Chapter 7 Flashcards
An air mass is a large body of air with generally uniform temp and humidity. How big are air masses? How do they form?
-Air masses are typically larger than 600 Km (1000 mi)
- The formation of air masses is usually associated with what are called source regions (regions of the Earths surface that are particularly well suited to generate air masses). An air mass develops its characteristics when it stagnates for a few days or remains over a uniform land or sea surface (source regions) long enough to acquire the temp, humidity, and stability characteristics of the surface below.
Know characteristics ad source regions of major North American
CP, cT, mP, mT air masses
Air masses classified on the basis of source region…
The latitude of the source region correlates directly with the temp of the air mass, and the nature of the surface strongly influences the humidity within the air mass.
Code cP,
type,
source region,
Source region properties
CP
Continental polar
High latitude plains of Eurasia and North America
Cold, dry, very stable
Code cT,
Type
Source region
Source region Properties
cT
Continental tropical
Low latitude deserts
Hot, very dry, unstable
Code mP
Type
Source region
Source region properties
MP
Maritime polar
Oceans in vicinity of 50-60 N and S latitude
Cold, moist, relatively, unstable
Code mT
Type
Source region
Properties
MT
Maritime tropical
Tropical and subtropical
Warm, moist, of variable stability
Be able to describe the differences (e.g., wind direction and speed, temp, humidity, potential for precipitation) associated with areas that experience the passing of these types of fronts, cold front, warm front, occluded front
Stationary front
Cold front
Warm front
Occluded front
Stationary front:
When neither air mass displaces the other. It is difficult to generalize about the weather along such a front, but often gently rising warm air produces limited precipitation similar to that of a warm front
Cold front:
The combination of steeper slope and faster advance leads to rapid lifting and adiabatic cooling of the warm air ahead of the cold. The rapid often makes the warm air ver unstable, and the result is blustery, violent weather along the cold front. Vertically developed clouds, such as cumulonimbus, are common along with turbulence and showery precipitation. Precipitation is usually more intense but shorter lived than that associated with a warm front.
Warm front:
The slope of a warm front is gentler than that of a cold front. As the warm air pushes against and rises over the retreating cold air, it cools adiabatically, usually resulting in clouds and precipitation. Because the frontal uplift is gradual, clouds form slowly, and turbulence is limited. Precipitation frequently occurs broadly if the rising air inherently unstable, however, precipitation can be showery and violent. —more moderate than cold front
Occluded front:
The occlusion process may result in a short period of intense precipitation and wind until eventually all of the warm air mass is forced aloft, and the ground level pressure enter is surrounded by cool air — a stable condition.
What is mid latitude cyclone?
- Most significant atmospheric disturbance
- Associated primarily with air mass convergence ( CCW in northern hemisphere)
- Wind flow pattern brings together cool air from the north and warm air from the south
- convergence of these two unalike air masses created two fronts 1) Cold front extending to SW from the enter of the cyclone 2) warm extending eastward from the center.
- essentially a vast cell of low-pressure air, with ground level pressure in the center typically between 990 and 1000 milibars
Where is the mid-latitude wave cyclone?
- Throughout the mid latitudes (30-70) degrees
- almost entirely within the band of westerly winds
What is the tropical cyclone?
- Intense, low-pressure disturbances that develop in the occasionally move poleward into mid latitudes
- Associated with trade winds
- Considerably smaller than midlatitude cyclones
- Approximately 100 atmospheric disturbances each year yet only a few strengthen into hurricanes
Where is the tropical cyclone
-tropics
What is the localized severe weather? Where is it?
-Smaller atmospheric disturbance that affect a more restricted area
-Localized
Cyclones are low pressure systems. What happens in cyclogenesis? How do fronts where air masses meet?
-Cyclogenesis is referred to as the “birth” of cyclones
-this is occurs in upper troposphere conditions in the vicinity of the polar front stream.
-Most midlatitude cyclones begin as waves along polar front
-A cyclone is unlikely to develop at ground level unless there is a divergence above it—> convergence of air near the ground must be supported by divergence aloft
-Various ground factors can initiate a wave along the front (such as topographic. Irregularities, temp contrasts between sea and land, or the influence of ocean currents)
In the warm sector. Of a mid-latitude cyclone, winds typically come from the south. Why?
-because of the counter clock wise motion of the winds around he low center.
How does the weather change with the passing of a warm front vs. the passing of a cold front?
We the passing of the cold front, we see a drop in the temp with the warm front it rise
What are the before/after differences in temp, pressure, wind, sky, & precipitation?
-as the cold front passes, temp drops abruptly because the cold front is the boundary between the cold and warm air masses of the storm.
-The trough of low pressure associated with the cold front extends south from the heart of the storm. As the trough along the front approaches, pressure decreases, reaching its lowest point at the front. Then as the cold front passes and the trough moves away, prressure begins to increase steadily.
-due to the overall converging counter clockwise wind pattern, winds is the warm sector come from the south. Once the front passes, winds shift from west to northwest
-The generally clear skies ahead of the cold front are replaced by cloudiness and precipitation at the front. They are generated by the adiabatic cooling of the warm air as it is lifted along the front— and are in turn replaced hours later by clear skies in he cold air mass behind the cold front.
What is an occlusion? Why does it signal the “death” of a mid-latitude wave cyclone?
-an occlusion is the process of he storm dissapating because the cold front overtakes the warm front.
-occlusion creates a stable condition of weather (warm air forced to the top, cool air all at the bottom). This stability weakens the pressure gradient and therefore the storm’s energy. When the storm has no more energy, it dies.
At any given time on average, how many mid-latitude cyclones are swirling in the midlatitudes?
- 5 - 10 midlatitude cyclones
Mid-latitude anticyclones
What are the characteristics of anticyclones high-pressure systems? What weather does it bring?
- Air converges into it from above, subsiding and diverging at the surface (clockwise)
- Contains no fronts
- weather is clear and dry with little or no opportunity or cloud formation
- wind movement is limited near the center of the anticyclone but increases progressively outward (may create very strong winds)
- Move toward the east either at the same rate or. Little slower than midlatitudes cyclones
Stagnation is possible for up to days at a time, this may enhance the likelihood of air pollutants will become concentrated
Anticyclones (highs) tend to follow in the wake of cyclones (low). Which typically moves faster?
Cyclones
Tropical are called hurricanes in the USA ( what are hurricanes called in other parts of the world?)
Typhoons’, ‘baguios’, or simply cyclones
Categories of tropical disturbances: Tropical depression—>tropical storm—>hurricane
-Tropical depression: Wind speeds up to 48mph but has developed a closed wind circulation pattern
-tropical storm: winds between 39 to 73 mph
-hurricane: winds reach or exceed 74 mph
In what parts of the world are hurricanes, common, and when? What are named storms?
-hurricanes from only over warm oceans in the tropical storm and it is assigned a name from an alphabetical list
How do hurricanes originate?
They form over warm oceans in the tropics. The exact mechanism of formation is not understood but hurricanes always develop out of a pre-existing disturbance in the tropical troposphere. Easterly waves provide low level convergence and lifting that catalyses the development of many hurricanes. Hurricanes only can evolve when there is no significant wind shear—a significant change in wind direction or wind speed with increasing elevation.
What are the characteristics and movement patterns of hurricanes?
They occur in low latitude regions such as the north pacific basin (east of the Philippines and west of southern Mexico and Central America) and the west central portion of the north atlantic basin (Gulf of Mexico).Once formed, hurricanes follow irregular tracks within the general flow of the trade winds.
What are the hurricanes tracks? What is the life span of a hurricane, and how does it die?
-hurricane tracks are specific paths a hurricane follows, very hard to pre determine
-the life span is 0-4 weeks
-a hurricane dies as it penetrates cooler environments
Damage and destruction caused by hurricanes:
high winds, torrential rains, tornadoes, flooding caused by storm surges and heavy rain
What is a Storm Surge? What causes the most fatalities in a hurricane?
-a storm surge is defined a a bulging of the ocean (up to 3 feet) due to VERY low pressure. Wind may drive the waves up even higher (as much as 25 feet)
-the most fatalities in a hurricane are brought on by flooding from high seas
How do we classify hurricane strength? What happens when a hurricane makes landfall?
-hurricane strength is classified on the Saffir-Simpson Hurrican scale. It is used to rank the relative intensity of hurricanes (primarily by wind speed) ranging from 1-5 (5 being the most severe)
-if a hurricane makes landfall, it begins to die because its energy source (warm moist air) is cut off. The hurricane is still able to create horrible damage and loss of life.
How might Hurricanes change if some predicted versions of Climate Change occur?
-Possibly a greater number of hurricanes; possibly fewer hurricanes but of greater intensity
Localized Severe Weather
-In the U.S., greatest severe weather threat from thunderstorms extends from TX to southern MN.
-Can develop either along cold fronts or in subtropical convective conditions near water.
-The Gulf Coast and central Florida actually experience the most thunderstorms.
What are the hazards to humans that are associated with thunderstorms?
lightning, tornadoes, high winds, flash floods
Why do so many tornadoes form in the U.S.? Where else do they occur in the world?
-the US has optimal environmental conditions (relatively flat terrain of the central and south-eastern United States providing an unhindered zone of interaction between two large air masses.
-the also occur in the mid latitudes and subtropics
May occur in swarms, but an individual tornado is rarely more than a few 100 yards across.
How do we measure the strength of tornadoes? When is this measurement usually done?
-the strength is measured by the Enhanced Fujita Scale (EF Scale). This scale is based off three second gust wind speeds as determined by observed damage after a tornado
