The next article in the Drone Pilots Guide. If you missed the first article How to be a Good, Legal and Safe Drone Pilot be sure to go back and read it.
Just what are we talking about? Well meteorology is the scientific study of the atmosphere. According to Wikipedia. Yep, I had to look that one up, I couldn’t nail the definition right off either. And on many a day I wonder if the “weatherman” knows.
What makes the weather?
The energy that drives our weather originates 150 million kilometres (93 million miles) away in the Sun. Incoming solar radiation warms the Earth’s surface, which in turn heats the air and sets up air currents.
The portion of the Earth’s surface most perpendicular to the Sun’s rays receives the most intense incoming solar radiation. This means that equatorial regions receive more heat than the poles. The air warmed at the equatorial regions tends to rise and flow towards the poles, while cold air from the poles slides along the surface to replace it. But the Earth is covered with large areas – such as oceans and deserts – that warm up and cool down at different rates, and these set up other airflows that complicate that basic model. And all the time the Earth is rotating and dragging the atmosphere around after it, which further confuses matters. But basically it all starts with the Sun!
The phenomena we call weather – pressure systems, wind, precipitation, cloud etc. – all occur in the very thin lower layer of the Earth’s atmosphere, called the troposphere. This layer is about 17 km (55,000 ft) thick at the equator, about 9 km (30,000 ft) deep at the poles. Within this layer, temperature generally decreases with height.
As mentioned above, the equator receives more heat than the poles. The air at the equator warms and rises. In doing so it creates a low pressure area near the surface. Meanwhile the cold air at the poles sinks, creating a high pressure area. And then this high pressure cold air at the surface flows toward the low pressure area at the equator.
At higher levels the air above the equator flows toward the poles, to replace the air that has descended – but only gets to about 30 degrees of latitude before it cools and sinks – causing high pressure areas in the subtropics. As this air descends to the surface some flows south (causing the ‘tradewinds’) and some flows north to collide with the air heading south from the poles. This ‘collision’ takes place around the latitude of the UK – causing much of our weather.
But the key point is that all this rushing about of air is wind – the enemy of R/C and drone pilots.
Other small pressure changes due to localized heating and cooling results in localized winds, such as sea breezes.
Basically any time there is wind, it is air in motion from a high pressure area to a low pressure area.
The amount of moisture that the air can carry as water vapour depends on its temperature. If warm air containing some moisture is cooled below its dew point, the moisture will condense into small droplets and form a cloud.
If the process is allowed to continue and the correct conditions prevail, the cloud droplets will amalgamate until they are too large to remain in suspension in the cloud – and they will then fall as rain.
One of the simplest ways to cool air below its dew point is to force it upwards. A stream of air flowing over flat ground will be forced upwards if it encounters a hill. If the hill is large enough, the moisture in the airstream will condense, and so-called ‘orographic’ cloud will form on the hilltop. Orographic rain can result.
A bubble of air rising after being heated by ‘hot spots’ on the ground may cool sufficiently to pass its dew point and form convection clouds. These clouds have a distinct puffy form with flat bases, often described as looking like a heap of cotton wool, and are consequently called by the Latin for heap: cumulus. These clouds mark the tops of thermals, and indicate bumpy flying conditions.
Sometimes the atmosphere is sufficiently unstable for large heap clouds with towering dark tops to develop. These are called cumulonimbus clouds (nimbus is the Latin for rain-cloud). These clouds are colloquially referred to as ‘cu- nimbs’.
If large enough, they produce thunderstorms. They also indicate dangerously severe turbulence – often with sudden strong winds on the ground from an associated ‘gust front’.
General lifting of masses of air can cause widespread areas of cloud. Layer, or stratus, cloud is a boringly familiar sight to all of us in Britain. If it is well developed, the thickness of the layer makes the cloud appear darker, and rain is likely. In this state the cloud is called nimbostratus.
At very high levels, the water droplets freeze and form rather prettier clouds. The thin, wispy ‘mare’s tail’ or cirrus clouds often seen high in a clear blue sky are an example of this type (cirrus means ‘curl of hair’ in Latin).
These clouds are formed at altitudes of 5–13 km (3–8 miles) in our latitudes. Their distinctive hooked shape is due to the strong winds at these high altitudes. Ice particles that fall to lower levels are left behind and form the hook. As they fall to still lower levels they evaporate away.
Getting and using a forecast
Drone pilots use forecasts to help them decide whether or not to make the journey to the flying location.
But never forget that a forecast can be wrong.
Look out of your window, and check to see which way the clouds are moving and how fast they are moving. If there’s half a chance that it will be flyable, go and have a look.
The only certainty is that your quadcopter will never fly sitting at home!
Obtaining a weather forecast is relatively simple, though there are a variety of sources, depending on the technology you have at your disposal.
The Internet is the best source of weather forecasts, if you know where to look. There is a vast amount of weather information on the Web, though much of it is of negligible value to the multirotor pilot.
http://www.weather.com/ in the U.S.
The Met. Office Web site (www.met-office.gov.uk) also has a ‘General Aviation services’ page where you can create a free account and then access various charts and forecasts, including the ballooning forecast which includes very useful information. There is also a handy booklet that you can download from the site: http://www.metoffice.gov.uk/aviation/ga
Television and radio
Perhaps the easiest and most obvious forecasts to find are the television forecasts that follow the news. These forecasts tend to vary in quality.
Using the forecast
Having obtained your forecast, the next step is using the information to your advantage. If your flying location is relatively close to your home, then you will already know if it is raining or snowing.
So generally the key information you are looking for is the wind strength – and any trends in the weather. Often in the weather may be unsuitable at say lunch time, but a lovely summer evening is forecasted – this advance warning gives you plenty of time to get your batteries on charge and ready.
Flying in different conditions
Don’t get frustrated here. It will take time to learn to navigate, but following the tips below will help you tremendously.
Flying in moving air
The speed of your drone through the air is its airspeed, which you as the pilot control. If your drone is flying with an airspeed of 20 mph on a day when there is not a breath of wind then its speed over the ground (groundspeed) will also be 20mph. More often there will be some wind, and this can significantly affect the drone’s speed over the ground.
Imagine you are again flying your drone at 20 mph, but directly into wind, and the windspeed is 20mph. So basically you are flying at 20 mph within an enormous mass of air moving in the opposite direction at 20 mph.
In this case your drone’s groundspeed will be 0 mph. If you now turn to fly downwind, maintaining the drone’s airspeed of 20 mph, its speed over the ground will be a rapid 40 mph – it is flying at 20 mph within an enormous block of air, at the same time as the block of air is being moved across the ground in the same direction at 20 mph.
Let’s now imagine that your drone’s top speed is 30mph, and the windspeed is 20mph, and you fly it to a position a mile downwind of you. (This takes less than a minute.) You now turn back into wind and the drone, at full speed is moving over the ground at only 10mph. That is 0.16 miles per minute.
It will take over six minutes at full power to cover that distance. If the battery starts to run out you will never get the drone back to base. And if the terrain is hostile (a lake, the sea, a forest) you will most likely never recover the drone.
The message is that you should always be conscious of the windspeed and direction, and be very careful of venturing downwind in breezy conditions.
Measuring wind strength
The severity of mechanical turbulence is directly proportional to the wind strength – if the wind strength doubles the turbulence quadruples – so it is vital that you accurately check the wind strength before flying.
To aid you in making a reliable assessment, you can buy a hand-held wind-strength meter (anemometer). These are reasonably accurate, and as you become familiar with a certain meter you will soon begin to relate its readings to the actual flying conditions experienced, and so will very quickly be able to use it to help you judge whether the wind conditions are within your model’s limits.
The problem with anemometers is that they can only measure the wind at your position – so they are therefore very susceptible to localized effects. It is worth thinking through the possible airflow pattern around your site, and measuring the wind strength at other positions if there is any doubt.
Besides the wind strength you must check for gustiness, and you should monitor the general weather.
In the absence of an anemometer you can fall back on the Beaufort Scale – which was devised over 200 years ago and still holds good. Basically this relates the observed actual conditions to a wind speed.
Wind Force Description Speed Specifications
0 Calm Smoke rises vertically
1 Light Air Direction shown by smoke drift but not by wind vanes
2 Light Breeze Wind felt on face; leaves rustle; wind vane moved by wind
3 Gentle Breeze Leaves and small twigs in constant motion; light flags extended
4 Moderate Breeze Raises dust and loose paper; small branches moved.
5 Fresh Breeze Small trees in leaf begin to sway; crested wavelets form on inland waters.
6 Strong Breeze Large branches in motion; whistling heard in telegraph wires; umbrellas 31 used with difficulty.
7 Near Gale Whole trees in motion; inconvenience felt when walking against the wind.
8 Gale Twigs break off trees; generally impedes progress.
9 Strong Gale Slight structural damage (chimney pots and slates removed).
10 Storm Seldom experienced inland; trees uprooted; considerable structural damage
11 Violent Storm Very rarely experienced; accompanied by widespread damage.
12 Hurricane Devastation
Whilst learning to fly your drone you should not venture out in anything more than a Force 3 gentle breeze. Once experienced you might also fly in a Force 4 moderate breeze.
Winds that are stronger than this would be beyond the capabilities of most drones and their pilots. (Small lightweight quads are less able to fight against turbulence and wind effects – with the very tiny ones best reserved for indoor flying and only ventured outside when it is calm.)
Turbulence is a swirling motion imparted to the air by some external disturbing force – and it can have very unwelcome effects upon low-mass aircraft.
When air flows around a smooth streamlined shape it will follow the shape. Sharp edges and sudden changes in shape will tend to produce turbulence.
When moving air (wind) encounters an object such as a building, eddies are produced, because the shape is too angular for the air to flow smoothly around it. Standing eddies can form: they do not change position, and because they spin around on the spot they are often referred to as rotors.
You should also avoid flying in the lee of trees and buildings.
Turbulence will also be found around thermals and other forms of wind shear, such as in a wind gradient.
Air flowing across the ground is slowed down by contact with the surface and with crops, hedges, trees, buildings and so on. Because air is slightly ‘sticky’ (viscous), the layer of air just above the slowed layer is also slowed to a lesser extent.
This effect carries on until eventually the true wind speed is found at some distance from the ground. It is not unusual to find that a very gentle surface breeze is a strong blow at 300 m (1000 ft). This situation is most common on relatively stable days when thermal currents are not vertically mixing the atmosphere.
The temperature has various effects on drone flying. Firstly low temperatures can result in clumsy pilots – but also it reduces the power output of your batteries quite considerably.
High temperatures result in thinner air, so your drone’s ability to lift its normal payload will reduce in significantly higher temperatures. Also, as the motors will have to spin the propellers faster to get the required amount of lift, your batteries will be drained more quickly.
Just as higher temperatures mean less dense air, higher altitudes also mean that your drone is operating in less dense air. So the worst combination is ‘hot and high’. If you attempted to operate your drone from even UK Lakeland fell tops on a hot summer’s afternoon you would notice a significant reduction in performance.
See the next section here: Laws of the Air