Note the listing of relevant figures from the text that you can look at as you read these summary notes.
Air Masses
Regional surface conditions in a "source area" imprint temperature, humidity, and therefore stability characteristics upon the air resting over it.
Air masses with different temperature and humidity characteristics interact to produce weather patterns.
Air Mass Classification
1) Moisture
m (maritime - wet)
c (continental - dry)
2) Temperature
A (arctic)
P (polar)
T (tropical)
E (equatorial)
AA (antarctic)
Air Masses Affecting North America (Fig 8.2)
cP - continental polar
cold, dry, stable, high pressure air from over Canada, especially strong in winter
mp - maritime polar
cool, moist, unstable, typically low pressure, from the north Pacific and north Atlantic
mT - maritime tropical
warm, moist, unstable air from the Gulf of Mexico/Atlantic
not as warm or moist air from off the Pacific
Air Masses are modified as they move from their source area and to areas with different temperature/moisture characteristics. For example, cold, dry cP air spilling out of Canada warms somewhat and picks up moisture as it moves over the Great Lakes, resulting in lake effect snow on the eastern and southern sides of the Great Lakes (Fig 8.5).
Atmospheric Lifting Mechanisms (Fig 8.6)
1) convergence: When air converges it must rise. The intertropical convergence zone (ITCZ) is the largest example of this.
2) convection: Local differences in temperature causes some spots to be warmer, and the air less dense than surrounding air, so it rises
The Florida peninsula has both convergence and convection active, producing large amounts of rain from the maritime tropical air of the Gulf of Mexico and Atlantic Ocean (Fig 8.8).
As convection causes air to rise and cool at the dry adiabatic rate (DAR) it may cool to its dewpoint. The elevation at which this occurs is called the lifting condensation level. The flat bottoms of clouds are found at this elevation. With continued upward convection the rising air cools with condensation at the moist adiabatic rate (MAR). (Fig 8.7)
3) orographic lifting: Winds must rise to cross over mountains. The rise results in expansion and cooling which may lead to condensation and precipitation. Orographic precipitation produces a blanket of green forest on the windward side of mountain ranges. But as the air descends back to low elevation on the lee side of the range, compression and warming leaves the air hot and dry (low relative humidity).
In the Sierra Nevada Mountains of California and the Cascade Range running from northern California through Oregon and Washington the west (Pacific-facing) slopes are green with forests. The eastern slopes are dry and barren, and the hot, dry chinook winds blow out onto the desert valleys of the Great Basin in the rain shaddow of the mountains. (Figs 8.9, 8.10)
4) frontal lifting: (Figs 8.11, 8.12, 8.13)
- cold front (cP or mP)
Colder, denser air displaces warmer, less dense air. The warmer air rises over the colder, denser air.
Cold fronts advance at speeds of around 25 mph.
High cirrus clouds usually lead the front by one or two days, indicating that atmospheric lifting is approaching.
The boundary between cold and warm air is steep. The warmer air is lifted abruptly by the advancing front.
Low pressure usually comes just ahead of the front.
A narrow belt of clouds (stratus) precedes the front.
A squall line of cumulonimbus clouds (thunderstorms, with heavy rain , strong winds, and possibly tornadoes) passes through with the front.
These are produced by the rapid lifting associated with the front's passage.
High pressure and cooler (colder), drier air follows the passage of the front.
- warm front (mT)
Warmer, less dense air gradually overrides colder, denser air in an ascending wedge shape.
Warm fronts advance at speeds around 10-15 mph.
There is a gradual cloud progression from cirrus and cirrostratus to altostratus, stratus, and nimbostratus (rain clouds).
This belt of clouds is about twice as wide as that associated with a cold front.
Midlatitude Cyclones (Wave Cyclones)
Cyclonic storms, with low pressure, convergence and rising of air and counterclockwise circulation (northern hemisphere), frequently develop along the polar front.
Life Cycle (Fig 8.14) Midlatitude cyclones develop through the following sequence over 3 to 10 days.
1) Cyclogenesis: Convergence develops along a northward undulation of the polar front, causing lifting of air which strengthens into a low pressure center.
2) Open Stage: Counter clockwise flow about the low drives a warm front eastwared and northward and a cold front follows. Precipitation occurs along the fronts and around the low pressure center. Maritime tropical (mT) air (expecially from the Gulf of Mexico) is pulled into the region between the cold and warm fronts by the cyclonic circulation. This warm moist air helps to power the storm, because condensation as the air lifted releases heat which helps to drive further lifting.
3) Occluded (closed) Stage: The cold front catches up with the warm front, closing up the large embayment in the polar front and the connection between the low pressure center and the source of warm, moist mT air.
4) Dissolving Stage: The remaining warm is now lies above the colder air. There is no connection to the source warm air and no further lifting. The clouds dissipate.
Storm Tracks: Cyclonic storms track generally west to east across the U.S. and then up the eastern seaboard. Cyclongenesis is principally in the Pacific, Gulf of Mexico, and eastern seaboard. (Fig 8.16)
Violent Weather
Thunderstorms (cumulonimbus clouds) are produced by strong lifting of warm moist air by convection on hot summer days, by cold fronts, or by orographic lifting. Vigorous uplift above the lifting condensation level produces large amounts of condensation, liberating large amounts of heat (latent heat of condensation) which helps to drive further lifting. Strong updrafts develop. Cloud droplets remain suspended in the strong updrafts and grow to large size by continued growth and collisions with other droplets. Eventually, heavy falling rain drags air down producing strong downdrafts which spread out near surface level to produce strong surface winds. Very strong downdrafts (macrobursts and microbursts) are dangerous for aircraft taking off or landing.
Lightning The vigorous movement within the cumulonimbus cloud results in the buildup of unbalanced electrical charges. Some areas of the cloud will have a buildup of negative charges and other areas positive. The charge of the ground may also become much different than the area of the cloud overhead. The charge imbalances are equalized by a sudden arc of electricity between two oppositely charged areas. Tens to hundreds of millions of volts of electricity instantly discharge, heating the air around the lightning bolt to 15 to 30 thousand °C. The sudden expansion of that heated air produces a shock wave (sound wave) - thunder. (Fig 8.21)
Hail Raindrops in a cumulonimbus cloud can be carried by the updrafts to heights where the temperature is below freezing. The raindrop will freeze but then fall back downward and collect a coating of water only to be blown back up to freeze the new layer. This can be repeated many times until the hailstones, thus formed, are heavy enough to fall to the ground. (Fig 8.22)
Tornadoes Some thunderstorms, especially those produced by strong cold fronts, spawn tornadoes. The mechanisms that produce tornadoes are not completely understood but, at least in some cases appears to work as follows.
1) Wind is slower at the surface than at higher elevation (1000 m). This may set the air spinning along a horiontal axis perpendicular to the wind direction. (Fig 8.24a)
2) The horizontal roll of spinning air gets dragged up into a strong updraft, pulling the roll into a vertical spinning orientation. It is still spinning relatively slowly. This is called a mesocyclone. It is an upward spiraling updraft. (Fig 8.24b)
3) The mesocyclone then stretches downward and narrows. The narrowing increases the rotating wind speed (like how a figure skater pulling in his/her arms spins faster). (Fig 8.24c)
Tropical Cyclones: Hurricanes, Typhoons, Cyclones
about 80 per year global average
about 36 to 45 per year strengthen to become a hurricane/typhoon/cyclone
The name hurricane is used for northern tropical Atlantic and northern tropical Pacific storms that affect Central and North America and the Carribean.
The name typhoon is used for northern tropical Pacific storms that affect eastern Asia (Taiwan, Japan, etc.)
The name cyclone is used for southern tropical Pacific and tropical Indian Ocean storms that affect Australia southern Asia and southeastern Africa.
Tropical cyclones form over warm tropical oceans, but not near the equator (Fig 8.28). Here the air is homogeneous, without frontal battles between air masses of different temperatures.
Requirements for the formation of a tropical cyclone:
- sea surface temperature at least 79-80 °F (26-27 °C) extending downward at least 60 meters below the surface
to provide plenty of warm moist air
- divergence aloft - to allow chimney-like action to intensify atmospheric lift
- no wind shear at mid altitudes - shear would prevent vertical uplift
- not on equator - no coriolis effect on/near equator and therefore no circulation
Tropical cyclones mostly form when the ITCZ is farthest north or south of the equator during at the height of the northern and southern hemisphere summers. The Atlantic hurricane season officially runs from June 1 to Novembrer 30, but most hurricanes strike the southeastern U.S. from August through October when the northern tropical Atlantic and Carribean are at their warmest.
Tropical storms need a boost in the form of large scale convergence, lift, or circulation to get established. This may form at the ITCZ, but only when it is farthest from the equator (b/c no coriolis effect & circulation near equator). In the Atlantic, only 1 in 6 hurricanes forms at the ITCZ.
Easterly waves (Fig 8.27) more commonly set the stage for hurricanes. Easterly waves are long troughs of low pressure aligned approximately north-south from 5° to 30° latitude that drift westward with the prevailing winds across the tropical oceans. A center of low pressure may develop along an easterly wave and grow through the following stages (Table 8.2):
- tropical disturbance - cloudy with light, variable winds
- tropical depression - light to moderate rain with stronger, gale-force winds up to 39 mph
- tropical storm - heavy rain with strong winds up to 73 mph; closed isobars - assigned a name
- hurricane - very heavy rain, very strong winds sustained over 74 mph; closed isobars, circular storm, storm surge, tornadoes
Saffir-Simpson Scale of Hurricane Intensity (Table 8.2)
Category |
Central Pressure (mb) |
Wind Speed (mph) |
Storm Surge (ft) |
|---|---|---|---|
Category 1 |
>979 |
74-95 |
4-5 |
Category 2 |
965-979 |
96-110 |
6-8 |
Category 3 |
945-964 |
111-130 |
9-12 |
Category 4 |
920-944 |
131-155 |
13-18 |
Category 5 |
<920 |
>155 |
>18 |
Hurricane Structure (Fig 8.29)
| hurricane structure figure |
|---|
Tropical cyclones typically range from about 150 to 300 miles across, though they may range from 100 to nearly 1000 miles across.
Winds spiral inward and upward. The rising warm, moist air expands, cools, and condenses. The condensation liberates heat which powers further lifting. Hurricanes form and grow over warm ocean water. They dissipate when they move over land or over cool ocean water. Cumulonimbus clouds rise in spiral bands, where the heaviest rainfall occurs, leading toward the eye of the hurricane. In between the bands of cumulonimbus are thick stratiform clouds. Winds are slowest in the outer reaches of the storm and the highest wind speeds occur just around the eye in the eyewall (the last band of cumulonimbus). Within the eye, air is descending (compressing, warming, drying) so the air is clear. Surrounding the hurricane the air is also descending, so hurricanes are typically surrounded by clear air.
Hurricane Impacts
Wind: Obviously the strong winds can cause significant damage to structures not designed to withstand them. Roofs are blown off. Trees are blown down. Loose coconuts, branches, etc. become projectiles. Wind is strongest on the right side of a hurricane as it travels because the wind speed there are a combination of the inherent velocity produced by the pressure drop plus the speed that the storm is moving forward. Winds are weaker on the left side of the moving hurricane because there the winds are blowing in the opposite direction from the motion of the storm. Tornadoes may also form near the eyewall particularly in the front-right quadrant of a hurricane.
Rain: Heavy rains cause major damage via flooding inland. The heavy rains can also cause disastrous mudslides.
Storm Surge: The low pressure of the hurricane means less atmospheric weight lying on the ocean. The ocean can rise nearly one meter (3 ft) beneath a very strong hurricane generating very low central pressure. More importantly, the strong wind on the right side of the hurricae blowing toward the shore piles up water faster than it can flow back out on the seafloor. Category 1 hurricanes produce a storm surge of 4 to 5 ft while category 5 hurricanes can produce storm surges as high as 25 ft. The storm surge floods all low lying coastal areas. High wind-driven waves ride on top of the surge and devestate coastal structures.