Altimeters
(Simple) Altimeters measure pressure above the datum set on the millibar scale. Altimeters are calibrated to ISA conditions which assume, among other things, that the sea level tempreature is +15 degrees C. If the actual surface temperature is not +15, then the altimeter will read incorrectly.
If the temperature is less than +15 (as happens in UK much of the year), then the altimeter will over-read. You can work out the error by a simple calculation: approximately 4 feet per thousand feet (indicated altitude) times the number of degrees difference from +15 (ISA). e.g. cruising at 2000 feet on QNH, with a surface temperature of zero degrees, produces an error of 4x2x15 =120 feet. Your altimeter over-reads by 120 feet, therefore you are really flying at 1880feet.