Mad About Mountains : Predicting the Weather

Forecast | Predicting

Predicting the Weather

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Red Sky on Kinabalu, Malaysia

For many, predicting the weather is nothing more than sticking to the saying

"Red sky at night, shepherd's delight; red sky in the morning, shepherd's warning."

Luckily, the picture on the left was taken at sunset!

However, leaving aside the folklore, it is possible to make an educated guess about the upcoming weather, and factor this in to your plans.

Of course, whenever you venture out into the mountains, you must always be prepared to deal with any sort of weather. Nowhere is this more true than in Britain where the weather in the mountains can change notoriously fast.

Atmospheric Pressure

One popular method of predicting the weather is to look at atmospheric pressure. Pressure is measured in millibars (mb) using a barometer. Many people have these in their homes, and they often indicate the likely weather pattern on their dial or readout. In general terms, low pressure (less than 1000mb) indicates poor weather, while high pressure (more than 1000mb) indicates better weather.

Areas of low pressure are known as depressions. They are characterized by a cold air mass pushing in and gradually replacing a warm air mass along with surface winds that are circulating anti-clockwise around the center of the depression. A frontal system is likely to form at the boundary of these air masses, and these typically bring cloudy and often wet weather.

More than just the pressure at any point in time, it's important to take account of how the pressure is changing over time. If pressure is dropping, it may indicate the approach of a depression. This is likely to bring with it the characteristic wet weather, either as rain or snow if it is cold enough.

Alternatively, if pressure is rising then this may indicate the passing of a depression and the return to more stable, better weather.

Clouds

Another approach to predicting the weather is to focus on the clouds. There are four primary Latin terms used to describe clouds:

cirrus which means curl or fringe;
stratus which means stretched out, like a blanket;
cumulus which means a stack or pile;
nimbus which means rain bearing.

These four primary terms are combined to describe the various cloud formations we see.

Cirrus

Cirrus clouds are the really high, wispy clouds you sometime see on a clear day. They often indicate that a frontal system associated with a depression is approaching and so the weather is likely to deteriorate.

Cumulus

These are the typical 'cotton wool' type of cloud, and they come in a number of varieties:

Cirrocumulus are the highest level type, often forming amongst cirrus-type clouds.
Altocumulus are medium level clouds forming at around 8,000ft and so well about the summit of British hills. When they form in the early morning, this can indicate that wet weather is on its way
Cumulonimbus are the massive towering 'thunder' clouds. Even if they don't bring thunder, they are usually associated with bad weather of some sort.
'Cotton wool' cumulus form at low weather typically in late morning once the ground has been warmed by the sun. They tend to be small and on their own, they indicate fair, stable weather.

Stratus

As the name suggests, this type of cloud typically has no distinguishable base or top and is more like a sheet or blanket of cloud. Fog is one common form of Stratus as is the typical British overcast type of cloud.

Other than with fog, which often forms in periods of stable weather, stratus is typically associated with wet weather. As a rule, however, the higher the cloud, the longer the wait until the wet weather arrives.

Rules of Thumb

The following observations provide a good indication that the weather is on the change.

The weather is getting worse if:

The cloud level appears to be getting lower, and especially if the clouds are getting thicker or heavier.
On a hot day if the clouds start to grow in early or mid afternoon, this is likely to indicate rain or thunder by the evening.