Met Questions

[Outkast]

I have a few met questions I'd love if someone could help me with .I should probably know this stuff but I don't , here goes;

At airports that have an AWIS, why is it quite regularly not the same as the metar (ie the metar is an automatic one)? Sure the differences are usually not huge - but if the source is the same, why aren't they?

What instruments are used to figure out visibility and cloud bases?

Thanks

[Capt Fathom]

BoM Website - Ceilometer and Visibility Sensors (http://www.bom.gov.au/general/reg/aviation_ehelp/cv.shtml)

[Gen. Anaesthetic]

The only real difference is that AWIS is real time, in that the AWIS is updated as soon as one of the observations (pressure, wind, etc') change. METAR is only issued every hour or half hour (depending on the airport), so there may be changes within those time periods that result in the AWIS being different.