Our Dynamic Weather
Some Important Points
about Winds and Pressure
|Dr. Dave Dempsey
Dept. of Geosciences
SFSU, Fall 2012
This handout poses and tries to answer the following questions:
Terms and concepts that come up in this handout include the following:
(1) Wind is air in motion. What creates wind? That is, how does air get moving?
A basic law of physics tells us that, for the motion of any object or substance to change, a net force must push or pull on it. (A force is any push or pull that can change the motion of an object.) This law applies to a mixture of gases such as air, too.
According to this principle, for wind to blow, some force must push air into motion.
This principle, like the principle of conservation of energy, is one of the handful of basic physical principles on which modern weather forecasting is based. The idea is that, if we know what the winds are at a particular moment today (based on observations), and if we know the forces that push and pull on air to change it's motion, then we can (at least in principle) predict what the winds will be in the near future.
So, what force or forces create wind?
(2) This is where pressure comes into the weather picture. What is pressure?
Any substance (gas, liquid, or solid) consists of molecules, tiny bits of matter that are always in random motion. (Recall that the total energy of those motions is what we call heat, and temperature is a measure of the average speed of those random motions.) As a result of these random motions, molecules of a substance constantly bump into neighboring molecules. Each such collision exerts a tiny force, a push, on a neighboring molecule.
Imagine a small blob or "parcel" of air. The parcel's molecules, moving randomly, will collide with molecules of whatever is in contact with it (that is, it's "surroundings"), which will be either other air parcels or the earth's surface. The collective force exerted by a parcel's molecules colliding with the parcel's surroundings, is called air pressure.
It help visualize this, see The Ideal Gas Law (a downloadable Java Applet, at http://www.colorado.edu/physics/phet/projects2/idealgas/idealgas.jnlp).
Air pressure can vary from place to place. For example, if the air temperature is different at a different place, the molecules of an air parcel there will be moving at a different average speed, so they will collide harder, or less hard, with their surroundings. Moreover, the parcel's molecules might be packed closer together or spread farther apart (that is, air might have higher or lower density). If so, then the number of molecular collisions with the parcel's surroundings (and the collective force that they exert) will be different. That is, air pressure can vary from place to place because air temperature and air density can vary from place to place.
(3) What units do we use to measure and express pressure?
Common units of pressure include:
Professional meteorologists usually use millibars (mb).
(4) What values of atmospheric pressure do we typically observe?
The long-term, global average pressure at sea level is 1013.25 mb.
However, pressure decreases rapidly as you go to higher altitudes. At the top of the troposphere (the tropopause), which averages around 11 kilometers (about 7 miles) above sea level, the pressure averages only about 200 mb (about 20% of the average sea-level value).
In contrast, pressure varies much less from place to place horizontally. Here's an example of sea-level pressure observations, recorded at 00Z on October 19, 2012, which are typical:
Sea-level pressures expressed in millibars typically range from the high 900s to the mid-1000s, which means that they don't usually differ from the average value (1013.25 mb) by much more than around ±3%. [However, the all-time record low sea-level pressure is 870 mb (in the center of a strong hurricane). That's 14% below the average. The record high pressure is about 1085 mb (in Mongolia in the winter of 2001), which is about 7% above the average.]
(5) What does atmospheric pressure have to do with wind?
For our purposes let's consider only horizontal air motions. Horizontal wind speed and direction is what weather stations observe and report.
Image a parcel of air in the atmosphere. It is surrounded by air in all horizontal directions. The surrounding air exerts pressure on the parcel on all sides. This means that the parcel gets pushed in all (horizontal) directions at once. These pressure forces don't necessarily cancel each other out, though. There will be a net force on the parcel due to pressure if the pressure is not the same on all sides. We know that pressure can differ from one place to another, so this is quite possible.
If pressure differs between one side of an air parcel and the other, then the parcel will get pushed harder on one side of the parcel than the other, producing a net force. The force exerted on the higher-pressure side will overcome the weaker force pushing in the opposite direction on the lower-pressure side, producing a net force from the higher pressure side toward the lower pressure side.
The net force exerted on air parcels due to pressure differences between places can push air into motion. The net force is in the direction from higher toward lower pressure.
(6) What determines the strength of the force on air due to pressure differences?
The bigger the difference in pressure between one side of an air parcel to the other, the greater the net force will be on the parcel.
How can we tell how large the pressure difference might be between opposite sides of an air parcel? The horizontal pressure gradient can tell us a lot about that. The horizontal pressure gradient is defined as the difference in pressure across each unit of distance (horizontally). It is a measure of how rapidly the pressure varies from place to place horizontally.
The greater the pressure gradient is, the larger the pressure difference will be from one side of an air parcel to another, and so the greater the net force will be due to pressure differences. For this reason, the net force on air due to pressure differences between places is called the pressure-gradient force (PGF).
To rephrase and restate the points made above, the pressure-gradient force (PGF) pushes on an air parcel from the higher pressure side toward the lower pressure side. The larger the pressure gradient is, the greater the PGF will be.
(7) How can we visualize the connections between pressure patterns and winds on weather maps?
To understand wind patterns, we must see patterns of pressure-gradient force (PGF). To see patterns of PGF we must see patterns of pressure.
At the earth's surface, a world-wide network of surface weather stations, ships, and buoys report pressure every 1 to 6 hours. These observations are extrapolated to sea level and plotted on weather maps like the one in (4) above. However, it's hard to see the patterns of pressure by looking at the numbers alone. Hence, to help us see patterns of sea-level pressure more easily, we draw lines (contour lines) of constant pressure on the map, called isobars.
By definition, an isobar connects all points on the map where the pressures have the same value. Different isobars are drawn corresponding to different pressures, at regular intervals of pressure (the contour interval). On sea-level pressure maps, meteorologists usually draw isobars (lines of constant pressure) at intervals of 4 millibars, with particular values of ..., 984, 988, 992, 996, 1000, 1004, 1008, 1012 millibars, etc.
(8) How should we interpret isobars on a pressure map?
Isobars have several properties that help us draw and interpret them. For example:
Isobars help us see spatial patterns of pressure. For one thing, we can see immediately that pressure doesn't vary along an isobar but does vary across it.
To help us see other aspects of the pressure pattern, we can add labels and symbols to the map. For example, places where the pressure is higher or lower than anywhere else nearby (local pressure maxima or minima, respectively) are usually at the center of "bull's-eye" isobar patterns (concentric closed loops) and are labeled with an "H" or "L", respectively.
(9) How can we interpret the pressure-gradient force on a pressure map?
The pressure gradient is larger where isobars are closer together. In a direction along an isobars the pressure gradient is zero. The pressure varies most rapidly in directions perpendicular to isobars, so at any point on a weather map, the pressure gradient force (PGF) pushes in a direction perpendicular to isobars, from the higher-pressure side toward the lower-pressure side of the isobars.