Thermals
A thermal-column (or thermal) is a column of rising warm air in the lower altitudes of the Earth's atmosphere. Thermals are created by the uneven heating of the Earth's surface from solar radiation, and are an example of convection. The Sun warms the ground, which in turn warms the air directly above it.
The size and strength of thermals are influenced greatly by the properties of the lower atmosphere (the troposphere). Generally, when the environmental air is cold, bubbles of warm air form by the ground heating the air above it and rises like a hot air balloon. The air is then said to be unstable.
Thermals are often indicated by the presence of visible cumulus clouds and are one of the three sources of lift used by glider pilots to climb.
Rising and descending air currents affect local air circulation. Surfaces such as planted fields, meadows and water tend to retain heat and cause descending air currents. Meanwhile rocky or sandy terrain, ploughed fields and barren land reflect heat and cause ascending air currents. These will cause a landing aircraft to overshoot or undershoot the runway if not accounted for.
Turbulence
Most of us are all too familiar with the occasional bumps and lurches that come with flying on an aircraft. Turbulence, the stealthiest of weather hazards, can strike from a sky that is literally clear and blue. It costs millions of dollars each year due to injuries and operational disruptions, such as delays and rerouting.
Turbulence refers to an irregular or disturbed flow in the atmosphere that produces gusts and eddies. It may be caused by many factors. These include:
- Thermals: Heating off the surface that produces plumes of rising air.
- Jet Streams: Fast, high-altitude air currents shift, disturbing the air nearby and interaction with strong winds in the jet stream and weather fronts.
- Mountains: Air passes over mountains and causes turbulence as it flows above the air on the other side.
- Wake Turbulence: Near the ground a passing plane or helicopter sets up small, chaotic air currents.
- Weather: Storms that generate powerful and unexpected gusts and microburst stir up a strong downdraft close to the ground.
Up to 60% of aircraft encounters with this phenomenon are due to turbulence associated with thunderstorms. Often pilots can avoid turbulence because they can see bad weather on weather radars. However, when a sudden change in wind velocity occurs in the absence of clouds, it is invisible to radar. Such “clear air” turbulence can buffet an aircraft without warning.
Clear air turbulence occurs at high altitudes, frequently in small pockets of the atmosphere. It is associated with a rapid change in wind that causes disturbances, or eddies, near the edge of the otherwise smooth flow of the jet stream.
Different Intensities of Turbulence
- Light Turbulence: Briefly causes slight, erratic changes in altitude and/or attitude.
- Light Chop: Slight, rapid and somewhat rhythmic bumpiness without noticeable changes in altitude or attitude.
- Moderate Turbulence: Greater intensity with rapid and somewhat rhythmic bumpiness and noticeable changes in altitude or attitude occur. Aircraft remains in control at all times with variations in indicated air speed.
- Moderate Chop: Similar to light chop, but greater intensity. Rapid bumps or jolts without obvious changes in altitude or attitude.
- Severe Turbulence: Pilots experience large abrupt changes in altitude/attitude. There are large variations in indicated airspeed. Aircraft may be temporarily out of control.
- Extreme Turbulence: Aircraft is violently tossed about and is impossible to control. This may cause structural damage.
The reactions inside aircraft vary from occupants feeling slight strain against their seat belts and unsecured items being slightly displaced, through to occupants being forced violently against seat-belts, and unsecured items being thrown about.
Improved computer forecasts can help pilots plan ahead to avoid turbulence, together with online displays that combine turbulence estimates from a weather prediction model and pilot reports of turbulence encounters to forecast clear-air turbulence up to 12 hours ahead.