This follows up nicely with net radiation because we just talked about
the fluxes at night. Mark said what happens at night. The incoming is 0
and if the sky is colder than the ground there's a continuous flux of radiation
to the cold night sky, and the surface can get very cold. It's under
appreciated how cold surfaces can actually get at night when the sky is
clear. One of the examples of the power of this is if you build an insulated box
especially a tall box. I'm going to draw this here. If we build
a circular cylinder like this, and it's well insulated.We fill
this. This is open on the top but it's full of insulation. Now we put at the
bottom, a dish of water. We paint this black to have a high emissivity. This is
open. We take this cylinder and we put it outside in in the summer on a clear
night. This a PhD comprehensive exam question I like to ask. What happens to
the water? This is room temperature. I should have little blue for
the water, and a shallow dish of water.
The air temperature out here, let's say it's 30
degrees C, it's a pretty warm night but it's clear. What happens to the water?
Does it change temperature? Yes. Does it get hotter or colder? Colder. It's
radiating to the clear night sky. When we went outside with those things and it said the
sky temperature is -30 or some cold temperature. The incoming radiation is
real tiny because it's cold. It can't warm up by conduction and convection
because we insulated this thing. If if the wind is blowing, it'll keep it
warm, but if it's a calm night and it sits there... I read this in
some physics book a long time ago. So I built this in my backyard,
and got all the instruments, I put thermocouples in here and sat out there
at night and measured this thing... It gets colder and colder. It's continuously
radiating to the sky and even when the air temperature is 30 degrees the water
freezes inside this column. It's coming to equilibrium with the sky,
a super cold temperature. This isn't just a physics experiment. People can use
this in the tropics to keep food cold. You can radiate to the sky at night and
then in the daytime you just cover it, and you cool it down at night. No
electricity just the physics of radiation to the sky. This gives you a
sense of the power of longwave radiation at night. What happens to a
surface, a leaf of a plant or a surface, at night when it's clear? If it's cloudy not
much happens because the clouds are low and they're similar to ground
temperature, but if it's clear they cool off.
You run models because we if you buy a net radiometer and you measure it and
model it. We see conditions where it's easily five degrees C below air
temperature and there's cases when it's in 10 degrees C below air temperature
at night. That led us to make this sensor, a radiation frost sensor, which is
a way cheaper version than having to buy a net radiometer and run all these
models any way to predict surface temperatures at night. It's black so
it has a high emissivity. It's similar, like 0.96, it's similar to leaf
surfaces, black disc. Then we put it outside and we measure then the
temperature of the surface of that disc with a tiny thermistor. I'll show you
some data in a minute but you can see how this simulates a leaf temperature. Or
there's another sensor here and a tiny little thermistor, for a bud of a plant.
This is called when a surface freezes and the air temperature never
got below freezing that's called a radiation frost. It's a huge deal
worldwide for commercial fruit producers where the sensitive flowers are
coming out, you can see those in the background, little buds in the spring
coming out on that plant and if those get below freezing they can lose
millions of dollars in commercial yield because they're flowers freeze.
Because it's such a big deal they start wind machines, they start sprinkler
irrigation to freeze water, they do all kinds of things to protect their crop,
but they need to know exactly when to start this. I call this radiation
frost sensor or also a nighttime surface temperature sensor. It measures these
physical principles of radiation transmission
and temperature. I think my next slide, we don't have one of these here.
We could have picked them even worse data than this but this is a
typical data for a night in our spring in the northern hemisphere where this is
that sensor and the temperatures are all coming down together but it's a clear
night hours from midnight ,we're getting close to midnight, now look at that big
leaf sensor cooling off, a little bit. When it's calm wind this is wind
speed it really starts to diverge a lot. The wind will warm it back up
when it gets cold but if there's not a lot of wind it just keeps getting
colder. Right in here somewhere if you are only measuring air temperature
you wouldn't be worried about a frost. The air temperature is + 3
degrees C but the leaves are already starting to freeze. It gets bigger
and bigger, and finally four hours after midnight the air finally gets below
freezing but the plant has been below freezing for quite a while. Frost is
the big economic thing for freezing of sensitive buds, but it's also gives you
an indication of nighttime surface temperatures. We talked about
measuring those in the day and predicting water loss at night. It's more
of models and predicting rate of development of plants. If we understand
the night temperature, we can predict rate of development. It's a pretty
simple sensor it's it's not an expensive sensor and it's been particularly useful
to commercial tree crop producers to tell when to turn on their fans to
protect their crop at night. This is a pretty simple story once
you understand the concept of net radiation.