Monday Mar. 18, 2013

Joe Bonamassa "Driving Towards the Daylight"

1st part of the next bunch of material:














This is on p. 83 in the ClassNotes.






Basically the same idea as teaspoons of sugar mixed into a cup of tea.






















The sugar dissolved in tea analogy is still helpful. 













Each of the open circles represents 1 gram of water vapor that the air could potentially hold.
We haven't said anything about how much water vapor is actually in the air.

The numbers 15 and 5 came from the table on p. 86.



Now we have gone and added some water vapor, the same amount of water vapor,
to both parcels of air.

Maybe you can begin to figure out what relative humidity might mean.
Note:  RH doesn't really tell you how much water vapor is actually in the air.














We'll be making use of job #2 a lot in the next class or two.



Both parcels contains the same actual amount of water vapor (they have the same mixing ratio)
They both have the same dew point temperature.
Now back to the student/classroom analogy.






Doe_J

quiz1 -55 (195 pts possible) 71.8%

quiz2 -52 (170 pts possible) 71.5%

1.0 EC points (2.1 pts possible,class average is 0.95)

writing scores: 34.0 (expt/book report) + 15.5 (1S1P pts (average is 15.5))

writing percentage grade estimate: 89.2%

average (no quiz scores dropped): 75.2% + 1.0 = 76.1%
average (lowest quiz score dropped): 76.1% + 1.0 = 77.1%












We need to try to learn a little bit more about saturation of air. 
In particular why is there an upper limit to the amount of water vapor that can be found in air?
Why does the upper limit depend on temperature?









Could you tell the difference between cups of hot and iced tea, even if you couldn't see the ice cubes in the iced tea?

Hot water evaporates more rapidly than cold water.










We'll  cover both cups so that water vapor can begin to buildup in the air above the water in both cups.
 

Arrows represent the different rates of evaporation.  I just made up the numbers 10 and 30.  The important thing is that the warm water is evaporating more quickly than the cold water.









Water vapor will start to buildup in the air above each cup.  And, even though it has just evaporated, some of the water vapor will condense and rejoin the water at the bottom of each cup.  Let's just assume that 1% of the water vapor molecules will condense (again just a made up number).

The amount of water vapor in each glass will increase until it reaches a point where
water evaporation rate = water vapor condensation rate

for the cup of cold water
10 = 0.01 x water vapor concentration

The 0.01 is 1% expressed in decimal form.  Solving this equation gives you a water vapor concentration of 1000.  The air is saturated when you reach this point and the RH = 100%.


The saturation water vapor concentration in the air in the warm cup would be 3000.
 
same in both glasses
 different
saturated air
RH = 100%
rate of evaporation = rate of condensation
 actual amount of water vapor in the air











The fact that the rates of evaporation and condensation are equal when air is saturated (RH = 100%) is something we'll be using later when we study the formation of precipitation.  Here's a picture of how that would look inside a cloud.

The air inside the cloud is saturated.  The rate of evaporation from the cloud droplet (2 green arrows) is balanced by an equal rate of condensation (2 orange arrows).  The RH = 100%.  The cloud droplet won't grow any bigger or get any smaller.
Here's something to test your understanding of this material.


What information can you add to this picture?  Is the water in Glass A  WARMER  or  COLDER  than in Glass B?  Is there MORE  or  LESS  water vapor in the air in Glass A than in Glass B?  Is the relative humidity in each glass MORE than 100%, LESS than 100% or is it EQUAL to 100%.  The rates of evaporation and condensation aren't equal in either glass, so the pictures will change with time.  What will the glasses look once they have reached equilibrium?  Click here when you think you know the answers.