Meteorological expressions

Meteorological expressions in aviation

Reading time: 8 min

Pilots and other contributors to aviation industry are aware of the important role that meteorological phenomenon and predicting their changes play in safety and efficiency of flights. In this blog we are going to explore some of the most common meteorological expressions.

Advection

This is the situation when heat or moisture is transferred horizontally. Advection fog comes happens when a warm, moist air mass flows along a colder surface, which impacts aviation.

CAVOK – Ceiling and Visibility OK

This means that visibility is 10km or more, no clouds below 5,000 ft. or minimum sector altitude, depending on which is higher, and no cumulonimbus clouds around.

Cloud base

This is the height of the lowest visible part of a cloud over an airfield. It is utilized where the cloud above the airfield is defined as few – FEW 1-2 Octas, or scattered – SCT 3-4 Octas). For example, in a METAR, you may see SCT030, it means that the amount of cloud over the airfield is scattered at 3000ft.

Ceiling

According to the ICAO definition, the ceiling is the height above the ground or water of the base of the lowest layer of cloud under 20,000 ft. that is covering more than half the sky.

Density altitude

Density altitude means pressure altitude corrected for non-standard temperature according to ISA (International Standard Atmosphere), meaning that any temperature which is not 15 degrees Celsius, will produce an altitude reading which does not correspond to the ISA equivalent.

Dew point

This is the temperature where the air has been cooled to the point of creating condensation. In other words, this is the temperature where we have 100% humidity. This is important because humid air is less dense than normal, thus it weaken the lift of air craft.

Pressure altitude

It is the altitude of the aerodrome or the place you are at, adjusted for the local pressure. This changes constantly along with the high and low pressure systems that pass over an area.

RVR – Runway Visual Range

Runway Visual Range is the distance that the pilot of the aircraft-considered the centerline of the runway-can see the marking outlining the runway of in a horizontal direction. RVR is normally expressed in meters.

Updraft/downdraft

An updraft is a small-scale current of rising air, while a downdraft, as the name suggests, is a small-scale column of air that quickly falls toward the ground. Downdrafts are generally found in mountainous terrains. The culprit is the wind, when it blows from a certain direction, it is forced upward when faced with an interference in in its path, say like a mountain, causing updraft; then the air is compressed on the top and pushed down from the other side, resulting in a downdraft and turbulence. This could happen multiple times if there are several peaks.

Visibility

This is the horizontal distance you can see dark objects clearly without the aid of magnifying instruments. At night, it would be the distance we could see if the illumination is raised to normal daylight level.

Wind shear

Wind shear is the sudden change in speed and/or direction of the wind over a short span of time. It can happen in any direction though, it’s more common along the vertical or horizontal axis and can cause unexpected threats to the flight, particularly during the take-off or approach phase.

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