expression.

2.2.2. High - The Continental Highs (illustration here). Relatively shallow tropospheric feature caused by cooling. Centered over continental areas above 40 degrees north and south. The Siberian High is the strongest and coldest. These systems generally begin as quasi-stationary features, but are often picked up and moved by upper-tropospheric waves. Ephemeral; stronger in winter. Strong surface expression.

2.2.3. Low - The Subpolar Lows (e.g. The Icelandic Low) (illustration here). Deep tropospheric features caused by three-cell circulation. Centered over ocean at 60 degress north and south, although positions shift poleward in the summer hemisphere and equatorward in the winter hemisphere. Semipermanent; strongest in winter, weakest in summer. NOTE: At 60 degrees south, there are no landmasses to break up this feature, so it is essentially a cold-core trough that runs all the way around the world. Strong surface expression.

2.2.4. Low - "Cut-Off" or Distal Lows (illustration here). Deep tropospheric features associated with narrow troughs in the polar jet stream and deep, cold columns of air displaced far to the south of their point of origin. May become quasi-stationary, remaining over a fixed location for weeks. Weak surface expression.

2.2.5. Low - Deeply Occluded Lows. Deep tropospheric features characteristic of mature migratory cyclones and deep, cold columns of air. Often slow moving; may persist for days. Weak surface expression.

2.3. Baroclinic -- Isobars and isotherms cross, therefore wind transports warmer (colder) air into colder (warmer) regions (illustrations here and here):

2.3.1. Low - The Dynamic Low (illustrations here and here). Complex, vertically-tilted, migratory feature. Pressure center moves from warmest air to coldest air over the lifetime of the system (several days to two weeks). Often begins with the link-up of a shallow, warm surface low, an upper-tropospheric cold trough, and a speed-maximum in the polar jet stream. Creates waves in the semi-continuous polar front -- advancing portion of the front called the "cold" front; retreating portion of the front called the "warm" front. Responsible for most of the well-organized summer thunderstorm activity and the all of the large winter storms. Usually ends its life as a deeply-occluded, cold-core low. Strong surface expression.

2.3.2. High - The Dynamic High (illustration here). Complex, vertically-tilted, migratory feature. Pressure center moves from coldest air to warmest air over the lifetime of the system (several days to two weeks). Often begins with the link-up of a shallow, cold continental high and an upper-tropospheric warm ridge. Responsible for blue skies and moderate temperatures in the summer, and cold waves in the winter. Usually ends its life as a part of the subtropical ridge. Strong surface expression.

3.1. Definition: Synoptic-scale body of air with relatively homogeneous temperature and moisture characteristics. Airmasses extend from the Earth's surface to a maximum height of about 3000 meters (10,000 feet). They may cover an area of more than a million square miles.

3.2. Source region: Large, relatively uniform area of the Earth's surface where a body of air can remain for an extended period of time. The airmass takes on the characteristics of the source region. E.g. A cool, moist ocean area (such as the North Atlantic) will create a cool, moist body of air.

3.3. Designators:

3.3.1. Latitude: Equatorial, tropical, polar, and Arctic.

3.3.2. Surfacial: Continental (dry) and Maritime (moist).

3.4. Airmass modification: The temperature and humidity characteristics of an airmass begin to change if it leaves its source region. E.g. A cold, dry airmass (originating on a poleward continent) will slowly warm up and gain moisture if it moves out over an oceanic region.

3.5. Specific airmasses:


North American airmasses.
(Graphics credits)

3.5.1. Equatorial (E)(not shown in figure) - Hot and moist. Associated with the region between the northern and southern subtropical ridges. Narrow seasonal and diurnal temperature range; year round.

3.5.2. Continental Tropical (cT) - Hot and very dry. Associated with desert regions and thermal lows. Large diurnal and seasonal temperature variation; usually disappears in winter.

3.5.3. Maritime Tropical (mT) - Hot and moist. Associated with oceanic or ocean-dominated regions and the Subtropical Ridge. Narrow diurnal temperature variation; moderate seasonal variation.

3.5.4. Continental Polar (cP) - Cool and dry. Associated with large continental regions (central Asia, North and South America, Europe, Australia), and Continental Highs. Moderate to large diurnal temperature variation; wide seasonal variation.

3.5.5. Maritime Polar (mP) - Cool and moist. Associated with poleward oceanic regions (North Atlantic, North Pacific, Southern Ocean), and the Subpolar Lows. Narrow diurnal temperature variation; narrow to moderate seasonal variation.

3.5.6. Arctic (A)(shown in figure as "cA") - Very cold and very dry. Associated with Arctic and Antarctic regions, and the Polar Highs. Quite similar, in spite of the fact that the Arctic is an ocean (permamently frozen over), and the Antarctic is a continent. The areal extent is seasonally variable. Considerable seasonal and diurnal temperature variation.

LAB:

Analyze surface chart for pressure (isobars) and identify locations and types of highs, lows, and airmasses.

HOMEWORK:

1. Read Lutgens and Tarbuck chapters 8 and 9.

2. Analyze another surface chart for the same parameters as you did in the lab. Download the pre-plotted chart from the SUNY Albany Weather Page.