February 1, 2008
Briefly review how sea level pressure is plotted on weather maps. Determining pressure gradients on sea level maps.
Pressure patterns on upper air maps
n On upper air maps, the contour lines are the heights above sea level of the specified pressure surface (e.g., 500 mb maps. There are also 850 mb, 700 mb, 300 mb, 250 mb, etc. maps, which show the height of these pressure levels).
n These height contours on a constant pressure surface are equivalent to pressure contours (isobars) on a horizontal surface. In other words, we can use the height patterns on upper air maps to visualize the pattern of horizontal pressure gradients and therefore to estimate the wind patterns on the corresponding pressure surface.
o This is best shown using the figures on page 150 textbook. These figures are linked under the lecture summaries for today.
o Look also at the current 500 mb height pattern
Up to this point, we have said that winds are caused by horizontal pressure gradients, and the stronger the pressure gradient, the stronger the pressure gradient force, and the stronger the winds. We have also looked at how to visualize the strength of horizontal pressure gradients by examining both surface and upper air weather maps. We will now consider several other forces that act on the air to determine the actual wind direction based on horizontal pressure patterns.
Before doing this, we will first describe, then apply
Newton’s Laws of Motion
n 1st law: An object at rest will remain at rest (or an object in motion will continue to move in a straight line at constant speed) as long as no net force is exerted on it.
o You can think of a force as something that pushes or pulls on an object. Net force is the sum of all individual forces acting on the object. Zero net force does not mean no forces are acting, only that the sum of all forces is zero. For example, we are forced downward by Earth’s gravity, but the ground surface provides an equal force upward, so net force is zero and we remain stationary.
n 2nd law: An object will always accelerate in the direction of the net force acting on it. The strength of the acceleration is related to the net force by the equation: (Net force) = (Mass of object) x (Acceleration of object) or more familiarly F = ma.
o Acceleration is the rate of change of velocity. An acceleration can be a change in speed or a change in the direction of motion or both.
n A few examples
o An object moving at 1000 mph in a straight line is not accelerating. No net force
o An object moving at 1000 mph is a circle is accelerating because the direction of motion is always changing. Thus a net force is acting on the object.
o An object that is slowing down is accelerating. Net force is opposite to the direction of motion.
Forces that influence wind / Wind direction on upper
level maps
n Pressure Gradient Force (PGF)
o We have already described this force. A pressure gradient is the change in air pressure divided by the corresponding change in distance. The pressure gradient force is directed from higher toward lower pressure.
o The PGF is the force that causes wind to blow. The stronger the pressure gradient the stronger the wind. If there is no pressure gradient there is no wind.
o We have already seen how to visualize pressure gradients in the atmosphere using weather charts.
n Coriolis Force (CF) of Coriolis Effect
o An apparent force required to explain the motion of free-moving objects (those not attached to the Earth’s surface) as observed from the surface of the Earth, which is rotating.
o The
details of the CF are difficult to understand, so we will try to keep it
simple. The bottom line for determining
wind direction in the middle latitudes of the Northern Hemisphere (where the
n
Show video clip of ball rolling on a turning
merry-go-round and explain in terms of
n A few other notes about the CF:
o The CF acts at right angles to the wind direction, only influencing the wind direction, but not the wind speed.
o The CF is zero for stationary air, and increases in magnitude as the speed of the wind increases.
o The CF is zero at the Equator and gets larger and larger as one moves toward the north and south poles, where it is largest.
o The CF is negligible for small scale winds, like local sea breezes or local mountain breezes. It is only important at large scales, like the pressure gradients that we visualize on surface and upper air maps.
§ Contrary to popular belief, the CF has no influence on the way water swirls down a sink drain (scale of motion too small). However, it has a significant effect on large scale ocean currents.
Apply PGF and CF to
explain straight wind flow on upper air charts (geostrophic
wind)
n Geostrophic winds occur at upper levels (not at the surface) only when the height contours are parallel straight lines.
n Geostrophic winds move in a straight line parallel to the height contours at constant speed
o Because the motion follows a straight line at constant speed, the net force acting on the air under the conditions of geostrophic flow is zero. There is an exact balance between the PGF and the CF.
n Show Figure 6.16 from textbook. This may help you to understand why geostrophic winds develop when contours are parallel and evenly spaced. (One thing that I do not like about the figure is that it shows upper air isobars rather than height contours of a given pressure surface, which is how upper air weather maps are drawn. However, the principle is the same … we can use height contours to estimate pressure gradients.)
o Keep in mind that steps 1-5 on the diagram are only for illustration. We would never observe those steps. We would only be able to measure position 5, the geostrophic wind.
Apply PGF, CF, and
n Gradient winds consider the more general case of upper level air flow when the height contours are curved.
n Gradient winds move parallel to the height contours
o Because the motion follows a curved path, there must be a net force acting on the air, since the direction of motion is changing. In this case the PGF and CF are not in balance. For curved flow, the net force is inward toward the center of curvature. This inward force is called the centripetal force.
n Show Figure 6.18 from textbook. This shows the forces that act on the air as it flows around closed lows and closed highs on upper air maps. (Again the figure shows upper air isobars rather than height contours).
o Notice that for either a close upper high or a closed upper low, the net force must be directed inward in order for the air to curve.
o The same thing must happen when air flows through a trough or a ridge. It follows a curved path, so the net force must be directed in toward the center of curvature.
§ Draw a simple picture of this during lecture.
End Quiz #1 Material
Frictional Force / Wind direction on surface maps
n Frictional Force (FF)
o Air
moving along the ground is slowed by friction.
Thus, the frictional force always acts in the direction opposite to the
wind flow.
Apply CF, PGF, and FF
to explain wind direction on surface maps
n
Draw a
diagram with parallel isobars to show surface wind direction
o FF
acts opposite to the wind direction, slowing it
o Slower
wind results in a weaker CF
o The
weaker CF is not able to balance the PGF as it does on upper air maps
o The result:
surface winds do not blow parallel to the isobars, but are directed
slightly across the isobars toward lower pressure.
§
Over land areas, a good rule of thumb is that
the wind direction makes an angle of about 30° with the isobars. This angle is generally smaller over the
oceans because the frictional drag (and FF) is less over water.
n
Draw
diagrams showing the air flow around closed surface high and surface low
pressure areas