The dominant cause behind movement of air in near horizontal conditions is the pressure gradient. As seen in this diagram, if a high is next to a low and the rate of pressure drop, as indicated by the spacing of isobars (lines of equal pressure) of different values is fairly wide, the pressure gradient is small so that the wind moving towards the low moves more slowly than when the isobars are close-spaced (higher gradient and faster wind flow).
Lets switch to the last idea in the first paragraph: The Earth is spinning on its rotational axis at a rate approximating 1700 kilometers per hour (1062 mph. The speed diminishes poleward, going to just above zero immediately beyond the point where the axis can be imagined to emerge at the surface. This is indicated in the following diagram.
But because of the rotation, the air moves to the right of its straight path as it moves equatorward. Think of it this way: As free flowing air just above the surface moves south over a short (finite) time span, the Earth underneath is moving counterclockwise; points to the west of the intended path, if non-rotational, move eastward as the motion progresses; at lower latitudes points further and further west move to meet the intended path; tracing this out over time yields a curving line that has an apparent deflection pathway to the right (this has an analogy of wind in a baseball stadium causing a fly ball to curve in response).
Consider this diagram (about which more will be introduced later on this page). Blue arrows denote a possible straight line path. Red arrows indicate the Coriolus-induced deflections.These are to the right in the northern hemisphere and to the left in the southern hemisphere.
The reason for the reversal - right or left - is just the consequence of motion direction in the two hemispheres. This diagram explains that statement. Those in the southern hemisphere are in a sense upside down relative to those in the north so the perception of motions is reversed (the Moon is upside down when viewed in this hemisphere, as was learned for the first time by the writer during a visit to New Zealand).
The next two diagrams further elucidate the Coriolis idea. In the top diagram, two forces are assumed to be acting on moving wind, a pressure gradient force (PGF) moving from higher to lower pressure states, and the Coriolis force (CF) in the northern hemisphere. The resultant curving path is shown in green.
Lets summarize this concept for both hemispheres:
We'll consider at this point another idea depending on the Coriolis force. The Geostrophic Wind is a special case in which rising air reaches a condition of balanced forces such that the wind flow becomes parallel to an isobar - a line representing the lateral extent of air having the same pressure (analogous to a contour line on a topographic map [recall Section 10]). The first diagram is a simplified map of the forces involved relative to an isobar; the second diagram shows the changes in wind direction with height until the geostrophic condition is met. Geostrophic wind are real and do happen but most of the time winds are not parallel to isobars.
Near the Earth's surface, friction of moving wind accross terrain or open water becomes a factor. Here the resultant wind direction is a vectorial sum of the PGF, CF, and FF (frictional force). At some altitude, where friction is nil, in this case a geostrophic wind has evolved.
One cause of change in wind speed is that of unbalanced forces. In the diagram below, the PGF is strong enough to counter-influence the CF and FF such that the net wind experiences an acceleration. (In a storm wind changes directions and speeds frequently in different places and times owing to local variations in pressure gradients and friction in part because of obstructions and surface topographic fluctuations.)
Assuming the ideal case in which isobars are closed and circular, the patterns of wind flow around highs and lows appear in this diagram such as to cross a contour in near surface condition (where friction is effective) and to parallel the contour at some elevation owing to the geostrophic effect:
In the diagram, the terms cyclone and anticyclone are introduced. For a cyclone, the winds circulate counterclockwise around a low. the air is warm at the surface so it rises in a column such that its winds spiral upward and cool adiabatically. The cyclone is associated with rain-making conditions. An anticyclone is developed where cold air aloft, being heavier and having a higher pressure, descends in spiraling motions to reach the surface as a pressure high.
The next diagram is an extension of this idea, establish a connection at higher altitudes between the low and high air masses that are adjacent. Around the surface low, air converges into the lower pressure zones. The rising air reaches some altitude(s) at which the air must then spill outwards as a divergence toward the upper region of the nearby high, where the flow is converging. That air, after moving downward will spread out (diverge) at the surface high locations.
On any given day in a region, such as North America or, more restricted, the United States, surface highs (H) and lows (L) will have developed from differential heating and other cause in various areas of the land mass. Weather forecast maps (treated below) can just show the general areas where these occur, as indicated by positions the H or L's in the center of each mass. This is shown in the next figure for Tuesday, November 11, 2003, the day in which this page was written. Sometimes these weather maps (which may show major isobars at some specified altitude) are depicted such that the precipitation conditions are shown in a general way, either by color-code patches or by contours outlining the extent of the predicted or observed precipitation.
