Transcript No Slide Title

Precipitation and Soundings
Precipitation comes in many different
types including snow, rain, drizzle, ice
pellets, freezing rain and hail. In this
Presentation we begin by looking at the
forms and shapes of the particles that
make the precipitation. We then look at
the different types of precipitation on the
ground. Finally, because the form of the
particles depends on the temperature
and humidity conditions that they
encountered in the air above, we learn
about soundings (the vertical profiles of
temperature, humidity and wind in the
atmosphere). And since soundings are
taken during clear weather as well as
storms, we learn to relate each sounding
to the type of weather it indicates.
A weather balloon about to lift a radiosonde (not the meteorologist) through the
atmosphere to measure T, p, RH, and wind speed and direction.
Snow and Ice Crystals
Note: Spikes on each branch
are parallel to the next branch.
Crystal Shapes
Snow crystals are built on
a hexagonal plan but
assume many different
shapes
including
flat
plates, long columns or
needles, clusters, bullets
with pyramidal endings,
capped columns, and the
classic 6-sided stars (i. e.,
dendrites). The next slide
shows that crystal shape
tells the temperature and
degree of supersaturation
at which the crystals form.
Stars form only at high
humidity at T  -15C.
At T 0C, snow crystals get wet. They then stick together
when they collide and can grow into giant clusters or colonies.
A blizzard (heavy snowstorm with cold temperatures and
strong winds) with huge snowflakes, Upper Saddle River, NJ
In the Good Old Days When Winter Was Really Winter
Great Lakes Effect Snowstorm, New York State, January 2007
Raindrops
Small raindrops and cloud droplets are spheres, not tears my dears.
(Surface tension pulls them into the most compact shape). Large
raindrops flatten on bottom (as they fall) and resemble hamburger buns.
Tear-shape drops only occur when the drops slide down surfaces that can
get wet (like windows or skin on our faces).
Most raindrops (except those that form in clouds over tropical oceans)
begin their lives as snow crystals or snowflakes several km above sea
level and melt when they fall into warmer air near the ground.
Some winter storms produce truly
unBEARable weather. In addition to
snow, there can be freezing rain,
which is rain that freezes on contact
with the ground. This is also called
black ice, the most dangerous,
paralyzing form of precipitation from
winter storms. Freezing rain creates
a world without friction.
When T < 0C at ground level, how
can we tell whether precipitation will
come in the form of snow, freezing
rain, or ice pellets?
The answer is that we must look at
the SOUNDINGS, vertical profiles of
T, Td, and winds.
http://severInewx.atmos.uiuc.
edu/06/online.6.1.html
Ice Pellets
Ice pellets (also called
sleet)
are
refrozen
snowflakes. They occur
when a layer of air aloft
is warm enough (> 0C)
to melt snowflakes into
raindrops but the air
near the ground is cold
enough to freeze the
drops. (The refrozen
crystals never regain
their virginal form.) Ice
pellets occur in winter
storms and sting when
they hit your face.
Ice pellets are not hailstones, which are much larger accretions of frozen
raindrops and cloud droplets. Hailstones only form in severe thunderstorms
because they need large updrafts to suspend them long enough to grow large.
FORMATION OF PRECIPITATION
To produce precipitation, cloud droplets and crystals must form and then grow
large enough to fall to the ground. There are three stages in the growth
process.
1. Nucleation. Vapor molecules collide and stick to aerosols to form an embryo
droplet or crystal. This begins to happen when RH < 100%. For example, salt
particles get wet and expand (deliquesce) when RH > 75%.
2. Diffusion. When air is slightly supersaturated with water vapor (RH > 100%),
vapor molecules drift or diffuse toward the droplet or crystal and stick. When
both supercooled liquid droplets and ice crystals are present the crystals grow
at the expense of the droplets, which shrink and often evaporate.
3. Collision and Collection. (Coalescence for drops and Accretion for snow
and hail.) Large particles fall faster than small particles, so they collide and the
large particles collect or engulf the smaller particles. At this stage, raindrops
grow like planets.
COALESCE
Snowflake Growth by Accretion (Riming)
As snowflakes fall through a
cloud full of tiny supercooled
droplets, they collect the
droplets in their path, which
then freeze on them. In this
accretion or riming process
they grow and fall to the
ground. If air near the ground
is warm they will melt to rain
drops.
During the accretion process
the original pristine crystal
form
is
covered
and
destroyed. This specimen
resembles an animal that has
been engulfed by parasites.
Planets form and grow by the
same accretion process (and
may melt for a time).
Hailstones
Coffeyville, NB Record Hailstone
Hailstones only form in thunderstorms. Large updrafts loft the stones, allowing
time to grow by accretion as raindrops and cloud droplets collide with them and
freeze. Hailstones often have alternating rings of cloudy and clear ice showing
that they have moved up and down within the storm. Cloudy ice has trapped air
bubbles due to rapid freezing in cold air. Clear ice forms in air just below 0C,
so it freezes slowly enough for bubbles to escape.
The largest recorded hailstone in the United States by diameter (8 inches) and
weight (1.93 pounds) fell in Vivian, South Dakota on July 23, 2010.
PLANETARY ACCRETION
Clouds (Nebula) of gases and particles
surround newly forming stars. The particles
collide and often stick (if they have velcro-like
facets like snowflakes).
Eventually they grow large enough so that
gravity holds them together. When this happens
they gather particles more efficiently and even
capture gases. In this way all the planets and
moons have formed.
Accretion
Although some
material may escape,
each meteor adds its
mass to the planet or
moon it crashes into.
Meteor Crater, Arizona
The meteorite, about 1/40th the width of the crater, slammed into Earth about
50,000 years ago. Although it disintegrated and produced what appears to be a
great hole, it added its mass to that of the Earth.
The Cratered Moon
has no atmosphere,
wind, or rain to wash
away ancient craters
But how did the Moon form?
The larger Earth should have
gathered most of the meteors.
In the early days of the solar system, a
planetoid about the size of Mars, crashed into
Earth. Debris from the impact circled the
Earth and accreted to form the Moon.
http://en.wikipedia.org/wiki/Giant_impact_hypothesis
ATMOSPHERIC SOUNDINGS AND WEATHER
Introduction to Atmospheric Soundings
A sounding is a vertical profile of temperature, humidity and wind in the atmosphere. It
helps to tell the weather in the air directly above, the vertical extent of clouds, the type of
precipitation and the likelihood of thunderstorms or air pollution.
How Soundings are Obtained
Every 12 hours at several hundred stations around the world, instrumented weather
balloons are released into the atmosphere. These balloons measure temperature,
pressure, and humidity as they ascend through the atmosphere, and radio the readings
to the ground-based weather station. The wind above the ground is determined by
tracking the balloon by a radar as it rises. The height of the balloon is determined from
the pressure and the temperature mathematically by the hydrostatic equation, which
equates changes of pressure to the weight of a column of air.
Units for the Weather Variables
Temperature, T, for soundings is expressed in Centigrade.
Dew Point Temperature, Td, is expressed in Centigrade and is a measure of Humidity.
Pressure, p, is expressed in millibars (mb) or hectoPascals. 1 mb = 100 Pa (Pascals)
Height, h, is expressed in meters.
Web sources: http://weather.uwyo.edu/upperair/sounding.html
http://vortex.plymouth.edu/u-make.html
The dew point temperature is the temperature at which the air becomes saturated with water vapor.
Dew and clouds first appear when the air is cooled to the dew point. Further cooling can produce
precipitation. Td is an indirect measure of the amount of water vapor in the air. For example, when
Td = 0C, each kilogram of air at a pressure of 1000 millibars (mb) is holding 3.84 grams of water
vapor. In rough terms,
The amount of water vapor that the air can hold doubles for each 10C increase of T.
The amount of water vapor the air actually holds doubles for each 10C increase of Td.
Thus, when Td = 10C, 1 kg of air at 1000 mb is holding 7.76 g (nearly double 3.84), while when Td
= 20C, 1 kg of air at 1000 mb pressure is holding 14.95 g (nearly 4 times 3.84).
Plotting a Sounding
Soundings are profiles of both T and Td. They are often
plotted on a graph known as an adiabatic chart. For most
adiabatic charts, the x-axis of this chart is proportional to T
(or Td) while the y axis is related to pressure or height. T
and Td are plotted in the same manner as you would plot
any graph. For each data point of T make a large dot and
then connect the dots with a solid line. For each data point
of Td, make a small x and connect x’s with a dashed line.
Data for Sounding
z(m)
0
1000
2000
4000
6000
T
10
15
10
-9
-23
Td
3
4
10
-9
-40
Illustrative Sounding
Clouds and Precipitation
Clouds and precipitation occur when the air is saturated, or when T  Td. In the illustrative
sounding, the cloud layer occurs from about 2000 to 4000 m. Many clouds are too thin or have
bases that are too high to produce precipitation at the ground. In order to produce precipitation a
cloud base should be within about 3000 m of the ground and should in general be at least 2000 m
thick. The thicker the cloud and the lower its base, the more likely it will produce precipitation.
Clouds often occur even where the sounding suggests that T is up to about 5C higher than Td!
This occurs in layers in which there are breaks in the clouds. As mentioned, this is common with
thunderstorms. It also occurs when T is much below 0C because of the difference between
saturated mixing ratios of water and ice.
Temperature Inversions
An inversion is a layer in which T increases with height. In the illustrative sounding, the inversion
occurs from 0 m to 1000 m. Inversions are most common just above the ground on clear nights and
early mornings. Pronounced inversions also occur in polar lands during winter. Inversions also
occur at elevated surfaces of fronts.
Inversions are associated with pollution episodes because they act like lids, suppressing vertical
motions in the atmosphere. Ironically, weak inversions often occur several hours before severe
thunderstorms, where they help intensify the thunderstorms by delaying them until the atmosphere
becomes extremely unstable, and by restricting the area over which they form. Instability will be
discussed below.
Thunderstorms
Thunderstorms, which rise to heights of 8 – 17 km are exceptional because they pump warm,
humid air to great heights and form in the midst of soundings that are colder and may be dry at
most altitudes. Therefore, thunderstorms do not match the sounding of the surrounding
atmosphere. Few soundings are actually taken within thunderstorms, because they are so narrow,
so short-lived, and because their weather is so severe they shred the balloons. But the
thunderstorm sounding can be estimated from the sounding of the surrounding atmosphere.
Soundings for Classical Weather Situations
Very often it is possible to tell the type of weather to expect from the sounding. Soundings for
different types of weather conditions are described and graphed below.
1. Fair weather, with possible fog and pollution. Here the air near the ground is almost saturated
(T - Td  0) so that fog is likely, particularly when the air cools at night. The air at 800 m is also
cooler (and denser) than the air at 1300 m. This inversion will trap any pollution near the ground if
the winds are too light to blow it away. By contrast, the air aloft is very warm and dry, (T - Td > 10)
so that no clouds can form in it and no humid but cooler and denser air from the ground can rise
into it. Soundings like this are very common along the coast of California, where fog is frequent, and
they also resemble the soundings that occurred during some of the worst pollution episodes in
London. In these cases, and on most clear nights, the coldest air is right at ground level and the
inversion typically extends several hundred meters above ground.
2. Unstable air with a possible thunderstorm. Here air near the ground is humid and much warmer
than air above. Temperature decreases rapidly with height (i. e., the sounding line slopes sharply to
the left). For these soundings, thunderstorms are possible whenever the sounding temperature
decreases more rapidly with height than the cooling rate for rising saturated air (discussed below).
This situation is called unstable.
3. Warm front above with snow. Here, a mass of humid, tropical air ascends a dome of cold air at
the surface. Clouds and precipitation usually form in the warm air above the frontal surface (i. e. the
inversion). The type of precipitation reaching the ground is determined by the temperature structure
of the sounding. Because temperatures remain below 0C at all heights in sounding #3, snow is
most likely. If temperatures near the ground are well above 0C, rain will occur. You should be able
to draw a sounding with a frontal inversion that produces rain.
4. Warm front above with freezing
precipitation. This is the same
general situation as sounding #3,
but now, T < 0C near the ground,
while T > 0C in some layer aloft.
Snow will form near the top of this
sounding, melt or partly melt as it
falls through the zone with T > 0C
and then, as it falls through the
cold air near the ground, either
refreeze to ice pellets in mid-air or
freeze on contact with the ground
or any other object as freezing
rain.
___________________________
The red dotted line in panel 2
indicates T inside the thunderstorm
or the adiabatic cooling rate of
rising, saturated air. It shows that a
rising parcel (balloon) of saturated
air will be buoyant because it is
warmer than the surrounding air.
T of surrounding
atmosphere
T in thunderstorm
2000
Topeka Sounding
12Z 20 Jan 2010
1600
1400
Height (m)
From the afternoon of
19 Jan 2010 to the
morning of 20 Jan
skies aloft were clear
over Topeka and
winds blew from the
East. As the ground
radiated heat to space
it cooled and fog
formed. The sounding
for the morning of 20
Jan shows that an
inversion has formed
in the lowest 150 m
and that the the air is
saturated (T = Td),
while air aloft is dry.
1800
T(oC)
1200
1000
800
T(oC)
600
400
Inversion and Ground Fog
200
0
-15
-10
-5
0
T (oC)
5
10
15
Vandenberg Air
Force Base, CA.
Sounding 12Z 26
Jul 2009
The cold Pacific
Ocean along the
coast of California
chilled the air just
above sea level
producing
stratus
and fog. Air aloft is
much warmer. Its
extreme
dryness
(note that T >> Td)
indicates
it
had
sunk. If you lived on
the hills 1000 m
above sea level, you
would be hot and
dry and look down
on a sea of fog.
3000
2500
2000
1500
1000
Subsidence
Inversion
500
Fog or Low Stratus
0
-5
0
5
10
15
20
25
30
Cross Sections
Soundings provide a one-dimensional view of the vertical structure of the atmosphere above one
point on earth. Cross sections provide a two-dimensional view of the atmosphere along a line on
earth. Cross sections are "slices through the pie" that reveal the structure of fronts and the
associated forms of precipitation. This is done by contouring the cross section for temperature.
Cross sections show the sequence of precipitation form. With warm fronts there is a sequence of
rain, freezing rain, sleet (ice pellets), and snow as you move from the front to the cold air. However,
freezing rain and sleet only occur near sharp fronts with large temperature contrasts (as in the
diagram below to the left), because only then is there band where a refreezing zone lies below a
melting zone. This does not occur with a weak front (as in the diagram below to the right). Even
when freezing rain and ice pellets do occur, they generally form in a narrower zone than either rain
or snow.
Global Precipitation
Precipitation averages just about 1 meter per year over Earth but, like wealth,
varies widely from place to place and over time. Rainfall is generally greatest
near the Equator where the air rises, small in the subtropics where the air sinks,
and tiny in the polar regions where the frigid air has a tiny vapor capacity. The
next slide shows how precipitation varies over the year, as the main rain belts
follow the Sun north and south with the seasons.