Friday Mar. 28, 2014

Some what I guess is called Gypsy Jazz before class this morning.  You heard "Jazz from the Tip"and "Don't Panic" from the Hot Club of San Francisco. 

The revised Expt. #2 reports were collected today together with the Optional Assignment Hidden in the suggested reading section on saturation of air with water vapor.  Here are the Hidden Assignment questions and answers.

There's one remaining Humidity Problems Optional Assignment due Monday.  The Experiment #3 reports are also due Monday.

Just for the purposes of planning ahead I've listed some of the up coming 1S1P report topics online.

This chart, started on Monday, is pretty much complete

humidity variable
 mixing ratio
 saturation mixing ratio
 relative humidity
 dew point temperature
"job"
 how much water vapor is actually in the air
 the maximum amount of water vapor that can be found in air
 how close is the air to being filled to capacity with water vapor
 1. like mixing ratio it gives an idea of the actual amount of water vapor in the air
2. cool the air to its dew point and RH becomes 100%
units
 g/kg
 g/kg
 %
 oF
to increase value
add water vapor

warm the air
RH = r / rs
incr r or decr rs




same as the mixing ratio
to decrease value
 remove water vapor (or cool the air below the dew point)
 cool the air
 decr r or incr rs

A handout with the following figure was distributed in class.  This is NOT an assignment.  Filling in the blanks will give you some more practice with the humidity variables and when they change and remain constant.



You don't need to fill in the blanks with numbers, just indicate whether the variables increase in value, decrease, or remain constant.  You'll find the answers at the end of today's notes.


A couple of items we didn't have time to finish at the end of class on Wednesday (the next two figures are on p. 87 in the ClassNotes).



The air indoors in the winter is often quite dry.  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 air outdoors usually has a relative humidity less than 100% and is drier). Bringing the air inside and warming it will cause the RH to drop from 100% to 20%..  This can cause chapped skin, can irritate nasal passages, and causes 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 essentially 0%.  Passengers often complain of dehydration on long airplane flights.  This may increase the risk of catching a cold (ref)

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 but isn't something you need to remember).  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.






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 mountains.  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.  Temperate rainforests are found in some of these coastal locations.

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 rain shadow side, of the Cascades.  That's not too much more than Tucson which averages about 12 inches of rain a year.

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. 



Death valley is found on the downwind side of the Sierra Nevada mountains (source of left image).  The Chihuahuan desert and the Sonoran desert are found downwind of the Sierra Madre mountains in Mexico (source of the right image)

Just as some of the world's driest regions are found on the downwind side (the rain shadow side) of mountain ranges, some of the wettest locations on earth are on the upwind sides of mountains.  There seems to be some debate whether Mt. Wai'ale'ale in Hawaii or Cherrapunji India gets the most rain per year.  Both get between 450 and 500 inches of rain per year.


Here's a pretty difficult question to test your understanding of this topic.


The mixing ratio, r,  is increasing and RH is remaining constant _______
The RH is increasing & r is remaining constant _______
The mixing ratio is decreasing and RH is remaining constant _______
The RH is decreasing & r is remaining constant _______
(fill in each blank with a, b, or c corresponding to the paths shown in the figure or
  use d for nowhere; each answer should be used once)
You'll find the answers at the end of today's notes.

Next in today's potpourri of topics was a short discussion of how you might measure humidity.  One of the ways 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 dry thermometer measures the air temperature. 

Would the wet thermometer be warmer or colder.  People seemed to have some doubt about how to answer this questions.  The best thing in that case is to go get one of your hands wet.  Does it feel the same as the dry hand?  You might blow on both hands to increase the evaporation from the wet hand.  I think you'll find the wet hand feels colder.

What could you say about the relative humidity in these two situations (you can assume the air temperature is the same in both pictures).  A number of people thought the cold picture would correspond with high humidity.  That surprised me because you would feel coldest on a dry day (the left picture indicates dry air).  The evaporative coolers that many people use in Tucson in the summer work much better (more cooling) early in the summer when the air is dry.  Once the thunderstorm season begins in July and the air is more humid it is hard to cool your house below 80 F.



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  Warm water evaporates at a higher rate than cool water (think of a steaming cup of hot tea and a glass of ice tea).

The evaporation is shown as blue arrows because this will cool the thermometer. 

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.  As the thermometer cools the rate of evaporation will decrease.  The thermometer will continue to cool until the evaporation has decreased enough that it balances the condensation.




The rates of evaporation and condensation are equal.  The temperature will now remain constant.

Here's the situation on a day with higher relative humidity.  There's enough moisture in the air to provide 3 arrows of condensation. 


The thermometer will only need to cool a little bit

before the rates of evaporation and condensation are again equal.

Here's the all important summary picture.

A large difference between the dry and wet temperatures means the relative humidity is low.  A small difference means the RH is higher.  No difference  means the relative humidity is 100%. 

Note the difference between air temperature and dew point temperature follows the same kind of rule.





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 might remind you of something similar that we covered earlier in the semester. 

If you said "wind chill" you're right.




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.

There's something like that involving heat and humidity.  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 relative humidity because 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). 

Evaporative cooling and saturation are involved in the "drinking bird".  Here's a video.


The drinking bird is filled with a volatile liquid of some kind (it used to be ether but now is freon?).  Initially the bird's head and butt are the same temperature.  The liquid evaporates and saturates the air inside with vapor.

Next you get the bird's head wet.  Water is normally used but I used isopropyl alcohol because it evaporates quickly and produces a lot of cooling (if you've got some available a few drops on your hands, rub them together, and feel how cold they get).

The amount of vapor in the cool saturated air in the head is less than in the warmer saturated air at the bottom.


The differences in vapor concentrations produce pressure differences.  The higher pressure at the bottom pushes liquid up the stem of the bird.  The bird becomes top heavy and starts to tip.

At some point the bottom end of the stem comes out of the pool of liquid at the base.  Liquid drains from the neck.

and the bird straightens back up.
 
You can arrange the bird so that when it tips its beak dips into a small cup of water or alcohol.  This keeps the head moist and cool and the dipping motion could go on indefinitely.

Here's where we will be headed next.



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. 

Next Monday we'll be looking at what happens when air above the ground is 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 scatter sunlight and make the sky hazy.  As humidity increases dry haze turns to wet haze and eventually to fog (Point 4).  And we'll finish class by creating a cloud in a bottle.  We'll look at the role that condensation nuclei play in this process.  Do they help or hinder cloud formation?
But let's get dew and frost out of the way today.  This explanation is a little bit different from the one in class.


This figure illustrates the formation of dew.  We start at the top and assume the temperature is 70 F at the end of the day.  The temperature will start to drop as we move from day to night.  We assume the dew point temperature is 40 F on this night (it is considerably lower than that at the present time in Tucson, the air is very dry).  The relative humidity (RH) reaches 100% when the air temperature has cooled to the dew point.  The air temperature continues to drop to a low of 35 F.  When the air is cooled below the dew point, the air finds itself with more water vapor than it can contain.  The excess condenses and things on the ground get wet with dew.

Conditions are similar in this example except for the nighttime temperature which is below freezing.  Dew would begin to form once the air temperature reached the dew point.  But once the air temperature dropped below freezing the dew would freeze.  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 air temperature reaches the dew point and the RH reaches 100%, water vapor turns directly to ice (deposition).  This is frost.



What would happen here?  In one case both the air temperature and the dew point are above freezing.  In the other case they're both below freezing.  In both cases the dew point is lower than the nighttime minimum temperature.  You'll find the answer at the very end of today's notes.


Here are the answers to the first set of questions embedded in today's notes.



Here are answers to the question about the rain shadow effect
The mixing ratio, r,  is increasing and RH is remaining constant ___d____
There isn't any part of the picture where water vapor is being added to the air.

The RH is increasing & r is remaining constant ____a___
The rising air is cooling and RH increasing, from a value less than 100% at ground level to a value of 100% at cloud base.

The mixing ratio is decreasing and RH is remaining constant ___b____
The air is being cooled below its dew point temperature as the air continues to rise along Path B.  Excess water vapor will condense.  Removal of water vapor from the air means the mixing ratio will decrease.
The RH is decreasing & r is remaining constant ___c____
Along Path C the air is sinking and warming.  This lowers the RH.

Here's a summary of the dew, frozen dew, and frost material earlier in the notes.

Note in the first 3 cases the nighttime minimum temperature (Tmin) drops below the dew point temperature (Td).  Nothing forms in the last case because Tmin never cools to the dew point and the relative humidity never reaches 100%. 

Nothing would form in the two situations drawn earlier in the notes.  In the first case Tmin = 40 F and Td = 35 F and in the second case Tmin = 30 F and Td = 25 F.  The RH would never reach 100% in either of those situations.

Currently in Tucson nighttime minimum temperatures are dropping into the upper 40s perhaps the low 50s.  Dew points meanwhile are much lower, perhaps 20 F.  The nighttime minimum temperature isn't getting anywhere close to the dew point and the ground is remaining dry overnight.