Geography 140
Introduction to Physical Geography

Lecture: Moisture as an Element of Weather

 IV. Moisture -- In discussing the indirect methods by which air temperature 
     is changed, I mentioned that they required mediations between themselves 
     and their ultimate cause, insolation. We just finished discussing one of 
     those mediations, air pressure, which causes the air movement both 
     methods depend on. In this section, I'll take up moisture. The change of 
     state of moisture in moving air is what heats the atmosphere under the 
     wet adiabatic process.
     A. The importance of water vapor in the atmosphere.
        1. Its condensation and/or freezing that releases heat into the 
           atmosphere.
           a. This slows down the rate of cooling in rising air.
           b. It produces an absolute gain in air temperature should that air 
              move back down to lower elevations.
        2. The evaporation of water from the hydrosphere into the atmosphere 
           requires that it absorb atmospheric heat.
           a. This slows down rises in temperature at the earth's surface.
           b. The motion of water vapor then allows the transfer of latent 
              heat to other parts of the earth, where its later release helps 
              even out global temperatures.
     B. Water vapor in the atmosphere is called humidity.
        1. Measurement
           a. Specific or absolute humidity
                i. Defined as the actual amount of water in the air. It can be 
                      defined in terms of mass per unit mass of air (specific 
                      humidity) or per unit of air volume (absolute humidity).
                   a. For example, it's given as the mass of water vapor in 
                      grams contained in a cubic meter of air (absolute 
                      humidity).
                   b. Or as the mass of water vapor in grams found in one 
                      kilogram of air (specific humidity).
               ii. Saturation vapor pressure is the maximum partial pressure 
                   that the water vapor molecules in a package of air can 
                   contribute to the total pressure of all the gas molecules 
                   in a given volume of air (e.g., cubic meter) at a given 
                   temperature.  
                   a. Saturation vapor pressure is dependent on temperature.
                   b. Saturation vapor pressure is, however, unaffected by 
                      total pressure, since we're considering absolute 
                      humidity here, grams of vapor to cubic meter of air, and 
                      pressure varies depending on whether our cubic meter of 
                      air is at sea level or up a few kilometers.
                   c. In any air mass with access to liquid or solid water, 
                      there are always water molecules evaporating or 
                      sublimating into vapor at a rate set by temperature, and 
                      there are always some vapor molecules condensing or 
                      depositing (sublimating) back into a liquid or solid 
                      state.
                   d. If there are few vapor molecules, the rate of 
                      evaporation/sublimation is greater than the rate of 
                      condensation/deposition, so evaporation dominates 
                      water's behavior.
                   e. The more water vapor molecules there are in a volume of 
                      air, the greater the rate of condensation/deposition, 
                      which means that, sooner or later, the vapor entering 
                      the air space is balanced by the vapor leaving the air:  
                      This steady-state equilibrium of water entering and 
                      water leaving is called "saturation."  The vapor 
                      pressure at this point is the "saturation vapor 
                      pressure."  It is also called the "saturation quantity" 
                      or the "saturation absolute humidity." 
              iii. For any given temperature, there is a maximum mass of water 
                   vapor that a kilogram or a cubic meter of air can hold.  In 
                   other words, the saturation vapor pressure is related to 
                   temperature.
                   a. Remember? I referred to this before, in discussing the 
                      wet adiabatic process in the last lecture: The warmer 
                      the air is the more water it can hold as vapor. And vice 
                      versa?  This is the key point to remember:  Higher 
                      temperatures allow higher amounts of vapor to be 
                      sustained in the air, and colder temperatures mean less 
                      vapor will be sustained in the air.
                   b. Graphed this relation between saturation quantity and 
                      air temperature looks like this:

                      

               iv. Saturation mixing ratio is a similar concept, but it 
                   relates saturation to temperature AND to pressure.  
                   a. It's the ratio of water vapor to the other gasses in 
                      air.  
                   b. If you considered the ratio of water vapor to ALL gasses 
                      (including the water vapor), you'd have specific 
                      humidity.
                   c. Because water vapor is such a small percentage of air 
                      (somewhere from 0-4%), there isn't all that much 
                      difference, really, between saturation mixing ratio and 
                      specific humidity at saturation.
                   d. If you took a given volume of saturated air at a given 
                      temperature (say an imaginary 1 cubic meter box) and 
                      stuffed some more air in there, it would have to be 
                      gasses other than water vapor (since our imaginary box 
                      is saturated).  The addition of dry air would increase 
                      the denominator of the mixing ratio, which would 
                      decrease that ratio (same water: more air).  It would 
                      also raise the air pressure (cramming more air in the 
                      same volume).  So, the saturation quantity, measured as 
                      specific humidity, drops with drops in temperature AND 
                      with increases in pressure.
                v. The important thing to remember from this whole section on 
                   actual moisture content is that specific and absolute 
                   humidities measure the amount of water that can be 
                   extracted, at least potentially, from a package of air as 
                   precipitation.  Cold air yields very little as snow or 
                   rain; warm air has the capacity to yield lots of 
                   precipitation.
           b. Relative humidity is a term you hear a lot in weathercasts.
                i. It can be defined as the amount of water vapor actually in 
                   the air relative to what it could hold at that temperature.  
                   In other words, it's the percentage of actual amount of 
                   water present over the saturation quantity of that 
                   temperature.

                   R = A x 100/S
                       Where:  A = Actual amount of moisture (absolute 
                                   humidity or specific humidity, g/Kg or 
                                   g/cubic meter)
                               S = Saturation quantity for that amount of air, 
                                   depending on temperature (absolute 
                                   humidity) or on temperature and pressure 
                                   (specific humidity).

                   So, looking at that graph above, let's say we knew that a 
                   given cubic meter of air was holding 5 g of vapor.  Let's 
                   say, further, that the current air temperature was 30° 
                   C.  What would the relative humidity be?  A = 
                   5g/m3 and S = ??? (look up 30° C on the 
                   graph, read straight up until you hit the curve and then 
                   hang a left and read the saturatio quantity on the Y axis).  
                   So, 500/30 = ???  (it's pretty dry!).
               ii. When the actual amount is the same as the potential amount, 
                   the air is "saturated". Its relative humidity is 100%.
              iii. When the air temperature drops to that point where the 
                   saturation quantity is equal to the actual amount of water 
                   vapor present, it is said to have reached the "Dew Point" 
                   for that particular package of air with its particular 
                   H20 content.
               iv. You can estimate the dew point for a particular package of 
                   air by using that graph in the reverse direction.  Look up 
                   the moisture content of a package of air on the Y axis, 
                   read straight across until you hit the curve, and then drop 
                   straight down.  That temperature on the X axis is the dew 
                   point temperature for air holding that much moisture.
                v. The table below illustrates the relationships among 
                   relative and specific humidities, dew point, and saturation 
                   quantity graphs.  Make sure to read the graph above to make 
                   sure you know where I'm getting the saturation quantities.  
                   What we're going to do is follow the change in relative 
                   humidity in a given package of air containing 6 grams of 
                   moisture per cubic meter as it goes through an 18 hour 
                   period of changing temperatures.  The drop in temperature 
                   in the afternoon and evening will lower the saturation 
                   quantity (the denominator in the relative humidity formula 
                   above), and this will raise the relative humidity levels, 
                   even though no new moisture has been added!
                   a. Time .................  3 pm
                      Air temperature ...... 25° C
                      Saturation quantity .. 24 grams/cubic meter (make sure 
                                             you convince yourself of this, 
                                             using the graph)
                      Actual quantity ......  6 grams/cubic meter
                      Relative humidity = 600/24 = 25%
                      Is there any reason for precipitation? 
                           No: R < 100%


                           Therefore, let's move the 6 g forward 3 hours
                   b. Time .................  6 pm
                      Air temperature ...... 13o C (whoa! it got COLD!)
                      Saturation quantity .. 12 grams/cubic meter (check this)
                      Actual quantity ......  6 grams/cubic meter
                      Relative humidity = 600/12 = 50%


                      Is there any reason for precipitation? 
                           No: R < 100%
                           Therefore, let's move the 6 g forward 3 hours
                   c. Time .................  9 pm
                      Air temperature ......  9o C 
                      Saturation quantity ..  8 grams/cubic meter (check this)
                      Actual quantity ......  6 grams/cubic meter
                      Relative humidity = 600/8 = 75%


                      Is there any reason for precipitation? 
                           No: R < 100%
                           Therefore, let's move the 6 g forward 3 hours
                   d. Time .................  midnight
                      Air temperature ......  4o C 
                      Saturation quantity ..  6 grams/cubic meter (check this)
                      Actual quantity ......  6 grams/cubic meter
                      Relative humidity = 600/6 = 100%


                      Saturation has been attained!!!
                      4o C is the dew point for this air!
                      Is there any reason for precipitation? 
                           Oh, oh: R = 100%


                           If the air chills any more, it will precipitate.  
                   e. Time .................  3 am
                      Air temperature ......  -5o C 
                      Saturation quantity ..  3 grams/cubic meter (check this)
                      Actual quantity ...... ?????????? (some of those 6 grams 
                                             per cubic meter have been lost as 
                                             rain and then snow and are now 
                                             sitting on the ground.  So, how 
                                             can we know what's left? Don't 
                                             panic -- we can safely assume 
                                             that there can be no more vapor 
                                             left than the saturation quantity 
                                             for air chilling to this 
                                             temperature:  3 grams per cubic 
                                             meter)
                      Relative humidity = 300/3 = 100% (still 100%)
                      Is there any reason for precipitation?  Yes, as long as  
                           R = 100% and temperatures continue dropping, as    
                           they likely would in the wee hours here
                   f. Time .................  6 am 
                      Air temperature ......  -10o C 
                      Saturation quantity ..  2 grams/cubic meter (check this)
                      Actual quantity ......  2 grams/cubic meter
                      Relative humidity = 200/2 = 100%


                      Is there any reason for precipitation?  Yes, if it 
                           continues chilling, but temperatures would begin 
                           rising at dawn, which would put an end to chilling 
                           and, therefore, further precipitation
                   g. Time .................  9 am 
                      Air temperature ......  4o C 
                      Saturation quantity ..  6 grams/cubic meter (check this)
                      Actual quantity ......  2 grams/cubic meter
                      Relative humidity = 600/2 = 33%


                      Is there any reason for precipitation?  Not any more:  
                           The warming of the air raises the saturation 
                           quantity, which lowers the relative humidity, and 
                           removes the reason for excess freezing and 
                           condensation (or precipitation).  Wow!  Is this air 
                           dry -- bet it'll get pretty warm there later!
               vi. Measurement of relative humidity
                   a. A hygrometer is a gizmo that shows relative humidity on 
                      a calibrated dial. One simple type just uses a strand of 
                      human hair.  You guys have had enough "bad hair days" to 
                      know that humidity affects your coiffure.  Hair 
                      lengthens in humidity and contracts in dry conditions. 
                      By lengthening (in high relative humidity) and 
                      shortening (in low relative humidity), the hair on the 
                      hygrometer activates a pointer on the dial. It can even 
                      activate a pen on a rotating drum to provide a 
                      continuous record of relative humidity (a "hygrograph").  
                      Want to make one?  Find out how here!
                   b. A sling psychrometer is more accurate than a hygrometer.  
                      It consists of two thermometers mounted on a long, 
                      skinny frame.  The frame is attached to a handle, so you 
                      can sling it around in circles.
                      1. One thermometer, called the "dry bulb thermometer," 
                         is a plain, ordinary thermometer. 
                      2. The other one is mounted so its end sticks out beyond 
                         the end of the frame, and it has a little muslin sock 
                         wrapped on the end: This is the "wet bulb 
                         thermometer." You dip the muslin in distilled water 
                         and sling the heck out of the psychrometer. The lower 
                         the relative humidity, the faster the rate of 
                         evaporation out of the muslin and the cooler the wet 
                         thermometer will get compared to the regular dry bulb 
                         one.

                         

                      3. You record the two resulting temperatures and then 
                         subtract the wet bulb temperature from the dry bulb 
                         temperature to get the "wet bulb depression."  Next, 
                         you consult a chart, which converts the difference 
                         into relative humidity.  To get into the chart for 
                         your relative humidity, you look up the air 
                         temperature (dry bulb temperature, Tdb) on 
                         the Y axis (left side) and then the wet bulb 
                         depression across the X axis on the top.  Reading 
                         across from the dry bulb temperature and down from 
                         the wet bulb depression, you find your relative 
                         humidity.  So, if the air temperature were, oh, 
                         32° C, and the wet bulb temperature were, hmmmmm, 
                         25° C, what would your wet bulb depression be?  
                         Now, what would your relative humidity be?  Yep, 56%.  
                         You've got the hang of it.

     C. The manifestations of humidity, or how we experience humidity.
        1. Dew occurs when very moist air contacts a surface colder than its 
           dew point. Such a cold surface will be provided by a good 
           absorber/reradiator, such as a car's windshield or the leaves of 
           grass. By radiation, then, they cool a thin layer of air above them 
           below its dew point, and so the water condenses onto the cold 
           surface.
        2. Frost is the same thing as dew, except that the cold surface is 
           basically below freezing.
        3. Fog develops when a relatively thick layer of air is cooled by 
           conduction and radiation below its dew point. The water 
           vapor condenses (or in some case freezes) onto the dust nuclei 
           throughout the chilled zone. The droplets are tiny enough to be 
           kept in suspension by normal minor air turbulence and by the 
           buoyancy of moist air (remember? water molecules are light and 
           displace nitrogen and oxygen molecules, so air with a high water 
           content is lighter and more buoyant than dry air). This fog can be 
           a few centimeters thick (Dracula B-movie) to a few hundred meters 
           thick. 
           a. There are two types of fog:
                i. Radiation fog or ground fog forms exactly as just described 
                   (largely courtesy of radiation and on and near the ground).  
                   Hmmm -- "ground fog day"???  Sorry!
               ii. Advection fog or transported fog is a radiation fog that 
                   has moved -- wafted away from its place of origin and onto 
                   some other place. This most commonly occurs on coasts. Fog 
                   forms offshore and rolls inland.
           b. Something weird to think about:  Airports.  LA-X and San 
              Francisco International are located where they can be shut down 
              both by ground fog and by advection fog.  Why?  The increased 
              risk of flying into fog is compensated by the coastal location:  
              Planes in trouble on takeoff or landing (which is where most 
              mishaps happen) can come down in the drink, rather than in the 
              middle of the city for the safety of urban residents and a bit 
              of a better chance for those on board. 
        4. Clouds:
           a. Clouds are similar to fogs in a couple of ways:
                i. Both depend on dust nuclei, and both are made up of tiny 
                   droplets of water or ice crystals held in suspension by air 
                   turbulence and the buoyancy of moist air.
               ii. Both are the result of moist air being cooled below its dew 
                   point.
           b. Clouds differ from fogs in a few key ways, too, however:
                i. Usually in elevation:  Fogs are low in the landscape and 
                   hug the surface and seek out depressions in the terrain; 
                   clouds are at higher elevation (though they'll touch the 
                   ground if the ground is at high elevation on a 
                   mountainside).
               ii. They also differ very importantly in the manner of their 
                   formation: Fogs are the result of heat loss due to 
                   radiation and some conduction; clouds are always the result 
                   of adiabatic cooling processes.  Air must be rising above 
                   the lifting condensation level or dew point elevation for 
                   it to create clouds.
           c. Considerations in cloud classification.
                i. Clouds may be classified along three main dimensions: 
                   general shape (whether layered or globular), altitude 
                   (high, medium, or low), and activity (precipitating or 
                   not).
               ii. For the shape dimension, we can differentiate two basic 
                   patterns:
                   a. Stratiform for layered, blanket-like clouds covering 
                      large areas. They are produced by air layers being 
                      forced to rise gradually over layers of greater density. 
                      This produces a slow adiabatic cooling and, hence, 
                      condensation over a wide area. Over the long time it 
                      takes them to pass by a given landscape, these clouds 
                      can yield a lot of rain or snow.
                   b. Cumuliform for globular, puffy clouds. They represent 
                      bodies of warm air spontaneously rising through cooler 
                      layers, a relatively rapid updraft. Their precipitation 
                      is concentrated over a smaller area and can be quite 
                      intense in the short time it lasts.
              iii. The height dimension produces "cloud families" based on 
                   altitude:
                   a. The highest cloud family is denoted by "cirrus" or 
                      "cirro" in cloud names. They occur between 6 km and 15 
                      km up.
                   b. The middle altitude family has "alto" in its name (kind 
                      of like an alto voice is below a soprano but above a 
                      baritone?). These clouds occur between 2 km and 6 km up.
                   c. The low family has "stratus" or "strato" in its name, 
                      which is a little confusing, since "stratus" is also 
                      used to designate layer-shaped clouds.  They form below 
                      2 km up.
                   d. Then there are clouds of vertical development, which can 
                      build up through all three of the preceding altitudinal 
                      ranges:  "Cumulus" or "cumulo."  Again, there is a 
                      chance for confusion, since "cumulus" also describes 
                      shape.  Then, again, the only clouds that can build up 
                      like this are puffy in shape.  Some of these clouds may 
                      get taller than they are wide and can range from bases 
                      as low as 200 m to tops of 15,000 m (15 km).
               iv. The activity dimension yields two classes:  If the cloud is 
                   precipitating, it has "nimbo" or "nimbus" in its name.
           d. List of high cloud family types:
                i. Cirrus -- delicate, wispy, stringy in appearance. These are 
                   thin ice clouds, icy because of the altitude (> 6 km) and 
                   thin because such cold air does not have a high equilibrium 
                   level of moisture.  These are often called "mares' tails," 
                   because they sort of look like horses' tails.  They are 
                   often seen on the leading edge of a storm, for reasons to 
                   be taken up in the next lecture.

                   

               ii. Cirro-stratus -- a complete layer of thin ice cloud 
                   covering the entire sky or a large portion of it. You can 
                   see the sun or moon through it, and it often creates that 
                   halo or veil around the moon or sun. This often is see 
                   ahead of a storm, and is the basis of folk sayings about 
                   the weather to the effect, "ring around the moon, rain two 
                   days' noon."

                   

              iii. Cirro-cumulus -- a high layer of closely packed globular 
                   little clouds, grouped or aligned in rows and columns, like 
                   the cells in a spreadsheet.  This is sometimes called the 
                   "mackerel sky" (looks sort of like fish scales) or 
                   "buttermilk sky." You often see these after a storm has 
                   passed and sometimes as a storm approaches.

                   

           e. List of middle cloud family types:
                i. Alto-stratus -- a blanket layer, smoothly distributed over 
                   the sky or large portions of it. Light grey in color. You 
                   can't clearly see the sun or moon through it, but they 
                   often show up as a bright spot in the cloud. The appearance 
                   of this cloud commonly means the approach of bad weather. 

                   

               ii. Alto-cumulus -- comprises a closely fitted layer of 
                   individual cloud masses, generally in that spreadsheet-like 
                   row and column format. Generally very white with a little 
                   greyish on the underside and bright blue sky showing 
                   through.  These clouds often mean generally fair or 
                   clearing conditions.

                   

           f. List of low cloud family types:
                i. Stratus -- a dense, low lying, dark gray layer. The 
                   appearance of this rather ominous-looking cloud means you 
                   have a very high probability of experiencing precipitation 
                   soon.  

                   

               ii. Nimbo-stratus -- a stratus cloud from which rain, snow, or 
                   sleet is falling.  You are IN the storm now!

                   


              iii. Strato-cumulus -- is a low lying cloud layer of distinct 
                   grayish masses of cloud with open sky between. Individual 
                   masses of cloud often like long rolls of cloud at right 
                   angles to the wind. Fair or clearing weather, though with 
                   snow or rain flurries at times.

                   

           g. Clouds of vertical development:
                i. Cumulus is a white woolpack cloud mass.  It has a couple of 
                   subtypes:
                   a. Small cumulus clouds are small, randomly scattered 
                      little puffball clouds.  There may be only one of them 
                      around or a couple dozen, but they don't form rows and 
                      columns.  These are sometimes called "fair weather 
                      clouds," because they aren't associated with storm 
                      fronts.

                   

                   b. Enlarged cumulus (sometimes called congested cumulus).  
                      These show flat, grey or dark grey bases and bumpy, 
                      often blindingly white upper surfaces, especially on the 
                      side facing the sun.  These may once have been small 
                      cumulus clouds that began to mound up and may be on the 
                      way to creating a thunderstorm.  They may also form in 
                      lines, which indicates that they are associated with a 
                      front or with a wind smacking into a mountain range.

                   

               ii. Cumulo-nimbus (thunderhead) -- individual cumulus masses 
                   grow into huge, towering clouds, producing heavy rain and 
                   often hail, thunder and lightning, and gusty winds. Violent 
                   updrafts of hot air produce some cumulo-nimbus as tall as 
                   12 or 15 km from a base of 200 m up. They are often topped 
                   with a plume or flat top called an "anvil head," which 
                   marks that the cloud has towered up to the top of the 
                   troposphere and is spreading out along the bottom of the 
                   tropopause. 

                   a. Hail can even be shot out the top of these clouds and 
                      drop far from the cloud if the hail is large enough to 
                      survive the trip. 
                   b. There are violent updrafts and downdrafts in a 
                      thunderhead, as well as winds gusting in from the sides, 
                      so they are extremely dangerous for aviation.  
                   c. Adding to the hazard is lightning and the associated 
                      thunder (hence the popular name for these clouds, 
                      "thunderheads").  
                      1. Lightning is basically a huge electrical spark.  
                         Normally, the ground has a bit of a negative 
                         electrical charge to balance the positive charge of 
                         the ionosphere.  The normal electrostatic gradient 
                         (about 100-200 V/m) is not enough to produce a spark, 
                         especially since the air is highly resistive.  
                      2. Cumulo-nimbus clouds, however, develop a strong 
                         positive charge in their upper sections (as ice 
                         crystals in the upper portions of the cloud fall and 
                         crash into microcrystals and strip electrons from 
                         them) and strong negative charge in the middle 
                         portions (as the falling crystals surrender their 
                         ill-got electrons to still other ice crystals in the 
                         middle parts of the cloud).  
                      3. The ground, normally negatively charged, becomes 
                         relatively positively charged when compared to that 
                         strong negative charge in the middle of the cloud 
                         overhead.  The electrostatic gradient can reach 500 
                         million volts between the ground and the middle of 
                         the cloud (about 6 km up) or over 80,000 volts per 
                         meter!  That's enough to create QUITE a spark! 
                      4. What happens is a pile of electrons shoot toward the 
                         ground (or sometimes to positively charged areas in 
                         the cloud) in a series of short (~50 m) strokes, 
                         which creates a hole or channel in the air, 
                         surrounded by an envelope of ionized gasses (all the 
                         molecules that suddenly lost electrons).  This is the 
                         "stepped leader." 
                      5. As the intensely negatively charged leader approaches 
                         the ground, the positive charge in the ground 
                         concentrates under it and sends up a "streamer" to 
                         grope towards it.  
                      6. When the two meet, there is a powerful discharge from 
                         the ground upward, travelling at a third of the speed 
                         of light!  
                      7. It heats the channel to some 30,000° C, which 
                         creates extremely high pressure (Amonton's Law that 
                         gas pressure is proportional to temperature), which 
                         results in a shock wave that we hear as thunder.  
                         Thunder is also the result of the collapse of the 
                         channel once the charge has passed through.  The 
                         light in lightning is a product of the neutralization 
                         of the ionized gas around the channel when the charge 
                         passes through and it is emitted, not just in visible 
                         light, but in X-rays, the ultraviolet, and the radio 
                         wavelengths as well (which is why your radio crackles 
                         during lightning). 
        5. Precipitation occurs in several types, depending on whether the dew 
           point is below freezing or not, whether there are violent updrafts 
           in a cloud, and the temperatures of the air through which the 
           precipitation falls.
           a. Rain is falling water droplets, produced when the dew point is 
              greater than freezing point.  The small droplets do not 
              evaporate as fast as they form once the air is saturated, and so 
              these persisting water droplets bump into one another and get 
              larger and larger until their weight is great enough to overcome 
              turbulence and buoyancy and fall to the ground. This is far and 
              away the most common type of precipitation on Earth.
           b. Snow is made up of collections of ice crystals.  When the air is 
              saturated at a temperature below freezing, tiny crystals of ice 
              form faster than they sublimate into vapor. Persisting longer, 
              they connect with one another in a cloud, forming ever larger 
              and more complicated collections, or snow flakes. Finally, they 
              may accumulate enough weight that they can fall.
           c. Sleet consists of small pellets of ice. This precipitation 
              originally falls as raindrops, which then freeze on the way down 
              by falling through air colder than freezing.  This is 
              associated, then, with inversion layers and is, therefore, more 
              of a nighttime phenomenon.  In a nasty variant on this process, 
              sometimes the precipitation makes it all the way to the ground 
              before freezing, thereby forming dangerous glazes of ice on the 
              ground, on plants, and on power lines (which can break them).  
              Sometimes this is differentiated from regular sleet as "frozen 
              rain" or "freezing rain."  Nasty stuff!
           d. Hail is made of pellets or stones of ice, some of them quite 
              large (like grapefruit-sized!) formed in cumulo-nimbus clouds. A 
              droplet of water grows to falling point but is then caught by a 
              violent updraft in the cloud. Thrust so high as to freeze, it 
              falls again, collecting water drops on the way down. Caught 
              again by an updraft and thrown high enough, it freezes the water 
              it collected onto it like a skin. This goes on until some 
              updraft forces it out the top of the thunderhead or until it 
              falls down under the cloud without getting picked up by another 
              updraft. Hailstones, then, are layered like onions, each layer 
              representing a separate trip up past the freezing point in the 
              cumulo-nimbus cloud.
     D. Causes of precipitation, or, more precisely, causes of the uplift that 
        creates the adiabatic cooling that can result in precipitation.
        1. Orographic uplift: adiabatic cooling is produced by air movement up 
           a mountain slope.
        2. Convectional uplift is the updraft of heated air, resulting in 
           cumulus clouds or cumulo-nimbus clouds, leading to the intense 
           adiabatic cooling and and concentrated precipitation of a 
           thundershower.  
           a. What happens is the heating of an air mass can destablize it, if 
              it becomes warmer than the other air at the same altitude.  
              Being warmer, it expands, which reduces its density, which 
              increases its buoyancy.  So, it rises.  By rising, it will cool 
              adiabatically at the dry rate of ~10° C/km.  The stable air 
              surrounding it, though, cools with altitude at the environmental 
              lapse rate (which varies a lot, but averages out to the normal 
              lapse rate of 6.5° C/km), and this is lower than the dry 
              adiabatic lapse rate.  So the rising air becomes cooler at a 
              faster rate than the stable air around it.  Eventually, it will 
              have lost so much heat adiabatically that it catches up (or is 
              that down?) with the temperature of the stable air at some 
              altitude.  At that altitude, then, the air has reached the same 
              temperature ... and the same density ... as the stable air.  It 
              thereby loses its relative buoyancy and ceases to rise further, 
              just becoming another area of stable air.  
           b. If it rises enough to cool to dew point, however, things change.  
              At that altitude, the saturation condensation level, the release 
              of latent heat during condensation or freezing slows the cooling 
              down to the wet adiabatic lapse rate.  This rate is also 
              variable but it is commonly (though not always) smaller than the 
              local environmental lapse rate.  This means the rising air is 
              less likely to cool down to the ambient level of the surrounding 
              air, so it is likelier to remain unstable and buoyant and 
              continue rising (precipitating as it goes).  This continues 
              indefinitely unless the environmental lapse rate somehow falls 
              below the wet lapse rate or an inversion is encountered.
           c. Your textbook has some excellent graphs of the factors governing 
              stablility on p. 80-83.
        3. Convergent uplift occurs when two opposed winds collide or converge 
           with one another, producing vertical uplift mechanically similar to 
           convectional uplift.  
        4. Cyclonic or frontal uplift occurs when warm air is pushed aloft 
           above a colder air mass. Air of different density does not tend to 
           mix, so the lighter, more buoyant air is forced aloft, cooling 
           adiabatically in this way, which leads to precipitation.  
           a. If the moving air mass is the warm air mass, the resulting 
              uplift is mechanically similar to orographic uplift (in both 
              cases an air mass is forced to climb a dense object in its way). 
           b. If the moving air mass is the cold one, the movement is quite 
              rapid and the warm air in front of it is forced aloft rather 
              suddenly and vertically, producing uplift that is similar to 
              convectional and convergent uplift.


Come away from this lecture with a sense of what specific and absolute 
humidity measure, how saturation vapor pressure relates to the balance between 
evaporation/sublimation and condensation/freezing, how specific and absolute 
humidity relate to relative humidity, how relative humidity is measured with a 
sling psychrometer, how to read a dry bulb temperature/wet bulb depression 
chart, how to read the saturation quantity from air temperature, how to read 
the dew point temperature from knowing absolute humidity, how dew differs from 
frost, how clouds differ from fogs, how to classify clouds, how rain and snow 
form, what the difference is between sleet and hail, and the four mechanisms 
of uplift (convectional, orographic, convergent, and frontal).

The next lecture will examine storms and frontal uplift in more detail as an 
element of weather in the troposphere.

Document and © maintained by Dr. Rodrigue
First placed on web: 10/21/00
Last revised: 06/16/07