|THE RATE AT WHICH RAIN FREEZES|
IN A FREEZING RAIN EVENT
METEOROLOGIST JEFF HABY
Three misconceptions about
freezing rain are that it, "(1) always covers everything with a sheet of ice" and
"(2) always freezes on contact upon reaching the earth's surface" and "(3) the raindrops are always
upon impacting the surface". In this essay we will explore these three misconceptions and discuss the processes
that determine how fast freezing rain freezes.
(1) Freezing rain does not always coat everything in
a sheet of ice. The temperature of each individual object
on the ground determines if the rain will freeze. It is elevated structures that have the greatest potential
of freezing the rain (i.e. power lines and trees). This is especially true if the soils are warm. I have seen
many instances where trees and elevated structures froze, but the streets and ground remained wet due to warm
soils. This will also happen also when it is a "marginal" freezing rain event (temperature in 30's F) and the
ground soil temperature is above freezing. Expect a freezing rain on elevated structures and not the ground
surface if the temperatures were warm a couple of day before the freezing rain event and the air temperature
has not dropped below 30 F. The opposite will be the case if a freezing rain event is preceded by below freezing
temperatures for several days and temperatures during the event are below 30 F. In that case, you will have a coating
of ice on just about everything exposed to the air outside.
(2) The freezing process is never instantaneous. Freezing rain
does not freeze on contact with the earth's surface,
even if the rain is supercooled. It takes time for water to turn to ice just as it takes time to make ice cubes
from water in a freezer. Because freezing rain does not freeze right away, icicles and "dripping icicles" will
develop. The rate at which a water droplet freezes upon hitting a below freezing object depends upon these three
items: (a) the temperature of the drop, (b) the temperature of the object the drop strikes, and (c) the size of
the drop. The quickest drops to freeze will be small supercooled drops striking an object that is well below
freezing. The slowest drops to freeze will be large warm drops that strike an object that is 32 F.
(3) Some definitions of freezing rain state that the rain is
supercooled as it strikes the surface. This may or
may not be true. Supercooled water is water that is below freezing. Water has difficulty freezing unless it has
something to freeze upon to instigate the freezing process. If a droplet is supercooled it will freeze much
faster than an above freezing raindrop once it strikes a below freezing surface. This is because a rain drop
below freezing will partially or completely counteract the fact that the freezing processes releases
and warms the air immediately surrounding the droplet. Supercooled droplets striking an object well below freezing
is the closest freezing rain will come to "freezing on contact".
The rain drops could be well above freezing before striking a freezing surface. The warmth of the rain drops could
even be enough to warm the temperature above freezing and end the freezing rain event. The depth and maximum warmth
elevated warm layer aloft and the depth and minimum chill of the low level
PBL determine the temperature of
a rain drop just before it strikes the surface. A warm and deep EWL and a shallow layer of lower PBL temperatures
below freezing will allow raindrops to fall that may be well above freezing. These "warm" raindrops will take much
longer to freeze than the "supercooled" raindrops. The warm raindrops must be both cooled to the freezing point
and crystallized to ice.
Freezing rain can be defined as "rain that freezes shortly after striking a surface that is at or below freezing".