# Effect of Circulation on TA MET

If you are flying towards a high pressure, true altitude is increasing as you are following a pressure level that it bending away from the ground owing to the increasing pressure. Conversely, if you are flying towards a low pressure the true altitude is decreasing, hence the saying "high to low, look out below."

If you are given wind information, firstly draw two adjacent circles, and label one as a high and one as a low (it does not matter which.) Now, think about which hemisphere you are in and annotate the wind direction around each circle/pressure system - remembering that in the northern hemisphere, winds are anticlockwise around a low and clockwise around a high, or to use Buy Ballot's law, low on your left, wind at your back. These are reversed in the southern hemisphere.

Now, imagine the plane directly between the two circles and apply what the question says to find out which way you are flying. The question may tell you where the wind is coming from, or a drift. The aircraft will then either be pointing towards the high or low (in which case, remember "high to low, look out below" to get your answer) - or, in the case of following an isobar, a headwind or a tailwind, there is no pressure change and so true altitude does not change.