Monday, Mar. 18, 2019

Blue Man Group with the Kodo Drummers (10:22)

We'll be using page 89, page 90, page 91, and page 92 in class today.

One way of measuring humidity - a sling (swing) psychrometer

A short discussion of how you might try to measure humidity (short because it's a topic that tends to put people to sleep).  One of the ways is to use a sling (swing is 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 or the same as the dry thermometer?   You can check it out for yourself - 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.  That's basically what happens with the wet bulb thermometer.


What could you say about the relative humidity in these two situations (you can assume the air temperature is the same in both pictures).  You would feel coldest on a dry day (the left picture indicates dry air).  The evaporative coolers that people like me 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 (but by then you're used to it and it doesn't matter too much).

You feel colder because energy is needed in order for water to evaporate.  The energy in the cases above come from your body.  When your body starts to lose energy you feel cold.

Here are a bunch of details that you can read through if you're so inclined.  My goal is that you understand the basic principle behind a sling psychrometer.  If you'd rather not worry about the details skip to the summary a few pictures further on. 

You need to be aware of a few things to understand the details that follow:
(1a) evaporation is a cooling process
(1b) warm water evaporates more rapidly than cold water (think of a steaming glass of hot tea and a glass of iced tea)

(2a) condensation is a warming process
(2b) whenever there is any moisture in the air there will be some condensation, the rate of condensation depends on how much water vapor is in the air

(3) these two phenomena, evaporation and condensation, operate independently of each other

Here's the situation on a day with low relative humidity.


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.

The evaporation is shown as blue arrows because this will cool the thermometer.   The water on the wet thermometer starts out at 80 F and evaporates fairly rapidly.

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 begin to 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.

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



The rate of evaporation stays the same, the rate of condensation is higher.  The rate of evaporation is still higher than condensation but not by much. 



There'll only be a little cooling before the evaporation is reduced enough to be in balance with condensation.

Here's the summary



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%. 

We saw the same kind of relationship between RH and the difference between air and dew point temperature.


The drinking bird
Evaporative cooling and the dependence of saturation mixing ratio on temperature are both involved in the "drinking bird".


I'm very proud of the bird I found online.  It is about twice as big as what you normally find.  The bird is filled with a volatile liquid of some kind (ether?).  Initially the bird's head and tail are the same temperature.  The liquid inside the bird evaporates and saturates the air inside with vapor.

Next you get the bird's head wet.  Instead of water I cheat a little bit and use isopropyl alcohol (rubbing alcohol) because it evaporates more rapidly than water.  The evaporation of alcohol, just as with water, cools the bird's head.

As we saw last week, the saturation mixing ratio (saturation vapor concentration) of water depends on temperature.  Warm air can contain more water vapor than colder air.  The same applies to the ether vapor in this case.  The head is still saturated with vapor but there is less vapor in the cool head than there is in warm saturated air in the bird's tail.

The differences in amounts of vapor 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 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 a video.

We took away the bird's supply of alcohol, the bird warmed up and stopped tipping.

Wind chill and heat index




Cold temperatures and wind make it feel colder than it really is.   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.  High temperature and high humidity makes it feel hotter than it really is.  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).  A thermometer (a dry thermometer) would measure the same 105 F temperature on both a dry and a humid day.


Condensation nuclei and the formation of dew, frost, haze, fog, and clouds
Here's a visual summary of a part of what we'll be covering next.




A variety of things can happen when you cool air to the dew point and the relative humidity increases to 100%.  When moist air next to the ground becomes saturated (RH reaches 100%) water vapor condenses onto (or, in the case of frost, is deposited onto) the ground or objects on the ground.  This forms dew, frozen dew, and frost. 

When air above the ground cools to the dew point, it is much easier for water vapor to condense onto small particles in the air called condensation nuclei.  It would be much more difficult for the water vapor to condense and form small drops of pure water.  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 because they scatter sunlight and make the sky hazy.  As humidity increases dry haze turns to wet haze and eventually to fog.  We'll try to make a cloud in a bottle and you'll be able to better appreciate the role that condensation nuclei play. 


Condensation nuclei and the role they play in cloud droplet formation

The air next to the ground cools during the night.  Sometimes it cools enough to reach the dew point.  Water vapor condenses onto objects on the ground and you find everything covered with dew (or frost) the next morning.  When this happens in the air up above the ground you might think that water vapor would simply condense and form little droplets.  This is not the case; we will find that small particles in the air called condensation play an essential role in cloud (and fog) formation.

it is much easier for water vapor
to condense onto small particles
called condensation nuclei 		it would be much harder for water vapor
to just condense and form
small droplets of pure water


We probably won't go into all of the details that follow in class, though they aren't hard to figure out and understand.  You're free to just skip the details, but do remember that particles make it much easier for cloud droplets and clouds to form. 

When the air is saturated with water vapor (the relative humidity is 100%) the rates of evaporation and condensation above a flat surface of water will be equal.


There's no real reason for picking three arrows each of evaporation and condensation, the important point is that they are equal when the RH is 100%.

It's hard for water vapor to condense and form a small droplet of water because small droplets evaporate at a very high rate.  This is known as the curvature effect and is illustrated below.
 



The surface of the smallest droplet above at left has the most curvature and the highest rate of evaporation (6 arrows).  If a small droplet like this were to form, it wouldn't stay around very long.  With it's high rate of evaporation it would quickly evaporate away and disappear. 

The middle droplet is larger and would stick around a little longer because it does not evaporate as quickly.  But it too would eventually disappear.

The drop on the right is large enough that curvature no longer has an effect.  This drop has an evaporation rate (3 arrows) that is the same as would be found over a flat surface of water.  A droplet like this could survive, but the question is how could it get this big without going through the smaller sizes with their high rates of evaporation.   A droplet must somehow reach a critical size before it will be in equilibrium with its surroundings.

Particles in the air, cloud condensation nuclei (CCN), make it much easier for cloud droplets to form.  The figure below explains why.

By condensing onto a particle, the water droplet starts out large enough and with an evaporation rate low enough that it is in equilibrium with the moist surroundings (equal rates of condensation and evaporation). 

There are always lots of CCN (cloud condensation nuclei in the air) so this isn't an impediment to cloud formation. 

Now back to material that we will cover in class.
The following information is from the bottom of page 91 in the ClassNotes.
Note that condensation onto certain kinds of condensation nuclei and growth of cloud droplets can begin even when the relative humidity is below 100%.   These are called hygroscopic nuclei.  Salt is an example; small particles of salt mostly come from evaporating drops of ocean water.

I might try to show a video tape, not a digital video but video recorded on a magnetic tape.  It will depend first of all on there being a VCR in the classroom.

 Here are some more of the details that we won't cover in class. 

To understand how condensation onto particles can begin even before the RH has reached 100% we first need to learn about the solute effect






solution droplet
 pure water droplet

Water vapor condensing onto the particle in the left figure dissolves the particle.  The resulting solution evaporates at a lower rate (2 arrows of evaporation).  A droplet of pure water of about the same size would evaporate at a higher rate (4 arrows in the figure at right).  Note the rates of condensation are equal in both figures above.  This is determined by the amount of moisture in the air surrounding each droplet.  We assume the same moist (the RH is 100%) air surrounds both droplets and the rates of condensation are equal. 

The next figure compares solution droplets that form when the RH is 100% (left figure) and when the RH is less than 100%.
the droplet is able to grow
 the droplet is in equilibrium with its surroundings
even when the RH is less than 100%

The solution droplet will grow in the RH=100% environment at left.  You can tell the RH is less than 100% in the figure at right because there are now only 2 arrows of evaporation.  But because the solution droplet only has 2 arrows of evaporation it can form and be in equilibrium in this environment.


We should remember that much of what we see in the sky is caused by scattering of light.  There was a pretty good demonstration of light scattering during one of the music videos played earlier in the semester ("Strak ha pak" from a group called Startijenn).
 

 The figure below is at the bottom of page 91 in the ClassNotes and illustrates how cloud condensation nuclei and increasing relative humidity can affect the appearance of the sky and the visibility.

The air in the left most figure is relatively dry.  Even though the condensation nuclei particles are too small to be seen with the human eye you can tell they are there because they scatter sunlight.  When you look at the sky you see the deep blue color caused by scattering of sunlight by air molecules mixed together with some white sunlight scattered by the condensation nuclei.  This changes the color of the sky from a deep blue to a bluish white color.  The more particles there are the whiter the sky becomes.  This is called "dry haze."  Visibility under these conditions might be anywhere from a few miles up to a few tens of miles.




(source of the image above)

A photograph of fairly severe air pollution in Paris that illustrates an extreme case of dry haze (this is more common and more severe in China and India).   In Paris cars with even numbered license plates weren't allowed into the city on certain days of the week, odd numbers were banned on other days.  Public transportation was free for a short time to try to reduce automobile use. 


The middle picture below shows what happens when you drive from the dry southwestern part of the US into the humid southeastern US or the Gulf Coast.  One of the first things you would notice is the hazier appearance of the air and a decrease in visibility.  It isn't that there are more particles.  The relative humidity is higher, water vapor begins to condense onto some of the condensation nuclei particles (the hygroscopic nuclei) in the air and forms small water droplets.  The water droplets scatter more sunlight than just small particles alone.  The increase in the amount of scattered light is what gives the air its hazier appearance. This is called "wet haze."  Visibility now might now only be a few miles.




Thin fog (perhaps even wet haze)
with pretty good visibility
(source of the image)
 Thick fog
(visibility was less than 500 feet)
(source of the image)

Pictures of fog like we sometimes get in Tucson (maybe once a year).  The picture at left is looking east from my house and was taken early in the morning at the start of the spring semester in 2015.  The picture at right is the view to the west.  Visibility was perhaps 1/4 mile.



Finally when the relative humidity increases to 100% fog forms and water vapor condenses onto all the condensation nuclei.  Fog can cause a severe drop in the visibility.  The thickest fog forms in dirty air that contains lots of condensation nuclei.  That is part of the reason the Great London Smog of 1952 was so impressive.  Visibility was at times just a few feet!

I'm guessing that we'll be running short of time at this point and will probably delay the cloud-in-a-bottle demonstration until Wednesday.