Friday Mar. 23, 2012
click here to download today's notes in a more printer friendly format

A song from the Cirque du Soleil presentation of Alegria which I went to see a year or two ago.  I believe you heard "Icare", but it might have been "Irna".

I handed out what I consider to be a fairly challenging Optional Assignment.  Students were free to turn it in at the end of class but could also hang onto it, work out the questions over the weekend, and turn it in at the start of class next Monday.  You can download the assignment here if your interested in doing the same.

Occasionally you need to eat up all the left overs in your refrigerator.  In Atmo 170 we sometimes need to tie up a bunch of loose ends.  That's what we did today.

The figure below is on p. 87 in the photocopied ClassNotes.  It explains how you can dry moist air.


At Point 1 we start with some 90 F air with a relative humidity of 25%, fairly dry air.   These are the same numbers in Example Problem #4 last Wednesday.  We imagine cooling this air to the dew point temperature where the relative humidity would reach 100% and a cloud would form (Pt. 2 in the figure above). 

Then we continue to cool the air below the dew point, to 30 F.  Air that is cooled below the dew point finds itself with more water vapor than it can contain.  The excess moisture must condense (we will assume it falls out of the air as rain or snow).  When air reaches 30 F it contains 3 g/kg, less than half the moisture that it originally did (7.5 g/kg).  The air is being warmed back up to 90 F along Path 4.  As it warms the mixing ratio remains constant.  At Point 5, the air now has a RH of only 10%.

Drying moist air is very much like wringing moisture from a wet sponge.


You start to squeeze the sponge and it gets smaller.  That's like cooling the air and reducing the saturation mixing ratio, the air's capacity for water vapor.  At first squeezing the sponge doesn't cause anything to happen (that's like cooling the air, the mixing ratio stays constant as long as the air doesn't lose any water vapor).  Eventually water will start to drop from the sponge (with air this is what happens when you reach the dew point and continue to cool the air below the dew point).  Then you let go of the sponge and let it expand back to its orignal shape and size (the air warms back to its original temperature).  The sponge (and the air) will be drier than when you started.

This sort of process ("squeezing" water vapor out of moist air by cooling the air below its dew point) happens all the time.  Here are a couple of examples (p. 87 again)


In the winter cold air is brought inside your house or apartment and warmed.  Imagine 30 F air with a RH of 100% (this is a best case scenario, the cold winter air usually has a lower dew point and is drier). Bringing the air inside and warming it will cause the RH to drop from 100% to 20%..  Air indoors during the winter is often very dry.  This can cause chapped skin, can irritate nasal passages, and cause cat's fur to become charged with static electricity.

The air in an airplane comes from outside the plane.  The air outside the plane can be very cold (-60 F perhaps) and contains very little water vapor (even if the -60 F air is saturated it would contain essentially no water vapor).  When brought inside and  warmed to a comfortable temperature, the RH of the air in the plane will be very close 0%.  Passengers often complain of dehydration on long airplane flights.  The plane's ventilation system probably adds moisture to the air so that it doesn't get that dry.

Next a much more important example of drying moist air (see p. 88 in the photocopied ClassNotes).


We start with some moist but unsaturated air (the RH is about 50%) at Point 1 (the air and dew point temperatures would need to be equal in order for the air to be saturated).  As it is moving toward the right the air runs into a mountain and starts to rise.  Rising air expands and cools.   Unsaturated air cools 10 C for every kilometer of altitude gain.  This is known as the dry adiabatic lapse rate.  So after rising 1 km the air will cool to 10 C which is the dew point.

The air becomes saturated at Point 2 (the air temperature and the dew point are both 10 C).  Would you be able to tell if you were outdoors looking at the mountain?  Yes, you would see a cloud appear. 

Now that the RH = 100%, the saturated air cools at a slower rate than unsaturated air (condensation of water vapor releases latent heat energy inside the rising volume of air, this warming partly offsets the cooling caused by expansion).  We'll use a value of 6 C/km (an average value).  The air cools from 10 C to 4 C in next kilometer up to the top of the mountain.  Because the air is being cooled below its dew point at Point 3, some of the water vapor will condense and fall to the ground as rain.  Moisture is being removed from the air and the value of the mixing ratio (and the dew point temperature) decreases.

At Point 4 the air starts back down the right side of the mountain.  Sinking air is compressed and warms.  As soon as the air starts to sink and warm, the relative humidity drops below 100% and the cloud disappears.  The sinking unsaturated air will warm at the 10 C/km rate. 

At Point 5 the air ends up warmer (24 C vs 20 C) and drier (Td = 4 C vs Td = 10 C) than when it started out.  The downwind side of the mountain is referred to as a "rain shadow" because rain is less likely there than on the upwind side of the mountain.  Rain is less likely because the air is sinking and because the air on the downwind side is drier than it was on the upslope side.

I went looking for some good specific illustrations of the rainshadow effect.   







We can see the effects of a rainshadow illustrated well in the state of Oregon.  The figure above at left shows the topography (here's the source of that map).  Winds generally blow from west to east across the state. 

Coming off the Pacific Ocean the winds first encounter a coastal range of moutains.  On the precipitation map above at right (source) you see a lot of greens and blue on the western sides of the coastal range.  These colors indicate yearly rainfall totals that range from about 50 to more than 180 inches of rain per year.  This is where temperature rain forests are found. 

That's the Willamette River, I think, in between the coastal range and the Cascades.  This valley is somewhat drier than the coast because air moving off the Pacific has lost some of its moisture moving over the coastal range. 

What moisture does remain in the air is removed as the winds move up and over the taller Cascades.  Yearly rainfall is generally less than 20 inches per year on the eastern side, the rainshadow side, of the Cascades.  That's not too much more than Tucson which averages about 12 inches of rain a year.

Wikipedia considers the Tibetan Plateau one of the best examples of a rain shadow. (here's the source of the picture). 



The Himalayan mountains stretch across the lower left 1/3 of the picture.  The land below and to the left of the mountains appears somewhat green in the picture.  This is because moist air moving from lower left toward the upper right leaves most of its moisture on this side of the mountain range.  The upper right 2/3rds of the picture, the Tibetan plateau, is in the rain shadow and appears very dry and brown in the photograph.

Most of the year the air that arrives in Arizona comes from the west, from the Pacific Ocean (this changes in the summer).  It usually isn't very moist by the time it reaches Arizona because it has travelled up and over the Sierra Nevada mountains in California and the Sierra Madre mountains further south in Mexico.  The air loses much of its moisture on the western slopes of those mountains. 

Next in our mix of topics was measuring humidity.  One of the ways of measuring humidity is to use a sling (swing might be more descriptive) psychrometer.




A sling psychrometer consists of two thermometers mounted side by side.  One is an ordinary thermometer, the other is covered with a wet piece of cloth.  To make a humidity measurement you swing the psychrometer around for a minute or two and then read the temperatures from the two thermometers.  The difference between the dry and wet bulb temperatures can be used to determine relative humidity and dew point (you look up RH and Td in a table, it's not something you can easily calculate). 

You know I like to beat some concepts to death.  But also it's a pretty good example of where you can take some pretty basic concepts that you understand and use them to really understand something else.


The figure shows what will happen as you start to swing the wet bulb thermometer.  Water will begin to evaporate from the wet piece of cloth.  The amount or rate of evaporation will depend on the water temperature (the 80 F value was just made up in this example).  Warm water evaporates at a higher rate than cool water.

The evaporation is shown as blue arrows because this will cool the thermometer.  The same thing would happen if you were to step out of a swimming pool on a warm dry day, you would feel cold.  Swamp coolers would work well (too well sometimes) on a day like this.

The figure at upper left also shows one arrow of condensation.  The amount or rate of condensation depends on how much water vapor is in the air surrounding the thermometer.  In this case (low relative humidity) there isn't much water vapor.  The condensation arrow is orange because the condensation will release latent heat and warm the thermometer.
Because there is more evaporation (4 arrows) than condensation (1 arrow) the wet bulb thermometer will drop. 


The wet thermometer will cool but it won't cool indefinitely.  We imagine that the wet bulb thermometer has cooled to 60 F.  Because the wet piece of cloth is cooler, there is less or slower evaporation.  The wet bulb thermometer has cooled to a temperature where the evaporation and condensation are in balance.  The thermometer won't cool any further.

You would measure a large difference (20 F) between the dry and wet bulb thermometers on a day like this when the air is relatively dry.

Here's the situation on a moister day.  There's enough moisture in the air to provide 3 arrows of condensation.  You wouldn't feel as cold if you stepped out of a pool on a warm humid day like this.  Swamp coolers wouldn't provide much cooling on a day like this.


The wet thermometer only cools a little bit before the rates of evaporation and condensation are equal.

Here's a summary


A large difference between the dry and wet bulb temperatures means the relative humidity is low.
A small difference means the RH is higher.
No difference (the bottom figure) means the relative humidity is 100%.  Any evaporation from the wet thermometer is balanced by an equal amount of condensation from the surrounding air.



Evaporative cooling will make you feel cold if you get out of a swimming pool on a warm dry day.  You won't feel as cold if the air is humid and the relative humidity is high.  This reminds me of something we covered earlier in the semester.




We learned that a 40 F day with 30 MPH winds will feel colder (because of increased transport of energy away from your body by convection) than a 40 F day with no wind.  The wind chill temperature tells you how much colder it will feel ( a thermometer would measure the same temperature on both the calm and the windy day).  If your body isn't able to keep up with the heat loss, you can get hypothermia and die.

Now something similar but new.  Your body tries to stay cool by perspiring.  You would feel hot on a dry 105 F day.  You'll feel even hotter on a 105 F day with high humidity; your sweat won't evaporate as quickly.  The heat index measures how much hotter you'd feel. The combination of heat and high humidity is a serious, potentially deadly, weather hazard because it can cause heatstroke (hyperthermia).  


A variety of things can happen when you cool air to the dew point and the relative humidity increases to 100%.  Point 1 shows that when moist air next to the ground is cooled to and below the dew point, water vapor condenses onto (or is deposited onto) the ground or objects on the ground.  This forms dew, frozen dew, and frost. 

Air above the ground can also be cooled to the dew point.  When that happens (Point 2 above) it is much easier for water vapor to condense onto something rather than just forming a small droplet of pure water.    In air above the ground water vapor condenses onto small particles in the air called condensation nuclei.  Both the condensation nuclei and the small water droplets that form on them are usually too small to be seen with the naked eye.  We can tell they are present (Point 3) because they either scatter (haze or fog) or reflect (clouds) sunlight. 

We'll learn a little bit about the formation of dew and frost today.  We look at condensation nuclei and the role they play in cloud formation on Friday. 

The following confusing figures are found on p. 90 in the photocopied ClassNotes.



It might be a little hard to figure out what is being illustrated here.  Point 1 is sometime in the early evening when the temperature of the air at ground level is 65.  During the course of the coming night the air will cool to 35 F.  When the air temperature reaches 40 F, the dew point, the relative humidity reaches 100% and water vapor begins to condense onto the ground.  You would find your newspaper and your car covered with dew (water) the next morning.

The next night is similar except that the nighttime minimum temperature drops below freezing.  Dew forms (condensation) and first covers everything on the ground with water.  Then the water freezes and turns to ice.  This isn't frost, rather frozen dew.  Frozen dew is often thicker and harder to scrape off your car windshield than frost. 

Now the dew point and the nighttime minimum temperature are both below freezing.  When the RH reaches 100% water vapor turns directly to ice (deposition).  This is frost.

What happens on this night?  Because the nighttime minimum temperature never reaches the dew point and the RH never reaches 100%, nothing would happen.  I've seen some textbooks refer to this as black frost but I don't like to use that term.  You have probably heard of black ice.  Black ice does sometimes form on road surfaces and is a very dangerous driving hazard.  Because it's hard to see you can hit it with your car and lose control.