Uplift of Air The basic mechanism for raising air temperature occurs at ground level with the heating of the surface by the Sun. Consequently, temperatures generally fall with increasing altitude above the Earth's surface. Such a temperature profile in the lowest 10 km of the atmosphere allows significant uplift of air to take place, generating much of the world's weather. When a packet of air near the Earth’s surface is heated, it rises, being lighter than the surrounding air. This type of air uplift is called convection . Whether or not an air packet continues to rise will depend upon how stable the surrounding air is. As convection continues, air pressure begins to fall, and the air packet expands. Such expansion consumes heat energy and results in a fall in temperature. After sufficient cooling the dew point is reached and condensation occurs in the form of clouds . If the atmosphere is fairly stable, convection will be limited. Cumulus clouds often form in such atmospheric conditions. If the atmosphere is particularly unstable, uplift of air will continue to much greater altitudes, and huge towering cumulonimbus clouds may form, generating significant rainfall or hail. Uplift of air also occurs along fronts , when huge masses of air come together from different directions and with different temperatures. They cannot mix together immediately owing to their different densities, any more than two liquids like water and oil. Mixing takes time. In the meantime, the lighter, warmer air mass begins to rise above the cooler, denser one. The boundary between the two air masses is called a front. Fronts are accompanied by clouds of all types, and very often by precipitation . In addition to convection and frontal uplift there is a third lifting mechanism which produces cloud , and sometimes precipitation on its own, or enhances cloud and precipitation which already exists. This is the necessary lift air must make to surmount large obstacles which obstruct its passage. On an otherwise clear day, lift over hills and mountains may be enough to produce clouds over their tops. As the air descends over the other side, the clouds may dissipate. If the air is fairly humid, considerable precipitation may be generated over hills and mountains. Once past these obstructions, precipitation ceases as the air warms up and condensation returns to its vapor state. On the leeward side of mountain ranges, rain shadows can exist where little precipitation penetrates.
Stability of Air The stability of air in the atmosphere depends on the temperature of rising air relative to the temperature of the stationary surrounding air that it passes through, which varies from place to place and with changing atmospheric conditions. Air stability determines whether clouds form when air is uplifted, and the type of cloud. When a packet of air near the Earth’s surface is heated it rises, being lighter than the surrounding air. Whether or not this air packet continues to rise will depend upon how the temperature in the surrounding air changes with altitude. The rising packet of air will lose heat because it expands as atmospheric pressure falls, and its temperature drops. If the temperature of the surrounding air does not fall as quickly with increasing altitude, the air packet will quickly become colder than the surrounding air, lose its buoyancy, and sink back to its original position. In this case the atmosphere is said to be stable. If the temperature of the surrounding air falls more quickly with increasing altitude, the packet of air will continue to rise. The atmosphere in this circumstance is said to be unstable. As uplifted air cools, it condenses excess vapor out as cloud. The more unstable the atmosphere the more prolonged the uplift. Small cumulus clouds are evidence of a fairly stable atmosphere. Large cumulonimbus clouds are evidence of a highly unstable atmosphere, conducive to the formation of thunderstorms. Within depressions, atmospheric pressure is low and there is considerable atmospheric uplift and cooling at altitude, increasing atmospheric instability. Low-pressure systems are usually associated with an abundance of cloud and precipitation. In high-pressure systems or anticyclones, air may be descending, compressing and gaining energy , such that temperature at altitude rises, thereby increasing atmospheric stability. Anticyclones are often associated with cloudless skies.
The Earth's weather system represent complex interactions between the oceans, the land, the sun, and the atmosphere.
Weather is the condition of the atmosphere at a particular time and place. It refers to such conditions of the local atmosphere as
humidity (the amount of water contained in the atmosphere),
precipitation (rain, snow, sleet, & hail),
wind velocity .
Vertical Structure of the Atmosphere General trends with increasing altitude: Air pressure decreases . At any given altitude, the air pressure is caused by the weight of air above. Constituent gases decrease in density . Because air pressure decreases with altitude, the amount of air per unit volume (density) also decreases with altitude. Temperature decreases in the troposphere where weather occurs . Water vapor decreases dramatically
Air pressure decreases with altitude. At any given altitude, the air pressure is caused by the weight of air above. This means that the air near the ground is compressed by the weight of the air above it.
Rising air does not mix substantially with the surrounding atmosphere.
Atmospheric stability is a property of air that describes its tendency to remain in its original position or sink (stable) or to rise (unstable) once the initial lifting force ceases. A parcel of air forced to rise will expand and cool adiabatically.
Atmospheric Stability Stable air - if an air parcel that is forced aloft cools faster than the surrounding environment. If the lifting forced ceased, the parcel would have the density to sink. High pressure system – an area characterized by descending cooler dry air and clear skies. Cloud formation may occur at an altitude where the saturation temperature is reached (LCL), but clouds would be layered without much vertical development - fair weather clouds.
Atmospheric Stability Unstable air - if an air parcel that is forced aloft cools slower than the surrounding environment. If the lifting force ceased, the parcel will continue to rise because it is warmer and more buoyant than its surroundings. Low pressure system – An area characterized by rising warmer and humid air and cloudy skies. If the air parcel rises to an altitude where the saturation temperature is reached (LCL), clouds with vertical development will form as the buoyant air rises on its own. (thunderstorm clouds).
Tornadoes How a Tornado Works - Associated with strong thunderstorms that develop when 3 main atmospheric conditions occur simultaneously in the central US 1) a northerly flow of warm, moist air from the Gulf of Mexico 2) a cold, dry air mass rapidly moving southward from Canada or the Rocky Mountains 3) strong easterly jet stream - These three air masses moving in different directions produce shearing conditions that are give thunderstorm clouds a "spin" - Funnel clouds begin to form, they may (or may not) touch down and develop into a tornado.
Warm moist air tropical air shoots upward as it meets colder, dryer polar air. As the warm moist air rises, it may meet varying wind directions at different altitudes due to a strong westerly jet stream. If these varying winds are staggered in just the right manner with sufficient speed, they will act on the upward rising air, spinning it like a top.
The rotational cell sags below the cloud base to form a distinctive slowly rotating wall cloud. Strong tornadoes form within and then descend from the wall cloud.
Around a low pressure cell, an inward counterclockwise flow develops; Centers of low pressure are called cyclones = convergent flow
Around a high pressure cell, an outward clockwise flow develops. Centers of high pressure are called anticyclones = divergent flow .
Vertical Flow Net downward movement of air and fair weather Net upward movement of air, often resulting in cloud formation and precipitation . Low pressure center generally related to unstable conditions and stormy weather
Around a surface high air is spiraling outward, which leads to a downward flow of air at the center of the high and convergence aloft. Around a surface low air is spiraling inward, which leads to an upward flow of air at the center and divergence aloft.
A hurricane is a massive tropical cyclone with rotary winds that exceed 74mph blowing counterclockwise around a relatively calm central area of very low pressure.
Hurricanes in the Atlantic and East Pacific, typhoons in the west pacific, cyclones in the Indian Ocean and Southern Hemisphere.
Form in late summer and early fall when ocean waters are warmest.
Hurricane Formation The process by which a tropical cyclone forms and subsequently strengthens into a hurricane depends on three conditions 1. A pre-existing disturbance with thunderstorms (typically emerging from the west coast of Africa) 2. Warm (at least 80ºF) ocean temperatures. 3. Light upper level winds that do not change much in direction and speed throughout the depth of the atmosphere (low wind shear)
Tropical systems are classified into four categories according to its degree of organization and maximum sustained wind speed. Hurricanes are designated by categories on the Saffir-Simpson scale.
Once hurricanes form, they are pushed west with the prevailing west-blowing winds in tropics. The Coriolis force causes storms north of the equator to travel in right-curving paths , and storms south of the equator to travel in left-curving paths .
This photo is a composite of three days' views (August 23, 24 and 25, 1992) of Hurricane Andrew as it slowly moved across south Florida from east to west.
Strong vertical wind shear tears the hurricane apart.
Moving over cooler water can lead to weakening (Top Right).
Moving over land shuts off the moisture source and reduces surface circulation due to friction (Bottom Right).
Above: Average sea surface temperatures in the world during the month of November. The warmest seas are colored red. Tropical cyclones can survive in tropical areas with warm seas, but when they move out over cooler waters such as the yellow and green areas, they quickly start to lose their power Above: When hurricanes move over land, they usually start to weaken quickly.
Hurricane Damage Storm surges are like a “hill” of ocean water (sometimes as high as 20ft above sea level) pushed up by a hurricane. Storm surges are caused by two factors: low atmospheric pressure that pulls the ocean surface up, and the spiraling, converging winds that push the ocean water in toward the center of the storm.
90% of all deaths in tropical cyclones result from storm surge.
Destruction of homes, buildings, roads, bridges, and piers.
Houses raised up on 4m posts .
Flooding Hurricanes frequently produce huge amounts of rain. A typical hurricane brings at least 6 to 12 inches of rainfall to the area it crosses.
Winds and Wind Damages Wind damages include blowing in windows, doors, and walls, lifting off roofs, blowing down trees and power lines, and flying debris. Most damage is from winds.
The Saffir-Simpson Hurricane Scale is based on barometric pressure and average wind speed. The Saffir-Simpson Hurricane Scale
Hurricane Names Since 1953, Atlantic tropical storms reaching tropical storm strength have been named from lists originated by the NHC. When first developed, the lists featured only women's names (figures!). Then in 1979 a six year rotating list with alternating male and female names was developed. The names of devastating storms are retired, and another name is selected by the WMO to replace it. * Hurricanes retired since 1985. Wilfred Wendy William Wilma Walter Wanda Vicky Van Valerie Vince Virginie Victor Teddy Tanya Tony Tammy Tomas Teresa Sally Sebastien Sandy Stan Shary Sam Rene Rebekah Rafael Rita Richard Rose Paloma Pablo Patty Phillippe Paula Peter Omar Olga Oscar Ophelia Otto Odette Nana Noel Nadine Nate Nicole Nicholas Marco Michelle* Michael Maria Mitch* Mindy Lili Lorenzo Leslie Lenny* Lisa Larry Kyle Karen Keith* Katrina Karl Kate Josephine Jerry Joyce Jose Jeanne Juan Isidore Iris* Isaac Irene Ivan Isabel Horrtense Humberto Helene Harvey Hermine Henri Gustav Gabrielle Gordon Gert Georges* Grace Fran Felix Florence Floyd* Frances Fabian Edouard Erin Ernesto Emily Earl Erika Dolly Dean Debby Dennis Danielle Danny Cesar Chantal Chris Cindy Charley Claudette Bertha Barry Beryl Bret Bonnie Bill Arthur Allison* Alberto Arlene Alex Ana 2002 2008 2001 2007 2000 2006 1999 2005 1998 2004 1997 2003 Melissa Michelle Lee Lenny Kirk Keith Ingrid Iris Gaston Georges Franklin Floyd Andrea Allison Replacement Name Retired Name