Transcript Document
Clouds Moisture in the air Humidity-the amount of moisture in the air Relative humidity- the amount of moisture in the air compared to the amount it could hold at a given temperature. Dew point-the temperature at which moisture in the air condenses. Saturated-when air is holding all the moisture it can. Cloud development As air rises it cools when it reaches the dew point it condenses. It needs something to condense on. This is called a condensation nuclei. This can be a speck of dust etc. Sometimes the relative humidity can be above 100%. If the air is too clean there is nothing for the moisture to condense on. This is when cloud seeding may be effective. Silver iodide is shot into the air to give the moisture the needed nucleus. FOUR MAJOR CLOUD TYPES High Middle Low Clouds with Vertical Development HIGH--above 6000 m(cirro-high) Cirrus thin and wispy Most common high cloud Composed of ice crystals Occur in fair weather Point in the direction of movement Cirrocumulus Cirrocumulus are thin, white patches, sheets or a layer of clouds without shading, composed of very small elements in the form of grains, ripples, etc., merged or separate, and more or less regularly arranged A patch or layer of cloud consisting of tiny individual cloudlets at high-level is called cirrocumulus Cirrocumulus are composed almost exclusively of ice crystals. Cirrocumulus Cirrocumulus differs from Altocumulus in that most of its elements are very small and without shading. Cirrocumulus differs from Cirrus and Cirrostratus in that it is rippled or subdivided into very small cloudlets Cirrocumulus Sometimes they look like the scales on a fish - a "mackerel" sky that may mean that unsettled weather is on its way Like all high-level clouds, cirrocumulus is made of ice crystals Formation of Cirrocumulus It forms when cirrus or cirrostratus is warmed gently from below. This causes air to rise and sink inside the cloud. Some of the ice crystals change into water vapor, and gaps appear. It can be difficult to tell cirrocumulus from altocumulus Cirrocumulus has no shading (which altocumulus usually has), and because it is so much higher, the cloudlets of cirrocumulus are much smaller than those of altocumulus Cirrocumulus Mackerel Sky Cirrostratus sheet-like and nearly transparent Cirrostratus can cover the entire sky and be up to several thousand feet thick they are relatively transparent, as the sun or the moon can easily be seen through them These high-level clouds typically form when a broad layer of air is lifted by large-scale convergence Halo Sometimes the only indication of their presence is given by an observed halo around the sun or moon. Halos result from the refraction of light by the cloud's ice crystals. Cirrostratus clouds, however, tend to thicken as a warm front approaches, signifying an increased production of ice crystals. As a result, the halo gradually disappears and the sun (or moon) becomes less visible. MIDDLE-- (2000-6000m) Altostratus Altocumulus Altostratus Altostratus clouds are gray or blue-gray mid-level clouds composed of ice crystals and water droplets. The clouds usually cover the entire sky. Altostratus clouds often form ahead of storms with continuous rain or snow. Altocumulus parallel bands or rounded masses Typically a portion of an altocumulus cloud is shaded, a characteristic which makes them distinguishable from the high-level cirrocumulus. Altocumulus Altocumulus clouds usually form by convection in an unstable layer aloft, which may result from the gradual lifting of air in advance of a cold front. The presence of altocumulus clouds on a warm and humid summer morning is commonly followed by thunderstorms later in the day LOW Stratus— layered uniformly gray, often cover the entire sky Stratocumulus low, lumpy layer of clouds that is sometimes accompanied by weak intensity precipitation Stratocumulus vary in color from dark gray to light gray may appear as rounded masses, rolls, etc., with breaks of clear sky in between. Nimbostratus dark, low-level clouds with precipitation Nimbus means rain Stratus means sheet Nimbostratus light to moderately falling precipitation Low clouds are primarily composed of water droplets since their bases generally lie below 6,500 feet (2,000 meters) However, when temperatures are cold enough, these clouds may also contain ice particles and snow CLOUDS HAVING VERTICAL DEVELOPMENT Cumulus Form where temperatures change quickly with height Cumulus means heap Cumulonimbus Fair Weather Cumulus puffy cotton balls floating in the sky Lifetime of 5-40 minutes Fair weather cumulus Known for their flat bases and distinct outlines, fair weather cumulus exhibit only slight vertical growth, with the cloud tops designating the limit of the rising air Given suitable conditions, however, harmless fair weather cumulus can later develop into towering cumulonimbus clouds associated with powerful thunderstorms called supercells. Cumulonimbus Clouds (Cb) Cumulonimbus clouds much larger and more vertically developed than fair weather cumulus They can exist as individual towers or form a line of towers called a squall line Fueled by vigorous, convective updrafts the tops of cumulonimbus clouds can easily reach 39,000 feet (12,000 meters) or higher, reaching high into the atmosphere Mammatus sagging pouch-like structures Mammatus are pouch-like cloud structures and a rare example of clouds in sinking air Mammatus Clouds Sometimes very ominous in appearance, mammatus clouds are harmless and do not mean that a tornado is about to form; a commonly held misconception. In fact, mammatus are usually seen after the worst of a thunderstorm has passed Mammatus CONTRAIL A contrail, also known as a condensation trail, is a cirrus-like trail of condensed water vapor often resembling the tail of a kite. Contrails are produced at high altitudes where extremely cold temperatures freeze water droplets in a matter of seconds before they can evaporate. Lenticular Clouds Lenticular clouds are caused by a wave wind pattern created by the mountains. They look like discs or flying saucers that form near mountains. Cloud Types at Fronts Fronts Fronts form where air masses collide Cold front Cold air invades warm air Warm front Warm air invades cold air We find a variety of weather at fronts When the two masses meet, the differences in temperature, moisture and pressure can cause one air mass to override the other. If cold air invades warm air--cold front If warm air moves into cold air--warm front If neither air mass is moving a stationary front Fronts are accompanied by wind, clouds, rain, and storms. Approaching cold front Approaching warm front Less dense air gradually rises over the cold denser air Less obvious and more gradual than cold front ¨ Cirrus clouds ¨ Thicken into altocumulus and altostratus ¨ Sky turns gray ¨ Light to moderate rain or snow develops ¨ At front rain or snow turns to drizzle Warm front animation source: http://www.learn-line.nrw.de/angebote/klima/medio/bilder/wfront.gif Thunderstorm Formation As cold air pushes into warm moist air, the cold more dense air stays low and the warmer less dense air is pushed up rapidly This rapid upward movement forms thunderstorms