Anyone who recreates outdoors in Colorado knows how important monitoring the weather can be. Since weather here can change on a dime, it’s important to understand what indicates it’s time to get off the mountain.

Unstable vs Stable

In order to determine what the atmosphere is capable of for the day, we must first understand the difference between unstable and stable air. When the air is stable, the weather is likely to be relatively uneventful. Precipitation can occur in stable air conditions, but it is usually slow and steady. Weather conditions are not likely to change quickly. Unstable air, by contrast, means rapidly changing weather conditions, heavy precipitation, and storms. A good way of determining the air stability is to observe clouds throughout the day. When the air is stable, if any clouds form they will tend to “spread” horizontally. In unstable conditions, clouds will grow uninhibited vertically, sometimes reaching massive heights.

Cloud Basics

One of the most important tools for forecasting weather in the field is understanding clouds. There are two main types of clouds, stratus and cumulus. Stratus are usually flat or shallow, and indicative of stable weather. Cumulus on the other hand, are those puffy popcorn balls you see on a summer day. Cumulus clouds are a great way to monitor the stability of the air, as they are usually associated with unstable air. Here’s some common clouds and what they mean.

Cloud Diagram



Mountain During SpringCirrus are high, whispy, thin clouds that indicate high winds aloft. They are usually a good indicator of high pressure and stable air. 


Walker Ranch RainThese thick, heavy precipitation producers are usually an indicator that conditions aren’t likely to change rapidly. They usually don’t contain enough energy to be lightning/thunder producers, but can still produce steady precipitation. 


High Altitude TerrainOften referred to as “puffy” or “cotton-candy like”, cumulus are popcorn-like in shape often with flat bases. Although these are usually associated with fair weather, under unstable atmospheric conditions they can grow rapidly upward into thunderheads.

Towering Cumulus

Blue SKy white cloudCumulus congestus, more commonly known as towering cumulus, form when cumulus clouds experience growth in unstable air conditions. Basically, they are the middle stage between cumulus and cumulonimbus clouds. If you see cumulus starting to grow upwards into towering cumulus, it’s a good indicator to start planning your descent. 


DIA lightning Storm

These clouds are a sign to get off the mountain now. Cumulonimbus are storm producers; their towering heights a result of very unstable, moist air. These clouds produce lightning, heavy precipitation, and sometimes hail and tornadoes.

The Denver Brown Cloud

The Denver Brown Cloud is a nickname given to the layer of smog that sits over Denver/Boulder and is visible from the Foothills/High Country. The phenomenon is caused by a layer of stable air, known by meteorologists as an “inversion” or “cap”, trapping smog and pollutants below it. While the Brown Cloud is associated with stable air, it’s not a reliable forecasting tool. Often in the summer, inversions act like a lid on a boiling pot; trapping convection below. When the atmosphere below the inversion is warmed enough to “break the cap”, strong storms can develop rapidly. 


Afternoon Thunderstorms

boulder rainbow

In Colorado, it’s common knowledge that when attempting a summit, you want to be off the peak before noon. Like clockwork almost every day in the summer, afternoon thunderstorms roll through the high country. But why is this the case? A thunderstorm requires a few basic ingredients: warm temps, moisture, and rising air. In Colorado, warm temps in the mountains (60+) occur mostly in our summer months. Warm air contributes to atmospheric instability, allowing clouds to grow upwards uninhibited. Colorado’s geographic location is responsible for the next ingredient, moisture. Colorado sits in the perfect position for upper level winds to bring in moisture from the Gulf of Mexico or the Pacific during the late summer monsoon season. Moisture is important because it is necessary to form clouds. The final main ingredient is rising air. In Colorado, we actually have two different methods of rising air. The first being our topography. The mountains can act as a mechanism for physically pushing the air upwards. When the air is forced to rise, it expands and with enough moisture, forms clouds. In unstable air conditions, these clouds then continue to rise upwards and grow into thunderstorm clouds. Another mechanism for rising air is convection. Convection occurs when the hot summer sun heats the ground, causing air to rise upwards. Because this “perfect storm” of ingredients is present on most summer days in Colorado, afternoon thunderstorms can be an almost daily occurrence. 


What to Look for When the Sky is Partially Obscured

Mountain Storm brewing

When you can’t see the full scape of the sky, there are other signs of a potential thunderstorm to be aware of:

  1. Sudden Increase in Winds Often times you can feel the inflow and the outflow of an approaching storm.
  2. Humidity The air in Colorado is generally very dry so when we have a humid day, you can often feel it in the air. One days where the air feels heavy and slightly wet, or when your hair begins to frizz, there is usually a good chance a thunderstorm will develop. 
  3. Thunder/Lightning You can often see lightning and hear thunder even when the storm is miles away. A good time to hightail it back to the car. 
  4. Sudden Changes in Temperature If you can feel the temp suddenly drop, it’s a good sign a thunderstorm is approaching.


Forecasting Tools

The National Weather Service is your best bet for an accurate forecast. Because their forecasting centers are localized, you get the most accurate weather readings and forecasts for your area. If you’re using a site that provides 10+ day forecasts, know that forecasts are generally the most accurate under 10 days out. We don’t yet have the tools to accurately predict the weather for the whole month or season. Sorry Farmer’s Almanac fans. Also, don’t rely heavily on hourly forecasts, like the ones available on your smartphone, for accuracy either. While they can sometimes be accurate, it’s very difficult to predict the weather down to an hour by hour basis. Especially in Colorado where the weather can change rapidly. A good doppler radar app can also be helpful. RadarScope is the best, but if you don’t want to drop $12 on an app there’s plenty of good free ones as well.

  • Written by Hayley Moser. Adventurer and Fly Fishing, Backpacking, and Hiking Guide for Colorado Wilderness Rides and Guides.

Rainbow During Storm 

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