CLOUDS and PRECIPITATION

Introduction

    In this class we consider the atmospheric part of the Hydrologic Cycle - something we shall return to at the end of the course (Classes 25-28)

    In this class we have a sequence of steps tracing the water through the atmosphere.  First we evaporate it from the earth's surface, then consider it as moisture in the air, particularly in the surface layer where we feel that moisture.  Next we move it higher into the atmosphere and create clouds, discussing both cloud type and cloud formation.  The we let the clouds (or some of the at least) create rain, and bring the water back to the earth's surface.

Evaporation

    Evaporation is the change in state of water from liquid to vapor and its removal from the surface into the air.  In practice there are two separate processes:

evaporation - which occurs from an 'open' water surface (which can be any water surface ranging in size from the Pacific Ocean, through a puddle on a road, to the space between individual soil particles); and

transpiration - removal of water from plant leaves through their stomata, with the plant being an active participant in this removal.

However, when emphasizing the atmosphere rather than the plant, we lump them together as evapotranspiration and treat them as one process.

(It is also possible to consider sublimation - the direct phase change of water from the frozen to the vapor state  - but we also treat that simply as part of evaporation here)

The rate of evapotranspiration (and thus the total amount evapotranspired in a given time) is a function of:

1 - the energy available (evapotranspiration is another way of looking at the latent heat flow)

        evapotranspiration is usually at a maximum in sunny conditions - say midday in July

2 - the humidity gradient - the vertical rate of change of moisture in the air:

        the surface is usually nearly saturated, so dry air above (say at the level of your head) is needed

3 - wind speed - high winds serve to mix the air through turbulence (the gusts & lulls in the wind)

        this maintains a steep humidity gradient

4 - water available - this seems obvious, but is important.  A concrete surface is either wet (during/after         rain) or dry, but a vegetated surface has to get water to the leaves from the roots, and at times when the above 3 components are encouraging evapotranspiration, tnhe plant may not be able to get it fast enough.  Wilting will result.

Humidity

   We commonly express vapor amounts as the specific humidity

        the mass of water vapor in a unit mass of air (usually expressed in grams per kilogram)

  we can think of this as the basic measure (although it turns out not to be too useful for practical application)

The two humidity measures of concern to us:

Relative humidity -

    (actual specific humidity of the air) / (saturation specific humidity at the same temperature) x 100 (%)

    where the saturation specific humidity at the same temperature is the specific humidity that the air would have if it were saturated at the current temperature.

    This therefore is an indication of how close to saturation the air actually is.  It gives a measure of how humid the air 'feels' to humans.  For our purposes, however, it has a great drawback - it depends both on temperature and moisture:

   relative humidity tends to be high at the ends of the day, low in the middle, even if the amount of moisture in the air does not change - simply because of the temperature change.

Dew Point Temperature (usually just called the Dew Point)

   the temperature to which the air must be cooled, without change in moisture content, for saturation to occur.

        i.e. cooling so that dew forms.

Cloud Types

    See the text

There are a great number of different cloud types, however, they can be summarized as:   Cirrus                                                                                     Alto (stratus/cumulus)          (small)               (tall) Stratus                                   Cumulus        Cumulonimbus   Horizontal Development               Vertical Development

Cloud Formation

    Clouds form when air is cooled to or below its dew point.  Almost all ways of doing this (except for fog formation) involve vertical motions.  There are four basic processes:

Orographic	Air flowing over a hill is forced to rise and cool		Air coming down the other side warms, and the air is usually cloud-free.  The area is a rain-shadow.  Many mountain ranges have winds coming consistently from one direction, so that one side is wet, the other dry.  This occurs for the Rocky Mountains.     In North Carolina the same process acts in the Appalachians, but air can, and does, approach from the east or the west.  Three is likely to be a rain shadow on any individual day, but overall there is not a wet side and a dry side to our mountains.  The driest areas are commonly the mountain basins - of which Asheville is the best known (having the biggest city - and also a rain gauge to measure things!)
Convergence		When two air masses converge, they are forced to rise.	Note that rarely do air masses crash together head-on, usually they approach at an angle.  A depression, considered later, is a major area of convergence, general uplift and thus of cloud and often rain.
Frontal	When two air masses with different temperatures meet, the warmer air flows over the colder air - forming a front	The clouds are created in the rising warm air, the rain frequently falls from this through the cold air, arriving at the ground on the cold side of te front.
Convective

Precipitation Formation

Various processes with clouds are needed in order that the small cloud droplets - which are small enough to float in the air - can grow into raindrops.  The process is complex, and it is sufficient to say here that the process does not succeed in all clouds - not all clouds give rain.  In some the formation processes simply cease to operate and the cloud droplets evaporate back into the atmosphere.  The cloud then dissipates.

Precipitation Amounts

The resultant precipitation at the earth's surface has a distinct pattern.  This is closely related to the wind and weather systems considered next time. However, we can say here that there are distinct latitudinal bands, and the orographic effects of the major mountain ranges can be seen even on the global scale.

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