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.
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.
I forgot to mention in class that 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
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.
I forgot to mention in class that 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
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.
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.
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.
We watched a couple of short video segments at 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.
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 Friday. 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.
We watched a couple of short video segments at 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.
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 Friday. 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 Wed., Feb. 03
class notes)
Topographic or Orographic lifting
is the 4th way of causing air to
rise. This
wasn't
mentioned
in class on Wednesday.
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).
Answers to a couple of questions embedded in today's online notes.
1.
It is the converging motions that cause air to rise. Once the air arrives at the center of the low pressure it has to go somewhere. It rises. Since winds also converge around low pressure in the southern hemisphere, you would expect to see rising air motions and cloudy skies associated with low pressure in the southern hemisphere.
2.
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).
Answers to a couple of questions embedded in today's online notes.
1.
It is the converging motions that cause air to rise. Once the air arrives at the center of the low pressure it has to go somewhere. It rises. Since winds also converge around low pressure in the southern hemisphere, you would expect to see rising air motions and cloudy skies associated with low pressure in the southern hemisphere.
2.
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.