How do ATC controllers obtain QNH reading ? Does QNH reading being affected during bad weather?

Question

We know that the ATC provides QNH to pilots, but how exactly the ATC recieves QNH information in the first place? ( through a system? a device?)

and

Does QNH reading being affected during bad weather?

Answer 1

You have got the right answer from ATCbuckyBN, of course, he is a controller and know his stuff. But I wanted to add this:

QNH is the pressure at sea level for that airfield. In order to obtain that, the WX officer reads the pressure at the airfield, corrected for its altitude over the sea level. That is done by using a standard value based on standard atmosphere (SA): 15 degrees centigrade. But e.g. in the winter, that column of air is denser and shorter. That's why when e.g. isobar lines are drawn, a correction factor is used based on a weekly or monthly variation and this is called QFF.

Pilots can ignore the above because, in any case, aircraft flying on QNH fly lower in the winter since their altimeter is also calibrated for SA.

I don't really agree with ATCbuckyBN that the altimeter is in In Hg and QNH in millibars. In fact, the standard metric unit now is called hectoPascal (hPa), which is the same as millibar but just a new name, just like GMT is now UTC.

One atmosphere is 1013.25 hPa or 760 mm of mercury or 29.9 inches of mercury. What the controller reads and tell is not important, barometers are often graduated in the two units. In the US, your altimeter, though, is in inches of mercury. Ours in Norway is in hectoPascal.

Of course, QNH varies with bad or good weather. That's why it is given by the tower, the control centers and the ATIS. When you fly long distance with a flight plan, it is not unusual that control centers give something called: regional QNH because it is not the QNH of a special airfield.

Note that during a 'normal' flight the QNH is hardly varying for more that two or three hPa, which is not really relevant to the safety of your flight. An old seafarer saying is that a drop or rise of 10 hPa in a period of 8 hours is a sure sign of a storm on its way.

Answer 2

These guys answered better than I could. But if your buddy was sitting in his cessna on a dirt strip in the boonies you could ask him. All he'd have to do is dial in the field elevation in his altimeter and read the kolsman window. Then you'd have qnh and never talked to ATC. Simple huh?

Answer 3

Air Traffic Controllers get there weather information directly from Weather reporting equipment that is updated on a screen that usually sits right next to them.  At places where their is not equipment that is right next to them then a weather office will call every hour or when a significant weather change occurs, with the current wx information.  Winds, sky condition, temp, dew point, altimeter, qnh.  The difference in altimeter and QNH is that altimeter is measured in inches of Mercury and QNH is measured in millibars. (metric)  The US uses altimeter setting but most of the rest of the world uses QNH.  

Yes weather has an effect of the QNH and altimeter, if a thunderstorm is moving in then usually the altimeter and the QNH will go down.  The altimeter and the QNH are used to measure air pressure so that the pilot can set his altimeter on his airplane to show the correct altitude.