Clouds and Fog Stability summary 3 types of

Clouds and Fog

Stability summary • 3 types of environmental stability: – Absolutely stable – Absolutely unstable (very uncommon) • Does occur in few tens of feet above parking lots & roads in summer (mirages are often co-located) – Conditionally unstable • Conditional on when the rising air parcel reaches saturation (i. e. , the height of the LCL) – If parcel can maintain its temperature WARMER than environment, then it remains unstable

Examples in stability 1. Assume the environmental temperature cools at a rate of 4°C per 1000 m. If the surface temperature is 24°C and the surface dewpoint temperature is 19°C, determine the atmospheric stability for surface air parcels lifted to 3 km. Will clouds form? If so, at what level? 2. Now assume the environmental temperature cools at a rate of 9°C per 1000 m. If the surface T is 31°C and the surface Td is 16°C, determine the atmospheric stability for surface air parcels lifted to 3 km. Will clouds form? If so, at what level? 3. Finally, assume the env. cools at 12°C/km. If sfc T is 11°C, sfc Td is 11°C, determine stability. Will clouds form? If so, at what level?

Water vapor in the atmosphere and the concept of relative humidity are important when we talk about the formation of fog, clouds and precipitation Fog - is actually a cloud formed or lying on the ground Ø Fogs form when air saturates (its relative humidity reaches around 100 percent), and the water vapor within the air mass condenses on small particles in the air to form liquid cloud droplets. Ø The air may become saturated either by lower its temperature to its condensation (dew) point or by adding water vapor into the air until it reaches saturation.

There actually different types of fog in the sense that fog can form under a variety of conditions Radiation fog generally forms when the air near the surface cools to its saturation temperature due to radiational cooling at night when the sun has set.

Advection fogs are fogs formed when air moves either over a cooler surface or over a warmer, moist surface, and as a result the air mass reaches saturation. Most often this occurs when a moist air mass moves over a cold surface, such as a large, cold body of water or snow/ice cover, whose temperature is below the dew point of the advecting air mass, and its lowest reaches are cooled to condensation.

Upslope and valley fogs are two special cases of advection fogs particular to hilly terrain. They form when air moving in hilly or mountainous terrain cools to condensation.

Frontal fog, which may also be called precipitation fog. This fog type generally occurs when rain falling from warm air aloft evaporates at or near the surface under light wind conditions. The evaporating precipitation as it falls through colder air thus increasing the surface air's moisture until condensation is achieved. Such fogs are most common in the vicinity of warm or stationary fronts, but they can form at cold fronts as well. However, cold fronts generally move and mix too quickly to allow the condition to persist for long.

CLOUDS In 1802 an Englishman by the name of Luke Howard invented the cloud naming system that is still in use today. Howard used Latin names to describe clouds. • part of a cloud's name describes height • part tell us something about the cloud’s shape Ø The prefixes denoting heights are: cirro, high clouds above 20, 000 feet; alto are mid level clouds between 6, 000 - 20, 000 feet. There is no prefix for low level clouds. Ø The names denoting shapes are: cirrus mean curly or fibrous, stratus means layered, while cumulus means lumpy or piled. Nimbo or nimbus is added to indicate that a cloud can produce precipitation.

Cumulus clouds have a flat base and fluffy tops, and range in size from very shallow to very large, with a dense, mound-shaped top that looks like a large cauliflower. Where the sun shines on these clouds, they are a brilliant white; while their sides opposite the sun are darker, because they are in shadow. Cumulus clouds are often at least as tall as they are wide, if not taller. Cumulus clouds are classified as low clouds

Stratus clouds are usually gray seen from below, fairly uniform horizontally, and lie close to the surface of the Earth. They usually look like a solid layer but are sometimes found in patches. Stratus clouds can produce light precipitation. They are usually water clouds. When the sun is visible through stratus clouds, its edges are distinct. Stratus is classified as a low cloud. Nice, flat, uniform lighting throughout the cloud and snow, and trees and building make a good contrast. Photo from Pequot Lakes Elementary School, Pequot Lakes, MN. January 2003

Stratocumulus clouds are gray or whitish. Both the tops and bottoms of these bases are rounded. They can look like waves on the ocean, joined flat cumulus, or a continuous but irregular layer. They can form from spreading out cumulus clouds, or from a stratus layer. The individual cloudlets are wider than two fingers at arm's length. Stratocumulus is classified as a low cloud. Very opaque and mostly cloudy stratocumulus. Notice how the cloud bases are all about the same height. Photo taken by Doug Stoddard in March 2002, in St. Thomas. Generally translucent stratocumulus clouds. Mostly cloudy to overcast cloud cover. Photo by Kevin Larman.

Nimbostratus A dark, gray cloud characterized by more or less continuously falling precipitation. It is not accompanied by lightning, thunder, or hail.

Altocumulus clouds take many forms. They can look like waves on the sea, tiny cumulus, balls of cotton, or even little castle towers. The individual "cloudlets" are usually between one and two fingers wide at arm's length. They are thick enough to have gray bases. Altocumulus clouds are almost always made up of water droplets, but sometimes contain ice crystals as well. Altocumulus is classified as a middle cloud. Photo by Mandana Khaiyer, March 1997, in San Antonio, TX. This is a nice picture of altocumulus, showing a tendency to form cloud 'streets' (i. e. , rows of cloud), which is an indicator that there is some wave phenomenon occurring in the atmosphere.

Altostratus clouds form a bluish or grayish veil that totally or partially covers the sky. The sun can barely be seen through altostratus and its edges are not distinct. Altostratus clouds are water or ice clouds, and they do not produce halos. Altostratus is classified as a middle cloud. Compare to stratus and cirrostratus photos to contrast the opacity (amount of light reduction) by these clouds. Photo by Lin Chambers.

Cirrus clouds look like white delicate feathers or horse tails. They are generally white, wispy, and fibrous. Cirrus clouds are made up of ice crystals. Cirrus is classified as a high cloud. This photo shows transparent to translucent cirrus streaks, and illustrates why cirrus clouds are often described as mare's tails. The streaks come from falling ice particles that make up the clouds. Photo by Lin Chambers, August, 2002. Feathery cirrus turned pink by the setting sun, with some very distinct persistent contrails running through the sky. Photo by Ed Donovan, South Carolina, July 2003. From http: //asd-www. larc. nasa. gov/

Cirrocumulus clouds are thin white layers that look like patches of cotton or ripples. They are not shaded. They are usually made up of ice crystals. Cirrocumulus is classified as a high cloud. Photo by Jeff Caplan, NASA Photographer in 1997. This is an example of fairly largecelled cirrocumulus, mostly cloudy and pretty much translucent. Photograph taken just after sunset, about 8: 30 pm, on July 8, 2005 by Dave Kratz in Poquoson, Virginia.

Cirrostratus clouds are a thin, transparent, whitish layer made up of ice crystals. They may partially or totally cover the sky. Cirrostratus clouds sometimes produce haloes around the sun or moon. Sometimes patches of rainbow-colored light called "sundogs" occur on the halo on both sides of the sun. Cirrostratus is classified as a high cloud. Photo taken by Doug Stoddard in March of 2002 in Puerto Rico The upper right of the picture shows a cirrocumulus, but the portion where the sun is shining is a nice uniform cirrostratus. Photo from Dr. Francisco Vazquez Colon School, Manati, Puerto Rico. January 2003

The most likely cause of the cloud lines stems from the exhaust of ocean going vessels. Large numbers of Aitken (CCN) nuclei form in this exhaust. These are carried upward by the buoyancy of the hot gases and ship’s air wake to form droplets at slight supersaturation. The phenomenon does not appear related to special characteristics of the vessel's power plant but to a critical condition of the atmosphere. This condition may be described as having 1) a convectively unstable layer from the surface to a low-level stable layer, 2) saturation or slight supersaturation near the top of the convective layer, and 3) a convective layer, presumably deficient in CCN

Cumulonimbus are large clouds, with a generally flat, dark base and large tops with the shape of an anvil or giant cauliflower starting to flatten and become fibrous at the top or sides. Mature cumulonimbus clouds have strong updrafts; the flat top of the anvil indicates where the updraft is no longer buoyant and the air spreads outward horizontally. The temperatures at the top of the cumulonimbus are well below freezing: the fibrous structure indicates the presence of ice crystals. Cumulonimbus clouds are made up of a mixture of water droplets (lower parts of the cloud), ice crystals (upper parts of the cloud), and precipitation. Cumulonimbus are classified as "clouds with vertical development" since their tops extend quite high.

Unusual Cloud Formations NASA photo Rows of clouds stream over the Bering Sea from the edge of the sea ice. Called cloud streets, these cumulus clouds form when cold air from the ice blows over the open ocean, chilling the moist air. As the temperature drops, water freezes into tiny clouds, which are arranged in neat rows in line with the powerful sweep of the wind. Though some clouds form over the cracking sea ice on the right side of the image, most are over the unfrozen water.

Unusual Cloud Formations Lenticular Clouds These clouds are formed by mountain waves of air created by strong winds forced over high mountains. A stack of lenticular clouds caps and blocks the view of the summit of Mount Rainier. From http: //www. livingwilderness. com/ mountains

Unusual Cloud Formations Wave Clouds Kelvin-Helmholtz wave clouds are formed when there are two parallel layers of air that are usually moving at different speeds and in opposite directions. The upper layer of air usually moves faster than the lower layer because there is less friction. In order for us to see this shear layer, there must be enough water vapor in the air for a cloud to form. Even if clouds are not present to reveal the shear layer, pilots need to be aware of invisible atmospheric phenomenon.

PRECIPITATION

The average cloud droplet is small enough to stay suspended in the atmosphere by the motion of the air. In order to fall as precipitation, these liquid (or solid) water particles must grow much larger so that gravity dominates. But how do we go from something the size of a cloud droplet to something 2 orders of magnitude larger? Cloud droplets are so small it may take a million or more to produce a single raindrop.

How do clouds really form? Humid air, purified of all foreign nuclei, can be expanded in cloud chambers to relative humidities of the order of 400% without any condensation taking place!! Why? Surface tension of a water molecule is very high and tension must be overcome to form small cloud droplet. This type of condensation is called homogeneous nucleation. However, cloud condensation occurs in our atmosphere at relative humidities near 100% only because nature provides an abundance of condensation nuclei. This process is called heterogeneous nucleation. • NEED Condensation Nuclei to form cloud drops

Condensation Nuclei: Small particles in air created from/by: • dust • volcanoes • factory smoke • forest fires • ocean salt • sulfate particles from phytoplankton in ocean • They are most abundant in lower troposphere over urban areas • They are quite small relative to a rain drop or cloud drop

Two types of CCN: • Hydroscopic - water seeking - H 2 O readily condenses on these • ocean salt is a good example (sticky salt shaker when humid) • Hydrophobic- water repellant - H 2 O does not readily condense on these (wax on car) Type of particle Approximate radius (μm) # of particles per cubic centimeter* Range (typical) Small (Aitken) < 0. 2 1000 – 10, 000 (1000) Large 0. 2 – 1. 0 1 – 1000 (100) Giant > 1. 0 <1 – 10 (1) Fog & cloud > 10. 0 10 – 1000 (300) *Note: 1 cm 3 is about the size of your thumb!!!

There are two basic mechanisms by which precipitation forms in clouds: Ø “Warm rain“ or collision-coalescence Ø “Ice crystal" or Bergeron processes

The term "warm rain" was derived after scientists noticed that rain in tropical regions often fell from clouds with temperatures never colder than 32°F (0°C). 1) There must be a high liquid water content within the cloud. 2) There must be sufficiently strong and consistent updrafts within the cloud. 3) A large range of cloud droplet sizes is very helpful. 4) The cloud must be thick enough so that the cloud droplets have enough time to gather surrounding smaller droplets.

Typical raindrop path: Starts as very small cloud droplet Rises (relatively slowly) through the cloud Collision-coalesence occurs both in the UP and DOWN paths of water drop

Collision-coalesence -Occurs in clouds with temps > -15 C -Larger drops fall faster than smaller ones -Drops collide & merge (coalesce)

The Bergeron Process, named after its discoverer, Tor Bergeron, involves supercooled water droplets. v The term supercooled, refers to water that is still a liquid at temperatures below the freezing point v Cloud droplets have been observed to remain liquid at temperatures well below freezing (as low as -40 C)! Why? ? Large amount of kinetic energy must be removed to initially form the lattice structure of ice crystal (to transform liquid water into ice). v. But: ice crystals routinely form between -5 C and -15 C. Why? Presence of ice nuclei (larger than CCN). Ice nucleus impacts supercooled cloud droplet, droplet immediately freezes onto it.

This process assumes that ice crystals and very small pure water cloud droplets (supercooled liquid) and water vapor all are present in the same region of a cloud. v Tiny ice crystals, in a supercooled cloud, grow larger at the expense of the surrounding liquid cloud droplets. v After growing large enough, the crystals fall from the clouds as precipitation. v This precipitation formation process is more common at higher altitudes as well as at middle and high latitudes.

Bergeron Process -- If we can get small ice crystals to form (not trivial, needs presence of “ice nuclei” in upper parts of cloud), then --water moves, in vapor phase, from supercooled liquid droplet onto crystal structure

Notice the small droplets have evaporated and redeposited onto the ice crystal

Four types of “winter precip” 1. Snow 2. Sleet 3. Freezing rain 4. Rain The type of precip we see at the surface is highly dependent on the vertical temperature structure of the atmosphere in & below the cloud: -Air is < 0 C entirely: snow -Air is > 0 C near sfc: rain -Warm layer above sfc: sleet or freezing rain

SNOW The green dashed line is the temperature in respect to elevation. The surface temperature is 25°F and increases with height before decreasing. However, since the temperature remains below freezing any precipitation that falls will remain as snow.

Snow along the Trans Labrador Highway in Quebec, Canada

SLEET Note that the surface temperature in this figure it 27°F. Also as elevation increases, the temperature increases to a point where some of the atmosphere is above freezing before the temperature lowers again below freezing. As snow falls into the layer of air where the temperature is above freezing, the snow flakes partially melt. As the precipitation reenters the air that is below freezing, the precipitation will re-freeze into ice pellets that bounce off the ground, commonly called sleet. The most likely place for freezing rain and sleet is to the north of warm fronts. The cause of the wintertime mess is a layer of air above freezing aloft.

FREEZING RAIN Freezing rain will occur if the warm layer in the atmosphere is deep with only a shallow layer of below freezing air at the surface. The precipitation can begin as either rain and/or snow but becomes all rain in the warm layer. The rain falls back into the air that is below freezing but since the depth is shallow, the rain does not have time to freeze into sleet. Upon hitting the ground or objects such as bridges and vehicles, the rain freezes on contact. Some of the most disastrous winter weather storms are due primarily to freezing rain.