Cloud

In meteorology, a cloud is a visible mass of liquid droplets or frozen crystals made of water or various chemicals suspended in theatmosphere above the surface of a planetary body. These suspended particles are also known as aerosols and are studied in the cloud physics branch of meteorology.

Terrestrial cloud formation is the result of air in Earth's atmosphere becoming saturated due to either or both of two processes: cooling of the air and adding water vapor. With sufficient saturation, precipitation will fall to the surface; an exception is virga, which evaporates before reaching the surface.

Clouds in the troposphere, the atmospheric layer closest to Earth's surface, have Latin names due to the universal adaptation of Luke Howard's nomenclature. It was introduced in December 1802 and became the basis of a modern international system that classifies these tropospheric aerosols into several physical forms, then cross-classifies them as low-, middle- and high-étage according to cloud-base altitude range above Earth's surface. Clouds with significant vertical extent occupying more than one étage are often considered a distinct group or sub-group. One physical form shows free-convective upward growth into low or vertical heaps of cumulus (cumuliform). Other forms appear as non-convective layered sheets like low stratus (stratiform), and as limited-convective rolls or ripples as with stratocumulus(stratocumuliform). Both of these layered forms have middle- and high-étage variants identified respectively by the prefixes alto- and cirro-. Thin fibrous wisps of cirrus are a physical form found only at high altitudes of the tropopshere (cirriform). In the case of clouds with vertical extent, prefixes are used whenever necessary to express variations or complexities in their physical structures. These include cumulo- for complex highly convective vertical nimbus storm clouds (cumulonimbiform), and nimbo- for thick stratiform layers with sufficient vertical depth to produce moderate to heavy precipitation. This process of cross-classification produces ten basic genus-types or genera, most of which can be subdivided into species and varieties. Synoptic surface weather observations use code numbers to record and report any type of tropospheric cloud visible at scheduled observation times based on its height and physical appearance.

While a majority of clouds form in Earth's troposphere, there are occasions when they can be observed at much higher altitudes in the stratosphere and mesosphere. Clouds that form above the troposphere have common names for their main types, but are sub-classified alpha-numerically rather than with the elaborate system of Latin names given to cloud types in the troposphere. These three main atmospheric layers that can produce clouds, along with the lowest part of the cloudless thermosphere, are collectively known as thehomosphere. Above this lies the heterosphere (which includes the rest of the thermosphere and the exosphere) that marks the transition to outer space. Clouds have been observed on other planets and moons within the Solar System, but, due to their different temperature characteristics, they are often composed of other substances such as methane,ammonia, and sulfuric acid as well as water.

Moon

The Moon (Latin: Luna) is the Earth's only natural satellite. Although not the largest natural satellite in the Solar System, it is, among the satellites of major planets, the largest relative to the size of the object it orbits (its primary)  and, after Jupiter's satellite Io, it is the second most dense satellite among those whose densities are known.

The Moon is in synchronous rotation with Earth, always showing the same face with its near side marked by dark volcanicmaria that fill between the bright ancient crustal highlands and the prominent impact craters. It is the most luminous object in the sky after the Sun. Although it appears a very bright white, its surface is actually dark, with a reflectance just slightly higher than that of worn asphalt. Its prominence in the sky and its regular cycle of phases have, since ancient times, made the Moon an important cultural influence on language, calendars, art, and mythology. The Moon's gravitational influence produces theocean tides and the slight lengthening of the day. The Moon's current orbital distance is about thirty times the diameter of Earth, causing it to have an apparent size in the sky almost the same as that of the Sun. This allows the Moon to cover the Sun nearly precisely in total solar eclipse. This matching of apparent visual size is a coincidence. The Moon's linear distance from Earth is currently increasing at a rate of 3.82±0.07 cm per year, but this rate is not constant.^[9]

The Moon is thought to have formed nearly 4.5 billion years ago, not long after Earth. Although there have been several hypotheses for its origin in the past, the current most widely accepted explanation is that the Moon formed from the debris left over after a giant impact between Earth and a Mars-sized body.

The Moon is the only celestial body other than Earth on which humans have currently set foot. The Soviet Union's Luna programme was the first to reach the Moon with unmanned spacecraft in 1959; the United States' NASA Apollo programachieved the only manned missions to date, beginning with the first manned lunar orbiting mission by Apollo 8 in 1968, and six manned lunar landings between 1969 and 1972, with the first being Apollo 11. These missions returned over 380 kg oflunar rocks, which have been used to develop a geological understanding of the Moon's origin, the formation of its internal structure, and its subsequent history.

After the Apollo 17 mission in 1972, the Moon has been visited by only unmanned spacecraft. Of these, orbital missions have dominated: Since 2004, Japan, China, India, the United States, and the European Space Agency have each sent lunar orbiters, which have contributed to confirming the discovery of lunar water ice in permanently shadowed craters at the poles and bound into the lunar regolith. The post-Apollo era has also seen two rover missions: the final Soviet Lunokhod mission in 1973, and China's ongoing Chang'e 3 mission, which deployed its Yutu rover on 14 December 2013.

Future manned missions to the Moon have been planned, including government as well as privately funded efforts. The Moon remains, under the Outer Space Treaty, free to all nations to explore for peaceful purposes.

sun

The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven withmagnetic fields. It has a diameter of about 1,392,684 km (865,374 mi), around 109 times that of Earth, and its mass (1.989×10³⁰ kilograms, approximately 330,000 times the mass of Earth) accounts for about 99.86% of the total mass of the Solar System. Chemically, about three quarters of the Sun's mass consists of hydrogen, while the rest is mostly helium. The remaining 1.69% (equal to 5,600 times the mass of Earth) consists of heavier elements, including oxygen, carbon, neon and iron, among others.

The Sun formed about 4.567 billion years ago from the gravitational collapse of a region within a large molecular cloud. Most of the matter gathered in the center, while the rest flattened into an orbiting disk that would become the Solar System. The central mass became increasingly hot and dense, eventually initiating thermonuclear fusion in its core. It is thought that almost all starsform by this process. The Sun is a G-type main-sequence star (G2V) based on spectral class and it is informally designated as ayellow dwarf because its visible radiation is most intense in the yellow-green portion of the spectrum, and although it is actually white in color, from the surface of the Earth it may appear yellow because of atmospheric scattering of blue light. In the spectral class label, G2 indicates its surface temperature, of approximately 5778 K (5505 °C, 9941 °F), and V indicates that the Sun, like most stars, is a main-sequence star, and thus generates its energy by nuclear fusion of hydrogen nuclei into helium. In its core, the Sun fuses about 620 million metric tons of hydrogen each second.

Once regarded by astronomers as a small and relatively insignificant star, the Sun is now thought to be brighter than about 85% of the stars in the Milky Way, most of which are red dwarfs. The absolute magnitude of the Sun is +4.83; however, as the star closest to Earth, the Sun is by far the brightest object in the sky with an apparent magnitude of −26.74. This is about 13 billion times brighter than the next brightest star, Sirius, with an apparent magnitude of −1.46. The Sun's hot corona continuously expands in space creating the solar wind, a stream of charged particles that extends to the heliopause at roughly 100 astronomical units. The bubble in the interstellar medium formed by the solar wind, the heliosphere, is the largest continuous structure in the Solar System.

The Sun is currently traveling through the Local Interstellar Cloud (near to the G-cloud) in the Local Bubble zone, within the inner rim of the Orion Arm of the Milky Way. Of the 50 nearest stellar systems within 17 light-years from Earth (the closest being a red dwarf named Proxima Centauri at approximately 4.2 light-years away), the Sun ranks fourth in mass. The Sun orbits the center of the Milky Way at a distance of approximately 24000–26000 light-years from the galactic center, completing one clockwise orbit, as viewed from the galactic north pole, in about 225–250 million years. Since the Milky Way is moving with respect to the cosmic microwave background radiation (CMB) in the direction of the constellation Hydra with a speed of 550 km/s, the Sun's resultant velocity with respect to the CMB is about 370 km/s in the direction of Crater or Leo.

The mean distance of the Sun from the Earth is approximately 1 astronomical unit (about 150,000,000 km; 93,000,000 mi), though the distance varies as the Earth moves from perihelion in January to aphelion in July. At this average distance, light travels from the Sun to Earth in about 8 minutes and 19 seconds. The energy of this sunlight supports almost all life on Earth byphotosynthesis, and drives Earth's climate and weather. The enormous effect of the Sun on the Earth has been recognized sinceprehistoric times, and the Sun has been regarded by some cultures as a deity. An accurate scientific understanding of the Sun developed slowly, and as recently as the 19th century prominent scientists had little knowledge of the Sun's physical composition and source of energy. This understanding is still developing; there are a number of present-day anomalies in the Sun's behavior that remain unexplained.

light

Light is radiant energy, usually referring to electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light is usually defined as having a wavelength in the range of 400 nanometres (nm), or 400×10⁻⁹ m, to 700 nanometres – between the infrared, with longer wavelengths and the ultraviolet, with shorter wavelengths. These numbers do not represent the absolute limits of human vision, but the approximate range within which most people can see reasonably well under most circumstances. Various sources define visible light as narrowly as 420 to 680 to as broadly as 380 to 800 nm. Under ideal laboratory conditions, people can see infrared up to at least 1050 nm, children and young adults ultraviolet down to about 310 to 313 nm.

Primary properties of visible light are intensity, propagation direction, frequency or wavelength spectrum, and polarisation, while itsspeed in a vacuum, 299,792,458 meters per second, is one of the fundamental constants of nature. Visible light, as with all types of electromagnetic radiation (EMR), is experimentally found to always move at this speed in vacuum.^{[citation needed]}

In common with all types of EMR, visible light is emitted and absorbed in tiny "packets" called photons, and exhibits properties of bothwaves and particles. This property is referred to as the wave–particle duality. The study of light, known as optics, is an important research area in modern physics.

In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not. This article focuses on visible light. See the electromagnetic radiation article for the general term.

meteoroid

A meteoroid is a small rocky or metallic body travelling through space. Meteoroids are significantly smaller than asteroids, and range in size from small grains to 1 meter-wide objects. Smaller objects than this are classified as micrometeoroids or space dust. Most are fragments from comets or asteroids, while others are collision impact debris ejected from bodies such as the Moon or Mars.

When such an object enters the Earth's atmosphere at a speed typically in excess of 20 km/s, aerodynamic heating produces a streak of light, both from the glowing object and the trail of glowing particles that it leaves in its wake. At this point, the word meteor can be used to refer to the meteoroid, or to the streak of light caused by the meteoroid. Meteors are colloquially called "shooting stars" or "falling stars". A series of many meteors appearing seconds or minutes apart, and appearing to originate from the same fixed point in the sky, is called ameteor shower. Incoming objects larger than several meters (asteroids or comets) can explode in the air. If a meteoroid, comet or asteroid or a piece thereof withstands ablation from its atmospheric entry and impacts with the ground, then it is called a meteorite.

Around 15,000 tonnes of meteoroids, micrometeoroids and different forms of space dust enter Earth's atmosphere each year.

Thunder

Thunder is the sound caused by lightning. Depending on the distance and nature of the lightning, thunder can range from a sharp, loud crack to a long, low rumble (brontide). The sudden increase in pressure and temperature from lightning produces rapid expansion of the air surrounding and within a bolt of lightning. In turn, this expansion of air creates a sonic shock wave, similar to a sonic boom, which produces the sound of thunder, often referred to as a clap, crack, or peal of thunder. The distance of the lightning can be calculated by the listener based on the time interval from when the lightning is seen to when the sound is heard.

Storm

A storm is any disturbed state of an environment or astronomical body's atmosphere especially affecting its surface, and strongly implying severe weather. It may be marked by significant disruptions to normal conditions such as strong wind, hail,thunder and lightning (a thunderstorm), heavy precipitation (snowstorm, rainstorm), heavy freezing rain (ice storm), strong winds (tropical cyclone, windstorm), or wind transporting some substance through the atmosphere as in a dust storm, blizzard, sandstorm, etc.

Storms generally lead to negative impacts on lives and property such as storm surge, heavy rain or snow (causing flooding or road impassibility), lightning, wildfires, and vertical wind shear; however, systems with significant rainfall can alleviate drought in places they move through. Heavy snowfall can allow special recreational activities to take place which would not be possible otherwise, such as skiing and snowmobiling.

The English word comes from Proto-Germanic *sturmaz meaning "noise, tumult".