Weather hazards — transcript

00:00:00.600 --> 00:00:05.746 Weather hazards: inversions, fog and thunderstorms, and what they mean for you.

00:00:06.400 --> 00:00:10.856 Normally it's colder the higher you go. In an inversion,

00:00:10.856 --> 00:00:17.345 warm air sits on top of colder air, and the heavy cold air stays put. Very stable.

00:00:17.900 --> 00:00:20.990 The radiation inversion forms on clear nights.

00:00:20.990 --> 00:00:25.461 The ground loses heat to space and chills only a thin layer of air.

00:00:25.461 --> 00:00:27.696 Cloud or strong wind prevents it.

00:00:29.900 --> 00:00:33.427 Rising air stops at the inversion, trapping smoke and dust.

00:00:33.427 --> 00:00:37.482 Expect turbulence and poor visibility below, smooth clear air above.

00:00:37.482 --> 00:00:40.009 Frontal inversions trap haze the same way.

00:00:41.900 --> 00:00:47.205 Now fog. Cooler air holds less water vapour. At the dew point it's saturated,

00:00:47.205 --> 00:00:52.715 and fog is simply cloud at the surface, with visibility under a thousand metres.

00:00:54.400 --> 00:00:59.456 Radiation fog needs three things. Clear skies at night. Light winds,

00:00:59.456 --> 00:01:03.046 because dead calm gives only dew. And humid air,

00:01:03.046 --> 00:01:06.051 because dry air rarely gets cold enough.

00:01:06.401 --> 00:01:09.660 It forms between midnight and dawn. After sunrise,

00:01:09.660 --> 00:01:14.645 heating and mixing raise its temperature and it clears, often into low cloud.

00:01:14.645 --> 00:01:16.562 But at first, it can thicken.

00:01:18.901 --> 00:01:22.403 Watch slant visibility. Straight up, you see through a shallow layer.

00:01:22.403 --> 00:01:26.306 Send the drone to a distant waypoint and you're looking through far more fog.

00:01:26.306 --> 00:01:28.357 You need visual conditions at all times.

00:01:32.501 --> 00:01:35.243 Fog at dawn? Be patient. Wait for it to clear.

00:01:38.401 --> 00:01:42.606 Quick check. Which night is most likely to bring radiation fog?

00:01:43.401 --> 00:01:46.509 Clear skies, light winds, humid air.

00:01:46.901 --> 00:01:51.324 Thunderstorms and drones don't mix. A storm needs three ingredients:

00:01:51.324 --> 00:01:56.644 an unstable atmosphere, plenty of moisture, and a trigger to start the air rising.

00:01:57.401 --> 00:02:03.515 Stage one, developing. All the air moves upward. The cloud towers quickly,

00:02:03.515 --> 00:02:06.857 with sharp white edges, and no rain yet.

00:02:07.207 --> 00:02:11.767 Stage two, mature. Drops fall as rain and drag air down.

00:02:11.767 --> 00:02:17.847 Updrafts and downdrafts rush past each other: severe turbulence, lightning,

00:02:17.847 --> 00:02:20.007 heavy rain and often hail.

00:02:20.357 --> 00:02:26.035 Stage three, dissipating. The updrafts stop, the top streams out into an anvil,

00:02:26.035 --> 00:02:27.880 and the cloud evaporates.

00:02:28.357 --> 00:02:33.640 Near a storm, turbulence makes control almost impossible. Add squalls, gusts,

00:02:33.640 --> 00:02:36.688 wind shear, microbursts, hail and lightning.

00:02:36.688 --> 00:02:40.008 Low-level effects can reach ten kilometres away.

00:02:40.358 --> 00:02:44.668 Storms can build around you while you fly. If one develops nearby, land.

00:02:46.858 --> 00:02:51.908 So before you launch: inversion, expect haze and turbulence down low. Fog,

00:02:51.908 --> 00:02:54.198 wait it out. Storms nearby, land.

00:02:54.858 --> 00:02:58.985 Final check. A storm builds nearby mid-flight. What do you do?

00:02:59.658 --> 00:03:02.191 Land. Storm effects reach a long way.