Upside-down temperatures
Be on the lookout: Warm over cold
Sea level standard temperature is 15 degrees Celsius/59 degrees Fahrenheit, but up at 10,000 feet it’s a brisk -5 degrees Celsius/23 degrees Fahrenheit. But those are standard temperatures. In the real world, temperatures can be topsy-turvy. Sometimes temperatures can increase with altitude and be colder at or near the surface than aloft. These situations are called temperature inversions, or simply inversions.
One of the ways inversions happen is when the ground cools overnight. The ground heats up and cools down more rapidly than air aloft, so if there’s a big high-pressure system overhead and skies are clear, surface temperatures drop faster than those aloft. If there’s enough moisture in this low-level air, surface temperatures can drop to the dew point temperature and fog can form.
Valleys are prime locations for nighttime or early morning inversions, and their fogs form as cold, dense air draining from high terrain that cools as it sinks. It will take the heat of the day to heat up the ground, burn off the fog, and break up the inversion layer. Coastal areas where warmer air aloft moves over lower layers created by cold ocean currents—think California’s coastal marine layers—can also create inversions.
Fronts can also create inversions. When fronts move through, they typically bring warmer air with them. With warm fronts, warm air aloft rides up and over the colder, denser air masses below. In the colder months, any precipitation falling from the frontal surface aloft will go from warm air to cold. Snow, freezing rain, and other icing conditions can be the result. Cold fronts can push and lift the warmer air ahead of them. This warm air aloft may act as what’s called a capping inversion, or “cap” to prevent any heat rising from low-level air percolating upwards during the day—but if that heated air is warm enough, and rising fast enough, it will “break the cap,” and the stage is set for thunderstorms and broad areas of rain.
Inversions are great for trapping air and creating reduced visibilities in fog, smoke, haze, and pollutants. It’s natural for air near the surface to make thermals that mix with the air above it, but if the air aloft is warmer than the air below, then that can’t happen. You might see a thin cumulus layer that marks the warm-over-cold border, but that’s about it. As always, warm air rises, but only if the air around it is colder.
