ATOC 181 Lecture Notes - Lecture 14: Downburst, Leading Edge, Tropopause
7 Jun 2018
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LESSON 14
PRECIPITATION
Review from Last Lesson
• Ice pellets are transparent and they bounce. That is the difference with snow pellets. Ice pellets cannot
be seen very often.
• Snow pellets start as ice crystals and they collide with super-cooled droplets. At the ground, the
temperature is slightly below 0deg. They will fall without having melted
• Ice pellets go through a region where temp is slightly above 0deg and then the temp decreases. Have
snowflakes falling, melt, become rain drop and as they go through a layer of colder air below 0deg, they
will freeze again and form ice pellets.
• The thickness of the deep freezing layer is important.
Freezing Rain
• Glaze
• Layer below the precipitation cloud too shallow for the melted snowflake to freeze again
• Can cause falling, car accidents
• Why can have freezing rain: have it when the deep freezing layer is very shallow (below 0oC), so the rain
does not have time again to freeze again to become ice.
o Fall as super-cooled water on the ground and will freeze as soon as reaches the cold surface
(below 0oC) and forms layer of ice.
• The thickness of the deep freezing layer is important for getting ice pellets vs getting freezing rain.
o Very difficult to know in advance the thickness of this layer, so difficult to forecast freezing rain.
o Depends on how well the forecasters were able to assess and determine the thickness of the
layer below 0oC.
• Thickness of layer = thickness of air below 0oC. It is not the temperature that defines the thickness of the
layer
• If in 1km near the surface and rain is falling and not have enough time to become ice, fall as super-cooled
liquid water and freeze
• If layer is 3km, rain has enough time to become ice and fall as ice pellets. Less dangerous.
• Can be measured in some place, very difficult to determine if layer is thick enough or not
• When the super-cooled liquid raindrops reach the cold ground or a cold surface near the ground, it
freezes instantly.
• Have surface that is below 0oC: if have water on this surface, it freezes. The air temperature increases
and then decreases in the case when have this type of precipitation, have a region in the middle where
above 0oC. Have snow as below 0oC , then enter region where above 0oC so become rain and then reach
region where below 0oC again but shallow layer, so rain start to freeze but still liquid and freezes when
reaches surface. The same case can apply and freezes in the last layer to become ice pellets and fall as
such as layer is thick enough.
• If the drops D < 0.5mm : freezing drizzle
• Devastating effects resulting from freezing rain: January 1998 in Quebec.
o Have freezing rain. Ice layer forms on the ground, on cables, on anything below 0oC.
o In the case of power lines, ice layer was so heavy that the lines broke.
o 2.5 million people without power for 5-6 days. All bridges linking Montreal to the rest of the
world were closed.
o If lasted one more day, would have been a huge catastrophe. At the end, 3.5 million people
without power.
• Ice storms can be lethal.
find more resources at oneclass.com
find more resources at oneclass.com
Forms of Precipitation
• The curve of temperature = red
• If have snow falling, temp always below 0oC
• If have deep freezing layer, ends up with ice pellets:
snow melts, become rain and freezes
• If have shallow deep freezing layer: freezing rain
• If temp always above 0 oC: have rain
• In 1998, 10cm of ice on the ground
• Average annual number of days with freezing rain:
here can expect up to 20-30 days of some type of
freezing rain. In Vancouver, practically never
Hailstones
• Ball or lump of ice with diameter > 5mm
• Not always round
• Form of precipitation
• Small/medium = spherical
• Large = oblate spheroidal
o Rough surfaces often with lobes
o Layered internal structure (like an onion)
o theory: fall through clouds, picked up by updrafts, go up, another layer of ice forms and the
process repeats again: lift theory → not a valid theory: demonstrated through experiments and
measurements
• may have devastating effects
• associated with Cb (cumulonimbus): initiated and created in this type of clouds. Not see it in other types
of clouds
• have an updraft, warm air creating an embryo, will
accumulate ice mostly through collision with super-cooled
droplets or with other ice particles, will become hail, will be
picked up by downdrafts, will shower and hail will fall. Hail can
be ejected by clouds and fall. In suspension, accumulate ice
and then fall
• largest hailstone in Coffeyville, Kansas in 1970: 14.2 in
diameter.
o Weight = 766g.
o Nancy Knight: one of the person at the origin of the lift
theory
• largest measured hailstone: 17.8cm in diameter
• the observer in the US/Canada named the hailstones based
on the size.
o Named after common objects
• get them in the Prairies: Alberta, Saskatchewan. Most of them in Alberta
• the greater frequency of hailstorms in Alberta, Prairies
• There is a symbol associated with different types of precipitation
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Ice pellets cannot be seen very often: snow pellets start as ice crystals and they collide with super-cooled droplets. At the ground, the temperature is slightly below 0deg. Ice pellets go through a region where temp is slightly above 0deg and then the temp decreases. Have snowflakes falling, melt, become rain drop and as they go through a layer of colder air below 0deg, they will freeze again and form ice pellets: the thickness of the deep freezing layer is important. It is not the temperature that defines the thickness of the layer. If in 1km near the surface and rain is falling and not have enough time to become ice, fall as super-cooled liquid water and freeze. If layer is 3km, rain has enough time to become ice and fall as ice pellets. The air temperature increases and then decreases in the case when have this type of precipitation, have a region in the middle where above 0oc.
