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

= 1 KilogramEvery side = 1 Meter

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