1. What are the states that water can exist in?
2. How much energy must be added or removed to convert from one state to another?
3. What happens when that energy is added or removed?
4. What is water vapor?
5. What are clouds?
6. How does water vapor affect air density?
7. What are the different ways of measuring or describing "Humidity"?
   
8. What controls the maximum possible humidity?
   
9. How is Relative Humidity calculated?

• Temperature (See Thompson and Turk, pp. A.10-A.11):
• conversion
• Fahrenheit scale (Univ. of Michigan)
• Fahrenheit scale (St. Louis Community College, MO)
• Fahrenheit scale (Rutgers Univ.)
• Celsius scale
• States (or) Phases of Matter
• We can see the change between the different phases with a diagram such as the one below.
  

• 
  
• Note: "Evaporation" and "Vaporization" are exactly the same thing.
• vapor (= gas)
• liquid
• solid
• The + and (-) signs signify whether energy is added or removed from the water to cause a particular phase change
  
• A transfer of heat energy is needed convert from one phase to the another:
• To melt: add 80 calories/gram
• (the Heat of Fusion)
• To freeze: remove 80 cal/gram
• also (the Heat of Fusion)
• To vaporize: add 540 cal/gm
• (The Heat of Vaporization)
• To condense: remove 540 cal/gm
• also (The Heat of Vaporization)
• To go directly from solid to gas: Sublimation
• To go directly from gas to solid: Deposition
• Note: the word "deposition" is also used for sediments, so make sure that you know from the context what KIND of "deposition" is being referred to.
• We're all familiar with melting, freezing, condensation, and vaporization (which is exactly the same as "evaporation")
• But have you ever seen sublimation?
• Dry ice undergoes sublimation, but that's frozen CO₂, which has no liquid phase at room temperatures.
• What about for water ice?  Well, when you put a steak into the freezer and forget about it for a couple of months, what happens to it?  Freezer burn!  All the water in the frozen steak sublimates, leaving it dry and gnarly.
• Have you ever seen deposition?  Most Floridians say no.  But:
• Where does frost come from?  Directly deposition of ice from water vapor in the air!  And if you've ever seen "frost" occurring in a freezer even in the supermarket, that's "deposition", too.
• Speaking of water vapor, what is it?
• Answer this question: Which of the following is water vapor?
• Mist?
• Fog?
• Clouds?
• All of these?
• None of these?
  
• To answer that question, click here.  You'll probably be surprised.
  

• A calorie is defined as the amount of Heat energy needed to raise the temperature of 1 gm of H₂O by 1^o Celsius.
• This is different from a dietary Calorie (which should always be capitalized).
• A Calorie = 1000 calories = 1 kilocalorie.
• A seemingly harmless candy bar (200 Calories) actually carries 200,000 calories of chemical energy, enough to raise the temperature of 100 kg of water (about a 220 lb person) by 2 full degrees Celcius.  That's a lot of energy!
  
• Specific heat: the amount of Heat needed to raise the temperature of 1 gm of any material by 1^oCelsius.
• For water, it is: 1 calorie/gram-^oCelsius (exactly 1, because that's HOW the calorie is defined).
• Calculations to find how much energy is needed for certain temperature changes.
• If you have 1 gram of water and you want to heat it by 1^oC, the calculation is:
• 1 gram H₂O x 1^oC x 1cal/gm-^oC = 1 cal (the amount of energy needed to raise the temperature of 1g by 1oC)
• If you have 1 gram of water and you want to heat it by 10^oC, the calculation is:
• 1 gram H₂O x 10^oC x 1cal/gm-^oC = 10 cal
• If you have 10 grams of water and you want to heat it by 1^oC, the calculation is:
• 10 gram H₂O x 1^oC x 1cal/gm-^oC = 10 cal
• If you have 10 grams of water and you want to heat it by 10^oC, the calculation is:
• 10 grams H₂O x 10^oC x 1cal/gm-^oC = 100 cal
• So overall, to calculate how much energy is necessary to raise the temperature of a certain mass of any material:
• Mass x temperature change x Specific Heat = amount of heat energy
  
• For most other materials, the specific heat is much less.
• Examples:
• Water: 1.0 cal/g-^oC.
• Aluminum: 0.22 cal/g-^oC.
• Copper: 0.09 cal/g-^oC.
• Asphalt: 0.22 cal/g-^oC.
• Quartz Sand: 0.19 cal/g-^oC.
• Dry soil: 0.19 cal/g-^oC.
• Wet soil: 0.35 cal/g-^oC.
• This means that most materials heat up and cool off far faster than water (or things made of liquid water, like oceans, lakes, etc.)
• As we shall see, this will become extremely important when we consider how fast the land heats up during the day, or cools during the night, compared to the ocean.
• Rock in general has a much lower specific heat than water (i.e. the ocean).
• During the daytime, with the sun shining, the land heats up much more quickly.  This is why when you go to the beach, the sand is usually much hotter than than the ocean next to it.
• During the evening, without sun both the land and the ocean lose heat energy.  But due to the lower specific heat of the land, losing the same number of calories as the ocean causes the temperature of the land to decrease much more rapidly.  As a result, by morning the land will usually be significantly cooler than the ocean next to it.
• Remember this when we next learn about WIND.
  

• ---

• In any given volume of gas, at any given pressure and temperature, there are always the same number of gas molecules.  This is known as "Avogadro's Law". 
  
• They are in constant motion, bumping into one another and (at a constant temperature and pressure) keep the same average distance between them.
• Because of that, if you have, for instance, 1 liter of gas at 27^oC (room temperature, more or less) and 1 atmosphere of pressure, it will always contain the same number of molecules, whether you have:
• pure oxygen,
• pure nitrogen,
• pure argon,
• pure natural gas,
• pure water vapor,
• or any mixture of any of those things, including air.
  
• When you add water molecules (as a gas) to the air, they don't go in between constantly-spaced other atmospheric gas molecules and atoms, they replace them by pushing away the other atoms, the volume expands, and the density decreases.  Effectively, heavier atmospheric gases are replaced by lighter (water) molecules in the original volume.
  
• It's similar to what would happen if you had a bucket of golf balls and mixed in a bunch of ping pong balls.
• Some of the balls will spill out, leaving the same number of balls in the bucket, but now some of those balls are ping-pong balls.
• The ping pong balls are about the same size but much lighter, so the overall density of the bucket contents decreases.
• Therefore, adding water vapor lowers the average molecular weight of air, decreasing its density compared to dry air.
• Why do people think that humid air is more dense than dry air?  It's mostly a physiological effect.  When it's humid, water (perspiration) doesn't evaporate as easily from our skin, making us feel sticky and uncomfortable, and it's as if the air is weighing us down.  But the physical reality is just the opposite.
  

• The actual amount of water vapor present in the atmosphere:
  
• Mixing Ratio
• Specific Humidity
• Absolute Humidity
  
• Vapor Pressure
• They are defined in slightly different ways, with different units:
• Mixing Ratio - the amount (in grams H₂O/kg Air) of moisture present as water vapor in 1 kg of dry air.
• Specific Humidity - the amount (in grams H₂O/kg Air) of moisture present as water vapor in 1 kg of the atmosphere.  (numerically VERY CLOSE to Mixing Ratio)
  
• Absolute Humidity - the mass of moisture present as water vapor in a volume of the atmosphere (in grams H₂O/m³ Air).  In other words, it is the density of the water vapor component of air.  Not very useful because the volume changes with altitude and temperature.
• Vapor Pressure - the pressure that water vapor exerts (in millibars [mB]) in the atmosphere.  Generally a small fraction of the overall atmospheric pressure, no more than 4% of the total.
• This is an empirically determined quantity.  It has to be measured using specialized techniques and tools.  For the purposes of our calculations, this will be a "given" value.  In other words, for all our calculations, the amount of water vapor present in the atmosphere will be explicitly stated.
  
• The maximum amount of water vapor that the atmosphere can hold is called "Saturation",  and is expressed in a number of ways:
  
• There IS a maximum because as more and more water evaporates from the liquid form, the rate of condensation also increases.  At some point, dependent on temperature, the rate of condensation will equal the rate of evaporation, giving that maximum:
  
• Saturation Mixing Ratio
• Saturation Specific Humidity (also known as "Saturation Humidity" or "Water Vapor Capacity")
  
• Saturation Absolute Humidity
• Saturation Vapor Pressure
• They are also defined in slightly different ways:
  
• The Saturation Mixing Ratio - the maximum amount (in grams H₂O/kg Air) of moisture present as water vapor in 1 kg of dry air at any given temperature.
• Saturation Specific Humidity - the maximum possible Specific Humidity (in grams H₂O/kg Air) at any given temperature.
• Saturation Absolute Humidity - the maximum possible Absolute Humidity (in grams H₂O/m³ Air) at any given temperature.
• Saturation Vapor Pressure - the maximum pressure that water vapor can exert (in millibars [mB]) in the atmosphere at any given temperature. 
  
• The Relative Humidity
• This is what is reported on the weather report.  It is the ratio of the current humidity divided by the maximum humidity for the current temperature, x 100%.  Your text uses the ratio of Vapor Pressure to Saturation Vapor Pressure.  We will use the Specific Humidity and Saturation Specific Humidity, both in units of grams H₂O/kg Air.  The principles are the same.
  
• We will learn how to calculate this:

 NOTE: Saturation Specific Humidity is controlled by the temperature:

Saturation   Temper-      Specific    ature       Humidity (Degrees oC) (gH2O/kg Air)     40           47     35           35     30           26.5     25           20     20           14     15           10     10            7      5            5      0            3.5     -10           2     -20           0.75     -30           0.3     -40           0.1

Relative Humidity = the Specific Humidity / Saturation Specific Humidity x 100%

The Relative Humidity (RH) is the ratio of the amount actually present to the maximum amount that can be present (which must come from that graph or table).  It is always given in a percent of the maximum for a given temperature.

Examples (Download this file, print it out, and fill it out as you read this):

• 1. At 25^oC, the maximum amount of water that can be evaporated into the air is 20 g H₂O/kg air**.  That is the "Saturation Specific Humidity" (SSH) or "Water Vapor Capacity".  You get that number either from the graph or the table above.
• ** - Note - this number comes from the graph and table above.  If you do not understand how to arrive at this number, SEE ME to find out.
• If we have a closed container with 5 gH₂O of water vapor present in a kg of air,
  
• then the "Specific Humidity" (SH) is 5gH₂O/kg air.
  
• The "Relative Humidity" is (5gH₂O/kg air / 20gH₂O/kg air) x 100% = 25%.
• Note that the units of SH and SSH (in red in the above equation) are the same, are in both the ^numerator and _denominator of the ratio, and therefore "cancel out", leaving only percent as the unit.
  
• If there is liquid in the container, more molecules of water will evaporate than will condense back into the water, and the humidity will increase:
• 
• Since we are well below saturation, water molecules evaporate (red arrows up) faster than they condense back into liquid (blue arrows down).  We are not yet at equilibrium and water will continue to evaporate and accumulate in the container.
  
• 2. If enough water evaporates to give 10 g H₂O/kg air, then the "Specific Humidity" is 10 g H₂O/kg air.
• Since the temperature has not changed, the Saturation Specific Humidity has not changed and is still 20 g H₂O/kg air,
• and the "Relative Humidity is (10/20) x 100% = 50%.  Water will continue to evaporate at a higher rate than the rate of condensation, and the humidity will continue to increase:
• 
• We are still below saturation, so water molecules will continue to evaporate (red arrows) faster than they condense back into liquid (blue arrows).  We are not yet at equilibrium and water will continue to evaporate and accumulate in the container.
  
• 3. Eventually, when 20 g H₂O have evaporated / kg air,
• Specific Humidity is now 20 g H₂O/kg air,
• Saturation Specific Humidity is now 20 g H₂O/kg air, and
• the "Relative Humidity" is (20/20) x 100% = 100%. 
• 
• the rate of condensation will equal the rate of evaporation, and a Dynamic Equilibrium will be reached.
• 4. We can change the relative humidity simply by changing the temperature as well.
• At 20^oC, the Saturation Specific Humidity is 14g H₂O/kg Air.
• If there is actually 7 g H₂O/kg Air (the Specific Humidity),
• then the Relative Humidity is 50%.
• Show how this is calculated on your worksheet
  
• 5. Rather than add water as in the previous example, we reduce temperature.
• So, if we reduce the temperature to 10^oC,
• the Saturation Specific Humidity is 7g H₂O/kg Air (See Table and Graph above),
• and the Relative Humidity is 100%.
• Show how this is calculated on your worksheet
• At this temperature, if the temperature drops any further, condensation occurs, so the temperature where the Specific Humidity equals the Saturation Specific Humidity is known as the "Dew Point".
  
• 6. If we further reduce the temperature to 5^oC,
• the Saturation Specific Humidity is 5 g H₂O/kg Air (See Table and Graph above),
• This is lower than the Specific Humidity was in #4 and 5, so some water has to condense, because you CANNOT HAVE MORE WATER than the saturation value,
  
• and 2 g of our original 7 g of H₂O condenses and precipitates,
• leaving an Specific Humidity of 5 g H₂O/kg Air,
• and the Relative Humidity remains at 100%.
• Show your calculation on your worksheet.
  
• Remember, you can NOT have a Relative Humidity greater than 100% (because 100% means that your Specific Humidity (SH) EQUALS your Saturation Specific Humidity (SSH), which is the MAXIMUM amount of water vapor it is possible to have in the atmosphere at any given temperature), so that if your SSH drops below your SH, condensation of liquid water or deposition of ice MUST happen so that you stay at saturation.
• This is the ultimate cause of ALL precipitation.  Why we REACH saturation is a much more interesting question, that will be covered in detail in the Weather link.
  
• 7. Your worksheet does not need to be handed in.  I am giving it to you so you can learn how to do these calculations.
• If you are having difficulty understanding this, look at this link.  If you still have trouble, contact your instructor.
• You can also look at this Youtube video (link below) on the calculation of Relative Humidity that I've prepared.  This has a direct application to the Humidity Quiz and Investigation assignments that you'll be doing, so please watch this video!

  Calculation of Relative Humidity