Originally Posted by
gerardflyagain
I think I've found the answer to my own question!
No you haven't, the phrase is purely about atmospheric pressure, not temperature. Have a little think about how altimeters work & read Heston's post again.
Altimeters do not measure your altitude, they sense the static (ie ambient) atmospheric pressure & display it on a scale calibrated in feet or metres. If you are in a floatplane on a calm sea in a high pressure area, the QNH might be, say, 1030 hPa. Now take off & climb to 2,000' asl, the ambient atmospheric pressure at 2,000' will be around 970 hPa. If you maintain an indicated altitude of 2,000' you will be flying at an altitude where the ambient pressure is 970 hPa. If you are flying towards a low pressure area you will be following the pressure gradient downhill. If the sea level atmospheric pressure in the low pressure area is at or below 970 hPa your altimeter will continue to display an indicated altitude of 2,000' even as you fly into the sea.
"From high to low, lookout below" is a cautionary remeinder to either reset your QNH regularly or be aware that your real altitude will be less than your indicated altitude when flying from high to low pressure.