Reading the wind and the sky

What you will learn

• Read wind direction (the bearing the wind blows from) and wind speed, and explain the difference between average wind speed and gusts.
• Explain how flying upwind or downwind changes the drone's speed over the ground (ground speed) and the time a trip takes, and what happens when the wind is more than the drone can handle.
• Explain how rain, lightning, fog and temperature affect the drone, the battery, visibility and the pilot's safety.
• Use wind and sky information to decide, with reasons, whether to fly, wait or call it off.

Reading the wind

Where does the wind blow "from"?

You can't see the wind, but for a small drone it is one of the main things that can upset a flight. You read the wind by three things: wind direction (where it blows from), wind speed (how strong it is) and gusts (sudden stronger wind).

Wind direction is the bearing the wind blows from, measured clockwise with north as 0°. (Weather and aviation reports often write north as 360°, and 0 can mean no wind.) A west wind, blowing from the west, is 270°. Reading "270°" as "wind heading west" gets it backward.

How a drone moves in the wind

A drone moves through the air at its airspeed (its speed relative to the air). Heading upwind, the wind pushes it back, and its speed as seen from the ground (ground speed) drops. Heading downwind, the wind pushes it along and it goes faster.

Try it yourself: You can try this scene yourself inside the lesson.

Exercises in this part (solved inside the lesson)

• Fly the gates and land on a windy day

Wind limits and the effects of weather

When the wind is stronger than the drone

Every drone has a limit to how hard it can push against the wind. While the wind is light, it can still make progress upwind, but the closer the wind gets to that limit, the slower it goes upwind.

Once the wind exceeds that limit, the drone can't make progress even with the stick pushed upwind, and when you let go it gets blown downwind. GPS mode is no different: it can hold position only because it keeps producing thrust against the wind.

How rain, lightning, fog and temperature affect a flight

A drone is a collection of precision electronics. Besides wind, rain, lightning, fog and temperature affect a flight. Here is what each one does, and how to prepare.

• Rain: Water getting into the electronics can cause failures, and the camera lens gets wet too. Use rain radar to anticipate when rain will start, and land before it does. Even with a drone advertised as water-resistant, stay within the conditions in the manual.
• Lightning: A cumulonimbus cloud, which builds high into the sky, can bring lightning, sudden strong winds, short bursts of heavy rain and hail. If you hear thunder, land immediately and take shelter.
• Fog and dusk: The distance you can see (visibility) shrinks, and you lose sight of the drone's position and heading. Fly only within what you can see, and land before you lose sight of it.
• Cold: Battery voltage tends to drop, and usable capacity shrinks. Keep the battery warm until just before use, and after takeoff, hover first and check the voltage.
• Heat: The drone and battery heat up, and the thinner air reduces your thrust margin. Keep them out of direct sun and let them rest between uses.

Exercises in this part (solved inside the lesson)

Reading the sky and deciding

Reading changes in the sky

Weather information isn't something you check once and forget. Update it again and again, the day before, on the morning of the flight, on site and in flight, from before takeoff right through the flight.

• Forecasts (the day before and the morning of): Tell you the expected wind, rain and lightning. They can be wrong, so check again on the morning of the flight.
• Rain radar: Shows where rain clouds are now and which way they are moving. Some include a short-term prediction, but its accuracy is limited.
• On-site observation: Wind direction, wind speed and the state of the sky. Use flags, swaying trees and an anemometer to check the wind here and now. A forecast can't tell you about "here and now."
• Watching during the flight: Look out for sudden changes in the wind, cold wind, distant thunder and dark clouds spreading.

Fly, wait or call it off

The decision belongs to the pilot in command (PIC). There are three possible answers. When in doubt, lean toward waiting or calling it off, not toward flying.

1. Fly: When the criteria are met and you have margin in both wind and battery. Keep reviewing the criteria while you fly.
2. Wait: When the criteria aren't met, or you don't have enough information. There are things you can do while you wait: keep observing, shift the time, shorten the plan, and so on.
3. Call it off: When the criteria aren't met, or the situation remains unclear. Explain the reason to the client and move it to another day.

Set your criteria in advance, before you have to decide

• If gusts reach the drone's wind resistance rating, don't fly.
• If you hear thunder, land immediately. If dark clouds are approaching, cut the plan short.
• If visibility drops and the drone's position and heading become hard to see, land.
• Don't use a battery as is when it is cold or hot.

Exercises in this part (solved inside the lesson)

• Riverbank at dusk, clouds approaching

Summary

• Wind direction is the bearing the wind blows from. A west wind is 270°; it is not where the wind is heading.
• Look at the gusts, not just the average. Use the wind resistance rating with a margin that accounts for gusts.
• You move slowly upwind and fast downwind. When the wind is more than the drone can handle, it gets blown away, even in GPS mode.
• Rain, lightning, fog, cold and heat each call for their own preparation. When in doubt, lean toward waiting or calling it off, and set your decision criteria before you fly.

In Lv2 you split the wind into headwind and crosswind components, estimate with numbers how air density and height change the wind, and read METAR, TAF and Japan Meteorological Agency (JMA) data to make decisions.

Terms in this lesson

wind direction

The bearing the wind blows from, measured clockwise in degrees with north as 0° (reports often write north as 360°). A west wind is 270°; the number is not where the wind is going.

average wind speed

The wind speed averaged over a set period (often about 10 minutes in weather observations). Gusts that are strong only briefly get hidden in this average.

gust

Wind that suddenly blows harder than the average for a short time, a few seconds or so. Its direction often shifts suddenly too, and it upsets the drone's attitude and position.

upwind

The side the wind blows from. Flying upwind, the wind pushes you back, so you make slower progress.

downwind

The side the wind blows toward. You travel fast flying downwind, but coming back upwind costs extra time and battery.

airspeed

The drone's speed relative to the air. In a headwind it is larger than the speed seen from the ground (ground speed).

ground speed

The speed at which the drone moves over the ground. With the wind dead ahead or dead behind, subtract the wind speed from the airspeed (headwind) or add it (tailwind). A crosswind is split into components.

wind resistance

A guide to how strong a wind the drone can fly in while still holding its attitude and position. Check it in the manual for your model, and leave a margin that accounts for gusts, not just the average.

cumulonimbus

A cloud that builds high into the sky. It can bring lightning, gusts, short bursts of heavy rain and hail. Stop flying before one gets close.

visibility

How far you can see. Fog, rain and dusk shorten it. End the flight before you lose sight of the drone's position or heading.

Try it yourself

This page is the text of the lesson. The exercises and simulators run inside the lesson.

Go to the exercises of “Reading the wind and the sky”

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