Weather is arguably the single most important aspect of a successful and enjoyable adventure in the mountains. That’s because the weather in the mountains is more inclement and volatile: thunderstorms are more severe, winds blow stronger, and snow piles up higher. Weather can showcase the beauty of the mountains and can also make for a miserable experience for the unprepared. But what’s the reason for this? This post aims to explore the basic themes of why elevated terrain dramatically alters weather. Hopefully this information will enable you to be better prepared for your next adventure!
Mountains receive much more precipitation than surrounding land. This is due to a process called orographic lift. When an air mass is forced from low to higher elevations as it moves over rising terrain, it cools and forms clouds and precipitation. During a storm those clouds are functionally like fog on the mountain’s surface often creating white-out conditions. The greatest precipitation is typically on the windward side of the crest of the mountain range where orographic processes are at their greatest. On the lee side of the mountains, the air descends, warms and dries out producing clear skies and rain shadows. The Sierra Nevada, Rockies and Cascades all have notable rain shadows where the mountains capture most of the moisture from Pacific weather systems leaving the east side of the ranges significantly sunnier and drier.
Thunderstorms are perhaps the most common severe weather encountered in the summer months. From the Rockies to the Sierra, thunderstorm patterns are often attributable to the North American Monsoon, which is a seasonal shift in the upper level winds from the westerlies (Pacific Ocean) that prevail from autumn through spring, to a wind from the southeast (Mexico) that is tropical. The peak time of year for thunderstorms related to the monsoon is late July and early August but the season can start in June and persist until mid September. Thunderstorms can include torrential downpours, significant drops in temperature, hail and wind. Daytime heating on mountainous surfaces and the steep terrain causes air to rise and become unstable, which can blossom into storms in the afternoon. Thunder is the result of vibrations from the electricity discharge. You see lightning flash instantly, but sound travels about a mile in five seconds. Thus five seconds between lightning and thunder means the source of the lightning is about 1 mile away. Pay particular attention to puffy cumulus clouds in the morning. Once these puffy cumulus clouds start developing structure there’s a pretty good chance of afternoon thunderstorms so aim to finish the adventure by early afternoon or be prepared for a thunderstorm.
In general, temperatures decrease about 3.5 degrees Fahrenheit for every thousand feet as one gains in altitude. This is because the atmosphere gets thinner as one travels higher above the surface and the lowest surfaces of the earth are better at absorbing solar heat. An adventure with a lot of elevation gain will need to take this into account. What may feel mild at the bottom can turn into a cold outing at the top. This is especially apparent during storms where there is a gradient with rain at lower elevations transitioning to snow at higher elevations. The exception to this general rule is an inversion which happens when cooler air sinks or is trapped in the valleys.
Wind increases with height where there is more of a pressure gradient between cold and warm air, less friction and the air is less dense. The terrain of mountains causes disruption of airflow as it is squeezed through constrictions or forced up and down terrain further enhancing winds. Expect the strongest winds on exposed ridges or prominent summits. The stronger the winds, the greater the perceived temperature drop on exposed skin. A metric used to quantify this perceived temperature is wind chill. Windy conditions in the mountains can become dangerous when combined with cold temps to produce dangerous wind chills. If it’s a windy day, be prepared for it feel much colder than the actual air temperature.
Considering all of the above, my most important tip is to know the forecast! I recommend your local National Weather Service (NWS) office for the most comprehensive forecast. The NWS routeinly highlights risks in advance of weather events, posts detailed forecast discussions and includes point and zone forecasts to focus on your mountain location. Some people rely on phone apps for hour-by-hour forecasts, and while these apps serve a purpose, they can also be misleading and inaccurate when used for mountain weather. Aside from truly extreme events, there’s only bad decision making and preparation, not bad weather. If one must go with a forecast that is not great, be prepared for conditions to be even worse than forecast, which means bringing extra clothing such as the Ultra Jacket V2 if thunderstorms are in the forecast, or UTMBeanie and Ultra Flip Glove if the temperatures are colder and winds are stronger at higher elevations. When in doubt, always err on the side of caution. I also encourage flexibility with plans. There’s always an appropriate adventure for the given weather. Save the summit for another day when you can actually enjoy the scenery!