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Lecture 7

ATMO 201 Lecture 7: ch 4

Atmospheric Sciences
Course Code
ATMO 201
Timothy Logan

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Lecture 7
Tuesday, February 7, 2017
3:52 PM
CH 4
Condensation - the evidence that there is water vapor in the air
You see clouds
And then there is precipitation
Meteoric Miracle(s)
Flared somewhere in the troposphere
READ PG. 95-113; ANSWER QUESTIONS 1,2,3,7,9,*11-17 ON PG. 119
Dew, Frost, and haze
Dew point temperature (Td) - temperature at which water vapor will condense
Dew - condensation of water droplets onto objects when Tobject = Td
o The air temperature (T) is also close to Td
Frost - also when Tobject = Td but close to or below freezing (also known as deposition)
o Air temperature is usually no more than 2C or a few degrees above 32F
Haze - condensation of water droplets onto atmospheric particles
o Condensation nuclei (CN)
o Visible light is scattered
Seasonal Humidity Values
Dew point temperature is a good measure of moisture content while wet bulb temperature is a
good measure of dryness
o Point out the various geographic and seasonal influences
Where is the dew point the highest - around a body of water (Gulf coast)
o Towards the poles, the dew point is low because the air is dryer
o And as you go up, the dew point goes down because there is not a lot of moisture in the air
Relative Humidity and Water Vapor
New England in the winter time (ice)
o Air temperature and dew point both 28F
o Relative humidity 100 percent
Arizona desert
o Air temperature 96F
o Relative humidity 21 percent
Which place is more humid?
o NOT NEW ENGLAND because there is only frozen water here (hardly any water vapor)
o ANSWER IS DESERT eause you a’t see ater apor ad the de poit is high
Cold, dry air is heated in your home
Dry air can be problematic
o Dry skin
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o Pulmonary issues
o Bloody nose "syndrome"
o Humidify your home
Condensation Nuclei
Significant concentration of particles in the atmosphere
o Submicron to micron size (10^-6 m)
Serve as centers for condensation
o Cloud condensation Nuclei (CCN) or simply Condensation Nuclei (CN)
Many different types of CN are present in the atmosphere
o Some become wetted when RH <100%; responsible for haze
o Relatively large wetted CN grown to cloud droplet size
Haze (humidity less than 100%)
Example of pollution haze
o Serious visibility issues
o Dallas picture
Optical effects
o Because it scatters light
City in the sky
Particles are forward scattering the light
Fog (humidity = 100)
Forms by cooling - air cooled until saturation occurs (T to Td)
o Radiation or steam fog
Forms by evaporation and mixing - add water vapor by evaporation and mix it with dry air
o Classic fog
o Your breath on a cold day
o Ice and snow on a tree branch that is laying over liquid water
When the ice is in the water, the temperature is brought down bringing it to the dew
o Large weather pattern can bring fog to a large area
Particles in this fog is backwards scattering the light, making it impossible to see
o Wolf breath
The wolfs body is warm, and when it howls the warm moist air is mixing with the cold
dry air, that creates fog
o Why can't you see your breath in the summer??
There is a law that says when you increase temperature, you increase pressure (vapor
If the temperature crosses the saturation curve, then you can make fog
Have to mix dry air and moist air
Texas: we mix moist air with moist air
The reason why we can't see our breath in the summer
More fog types
Valley fog - cold air flowing down slope mixing with moist valley air
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