Winter Weather Q&A!

An unusually early freeze occurred from November 29 to December 4, with a low temperature of 26F on the last night. This prompted the usual flurry of breathless media reports and phone calls about frost protection measures. This was not a major weather event, but we don’t usually see these temperatures until later in December.

Then a couple of dozen cars piled up in the fog on Highway 5 near Woodland on December 15. Our famous tule fog led to very poor visibility, and people were driving too fast. Fog and imprudent drivers don't mix. But fog is a gardener's ally, providing moisture and natural frost protection in winter. Landscapes can be very pretty in the fog.

A customer asked me 'why does frost happen?' Leaving aside the metaphysical aspects of her question, I decided it was a good time for a Winter Weather Q & A.

What causes frost?

Frost occurs when the temperature drops below the dew point, then below freezing.

A surface loses heat to the sky during the night. On a cold, still night the temperature of that surface may drop below freezing. If the dew point is above freezing, the water vapor that is in direct contact with the surface will form droplets of water, which will then freeze. Then the water vapor in contact with those frozen droplets will freeze, and so on. The more water vapor (i.e., the higher the humidity), the more frost.

Ok, what's the dew point?

It is the temperature at which water vapor (the gaseous water that is in our air) converts to liquid. That is visible in the form of fog (tiny suspended droplets), or dew (larger droplets that form on a surface). Dew, by the way, doesn't drop out of the air onto a surface. The droplets condense directly onto the surface.

Why do you see frost on some surfaces before it occurs on others?

Because some surfaces get to freezing before others do. The temperature reported on the news is the air temperature. If you had little sensors all over your yard, house, and car (wouldn't that be an interesting study?), you'd see that some surfaces get cold much faster than others do. They are more efficient -- faster -- at losing heat.

Metal and glass lose heat very rapidly, and may have visible frost before midnight. Grassy surfaces are less efficient. Concrete and asphalt that are in contact with soil lose heat more slowly -- good thing, or our roads would be very dangerous! The soil below acts as a heat reservoir. Bridges become icy because they don't have that soil contact.

Open, plowed soil, a pile of leaves, a layer of bark mulch -- all those lose heat pretty quickly. A leaf exposed to the air, such as one sticking up above all the others, loses heat faster than those that are nestled together. So water will freeze first on the more exposed surfaces, and those which conduct heat efficiently.

How can I know when frost is likely?

The temperature on a cold, still night drops steadily at 1 - 2 degrees per hour until sunrise. So if we're in the low 40's at sunset there's a pretty good chance of frost by morning. But it tends to 'stall' at 32 degrees because water releases a bit of heat as it changes from gas to liquid, and from liquid to ice. That principle is at work when growers flood or sprinkle their orchards and vineyards on a freezing night--the large amount of water releases a little heat as it freezes. Until the pipes freeze. Then you have an icy mess.

What do I do?!

Remain calm. Plant protection on a single night of frost is not necessary. Expect your summer annuals to die. Tender plants in pots can be moved under an overhang on the east or south side. Make sure container plants are watered. Don't worry about your Citrus or most other subtropicals.

Why does it matter if the night is still?

The heat that has radiated from surfaces is trapped in a layer many feet above the ground. If you watch the smoke rise from a fire you can see this layer (called a temperature inversion). Unfortunately, it may be just a few feet above the ground where we are breathing…. Wind mixes this warmer air back with the colder air below, and can prevent a frost. Hence the large fans you see over vineyards in the Napa Valley: they push the warmer air back down.

What causes fog?

Fog occurs when the temperature drops below a higher dew point. If there is lots of water vapor, the dew point is higher. It may even be high enough near the ocean, or lakes or rivers, to cause fog during the summer.

Why is it rare to get fog and frost on the same night?

Water droplets condense out into visible fog. That process releases a little heat, and the fog itself traps the heat that the surfaces are losing and slows the process. It still gets colder, but usually not below freezing.

What is a tule fog?

Our local 'tule fog' occurs on a night when the humidity is moderately high. Newly plowed fields lose heat pretty quickly, and a dense layer of fog can occur just a few feet above them--much as it did over the native tule reeds, which grow in moist loamy soil. Under similar conditions, but with lower humidity, we are more likely to get frost.

Fog occurs first over Putah Creek, because the water loses heat very quickly at night (i.e., it is an efficient conductor of heat). And the humidity above the water tends to be higher because of evaporation from the surface.

What is a freeze?

It is a large, cold air mass that covers the entire region.

A freeze of several days' duration can occur when colds air moves over us from the north, draining and settling into our bowl-like valley (hot air rises, cold air settles). Damage to plants is due to desiccation and repeated exposure to low temperature. A freeze that moves in on a strong north wind has the greatest potential for harming plants, and the longer the cold air stays around the more damage there will be. The epic freeze of December 1990 blew in on a strong, dry wind; it dropped to 16 - 19F in many areas, and got below freezing every night from Dec. 19 - Jan. 4. Many plants were dry because there had been little rainfall that year.

What causes the plants to die?

Damage results from drying (desiccation) of leaves and twigs because of the very low humidity and the wind that occurs as the air mass moves in. Wind is simply the movement of air from a region of high to low pressure; the greater the difference between high and low pressure, the stronger the wind. A well-watered plant will resist damage up to a point. If the wind continues and the humidity is very low leaves may be 'burnt' because the roots just can't take water up fast enough.

What can I do?!

Plant protection in these extreme situations involves reducing the drying of individual plants, as well as applying heat if possible. A strand of holiday lights on a plant that has been draped with 'floating row cover' will be well protected. A good soaking before hand, especially if it is in a container, can make a big difference. Other freeze protection measures will depend on how cold it gets, and for how long.

These conditions can all be observed close together. It isn't uncommon for me to leave my house in Dixon on a sunny morning, with frost on the ground, and then hit a wall of fog where Highway 113 splits from 80 near Putah Creek-on the edge of the UC Davis campus. That, I guess, would be what we call an 'academic fog'. (Rimshot, please).

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