Like all aircraft drones have weather limitations. For example they cannot fly in precipitation or strong winds. Nor do they like cold temperatures. It is important to understand the weather in order to make sound jugements on whether to fly and when.
The important aspects to grasp when considering the weather and forecasting are:
Remember back at school we were taught that solids are good conductors of heat whereas gasses are not. The air around us is not warmed by the Sun's radiation. It is the earth that is heated by the Sun and through conduction, the first few feet of earth are warmed. Air is heated as it passes over the warm earth. We were also taught that warm air rises. This is because as it warms the air expands and decreases in density. Rising warm air, known as convection currents, carries the earth's heat into the atmosphere. As the warm air ascends it is replaced by descending cold air and it is this continual cycle that affects the weather patterns wherever you are in the world.
Increases in the Earth's temperature is dependent on:
As the air is heated from below it can be assumed that the air is cooler at height. A rule of thumb for aviators is that the air temperature decrease by 1.98 degrees C (call it 2 degrees!) for every thousand feet. For example, if it is +20 degrees at sea level it would be close to +10 degrees at 5,000 feet. On a cold day, say 0 degrees at sea level it would be close to -10 degrees at 5,000 feet.
For planning purposes the international standard of sea level temperature is 15 degrees C and at an average lapse rate of 1.98 degrees per thousand feet it is -56.5 degrees at 36,080 feet, above which the temperature remains constant at -56.5 degrees.
Air is elastic, it is compressible. Think of a column of air, at the bottom of the column the weight from above is large whereas at the top of the column it is small. At height the air is thinner (less dense) and therefore the air pressure at height is low.
The pressure of the air is due to the weight of the air column. The average sea level pressure is 1013.25 millibars (mb) or 29.92 inches of mercury but the air pressure changes from day to day and hour to hour. It is the observation of changes in temperature and air pressure that allow weather forecasting.
In aviation, a good rule of thumb is that 1 millibar of pressure equates to 30 feet. For example, if the air pressure at sea level is 1013 mb, it is likely that it is 983mb at 900 feet high.
Warm air can hold more water vapour than cold air. In forecasting it is important to understand where the air has come from! For example, if the air has travelled across a mass of water it is likely to be humid (wet). Whereas if the air has travelled over land, it is likely to be dry.
If warm moist air is cooled it will reach a point when it becomes saturated (it cannot hold any more moisture). If it cools further water droplets (cloud) will be formed. The temperature at which air becomes saturated is known as the Dew Point.
Lapse rate is the rate of change in temperature with height. The lapse rate is different from one day to another but the average at lower levels is 1.6 degrees C per 1000 feet. If air is unsaturated the lapse rate of warm rising air is 3 degrees C, this is known as the Dry Adiabatic Lapse Rate. When the rising air cools to the Dew Point (which also differs from day to day) condensation occurs and the lapse rate false to 1.1 degrees C per 1000 feet. This is known as the Wet Adiabatic Lapse Rate.
Figure 1 demonstrates the concept of convection and lapse rates. Air is warmed by the earth's surface and as it expands it starts to rise. As it climbs higher the warm air cools at the Dry Adiabatic Lapse Rate. When the temperature reaches the Dew Point of the day, condensation occurs and water droplets become visible in the form of cloud. If the rising air is still warmer than the surrounding air it will continue to rise but now at the Wet Adiabatic lapse rate. When the rising air matches the temperature of the surrounding air, it will stop rising delineated by the top of the cloud.
Figure 1 - How clouds form
Air moves from areas of high pressure to areas of low pressure. However, it is not linear because of the rotation of the Earth. This geostrophic effect causes the air to turn to the left in the Southern Hemisphere and to the right in the Northern Hemisphere. Centrifugal (or cyclostrophic) force pushes air that is moving in a curved motion away from the centre of the curve.
Figure 2 - Pressure distribution
Meteorological forecasts use charts with isobars that join all points of the same atmospheric pressure. The wind at 1500 feet above the ground follows the direction of the isobars. The closer together the isobars, the stronger the wind and the further apart the isobars, the lighter the wind.
Buys Ballots Law states that in the Northern Hemisphere, if you stand with your back to the wind the area of Low pressure will be on your left and the area of High pressure on your right. The opposite is true in the Southern Hemisphere!
In general, drone operations are most suited to the stable conditions and light winds brought by high pressure distribution.
Forecast charts indicate where the front touches the surface of the earth. The slope of the cold front is generally 1 in 50 and always behind where it touches the earth. In general the band of rain is 50 miles wide. If the front is moving at 20 knots, then citizens can expect it to be raining for 2.5 hours. Behind the cold front there should be fine flying conditions but it could remain windy for an hour or two.
Figure 3 - Cold Front
The slope of the warm front is generally 1 in 150 and always ahead of where it touches the earth. In general the band of rain is 200 miles wide. If the front is moving at 20 knots, then citizens can expect it to be raining for 10 hours. Not good news for drone operations!
Figure 4 - Warm Front
The land warms at least 5 times faster than the sea. The air above the land will warm, expand and rise. The hotter the land, the greater the effect. The cooler air over the sea will be drawn in to replace the rising air and a sea breeze is experienced. It will be at a maximum around 1500 hours (3 o'clock in the afternoon).
At night the opposite happens and a Land breeze is experienced. The land cools the air above it. Out at sea the air remains relatively constant and warmer, so it rises. The colder air over the land is drawn in to replace the rising air column. This is called a Land breeze.
Figure 5 - Sea and Land breeze
On calm summmer days the mountainsides heat up and the air in contact slowly flows up the slopes. This is an anabatic wind. If this occurs colder air from above sinks into the valley to replace the rising warm air.
At night the opposite happens and a katabatic wind is experienced. The warmer air in the valley rises and is replaced by the cooler air from the mountain slopes. This effect is quite prevalent if there is snow on the high ground. Mists can form in the valleys.
Figure 6 - Anabatic and Katabatic winds
The Cumulonimbus, or thunderstorm cloud, is avoided by aircrew at all costs!
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