Air will
start moving
toward low
pressure (like a rock sitting on a hillside that starts to roll
downhill), then something called the Coriolis force will cause
the
wind to start to spin (we'll learn more about the Coriolis force later
in the semester). In the northern hemisphere winds spin in a
counterclockwise (CCW) direction
around surface
low pressure
centers. The winds also spiral inward toward the center of the
low, this is called convergence. [winds spin clockwise around low
pressure centers in the southern hemisphere but still spiral inward,
don't worry about the southern hemisphere until later in the semester]
When the converging air reaches the
center of the low it starts to rise.
Rising air expands (because it is moving into lower pressure
surroundings at higher altitude), the expansion causes it to
cool. If the air is moist
and it is cooled enough (to or below the dew point temperature) clouds
will form and may then begin to rain or snow. Convergence is 1 of 4 ways of causing air
to rise. You often
see
cloudy skies and stormy weather associated with surface low pressure.
Surface high pressure
centers are pretty much just the opposite situation. Winds
spin
clockwise
(counterclockwise
in
the
southern
hemisphere) and spiral outward.
The
outward motion is called divergence.
Air sinks in the center of surface high pressure to replace the diverging air. The sinking air is compressed and warms. This keeps clouds from forming so clear skies are normally found with high pressure (clear skies but not necessarily warm weather, strong surface high pressure often forms when the air is very cold).
2.
The pressure pattern will also tell you something about where you might expect to find fast or slow winds. In this case we look for regions where the isobars are either closely spaced together or widely spaced.
Air sinks in the center of surface high pressure to replace the diverging air. The sinking air is compressed and warms. This keeps clouds from forming so clear skies are normally found with high pressure (clear skies but not necessarily warm weather, strong surface high pressure often forms when the air is very cold).
2.
The pressure pattern will also tell you something about where you might expect to find fast or slow winds. In this case we look for regions where the isobars are either closely spaced together or widely spaced.
Closely spaced contours means
pressure is changing
rapidly
with
distance. This is known as a strong pressure gradient and
produces fast winds. It is analogous to a steep slope on a
hillside. If you trip, you will roll rapidly down a steep
hillside, more slowly down a gradual slope.
The winds around a high pressure
center are shown above using both the
station model notation and arrows. The winds are spinning clockwise and
spiraling outward slightly. Note the different wind speeds (25
knots and 10 knots plotted using the station model notation)
Winds spin counterclockwise and
spiral inward around
low
pressure
centers. The fastest winds are again found where the pressure
gradient is strongest.
This figure is found at the bottom
of p. 40 c in the photocopied ClassNotes. You should be able to
sketch in the direction of the wind at each of the three
points and determine where the fastest and slowest winds would be
found. (you'll find the answers at the end of today's notes).
3.
The pressure pattern determines the wind direction and wind speed. Once the winds start to blow they can affect and change the temperature pattern. The figure below shows the temperature pattern you would expect to see if the wind wasn't blowing at all or if the wind was blowing straight from west to east. The bands of different temperature are aligned parallel to the lines of latitude. Temperature changes from south to north but not from west to east. The figures below are more cleaner and neater versions of what was drawn in class.
This isn't a very interesting
picture. It gets a
little
more interesting if you put centers of high or low pressure in the
middle.
The clockwise spinning winds
move warm air to
the north on
the western
side of the High. Cold air moves toward the south on the eastern
side of the High. The diverging winds also move the warm and cold
air away from the center of the High.
Counterclockwise winds move cold air toward the south
on the
west side
of the Low. Warm air advances toward the north on the eastern
side of the low.
The converging winds in the case of low pressure will move the air masses of different temperature in toward the center of low pressure and cause them to collide with each other. The boundaries between these colliding air masses are called fronts. Fronts are a second way of causing rising air motions (rising air expands and cools, if the air is moist clouds can form)
Cold air is moving from north toward the south on the western side of the low. The leading edge of the advancing cold air mass is a cold front. Cold fronts are drawn in blue on weather maps. The small triangular symbols on the side of the front identify it as a cold front and show what direction it is moving. The fronts are like spokes on a wheel. The "spokes" will spin counterclockwise around the low pressure center (the axle).
A warm front (drawn in red with half circle symbols) is shown on the right hand side of the map at the advancing edge of warm air. It is also rotating counterclockwise around the Low.
This type of storm system is referred to as an extratropical cyclone (extra tropical means outside the tropics, cyclone means winds spinning around low pressure) or a middle latitude storm. Large storms also form in the tropics, they're called tropical cyclones or more commonly hurricanes.
3.
The pressure pattern determines the wind direction and wind speed. Once the winds start to blow they can affect and change the temperature pattern. The figure below shows the temperature pattern you would expect to see if the wind wasn't blowing at all or if the wind was blowing straight from west to east. The bands of different temperature are aligned parallel to the lines of latitude. Temperature changes from south to north but not from west to east. The figures below are more cleaner and neater versions of what was drawn in class.
The converging winds in the case of low pressure will move the air masses of different temperature in toward the center of low pressure and cause them to collide with each other. The boundaries between these colliding air masses are called fronts. Fronts are a second way of causing rising air motions (rising air expands and cools, if the air is moist clouds can form)
Cold air is moving from north toward the south on the western side of the low. The leading edge of the advancing cold air mass is a cold front. Cold fronts are drawn in blue on weather maps. The small triangular symbols on the side of the front identify it as a cold front and show what direction it is moving. The fronts are like spokes on a wheel. The "spokes" will spin counterclockwise around the low pressure center (the axle).
A warm front (drawn in red with half circle symbols) is shown on the right hand side of the map at the advancing edge of warm air. It is also rotating counterclockwise around the Low.
This type of storm system is referred to as an extratropical cyclone (extra tropical means outside the tropics, cyclone means winds spinning around low pressure) or a middle latitude storm. Large storms also form in the tropics, they're called tropical cyclones or more commonly hurricanes.
Clouds can form along fronts (often
in a fairly narrow band along
a
cold front and over a larger area ahead of a warm front). We need
to look at the crossectional structure of warm and cold fronts to
understand better why this is the case.
The top picture below shows a crossectional view of a cold front
The top picture below shows a crossectional view of a cold front
At the top of the figure, cold
dense air on the left is
advancing into
warmer lower density air
on the right. We are looking at the front edge of the cold air
mass, note the blunt shape. The warm low density air is lifted
out of the way
by the cold air. The warm air is rising.
The lower figure shows an analogous situation, a big heavy Cadillac plowing into a bunch of Volkswagens. The VWs are thrown up into the air by the Cadillac.
We watched a couple of short video segments at about this point. The first used colored liquids with slightly different densities to show how a cold air mass can lift a warmer, less dense air mass. The video segment also tried to show how warm air overruns a receding mass of colder denser air. The second video was a time lapse movie of an actual cold front that passed through Tucson on Easter Sunday in 1999.
The lower figure shows an analogous situation, a big heavy Cadillac plowing into a bunch of Volkswagens. The VWs are thrown up into the air by the Cadillac.
We watched a couple of short video segments at about this point. The first used colored liquids with slightly different densities to show how a cold air mass can lift a warmer, less dense air mass. The video segment also tried to show how warm air overruns a receding mass of colder denser air. The second video was a time lapse movie of an actual cold front that passed through Tucson on Easter Sunday in 1999.
Here's a crossectional view of a
warm front, the structure is a
little different.
In the case of a warm
front we are looking
at
the
back,
trailing edge of cold air (moving slowly to the right). Note the
ramp
like shape of the cold air mass. Warm air overtakes the cold
air. The warm air is still less dense than the cold air, it can't
wedge its way underneath the cold air. Rather the warm air
overruns the cold air. The warm air rises again (more gradually)
and clouds form. The clouds generally are spread out over a
larger area than with cold fronts. The front can advance only as
fast as the cooler air moves away to the right.
In the automobile analogy, the VWs are catching a Cadillac. What happens when they overtake the Cadillac?
In the automobile analogy, the VWs are catching a Cadillac. What happens when they overtake the Cadillac?
The Volkswagens
aren't heavy
enough to lift the
Cadillac.
They run up and over the Cadillac.
Fronts are a second way of causing air to rise. Rising air cools and if the warm air is moist and cooled enough, clouds and precipitation can form. That's why the clouds were drawn in along the fronts in the middle latitude storm picture above.
We will come back to the topic of fronts again next Tuesday. We will, in particular, learn about some of the weather changes that take place as a front approaches and passes through. We will also look at how fronts can be located on surface weather maps.
As long as we're listing weather processes that can cause air to rise we might as well mention the 3rd process. It's something we've already covered as a matter of fact (see the Tue., Feb. 2 class notes)
At (1) sunlight reaching the ground
is absorbed and warms the
ground. This in turns warms air in contact with the ground
(2) Once this air becomes warm and its density is low enough,
small "blobs" of air separate from the air layer at the ground and
begin
to rise, these are called "thermals." (3) Rising air expands and
cools (we've haven't covered
this yet). If it cools enough (to the dew point) a cloud will
become visible as shown at Point 4. This whole process is called
free convection; many of our summer
thunderstorms start this way.
Speaking of free convection, here are a couple of statements that might seem contradictory.
It is important to remember that two separate things are going on here. Air that is warmer than the air surrounding it will rise because it is less dense than the air around it. Once the air starts to rise it will expand and start to cool. This might best be illustrated in a picture.
Will the air keep on rising and cooling? That depends on how quickly the rising air cools and what the temperature of the air around it is. Again two pictures can be used to illustrate two possible outcomes.
Points a and b are the same as in the figure above. Now we
have added the temperature of the surrounding air. In the left
figure the atmosphere has cooled from 64 F to 61 F at (c). With a
temperature of 60 F the rising air parcel now finds itself colder than
the surrounding air and will sink (d). In the right figure the
atmosphere has cooled from 64 F to 59 F (c). The rising air
parcel is now warmer than the surrounding air and will continue to rise
(d).
Topographic or Orographic lifting is the 4th way of causing air to rise.
When moving air encounters a mountain it must pass over it. You often find clouds and rain on the windward side of the mountain where the air rises. Drier conditions, a rain shadow, is found on the leeward side where the air is sinking (assuming that the winds mostly blow in the same direction over the mountain).
Speaking of free convection, here are a couple of statements that might seem contradictory.
warm
air
rises
rising air cools
rising air cools
It is important to remember that two separate things are going on here. Air that is warmer than the air surrounding it will rise because it is less dense than the air around it. Once the air starts to rise it will expand and start to cool. This might best be illustrated in a picture.
At Point a a parcel of air (65 F)
finds itself warmer (and less dense) than the surrounding air (64
F). It will rise and expand. At Point b the air temperature
has cooled to 60 F.
Will the air keep on rising and cooling? That depends on how quickly the rising air cools and what the temperature of the air around it is. Again two pictures can be used to illustrate two possible outcomes.
Topographic or Orographic lifting is the 4th way of causing air to rise.
When moving air encounters a mountain it must pass over it. You often find clouds and rain on the windward side of the mountain where the air rises. Drier conditions, a rain shadow, is found on the leeward side where the air is sinking (assuming that the winds mostly blow in the same direction over the mountain).
Here's the answer to a question
that was embedded in today's notes
The winds are blowing from the NNW
at Points 1 and 3. The winds are blowing from the SSE at Point
2. The fastest winds (30 knots) are found at Point 2 because that
is where the isobars are closest together (strongest pressure
gradient). The slowest winds (10 knots) are at Point 3.