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### Weather and Climate

September 11, 2000

Last Class

- Defined weather, climate, meteorology, atmosphere, etc.
- History of meteorology
- Composition of the atmosphere
- Vertical Structure of the atmosphere

Density

- Density = Mass/ Volume
- As you go up in the atmosphere density goes down.
- How does temperature change the density of air?
- When air is heated the molecules vibrate faster and become farther apart…….
therefore there are fewer molecules (less mass) in a given volume, and density goes down.

Equation of State

where: P=pressure

V=volume

n=moles of molecules

R=gas constant

T=temperature

PV=nRT

Equation of State(Ideal Gas Law)

- There are many different forms of the equation of state.
P = rRT

Where r = density

- Thus, if T is constant => P ~ r
if P is constant => T ~ 1/r

if r is constant => P ~ T

Also called “Barometric Pressure”

We measure pressure with a barometer.

The first barometers were mercury barometer.

The aneroid barometer is commonly used in homes.

Atmospheric PressureAtmospheric Pressure

- An “altimeter” is a type of aneroid barometer which is calibrated to indicate altitude.
- A “barograph” is a recording aneroid barometer.

Reading Pressure

- Several errors need to be accounted for when reading the pressure.
- Temperature
- Gravity
- Instrument Error

- The corrected pressure at a particular location and elevation is called the “station pressure”.

Sea Level Pressure

- Altitude of stations causes more changes in pressure than horizontal changes. (i.e. The pressure change at top of Sears Tower is greater than difference in pressure between NYC and Miami, FL)
- Thus pressure is adjusted to altitude of 0 meters, known as mean sea level (MSL).

What is “Energy”?

- By definition, energyis the ability or capacity to do work on some form of matter.
- Energy comes in many different forms:
- Kinetic energy
- Potential energy
- Radiant energy
- Chemical energy
- Electric energy
- Magnetic energy

- Energy is typically given in units of : erg, joule, or calorie.

Potential Energy

- The energy that a body possesses by virtue of its position with respect to other bodies in the field of gravity.
- Huh?
PE=mgh

Where m=mass of object,

g= gravity

h=height of object above ground

DEMO

Kinetic Energy

- The energy within a body that is the result of its motion.
KE=1/2mv2

Where m=mass of object

v=velocity of object

- If a volume of water and an equal size volume of air were moving at the same speed, which would have the greater kinetic energy?

Kinetic Energy

- The molecules and atoms that comprise all matter have kinetic energy due to their motion.
- This type of kinetic energy is often referred to as heat energy.

1st Law of Thermodynamics

“Energy cannot be created nor can it be destroyed.”

- Energy can change form………
that is, the energy lost during one process must equal the energy gained during another.

Temperature

- Temperature is a measure of the average speed of atoms and molecules comprising a matter.
Higher Temperature=> Faster Molecules

Lower Temperature=> Slower Molecules

- Temperature is a measure of the average kinetic energy.

Temperature

- When molecules move faster they also move farther apart.
- From the equation of state recall:
cold air => more dense air

warm air => less dense air

- Kinetic energy is only part of a matter’s total energy…….
there is also potential energy!

Temperature Scales

- If we allowed some molecules to cool to a point where they no longer had any motion, that point would be called absolute zero.
- At absolute zero there is a minimum amount of energy, and theoretically no thermal motion.
- Absolute zero is:
-459 oF (Fahrenheit)

-273oC (Celsius)

0 K (Kelvin)

Kelvin Scale

- This scale begins at absolute zero, so there are no negative numbers in this scale.
- Introduced by a British scientist, Lord Kelvin.
- Has the same increment as the celsius scale.
K = oC + 273

Example: -20 oC = ? In Kelvin

Answer: -20 oC + 273 = 253 K

An increment of 1 oC is equal to an increment of 1.8 oF.

oC= 5/9 (oF – 32)

oF = 9/5 oC + 32

Fahrenheit <=> CelsiusExample: 50 oF = ? oC

Answer: 5/9 (50 – 32) = 10 oC

Example: -5 oC = ? oF

Answer: 9/5(-5) +32 = 23 oF

What is “heat”?

- Heat is energy in the process of being transferred from one object to another because of the temperature difference between the the two objects.
- After heat is transferred it is stored in “internal energy”.
- “Internal energy” is the total kinetic and potential energy stored in molecules.

Heat Capacity

- “Heat capacity” is the ratio of the heat absorbed (released) by a system to the corresponding temperature rise (fall).
- “Heat capacity” is also known as “specific heat”.
- Water has a high heat capacity/specific heat.
- It takes 1 calorie to heat 1gram of water by 1oC.
- It only takes .2 calories to heat 1 gram of soil by 1oC.
- The high heat capacity of water is a major influence of climate.

Latent Heat

- “Latent Heat” is the heat that is either released or absorbed by a unit mass of a substance when it undergoes a change of state, such as during evaporation, condensation, or sublimation.
- Remember that water on earth is important to the energy budget because of latent heat.

Sensible Heat

“Sensible heat” is the heat we can feel and measure with a thermometer.

How is heat transferred in the Atmosphere?

- Conduction
- The transfer of heat by molecular activity from one substance to another, or through a substance.
- The transfer is always from warmer to colder regions.

How is heat transferred in the Atmosphere?

- Convection
- Motions in a fluid that result in the transport and mixing of the fluid’s properties.
- In atmosphere, air near the surface is warmed by conduction, and then becomes less dense and rises.
- The rising air bubbles are referred to as “thermals”.
- “Advection” is the horizontal transfer of any atmospheric property by the wind.

How is heat Transferred in the Atmosphere?

- Radiation
- Energy propagated in the form of electro-magnetic waves.
- These waves do not need molecules to propagate them, and in a vacuum they travel at nearly 186,000 miles per second.
- We can think of radiation as streams of discrete packets of energy called photons.
- Photons make up waves, and groups of waves make up a beam of radiation.

Radiation and Temperature

- All things (whose temperature is grater than absolute zero), no matter how big or small, emit radiation.
- The wavelengths at which an object emits radiation depends mostly on the temperature of the object.
- The higher the temperature, the shorter the wavelengths of emitted radiation.

Can we see this radiation being emitted?

- Objects at about 500 oC emit at wavelengths short enough for us to see the object glow red.
- Objects cooler than 500 oC , are emitting at wavelengths too large for us to see.
- Light bulbs work because the filament is emitting at the correct temperature for us to see the wavelengths.

Stefan-Boltzmann Law

E = sT 4

- Objects at a higher temperature emit radiation at a greater rate.
=> As the temperature of an object increases, more total radiation is emitted per second.

- A small increase in temperature leads to a large increase in amount of radiation emitted.

Sun Vs. Earth

- The surface temperature of the sun is 6000K (10,500 oF).
- The surface temperature of the earth is 288 K (59 oF).
lmax = constant/T

- The above equation knownas Wein’s Law allows us to determine the maximum wavelength at which the sun and earth emit radiation.

The Sun’s Electromagnetic Spectrum

While the sun has a maximum emitted at .5 mm, it still emits at almost all other wavelengths.

The Colors We See

DEMO

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