5b_AtmosphericWeather
Water, Weather, and Climate Systems
Weather Overview
Weather is influenced by a variety of factors, including air masses and atmospheric lifting mechanisms.
Major components of weather systems include:
Air Masses
Atmospheric Lifting Mechanisms
Mid-latitude Cyclonic Systems
Violent Weather
Air Masses
An air mass is a large body of air that reflects the characteristics of its source region. Examples include:
Cold Canadian air mass
Moist tropical air mass
Classification of Air Masses:
Moisture:
Maritime (m): wetter air
Continental (c): drier air
Temperature:
Arctic (A)
Polar (P)
Tropical (T)
Equatorial (E)
Antarctic (AA)
Air Mass Modification
When air masses migrate, they modify temperature and moisture characteristics.
Examples of modification:
Warm, humid maritime air moving into cooler continental areas
Dry, cold continental air from polar regions moving south and east over the Great Lakes.
Atmospheric Lifting Mechanisms
Types of Lifting Mechanisms
Convergent Lifting
Air flows toward an area of low pressure, causing uplift.
Example: Trade winds converge at the Intertropical Convergent Zone.
Convectional Lifting
Caused by local surface heating, where cooler air moves over warmer land.
Includes: urban heat islands and darker soils.
Orographic Lifting
Air is forced upwards over barriers like mountains.
Windward side experiences cooling and potential precipitation, while leeward side may experience drying (rain shadow).
Frontal Lifting
Occurs along boundaries of differing air masses (cold and warm).
Warm air is lifted over denser, cold air, resulting in precipitation and potential storm activity.
Mid-latitude Cyclonic Systems
Cyclogenesis and Life Cycle of a Midlatitude Cyclone
Cyclogenesis: conflict between cold and warm air masses leads to cyclone formation.
Stages of Cyclone:
Open Stage: Warm air moves north while cold air advances south.
Occluded Stage: Cold air overtakes warm front.
Dissolving Stage: Continuous layer of cooler air beneath warmer air.
Storm Tracks
Cyclonic storms are guided by the jet stream and shift in latitude with changing seasons.
The most significant frontal activity occurs in spring, leading to thunderstorms and tornadoes.
Violent Weather Phenomena
Thunderstorms
Can develop in various conditions, including air mass instability and orographic lifting.
Characterized by: large water vapor, latent heat release, and cumulonimbus clouds with heavy precipitation, lightning, and potential for tornadoes.
Lightning Formation
Lightning occurs due to charge differentiation within cumulonimbus clouds, creating a path of electrical discharge.
A lightning stroke can heat the air up to 30,000 °C, causing a rapid expansion that creates thunder.
Hail Formation
Hailstones develop in strong updrafts within cumulonimbus clouds, gaining layers of ice as they circulate above and below the freezing level.
Derechos
Derechos are strong linear winds associated with thunderstorms, capable of causing significant damage.
Tornado Dynamics
Formation of a Tornado
Begins with unstable warm, moist air beneath a ceiling of cold air.
Abrupt lifting can create a narrow area of intense low pressure, leading to a vortex.
Tornadoes primarily form in areas known as "Tornado Alley."
Tornado Ratings
Tornadoes are rated based on the damage they cause and their wind speeds.
Tropical Cyclones
Formation and Characteristics
Develop within tropical air masses and start with a wave of low pressure in trade wind belts.
A hurricane forms when sea temperatures are above 26 °C (79 °F). Key features include:
Counterclockwise wind rotation in the Northern Hemisphere.
Strong winds and low pressure lead to storm surges.
Historical Hurricanes
Galveston Hurricane of 1900: Category 4 with severe destruction and high fatality rate.
Hurricane Katrina (2005): Category 5 with the highest economic loss in U.S. history.
Hurricane Ike (2008): Notable for its size and destruction as a Category 2 hurricane.