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<br />frontal lifting, (3) orographic lifting, and/or
<br />(4) atmospheric instability. More often than not,
<br />two or more of these processes are active in pro-
<br />ducing the lifting associated with the heavier
<br />rainfall intensities and amounts. All four act
<br />simultaneously in some situations.
<br />1.3.4 Horizontal con.vergence, commonly re-
<br />ferred to simply as convergence, occurs when the
<br />pressure and wind fields act to concentrate inHow
<br />of air into a particular area, for example, a 10'''-
<br />pressure area. If this convergence takes place in
<br />the lowest layers of the atmosphere, the tendency
<br />to pile up forces the air upward, resulting in its
<br />cooling.
<br />1.3.5 Frontal lifting takes place when rela-
<br />tively warm air Howing towards a colder, hence
<br />denser, air mass is forced upward as the cold air
<br />acts as a wedge. Cold air overtaking warmer air
<br />will produce the same result by "wedging" the lat-
<br />ter aloft. The surface of separation (strictly
<br />speaking, a transition zone) between the two diff-
<br />erent air masses is called a frontal surface. A
<br />frontal surface always slopes upward toward the
<br />colder air mass, and the intersection of the surface
<br />with the ground is called a front. A warm frontal
<br />surface (between advancing warm air and re-
<br />treating or stationary cold air) usually has a slope
<br />of 1 :100 to 1 :300. The cold frontal surface (be-
<br />tween advancing cold air and retreating warm
<br />air) has a steeper slope, usually 1 :25 to 1 :100.
<br />Consequently, the upward velocity component of
<br />air forced upward by frontal surfaces alone is
<br />usually relatively small, even under strong wind
<br />conditions.
<br />1.3.6 Orographic lifting occurs when air
<br />Howing toward an orographic barrier is forced to
<br />rise in order to pass over it. The slopes of oro-
<br />graphic barriers are often appreciably steeper than
<br />the steepest slopes of frontal surfaces. Conse-
<br />quently, other conditions being equal, air may be
<br />cooled much more rapidly by orographic lifting
<br />than by frontal lifting.
<br />1.3.7 Atmosphem instability may be defined,
<br />for the purposes of this report, as a state in which
<br />the vertical temperature and/or moisture distribu-
<br />tion is such that if a quantity of air is given an
<br />initial upward impulse, it will tend to contiuue
<br />rising because of having a lower density than the
<br />surrounding air-in other words, buoyancy, Un-
<br />saturated air rising in the atmosphere cools prac-
<br />tically adiabatically; that is, without heat being
<br />added or removed. The adiabatic lapse rate, or
<br />
<br />change of temperature with elevation, is about 5.4
<br />F.O per 1,000 ft. Rising saturated air behaves in
<br />a similar manner except that, because of the latent
<br />heat released by condensation, it cools at a slower
<br />rate. For practical purposes, ascending saturated
<br />air is considered to cool pseudoadiabatically; i.e.,
<br />the water is precipitated immediately upon con-
<br />densation. The pseudoadiabatic lapse rate in-
<br />creases with elevation because the moisture
<br />content of saturated air (hence, amount of latent
<br />heat of condensation released) decreases with ele-
<br />vation. It averages about 3.3 1<'.0 per 1,000 ft. in
<br />the lower layers of the atmosphere and approxi-
<br />mates the dry-adiabatic lapse rate (5.41<'.0/1,000
<br />ft.) at high altitudes. A layer of unsaturated or
<br />saturated air with a vertical temperature gradient
<br />tending to exceed the dry-adiabatic or pseudo-
<br />adiabatic lapse rate, respectively, is thus unstable
<br />since the temperature of a lifted parcel of air is
<br />warmer than that of the surrounding air.
<br />1.3.8 Instability may also be realized in an
<br />unsaturated air mass having a lapse rate between
<br />the dry-adiabatic and the pseudoadiabatic. If,
<br />within this air mass, a parcel of air having a rela-
<br />tively high moisture content is lifted high enough,
<br />it cools dry-adiabatically to the condensation
<br />temperature at what is called the lifting conden-
<br />sation level. Above that level the parcel cools at
<br />the much slower pseudoadiabatic rate. As the
<br />lapse rate of the air mass is greater than the
<br />pseudoadiabatic, there is a level, called the level of
<br />fTCe convection, where the temperature of the
<br />lifted parcel is the same as that of the surround-
<br />ing air. Above the level of free convection the as-
<br />cending parcel is warmer, hence lighter, than the
<br />surrounding air and continues to rise through
<br />buoyancy even if no other lifting forces exist.
<br />1.3.9 Instability may also result from the lift-
<br />ing of a layer of air having a relatively high vapor
<br />content at the bottom and being relatively dry at
<br />the top. "''hen lifted, the lower part of the layer
<br />soon reaches the lifting condensation level, above
<br />which it cools at the pseudoadiabatic rate. The
<br />top part of the. layer, being relatively dry, cools
<br />at the more rapid dry-adiabatic rate. Continued
<br />lifting results in an increase of the vertical tem-
<br />perature gradient of the layer until the instability
<br />of the layer is realized.
<br />1.3.10 As discussed in paragraphs 1.3.7-1.3.9,
<br />the instability of an air mass is released when the
<br />lapse rate is increased until it reaches critical
<br />values. The increase may originate from: (1)
<br />
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