A Pilot's Guide to Inflight Icing
Module I - Before You Fly
Check the Weather
Section: TAFs AND METARs
Start This SectionTAFs AND METARs Check the Weather
Step 6. Check the TAFs & METARs
One way to exit from icing conditions is to descend below the cloud bases. In some cases, the only way to get to warmer or dryer air will be to shoot an unplanned approach. Consult the Terminal Aerodrome Forecasts (TAFs) for airports along your route of flight to obtain cloud base and surface precipitation forecasts to determine where these exit strategies can be used.
The TAF forecasts the wind, visibility, weather, sky condition, and wind shear in the vicinity of the airport, but does not explicitly include icing information.
TAFs are issued four times a day (0000Z, 0600Z, 1200Z, and 1800Z) and are valid for 24 hours.
Check TAFs and METARs to determine cloud bases and surface precipitation
Related Information
TAF EXAMPLE:
KDLH 181545Z 181612 33010KT P6SM BKN035
TEMPO 1619 5SM -SHRA BKN025
FM1900 29014G23KT P6SM SCT050 SCT150
BECMG 0103 28009KT
FM0900 25009KT P6SM SCT060 BKN120=
TAF DECODED:
Forecast for: KDLH (Duluth, MN) on 10/18/2001 15:45 UTC
Valid 10/18/2001 1600 UTC to 10/19/2001 1200 UTC
From 1600 to 1900 Z
Wind: from 330 at 10 knots
Visibility: More than 6 miles
Sky Condition: Broken 3500
Temporarily between 1600 Z and 1900 Z:
Visibility: 5 miles, light rain showers
Sky Condition: Broken 2500
From 1900Z to 0900Z:
Wind from 290 at 14 knots gusting to 23 knots
Visibility: More than 6 miles
Sky Condition: Scattered 5000, Scattered 15,000
Between 0100 Z and 0300Z: Wind from 280 at 9 knots
From 0900 Z: Wind 250 at 9 knots
Visibility: more than 6 miles
Sky Condition: 6000 Scattered, 12000 Broken
TAFs AND METARs Check the Weather
METARs (METeorological Aviation Routine) report wind, visibility, weather, sky condition, temperature, dew point, and the altimeter setting in the vicinity of an airport. METARs can also be used to pinpoint the actual surface location of a front (by changes in temperature, pressure, and wind direction), to find information about cloud bases along the route of flight, and to confirm forecasts. Reports of freezing precipitation, or frozen precipitation such as ice pellets, at the surface may also be indicative of conditions aloft.
METARs are reported each hour. Normally, the elements of the report are observed between 45 and 55 minutes past the hour. Whenever a substantial change occurs, a SPECI (Special) report is released. Generally, SPECIs are generated when the weather improves above or degrades below VFR or IFR minimums, or when there is a change in freezing precipitation or thunderstorm activity. Automated stations such as ASOS may provide METARs more frequently (e.g. every 20 minutes). Be aware that some automated stations have limited reporting capabilities, that some stations do not report precipitation at all, and that none are capable of reporting freezing drizzle or drizzle without human augmentation.
Use METARs to check the accuracy of the forecasts. Note that some ASOS have limited capabilities.
METAR EXAMPLE:
KDLH 181555Z 32008KT 6SM -RA BR BKN025
OVC031 05/04 A2974
METAR DECODED:
Conditions at: KDLH 10/18/2001 15:55 UTC
Wind: from 320 degrees at 8 knots
Visibility: 6 miles
Present Weather: Light rain, Mist
Sky Conditions: Broken 2500, Overcast 3100
Temperature: 5°C
Dew Point: 4°C
Pressure (altimeter): 29.74 in. Hg
Using the standard lapse rate as a rough estimate, the freezing level over KDLH would be expected to be about 2500 AGL. Because there are clouds at this altitude there is a possibility of icing. The narrow spread between the temperature and the dew point suggests that the visibility may decrease quickly and that a missed approach could be necessary.