If you hike in the mountains or above-treeline, you’ll find frequent mention of the term “diurnal heating” in mountain weather forecasts, often accompanied by thunderstorms, hail, and high wind. What is diurnal heating and why should you be cautious when it’s in the forecast?
Where do Thunderstorms Come From?
Thunderstorms need three ingredients to form:
- Unstable air
Moisture in the air comes from the oceans or large bodies of water that evaporate moisture into the air. This moisture is responsible for making clouds. Unstable air occurs when warm moist air near the ground encounters cold drier air at higher altitudes. Since warm air rises, anything that warms the air close to the ground will nudge it higher. As the warm air lifts higher and higher, it causes clouds to grow taller and tall, resulting in the towering anvil-shaped could characteristic of thunderstorms.
Where does this “lift” come from?
Each day, the sun heats the earth, creating a warm layer of air close to the ground which rises as it gets hotter and hotter. This heating effect is very noticeable if you’re hiking over bare rocks or boulders near the top of a mountain, where you can feel the heat radiating up and at you.
The heat radiated by the earth reaches its maximum by 3 to 4-hours after solar noon, which is why it feels hottest in the mid-afternoon and not at midday. This also explains why most thunderstorms occur in the afternoon when the degree of lift caused by diurnal heating reaches its peak.
Thunderstorm Awareness and Safety
Hiking in a thunderstorm can be quite unpleasant or dangerous if you get caught out in the open when the high winds, hail, and lightning that accompany them strikes.
If the weather forecast calls for diurnal heating with a chance of thunderstorms later in the day, it can be prudent to time your hike so that you get below treeline or undercover before mid-to-late afternoon to avoid the accompanying high winds and lightning. In the absence of a forecast, you can usually recognize that a thunderstorm is coming and take evasive action if you see the towering anvil shape forming overhead.
Weather Forecasting For Hikers and Backpackers
If you’re interested in learning more about the weather and how to read or predict it while hiking and backpacking off the grid, NOAA has a lot of excellent information posted at https://weather.gov although it might take a little googling to find it. I can also recommend
- Mountain Weather: Backcountry Forecasting for Hikers, Campers, Climbers, Skiers, and Snowboarders (Kindle)
- Reading the Clouds: How You Can Forecast the Weather (Kindle).