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Nori cumulus deasupra Australiei

Un nor este o masă vizibilă de picături de lichid condensat (apă pe planeta Pamânt) sau de cristale de gheaţă condensate care se găseşte în atmosferă deasupra suprafeţei Pamântului sau deasupra unei alte planete ce posedă atmosferă. Există o ramură specială a meteorologiei care studiază norii, nefologia.

Pe planeta Pamânt, substanţa care se condensează este apa, care formează picături foarte mici de apă sau de cristale de gheaţă (de obicei de 0,01 mm în diametru), care fiind înconjurate de un număr imens de alte picături asemănătoare, produc efectul vizibil de nori având culori variind de la albul pur (când proporţia de cristale de gheaţă este mare) până la nuanţe foarte închise de gri (pentru norii ce conţin picături de apă în proporţie majoritară). O altă cauză a culorii norilor variind între alb şi negru este grosimea acestora, deoarece norii reflectă la fel toate lungimile de undă ale luminii solare albe. Totuşi, cu cât norul este mai gros şi mai dens, cu atât culoarea este mai închisă, din cauza absorbţiei luminii în interiorul norului.

[modifică] Formarea norilor şi precipitaţiile

Nori cumulus de vreme bună.

[modifică] Cloud formation and properties

A variety of cloud formations.

Global scheme of cloud optical thickness.

Clouds form when the invisible water vapor in the air condenses into visible water droplets or ice crystals. This can happen in three ways.

1. The air is cooled below its saturation point. This happens when the air comes in contact with a cold surface or a surface that is cooling by radiation, or the air is cooled by adiabatic expansion (rising). This can happen

• along warm and cold fronts (frontal lift),
• where air flows up the side of a mountain and cools as it rises higher into the atmosphere (orographic lift),
• by the convection caused by the warming of a surface by insolation (diurnal heating),
• when warm air blows over a colder surface such as a cool body of water.

2. Clouds can be formed when two air masses below saturation point mix. Examples are breath on a cold day, aircraft contrails, and Arctic sea smoke.

3. The air stays the same temperature but absorbs more water vapor into it until it reaches saturation.

The water in a typical cloud can have a mass of up to several million tonnes. However, the volume of a cloud is correspondingly high, and the net density of water vapor is actually low enough that air currents below and within the cloud are capable of keeping small droplets suspended. As well, conditions inside a cloud are not static: water droplets are constantly forming and re-evaporating. A typical cloud droplet has a radius on the order of 1 x 10^-5 m and a terminal velocity of about 1-2 cm/s. This give these droplets plenty of time to re-evaporate as they fall into in the warmer air beneath the cloud.

Most water droplets are formed when water vapor condenses around a condensation nucleus, a tiny particle of smoke, dust, ash, or salt. In supersaturated conditions, water droplets may act as condensation nuclei.

Water droplets large enough to fall to the ground are produced in two ways. The most important is through the Bergeron Process, theorized by Tor Bergeron, in which supercooled water droplets and ice crystals in a cloud interact to produce the rapid growth of ice crystals, which precipitate from the cloud and melt as they fall. This process typically takes place in clouds with tops cooler than -15°C. The second most important process is the collision and wake capture process, occurring in clouds with warmer tops, in which the collision of rising and falling water droplets produces larger and larger droplets, which are eventually heavy enough to overcome air currents in the cloud and the updraft beneath it and fall as rain. As a droplet falls through the smaller droplets which surround it, it produces a "wake" which draws some of the smaller droplets into collisions, thus perpetuating the process. This method of raindrop production is the primary mechanism in low stratiform clouds and small cumulus clouds in trade winds and tropical regions, and produces raindrops of several millimeters diameter.

This wave cloud pattern formed off of the Île Amsterdam in the far southern Indian Ocean.

The actual form of cloud created depends on the strength of the uplift and on air stability. In unstable conditions convection dominates, creating vertically developed clouds. Stable air produces horizontally homogeneous clouds. Frontal uplift creates various cloud forms depending on the composition of the front (ana-type or kata-type warm or cold front). Orographic uplift also creates variable cloud forms depending on air stability, although cap cloud and wave clouds are specific to orographic clouds.

[modifică] "Hot Ice" and "Ice Memory" in cloud formation

Perhaps somewhat confusingly, in addition to being the colloquial term sometimes used to describe dry ice, hot ice is the name given to a surprising phenomenon in which water can be turned into ice at room temperature by supplying an electric field of the order of 1 million volts per meter. (Choi 2005). The effect of such electric fields has been suggested as an explanation of cloud formation. Though, this theory is highly controversial and is not, by any means, widely accepted as being the actual mechanism of cloud formation. The first time cloud ice forms around a clay particle, it requires a temperature of -10°C, but subsequent freezing around the same clay particle requires a temperature of just -5°C, suggesting some kind of "ice memory" (Connolly, P.J, et al, 2005)

[modifică] Clasificare norilor

[modifică] Culorile norilor

[modifică] Nori pe alte planete

[modifică] Legături externe

• Vreme rea în Australia (Australia Severe Weather} Sistem de clasificare al norilor
• Nori Chitambo --- Nori şi alte fenomene meteorologice - Fotografii şi informaţii despre diferite tipuri de nori
• Explicarea formării norilor
• Societatea de apreciere a norilor (Cloud Appreciation Society) - Estetica norilor