AOS 100: Weather and
Climate
Instructor: Nick Bassill
Class TA: Courtney Obergfell
Miscellaneous
• Class website:
http://www.aos.wisc.edu/~aos100nb/
• Extra copies of the syllabus
• Exam dates
• Class availability
• Willing note-takers?
Notecards I
• Commonly suggested topics:
- Hurricanes
- Tornadoes
- Predicting the above
- Global Warming/Climate Change
- Lightning/Thunder
- Earthquakes/Volcanoes/Tsunamis
Notecards II
• Other suggestions:
- Slow down
- Visuals/Models/Etc.
- Post lectures before class
Review of September 3rd: Remote
Sensing
• Two primary types: RADAR & Satellites
• RADARS:
- Located on the Earth’s surface
- Send out an electromagnetic pulse; the length
of time it takes to return determines the object’s
distance, and the intensity its size
- Doppler RADAR is used to determine whether
objects are coming towards or away from the
RADAR site
Review Continued
• Two types of satellites: GOES and POES
• GOES Satellites:
- Remain above the same location on the
equator (geostationary) at a great distance
- Good temporal resolution, poor spatial
resolution
• POES Satellites
- Orbit around the poles at a close distance
- Very poor temporal resolution, but excellent
spatial resolution
Review Continued
• Satellite imagery: Visible, Infrared, Water Vapor
• Visible relies on sunlight reflected off the Earth
• Infrared (IR) detects radiation emitted by the
Earth, so the strength of the radiation
determines how warm the object (surface, cloud,
etc.) is
• Water Vapor detects radiation that is strongly
absorbed by water, so it is a useful detector of
dry/moist areas
The Ideal Gas Law
Pressure = Density * Gas Constant
* Temperature
The Ideal Gas Law
P=ρRT
Pressure
Density
Temperature
Gas Constant
Density
• Density equals Mass divided by Volume
ρ=(kg)/(m3)
• Imagine air molecules in a box
Every side = 1 Meter
= 1 Kilogram
Which has the greater density?
• Box 1: 9 kg / (1 m * 1 m * 1 m) = 9 kg/m3
• Box 2: 3 kg / (1 m * 1 m * 1 m) = 3 kg/m3
Box 1 is more dense
Pressure
• Pressure equals force per unit area
• Continuing the molecules in a box idea:
Which box has more pressure
exerted on the inside walls?
• Box 1 has more molecules hitting the side, and
therefore has a greater force per area is exerted
on the walls
Pressure Continued
• In the context of the atmosphere, the surface
can be thought of as the wall of the box
• Atmospheric pressure is most commonly
reported in “millibars” (which are equivalent to
“hectoPascals”)
• A standard pressure at sea-level is about 1013
millibars (or mb), which is equivalent to about
14.7 pounds per square inch
• For reference, the strongest hurricane (an area
of low pressure) reached a minimum pressure of
870 mb at the sea surface
• The maximum sea-level pressure is 1083.8 mb
Temperature
• To understand temperature, we must
understand Kinetic Energy (KE)
• Kinetic means motion, and energy
measures an object’s ability to do work
• So KE is a measure of an object’s ability to
do work via its motion
• KE=½*Mass*Velocity2
• Therefore, KE is greatly dependent on the
object’s velocity
Temperature Continued
• Temperature, then, is a measure of the average
kinetic energy of the molecules in a substance
(or amount of air)
• Therefore, faster moving molecules have a
higher temperature
• Scales, starting with no molecule movement:
Kelvin (K): begins at 0 K,
Celsius (C): begins at -273.15 ºC
Fahrenheit (F): begins at -459.67 ºF
Conversions:
K = ºC + 273.15
ºF = 9/5 * ºC – 32
ºC = 5/9 * ºF + 32
Records:
Low Temp: -128.2 ºF
High Temp: 136.4 ºF
Now, for some thought experiments …
• Remember the Ideal Gas Law:
Pressure = Density * Gas Constant * Temperature
• Imagine an airtight metal box, that is placed
above a fire
• The air in the box gets warmer (T↑) with time
• This means that the molecules will move faster
• Which in turn means that they hit the walls of the
box with more force (P↑)
• This makes sense, since P=ρ*R*T, where ρ, R
stay constant
Next Thought Experiment
• Begin with the same situation, except this time,
we’ll let the box expand or contract as needed
(i.e. like a balloon)
• Now, as the box is heated and the air warms, it
expands, so that the pressure on the inside of
the walls remains equal to the pressure on the
outside of the walls
• This means that the density decreases (the
volume goes up but the mass stays the same)
• Therefore, if pressure stays the same, and the
temperature increases, the density must
decrease
• Remember P=ρRT
Final Thought Experiment
• Again, start with a metal box. Imagine a
number of molecules are suddenly added
to the box at the same temperature as the
original molecules
• If the volume is the same, the density must
increase
• Since density increases, and we said the
temperature stayed constant, the pressure
must increase
• Remember P=ρRT
The Real Atmosphere
• Density is greatest near the Earth’s surface (due
to gravity)
• In the part of the atmosphere closest to the
Earth’s surface (where weather occurs), the
temperature decreases with height
• Therefore, since density and temperature
decrease with height, we must know that
pressure also decreases with height, based on
the ideal gas law
• Remember P=ρRT
Station Models
These display virtually all the relevant current weather observations for a given
weather station
http://weather.cod.edu/notes/stnmodel.html
Meanings
Wind Speeds