By Leigh Matheson, MetService Forecaster
Have you ever wondered why you can fight through screaming winds on a saddle to later sit on the peak in relatively light conditions? Or that some parks are constantly battered by strong winds, while others aren’t so much? Well, I did.
The answer is not a particularly simple one, but the best analogy is that mountains (in most cases) are like rocks in a river.
Watching changes in river flow, and how the water behaves as it flows around the rocks, is largely how air behaves when encountering the mountains. However, there are some differences, because air is not uniform in density and temperature like a river, and water vapour readily changes state within the atmosphere, adding or removing energy as it does so. So, in some cases the air is encouraged to flow around, rather than over the mountains, while in other instances air will readily lift, or is forced to lift.
The strength of mountain winds therefore depends largely on the state of the atmosphere, or in other words, the large-scale weather situation.
In the region of a broad trough or low pressure system, the air is in a ‘lifting environment’, so when pushed against the ranges it will often freely rise over the mountains. In these cases, the wind is often less markedly different between saddles and the peaks and you can expect the air speed to increase with height.
In the region of high pressure, the air is in a ‘sinking or stable environment’, so is resistant to being pushed over higher peaks and will instead flow through gaps and saddles. Contrary to the idea that air speed increases with height, in light and moderate flows, the wind will instead increase about the saddles and gaps.
However, as the wind speed becomes stronger for example, as a front approaches behind a retreating ridge the strengthening flow will eventually force air over the higher ranges and peaks, greatly enhancing the wind speed as it does so. The most common situations to experience these very strong winds occur when a front moves in from the south behind a retreating ridge, or when a ridge strengthens rapidly behind a southerly change. In these advancing or retreating ‘stable’ environments, the strongest winds can be experienced, with hurricane force winds not unusual.
The photo taken in Otago highlights a classic lenticular cloud, which forms in such environments when the wind becomes very strong. As the air ‘pours’ over the ridge-tops, then rapidly descends on the lee-side, it causes the air to bounce and create waves downstream (you can see this downstream of boulders in a river). At the top of the wave, the water vapour briefly condenses before it sinks again creating these clouds. The air speed through these clouds can be in excess of 180 kilometres per hour, yet the cloud itself remains stationary as the wave remains stationary.
This particular lenticular cloud is known as the ‘Taieri Pet’ which is fairly common over Otago’s Taieri Plains. In this case, the wind has met a number of ranges upstream in just the right sequence, which amplifies the wave, creating a greater area of lift in that region and hence causing a thicker, layered-looking cloud to form.
If you are concerned about extreme winds over a particularly sensitive part of your trip, ask yourself these questions.
- Is the park exposed to that particular direction of wind? Will it be lying parallel or perpendicular to the flow? (The air speed will be enhanced if the flow is directly against the range.)
- What does the weather map tell you? Do you have a retreating or developing ridge of high pressure? (This could be an indicator of very strong winds – check the forecast for confirmation.)
Check the forecast. The forecast gives a general view of the flow strength over the range, expect differences in the strength depending on your location within the range, but if the forecast wind is high, expect a rough day out on the tops!
This column was originally published in the August 2012 FMC Bulletin. FMC thanks Leigh for her valued contribution to the FMC Bulletin weather column. If you have mountain weather questions, email them to backcountry@fmc.org.nz, and we will ask Leigh to answer them in future weather columns.
