Weather in Davis!
Originally published in the Davis Enterprise, Wed. 26 Nov 2003
Can't do anything about the weather?
Well, you can plan for it!
There's no such thing as 'normal' weather, so
let's talk about averages.
Thanksgiving usually marks the date of our first frost. Frost is unusual
after Valentine's, although we had a rare April frost in 2002. It's hard
to imagine that just a few weeks ago temperatures may have been in the 90's!
Frost occurs on still, clear nights when heat from the ground radiates to
the sky until it drops below freezing. As water vapor turns to liquid and
then to ice, some heat is released, so temperatures on these frosty nights
hover at 29 - 30F.
This nips the newest foliage on subtropicals such as
Citrus, but a string of holiday lights (not the little twinkle lights)
generates enough heat to prevent damage.
The Valley Fog
Sacramento's tule fogs are famous. These occur under similar conditions to
frost, except that the 'dew point' (the temperature at which water vapor
condenses into liquid droplets) is higher. Invisible water vapor becomes
They happen first over Putah Creek and open fields
because those surfaces lose heat rapidly, and hover just a few feet above
the ground. Very pretty to look at and walk in, but very dangerous to bike
or drive in.
A ridge of high pressure then settles over us; conditions are stagnant and
air is 'trapped'. Temperatures drop steadily, ground and river fogs
coalesce into one big foggy mass. Satellite images show a big bank of fog
nestled in the soup bowl of the Sacramento Valley: an hour's drive east or
west gets you up and out of it.
This gloom can settle in for days, lifting by
mid-day (but not clearing) and settling back down at night. In one record
event we went 15 days without seeing the sun, and the day's high
temperatures barely topped the mid 40's. That can make folks pretty
cranky, and it's usually right around the holidays. "White Christmas": has
a different meaning here.
occur infrequently and are memorable. In 1998 the Northern California
citrus crop was destroyed. In 1990 mature Eucalyptus and Citrus trees, and
many other landscape plants, were badly damaged or killed. A very cold air
mass moved over the entire region, with a dry wind, and temperatures
dropped into the low 20's or even the teens.
Weather folks on TV give us a day or so warning, but they tend not to
differentiate between frosts and freezes. Don't panic on a frosty night,
but take action when a regional freeze threatens. Here's a hint: still &
cold = frosty. Windy & very cold = freezing. Don't pick all your lemons
on a frosty night, but you may lose your crop during a prolonged freeze.
Freezes occur between mid-December and early January, when days are
shortest and preceding days have already been foggy and cold. The 1998
freeze happened December 26, so folks returned from holiday travel to find
dead plants. Going away? Tender plants should come inside, and marginal
ones should be tucked into south or east facing nooks near the house for
The greatest risk to plants in a freeze is desiccation (damage to leaves
and stems caused by drying), so soak well (especially container plants).
Succulent leaves rupture as the water inside freezes, so your jade plant
should be brought indoors. Our goal with woody plants is to keep them
alive-don't worry about the foliage of Citrus or Bougainvilleas at this
Protection measures will each give a couple of degrees of
protection. Securely drape with plastic to trap heat and prevent drying;
spray the plant with an anti-transpirant; leave the aforementioned holiday
lights (or a shop light with a 40-watt bulb) on all night.
Rainfall in a Mediterranean climate.
We average 17 - 20" of rain, nearly all of it between mid-October and April.
January and February have the most. 1980-'81 we had 42"; in the worst
drought year of 1977 we had less than 10".
Rainfall May - Sept. is negligible; an occasional storm spins up from the
southwest and startles us with a half inch of rain-barely enough to settle
the dust or wet the lawn chairs.
Winter storms mostly originate in the Gulf of Alaska and head towards the
Pacific Coast. When the "storm door" is open these hit every few days.
Whether they hit us (sometimes) or Oregon and Washington (usually) depends
on how far south the jet stream has shifted.
These storms are cold and
fast-moving, dumping 1/2" to 1" of rain in less than a day. The snow level
drops to a couple of thousand feet, and there may even be little snow
flurries on the valley floor. Although water may puddle for a few hours,
our soils can readily absorb this amount of water.
Some storms come off the warmer part of the Pacific, and warm air can hold
more moisture. These storms are bigger, wetter, and slower, lasting a
couple of days. The snow level is usually above 5000'. When these hit the
coastal mountain range the clouds elevate and cool rapidly, leading to
some very high rainfall totals on this side of the Valley. Last December 
folks behind Winters recorded 6 - 7" of rain during one storm in early
December! These "Pineapple Express" storms really pack a punch when they
collide with cold air from the Gulf of Alaska as the sudden cooling
squeezes out the moisture.
Clay soils can't absorb water when it comes down this fast, so runoff and
standing water are inevitable. Plants are surprisingly tolerant of having
their roots stand in water for a day or so in the winter, because the
fungi which attack roots are mostly inactive in cold weather.
More than a day without oxygen in the root zone can be harmful, and root damage
isn't noticeable until warmer weather. Digging a quick channel (a fun
project for kids!) to drain the water to the curb or lawn can save
These are the storms which break branches and topple trees. Fall or early
winter would be good times for a visit from a certified arborist to
correct poor branch angles or reduce the weight on large-limbed
Check the staking of young trees, as these storms whip the
branches and tops. Trees that fall suddenly usually had defective roots to
begin with. A young tree whose trunk is an inch or more in diameter may be
impossible to straighten, and should probably be replaced-but check with a
Climatologists and oceanographers know that ocean temperatures follow a
pattern along the Pacific coast: some years they are warmer than average
(El Niño), and some years they are cooler (La Niña). This gradual movement
of warm water back and forth across the Pacific is called the Southern
Oscillation. El Niño years tend to be wet here; La Niña years tend to be
drier and colder overall (but with more Pineapple Express storms hitting
Northern California). In between are years like this  (La Nada?).
So expect many days this winter with morning fog, a week or so of Valley Fog, a dozen or so storms at 1 - 2 week intervals, 17 - 20" of rain, and about a dozen
frosts between Thanksgiving and Valentine's. But watch that Weather
Channel, especially around the holidays!
© 2016 Don Shor, Redwood Barn Nursery, Inc., 1607 Fifth Street, Davis, Ca 95616
Feel free to copy and distribute this article with attribution to this author.
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