Download Morgan Evans December 9, 2014 Metr 302 Class Notes

12/12/14 12:54 PM
Morgan Evans
December 9, 2014
Metr 302
Class Notes
Assignments
Read from the website:
-The Role of Vertical Shear and Storm Severity
-Pattern Favoring Long Lived Severe Thunderstorms in the Great Plains
-A direct measure of the vertical level shear's contribution to a rotating updraft: deep layer shear and
helicity
-Rotation on Doppler Radar: The Hook Echo
-Chaos Theory
Read from textbook:
Williams: Chapters 10, 12. The completion of the Williams book readings.
Housekeeping
- It is very important for students to check over their midterm scantron's to not only use as a study tool, but
also to make sure that there wasn't any errors.
- We watched a precipitation forecast clip that shows a wave cyclone just west of San Francisco and
expected strong winds and heavy precipitation the next few days. The close isobars are together the stronger
the winds are.
- We looked at the cloud forecast in a satellite image and noticed that water vapor is unstable in the
atmosphere and the deep blue shows deeper clouds. There is expected flooding in the rainfall forecast and in
the red areas of the rainfall forecast satellite it shows where there will be 5 to 10 inches of rain. It is expected
to stay like this until 4am on friday.
- Although there is no relation of the snow forecast to hurricanes, highways in Tahoe will be closed and they
are expected to have 2 to 4 feet of snow. In the satellite image that we reviewed in class the yellow shows 24
to 48 inches of snow. People are warned to gather supplies ahead of time in case they cannot leave their
homes for a few days during the storm.
Definitions
-Instability: When an air parcel is warmer than the air surrounding it at it same pressure or elevation. The air
parcel is buoyant and will rise because air parcels that are warmer than the surrounding air at the same
elevation are less dense, making them lighter.
-Stability: When an air parcel is colder than the air surrounding it at its same pressure or elevation. Air
parcels will sink because air parcels colder than the surrounding air are more dense, making them heavier.
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-CAPE: Which stands for Convective Available Potential Energy, refers to the strength of the thunderstorms
updraft.
Convective Available Potential Energy (CAPE)
In the map of CAPE that we viewed in class today, the colors relate to how warm the ascending parcels
are relating to how strong the thunderstorms updraft is.
Vertical Shear and Storm Severity
The impact of increasing vertical shear on a thunderstorm is that the storm moves only slightly and takes
away precipitation from the updraft. The storm is not immediately taken out it just creates more time for
large masses of hail to grow. This can lead to a very severely dangerous thunderstorm. Winds aloft are
stronger than winds on the ground during this occurrence. The updrafts stay alive longer so hail sizes grow
bigger and the downdraft is what eventually ends the thunderstorm. Although, in the Great Plains we do not
see this!
Common Thunderstorms
There are three major stages in the formation of common thunderstorms. In the first stage, which is called
"Cumulus Stage", the width of the cloud is larger than the height and the updrafts are moving at 5 mph. In
the second stage, referred to as "Cumulus Congestus Stage", the height of the cloud is larger than the width of
the base and updrafts are moving at 10 to 20 mph. There are mostly water droplets and it contains all updraft.
During the final stage of development of a common thunderstorm, the "Cumulonimbus" stage, the cloud
penetrates to the top of the troposphere and hail forms at freezing level just below the anvil top. The
downdrafts have to do with precipitation and often lightning occurs.
Horizontal Wind Shear
This "Cumulonimbus Stage" can cause a great horizontal wind shear, which can be deathly for planes
trying to fly through. The horizontal wind shear is between the updrafts and the downdrafts and the area can
drop up to 30 degrees. The downdrafts in this stage of thunderstorm development, move downward and have
an outflow at the bottom of cold air, releasing the thunderstorm. Although, this can also cause the
development of another thunderstorm nearby. Unstable air on the side of a thunderstorm is lifted and can
cause another thunderstorm to then occur.
The horizontal wind shear is the region where hail and crystals chill. Then as they drop down sometimes
they can be heated up so that when they reach the ground their not as large of hail. Although when updrafts
are moving at 20 to 40 mph the hail stones can be a lot larger like golf ball size when they hit the ground
causing severe damage and can be deathly if someone is hit by them. If the hail melts on its way down that is
when on the ground we get big rain drops causing floods.
Lightning
During the "Cumulonimbus Stage", when the hail and crystals form at freezing level, portions of the cloud
becomes "electrified". They have electric charge differences and when the differences become large enough
that is when lightning occurs. Since lightning and hail both occur at the freezing level of the cloud, it goes to
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show that the two occurrences are very related.
Cloud to ground Lightning
Clouds get an electric charge and when they come into contact with either trees or tall rods or anything
that is higher up it can hit the ground. Lightning can be as hot as 30,000 degrees, making it very unlikely for
survival if struck by lightning. A good idea if outside during "cloud to ground" lightning is to lay down flat
on the ground to wait it out. Standing up can increase your chances of getting struck by lightning.
Fun Facts:
-The more dust there is in an area, the more yellow and red the lightning will appear.
-The higher above ground lightning is the longer the thunder will take to hit your ears.
!
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