Reading Weather in the Backcountry: The Basics
Anyone who recreates outdoors knows how important monitoring the weather can be. Since weather in the mountains can change on a dime, it’s important to understand what indicates it’s time to get off the mountain. In the beginning of my hiking career, I’ve found myself in less than ideal situations (cowering under a rock in a sudden, above-treeline hailstorm anyone?) because I didn’t heed the warning signs. So I put together a list of tips, tricks and info from my years of experience recreating in the mountains as well as knowledge I gained from my degree in meteorology and decades of being a weather nerd. So, let’s get into it.
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 are 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.
Valentin de Bruyn / CotonThis illustration has been created for Coton, the cloud identification guide for mobile., CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
Cirrus
Cirrus are high, whispy, thin clouds that indicate high winds aloft. They are usually a good indicator of high pressure and stable air.
Nimbostratus
These 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.
Towering Cumulus
Cumulus 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.
Cumulonimbus
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.
Special Type: The Denver Brown Cloud
This is for my fellow Front Rangers. The Denver Brown Cloud is a nickname given to the layer of smog that sits over Denver 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 Hazards
In parts of the Intermountain Region of the US, it’s common knowledge that when attempting a summit, you want to be off the peak before noon. In the hiker’s paradise of Colorado, 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 the 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 mountainous regions, 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.
In regions where afternoon thunderstorms aren’t as common, you still want to get an early start on your above-treeline adventures, especially when there’s still a layer of snow on the peak. This is because as the temperatures reach their climax around the afternoon hours, more snow and icemelt occurs. Not only does this make hiking conditions more treacherous and difficult, but it can also increase the chances for dangerous rock falls. You may be groaning at the thought of waking up at 4am to start hiking, but the view of the sun rising over the mountains makes it all worth it, trust me.
Things to look for when the sky is partially obscured
When you can’t see the full scape of the sky, there are other signs of a potential thunderstorm to be aware of:
Sudden increase in winds. Often times you can feel the inflow and the outflow of an approaching storm
Humidity, When we have a humid day, you can often feel it in the air. One days where the air feels heavy and wet, or when your hair begins to frizz, there is usually a good chance a thunderstorm will develop.
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.
Sudden Changes in Temp If the temp suddenly drops, it’s a good sign a thunderstorm is approaching
Forecasting Tools
The National Weather Service is the most accurate forecasting service. 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 generally forecasts are the most accurate 1-3 days out. According to the National Atmospheric and Oceanic Administration, a 7 day forecast is accurate about 80% of the time while a 5 day forecast is accurate about 90% of the time. Contrast this with a 10 day forecast which is generally only right about half of the time. 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 on hourly forecasts, like the ones available on your smartphone, for accuracy either. While they can sometimes be accurate, it’s extremely difficult to accurately predict the weather down to an hour by hour basis. Especially in places like Colorado where the weather can change rapidly. (Cuts to NOAA and the NWS aren’t helping either, but that’s a discussion for another day). A good doppler radar app can also be helpful for the backyard forecaster. RadarScope is the best, but if you don’t want to drop $12 on an app RadarOmega is a good, free alternative.
Hopefully now you have a better understanding of what to look for next time you hit the trails. As always, if you’re unsure about the clouds gathering in the distance, it’s better to err on the side of caution and turn around. The trail will still be there next time.
