Stable air mass – an air mass in which the temperature of the evaporated air from the ground is lower than the surrounding air at the same pressure. This is most common during the fall and winter season. As a result, stratus clouds form – a flat, foggy “blanket” at low altitudes with colors ranging from dark gray to almost white. They do not develop vertically. These clouds pose no threat to drone flights other than the precipitation/humidity they may bring. Also cirrus clouds are observed in the stable mass – tall, thin clouds that first appear in front of an area of low pressure; they usually foretell its decrease and, consequently, worsening weather conditions in the coming hours.
Unstable air mass – an air mass in which the temperature of the air evaporating from the ground is higher than the surrounding air at the same pressure. The air will rise spontaneously and cause the formation of cumulus clouds. The most obvious impact of this air mass on UAVs is the safety hazard caused by associated precipitation, turbulence/downdrafts and hail. Precipitation and hail can damage the body of a small UAV and lead to an uncontrolled decrease in altitude. Turbulence caused by convection may disrupt the aircraft’s ability to maintain its intended position.
There are one main category of clouds that create unstable conditions:
Cumulus clouds – fluffy and usually freestanding, they exhibit limited vertical movement and thermal rise;
Let us summarize the main weather phenomena observed in air masses
Stable air mass
- Stratus cloud cover
- Smooth air (no turbulence)
- Prolonged precipitation (not like showers)
- Low visibility
Unstable air mass
- Cumulus clouds (high and fluffy)
- Turbulent air
- Pouring rain
- Thunderstorm, squalls, hail
Pilots of “open” category UAVs must rely on visual observation of clouds and (especially) unstable aircraft to make flight decisions.
Strong wind and heavy precipitation are the main significant risk factors due to the light weight and water sensitivity of typical small drones