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PRECIPITATION
Precipitation is the general term for all forms of moisture emanating from the
clouds and falling to the ground.
Types of Precipitation
The formation of precipitation requires the lifting of an air mass in the
atmosphere so that it cools and some of its moisture condenses. Precipitation
is often typed according to the factor responsible for the lifting. The three
main mechanisms of air mass lifting are
frontal lifting, where warm air is lifted over cooler air by frontal passage or
cyclone, thus, Cyclonic precipitation occurs;
orographic lifting, in which an air mass rises to pass over a mountain range,
then there is Orographic precipitation ; and
convective lifting, where air is drawn upwards by convective action, such as
in the center of a thunderstorm cell, resulting into a Convective precipitation.
Convective cells are initiated by surface heating, which causes a vertical
instability of moist air, and are sustained by the latent heat of vaporization
given up as water vapor rises and condenses.
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Hydrology
Formation of Precipitation
The formation of precipitation in clouds is illustrated in Fig. 3.3.1. As air rises
and cools, water condenses from the vapor to the liquid state. If the
temperature is below the freezing point, then ice crystals are formed instead.
Condensation requires a seed called a condensation nucleus around which
the water molecules can attach or nucleate themselves. Particles of dust
floating in air can act as condensation nuclei; particles containing ions are
effective nuclei because the ions electrostatically attract the polar-bonded
water molecules.
The tiny droplets grow by condensation and impact with their neighbors as
they are carried by turbulent air motion, until they become large enough so
that the force of gravity overcomes that of friction and they begin to fall,
further increasing in size as they hit other droplets in the fall path. However,
as the drop falls, water evaporates from its surface and the drop size
diminishes, so the drop may be reduced to the size of an aerosol again and
be carried upwards in the cloud through turbulent action. The cycle of
condensation, falling, evaporation, and rising occurs on average about ten
times before the drop reaches a critical size of about 0.1 mm, which is large
enough to fall through the bottom of the cloud.
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CE 7d
Cloud seeding is a process of artificially nucleating clouds to induce
precipitation. Silver iodide is a common nucleating agent and is spread from
aircraft in which a silver iodide solution is evaporated with a propane flame
to produce particles. While there have been many experiments wherein cloud
seeding was considered to have induced precipitation, the great variability of
meteorological processes involved in producing precipitation make it difficult
to achieve consistent results.
Forms of Precipitation
Drizzle consists of waterdrops under 0.02-in. diameter, and its intensity is
usually less than 0.04 in./hr.
Rain consists of drop usually greater than 0.02-in. diameter. Drops greater
than 0.25-in. diameter tend to break up as they fall through the air, so that
0.25 in. may be accepted as a practical upper limit of raindrop size.
Glaze is the ice coating formed when drizzle or rain freezes as it comes in
contact with cold objects at the ground.
Sleet is frozen raindrops cool d to the ice stage while falling through air at
subfreezing temperatures.
Snow is precipitation in the form of ice crystals resulting from sublimation,
i.e., from water vapor directly to ice. A snowflake is made up of a number of
ice crystals fused together.
Hail is precipitation in the form of balls or lumps of ice over 0.2-in. diameter
formed by alternate freezing and melting as they are carried up and down in
highly turbulent air currents. Single hailstones weighing over a pound have
been observed.
Measurement of Precipitation
All form of precipitation are measured on the basis of the vertical depth of
water that would accumulate on a level surface if the precipitation remained
where it fell. An amount less than 0.005 in. is recorded as a trace.
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Precipitation gages. Any open receptacle with vertical sides is a convenient
rain gage, but because of varying wind and plash effects the measurements
would not be comparable unless the receptacles were of the same size and
shape and similarly exposed.
The standard 8-in precipitation gage. Rain passes from the collector into a
cylindrical measuring tube inside the overflow can. The measuring tube has a
cross-sectional area one-tenth that of the collector so that 0.I-in. rainfall will
fill the tube to a depth of 1 in. With a measuring stick marked in tenths of an
inch, rainfall can be measured to the nearest 0.01 in.
The weighing-type gage weighs the rain or snow which fall into a bucket set
on a platform of a spring or lever balance. The increasing weight of the bucket
and its contents is recorded on the chart held by a clock-driven drum. The
record thus shows the accumulation of precipitation.
The tipping-bucket gage. The 12-in. collector funnels the rain into a twocompartment tipping bucket. One-hundredth inch of rain will fill one
compartment and overbalance it so that it tips, emptying into a reservoir and
moving the second compartment of the bucket into place beneath the funnel.
As the bucket is tipped by each 0.01 in. of rain, it actuates an electrical circuit
causing a pen to mark on a revolving drum. Unfortunately, this type of gage
is not suitable for measuring snow without heating the collector.
The standpipe-type gage is made from 12-in. thin-walled pipe in 5-ft
sections, so that any height in multiples of 5 ft is possible. Storage gages are
customarily charged with a calcium chloride solution (antifreeze) to liquefy
the snow and prevent damage to the gage.
When rain is falling vertically, a gage inclined 10° from the vertical will catch
1.5 per cent less than it should. If a gage on level ground is inclined slightly
toward the wind, it will catch more than the true amount. Some
investigators2 feel that gages should be perpendicular to land slopes.
However, the area of a basin is its projection on a horizontal plane, and
measurements from tilted gages must be reduced by multiplying by the
cosine of the angle of inclination. Considering the variability of land slope,
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CE 7d
aspect, and wind direction, it is virtually impossible to install a network of
tilted gages for general purpose.
Hydrology
When rain is falling vertically, a gage inclined 10° from the vertical will catch
1.5 per cent less than it should. If a gage on level ground is inclined slightly
toward the wind, it will catch more than the true amount. Some
investigators2 feel that gages should be perpendicular to land slopes.
However, the area of a basin is its projection on a horizontal plane, and
measurements from tilted gages must be reduced by multiplying by the
cosine of the angle of inclination. Considering the variability of land slope,
aspect, and wind direction, it is virtually impossible to install a network of
tilted gages for general purpose.
Characteristic of Precipitation
Duration -Measured in units of minutes or hours.
Depth -Measured in units in mm or inches.
Rate Intensity -Measured in units of mm/hr or inch/hr.
Storm Frequency–Describe the probability of occurrence.
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Hydrology
INTERPRETATION OF PRECIPITATION DATA
In order to avoid erroneous conclusions it is important to give the proper
interpretation to precipitation data, which often cannot be accepted at face
value. For example, a mean annual precipitation value for a station may
have little significance if the gage site has been changed significantly during
the period for which the average is computed. Also, there are several ways
of computing average precipitation over an area, each of which may give a
different answer.
Estimating Missing Precipitation Data
If the normal annual precipitation at each of the index stations is within 10
per cent of that for the station with the missing record, a simple arithmetic
average of the precipitation at the index stations provides the estimated
amount.
If the normal annual precipitation at any of the index station differ from that
at the station in question by more than 10 per cent, the normal-ratio
method is used. In this method, the amounts at the index stations are
weighted by the ratios of the normal-annual-precipitation values. That is,
precipitation at Station X, Px, is
Where N is Normal Precipitation.
Double-Mass Analysis.
Double-mass analysis tests the consistency of the record at a station by
comparing its accumulated annual or seasonal precipitation with the
concurrent accumulated values of mean precipitation for a group of
surrounding stations.
A procedure has been devised to evaluate the significance of any slope
change in terms of the probability of getting such a change purely by
chance. However, the procedure itself is subject to some error, its results
are not altogether conclusive, and its application is somewhat cumbersome.
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Average Precipitation Over Area.
The average depth of precipitation over a specific area, either on a storm,
seasonal, or annual basis, is required in many types of hydrologic problems.
The simplest method of obtaining the average depth is to average
arithmetically the gaged amounts in the area. This method yields good
estimates in flat country if the gages are uniformly distributed and the
individual gage catches do not vary widely from the mean. These limitations
can be partially overcome if topographic influence and areal
representatively arc considered in the selection of gage sites.
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Hydrology
average of the two isohyetal values) by the area between isohyets, totaling
there products, and dividing by the total area.
Thiessen Method.
The Thiessen method attempts to allow for non-uniform distribution of
gages by providing a weighting factor for each gage. The stations are plotted
on a map, and connecting lines are drawn (Fig. 3-10b). Perpendicular
bisectors of these connecting lines form polygons around each station. The
sides of each polygon are the boundaries of the effective area assumed for
the station. The area of each polygon is determined by planimetry and is
expressed as a percentage of the total area. Weighted average rainfall for
the total area is computed by multiplying the precipitation at each station
by its assigned percentage of area and totaling. The results are usually more
accurate than those obtained by simple arithmetical averaging.
Isohyetal Method.
The most accurate method of averaging precipitation over an area is the
isohyetal method. Station locations and amounts are plotted on a suitable
map, and contours of equal precipitation (isohyets) are then drawn (Fig. 310c). The average precipitation for an area is computed by weighting the
average precipitation between successive isohyets (usually taken as the
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CE 7d
Variability of Precipitation
Precipitation varies in space and time according to the general pattern of
atmospheric circulation and according to local factors. The average over a
number of years of observations of a weather variable is called its normal
value. Figure 3.3.5 shows the normal monthly precipitation for a number of
locations in the United States. Higher precipitation occurs near the coasts
than inland because the oceans supply the bulk of the atmospheric moisture
for precipitation. Precipitation is very variable in the mountain states in the
west where orographic effects influence precipitation. Precipitation
increases going east across the great plains and is spatially more uniform in
the east than in the west.
Hydrology
Rainstorms vary greatly in space and time. They can be represented by
isohyetal maps; an isohyet is a contour of constant rainfall. Figure 3.4.1 shows
an isohyetal map of total rainfall depth measured for two storms: one a storm
of May 30-June 1, 1889, which caused about 2000 deaths in Johnstown,
Pennsylvania, following a dam failure, and the other a storm of May 24-25,
1981, in Austin, Texas, which caused 13 deaths and $35 million in property
damage (Moore, et al., 1982). The Johnstown storm is plotted on a scale 50
times larger than the Austin storm. The maximum depth of precipitation in
both storms is nearly the same (~ 10 in), but the Austin storm was briefer and
more localized than the Johnstown storm. The Austin storm was caused by a
convective cell thunderstorm of the type analyzed in Example 3.3.2.
Isohyetal maps are prepared by interpolating rainfall data recorded at gaged
points. A rain gage record consists of a set of rainfall depths recorded for
successive increments in time, as shown in Table 3.4.1 for the data in 5minute increments from gage 1-Bee in the Austin storm. A rainfall hyetograph
is a plot of rainfall depth or intensity as a function of time, shown in the form
of a histogram in Fig. 3.4.2(a) for the 1-Bee data. By summing the rainfall
increments through time, a cumulative rainfall hyetograph, or rainfall mass
curve, is produced, as shown in Table 3.4.1 and Fig. 3.4.2(Z?). The maximum
rainfall depth, or intensity, (depth/time) recorded in a given time interval in
a storm is found by computing a series of running totals of rainfall depth for
that time interval starting at various points in the storm, then selecting the
maximum value of this series. For example, for a 30-minute time interval,
Table 3.4.1 shows running totals beginning with 1.17 inches recorded in the
first 30 minutes, 1.65 inches from 5 min to 35 min, 1.81 inches from 10 min
to 40 min, and so on. The maximum 30 minute recorded depth is 3.07 inches
recorded between 55 min and 85 min, corresponding to an average intensity
of 3.07 in/0.5 h = 6.14 in/h over this interval.
RAINFALL
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Hydrology
It can be seen that as the time period increases, the average intensity
sustained by the storm decreases (5.56 in/h for one hour, 4.10 in/h for two
hours), just as the average intensity over an area decreases as the area
increases, as shown in Fig. 3.4.1.
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PROBLEMS
Hydrology
All coordinates are expressed in miles. Compute the average rainfall in the
area by the Thiessen method.
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