For those who are not familiar with it, this article will introduce you to and help you understand the Lapse Rate. Of course, this is in the context of mountaineering not anything else.
The ‘Lapse Rate’ is the rate at which temperature changes with height in the atmosphere. Most people already have a sub-conscious awareness of this. At its most basic, this is perhaps gained from seeing images of sunny beaches (obviously at se-level) and images of Himalayan peaks with snow on the tops.
But even if we know that it’s very cold on the top of Everest, does this have any relevance to us going up Scafell Pike or Ben Nevis? In short, yes, it absolutely does.
Table of contents
- Describing the Lapse Rate
- Practical Relevance of the Lapse Rate in the UK
- Particular Importance of the Lapse Rate in Winter
- Finding Forecasts of Mountain Temperatures
Describing the Lapse Rate
Simply put, as warm air rises from the surface of the earth, it also cools down. The rate at which it cools is the ‘lapse rate’. Very dry air, (usually associated with high altitudes and/or high pressure weather) cools at a rate of approximately 1°C per 100 metres. Saturated (damp) air cools at a rate of approximately 1°C per 200m.
Although simplified, those are the key facts to understand and remember about the lapse rate.
Practical Relevance of the Lapse Rate in the UK
To understand the direct practical relevance of the lapse rate for mountain-goers in the UK, let’s look at two examples.
Example 1 – Fort William/ Ben Nevis in July
Fort William Sea-level Forecast
- Temperature – 12°C
- Wind-speed – 10mph
- Weather – Clear and very dry. (Let’s assume this is dry enough for the faster lapse rate to occur, as a worst case).
Ben Nevis Summit Conditions (1344 metres above sea-level)
- Temperature – Minus 1°C. (There is a difference in height of 1300m, so 13 x 100m = a loss of 13 x 1°C)
- Wind-speed – 20mph. (The wind-speed typically doubles at 1000m of altitude).
- Weather – Clear and dry
- Wind-chill (or ‘feels-like’) – Minus 7°C
That’s a very typical weather forecast and although the dry-air rate of cooling has been used, it may come as a shock to some to discover that the chilling effect on the body would be the equivalent of Minus 7°C.
Pack those extra layers, even on a ‘summer’ day.
Example 2 – Fort William/Ben Nevis in February
Fort William Sea-Level Forecast
- Temperature – 3°C
- Wind-speed – 20mph (gusts 30mph)
- Weather – Drizzle, with occasional heavier showers. (This will certainly be ‘saturated’ air.)
Ben Nevis Summit Conditions (1344 metres above sea-level)
- Temperature – Minus 3.5°C. (There is a difference in height of 1300m, so 6-and-a-half x 200m = a loss of 6.5 x 1°C)
- Wind-speed – 40mph, with gusts of 50-60mph. (The wind-speed typically doubles at 1000m of altitude).
- Weather – The drizzle and rain will now be snow/hail/sleet. Coupled with a 40mph wind this is blizzard conditions.
- Wind-chill (or ‘feels-like’) – Minus 12-14°C
In this example, what is no more than a cold, dreary, blustery day in Fort William is likely to be producing full-on winter blizzard conditions on the summit of Ben Nevis.
Particular Importance of the Lapse Rate in Winter
It should now be clear that the lapse rate is important even in summer in the UK and that’s why we must understand it.
In winter, though, it becomes even more relevant for a couple of main reasons.
One very important piece of information for a winter mountaineer to know is where ‘freezing level’ will be. The lapse rate helps us understand this and estimate it for ourselves even in the absence of detailed forecast data.
At 0°C water will generally start to freeze and turn to ice. So knowing how high freezing level will be gives a good indication of where crampons and ice axes may be required. This isn’t an absolute – in super-dry conditions if there is no moisture to freeze then there will still be no ice. And at times when there is already an established snow-pack, crampons and axes may still be required much lower down than freezing level. But it’s important that we take the time to find out or estimate where freezing level will be.
Effect of Temperature on Snowpack and Avalanche Hazard
Changes in temperature will affect the stability of the snow-pack and make it more (or less) prone to avalanche. Fortunately, we don’t need to engage in guess-work and amateur predictions but can instead make use of the excellent forecasts published by the Scottish Avalanche Information Service.
Nevertheless, an understanding of the lapse rate helps improve our understanding of why the avalanche forecast is what it is.
Finding Forecasts of Mountain Temperatures
These days there are numerous options available to us for finding forecasts that give details of the temperatures at different heights, caused by the lapse rate. All of these tools can help us make informed decisions about when and where to go in the mountains, and where to avoid.
- Met Office (and other) phone apps. There are many to choose from, including the Met Office. These allow you to search for named places, which include mountain summits. I tend to save them as favourites, grouped together. For example, Fort William and Ben Nevis; or Capel Curig and Snowdon. I can then easily check valley forecasts and compare them with summit conditions.
- MWUK phone app. The Mountain Weather UK (MWUK) app pulls together data from a variety of sources. Their regional forecasts (eg ‘Southwest Highlands’ for Ben Nevis and Glencoe) allow you to drill down and see the temperature at different altitudes at different times of day. The named summit forecasts (eg ‘Ben Nevis’ allow you to see the summit forecast at different times of the day.
- Met Office website. The main Met Office website has a specialist section for Mountain Forecasts. As well as forecasting for mountain areas (eg ‘Southwest Highlands‘), there are also individual summit forecast with amazing levels of detail (eg Ben Nevis).
- MWIS website. The Mountain Weather Information Service (MWIS) was at one time the only decent source of mountain weather forecasts. Their presentation of the forecast always included important information that was dependent on the lapse rate. They state how cold it will be at 900 metres (a good proxy for many mountain summits). They also state where freezing level will be – either by giving the altitude or simply saying that it will be above the summits (if that is the case).
The key points to remember and understand about the lapse rate are:
- Air cools with altitude (to an extent that we need to be concerned about it). This is called the ‘lapse rate’.
- In dry air, the lapse rate is about 1°C per 100m.
- In saturated air, the lapse rate is about 1°C per 200m.
- These amounts may seem small. But even on a UK mountain summit of 1000m the lapse rate can make a huge difference between what you experience in the valley and what conditions are like on the summit.
- There are plenty of sources available to get a good forecast of mountain conditions as you plan your day out. My own favourites are the Met Office website and the MWUK phone app.