Where wind comes from — transcript

00:00:00.600 --> 00:00:03.656 Where does wind come from? It starts with air pressure.

00:00:06.000 --> 00:00:11.096 Air pressure is the weight of the air above you, measured in hectopascals.

00:00:11.096 --> 00:00:16.192 Sinking air raises surface pressure and spreads out. Rising air lowers it,

00:00:16.192 --> 00:00:17.416 and air flows in.

00:00:20.000 --> 00:00:25.164 In the Southern Hemisphere, Earth's rotation deflects moving air to the left.

00:00:25.164 --> 00:00:29.666 This is the Coriolis effect. Air spins anticlockwise around a high,

00:00:29.666 --> 00:00:31.520 and clockwise around a low.

00:00:32.400 --> 00:00:38.905 Highs: stable, mostly clear, light winds. Lows: cloud, rain, stronger winds.

00:00:38.905 --> 00:00:42.875 The lower the pressure, the worse the weather.

00:00:43.225 --> 00:00:48.551 A synoptic chart maps it all. Isobars join places of equal pressure,

00:00:48.551 --> 00:00:53.491 usually four hectopascals apart. H marks a high, L marks a low.

00:00:55.225 --> 00:00:59.516 Air flows from high to low pressure, like a skier down a slope.

00:00:59.516 --> 00:01:03.137 Tight isobars mean a steep gradient and strong winds.

00:01:03.137 --> 00:01:05.282 Wide spacing means light winds.

00:01:07.225 --> 00:01:13.727 Ridge: a finger of high pressure, usually stable. Trough: a finger of low,

00:01:13.727 --> 00:01:20.835 often unsettled. Col: between two highs and two lows, with light, variable winds.

00:01:21.185 --> 00:01:26.644 Isobars packed tight over your site? Expect strong winds. Rethink the flight.

00:01:27.585 --> 00:01:32.548 Quick check. Isobars are close together over your site. What should you expect?

00:01:33.285 --> 00:01:36.211 Strong winds. Steep gradient.

00:01:36.561 --> 00:01:40.513 Above the friction layer, the gradient wind blows along the isobars.

00:01:40.513 --> 00:01:42.574 Near the ground, friction slows it,

00:01:42.574 --> 00:01:45.782 so the surface wind crosses the isobars toward the low.

00:01:48.561 --> 00:01:53.870 Turn right to keep facing the wind? It has veered. Turn left? It has backed.

00:01:53.870 --> 00:01:57.939 The surface wind has always veered from the gradient wind.

00:02:00.061 --> 00:02:04.364 Wind is reported as the direction it comes from, in degrees true,

00:02:04.364 --> 00:02:08.864 then speed in knots. Two-three-zero at fifteen: from the south-west.

00:02:11.561 --> 00:02:17.623 By day, land heats much faster than the sea. Warm air rises over the land,

00:02:17.623 --> 00:02:23.525 and cool sea air flows in: the sea breeze. It's strongest mid-afternoon.

00:02:23.875 --> 00:02:26.663 At night it reverses. The land cools faster,

00:02:26.663 --> 00:02:30.876 and a gentle land breeze blows out to sea, strongest early morning.

00:02:32.875 --> 00:02:36.950 Calm at the beach this morning? Check the windsock again this afternoon.

00:02:39.375 --> 00:02:44.154 Watch for other local winds: katabatic flow downhill at night,

00:02:44.154 --> 00:02:48.705 anabatic flow up sunny slopes, channelling through valleys,

00:02:48.705 --> 00:02:51.208 and turbulence behind obstacles.

00:02:51.558 --> 00:02:56.260 So: pressure drives the wind, tight isobars mean strong winds,

00:02:56.260 --> 00:02:59.395 and local effects can override the chart.

00:02:59.745 --> 00:03:05.361 Final check. It's mid-afternoon on a sunny coast. Which way is the breeze blowing?

00:03:05.745 --> 00:03:07.547 Onshore. A sea breeze.