1. Buys Ballots rule applies at any height, but refers to the pressure distribution at that height. Thus at low level it tells you about the MSL pressure and the resulting low level winds.
2. As you go up the pressure is increasingly affected by the temperature in the air masses, so Buys Ballot now tells you about the the upper winds, a combined effect of MSL pressure and horizontal temp gradient.
3. You get a better picture of the upper air if you plot and use a contour chart. These show the height AMSL of a specific pressure level, like the 500mb or 300mb surfaces. Because in calculating the contour height you have factored in the MSL pressure the resulting contours show the upper wind - the combined MSL pressure and thermal effect - and you read them just like a constant height pressure chart.
4. Thus, using Buys Ballot, wind from the left in the N hemi says you are heading to lower height contours. Thus, if you are holding a constant altimeter reading, which is flying on a constant pressure level, wind from the left etc means you are going down.
5. This doesn't exactly say whether you are going down because the MSL pressure ahead is low, or because the air mass ahead is low temperature, for the upper wind comes from a combination of these two causes. HOWEVER, at any significant height, say FL180 and above, you can bet with some confidence that the overriding factor is the air mass temperature. Reasonable assumption therefore, as has already been given, is that the air ahead is cold.
Dick W