The orographic effect occurs when air masses are forced to flow over high topography. Once the airmass temperature reaches the dewpoint during continued rise, water droplets begin to condense (forming clouds) and the airmass follows a moist adiabatic lapse rate (Figure 14), for which the rate of cooling with elevation decreases because of the addition of some offsetting heat to the airmass from the process of condensation (termed latent heat). If an air mass begins rising and has not reached the dewpoint temperature, it follows a dry adiabatic lapse rate, with the rate of cooling due nearly entirely to decreasing pressure, as shown in Figure 14. A typical average lapse rate is around 7° C per km of altitude change. The decrease in air temperature with elevation is known as the atmospheric (or adiabatic) lapse rate, as shown below, and is related to decreasing air density and pressure with increasing altitude (as air rises, it expands due to decreased pressure, leading to lower temperature). In both cases, cooling and warming of air masses occurs because they are forced upward or downward in the atmosphere. These are the orographic effect, and atmospheric convection. Both are related to the transport, rise, and fall of air masses that lead to temperature changes, and ultimately in the amount of water vapor that the air can hold. This is a banner cloud and should not be confused with snow blown from the crest or peak.To take the concept of relative humidity outdoors, let's consider why it rains in some areas and we have deserts in others. When the wind is strong, the orographic clouds formed near the summit may be observed streaming away from the mountain on the leeward side. Orographic cloud in stable conditions and low moisture windward Under drier conditions, waves may be present without cap clouds.įigure 4. It is important to remember that their absence does not mean that waves are absent. Sometimes, the clouds resemble a bank or wall that follows mountain contours. In the case of a mountain barrier, observed from the leeward side, cap clouds indicate likely wave activity downstream. These clouds give little or no precipitation. In the case of an isolated mountain, orographic clouds often have the form of a collar, surrounding the mountain or that of a cap covering the peak (Figure 4), both of which are fairly symmetrical. This process, referred to as orographic enhancement of precipitation, requires specific synoptic conditions not related to topography.Ĭlouds thin out and dissipate on the leeward side, where the relief causes descending motion and the precipitation is notably less ( rain shadow). Quite often, heavy orographic precipitation occurs on the upwind side of the barrier, particularly for barriers located near the sea. Precipitation may start or increase in intensity. The clouds become denser, the size and concentration of water droplets or ice particles increase and frequently develop greater vertical extent. Therefore, hilly areas are often cloudier than nearby lower land.įigure 3. Orographic cloud in unstable conditions and high moisture windwardĮxisting clouds arriving over mountainous or hilly areas, situated at heights comparable with that of the obstacle, may change in shape and structure as a result of the topography. Air also rises up a slope due to daytime heating so both orographic and thermal lifting often work together to produce tall, vertically developed Cumulus clouds (Figure 3). The type of cloud that forms depends on the air stability and moisture content. Orographic cloud in stable conditions and high moisture windward If the flow is sufficiently humid, clouds form on the windward side of mountains and are called orographic clouds (Figure 2).įigure 2. As airflow encounters a mountain or hill, it is forced to rise this is referred to as orographic lift.
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