SUPPLEMENTAL LECTURE MATERIALS...
Definition of Pressure, Pressure Measurement and Variation
Pressure can be thought of as the weight of the air above a given point. Average sea level pressure value is 29.92 inches of Mercury or 1013.25 millibars (mb).
Pressure is measured with a barometer where mercury rises and falls within a tube as pressure changes.
Important fact: pressure decreases with height above sea level.
Because pressure decreases with height, stations recording pressures at higher elevations would show lower pressures. We need to remove this effect. To do this, all pressures shown on a surface weather map have been corrected to remove the effects of topography. Therefore, the weather map shows "sea level pressures" and we only see pressure variations due to "weather" not elevation.
Highs, Lows and the Surface Weather Map
The surface weather map is a constant height surface (all stations at sea level) and the map shows variations in pressure due to weather. The two main pressure features are highs and lows.
Lines of equal pressure values are called isobars (see map above). It is standard to show isobars on maps at 4 mb intervals where 1000 mb is the base value. Notice the values of the pressures are higher near the "high" and lower near the "low". Interpreting pressure values is very straight forward, the bigger the number, the higher the pressure.
High pressure systems, also called anticyclones, have air moving clockwise and outward from the center in the Northern Hemisphere. High pressure areas are generally associated with fair, clear weather.
Low pressure systems, also called cyclones, have air moving counterclockwise and into the center in the Northern Hemisphere. Low pressure areas are generally associated with clouds, precipitation and storminess.
Upper Level Maps
In contrast to the surface map, the upper air maps show variations in height along a constant pressure surface. Each map is produced at a specific pressure level, say 500 mb, and the variations are "how high do we have to go into the atmosphere for the pressure to have dropped to 500 mb". The height varies with temperature such that, heights are higher where the atmosphere is warmer and lower where the air is colder.
On these maps, higher heights are equated to higher pressure and lower heights are associated with lower pressure at the same level. A ridge is an elongated area of high pressure and a trough is an elongated area of low pressure. Lines of equal height values are called contour lines.
Wind and the Forces that Create it
Wind is a horizontal movement of air and there are also vertical motions associated with high and low pressure areas.
The underlying cause of wind flow is the uneven heating of the earth's surface. This causes pressure differences between locations which lead to the formation of wind as air flows from high pressure to low pressure areas.
On a large scale, wind functions to move excess heat from the tropics to the polar regions to try to reach an equilibrium or balance in the earth's distribution of energy.
Three Forces Affecting Wind Flow
- Pressure Gradient
- Coriolis Effect
Pressure Gradient Force (PGF)
Pressure gradient force is created by the uneven heating of earth which results in a pressure difference between locations, this creates a force: Pressure Gradient Force (PGF)
Due to PGF, air moves from higher to lower pressure. PGF operates at right angles to the isobars.
In the diagram below you can see that with pressure gradient as the only force, the wind is flowing in the same direction as the force itself.
PGF is responsible for beginning air movement - it "gets the wind going".
There is a strong relationship between the pressure gradient (difference in pressure between locations) and the wind speed. The greater the pressure difference between places (stronger pressure gradient), the higher the wind speed. We can see this by looking at the spacing between isobars on a weather map.
- Closely spaced isobars = strong (or steep) pressure gradient = strong winds.
- Widely spaced isobars = weak pressure gradient = weak winds
If PGF were the only force operating, then air would flow straight out of a high and straight into a low as shown below.
- An apparent force created by Earth's rotation
- Causes objects to be deflected from their path of motion.
- Coriolis effect varies as a function of latitude:
- There is no coriolis at the equator
- Maximum coriolis at the poles
Coriolis is also affected by the speed of the wind - faster the wind, the greater the deflection.
- Operates on wind and ocean currents. The direction of deflection is...
- To the right in the Northern Hemisphere
- To the left in the Southern Hemisphere
- Coriolis only influences wind direction and never the wind speed.
Friction slows surface wind speed and weakens the Coriolis effect. Near the surface, winds flow differently than they do in the upper levels of the atmosphere. We'll look at the differences between the upper and lower atmospheric wind flows.
Upper Level Winds: The Geostrophic Wind
The geostrophic wind results from a balance between PGF and Coriolis only. It flows parallel to the contours and occurs above 1 km in the atmosphere.
Air flow around a curved surface:
If winds are geostrophic, air flow looks like it does in the figure below (in the Northern Hemisphere). Remember these are upper level winds found above 1000 m or 1 km above the ground. Southern hemisphere circulations will be in the opposite direction.
From Buys-Ballot's Law for the Northern Hemisphere...
If you stand with the wind at your back, the area of low pressure is directly to your left.
Lower Level Winds: The Surface Wind
Due to the effects of friction and the reduction in Coriolis, the winds below 1km are not geostrophic. They blow across the isobars towards low pressure. The winds here are a balance between PGF, Coriolis and friction. Now the winds in the Northern Hemisphere are...
- Winds blow counterclockwise and INTO a low
- Winds blow clockwise and OUT OF a high
In the Southern Hemisphere...
- Winds blow clockwise and INTO a low
- Winds blow counterclockwise and OUT OF a high
Wind flow at the surface in the N.H. looks like the figure below.
From Buys-Ballot's Law for the Northern Hemisphere...
If you stand with the wind at your back, stick out your left arm directly to the side and move it forward about 30 degrees. That is where the low pressure is located.
Air converges near the ground into a low pressure area. The air can't just pile up, so it rises and diverges aloft (in upper atmosphere).
With high pressure, air converges aloft, descends toward the surface and then diverges at the ground level.
Rising air with low pressure is associated with clouds and precipitation while high pressure is associated with fair weather.
GLOBAL WIND CIRCULATION
Your textbook does an excellent job with a thorough discussion of atmospheric circulation. Read the chapter carefully.
The underlying cause of our global wind circulation is the unequal heating of Earth's surface. Unequal heating creates pressure differences.
- where intense surface heating occurs - warm air rises creating low pressure
- where intense surface cooling occurs - air cools, gets heavier, sinks and creates high pressure
Single Cell Model - Simple Convection System
Assumptions for the single cell model:
Given these conditions, air rises at the equator due to intense surface warming (low pressure), travels poleward in the upper levels, descends to the surface at the poles (high pressure) and then returns to the equatorial region as surface flow.
Air flow would be very simple under these conditions with a single convective circulation cell in each hemisphere.
Three Cell Model
We now allow the Earth to spin introducing the Coriolis effect which complicates the pattern of circulation markedly - see the two figures below.
Again, we see air rising from the equator (equatorial low) and diverging (spreading apart) aloft. The air then moves poleward in both hemispheres. As the air moves towards the poles, it begins to converge in the range of 25° and 35° north and south latitude, let's just call it 30° for now. You can see how this occurs by placing your fingers on the lines of longitude printed on a globe. As you move your hands poleward keeping your fingers on the longitude lines, you should notice that your fingers converge (move towards each other). A similar thing happens to the air. This upper air convergence, coupled with radiational cooling cause the air to subside (sink towards the earth's surface) in the subtropics(around 30° lat). As the air reaches the surface, atmospheric pressure increases forming the subtropical highs. This region is also known as the horse latitudes and is characterized by light or non-existant winds. This area was so named by the early Spanish sailors who threw their horses overboard to conserve water when their ships were becalmed in this region.
After sinking back to the surface near 30° latitude, some of the air returns to the equator (creating the trade winds in each hemisphere) and some continues poleward (now at ground level), creating the westerlies in both hemispheres. Air returning to the equator converges there and causes air to rise. This area is therefore called the Intertropical Convergence Zone (ITCZ) and is associated with clouds and precipitation. This region is also known as the doldrums and is also a region of very light winds.
Near 60° latitude air is forced to rise along the polar front creating clouds and precipitation. This is the region of the subpolar low and the polar front (and polar jet stream). Some air continues to the pole where the air descends (creating the polar high) and travels toward 60° latitude (winds between 60° and 90° are the polar easterlies). The top diagram you see below is the one found in your textbook.
In the bottom figure, representing the same 3-cell circulation, shows a distinct circulation cell in the midlatitudes (approx. 30° - 60° latitude). This is called the Ferrel cell and isn't 100% correct, but it is often shown this way to ease student comprehension of the complicated pattern. Otherwise, the two pictures are showing the same general circulation of the atmosphere. This bottom diagram shows that at 60° the air rises (at the polar front) and some air moves to the poles and some returns aloft to the region of the subtropical highs. In actuality, there isn't really a well defined upper level return flow to complete this cell, in fact the winds are westerly at high altitudes which is inconsistent with a return flow aloft (winds would be easterly due to coriolis and they are not). Regardless, it's okay if you learn it this way.
Also note in this bottom picture that the polar front is shown as a blue line with triangles on it and it "wiggles" (not straight and fixed at 60° latitude). This is a correct depiction. Also note the movement in the position of the ITCZ. This is also correct and will be discussed briefly later (and also, thoroughly in your textbook).
See in the bottom diagram that there are three convection cells in each hemisphere: Hadley Cell, Ferrel Cell, and Polar Cell (Hadley and Polar are best defined)
In both diagrams, you can see the following features:
Pressures are: Equatorial Low, Subtropical Highs, Subpolar Lows, Polar Highs
Wind Belts: Tradewinds (NE and SE), Westerlies, Polar Easterlies.
Two areas with lack of wind: Doldrums (near equator) and Horse Latitudes (near 30° latitude in each hemisphere)
**You should be able to identify all of the features (pressures, winds, fronts, jets, etc) regardless of what type of diagram I show you (in other words, on an exam you may see a diagram different than either one of those shown below and you are responsible for the identification of circulation features that you have now been introduced to).
Another site for further information is from NASA's Jet Propulsion Laboratory. Check it out.
At 0°: Equatorial Low, Intertropical Convergence Zone (ITCZ), Doldrums
30° N and S: Subtropical High, Horse Latitudes
60° N and S: Subpolar Low, Polar Front, Polar Front Jet Stream (or just Polar Jet Stream)
90° N and S: Polar High
Winds in between major pressure regions (so latitudes are approximate):
0°- 30° N = NE Tradewinds
0°- 30° S = SE Tradewinds
30° - 60° N and S = Westerlies
60° - 90° N and S = Polar Easterlies
Polar Front and Polar Jet Stream
The region of the polar front is a place of convergence between cold, polar air from the north with warm, tropical air from the south. Lifting occurs at the boundary leading to clouds and precipitation. The boundary is characterized by waves known as Rossby waves or longwaves (real life: subpolar low region is not a straight zone like the three cell model presents it (in the top picture), but rather, a wavy boundary like that shown in the bottom figure). Along this boundary with a large temperature difference, a strong horizontal pressure difference is created. This creates the Jet Stream, a zone of high velocity winds found high in the atmosphere. Winds can be around 200-250 mph.
The region of the polar front is a zone of always changing weather. It is the location where mid-latitude cyclones begin their life cycle. There are clouds and storminess as lifting is occuring at this boundary.
The other two assumptions from our 3-cell model:
Let's add the land back and allow the sun to migrate as it does over the course of the year.
The removal of the homogeneity assumption puts land back on earth. This causes the circulation belts to break up into cells. Pressures that persist over the entire year are called "semipermanent highs and lows".
The semi-permanent pressure systems (cells) around North America are:
- Azores-Bermuda High (centered over the Atlantic Ocean)
- Pacific High (also called the Hawaiian High) and centered over the Pacific Ocean
- Aleutian Low
- Icelantic Low
See pressure maps in either your textbook (Fig. 5-17) or the atlas to find these pressure cells.
There are also temperature generated areas of pressure - remember intense heating creates low pressure and intense cooling creates high pressure...
Siberian High - some of the highest pressures on earth occur in Asia during the long winters
Thermal Lows - largest is in SE Asia associated with the Asian Monsoon, but there is a thermal low generated each summer near the border of Arizona, Nevada and California.
Finally, allow the sun to migrate...
Once we allow the sun to move between the two tropics over the course of a year, we note that the pressure systems and wind belts shift to follow the sun (northward for the N.H. summer and southward for the S.H. summer). Basically, this includes the entire 3-cell model diagram - shift the entire picture to higher or lower latitudes depending on season.
As shown in the bottom 3-cell model diagram, the polar front and polar jet change position in a wavy fashion with their average position following the sun. As an example, the polar jet/polar front come as far south as our latitude (Phoenix) during the winter so you can see how much it can vary in position! You would notice these occurrences as the times during the winter when it gets 'cold' here in the southwest. The ITCZ, of course, also moves, see Fig. 5-30 in your textbook to see how much its position can change during the year. Notice that it doesn't move a uniform distance away from the equator - pay special attention to the type of surface over which it moves the greatest distance to higher latitudes.
Which side of the "HIGH"?
It matters which side of the "high" you are on as to the exact character of the weather and climate. Let's compare the east coast to the west coast of the U.S. since both sides are influenced by semi-permanent areas of high pressure. You would think given what you know about high pressure that both coasts should have similar weather that is clear and generally dry. Not so!
Locations on the west coast are situated on the eastern side of the "high". The cold water offshore helps stabilize the atmosphere and conditions are indeed dry. On the east coast of the U.S., the cities are now on the west side of the "high" with warm surface waters. These warm water temperatures tend to make the atmosphere unstable and increase the amount of water vapor in the atmosphere. Therefore, on this side of the U.S., weather is warmer, cloudier, and wetter.
Coincidence of Pressure Patterns/Wind Belts and Precipitation
Zone of maximum surface heating shifts to follow the sun. This results in the movement of pressure systems, wind belts and regions of precipitation and drought over the course of the year. Overall we can summarize the major rainy and dry regions, keeping in mind that they migrate north and south of their average location at various times during the year.
There are three major rainy regions...
- The polar front regions in each hemisphere with associated mid-latitude cyclones (low pressure)
- The Intertropical Convergence Zone near the equator. Here tradewinds converge at the equatorial low with convectional lifting also at this location. The result is daily production of rainshowers and thunderstorms.
There are four major dry regions...
- Subtropical highs in both hemispheres - air is descending, no clouds, no rain. Coincide with some of the world's great desert locations.
- Polar high regions in both hemispheres - extremely cold air, few clouds, little precipitation all year.
Thermal Circulations - circulations created by changes in air temperature
Land - Sea Breeze
Unequal heating between land and water along a coast create unequal pressures that cause pressure gradient force to move air from high to low pressure areas. During the day, the sea breeze develops and at night, these locations experience the land breeze. This type of circulation is also found near large lakes and is called the lake/land breeze.
Mountain - Valley Breeze
Differences in heating create pressure differences that create the mountain and valley breezes. Mountain breezes occur at night when air descends from the mountains. Valley breezes are created during the daytime in response to strong heating and air rising up toward higher terrain.
Chinook / Santa Ana Winds
Air descending on the leeward side of mountains warms by compression and creates hot, dry winds such as the Chinook and Santa Ana winds.
The Asian Monsoon
Monsoon, by definition, is a wind system that changes wind direction seasonally. The wind shift needs to be a minimum of 120 degrees. Note: We experience a monsoon circulation here in Arizona, but for all discussions here in the notes, in the book, and on exams, we will never be speaking about the Arizona Monsoon, just the Asian Monsoon.
During the winter, the large continent of Asia gets extremely cold and the Siberian high pressure develops. Air flow is offshore and dry. During the summer, the continent develops low pressure in response to heating and the airflow reverses. Moisture-laden air from the ocean is brought inland where it rises over the terrain and produces extremely large amounts of rainfall.
Dust Devils (Whirlwinds)
Rapidly circulating column of air that forms on clear, hot days. They usually form when wind is deflected by a small topographic barrier.
See an mpg movie of a dust devil shot by NASA