Clouds & Precipitation What is required? 1

Humidity Measuring humidity – Relative humidity – Ratio of the air's actual water vapor content compared with the amount of water vapor required for saturation at that temperature (and pressure) 2

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Relative humidity can be changed in two ways Add or subtract moisture to the air Adding moisture raises the relative humidity Removing moisture lowers the relative humidity 4

Changing the air temperature Lowering the temperature raises the relative humidity Dew point temperature Temperature to which a parcel of air would need to be cooled to reach saturation Cooling the air below the dew point causes condensation e.g., dew, fog, or cloud formation Water vapor requires a surface to condense on 5

Typical daily variations in temperature and relative humidity 6

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Adiabatic cooling of rising air 8

Other Processes that lift air 9

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Stability of air Types of stability Stable air Resists vertical displacement Cooler than surrounding air or Denser than surrounding air Wants to sink No adiabatic cooling 13

occurs when the environmental lapse rate is less than the wet adiabatic rate Stable air Often results in widespread clouds with little vertical thickness Precipitation, if any, is light to moderate Environmental lapse rate is greater than the dry adiabatic rate Warmer & Less dense than surrounding air Continues to rise until it reaches an altitude with the same temperature Adiabatic cooling Clouds are often towering 14

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Conditional instability occurs when the atmosphere is stable for an unsaturated parcel of air but unstable for a saturated parcel 16

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Height High clouds - above 6000 meters Types include cirrus, cirrostratus, cirrocumulus 22

Height Middle clouds – 2000 to 6000 meters Types include altostratus and altocumulus 23

Height Low clouds – below 2000 meters Types include stratus, stratocumulus, and nimbostratus (nimbus means "rainy") 24

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The sun (or moon) is not visible through nimbostratus clouds, which distinguishes them from mid-level altostratus clouds. Because of the fog and falling precipitation commonly found beneath and around nimbostratus clouds, the cloud base is typically very diffuse and difficult to accurately determine. 33

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Fair weather cumulus are fueled by buoyant bubbles of air, or thermals, that rise upward from the earth's surface.thermals As they rise, the water vapor within cools and condenses forming cloud droplets.water vapor within cools and condenses Young fair weather cumulus have sharply defined edges and bases while the edges of older clouds appear more ragged, an artifact of cloud erosion. Evaporation along the cloud edges cools the surrounding air, making it heavier and producing sinking motion (or subsidence) outside the cloud. 35

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Particle sizes involved in condensation and precipitation 41

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o Upon reaching the bottom of the cloud, some of the ice pellets are carried by the updraft back up to the top of the storm. 45

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The vertical line in the center of the diagram is the freezing line. Temperatures to the left of this line are below freezing, while temperatures to the right are above freezing. 47

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Sleet is less prevalent than freezing rain and is defined as frozen raindrops that bounce on impact with the ground or other objects.freezing rain Sleet is more difficult to forecast than freezing rain because it develops under more specialized atmospheric conditions.freezing rain It is very similar to freezing rain in that it causes surfaces to become very slick, but is different because its easily visible. 51

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an aggregate of ice crystals Progressing even further away from the warm front, surface temperatures continue to decrease and the sleet changes over to snow.warm front Snowflakes are simply aggregates of ice crystals that collect to each other as they fall toward the surface. Since the snowflakes do not pass through a layer of air warm enough to cause them to melt, they remain in tact and reach the ground as snow. Snow 53

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Rain – Standard Rain Gauge Snow - has two measurements Depth Water equivalent General ratio is 10 snow units to 1 water unit Varies widely Radar is also used to measure the rate of rainfall Measuring precipitation 55

Other Weather Instruments TEMPERATURE WIND direction WIND speed AIR PRESSURE HUMIDITY 56