+ Cloud Formation A cloud is a visible aggregate of small water droplets or ice crystals that are suspended in the atmosphere above the earth’s surface Lifting condensation level The ascending parcel of air is cooled to its dew point temperature and triggers condensation
+ Cloud Formation Condensation aloft: Adiabatic cooling causes clouds to form as water vapor condenses in the atmosphere. The air becomes saturated and there must be a surface. Condensation nuclei act as surfaces, on which the water vapor can condense.
+ Cloud Formation Growth of cloud droplets: Hygroscopic (water-seeking) nuclei are most effective for condensation. Growth is rapid at first, then slows as water vapor is consumed. Hygrophobic (water-fearing) nuclei are not efficient condensation nuclei but could droplets will form on them when the relative humidity reaches 100%
+ Cloud Classification Clouds are classified on the basis of two criteria. 1. Form: Cirrusare high, white, and thin forming delicate veil like patches or wisplike strands often have a feathery appearance Cumulus clouds are globular, usually exhibiting a flat base and appear as rising domes or towers. Stratus clouds are best described as layers or sheets covering much of the sky.
+ Cloud Classification 2. Height: High clouds are above 6000 m. Middle clouds range between 2000–6000 m. Low clouds are at altitudes of less than 2000 m. Clouds of vertical development extend upward to span more than one height range.
+ Cloud Classification- High Clouds Low temp and small amount of water vapor at high altitudes equal high clouds, thin, white and made of ice crystals Cirrus Delicate, icy filaments. Winds cause the filaments to bend or curl
+ Cloud Classification- Medium Clouds Altocumulus Tend to form in large patches composed of rounded masses or rolls that may or may not merge Composed of water not ice droplets The individual cells have a more distinct outline
+ Cloud Classification- Medium Clouds Altostratus Formless layer of grayish clouds that cover all or portions of the sky. Sun is visible as a big bright spot with the edge of it’s disc not discernible; no halos Infrequent snow or drizzle. Commonly associated with approaching warm fronts thicken into nimbostratus (which = lots of rain)
+ Clouds of Vertical Development Cumulonimbus Large dense billowly clouds of considerable vertical extent in the form of huge vertically towers Tops spread out into an anvil shape Towers produce heavy precipitation with lightening, thunder, and occasionally hail
+ Cloud Classification Cloud varieties: Uncinus are hooked shaped clouds, often precursors to bad weather. Fractus or fractured clouds are stratus or cumulus clouds that appear broken. Mammatus clouds have udder-shaped protuberances on their bottom surfaces and are associated with stormy weather. Lenticular clouds are lens shaped and are common in rugged or mountainous topographies.
+ Types of Fog Fog is defined as a cloud with its base at or very near the ground. Fog results from cooling or when air becomes saturated through the addition of water vapor Radiation Advection Upslope Evaporative
+ Fog Formed by Cooling Radiation fog results from radiation cooling of the ground and adjacent air. Night time phenomena requiring clear skies and relatively high humidity The high humidity can cause a small amount of cooling to lower the temperature to the dew point. To be extensive, there should be a slight breeze. It is usually thickest in valleys.
+ Fog Formed by Cooling Advection fog is a blanket of fog caused by warm, moist air blowing over a cold surface. Some turbulence is needed (10–30 kph winds). Turbulence facilitates cooling through a thicker layer of air and carries it to greater heights Wintertime fog in the midwest This fog is thick and produces hazardous driving conditions
+ Fog Formed by Cooling Upslope fog is created when relatively humid air moves up a sloping landform or up the steep slopes of a mountain. Theupward flow causes the air to expand and cool adiabatically resulting in fog.
+ Fog Formed by Cooling There are two types of evaporation fog. Frontal (precipitation) fog Occurs when rain droplets falling from relatively warm air above a frontal surface evaporates into the cooler air below and causes it to become saturated.
+ Fog Formed by Cooling Steam fog Occurs when cool air moves over warm water. Moisture evaporates and saturates the air above it. Rising water vapor meets the cool air, condenses and rises. Looks like steam. Steam fog is very common over lakes.
+ How Precipitation Forms Clouddroplets are about 20 micrometers in diameter. A human hair is about 75 micrometers A rain droplet is about 2 mm or 100 times the average cloud droplet Volume is 1 million times greater
+ How Precipitation Forms Process that generates precipitation in the middle latitudes The Bergeron Process depends on the process of the coexistence of water vapor, liquid cloud droplets and ice crystals. Cloud droplets do not freeze at 0°C. It freezes at -40°C. it is super cooled. The saturation vapor pressure above ice crystals is slightly lower than above super cooled liquid droplet
+ How Precipitation Forms So, imagine a cloud at -10°C where each ice crystal (snow crystal) is surrounded by thousands of liquid droplets. Because air is saturated at 100% with respect to liquid water, it will be super saturated (above 100%) with respect to the newly formed ice crystals. At this result of this supersaturation, the ice crystals collect water, lowering relative humidity, the water droplets shrink to replenish what was lost
+ How Precipitation Forms So the growth of ice crystals is fed by continued evaporation of liquid droplets When ice gets large enough they fall, they grow as they intercept cloud droplets on their fall. A chain reaction ensues and produces many snow crystals= snow flakes
+ How Precipitation Forms The type of precipitation (snow, sleet, rain, or freezing rain) depends on the temperature in the lower few km of the atmosphere When the surface temperature is above 39°F snow usually melts before it hits the ground
+ How Precipitation Forms The Bergeron process (precipitation from cold clouds) depends on the coexistence of water vapor, liquid cloud droplets, and ice crystals
+ How Precipitation Forms The collision-coalescence process (precipitation from warm clouds) occurs as copious rainfall associated with clouds located below the freezing level (called warm clouds), especially in the tropics. Small droplets hit other droplets and become larger. They collide with more droplets and their falling velocity increases.
+ Forms of Precipitation Rainis restricted to droplets of water with a diameter at least 0.5mm. Cloudbursts are unusually heavy rainfalls. Drizzleare fine, uniform droplets with a diameter less than 0.5mm. (not really considered rain)
+ Forms of Precipitation Virga is rain that evaporates above ground. Fallstreaks are ice crystals that sublime in the dry air below Mist contains the smallest droplets.
+ Forms of Precipitation Snow is precipitation in the form of ice crystals or aggregates of ice crystals. Size, shape and concentration depend to great extent on the temp at which they form Low temperature, the moisture is low = very light fluffy snow made up of individual 6 sided crystals Warmer temperature at about 23°F, ice crystals join together into large clumps of tangled crystals
+Forms of PrecipitationSleet is clear to translucent particles of ice, rain drops freeze while falling.Freezing rain (glaze) are rain drops that become super-cooled, hit a surface, and freeze immediately.
+ Forms of Precipitation Hail is precipitation in the form of hard, rounded pellets of ice. Itis produced in cumulonimbus clouds. Hail stones begin as small ice pellets and grow as they are propelled by updrafts and downdrafts through the cloud.
+ Forms of Precipitation Rime is a deposit of ice crystals, formed on surface objects by super-cooled fog or cloud droplets. Itoccurs when the surface temperature of an object is below freezing.
+ Precipitation Measurement Standard instruments: A standard rain gauge catches rain water and conducts it through a narrow opening into a cylindrical measuring tube. The gauge is 20 cm in diameter and can measure rainfall to the nearest 0.025 cm. < 0.025 cm = trace
+ Precipitation Measurement A tipping bucket has two compartments (or buckets). When one bucket fills, it tips and empties its water and the other bucket takes its place at the funnel. A weighing gauge collects rain fall in a cylinder that rests on a spring balance. As the cylinder fills, the movement is transmitted to a pen that records the data.
+ Precipitation Measurement Whenmeasuring snowfall, two measurements are normally taken. The depth is measured with a calibrated stick. To obtain the water equivalent, snow is melted and then weighed or measured as rain. Weather radar uses radio waves to measure precipitation. The radio waves penetrate small droplets, but are reflected off larger ones. Echoes are sent back and displayed.
+ Intentional Weather Modification Intentional weather modification, such as cloud seeding, is deliberate human intervention to influence processes that constitute the weather. Snow and rain making Silver iodide crystals act as freezing nuceli. Fog and cloud dispersal Cloud seeding with dry ice into super-cooled fog or stratus clouds helps to disperse them to improve visibility.
+ Intentional Weather Modification Hailsuppression has been shown to be ineffective. Anti-hail cannons produced a loud whistling noise and a large smoke ring thought to suppress hail. Cloud seeding with silver iodide crystals was also employed to disrupt the growth of hailstones.
+ Intentional Weather Modification Several methods of frost prevention are being used. Water sprinklers add heat from water and from the latent heat of fusion when the water freezes. Air mixing uses wind machines to mix warm and cool air. Orchard heaters produce the most successful results, but fuel cost can be significant.