Weather fronts in the mountains


July 10, 2017 Facebook Twitter LinkedIn Google+ How to



If you’re interested in being able to forecast weather changes in the backcountry, it helps to know a little bit about how weather fronts work. If you spend time in the mountainous backcountry, 

Weather fronts will bring in either cold or warm air. When a weather front approaches, you can expect the temperature to change. The winds will also shift, and you’ll eventually get clearing after the front has passed, which could take a few days. 

Weather fronts passing through the mountains are amplified when compared to when they pass through non-mountainous terrain (like cities), because mountains lift the air masses more. Frontal passage is usually quicker and more dramatic in the mountains, especially the passage of a cold front.  

Why is mountain weather unpredictable?

isolated mountain weather iceland bc25

The mountains can create localized unpredictable weather. Here, drizzle and fog in the mountains in Iceland.

 

The reason mountain weather is so unpredictable is because of orographic lifting. Orographic means mountains – so when a weather front encounters them, the air is pushed up. As air is pushed up, it cools and condenses into cloud. From these clouds we get our weather. Often, bad weather will exist close to the mountains or on the mountain summit itself, while nearby areas are dry and hot. 

Because mountain weather can be very localized, a decent forecast given for a large area does not necessarily mean that you won’t have bad weather if you’re up in the alpine. Generally, mountains create barriers to moving air, causing it to rise and condense, creating cloud and usually rain or snow. It’s not uncommon to descent a mountain in a whiteout with rain and cold and to find pleasant conditions in the valley below.  

Treat forecasts with caution. For example, if the forecast calls for mostly sunny, and 30% chance of precipitation, for a town in the valley, I’ll know to expect clouds, fog and possible whiteout conditions up high in the alpine. This becomes especially important if planning glacier or snow travel (such as backcountry skiing or mountaineering), because the white glacier will be undistinguishable from the low, overcast sky.

As weather can change very quickly, it’s another reason we should be prepared with some essential safety items we should always bring in the backcountry.

What does low pressure mean?

Low pressure systems bring in weather fronts. You can predict them by watching the clouds as they come in – read more about how to predict weather in the backcountry here

There are four main kinds of weather fronts: the cold front, the warm front, the occluded front the stationary front. The first three are the ones you’ll need to pay the most attention to, since the stationary front doesn’t move very fast and produce very much weather. 

  1. The cold front

ColdFront oklahoma climatological survey

How a cold front moves. Graphic from the Oklahoma Climatological Survey at http://climate.ok.gov/.

In a cold front, cold air replaces warm air at the surface. These are the fastest moving fronts, and are typically associated with thunderstorms. Be careful of these in the mountains in the summertime. Don’t get caught up high on a mountain if you see thunderstorm clouds approaching, these storms move very fast and can be very violent.

Weather ahead of the front: You’ll see high cloud, and decreased visibility (haze) with light, patchy rain. In the summer, you’ll see thunderstorms.    

During: A sudden cooling will occour with gusty and shifting winds. Extended periods of rain or showers will follow, with thunderstorms in the summer. Visibility will be poor but slowly improving.

After a cold front passes: obviously, the weather will be much colder than before, with steady cooling. Showers will eventually taper off and the sky will clear. 

 

  1. The warm front

HA warm front approaching. Graphic from the Oklahoma Climatological Survey

A warm front approaching. Graphic from the Oklahoma Climatological Survey at http://climate.ok.gov/.

In the warm front, the opposite happens: warm air replaces cold air at the surface. After a warm front, it will be much warmer and more humid.

Weather ahead of the front: You’ll see high cloud going to middle cloud, and maybe some precipitation, usually in the summer only. Visibility will be poor with some haze in the air. The visibility will be worse when a warm front passes than it is when a cold front passes. The winds will be variable but not particularly strong or gusty. 

During: Persistent rain and slight warming temperature. Visibility will slowly improve.

After a warm front passes: the air temperature will be warmer than before, and hold steady. There will be light drizzle, then clear, sunny weather. 

  1. The occluded front

This happens when a cold front overtakes a warm front. Cold fronts move faster than warm, so when they catch them, an occluded front is created. This weather front is the result of “mature” weather systems. The weather behind the occlusion will be similar to when a cold front passes. The weather ahead of the front will be similar to a warm front.

You can still get out during these weather changes, just be prepared, and stay dry. Make sure you have some critical safety items in your backpack and be prepared, you can get very cold in the mountains very fast. 

If you want to read more about weather, find out how to predict weather in the backcountry when you don’t have access to a weather forecast. 

Read more cool facts about mountains here. 

Alicja

Alicja

Alicja is an economist, enjoys climbing, mountaineering, backcountry skiing, cycling and gets out into the backcountry as much as possible. See all of Alicja's Blog Posts
Alicja
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