6. METEOROLOGY

6. METEOROLOGY
   1. Introduction
   2. METAR – Aviation Routine Weather Report
   1. Wind
   2. Visibility
   3. Runway Visual Range
   4. Weather Phenomena
   5. Clouds
   6. CAVOK
   7. Temperature
   8. Dew point
   9. Air pressure QNH
   10. Trend
   11. Runway Conditions
   3. VMC Visual Meteorological Conditions
   4. Figure 6.1 - METAR

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6.1 Introduction

Weather knowledge is essential in aviation: Visibility, air pressure, thunderstorms, temperature, clouds, rain and dew point etc.
All affect flying in one way or another. You are most affected by weather when flying VFR, but also when flying IFR, there are restrictions how bad weather can be before plans must be changed. In this section, we'll look into how weather is reported, but not how weather affects flight.
There are many acronyms and concepts in weather reporting, which we will learn throughout the rest of this document. Below we'll cover rather completely the weather report. There are many acronyms and concepts. You need not know them all from the beginning, but you must be able to get the name of the airport, wind and air pressure. Since you as a controller read the weather report to the pilots, you'll soon learn the rest of the acronyms.

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6.2 METAR – Aviation Routine Weather Report [S/S+*]

* As a Student you should have knowledge about what a METAR is and recognize the most essential parts (Where, when, winds and QNH) by heart and know where to find information about the other elements. As a Senior Student you should be able to read out a whole METAR.

At major airports, weather observations are made every half hour, day and night.
The weather in the METAR is the weather observed 10 minutes before the report.

The following is part of a METAR:

• Airport (for example EKCH for Kastrup)
• Observation time (day and time followed by Z)
• Wind at ground level: direction, speed and unit (knots)
• Meteorological visibility
• Runway Visual Range (RVR) if visibility is below 1500 m.
• Present weather
• Clouds, amount, type and base
• Air temperature and dew point
• QNH (air pressure), preceded by Q.
• Other information, for example wind change.
• Landing forecast, trend.

The following is a brief breakdown of some of the basic METAR elements:
 

       6.2.1 Wind [S]

Wind is measured 10 m above ground.
The direction is from where the wind is coming. The precision is 10 degrees.
In the METAR, the wind velocity is a 10 minute average and given in knots (kt). If there are gusts 10 KT over the average value, this is reported as well. The gusts are reported as G17 and should be read “gusting” or “maximum”.

When flying, the wind direction in itself isn't the most important factor, but it’s the side-wind component. If the wind direction varies more than 60 degrees and if there's more than 3 kt wind, it is reported as V (variable). If there is less than 3 kt wind and it varies, this is reported as VRB. Calm is reported as 00000KT.

Example:

• VRB02KT – variable two knots
• 25020KT – two five zero degrees two zero knots
• 15015G25KT –  one five zero degrees one five knots maximum (or gusting) two five knots
• 24018G35KT 160V290 – two four zero degrees one eight knots maximum (or gusting) three five knots variable between one six zero and two niner zero degrees

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       6.2.2 Visibility [S+]

Visibility is often of vital importance. There are three ways of obtaining the visibility

• Flight Visibility is the visibility forward from the cockpit of an aircraft in flight.
• The Ground Visibility, which is given in the ATIS and METAR, is the visibility at an aerodrome, as reported by an accredited observer.
• Ground visibility reading, which is measured automatically.

The two latter is the distance measured to a large unlit object, where the contours of this object can be seen.
Visibility is affected by moisture, ice crystals, salt or dust in the air.
Visibility in darkness is today most often measured with infrared light.
Visibility is measured in steps up to 10 km. Visibility below 50 m is written as 0000 and 10 km or more as 9999. Visibility below 5000 m is given in m, and above in km.

Example:

• 450 – visibility four five zero meters
• 1500 – visibility one five zero zero (or one thousand five hundred) meters
• 9999 – visibility more then ten kilometres
• Special case: 1800N 7000S – visibility to north one eight zero zero meters, to south seven kilometres.

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       6.2.3 Runway Visual Range [S+]

Runway Visual Range is only measured if visibility is below 1500 m.
The RVR instruments (transmissometers) are located at both ends, and in the middle of the runway. These are referred to as RVR Touchdownzone (TDZ), Midpoint and Stop-end. Most important for the landing is the value of the TDZ.
The RVR report starts with an R followed by the runway designator. If there is more than one runway in the same direction, L, R and C are used.

Other RVR codes:

• M: visibility is below what can be measured, or below 50m
• P: RVR is more than 1500 m
• V: RVR varies
• U: RVR is going Up
• D: RVR is going Down

Examples:

• R29L/0700 – R-V-R two niner left seven zero zero meters
• R19R/M0150D – R-V-R one niner right below one five zero meters going down
• R26/0350V0600U – R-V-R two six variable between three five zero and six zero zero meters going up

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       6.2.4 Weather Phenomena [S+]

Current weather is included in the METAR. It is abbreviated with two letters. If this isn't enough, the abbreviations can be combined. The abbreviations are listed at the end of this chapter. Some abbreviations can be preceded with a ”+” or ”-”, this lists the intensity. VC means Vicinity, which means within 8 km from the airport. Exception: thunder and cumulonimbus clouds. Sometimes the recent weather is also reported, in this case ”RE” precedes the weather condition.
Examples:

• SN BLSN – Snow and blowing snow
• +FZDZ FG – Heavy freezing drizzle with fog
• RESN – Recent snow

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       6.2.5 Clouds [S+]

There are a number of reported clouds types, but only two are important to differentiate: Cumulonimbus (CB) and Towering Cumulus (TCU). Otherwise, only the fraction of sky covered by clouds is measured. This was previously reported in 1/8's, but nowadays this is described in words:

• 0/8: Sky clear (SKC)
• 1-2/8: Few (FEW)
• 3-4/8: Scattered (SCT)
• 5-7/8: Broken (BKN)
• 8/8: Overcast (OVC)

The cloud base above the airport's reference height AGL (Above Ground Level) is measured in hundreds of feet. 001 means 100 ft, 012 = 1200 ft and 120 = 12000 ft. Vertical visibility is reported as VV, and if this is not measured VV///.

Examples:

• BKN002 – Broken two hundred feet
• SCT013 BKN120 – Scattered one thousand three hundred feet broken one two thousand feet.
• SCT035TCU – Scattered three thousand five hundred feet, towering cumulus.

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       6.2.6 CAVOK [S+]

CAVOK (read CAV-OH-KAY) replaces visibility, weather and clouds if: visibility ≥10km; no cloud below 5000 ft (1500m) or below the highest minimum sector altitude, whichever is greater and no CB or TCU (Cumulonimbus); and no precipitation, TS, DS, SS, MIFG, DRDU, DRSA, or DRSN.

The full readout of CAVOK can be either “Clouds and Visibility OK” or “Ceiling and Visibility OK”. The most common is the latter, but the most correct ought to be the first. This is because ‘ceiling’ means BKN or OVC when it comes to clouds and the definition of CAVOK is that NO clouds should be present below 5000ft.

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       6.2.7 Temperature [S+]

The air temperature is measured in degrees Celsius.
If below 0, it is preceded by an M.

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       6.2.8 Dew point [S+]

The dew point is defined as the temperature the air must be cooled to, to get saturation, i.e. relative humidity 100%.
If below 0, it is preceded by an M.

Dew point is important to the pilot since this value gives information about visibility, clouds and together with the temperature indicates the risk of ice-formation. The closer the temperature and dew point are, the more humidity is in the air and the worse is the visibility.
The difference between temperature and dew point is called spread. If you calculate SPREAD x 400ft you will get the lowest cloud base.

Example:

• 02/M04 – Temperature two dew point minus four

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       6.2.9 Air pressure QNH [S]

As described in other sections in this manual, the air pressure is vital to know, since it affects the altitude measuring system. Air pressure can be measured in different ways, and relative different levels.

QNH is air pressure at sea level (or reduced to sea level in standard atmosphere if it’s measured at another point).
QFE is air pressure at the airport.

A high value means high air pressure and vice versa.
Standard pressure is 1013.25 hPa or 29.92 inch Hg.

In the METAR, the value is preceded by a Q if the unit is hPa and A if it's inch Hg. Q is used in Europe.

Example:

• Q0987 – Q-N-H niner eight seven

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       6.2.10 Trend [S+]

The trend prognosis should indicate expected changes within next two hours.

There are three MAIN concepts used in Trend:

• Becoming (BECMG)
• Temporary (TEMPO)
• No Significant Change (stable) (NOSIG)

The first two can be given with a time reference.

Examples:

• BECMG FM1250 TL1340 – Becoming from 1250 till 1340 (the change will take place between 12:50 to 13:40)
• BECMG AT 1400 – Becoming at 1400 (will change at 14:00)
• TEMPO FM 1400 – Tempo from 1400 (One or more changes shorter than one hour, from 14:00 to two hours after the METAR was reported.)

Additional Trend prognosis can be From, To and At.

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       6.2.11 Runway Conditions [C]

Runway Condition is not always included in a METAR. It is only when the conditions on the runway might affect aircrafts landing and departing.

First the runway number is indicated. This is done with the runway two digit numbers. If the report covers all runways at the airport, ”88” is reported. Repeats of earlier given information is indicated with ”99”.
 
The runway report is given as AABCDDEE as follows:

AA: Runway
01 – 36 Runway (Left if parallel)
51 – 86 Right runway (if two parallel)
88 All runways at airport