Buys Ballot’s Law
It’s well worth knowing about Buys Ballot’s law. Buys Ballot was a Dutch meteorologist in the 19th Century who stated what has become known as his ‘law’. In the Northern Hemisphere, if a person stands with their back to the wind then the low atmospheric pressure is to their left and the high pressure to their right.
These days we have very reliable weather forecasts. So with all the information available to us through handy apps, an ‘old school’ tip like this may seem irrelevant. However, for a variety of reasons it’s not – forecasts can still be wrong; the path of a weather system may have altered by the time you are there; or you may be in a part of the world with unreliable forecasts (or have no access to them).
“So what?”, you may ask. Well, coupled with an understanding of winds ‘veering’ or ‘backing’, it is possible to deduce where in a low pressure system (storm) you are positioned. A wind that changes direction over time in a clockwise direction is said to be ‘veering’ and one that changes in an anti-clockwise direction is described as ‘backing’. While a depression passes overhead an observer in a fixed location will experience changes in wind direction that will differ depending on where they are in relation to the overall depression.
This was originally of most interest to ships’ captains. It enabled them to steer their ships away from the ‘dangerous quadrant’ of the storm. As mountaineers, we have little opportunity of changing our location relative to the path of the storm, but this understanding does give us the ability to judge whether the forecast is not as expected and why, and, if absolutely necessary, simply to abandon our route.