SHW Ch. 8 - Air Masses
A large volume of air that is characterized by similar temperature and moisture properties at any given altitude. Can cover many thousands of square km horizontally and can extend several km in the vertical (a 3-D chunk of air); can be as deep as the depth of the troposphere or as shallow as 1 to 2 km deep.
Horizontal variations in temperature and moisture usually exists within an air mass, but are small compared to the difference between air masses.
Source Region: a large expanse (several hundred thousand square km) where air can stagnate (light winds) long enough to "acquire" the thermal and moisture properties of the underlying surface.
In summer, when the oceans are cooler than the continents, semi-permanent high pressure areas form; Bermuda High in the Atlantic Ocean; Pacific High in the Pacific Ocean
In winter, semi-permanent low pressures are form over the northern oceans; Icelandic Low in the Atlantic Ocean; Aleutian Low in the Pacific Ocean
See Figure 8.1 for typical North American air mass source regions
Continental Polar (cP) & Continental Arctic (cA)
Cold (sometimes, bitterly cold) and dry; stable; cA air masses are colder than cP
Originate over the ice-and snow-covered regions of northern Canada & Alaska, where long, clear nights allow for strong radiational cooling of the surface
In winter, outbreaks of bitterly cold air into continental U.S. (e.g., "Texas Blue Norther"); will create lake effect snow in late autumn and winter
In summer, can provide relief from heat waves.
Maritime Polar (mP)
Cool and moist; unstable
Form over high latitudes over ocean water; usually a modified cP air mass from northern Asia or AK
Affects west coast of U.S.; most often from Pacific Ocean
Atlantic mP are often colder than Pacific mP, due to smaller size of water expanse.
Maritime Tropical (mT)
Warm and moist; usually unstable; form over sub-tropical bodies of water.
Gulf of Mexico, Caribbean Sea and western Atlantic Ocean all affect eastern and central U.S.
In Winter, mT air is usually confined to the Gulf Coast states.
In Summer, mT air can provide moisture for "monsoon" over SW-U.S.
"Pineapple Connection": sub-tropical jet stream sends ample mT air from Hawaii to Western U.S. and is associated with "El Nino"; heavy rains and flooding in central and southern CA.
Continental Tropical (cT)
Hot, dry stable air aloft; conditionally unstable surface air; usually comes down from the southern Rockies & the northern Mexican Plateau
Marked by large diurnal temperature ranges (i.e., hot days and cool nights)
Can set up a "dry line" over south-central U.S. (especially TX & OK).
SHW Ch. 8 - Fronts
A transition zone between two air masses of different densities (e.g., oil & water), due to contrasts of temperature and humidity
Polar and tropical air masses typically meet in the mid-latitudes
Isotherms and isodrosotherms are usually tightly packed (relative to nearby areas) in the vicinity of a frontal boundary. (Note: a change in elevation or a change in underlying surface [e.g., water to land, snow to bare ground, etc.] may lead to large temp/dew point gradients in the air above)
Fronts tend to be found in troughs of low pressure and winds tend to converge at a front
Clouds and precipitation usually accompany fronts
Frontogenesis: when a frontal boundary forms or becomes stronger due to increased temperature contrast across the front
Frontolysis: when a frontal boundary weakens and dissipates due to a decrease in temperature contrast across the front
Anafront: front with clouds and precip in the cold air, as with nearly all warm fronts; precip behind a cold front
Katafront: front with clouds in the warm air; precip ahead of a cold front
Frontal Naming Criteria
Cold Front: cold air is moving toward the frontal boundary.
Warm Front: cold air is moving away (i.e., retreating) from the frontal boundary.
Stationary Front: cold air is moving parallel to the frontal boundary.
Occluded Front: difficult to determine from a surface map; cold front has overtaken warm front at surface (warm air aloft); cold occlusion and warm occlusion
Dry Line: narrow boundary where there is a steep horizontal change in moisture; a "dew point front"; often a leading edge of a cT air mass; common in TX, OK & KS, especially during the spring and early summer
Upper-Level Front: forms when two air masses aloft meet (a cold front aloft); modest to little change in temp, but very sharp change in dew point (RH); air originates in upper troposphere and lower stratosphere; compare temp and dew point data on the 700 mb and 500 mb charts
Frontal Location Criteria
Sharp temperature contrast over a relatively short distance
Changes in the air's moisture content (as shown by marked changes in the dew point)
Shifts in wind direction
Pressure and pressure changes
Clouds and precipitation patterns
(Fronts continued)
Warm Front
Cooler temps in advance of front; sharply warmer following frontal passage
Dew point temps cooler in advance of front; higher following frontal passage
Wind has N to SE component in advance; SE to SW behind
Pressure falls steadily as front approaches; rises following front, but may fall again
OVC (-RA/-SN) in advance of front; partial clearing behind (sometimes Cb/TSRA)
Cold air damming (trapping) occurs east of a mountain range; can distort front
Cold Front
Temps will begin to fall behind front; front located at leading edge of baroclinic zone
Dew point temps turn sharply colder (less humid)
Wind has SE to SW component in advance; SW to NW behind
Pressure falls in advance; sharply rises following frontal passage
Cb/Cu (TSRA/SHRA) can precede/accompany front; Sc (SHRA) or CLR can follow
Occluded Front
Usually has cool/cold air on both sides of front; cold<warm>cool; cool<warm>cold
Dew point temps usually within a few degrees of temps (i.e., high RH)
Sharp wind shift from E to S component to W to N
Pressure falls steadily in advance, then rises steadily after passage
Usually OVC, with light precip (-RA/-SN/-PL/-FZRA) on both sides of front
Stationary Front
Sharp temperature contrast on both sides of front
Sharp dew point contrast on both sides of front
Wind E to SW on warm side of front; NW to NE on cold side of front
Pressure remains fairly steady
Katafront: clouds + precip on warm side; Anafront: clouds + precip on cold side
Dry Line
Warm to hot temps on both sides of dry line
Much lower (drier) dew points on west side; higher dew points on east side
Wind E to SW in advance of dry line (warm/humid); SW to NW behind (warm/dry)
Pressure falls in advance; rises behind
Many times just CLR, but can initiate severe weather (+TSRA/+FC)
Upper-Level Front
No significant change in surface temps
No significant change in surface dew points
Slight shift in surface wind direction as it passes; from E to SE, to perhaps SE to SW
Pressure falls in advance; rises behind; a surface trough may be noted
Sharp line of TSRA/SHRA exists, with CLR/SCT to the west of the line