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<br />Chapter 2
<br />
<br />PRECIPITATION RATES
<br />
<br />2.1 Introduction
<br />2.1.1 Precipitation rates are a function of
<br />(1) the availability of moisture, and (2) the rate
<br />at which the moisture can be converted into pre-
<br />cipitation. Both these factors, hence precipitation
<br />rates, exhibit marked seasonal and geographic
<br />variations and are not completely jude'pendent of
<br />one another.
<br />2.1.2 Gage measurements of precipitation
<br />rates ma'y occasionally be very inaccurate. Al-
<br />t.hough measure.ments, in g-enera], tend to be too
<br />low, some of the higher rates measured are difficult.
<br />to explain on the basis of current theories of pre-
<br />cipitation formation. Areal measurements, which
<br />involve interpolation and extrapolation of gage
<br />measurements, are also subject to appreeiable
<br />error.
<br />2,1.3 Since the results of this report are to be
<br />used as design criteria for hydraulic structures
<br />controlling streamflow from watersheds not ex-
<br />ceeding 400 sq. mi., the primary concern is with
<br />high rainfall rates for durations of no more than
<br />24 hr. Rainfall for lower intensities and longer
<br />durations is given little consideration.
<br />2.2 Availability of moisture
<br />2.2,1 The rate at which moisture is made
<br />available to the precipitation-producing, or storm,
<br />uleehanism is a veTY important fa.ctor in determin-
<br />ing the precipitation rate. If it were not for the
<br />continuing moisture supply into a storm, the total
<br />amount of precipitation produced could not exceed
<br />the maximum amount of lV, plus liquid water in
<br />the air above the prec,ipitation area, or a total of
<br />about 6 to 7 in. in southern United States. Storms
<br />producing more than 7 in. of precipitation are
<br />fairly common so there must be some replenish-
<br />ment of the atmospheric moisture precipitated
<br />when greater amounts of precipitation are ob-
<br />served. Actually, it is very likely the above ex-
<br />treme amount of water in the ail' has never been
<br />observed and since no natural precipitation process
<br />removes an water vapor from the air, replenish-
<br />ment is a very important factor even in storms
<br />producing much less than 7 ill. of preeipitation.
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<br />2.2.2 Inflow of air into a storm is a natural
<br />feature of any storm mechanism. Lifting of air,
<br />the prime cooling factor in the precipitation
<br />process (ch. 1), is associated with a horizontal
<br />inflow, or convergence, of the air into the space
<br />vacated by the ascending air. The inflowing air is
<br />in turn lifted, leaving space for anew inflow. The
<br />process is, of course, continuous during the storm.
<br />The amount. of moisture in the inflowing air and
<br />the rate of inflow are the two most import,ant
<br />factors in determining precipitation rates.
<br />2.2.3 The amount of lV, in the atmosphere
<br />varies with (1) distance from the moisture source;
<br />(2) latitude, (3) season,and (4) elevation. These
<br />effects are clearly indicated in tables and charts
<br />of mean lV, over the United States [2]. Thechief
<br />source of water vapor in the atmosphere is wate.r
<br />evaporated from the seas. Consequently, other
<br />conditions being equal, ail' moving inland from
<br />the sea has a much higher water-vapor content
<br />than does air with a long trajectory over land.
<br />Furthermore, since the air temperature determines
<br />the upper limit of the water-vapor capacity of
<br />the air and since evaporation from a water surface
<br />tends to be greater with warmer water tem-
<br />peratures, warm air over a warm body of water
<br />has a tendency for higher lV p values. The Gulf of
<br />Mexico and Caribbean Sea, for instance, are the
<br />most favorable sources of moisture for precipita-
<br />tion in the United States.
<br />2.2.4 lVp values tend to be higher at low lati-
<br />tudes than at high latitudes because the tempera-
<br />ture, hence water-vapor capacity of the ail', is, in
<br />general, higher at low latitudes. Similarly, lV,
<br />values tend to be higher in summer than in
<br />'winter bee,ause of the warmer air te.mperatures.
<br />2.2,5 Other conditions being equal, a thin
<br />layer of air naturally contaius less lVp than does a
<br />thicker layer. Thus, for example, the atmosphere
<br />above a high plateau tends to haye less lV, than
<br />does the atmosphere aboye low-lying plains.
<br />Since air temperatures are generally warmest ,at
<br />low elevations and much of i.he water yapor is in
<br />the lowest levels, lV, in the atmosphere above a
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