Metar is a format for reporting weather information about the airport and it can be sent every hour or every 30 minutes. If there are weather change between the bulletins, AMO employees must send another bulletin called SPECI. This bulletin is sent to international network (AFTN) and everyone can see the last Metar on web such as this.    



An example of Metar is:

METAR LIRF 201650Z 21008KT 180V260 CAVOK 18/09 Q1019 NOSIG​

A typical Metar contain the following data:
Type of Bulletin
ICAO airport code
Date e Time (indicates the time of the observation, day of the month followed by the Zulu time of day)
Wind direction and Speed  ( wind direction is indicated by degrees, wind speed is indicated by knots -KT- or meter per second -MPS- )
Prevailing Visibility
Runway Visual Range ( this data is posted if the prevailing visibility is under 1500 meters or if the visibility of one or all RVR sensors give a visibility under 1500 meters )  
Present Weather ( the following table don't have all PW but only the most common, In addition you can find the VC = vicinity. For example you can find the VCTS = Vicinity thunderstorm )
Sky coverage
Temperature and Dew Point
Pressure
Additional information
Recent Weather
Condition of Runway
Remark (is not sent in the AFTN Network)
Trend




Some Present Weather Code

DZ = Drizzle 
RA = Rain 
SN = Snow 
SG = Snow grains 
IC = Diamond dust 
PL = Ice Pellets 
GR = Hail Grandine
GS = Small hail and/or Snow Grains 
BR = Mist 
FG = Fog 
FU = Smoke 
VA = Volcanic ash 
DU = Widespead dust 
SA = Sand 
HZ = Haze 
PO = Well-Developed Dust/Sand Whirls 
SQ = Squalls 
FC = Funnel cloud 
SS = Sand storm 
DS = Dust storm