Our Dynamic Weather
Some Important Points
about Winds and Pressure
|Dr. Dave Dempsey
Dept. of Geosciences
SFSU, Spring 2011
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, the principle of conservation of momentum (also known as Newton's 2nd Law*), 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. Moreover, once air is moving, it's motion won't change unless some (net) force acts on it.
This principle, like the principle of conservation of energy, is one of the fundamental 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 or sensible 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 on a neighboring molecule.
Imagine an object, or a blob or "parcel" of a substance such as air or water (say), and focus on a unit of area (say, 1 square inch [1 in2] or 1 square centimeter [1 cm2]) of the object or parcel's surface. The object or parcel's molecules, moving randomly, will collide with molecules of whatever substance or object is in contact with it on that surface (that is, with the object or parcel's "surroundings").
Now add up the total force exerted by the object or parcel's molecules colliding with the molecules of the surroundings on the unit of surface area. Pressure is the collective force exerted by random molecular collisions on a unit of surface area of an object or substance's surroundings. (As a result, we express pressure in terms of force per unit area.)
It help visualize this, see The Ideal Gas Law (a downloadable Java Applet, at http://www.colorado.edu/physics/phet/projects2/idealgas/idealgas.jnlp).
Because molecular motions are in random directions, at any particular place the pressure that an object or substance exerts on its surroundings is the same in all directions.
However, at a different place the molecules might be moving faster or slower on the average (that is, the substance might have a different temperature), in which case the molecular collisions would exert more or less force on surrounding molecules. In addition, in a parcel of gas such as air at a different location, the molecules might be packed closer together or spread farther apart (that is, air might have higher or lower density), in which case there will be more or fewer molecules colliding with a unit of surface area of the parcel's surroundings and hence more or less collective force exerted against that unit of surface area. That is, air pressure can vary from place to place because the temperature and density of air can differ from place to place. This relationship between air pressure, temperature, and density is called the ideal gas law. It's a third fundamental physical principle on which modern weather forecasting is based.
(3) How do we measure pressure?
Any instrument that measures atmospheric pressure is called a barometer. There are several types of barometers. One very common type is the aneroid barometer. ("Aneroid" means "without air".) It consists of a sealed container with very little air in it. One wall of the container can move in and out like a piston and is attached on the inside to a spring. The spring exerts a force against the the movable container wall, pushing on it outward. (The relative absence of air inside the container means that the spring exerts virtually the only force because there is very little air pressure inside.) Air on the outside exerts a force due to pressure on the outside of the wall. The greater the air pressure on the outside, the greater the force on the outside wall will be, pushing the wall in and compressing the spring. The more compressed the spring becomes, the harder it pushes back, until the two forces equal each other. At that point there is no net force and the wall won't moving. The spring is attached to a gauge that indicates how great the outside atmospheric pressure is, based on how compressed the spring is.
(4) What units do we use to measure and express pressure?
Common units of pressure include:
Professional meteorologists usually use millibars (mb) or hectopascals (hPa), which are equivalent.
(5) 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 with increasing altitude. From sea level, pressure typically drops at a rate of about 1 millibar every 8 meters (about 26.4 feet), though at higher and higher altitudes it decreases less and less rapidly. (See a plot of pressure vs. altitude.) 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 at 06Z on April 15, 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 typhoon [another name for a hurricane], Typhoon Tip, in the western Pacific Ocean in 1979. 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.
(6) 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 on all sides (that is, 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 due to surrounding pressure. However, the principle of conservation of momentum says that it takes a net force to change the motion of an object. There will be a net force on the parcel due to pressure only 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.
(7) What determines the strength of the force on air due to pressure differences?
The more rapidly that pressure varies from one side of an air parcel to the other, the larger the pressure difference will be and the greater the net force will be on the parcel.
One measure of how rapidly pressure varies from place to place horizontally is the horizontal pressure gradient. The horizontal pressure gradient is defined as the difference in pressure across each unit of distance (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 air parcels from the higher pressure side toward the lower pressure side. The larger the pressure gradient is, the greater the PGF will be.
(8) 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 hour, every 3 hours, or every 6 hours. These observations are extrapolated to sea level and plotted on weather maps like the one in (5) 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.
(9) 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.
As another example, isobars encircling a high or low pressure area aren't usually perfect circles. Sometimes they elongate away from the center and curve more sharply at their outermost point. A concentric series of such elongated isobars defines a ridge (extending out from a high pressure center) or trough (extending out away from a low pressure center). We mark ridges using a jagged line connecting the most sharply curved points of isobars defining the ridge, and we mark troughs using a dashed line connecting the most sharply curved points of isobars defining a trough.
(10) How can we interpret the pressure-gradient force on a pressure map?
The sea-level pressure gradient is larger where isobars of sea-level pressure are closer together (because isobars crowd close together where the pressure varies rapidly with distance). Along 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.
*Newton's 2nd Law of Motion is named after Isaac Newton, an Englishman who figured it out in the late 1600s by making many very careful observations of the way that objects behave when forces push or pull on them.