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&apos;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&apos;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.
Climate and Weather Related to Hazards
EXTREME WEATHER PHOTOS
methane, nitrous oxide,
(CFC's) are some
greenhouse gases of
Left: Greenhouse gases trap heat
in the Earth’s atmosphere much
as the glass in a greenhouse
permits the sun to shine in but
prevents most heat from
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),
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
Water vapor decreases dramatically
Water Vapor in the Atmosphere
Water can exist in all 3 states at
the normal range of earth
temperature and pressure.
Whenever matter changes from one
state to another, energy is
either absorbed or released.
• From liquid to gas - evaporation
- heat energy is absorbed
• From gas to liquid -
condensation - heat energy is
The Hydrologic Cycle
oceans and other water
bodies, falls as rain or
snow, is transpired by
plants, and flows
through streams and
groundwater back to
Saturation vapor pressure = Maximum amount of moisture air can
•cannot be exceeded.
•evaporation = condensation
Relative Humidity = The percentage of moisture in air relative
to the maximum amount it can hold (Saturation Vapor Pressure)
under its given temperature and pressure.
Therefore, relative humidity can be changed by...
• Changing the water vapor content.
– Add water, increase relative humidity.
– subtract water, decrease relative humidity.
• Changing the temperature.
– Increase temperature, decrease relative humidity.
– Decrease temperature, increase relative humidity.
Processes that occur without the addition or
subtraction of heat from an external source.
Because air pressure decreases with increasing
altitude, rising air expands and sinking air is
Compressional warming - when air is
compressed, the temperature rises.
Expansional Cooling - when air expands, the
The adiabatic lapse rate - the way
temperature changes with altitude in rising or
falling air (top right).
Lifting condensation level = altitude at which
the rising parcel reaches saturation
temperature and cloud forms (bottom right).
Upward movement of air results from:
• Convergence lifting - when
flowing air masses of equal
density converge and are
• Convective (Density) lifting -
When warm, low-density air
rises convectively and
displaces cooler, denser air.
• Orographic lifting - When
flowing air is forced upward
over a mountain range.
• Frontal lifting - when two
flowing air masses of
different density meet.
Warm Front (Left):
Warm air mass
Cold Front (Right):
Cold air mass
– Lifting processes force air upward.
– Rising air does not mix substantially with the
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.
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
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).
Horizontal differences in
air pressure between high
and low pressure systems
Wind results when air flows
from a place of high
pressure to one of low
Magnitude is determined
from the spacing of the
Isobars - lines connecting places of equal air
pressure on a map (Compare to the slope of a
The spacing of the isobars indicates the
amount of pressure change over a given
distance = pressure gradient.
Due to the rotation of the
earth on its axis.
Deflects all free moving
objects to the right of their
path in the Northern
Hemisphere and to the left
in the Southern
Deflection increases with
Deflection is strongest at
the poles and weakens
In the Northern Hemisphere
•Around a low pressure cell (rising air), an inward counterclockwise
flow develops near the ground surface; Centers of low pressure are
called cyclones = convergent flow
•Around a high pressure cell (sinking air), an outward clockwise flow
develops near the ground surface. Centers of high pressure are
called anticyclones = divergent flow.
Right Hand Rule for Rising or Falling Air in the
(Left Hand Rule for the Southern Hemisphere)
air and fair
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
Around a surface low air is
spiraling inward, which leads
to an upward flow of air at
the center and divergence