Is there really less diurnal temperature variation in the mountains than at sea level?

Question

I live in the Los Angeles area, at sea level. Nearby we have tall mountains where I like to go hiking. I've noticed something strange about the NOAA forecasts, which is that the forecasts in the mountains seem to predict day-night temperature differences that appear implausibly small. Example:

NOAA forecast for where I live, at sea level

NOAA forecast for the high elevation area (9000-10000') surrounding a nearby summit

These are live, so here are a couple of static screenshots for today and tonight:

The day-night difference is predicted to be about 20 degrees at sea level, but only 8 degrees on the mountain, at about 10,000'. This is a pattern that I've seen repeatedly in the forecasts, not just when the weather is changing or something. For the sea-level temps, I'm able to verify with a thermometer that the forecast is just about right for this morning, and in general the daytime temps at my house are about right, i.e., I believe the 20-degree difference at sea level based on empirical data.

The tiny 8-degree difference for the mountains just seems impossible. I don't bring a thermometer with me when I hike, and I don't think there is any actual weather station on top of any of our local mountains, but for instance when I visited this mountain last Friday, it seemed obvious to me that temperatures were increasing rapidly as the day went on.

Admittedly I don't have hard data for the temperatures in the mountains. Am I just wrong? Is there some actual physical reason that diurnal differences in the mountains would be so small compared to the ones at sea level? Or is there some bias in the way NOAA's software works, perhaps because they have ground station data at sea level, but not at high elevations?

Answer 1

This might has more to do with your proximity to the ocean than it does with the elevation. The ocean and all larger bodies of water act as insulators for their locales. During the day, when solar heating creates temperature differences on the landscape, little pockets of wind pop up and bring the ocean regulated air on-land. This is called the marine layer, and it's the reason for increased instances of fog and cooler temperatures in summer along the coast while areas inland bake. In late fall and winter, the marine layer acts oppositely and warms areas along the coast. Water has higher specific heat than the land or air and holds onto its temperature longer, creating a warming insulator during the early cool months. The effect is mostly negated at night as winds calm and the land radiates off its heat more effectively than the ocean, which brings the temperature down quicker.

Higher up in the mountains, the daily temperature changes are dictated by the air aloft. While on this particular day, the diurnal range was somewhat small, this range can vary wildly depending on the day and particular atmospheric setup. The reason for this volatility is because higher up in the atmosphere there isn't as much terrain to influence the temperature of the air so sharper fronts (be they cold or warm) are more common. Another thing to consider is how the daily temperature change is influenced by the fact that rising air cools naturally and sinking air warms naturally. So if you're on the western side of the mountains, air is more than likely naturally cooling which could be offsetting heating from the sun.

These are mostly theories taking into account atmospheric physics, but with some data a better theory could be drawn.