Weather and plants

Robert Parks via pbs pbs@lists.pacificbulbsociety.net
Thu, 21 Jan 2021 12:11:52 PST
As Robert Lauf points out, your local conditions make a great difference in
what actually happens in the garden. Radiative frost is almost completely
blocked if there is any kind of tree cover, at least to the edge of the
drip line. I live in a more or less standard house on a small lot with
houses on both sides, the alleys are much cooler, and stay cooler into the
day. We have high wood fences on three sides, so I am blocked from the cool
air flowing down the hillside so the back yard is fairly clement. The front
yard is much more exposed to wind, but has enough tree cover to avoid the
frost that covers the cars parked at the sidewalk. Try moving the
thermometer around to find your yard's microclimates! I have a set of cheap
remote thermometers that vary by a degree or two, but still show the
temperatures in the various locations.

Robert
in cool San Francisco (10A/10B) where it is time to irrigate again!

On Thu, Jan 21, 2021 at 10:51 AM Robert Lauf via pbs <
pbs@lists.pacificbulbsociety.net> wrote:

> Dew point is another way of saying the air is very dry.  Frost is the
> result of radiative cooling, and the biggest things to worry about are: a
> very clear sky and no air movement.  So low humidity will definitely
> contribute to a clear sky.  Air movement can depend on local geography.  If
> you're in a protected valley or dip, you are in more danger than if you are
> on top of a ridge, ceteris paribus.  I'm at the top of a ridge and my first
> frost in fall is two weeks later than down at the bottom of the hill 1/2
> mile away.  And my last frost in spring is correspondingly earlier.  So an
> extra month of growing season overall.
>
> So you can see why orange groves often fight frosting conditions just with
> big fans, then maybe some smoke, and when the air finally approaches
> freezing, then the sprinklers come on.
>
> My advice is to pay attention to the rhythms of nature as they play out in
> your garden, because that's the only thing that matters to you, and it can
> be surprisingly different from even a nearby area.  If you want to get a
> dramatic feel for microclimates, just ride a motorcycle for a while!
>
> Bob    E Tenn and a gloomy 41 degrees
>
>
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