# Air Pressure and Wind Chapter 6.

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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

Air Pressure and Wind 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

Air Pressure and Wind 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…..

Air Pressure and Wind 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 density Cold air is dense and sinks Just like we’ve been saying all along

Air Pressure and Wind 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

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

Atmospheric Pressure 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

Atmospheric Pressure 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

Atmospheric Pressure 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 L H

Atmospheric Pressure Points: L H
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 L H

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

Atmospheric Pressure 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!!

Atmospheric Pressure 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

Measuring Air Pressure
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 inches (avg. sea level pressure) Or millibars

Measuring Air Pressure
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

Measuring Air Pressure

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

Atmospheric Pressure 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?

Atmospheric Pressure 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 996 1000 1004 1008 1012 1016

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

Isobaric Map (Upper Air Chart)
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

Isobaric Map (Upper Air Chart)
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

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 subject An “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 earth 2) Latitude 3) 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 pressure Stronger PGF = stronger wind 2) Coriolis force Deflects wind to right in NH Faster wind = stronger CF Zero at equator, max at poles These two forces are roughly in balance above the surface Causes 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

Convergence and 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?)

Convergence and Divergence
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?

Convergence and Divergence
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

Convergence and Divergence
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!

Convergence and Divergence
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

Convergence and Divergence
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

Convergence and Divergence
In summary: Pressure at the surface is largely dependent on wind patterns at both the surface and aloft This 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 2) Strength
Always direction it’s coming FROM Can be N/S/E/W or in degrees on a compass 2) Strength Usually 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 Anemometers
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