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 localised heating and cooling results in localised 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. You should start with the ‘XC weather’ site.
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, though they all originate from the Met. Office in Exeter. Probably the best are the BBC forecasts following the evening and late evening news.
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 UK the weather may be unsuitable at say lunch time, but a lovely summer evening is forecast – and this advance warning gives you plenty of time to get your batteries on charge ready.
See the next section here: Flying in Different Conditions