AOS 100: Weather and Climate

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Transcript AOS 100: Weather and Climate

AOS 100: Weather and
Instructor: Nick Bassill
Class TA: Courtney Obergfell
• Homework due Thursday
• Exam next Tuesday
Review of October 22nd: Thickness and
the Thermal Wind
500 mb
500 mb
600 mb
1000 mb,
0 meters
is present
600 mb
Review Continued
• The thermal wind is calculated by subtracting
the upper level geostrophic wind vector from the
lower level geostrophic wind vector
• Therefore, the thermal wind isn’t an actual
observable wind, but it does tell us some useful
things about the atmosphere
• It blows parallel to lines of constant thickness,
and its magnitude gives some indication of how
strong the horizontal temperature contrast is
within the layer it was calculated from
• A stronger thermal wind indicates a greater
temperature gradient in that layer
Review Continued
• The thermal wind always blows with cold
thickness to the left
• Most importantly, the direction it points in is
roughly the direction you would predict a surface
cyclone to move
• This should be intuitive, since the thermal wind
blows parallel to temperature gradients, and we
know that cyclones exist to spread out these
temperature gradients
• These properties mean that we can predict the
movement of cyclones knowing only what the
thickness pattern is (i.e. what the temperature
distribution is)
• Obviously, clouds need to form first in order for
precipitation to form
• Clouds will only form when the air reaches
saturation (so where relative humidity = 100%)
• This can occur either by adding moisture to the
air, or by cooling the air
• Of these two options, the second is a much
more common method of forming clouds
• One of the easiest ways to make the air cool is
by forcing it to rise
Lapse Rate(s)
• A “Lapse Rate” is merely the rate at which temperature
decreases with height
• The Environmental lapse rate is therefore simply the rate
at which the temperature of the atmosphere decreases
with height
• Sometimes, a “parcel” of air is considered:
- The dry adiabatic lapse rate (DALR) is for a parcel with
0% RH, and is about 9.8ºC/km
- The moist adiabatic lapse rate (MALR) is for a parcel
that is saturated, and is close to 6.5ºC/km in the lower
- This difference is caused by the release of latent heat as
water changes phase (like from a gas to a liquid), which
warms the air
- Adiabatic means no energy is added or taken from the
parcel (i.e. no net radiation loss or gain)
Surface dewpoint = 20ºC
End of Exam 2 Material
• Beginning exam 3 material …
Adiabatic Lapse Rate Mixing Ratio
Moist Adiabatic
Lapse Rate
Cloud Formation
• Soundings can be used to figure out not
only where clouds currently are, but where
they would form
• The Lifted Condensation Level (LCL) tells
you where clouds would form if you forced
a parcel of air to rise
• To find this, you need to know both the
temperature and mixing ratio
• In order to find the LCL, use the dry adiabatic
lapse rate (DALR) to find how far up it would
take for the parcel to cool to the dewpoint
• This is done by following the DALR up from the
temperature, and the mixing ratio up from the
dewpoint, and figuring out where they cross
• Once this happens, clouds should form
• If the parcel is forced to keep rising, it will now
cool by the moist adiabatic lapse rate
Level of Free Convection (LFC)
• The LFC is defined as “The level at which
a parcel of air lifted dry-adiabatically until
saturated and saturation-adiabatically
thereafter would first become warmer than
its surroundings in a conditionally unstable
An Example
The red arrow
indicates the
LCL, while the
yellow arrow
indicates the
Mountain Effects and Temperature
• The air ends up warmer due to the extra
latent heat release