Precipitation type and temperature
We’re getting close to Halloween; which means not only are
we on the lookout for Vampires and Hobgoblins, but also questions, on How come
its 36°F out and it’s snowing?
First, I will mention rain below 32°F:
Most of us know that it can rain even if the thermometer
shows a temperature below 32°F. If there is a layer air aloft is above
freezing, but the air at the surface is below freezing, As the raindrops
approach the ground, they move through that thin layer of cold air and cool to
temperatures below 32°F. This phenomenon is called supercooling (or forming
"supercooled raindrops").
These supercooled rain drops are called freezing rain, where the rain
freezes as soon as it hits something, like the ground, streets, cars, or trees.
Sleet is a result of the same process, except the layer of
freezing air is thicker. So, the raindrops end up freezing before reaching the
ground.
Because of this supercooling, drizzle, which is composed of
small liquid droplets, can form as liquid and remain unfrozen even when
temperatures are continually below freezing.
In these cases, the clouds form as tiny liquid drops, even though the
air temperature is below 32°F. This
happens in relatively shallow clouds in which no part of the cloud has
temperatures too far below freezing, so the cloud doesn’t contain any
snow. Sometimes the drops grow large
enough to become freezing drizzle at the ground, or liquid drizzle if there is
warm air below. Or it might even be
crunchy round snow pellets if the air is very cold below the cloud.
Snow above 32°F:
We see it here all the time when we get into late fall. The
air temperature is 38°F, 40F and looking out the window we see it’s snowing!
Weren’t we taught in school, that 32°F is the freezing point (of pure water)
and that snow/ice melts at 33°F and above? So, how can it be snowing?
To get a snowflake to form, the temp must be 32F or lower.
No exceptions. That flake then falls. Usually a few thousand feet or more, from
the cloud to the ground. Once formed and falling, it has several thousand feet
of air to deal with, before reaching the ground. Almost all precipitation
begins as snow, as ice crystals in clouds absorb super-cooled water droplets
(small liquid droplets in clouds), and grow big and heavy enough to fall from
the cloud. As long as the air temperature is below freezing on the way to the
ground, the precipitation will stay in snow form.
Suppose the air was below freezing, all the way down…until
about say, 50 feet above the surface. That would mean that the flake only has
to fall through 50 feet of warmer air before it reaches the ground.
Moisture falling through air, is not only affected by the
air; it also effects the air it moves through So, in that last 50 feet, the air
may be beginning to melt the flake, but, the flake…by the process of melting in
and of itself, is evaporatively cooling that air! evaporative cooling is a
basic thermodynamic principle surrounding evaporation. When liquid water
evaporates, it requires energy to change from a liquid to gaseous state, which
it draws in the form of sensible heat from its surroundings, thus cooling the
air. This is how sweat keeps us cool - when it evaporates, it takes in heat
from your skin. Exactly the same happens when snow melts into rain as it falls.
So, those first few minutes of flakes never reach the
ground, because they are evaporating in to the air and are cooling that air,
often several degrees or more. As I said
above the warm layer has to be thin; if it’s too thick the snowflakes will melt
into raindrops; but if the layer is thin, the flakes evaporate at first,
cooling down that 40°F air to close to 32°F. This allows the flakes behind the
ones who melted to make it to the surface.
Yes, Virginia, there can be snow falling by your window, even if it's 41°F.
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