Air Pressure and Wind Chapter 6. Pressure zHear this term often in weather forecasts but what does...

Air Pressure and Wind

Chapter 6

Pressure

Hear this term often in weather forecasts but what does it mean in the atmosphere?- From earlier, it’s the weight of the air above- How about weather?

High pressure?- Usually nice weather

Low pressure?- Associated with stormy weather

Wind

Another weather element we deal with on a regular basis- Anyone know why the wind blows?- Turns out that wind and pressure are related- In fact, wind blows due to horizontal differences

in pressure

If there is high pressure over one part of the country and low pressure over another part:- The atmosphere is out of balance and the wind

blows in an attempt to restore balance


A couple of chapters ago we talked about temperature

Before that, pressure and density In the atmosphere, these variables are all

related such that a change in one will cause changes in the others- Ex. If temperature changes, there will be a

corresponding change in pressure and/or density


The “Ideal Gas Law” or “Equation of State” illustrates this:

Pressure is approximately equal to density x a constant x temperature- We can ignore the constant and just go with…..

RTp


If the pressure doesn’t change:- An increase in T results in a decrease in density

Warm air is less dense and therefore rises

- A decrease in T results in an increase in densityCold air is dense and sinks

Just like we’ve been saying all along

Tp ~


If the temperature doesn’t change:- An increase in pressure results in an increase in density- A decrease in pressure results in a decrease in density

At the same temperature, air at a higher pressure is more dense than air at a lower pressure

Tp ~

Atmospheric Pressure

How does all of this stuff relate to the atmosphere?

We’ve said from the beginning that pressure is basically the weight the air above us.- Like at the right- Pressure at the surface is

due to the weight of all air molecules in the colum


Simplified model- Assumes air can’t leave

the column

Columns of air have the same # of molecules and are at the same temperature- The pressure at the

surface is the same

What would happen if the temperatures of the air changed?- Cool #1, Warm #2


City 1, temp decreased so density increased

City 2, temp increased causing density to decrease- Just like the gas law said

Pressure stayed the same Bottom line: It takes a

shorter column of cold air to exert the same amount of pressure as a taller column of warm air


Now let’s go up to a certain height in the atmosphere- At this level, in which

column is the pressure greatest?

- So, relatively speaking, the pressure is high at this level in column 1 and low in column 2

HL


Points:- Pressure changes more

rapidly w/ height in cold air masses

- Warm air aloft is associated w/ high pressure aloft

- Cold air aloft is associated w/ low pressure aloft

- Differences in temperature cause differences in pressure

HL


Finally, notice that the heights of pressure surfaces (500 mb in this example) are lower in the cold air column

500 mb

500 mb


The difference in pressure establishes a force we call the “pressure gradient force” (directed from H to L)

Now, if we remove the side barriers of the columns, air will rush from high to low pressure in order to equalize things …. - WIND!!


Pressures at the surface will also change due to molecules moving- Pressure will rise at City 1

and fall at City 2

To make a long story short:- Differences in temp from

place to place can cause differences in pressure resulting in the movement of air.

Sea-level Pressure

Measuring Air Pressure

Even though pressure is exerted on us at all times, it’s hard to detect small changes- Can you tell difference between high and low?

We can detect big changes in pressure though- Like popping of ears in the mountains or in planes

Air pressure equalizing inside/outside ears


Mercury Barometer- Just a large, hollow glass

tube immersed in mercury- As air pressure changes,

mercury is forced up or down the tube…pretty simple right

On average, the height of the mercury would be 29.92 inches (avg. sea level pressure)- Or 1013.25 millibars


Aneroid Barometer- Has a hollow metal “cell”

which expands or contracts as pressure changes

- Same type as in the 3-dial weather instruments people hang on walls

Altimeters

Just an aneroid barometer- Calibrated to equate pressure to height- Must be corrected constantly by pilots!

Altimeters

Or this might happen w/ poor visibility


Seen something like this on TV right?

Lines are called “isobars”- These are lines of equal

pressure (in millibars)

Question:- If elevation varies across the

US (and it does), and we know pressure changes quickly w/ height, then why are these types of maps nice and neat?

- Shouldn’t they be really screwy looking?


This kind of map depicts “sea-level pressure”, not surface pressure- We wouldn’t always be able

to tell where high and low pressure systems were otherwise

How do we figure out what sea-level pressure is at each location where measurements are taken?

Sea-level Pressure

To get a sea-level pressure chart:- 1) Measure surface pressure- 2) Correct for instrument error

Temperature, gravity, materials of barometer, etc.

- 3) Correct for altitude- 4) Draw isobars (usually at 4 mb increments)

Connect the dots essentially

1) and 2) are easy. What about 3) and 4)?

Altitude Correction

Near the earth’s surface, pressure changes at 10mb per 100m

So, if a station is 300m altitude, 30mb needs to be added to the surface pressure to get sea-level pressure

Once the altitude correction is done everywhere, we can draw the isobars

Isobars

995 999 1005 1010

998 1002 1007 1011 1014

1006 1010 1015

1009 1013 1020

996

1004

1000

1008

10161012

Surface Chart

End result is a “sea-level pressure chart” or “surface chart”

“Closed” highs and lows show where centers of pressure systems are

Pressure and Wind

Northern Hemisphere: surface winds blow clockwise and outward from high pressure (anticyclones)

counter clockwise and inward around low pressure systems (cyclones)

Note: - Winds cross isobars slightly- Tightly packed - stronger winds- Reversed flow in Southern

Hemisphere

Isobaric Map (Upper Air Chart)

Shows the height of a pressure surface - constant pressure chart- In meters (60m

intervals)

This one is 500mb From Monday -

pressure surfaces are higher up in warm air- So, the 500mb heights

are higher toward the south


Where heights are low - cold air

High heights - warm air

Elongated areas of low heights/pressure are called troughs- cold air

Elongated areas of high heights/pressure are called ridges- warm air


Other uses?? Wind - shows us

direction and speed- Like surface map except

winds tend to blow parallel to height lines

- Closer lines - stronger wind speeds

Steering - upper level winds determine where surface systems go and whether or not they strengthen (more later)

Surface and Upper Air Charts

Both surface and upper air charts are extremely valuable to meteorologists- Surface charts identify where pressure systems are

located and their intensities- Upper air charts indicate where these systems will

move and how they will strengthen/weaken in time

Wind

Now with all this background, we can determine why the wind blows

More specifically, what the direction and speed will be

Anything that moves does so due to the forces acting upon it- Throwing a ball - pushing away by the hand, friction

from the air, gravity, etc.- Same thing for the wind

Wind

Actually 4 forces acting to influence wind speed and direction- 1) Pressure gradient force- 2) Coriolis force- 3) Friction- 4) Centripetal force

Pressure Gradient Force

Due to the difference in pressure over a distance

Greater pressure gradients lead to a greater PGF- like at the right

Hurricanes are a good example- Very low pressures at the center- Pressure increases rapidly as

you move away from the center- Strong PGF


ALWAYS directed from high to low pressure

Direction is at right angles to the isobars

This does not mean that wind blows directly from high to low though…….other forces…..


Wait just a second. Since pressure changes much faster w/height than horizontally, isn’t the PGF incredibly strong in the vertical?

I’ve already said that vertical air motions are very small (usually) compared to horizontal winds…..what’s up with that?


Gravity almost exactly balances the upward directed PGF - “Hydrostatic balance”

High or Low Pressure?

Coriolis Force (Effect)

Tricky subjectAn “apparent” force due to the rotation and

curvature of the earth- Causes wind to deflect to the right in the Northern

Hemisphere Left in SH

- Force is at a right angle to the wind- Things drain differently in SH??? Umm..no

Coriolis Force

Coriolis Force

Maximum at poles, zero at the equator Faster speeds - stronger Coriolis force It’s why aircraft fly in “circular” paths

Coriolis Force

Summary:- Causes objects to deflect to the right of a straight

path in the NH (left in SH)- Amount of deflection depends on

1) Rotation of the earth2) Latitude3) Speed of object (wind, airplane, etc.)

PGF - Coriolis Balance

Above the friction layer near the surface, the PGF and CF roughly balance each other- That’s why air aloft flows parallel to isobars

Wind which flows at a constant speed parallel to evenly spaced isobars is called- Geostrophic Wind

Geostrophic Wind

Always low pressure to the left and high pressure to the right (Northern Hemisphere)

Speed depends on the “packing” or “tightness” of isobars- loosely packed = weak wind : tightly packed = strong wind

Geostrophic Wind

Only a theoretical wind but still, a good approximation of winds above the surface

Why only theoretical?- Isobars are rarely evenly spaced OR straight

Gradient Wind

In this case, the wind is called a gradient wind- Basically the same as geostrophic wind except that it

blows parallel to curved isobars- Note: both geostrophic and gradient winds refer to

air flow well above the surface

Wind Review

4 forces (talked about 2 so far)- 1) Pressure gradient force

Directed from High to Low pressureStronger PGF = stronger wind

- 2) Coriolis forceDeflects wind to right in NHFaster wind = stronger CFZero at equator, max at poles

- These two forces are roughly in balance above the surfaceCauses upper level winds to generally flow west to east in

mid-latitudes (parallel to isobars or height contours)

Friction

Friction affects air flow near the surface (lowest 1 km or so)

Slows down the wind (drag) If wind slows, what happens

to the Coriolis Force??- Weaker

So, the PGF is now greater than the CF and flow is across isobars- ~ 30º angle

Centripetal Force

A little confusing so just remember it is:- The force required to keep an object (wind) moving

in a circular path- Directed inward toward the center in both high and

low pressure systems

Convergence and Divergence

Now that we know a little about surface and upper level winds….- How do they affect vertical air motions?

By convergence and divergence patterns

ex. Convergence Divergence


Remember what winds are like around high and low pressures?

- Winds diverge from high pressure and converge at low pressure

If air converges at low pressure (at the surface for ex.), what must it do?- Must rise (can’t go into the ground

right?)


What about air diverging from a high pressure center (again at the surface)?- Some air will have to sink from

above to replace it- This explains why we have clear

weather w/ highs and cloudy weather w/ lows

So far we’re just talking about the surface. But what’s going on above the surface high and low pressure systems?


Air that is forced to rise due to convergence at the surface low eventually diverges aloft.

Air converges aloft above the surface high and sinks to replace the diverging air at the surface.- Think of it like columns of

air above the surface pressure systems


In either of these examples, if convergence = divergence, what happens to the surface pressure.

Hint: - This means the # of

air molecules over the surface does not change.

Pressure stays the same!


What if convergence and divergence are not equal??

Over the low: Divergence aloft >

Convergence at the surface?- Net loss of air over low -

pressure gets even lower- Hurricanes?? (not Miami)

Div < Conv?- Net gain of air - pressure

increases


Over the high: Convergence aloft >

Divergence at the surface?- Net gain of air over high -

pressure gets even higher

Conv < Div?- Net loss of air - pressure

decreases


In summary:- Pressure at the surface is largely dependent on wind

patterns at both the surface and aloftThis is why meteorologist actually care about what is

happening above the surface of the earth

- If this seems a little fuzzy to you, look at figure 6.21 and convince yourself of it.

Measuring Wind

Described by:- 1) Direction

Always direction it’s coming FROMCan be N/S/E/W or in degrees on a compass

- 2) StrengthUsually mph, m/s (meters per second), knots (nautical miles

per hour)- NOTE: Nautical mile > statute mile

Measuring Wind

Wind directions: Real easy, just think of a

360° circle- East wind - 90° - South wind - 180° - West wind - 270° - North wind - 360°

Again, always described in terms of direction from- ex. NW wind is out of the

northwest, not toward the northwest

Measuring Wind

Wind vane- Simple instrument which

measures direction only

Anemometers- Measures speed only- Example is of a “cup”

anemometer

Measuring Wind

Aerovane- Measures both wind speed

and direction- Will face into the wind

giving direction- Propellers rotate to yield

wind speed- Info is transmitted

electronically

- One on top of the Love Building

Measuring Wind

Radiosonde- Balloon is tracked from the surface- Simple calculations to determine its speed (wind)

RADAR- Doppler in particular- Can determine wind speed and direction by

frequency changes in the emitted RADAR pulse

Satellites- Cloud drifts

Air Pressure and Wind Chapter 6. Pressure zHear this term often in weather forecasts but what does...

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Transcript of Air Pressure and Wind Chapter 6. Pressure zHear this term often in weather forecasts but what does...