Lesson cancelled today due to a high cross wind, almost at a right angle to the run way. Rather than dwell on the fact that I now seem to have complete control of the wind and rain (but cannot tame the sun!!). Time to look back at the cause of today’s cancellation…… The Wind.
Air Pressure
First thing to remember when looking at a map plotted with measurements of pressure (e.g. 1010mb), is that you’re actually looking at a plot of ‘elevation corrected‘ pressures – not the actual measured pressures. This is because if you didn’t correct all the measurements taken across a large chunk of land, all you’d have a is a map of pressure with respect to elevation. This will tell you nothing about wind, it would just be telling you that the high point had low pressure and the lower point had a higher pressure….. Which is obvious because pressure decreases with altitude.
The correction is roughly 1mb per 27ft above mean sea level of the location, though is often rounded to 1mb = 30ft.
If lots of points are taken at the same time across a large area (i.e. a country), and the measurements are elevation corrected to put them all at mean sea level. Then you’ve effectively got a map of pressures which you can then use to see how steep the pressure gradients are, which will tell you about the wind.
Isobars
With all the pressure measurements elevation corrected and plotted on a map. Lines can be drawn to connect points of equal sea level pressure. These lines are isobars.
Lines close together indicate: A steep gradient in pressure over a short distance.
Lines far apart indicate: A slack pressure gradient.
Pressure Gradients
Air flows down a pressure gradient, from high pressure to low pressure.
The steeper the pressure gradient, the faster the air flows…. The greater the wind!
As the proximity of the isobars dictate the steepness of the pressure gradients: The closer the isobars are together, the greater the wind will be.
Coriolis Force
Coriolis force is caused by the earths rotation, instead of traveling in a straight line along the pressure gradient line – Coriolis force causes the wind to be deflected and flow in an arc.
The force is strongest at the poles (north/south) and acts at 90 degrees to the Right of the moving air particle in the Northern hemisphere. While it acts at 90 degrees to the Left in the Southern hemisphere.
Because air will flow from high pressure to low pressure and the Coriolis force can act at a different angle to that flow. It is feasible for the air flow caused by the pressure gradient and the Coriolis force to cancel each other out.
Wind Measurements
Finally wind is always measured as a Velocity.
This means it must always be expressed as having two components:
- Direction
- Speed
There’s lots more to wind: Local Winds, Wind Velocity with altitude, ground obstacles & turbulence etc. etc. All for another post….