Estimation of the height of the cloud base
An accurate cloud height can be obtained via a pilot report. Bear in mind that aircraft usually operate with the altimeter barometric subscale set to QNH, so any information being relayed may likely be referenced to the height above sea level. Be sure to convert this information to the height above the station (ground level) for reporting/broadcasting where required.
A pilot reports entering an overcast cloud base on departure at 6500 feet “on aerodrome QNH”. This cloud base is 6500 feet above mean sea level. To convert this to the height above the aerodrome, the observer must subtract the aerodrome elevation. The aerodrome elevation is 1500 feet.
→ 6500ft - 1500ft = Cloud base 5000ft above the aerodrome.
More about altimetry
An aircraft’s altimeter uses a similar operating principle to an aneroid barometer. On an altimeter, the pilot will set a known pressure in the barometric subscale (for a particular datum – usually mean sea level), and the pointers will indicate the height above that datum. When the mean sea level pressure (QNH) setting is used, the altimeter will indicate the height of the aircraft in feet above mean sea level (AMSL), or altitude as it is known.
To obtain the correct altitude, an accurate QNH must be entered or dialled on the barometric subscale on the altimeter. An error of 1hPa entered on the subscale will result with an approximate 30 foot error in the height indicated on the altimeter.
If the pilot of an inbound aircraft was notified that the QNH is 1031hPa when in fact it is actually 1013hPa, the altimeter would over-read by 540 feet; that is, the pilot would think the aircraft was 540 feet higher than it actually is. If the aircraft is operating close to terrain without visual reference to the surrounds, the consequences could be disastrous.
Accurate pressure information is vital for the safety of aircraft operations.