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If you hike in the mountains or above the 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:
- Moisture
- Unstable air
- Lift
Moisture in the air comes from the oceans or large bodies of water that evaporate moisture. 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 taller, resulting in the towering anvil-shaped cloud characteristic of thunderstorms.
Diurnal Heating
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 peaks.
Thunderstorm Awareness and Safety
Hiking in a thunderstorm can be unpleasant or dangerous if you get caught out in the open when the high winds, hail, and lightning that accompany them strikes near you.
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. Without 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
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Always wondered what that meant! Thanks for the explanation. I never knew that heat from the earth could affect the weather.
A photo of Lakes of the Clouds in a post about bodies of water, clouds, and heat radiating from the earth – well selected! Thanks for the information and the links.
Smalls lakes, but lots of rocks!
Once I was leading a hike to Garfield and about a quarter of a mile from the summit the sky went black in the valley. We were below tree line so we spread out on the trail. Ironically we were at a high enough elevation that we could get a radar image on our phones from Cannon cell towers. We could tell that if we sat tight, the thunderstorms in the valley would pass and we had an hour and an half to summit Garfield until the next cells moved in.
Similar experience – watching thunderstorms approaching from 20 miles out while you’re hiking across an open ridge above treeline. You know you have time before they arrive, but you hurry because you know you’re very very exposed. This has happened to me on Saddleback, Success, and the Osgood Trail.
weather.org seems like a good website, but isn’t the NOAA site weather.gov?
sorry – fixed. I should know this. I only check it about a dozen times a day!
I got caught in a Derecho and only got a warning about 30 mins before it was hit by a inreach text. I did know what it was or how serious it was. Likewise, I have never heard of diurnal heating and was interested in learning more about it. Weather is no joke in the mountains and I have learned that understanding weather is and important outdoor skill.
Good explanation of Diurnal Heating, and yes 3 ingredients to Tstorms are Moisture, Instability and Lift. But while the moisture comes from the ocean, it is the amount of moisture in the specific air mass that determines likelihood of Tstorms. Also instability is not created by warm moist air rising into cold dry air but rather instability (lower pressure) of an air mass that allows the air to rise and mix. This is why when you have a large high pressure system Tstorks are unlikely. Also why do dont get Tstorms in the winter because the air is too cold to hold enough moisture.
We get thundersnow storms in Massachusetts and New Hampshire 1-2 times a year in winter. I suspect it’s because we have a Maritime Mountain climate that’s heavily influenced by lows sitting in the Gulf of Maine.
While not common, I’ve certainly experienced thundersnow. It’s basically like a thunderstorm except the precipitation is snow rather than rain.
I should have said don’t generally get Tstorms in winter. I live in New England and have experienced thundersnow a few times. What’s happening there is a comparatively warm and moist air mass (by winter standards) is in place and a fast moving cold front associatef with a low pressure system sweeps in and drives that warm moist air up (lifting force) into the atmosphere where it condenses into rain. When this happens with enough force it can produce Tstorms. This sort of frontal convective activity happens in the Summer too and is differentiated from typical afternoon Tstorms by the lifting force. A meeting of two airmasses vs Diurnal Heating.
Thundersnow actually starts as rain and then freezes into snow as is falls through the underlying cold air mass.
Cool! I love learning new things.