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 (normally moisted with distilled water).  To make a humidity measurement you swing the psychrometer around for a minute or two and then read the temperatures from the two thermometers.  Measurements of the air temperature (dry bulb) and the dry-wet thermometer temperature difference (wet bulb depression) can be used to determine relative humidity and dew point. 

The figure at upper left 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 and the 4 arrows of evaporation were just made up for 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.  Evaporative coolers (swamp coolers) work well (sometimes too well) on days 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 and not much condensation.  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, the water is evaporating more slowly.  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.

The air temperature is the same in this example, but there is more water vapor in the air.  You wouldn't feel as cold if you stepped out of a pool and swamp coolers wouldn't provide much cooling on a warm humid day like this.

There are four arrows of evaporation (because the water temperature is still 80 F just as it was in the previous example) and three arrows now of condensation (due to the increased amount of water vapor in the air surrounding the thermometer).  The wet bulb thermometer will cool but won't get as cold as in the previous example. 

The wet bulb thermometer might well only cool to 75 F.  This might be enough to lower the rate of evaporation (from 4 arrows to 3 arrows) enough to bring it into balance with the rate of condensation.

You would measure a small difference (5 F) between the dry and wet bulb thermometers on a humid day like this.

There won't be any difference in the dry and wet bulb temperatures when the RH=100%.  The rates at which water is evaporating and water vapor is condensing are equal.  That's one of the things that happens when air is saturated.  The dry and wet bulb thermometers would both read 80 F.

The chart below summarizes the relationship between relative humidity and the Tdry - Twet difference.  The chart also includes something we mentioned a lecture or two ago, the difference between air temperature and dew point temperature.


We learned about wind chill earlier in the course.



A 40 F day with 30 MPH winds will feel colder (because of increased transport of energy 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.


Evaporative cooling will make you feel cold if you get out of a swimming pool on an 80 F day with dry air.
You won't feel as cold if the air is humid.  Sling psychrometers make use of this to measure relative humidity and dew point.


Your body tries to stay cool by perspiring.  You would still feel hot on a hot dry day.  The heat index measures how much hotter you'd feel on a hot humid day.  The combination of heat and high humidity is a serious 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 very small particles in the air (condensation nuclei) rather than just forming a small droplet of pure water.   The small water droplets that form are themselves 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 start by looking at the different conditions that can lead to the formation of dew and frost.

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 (the nighttime minimum temperature, Tmin.  Sometime during the night, at Point 2, the air temperature reaches 40 F, the dew point.  The relative humidity reaches 100% and water vapor begins to condense onto the ground.  This continues as the air cools to Tmin.  You would find your newspaper and your car covered with dew the next morning.

This night is similar except that the nighttime minimum temperature drops below freezing.  Dew forms 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, the RH never reaches 100%.  Nothing would happen.

As mentioned earlier, when the relative humidity in air above the ground (and away from objects on the ground) reaches 100%, water vapor will condense onto small particles called condensation nuclei. 


It would be much harder for the water vapor to just condense and form small droplets of pure water.  This is because very small droplets have an unusually high rate of evaporation.  This is known at the curvature effect.  Saturated air (RH=100%) is not able to supply enough condensation to offset the high rate of evaporation.  If a small droplet suddenly forms it will quickly evaporate away.

The figure below shows 4 droplets of varying radii.  Note that the rates of condensation (3 arrows) are equal in all 4 cases.  The rate of condensation depends on the amount of moisture in the air surrounding each drop and that is the same for each droplet.

A very small droplet is shown in 1 with a high rate of evaporation (6 arrows of evaporation).  The droplet is a little larger in 2 and the rate of evaporation has decreased to 4 arrows.  But, because evaporation exceeds condensation in both 1 and 2, both droplets would quickly evaporate away shortly after they had formed. 

Once the droplet grows to a certain size as in 3, the rate of evaporation has decreased to a point where it is balanced by an equal amount of condensation.  The rate of evaporation won't decrease further if the droplet grows beyond this size.  Droplets 3 and 4 have the same rates of evaporation.  Both droplets are in equilibrium with their surroundings (equal rates of evaporation and condensation).

One way of avoiding the difficult shown above is for water vapor to condense onto a small particle of some kind.

In this case you effectively start out with a droplet that is large enough that it won't have the high rate of evaporation found with very small droplets.  The droplet shown above is in equilibrium with its surroundings.

Small droplets can also form on CCN particles that dissolve.  The rate of evaporation from the resulting water solution is less than that of pure water.  This is known as the solute effect.


Water vapor has condensed onto a CCN particle at left in the figure above.  In the middle picture the CCN particle has dissolved and formed a solution with strong concentration.  This reduces the rate of evaporation.  Since condensation exceeds evaporation the droplet will grow.  Without the solute effect, a drop this small would have a high rate of evaporation that would exceed the rate of condensation and the droplet would evaporate.

Eventually the solution concentration weakens enough that it no longer affects the evaporation rate.  In the right picture the rates of evaporation and condensation are equal and the droplet is in equilibrium with its surroundings.

Because of the solute effect, it is possible for droplets to form when the relative humidity is less than 100%.  Condensation nuclei that allow this to happen are called hygroscopic nuclei.  This is illustrated in the figure below.  A small droplet has formed.  Because the resulting solution concentration is strong there are only 2 arrows of evaporation.  The droplet is in equilibrium with surroundings that are only able to supply 2 arrows of condensation (2 arrows of condensation in this figure compared with 3 arrows in the previous figures implies the RH is less than 100%).

In the classroom version of this course we show a short video that demonstrates how water vapor would, over time, preferentially condense onto small grains of salt rather than small spheres of glass. 


At the start of the demonstration (left figure above) small grains of salt (soluble in water) were placed on a platform in a petri dish containing water.  Some small spheres of glass (insoluble) were placed in the same dish.  After about 1 hour small drops of water form around each of the grains of salt but not the glass grains (shown above at right).

In humid parts of the US, water will condense onto the grains of salt in a salt shaker causing them to stick together.  Grains of rice apparently absorb moisture which keeps this from happening and allows the salt to flow freely out of the shaker when needed.


This figure shows 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."

The middle picture shows what happens when you drive from the dry southwestern part of the US into the humid southeastern US.  One of the first things you would notice is the hazier appearance of the air and a decrease in visibility.  Because the relative humidity is high, 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."

Finally when the relative humidity increases to 100% fog forms.  Fog can cause a severe drop in the visibility.  The thickest fog forms in dirty air that contains lots of condensation nuclei. 


Fog can be produced in a variety of ways.  To produce fog you first need to increase the relative humidity (RH) to 100%


You can do this either by cooling the air (radiation fog) or adding moisture to and saturating the air (evaporation or steam fog).  Both will increase the ratio in the RH formula above.

Probably the most common type of fog in Tucson is radiation fog.  The ground cools during the night by emitting IR radiation (left figure below).  The ground cools most rapidly and gets coldest when the skies are free of clouds and the air is dry (except for a thin layer next to the ground.
 
Air in contact with the ground cools and radiation fog can form (right figure above).  Because the fog cloud is colder than the air right above, this is a stable situation.  The fog clouds "hugs" the ground.

Radiation fog is sometimes called valley fog.


The cold dense foggy air will move downhill and fill low lying areas.   Because the fog reflects sunlight, it is often difficult for the sun to warm the air and dissipate thick clouds of valley fog.

Steam fog or evaporation fog (also sometimes known as mixing fog) is commonly observed on cold mornings over the relatively warm water in a swimming pool.



Water evaporating from the pool saturates the cold air above.  Because the fog cloud is warmer than the cold surrounding air, the fog clouds float upward.

When you "see your breath" on a cold day


you're seeing mixing fog.  Warm moist air from your mouth mixes with the colder air outside.  The mixture is saturated and a fog cloud forms.

Here's another demonstration from the classroom version of this course that puts together many of the concepts we have been covering.  Cooling air and changing relative humidity, condensation nuclei, and scattering of light are all involved in this demonstration.


We used a strong, thick-walled, 4 liter flask (vaccum flasks like this are designed to not implode when all of the air is pumped out of them, they aren't designed to not explode when pressurized).  There was a little water in the bottom of the flask to moisten the air in the flask.  Next we pressurized the air in the flask with a bicycle pump.  At some point the pressure blows the cork out of the top of the flask.  The air in the flask expands outward and cools.  This sudden cooling increases the relative humidity of the moist air in the flask to 100% ( probably more than 100% momentarily ) and water vapor condenses onto cloud condensation nuclei in the air.  A faint cloud became visible at this point.  The cloud droplets are too small to be seen with the human eye.  You can see the cloud because the water droplets scatter light.





The demonstration was repeated an additional time with one small change.  A burning match was dropped into the bottle.  The smoke from the match added lots of very small particles, condensation nuclei, to the air in the flask.  The cloud that formed this time was quite a bit "thicker" and much easier to see.





Clouds are one of the best ways of cleaning the atmosphere (cloud droplets form on particles, the droplets "clump" together to form a raindrop, and the raindrop carries the particles to the ground).  A raindrop can contain 1 million cloud droplets so a single raindrop can remove a lot of particles from the air.  You may have noticed how clear the air seems the day after a rainstorm; distant mountains are crystal clear and the sky has a deep blue color.  Gaseous pollutants can dissolve in the water droplets and be carried to the ground by rainfall also.


A cloud that forms in dirty air is composed of a large number of small droplets (right figure above).  This cloud is more reflective than a cloud that forms in clean air, that is composed of a smaller number of larger droplets (left figure).   Just like in the cloud-in-a-bottle demonstration, the cloud that was created when the air was full of smoke particles was much more visible than the cloud made with cleaner air.

This is has implications for climate change.  Combustion of fossil fuels adds carbon dioxide to the atmosphere.  There is concern that increasing carbon dioxide concentrations will enhance the greenhouse effect and cause global warming.  Combustion also adds condensation nuclei to the atmosphere (just like the burning match added smoke to the air in the flask).  More condensation nuclei might make it easier for clouds to form, might make the clouds more reflective, and might cause cooling.  There is still quite a bit of uncertainty about how clouds might change and how this might affect climate (remember too that clouds are good absorbers of IR radiation).