Meteorology 3

Weather reports

• An observation of the actual state of the weather at a specific time is a report

• Most commonly used report in aviation is the METeorological Aerodrome Report (METAR)

• Formalised report of key information

  • Wind

  • Visibility

  • Precipitation

  • Cloud

  • Temperature

  • Pressure

METAR

METAR Date & Time

• Format:ddhhmmZ

  • dd - day of the month

  • hh - hour in 24 hour time

  • mm - minute

• Represents the time at which the report was valid

  • Immediately out of date as soon as it’s published

• No month or year given as reports are typically only useful over short timescales

• Z - Zulu Time

  • Universal Coordinated Time (UTC)

  • Equivalent to Greenwich Mean Time (GMT)

  • Does not change with British Summer Time (BST)

METAR Wind

• Default format: dddssKT

  • ddd - direction the wind is blowing from relative to true north

    • May be given VRB (variable) if wind speed is low

  • ss - speed of wind in knots

• May include gusts: dddssGggKT

  • gg - speed of wind gust in knots

• May be followed with variable wind direction: aaaVbbb

  • Direction varying between aaa and bbb

METAR visibility

• Horizontal visibility reported in metres

• Always reported as 4 digits

• 9999 means “at least 10km”

• May also include Runway Visual Range (RVR) information

  • Not relevant to (most) PPL flying

• May be omitted if CAVOK reported

METAR Precipitation

• DZ - Drizzle

• RA - Rain

• SN - Snow

• SG - Snow grains

• IC - Ice crystals

• PE - Ice pellets

• GR - Hail

• GS - Small hail

• BR - Mist

• FG - Fog

• HZ - Haze

• Modifiers

• SH - Showers

• TS - Thunderstorm

• FZ - Freezing

• VC - …in the vicinity

• - Light

• + Heavy

METAR Clouds

• Format: ccchhh

  • ccc - amount of cloud cover

  • hhh - height of cloud base above aerodrome level in hundreds of feet

• Amounts given as

  • 1 okta = 1/8^th of the sky

    • FEW - few, 1-2 oktas

    • SCT - scattered, 3-4 oktas

    • BKN - broken, 5-7 oktas

    • OVC - overcast, 8 oktas

• Cumulonimbus (CB) or Towering Cumulus (TCU) clouds may be mentioed specifically

METAR Temperature

• Format: tt/dd

  • tt - Temperature in degrees Celsius

  • dd - Dew point in degrees Celsius

• Either may be preceded by M to indicate minus

METAR Pressure

• Format: Qpppp

  • pppp - sea level pressure in hectopascals

• Q refers to QNH

METAR CAVOK

• Visibility and cloud reports may be omitted and replaced by CAVOK

• Cloud And Visibility OK

• No clouds below 5000ft, no CB or TCU

• No significant weather in the vicinity

• Visibility of at least 10km

Weather forecasts

• A prediction of the weather at some future time

• Forecasts are either textual or graphical

• Textual forecasts

  • Terminal Aerodrome Forecast (TAF)

  • General Aviation METeorological forecast (GAMET)

• Graphical

  • Synoptic chart

  • Form 214

  • Form 215

TAF

TAF - Period of Time

• The Zulu time given at the start of the forecast indicates when the forecast was published

  • It does not mean the forecast is accurate at this time

• Periods of time are formatted: aabb/ccdd

  • aa - day of the week the period starts

  • bb - hour of the day the period starts (UTC)

  • cc - day of the week the period ends

  • dd - hour of the day the period ends (UTC)

  • Short term forecasts may just use bbdd

• First period indicates duration of validity of the forecast

• Subsequent periods indicate a change

TAF - Types of change

• BECMG - Becoming

  • Between the specified time period the weather will change from that preceding to that following

• TEMPO - Temporarily

  • During the specified time period the weather may change temporarily to that following

TAF - Likelihood of occurrence

• PROB - probability

  • Indicates a particular probability that the following change will occur

• PROB30 - 30% probability

Form 214

• Met Office provided spot wind forecast

• Forecast at various altitudes for specific locations

  • Wind speed

  • Direction

  • Temperature

• Extremely useful for flight planning purposes

Form 215

• Met Office provided low altitude significant weather forecast

• Shows weather fronts and areas of similar weather

• Forecasts surface visibility, precipitation, cloud and freezing level

• Also forecasts turbulence and icing potential inside cloud

• Notation is broadly similar to TAFs with some spatial terms

  • ISOL - `Isolated - present in only a small part of the area

  • OCNL - Occasional - occurring occasionally throughout the area

  • WDSPR - Widespread - Likely to be encountered throughout the area