AOS 100: Weather and Climate

Instructor: Nick Bassill

Class TA: Courtney Obergfell

Miscellaneous

• Homework Reminder

• Exam Reminder

Review of September 22nd: More Radiation

• Kirchoff’s Law: The efficiency of absorption at a certain wavelength of radiation is exactly equal to the efficiency of emission in the same wavelength

• If something is perfect at absorbing all wavelengths of radiation (and thus perfect at emitting all wavelengths), then we call it a blackbody

• We can assume the Earth is a blackbody, which means Wien’s Law and Stefan-Boltzmann’s Law can be used

Review Continued• Using Stefan-Boltzmann’s Law (E=σT4), and what we

know of the incoming solar radiation, we get a surface temperature that is much too cold (about 255 K)

• The reason we are warmer is that our atmosphere absorbs some of the Earth’s outgoing longwave radiation and emits some of that radiation back towards the surface

• The combination of the Sun’s shortwave radiation and the atmosphere’s longwave radiation make the surface as warm as we observe it

Radiation Balances

• Radiative equilibrium: the state where the rate an object emits radiation is equal to the rate the object absorbs radiation

• The Earth is in radiative equilibrium as a whole, but any given location on the Earth is probably not in radiative equilibrium

• Whether or not a particular location is receiving more radiation than it is emitting, or vice versa, largely determines whether that place will warm or cool

• This process determines everything from the diurnal cycle of temperature to the seasons

Tuesday’s Rain

• http://www.crh.noaa.gov/news/display_cmsstory.php?wfo=mkx&storyid=31341&source=0

What can modify this cycle?

Moisture (Variables)

• Relative Humidity (RH) is defined as the ratio of the amount of water vapor in the air to the amount of water vapor the air can hold (given as a percentage)

• Dewpoint is defined as the temperature the air would have to be cooled to reach saturation (RH=100%)

• Warmer air can hold more water vapor, so warmer air will by definition have a higher dewpoint

• Mixing Ratio is the ratio of the mass of water to the mass of dry air

A Comparison

• Consider two air parcels:- Parcel 1 has a temperature/dewpoint of

90º/60º- Parcel 2 has a temperature/dewpoint of

30º/29º• Parcel 1 holds much more water vapor

(i.e. has a higher mixing ratio), but parcel 2 has a much higher relative humidity (i.e. is closer to saturation)

Types of Heat

• Sensible Heat is the sort of heat you can measure with a thermometer

• It’s also the type of heat you feel when you step on a hot surface with bare feet

• Latent Heat is the heat required to change a substance from one phase to another

• This is most commonly important with water, which is the only substance that exists on the Earth is three different phases

• Gases are more energetic than liquids, which are more energetic than solids, so to move up in energetic states, energy is taken from the environment, and vice versa

Latent Heat

Expanded Energy Budget

Latent Heat Continued

• Since water going from a gas to a liquid releases energy, this means that condensing water in the atmosphere releases energy to the surrounding air

• If a parcel of air is at 100% RH, and is forced to cool, some of that water vapor will condense, which will warm the air

• This is important for cloud formation, as oftentimes air at 100% RH is forced to rise, which warms the air, and in some cases can make it keep rising (because warm air is less dense), which causes more condensation, creating a positive feedback

More Energy Balance

Lapse Rate(s)

• Remember, 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 atmosphere

- The difference is due to the energy released through phase changes

Atmospheric Soundings

• Weather balloons (radiosondes) are launched at least twice daily from locations across the United States and the world

• The purpose of radiosondes is to “observe” the atmosphere above the surface of the Earth

• These radiosondes send back data about the vertical structure of temperature, moisture, pressure, and wind in the atmosphere (or any other variable)

• This information is most commonly depicted in what is known as a “Skew-T”

Mixing RatioAdiabatic Lapse Rate

Temperature

Moist Adiabatic Lapse Rate

TemperatureDewpoint

What Skew-T’s Tell Us

• Skew-T’s allow us to quickly determine where:- Extremely dry layers are- Saturated layers are (often clouds)- Inversions are present- Determine the likelihood of severe weather

(more on this later)- Find “well-mixed” layers (more on this later)- Find the lifted condensation level (LCL) and the

level of free convection (LFC) (more on these later)

Well-Mixed Layer

Inversions Dry Layers Clouds

Inversions Clouds

Inversions Dry Layers Clouds