{"id":188,"date":"2011-09-11T18:51:03","date_gmt":"2011-09-11T17:51:03","guid":{"rendered":"http:\/\/www.fortyfivehours.co.uk\/?p=188"},"modified":"2011-09-11T18:53:45","modified_gmt":"2011-09-11T17:53:45","slug":"wind-pressure-and-coriolis-force","status":"publish","type":"post","link":"http:\/\/www.fortyfivehours.co.uk\/?p=188","title":{"rendered":"Wind, Pressure and Coriolis Force"},"content":{"rendered":"

Lesson cancelled today due to a high cross wind, almost at a right angle to the run way.\u00a0\u00a0 Rather than dwell on the fact that I now seem to have complete control of the wind and rain (but cannot tame the sun!!).\u00a0\u00a0 Time to look back at the cause of today’s cancellation…… The Wind<\/strong>.<\/p>\n

Air Pressure<\/strong><\/span><\/p>\n

First thing to remember when looking at a map plotted with measurements of pressure (e.g. 1010mb),\u00a0 is that you’re actually\u00a0 looking at a plot of ‘elevation corrected<\/span>‘ pressures – not the actual measured pressures.\u00a0\u00a0 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<\/em>.\u00a0\u00a0\u00a0 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…..\u00a0 Which is obvious because pressure decreases with altitude.<\/p>\n

The correction is roughly 1mb per 27ft above mean sea level of the location, though is often rounded to 1mb = 30ft.<\/em><\/p>\n

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.\u00a0\u00a0 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.<\/p>\n

Isobars<\/strong><\/span><\/p>\n

With all the pressure measurements elevation corrected and plotted on a map.\u00a0 Lines can be drawn to connect points of equal sea level pressure<\/span>.\u00a0\u00a0 These lines are isobars.<\/p>\n

Lines close together indicate:\u00a0 A steep gradient in pressure over a short distance<\/span>.<\/p>\n

Lines far apart indicate: A slack pressure gradient. <\/span><\/p>\n

Pressure Gradients<\/strong><\/span><\/p>\n

Air flows down a pressure gradient, from high pressure to low pressure.<\/p>\n

The steeper the pressure gradient, the faster the air flows….\u00a0 The greater the wind!<\/em><\/p><\/blockquote>\n

As the proximity of the isobars dictate the steepness of the pressure gradients: \u00a0 The closer the isobars are together, the greater the wind will be.<\/p>\n

Coriolis Force<\/strong><\/span><\/p>\n

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.<\/p>\n

The force is strongest at the poles (north\/south) and acts at 90 degrees to the Right<\/em> of the moving air particle in the Northern<\/em> hemisphere.\u00a0\u00a0 While it acts at 90 degrees to the Left<\/em> in the Southern<\/em> hemisphere.<\/p>\n

Because air will flow from high pressure to low pressure and the Coriolis force can act at a different angle to that flow.\u00a0\u00a0 It is feasible for the air flow caused by the pressure gradient and the Coriolis force to cancel each other out.<\/p>\n

Wind Measurements<\/strong><\/p>\n

Finally wind is always measured as a Velocity<\/span><\/a>.<\/p>\n

This means it must always be expressed as having two components:<\/p>\n

    \n
  1. Direction<\/li>\n
  2. Speed<\/li>\n<\/ol>\n

    There’s lots more to wind:\u00a0\u00a0 Local Winds, Wind Velocity with altitude, ground obstacles & turbulence etc. etc.\u00a0\u00a0 All for another post….<\/em><\/p>\n","protected":false},"excerpt":{"rendered":"

    Lesson cancelled today due to a high cross wind, almost at a right angle to the run way.\u00a0\u00a0 Rather than dwell on the fact that I now seem to have complete control of the wind and rain (but cannot tame the sun!!).\u00a0\u00a0 Time to look back at the cause of today’s cancellation…… The Wind. Air […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":[],"categories":[12],"tags":[],"_links":{"self":[{"href":"http:\/\/www.fortyfivehours.co.uk\/index.php?rest_route=\/wp\/v2\/posts\/188"}],"collection":[{"href":"http:\/\/www.fortyfivehours.co.uk\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/www.fortyfivehours.co.uk\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/www.fortyfivehours.co.uk\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/www.fortyfivehours.co.uk\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=188"}],"version-history":[{"count":13,"href":"http:\/\/www.fortyfivehours.co.uk\/index.php?rest_route=\/wp\/v2\/posts\/188\/revisions"}],"predecessor-version":[{"id":200,"href":"http:\/\/www.fortyfivehours.co.uk\/index.php?rest_route=\/wp\/v2\/posts\/188\/revisions\/200"}],"wp:attachment":[{"href":"http:\/\/www.fortyfivehours.co.uk\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=188"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/www.fortyfivehours.co.uk\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=188"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/www.fortyfivehours.co.uk\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=188"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}