Weather and Climate Test 5
Source Region
* The geographic origin of the air mass. * Lower Case * (p) Pacific * (a) Atlantic * (g) Gulf of Mexico * (c) Canadian * (s) Sonoran
Continental Tropical Sonoran Air Mass (cTs)
* Very hot and dry (RH often <10%). *Clear skies- air must rise more than 10,000 feet for condensation *These are dry and hot air masses that form over subtropical desert and plateau regions of Mexico and the southwestern U.S. *These air masses often bring warm, dry weather well into west Texas.
Cold Front
*A cold air mass overtakes and replaces a warm air mass. *In the United States, the most common air masses from Canada or Alaska, and mTa or mTg air masses from the Atlantic or Gulf of Mexico.
Warm Front
*A warm front occurs when a warm air mass replaces a cold air mass. *In the U.S, this generally means that an mTg air mass replaces a cPc air mass.
Characteristics of Air Masses
*Air masses are literally giant masses of air that average 500 to 1500 miles or move in diameter #. *Fairly shallow layers of air that average only a few thousand feet in depth.
Warm fronts vs Cold fronts
*Cold fronts generally move faster than warm fronts and have an average speed of about 25 to 30 mph. *The frontal boundary of a cold front is steep relative to that of a warm front. *Generally speaking, the boundary rises one foot for every 40 to 80 feet of horizontal distance (slope=1/40 to 1/80). *Warm, moist low-pressure conditions exist before a COLD front arrives, and cold, dry, high pressure conditions exist after it passes. *Cooler drier, high pressure conditions are slowly replaced by warmer, moister, low pressure conditions after a WARM front passes. *Warm front weather typically lasts much longer than cold front weather, and precipitation tends to be of lower intensity than that associated with cold fronts.
New England Nor'easters
*Deep low pressure systems in New England can sometimes draw heavily on mPa air masses and their moisture. When this occurs, local residents experience a very different kind of winter storm. *In addition to heavy snows, Nor'easters can wreak havoc on the coast. *Like a hurricane, they generate strong winds, big waves, and a storm surge that can destroy homes and buildings just as deadly as a hurricane.
Anticyclones
*High pressure center *Develop when cold, dry air sinks (subsides) toward the surface, it is forced to flow outward (divergent flow). *Because of the coriolis force, the outward-flowing air is deflected to the right of its diretion of motion (in a clockwise fashion) in the NH. *In the NH, an anticyclone is characterized by air that flows downward, outward, and clockwise.
Hurricanes
*Hurricanes are rotating low-pressure systems, so they are also cyclonic storms, but on a much larger scale than tornadoes.
Temperature
*Indicated by latitudinal zone of formation. *Upper Case *Polar = Cold (P) , Arctic = extremely cold (A) , Tropical = Warm (T) , Equatorial = extremely warm (E).
Moisture
*Indicated by surface nature of source region. *Lower Case *(c) continental = dry; (m) marine = wet
Cyclones
*Low pressure system *Characterized by convergent rising air *Because of the coriolis force, the convergent air is deflected to the right of its direction of motion (in a clockwise action) in the NH. *In the NH, cyclones are cahracterized by convergent, countercockwise surface flow, and vertical uplift.
Stationary Front
*Stationary fronts develop when a front shows no appreciable movement over a period of hours. *Stationary fronts may be linked to cold or warm fronts depending upon the relative movement of the frontal boundary.
Maritime Tropical air masses from the Gulf of Mexico (mTg) and Atlantic (mTa)
*These air masses dominate the warm-season weather in the entire southeastern U.S. *The warm waters of the Gulf and tropical Atlantic may exceed 85 F (29 C), and pump tremendous amounts of moisture into the air. *The collision between cPc and mTg air masses frequently triggers severe storms, even during the winter time.
Maritime Polar Atlantic (mPa)
*These air masses normally have little impact on North American weather because the prevailing westerlies normally cary these air masses to the east away from the U.S. *They generate most of the winter storms that move into Europe from the North Atlantic. *They are responsible for the frequent grey skies in Europe, especially in winter. In a sense, this air mass source affects Europe the same way that the mPp source region in the Gulf of Alaska affects the U.S.
Continental Polar Canadian Air Masses (cPc)
*These air masses tend to be cold to severely cold and dry during the winter and spring months, and cool and dry during the summer and fall. *In the middle of winter, cPc air masses can drop well below minus 30 F, with wind chill equivalent approaching minus 100 F. *Some cPc air masses travel straight from the Canadian Plains south into Texas and out into the Gulf. *The relatively flat nature of the Great plains offer little resistence to their movement, and the Rocky Mountains and Appalachian Mountains help to funnel these air masses toward the south.
Maritime Polar Pacific Air Masses (mPp)
*These air masses tend to be cool and moist. *Typically bring overcast, drizzly, rainy or snowy conditions. *Prolonged periods of rainy weather over the Pacific Northwest during the winter are caused by mP air masses.
Occluded Front
*These occur when 3 air masses meet - cold, warm, and cool *The process of occlusion usually occurs in the later stages of a mid-lattitude cyclonic storm. *Occlusion causes the warm air mass in between the cooler air masses to be lifted upward off the ground.
Tornadoes
*Tornadoes are intense, rotating low pressure systems, they can be considered an extreme type of cyclonic storm. *Most tornadoes occur in association with cold fronts characterized by intense thunderstorm activity.
Hurricane Classification
*Tropical Depression (23-39 mph) *Tropical storm (40-74 mph) *Hurricane (above 74 mph)
Maritime Tropical Pacific (mTp)
*Typically bring heavy downpours of rain to the southwestern U.S.
Thunderstorm hazards
*strong winds *lightening *hail *floods *tornadoes
Stages of thuderstorm development
1. Cumulus Stage (Updraft) 2. The Mature Stage (Warm Uplift) 3. The Dissipating Stage
Atributes of a Source Region
1. Large size,usually flat, homogenous, temperature and moisture characteristics. (ex: clod and dry or warm and wet.) 2. Light upper level winds- allows air to reamin in a source region long enough to take on the characteristics of the source region. 3. Air masses are the best developed in areas that favor the development of a HIGH PRESSURE zone and associated AIR SUBSIDENCE. Specifically, upper level convergence of cold, dry air favors air mass development.
What changes in weather conditions can be expected when a front passes and an air mass is replaced by a different air mass?
1. Sharp temperature changes 2. Changes in the air's moisture content 3. Shifts in wind direction 4. Changes in air pressure 5. Clouds and precipitaion patterns
How do Stationary fronts occur?
1. The momentum associated with the cold air equals that of the opposing warm air and do the frontal boundary ceases to move. Example: This can occur when a cPc air mass moves far enough southward that it begins to lose its identity and/or when a mTg air moves far enough northward that it loses its identity. By the time the two air masses meet, they are so similar that neither can dominate the other. 2. The air masses are prevented from advancing by a mountain range that lies between them. This frequently occurs, when for example, a cPc air mass moves south out of Canada and to the west of the Rockies and a mTg air moves northward from the Gulf of Mexico to the east of the Rockies. The Rockies then act as a wall that separates the cPc and mTg air masses. Because neither air mass can displace the other in this situation, it is considered to be a stationary front.
Thunderstorm Severity
1. Typical thunderstorms 2. Severe thunderstorms 3. Supercell thunderstorms 4. Mesoscale Convective Complex (MCC)
What does the rate and amount of air modification depend on?
1. speed of motion = more rapid change 2. distance of travel = more change 3. dissimilarity of the areas traveled over
Types of thunderstorms
1.Air mass thunderstorms (divergence) 2.Orographic Thunderstorms 3.Frontal Thunderstorms
What is a front?
A front is the boundary/transition zone between unlike air masses. The upper extension of the front is called the frontal surface (frontal zone).
Air Mass
A large mass of air taht is characterized by relatively uniform temperature and moisture characteristics over a broad geographic region near the surface of the earth.
What is a squall line?
A narrow line of intense thunderstorm activity that may develop along a frontal boundary.
Storm Surge
A storm surge is an elevated dome of water that develops under a hurricane as a result of both the low pressure (which elevates the sea surface), the flow of water toward the eye as wind pushes it along, and astronomical high tides.
Uniform Characteristics
Air mass' uniform characteristics are derived from the area in which they form; an area known as an air mass source region.
Air Mass Classification
Air masses are classified according to their source region characteristics. The characteristics include: Temperature, Moisture, and Stability.
What are the developemental cycles for frontal cyclones?
Early stage, open stage, occluded stage, and dissolving stage.
How are fronts named?
Fronts are named according to which air mass replaces the other. Ex: A "cold front" a cold air mass replaces a warmer air mass.
What happens when air masses move out of their source region?
Long after they move out of their source regions air masses continue to retain their temperature, moisture, and stability characteristics they inherited from their source region.
Modification of Air Masses
Once an air mass leaves its source region (because the wind blows it to another location), it begins to change as it moves over areas that are unlike its source region. This changes both the temperature and moisture charcteristics of the air mass.
How are hurricanes measured?
Saffir-Simpson Scale (wind speed)
How do fronts form?
They form when air masses move in response to pressure differences and, as a result, they come into contact with other, unlike air masses, which form fronts.
How are hurricanes named?
Using alternating male and female names through the alphabet, after that Greek alphabet is used.
Hurricane Structure
eye, eye wall, collar cloud, rainbands, and dense cirus overcast.
