Environmental Geology
Vertical Structure of the atmosphere
Thin membrane represents about 1 millionth of total mass of the Earth
Average pressure exerted by the atmosphere at sea level is 14.7 pounds/inch^2 = 1013.25 millibars.
Atmosphere is stratified by temperature into 5 major layers
Troposphere – layer closest to Earth to a height of about 14 km. Greater height at the equator and closer to the Earth at the poles. About ˝ the mass of the atmosphere is within about 18,000 feet of the Earth’s surface.
Most weather occurs in the troposphere, and it contains about 99% of all the water vapor and aerosols.
Next layer up is the tropopause – temperature inversion: temperature in the troposphere decreases with increasing altitude at about 6.4 degrees/1000 meters of rise (from about 18 degrees C at surface to about -58 degrees C at the tropopause. This decline rate is called the lapse rate.
The next layer up is the stratosphere – this is the zone that contains the Earth’s protective ozone shield that helps block UV radiation from the sun. temperatures rise in the stratosphere from -58 degrees C at the base to about 0 degrees C at the top.
The stratopause marks another temperature break where temperatures begin to fall as you continue to go up in the mesosphere. Within the mesosphere temperatures drop from near 0 degrees C at the base to about -97 degrees C at the mesopause.
The layer above the mesopause is the thermosphere where temperatures increase rapidly from -97 degrees C to over 100 degrees C.
The last layer is the ionosphere.
We will concentrate in this class on the layers below the stratopause.
Atmospheric Circulation:
General circulation results from differential heating of the globe resulting in the development of huge low and high pressure cells.
What is wind?
Measure wind with an anemometer (wind velocity) and wind direction with a wind vane.
Measure in knots or nautical miles: 1 nautical mile = 1.15 statute miles
Report direction of the wind as the direction from which the wind is blowing
Air parcels (balloon size to room size)
Air masses subcontinental size – large bodies of air with uniform properties of temperature, pressure, and moisture content.
Characterize air masses by:
Temperature
Pressure
Moisture content
Aerosol load
Wind speed
Major categories of air masses
mT – maritime tropical, hot and moist
cT – continental tropical, hot and dry
mP – maritime polar, cool and moist
cP – continental polar, cold and dry
cA – continental arctic, very cold and dry
Air masses represent mesoscale
meteorology which collide as they move about the globe in the mid-latitudes
(like here in
Wind blows in response to pressure differentials between air masses.
Measure air pressure with a barometer which was first developed by Evangelista Torricelli in 1643.
Barometric Pressure
Air moves in response to pressure gradient from high pressure to low pressure
Contour pressure using isobars (lines of equal pressure)
Warm air expands: molecules spread out, air becomes lighter (less dense) and
rises; this leaves low pressure at the surface but creates high pressure in the
upper atmosphere
Cooler air is heavier (more dense); air molecules contract, air sinks; this
creates high pressure at the surface and low pressure in the upper atmosphere
Solar heating over tropics causes air to expand, air rises creating low
pressure at the surface along the equatorial region. Surface winds
converge (flow in) to fill this low pressure belt known as the ITCZ (Intertropical Convergence Zone). This zone is also
known as the Doldrums because the weather pattern is so boring!! Heating
in the morning leads to mid day rain followed by clearing in the
afternoon. Temperatures in the 80’s everyday. This is the location
of the worlds tropical rain forests.
Other Factors Affecting Air flow (wind)
Coriolis Effect
Deflection due to Earth’s rotation: objects appear to be deflected away from
their path of motion. First discovered by George de Coriolis,
an artillery officer for Napolean, who notice long
range artillery was consistently missing the target. The deflection is
always to the right in the northern hemisphere and to the left in the southern
hemisphere.
Low pressure cells: cloudy weather, precipitation. Along the ITCZ air is
heated by sun, expands and rises. As it rises it cools eventually
reaching its dew point (the temperature at which condensation occurs). As
the air cools it becomes more dense and diverges (flows outward) aloft. This
convergence at the surface and divergence aloft creates a primary pressure cell
that moves energy from the equator toward the north and south. This is
called a Hadely cell.
Descending air at about 30 degrees north and 30 degrees south latitude results
in high pressure at the surface and low pressure in the upper atmosphere.
This is the latitude where most of the world’s deserts are located. As
the air sinks it warms, as it warms it is able to hold more moisture which is
unlikely to condense and cause precipitation. These latitudes are also
known as the Horse latitudes because the early sailing vessels became trapped
in the calm waters and had to jettison cargo including livestock and also had
to eat some of their horses for survival.
High pressure is associated with fair weather.
Low pressure cells have counterclockwise rotation with converging winds
High pressure cells have clockwise rotation with diverging winds
A weaker global circulation cell known as a Ferrel
cell moves energy from 30 degrees north and 30 degrees south latitude to about
60 degrees north and south latitudes.
The final global circulation cell is called the Polar cells. In the
vicinity of the north and south poles the extreme cold caused air to descend at
the poles creating high pressure at the surface. This cold air flows out
(diverges) from the poles to about 60 degrees south latitude where it rises and
returns to the poles.
The polar highs are desert areas.
The front created where the polar cells meet the Ferrel
cells is the boundary where two very dissimilar air masses collide and is the
zone where many severe weather events are observed.
Weather
Air masses move from their source regions in response to pressure
gradients. These air masses move as a unit and do not easily mix with
other air masses.
Well defined boundaries develop between air masses that extend from the Earth’s
surface to near the top of the troposphere. These boundaries are called
weather fronts.
Four primary types of fronts:
1. cold front – shown on map as saw tooth pattern with teeth
pointing in direction of movement
2. warm fron – shown on map with
semicircles pointing in direction of front movement
3. Stationary front – shown on map with saw tooth and
semicircles on opposite sides of line indicating no movement
4. Occluded front – shown on map with saw tooth and
semicircles on same side of line indicating overrunning of the warm front by
the faster moving cold front.
Lifting mechanisms:
To cause condensation and precipitation the air mass must be cooled. The
primary method of cooling is to lift the mass (cause it to rise) and cool at
the lapse rate.
Three primary lifting mechanisms;
1. Convection – as described for the ITCZ. Sun heats
ground, ground heats air, air expands and rises
2. Orographic: lifting due to
forcing of flow over mountains. Example is westerly flow from Pacific
Ocean rising over the western U.S. mountain ranges.
Flow from Pacific Ocean onto the U.S. landmass is called the windward
flow. As the air rises it eventually condenses and precipitation
occurs. This is why we observe rain forests in Western Washington and
Oregon.
Once the air moves over the mountains and descends on the eastern flank (the
lee side) most of the moisture has been squeezed out. There is little
precipitation on the lee side and so we observe desert environments here.
Precipitation pattern observed with various fronts:
Cold Front: precipitation occurs along the leading edge of the front in a
relatively narrow band. Often this is the zone of severe thunderstorms.
Storms tend to be relatively short duration (0.5 to 2 hours or so).
Warm Front: precipitation occurs along a broader area on the downwind side of
the front. Area of precipitation is greater than that observed with cold
front. Some severe storms do occur with warm fronts but generally not as
severe as those associated with the cold fronts.
Occluded front: Lots of precipitation over long period: slow steady rain and
drizzle.