Friday Oct. 20, 2006

The 1S1P Assignment #2a reports were collected today.  The Assignment #2b reports are due next Monday.  Remember you can only do a maximum of 2 reports.
One of the ways relative humidity and dew point temperature can be measured.

A sling psychrometer consists of two thermometers mounted side by side.  One 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 following explanation is an expanded and enhanced version of what is found on p. 89 in the photocopied class notes.

On a warm dry day there will be a large difference (20 F in this example) between the dry and wet bulb thermometer readings.  You could use the dry and wet bulb temperature measurements and a chart (see pps 433-436 in the text) to determine the relative humidity and the dew point temperature.

The difference between the dry and wet bulb thermometers will be smaller on a humid day (only 5 F here).

There won't be any difference in temperatures when the RH=100%.
When you cool air that is next to the ground to the dew point, water vapor condenses onto objects on the ground such as blades of grass, your automobile, your morning newspaper, maybe even your sleeping cat.

In the first example air  starts out with a temperature of 65 F early in the evening.  It cools to 35 F during the night.  When the air reaches 40 F, the dew point, the RH reaches 100%.  As the air temperature drops below the dew point and cools to 35 F water vapor will condense onto the ground or objects on the ground (such as an automobile).  This is dew.

The dew point is the same but the nighttime minimum temperature is below freezing in the second example.  Dew will form again on this night when the air temperature reaches 40 F.  Once the air temperature drops below 32 F though the dew will freeze and form frozen dew.

In the third example both the dew point and nighttime minimum temperatures are below freezing.  When the air temperature drops below the dew point, water vapor turns directly to ice (deposition) and forms frost.  The dew point in this case is sometimes called the frost point.

The air never becomes saturated in the fourth example because the nighttime minimum temperature never cools to the dew point.  You wouldn't see anything on this night.
When air above the ground reaches 100% relative humidity it is much easier for water vapor to condense onto small particles in the air called condensation nuclei.  It would be much harder for the water vapor to condense and form small drops of pure water.  You can learn why this is true by reading p. 92 in the photocopied class notes.

Water vapor will condense onto certain kinds of condensation nuclei even when the relative humidity is below 100% (again you will find some explanation of this on p. 92).  These are called hygroscopic nuclei.
A short video showed how water vapor would, over time, preferentially condense onto small grains of salt rather than small spheres of glass.

The start of the video at left showed the small grains of salt were placed on a platform in a petri dish containing water.  Some small spheres of glass were placed in the same dish.  After about 1 hour small drops of water had formed around each of the grains of salt (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 will keep this from happening and allow 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 together with white light scattered by the larger 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.
Now armed with some knowledge of condensation nuclei, humidity, and scattering of light we are ready for another demonstration.  We will try to make a cloud in a bottle.






We used a strong thick-walled 4 liter flask (flasks like this are designed not to implode when all of the air is pumped out of them, they aren't designed 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.  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 cooling increases the relative humidity of the moist air in the flask to 100% (probably more than 100%) 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 a second time (perhaps a third time) with one small change.  A burning match was dropped into the bottle.  The smoke from the match consisted of lots of very small particles that act as condensation nuclei.  The cloud that formed this time was somewhat "thicker" and 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 contains about 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.  Gaseous pollutants can dissolve in the water droplets and be carried to the ground by rainfall also.

A cloud composed of a large number of small droplets is more reflective than a cloud composed of a smaller number of larger droplets.  This something that interests the people studying climate change.  Combustion of fossil fuels adds carbon dioxide to the atmosphere but also adds condensation nuclei.  The increasing greenhouse gas concentrations are expected to warm the earth.  More particles might make clouds more reflective though which could cool the earth.