24.2 Cloud Formation 2/3/2014. Orographic Lifting. Processes That Lift Air Frontal Wedging. Convergence and Localized Convective Lifting

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1 2/3/2014 Orographic Lifting Processes That Lift Air Frontal Wedging A front is the boundary between two adjoining air masses having contrasting characteristics. Convergence and Localized Convective Lifting Processes That Lift Air Convergence Convergence is when air flows together and rises. Localized Convective Lifting Localized convective lifting occurs where unequal surface heating causes pockets of air to rise because of their buoyancy. #2 Mixing of Different Bodies of Air Clouds will form when one body of moist air mixes with another body of moist air at different temperatures. The mixing of the two bodies of air into one will cause the air s temperature to change, possibly below its dew point which will produce condensation and cloud formation. #3 Advective Cooling Advective cooling is cooling produced when wind carries warm, moist air across a cold ocean or region of land. The cold water or land absorbs heat from the air, and the temperature of the air decreases. If the air is cooled below its dew point, then condensation and fog or cloud formation takes place. These clouds are typically very low in the atmosphere. 1

2 Clouds are classified on the basis of their form and height. The three forms of clouds are: Cirrus (cirrus = curl of hair) are clouds that are high, white, and thin. Cumulus (cumulus = a pile) are clouds that consist of rounded individual cloud masses. Stratus (stratus = a layer) are clouds best described as sheets or layers that cover much or all of the sky. Cirrus Clouds High Clouds Cirrus clouds are high, white, and thin. Cirrostratus clouds are flat layers of clouds. Cirrocumulus clouds consist of fluffy masses. Middle Clouds Altocumulus clouds are composed of rounded masses that differ from cirrocumulus clouds in that altocumulus clouds are larger and denser. Altostratus clouds create a uniform white to gray sheet covering the sky with the sun or moon visible as a bright spot Cloud Types Cloud Classification Low Clouds Stratus clouds are best described as sheets or layers that cover much or all of the sky. Stratocumulus clouds have a scalloped bottom that appears as long parallel rolls or broken rounded patches. Nimbostratus clouds are the main precipitation makers. m Movie Clip 2

3 Clouds of Vertical Development - Cumulonimbus Some clouds do not fit into any one of the three height categories mentioned. Such clouds have their bases in the low height range but often extend upward into the middle or high altitudes. Fog Fog is defined as a cloud with its base at or very near the ground. Radiation Fog = Fog Caused by Cooling At night, as the air cools, it becomes denser and drains into low areas such as river valleys, where thick fog accumulations may occur Forms of Precipitation 1.Drizzle 2.Rain 3.Snow 4.Sleet 5.Hail For precipitation to form, cloud droplets must grow in volume by roughly one million times. Warm Cloud Precipitation The collision-coalescence process is a theory of raindrop formation in warm clouds (above 0oC) in which large cloud droplets collide and join together with smaller droplets to form a raindrop. Large drops form on large condensation nuclei or through random collisions of droplets. As the droplets fall (larger drops fall faster than smaller drops) the larger droplets overtake and collide with smaller drops in their path. The merging of cloud droplets by collision is called coalescence. (Note: collision does not always guarantee coalescence) 3

4 Cold Cloud Precipitation The Bergeron process is a theory that relates the formation of precipitation to supercooled clouds, freezing nuclei, and the different saturation levels of ice and liquid water. Precipitation may form in clouds that contain water vapor, ice crystals, and water droplets that have gone through supercooling. The Bergeron Process Cold Cloud Precipitation Supercooled water is the condition of water droplets that remain in the liquid state at temperatures well below 0ºC; the water can reach temperatures as low as -10ºC. The water droplets at this cold temperature are not able to freeze because there are too few freezing nuclei. Freezing nuclei are kinds of condensation nuclei with a crystalline structure similar to that of ice. These can consist of clay materials, certain types of bacteria, and other ice crystals. Water from the supercooled water droplets condenses onto existing ice crystals. These ice crystals grow larger and eventually fall as rain or snow Water in the Clouds 24.3 Cloud Seeding Cloud Seeding To inject (or seed) a cloud with small particles that will act as nuclei, so that the cloud particles will grow large enough to fall to the surface as precipitation. First experiments in late 1940s using dry ice. Silver Iodide is also used today because it s structure is similar to that of ice crystals. Under some conditions, cloud seeding has increased precipitation while decreasing precipitation in other conditions. Research continues... 4

5 24.3 Measuring Precipitation Measuring Precipitation Rain Gauge A rain gauge is an instrument for measuring the amount of rainfall. These instruments only measure precipitation that falls in one spot. Largest Recorded Hailstone 5