In the last lecture we learned that the ice crystal process works in the middle part of a cold cloud where there is a mixture of ice crystals and lots of supercooled water droplets.  Now we'll see how ice crystals can quickly grow in that environment into precipitation size particles.


This first figure above  shows a water droplet in equilibrium with its surroundings..The droplet is evaporating (the 3 blue arrows in the figure).  The rate of evaporation will depend on the temperature of the water droplet.  The droplet is surrounded by air that is saturated with water vapor (the droplet is inside a cloud where the relative humidity is 100%).  This means there is enough water vapor to be able to supply 3 arrows of condensation.  Because the droplet loses and gains water vapor at equal rates it doesn't grow or shrink.


This figure shows what is required for an ice crystal (at the same temperature) to be in equilibrium with its surroundings.  First, the ice crystal won't evaporate as rapidly as the water droplet (only 1 arrow is shown).  Going from ice to water vapor is a bigger jump than going from water to water vapor.  There won't be as many ice molecules with enough energy to make that jump.  A sort of analogous situation is shown in the figure below.  The class instructor could and most of the people in a classroom could jump from the floor to the top of a 12 or 15 inch tall box.  It would be much tougher to jump to the top of the cabinet (maybe 30 inches off the ground).  There wouldn't be as many people able to do that.

To be in equilibrium the ice crystal only needs 1 arrow of condensation.  There doesn't need to be as much water vapor in the air surrounding the ice crystal to supply this lower rate of condensation.




Now what happens in the mixed phase region of a cold cloud is that ice crystals find themselves in the very moist surroundings needed for water droplet equilibrium. This is shown below.

The water droplet is in equilibrium (3 arrows of evaporation and 3 arrows of condensation) with the surroundings.  The ice crystal is evaporating more slowly than the water droplet.  Because the ice crystal is in the same surroundings as the water droplet water vapor will be condensing onto the ice crystal at the same rate as onto the water droplet.  The ice crystal isn't in equilibrium, condensation (3 arrows) exceeds evaporation (1 arrow) and the ice crystal will grow.  That's what makes the ice crystal process work.

The equal rates of condensation are shown in the figure below using the earlier analogy.


Most everyone can manage to make the big or the small jump.
Now we will see what can happen once the ice crystal has had a chance to grow a little bit.

 

Once an ice crystal has grown a little bit it becomes a snow crystal (this figure is on p. 102 in the photocopied classnotes).  Snow crystals can have a variety of shapes (plates, dendrites, columns, needles, etc.; these are called crystal habits) depending on the conditions (temperature and moisture) in the cloud.  Dendrites are the most common because they form where there is the most moisture available for growth.  With more raw material available it makes sense there would be more of this particular snow crystal shape.


Here are some actual photographs of snow crystals (taken with a microscope).  Snow crystals are usually 100 or a few 100s of micrometers in diameter (tenths of a millimeter in diameter).  The different shapes are called "habits", I probably didn't mention that in class.

You'll find some much better photographs and a pile of addtional information about snow crystals at www.snowcrystals.com

A variety of things can happen once a snow crystal forms.  First it can break into pieces, then each of the pieces can grow into a new snow crystal.  Because snow crystals are otherwise in rather short supply, ice crystal multiplication is a way of increasing the amount of precipitation that ultimately falls from the cloud.

Several snow crystals can collide and stick together to form a snowflake.  Snow crystals are small, a few tenths of a millimeter across.  Snowflakes can be much larger and are made up of many snow crystals stuck together.  The sticking together or clumping together of snow crystals is called aggregation (I frequently forget this term.  If I can't remember it I don't expect you to remember it either)


Snow crystals can collide with supercooled water droplets.  The water droplets may stick and freeze to the snow crystal.  This process is called riming or accretion (note this isn't called collision coalescence even though it is the same idea).  If a snow crystal collides with enough water droplets it can be completely covered with ice.  The resulting particle is called graupel.  Graupel is sometimes mistaken for hail and is called soft hail or snow pellets.  Rime ice has a frosty milky white appearance.  A graupel particle resembles a miniature snow ball.  Graupel particles often serve as the nucleus for a hailstone.


This figure gives you an idea of how hail forms.


In the figure above a hailstone starts with a graupel particle (Pt. 1, colored green to represent rime ice).  The graupel falls or gets carried into a part of the cloud where it collides with a large number of supercooled water droplets which stick to the graupel but don't immediately freeze.  The graupel gets coated with a layer of water (blue) at Pt. 2.  The particle then moves into a colder part of the cloud and the water layer freeze producing a layer of clear ice (the clear ice, colored violet, has a distinctly different appearance from the milky white rime ice), Pt. 3.  In Tucson this is often the only example of hail that you will see: a graupel particle core with a single layer of clear ice.

In the severe thunderstorms in the Central Plains, the hailstone can pick up a new layer of rime ice, followed by another layer of water which subsequently freezes to produce a layer of clear ice.

This cycle can repeat several times; large hailstones can be composed of many alternating layers of rime and clear ice.  An  unusually large hailstone (around 3 inches in diameter) has been cut in half to show (below) the different layers of ice.  The picture below is close to actual size.

Hail is produced in strong thunderstorms with tilted updrafts.  You would never see hail (or graupel) falling from a nimbostratus cloud.  


The growing hailstone can fall back into the updraft (rather than falling out of the cloud) and be carried back up toward the top of the cloud.  In this way the hailstone can complete several cycles through the interior of the cloud.


One last figure showing some of the things that can happen once a precipitation particle falls from a cloud


Moving from left to right, a falling graupel particle or a snow flake can move into warmer air and melt.  The resulting drops of water fall the rest of the way to the ground and would be called RAIN.  Note sometimes the grauple will reach the ground before melting.

Sometimes the falling raindrops will evaporate before reaching the ground.  This is called VIRGA and is pretty common early in the summer thunderstorm season in Arizona when the air below the thunderstorm is dry.  Lightning that comes from thunderstorms that aren't producing much precipitation is called "dry lightning" and is a common cause of brush fires.

Rain will sometimes freeze before reaching the ground.  The resulting particle of clear ice is called SLEET.  FREEZING RAIN by contrast freezes once it reaches the ground.  Everything on the ground can get coated with a thick layer of ice.  It is nearly impossible to drive during one of these "ice storms."  Sometimes the coating of ice is heavy enough that branches on trees are broken and power lines are brought down.  It sometimes takes several days for power to be restored.

Satellite photographs are a good way of observing clouds (especially out over the ocean).  Using both visible and infrared light satellite photographs, you can get a good idea of cloud type.  However satellite photographs don't really tell you whether a cloud is producing precipitation or how much precipitation a cloud is producing.  For that you need radar.

In some ways a radar image of a thunderstorm is like an X-ray photograph which provides a partial view of the insides of a human body.  The Xrays pass through the flesh but are partially absorbed by bone. 


The radio signals emitted by radar pass through the cloud itself but are reflected by the much larger precipitation particles.  The intensity of the reflected signal (the echo) depends on the number and the size of the precipitation particles.  Red generally means an intense reflected signal and heavy precipitation.  The edge of the cloud isn't normally seen on the radar signal.




Weather radar can be used in a variety of ways.  An ordinary radar periodically transmits a short burst of microwave radiation.  The radar keeps track of what direction the antenna is pointing and determines how long it takes for a signal to go out and return.

Conventional radar can thus determine the direction and distance to a precipitating cloud and make an estimate of the rainfall rate or intensity.

The radar antenna slowly spins as it is transmitting so it scans a full 360 degrees of azimuth in a minute or two.  Information from a single radar or a combination of data from many radars are drawn on weather maps (the PPI display above shows the data from a single radar, the radar would be at the center of the picture).  


The radar antenna can also be pointed at an interesting storm and the radar antenna can then be caused to scan vertically through the storm.  This produces the RHI display shown in the figure above.

By detecting changes in the frequency of the reflected signal, a doppler radar can measure the speed at which precipitation particles are moving toward or away from a radar antenna.  By combining data from 2 or more radars (and some complicated computer processing), three-dimensional wind motions inside a cloud can be mapped out. 

Doppler radars can detect a rotating thunderstorm updraft (a mesocyclone) that could indicate a thunderstorm capable of producing tornadoes.  Small mobile doppler radars are being used to try to measure wind speeds in tornadoes.  Police use doppler radar to measure the speeds of automobiles on the highway (moving toward or away from the police car).

Dual polarization radar is a research tool that can be used to learn something about the kinds of precipitation particles inside a cloud.