Heres the unpleasant technical answer.....Enjoy !
Taken from CAP 746 which is utterly thrilling (onist)
5 Requirements for Temperature and Dew Point Measurement
5.1 Introduction
This section details the performance criteria and siting of temperature and dew point measuring equipment installed at UK aerodromes.
5.2 Siting
The sensors should be mounted at a height of 1.25 to 2 metres, above an earth or grass surface. The sensors shall be exposed in an instrument housing, which provides protection from atmospheric radiation and water drops either as precipitation or fog.
5.3 Performance
5.3.1 The equipment shall be capable of measurement to an accuracy better than plus or minus 1.0 degrees Celsius for air temperature and dew point, over the following range:
Table 7 Tolerance values of sensors and equipment
Variable - In Tolerance-Operating Range-Recoverable Range
Temperature - minus 25° C to plus 50° C minus 30° C to plus 70° C
Humidity - 5 to 100% RH condensing 0 to 100% RH condensing
NOTE: Dew point shall be displayed for temperatures below zero; frost point should not be displayed.
5.3.2 Temperature and dew point measurements shall be measured to a resolution of 0.1 degrees Celsius.
5.3.3 Electronic sensor(s) shall be sampled at minimum of once per minute.
5.4 Backup
Alternative sensors may be used and mounted in an appropriate location on the aerodrome for use e.g. in a screen attached to the North facing wall of the observer’s building. The accuracy of the sensor shall be capable of measurement to an accuracy better than plus or minus 1.0 degrees Celsius for air temperature and dew point.
DTUP
So in short screen YES mercury NO