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A, or a, is the first letter and the first vowel of the modern English alphabet and the ISO basic Latin alphabet.

Its name in English is a (pronounced ), plural aes.

It is similar in shape to the Ancient Greek letter alpha, from which it derives.

The uppercase version consists of the two slanting sides of a triangle, crossed in the middle by a horizontal bar.

The lowercase version can be written in two forms: the double-storey a and single-storey ɑ.

The latter is commonly used in handwriting and fonts based on it, especially fonts intended to be read by children, and is also found in italic type.

In the English grammar, "a", and its variant "an", are indefinite articles.

History The earliest certain ancestor of "A" is aleph (also written 'aleph), the first letter of the Phoenician alphabet, which consisted entirely of consonants (for that reason, it is also called an abjad to distinguish it from a true alphabet).

In turn, the ancestor of aleph may have been a pictogram of an ox head in proto-Sinaitic script influenced by Egyptian hieroglyphs, styled as a triangular head with two horns extended.

When the ancient Greeks adopted the alphabet, they had no use for a letter to represent the glottal stop—the consonant sound that the letter denoted in Phoenician and other Semitic languages, and that was the first phoneme of the Phoenician pronunciation of the letter—so they used their version of the sign to represent the vowel , and called it by the similar name of alpha.

In the earliest Greek inscriptions after the Greek Dark Ages, dating to the 8th century BC, the letter rests upon its side, but in the Greek alphabet of later times it generally resembles the modern capital letter, although many local varieties can be distinguished by the shortening of one leg, or by the angle at which the cross line is set.

The Etruscans brought the Greek alphabet to their civilization in the Italian Peninsula and left the letter unchanged.

The Romans later adopted the Etruscan alphabet to write the Latin language, and the resulting letter was preserved in the Latin alphabet that would come to be used to write many languages, including English.

Typographic variants During Roman times, there were many variant forms of the letter "A".

First was the monumental or lapidary style, which was used when inscribing on stone or other "permanent" media.

There was also a cursive style used for everyday or utilitarian writing, which was done on more perishable surfaces.

Due to the "perishable" nature of these surfaces, there are not as many examples of this style as there are of the monumental, but there are still many surviving examples of different types of cursive, such as majuscule cursive, minuscule cursive, and semicursive minuscule.

Variants also existed that were intermediate between the monumental and cursive styles.

The known variants include the early semi-uncial, the uncial, and the later semi-uncial.

At the end of the Roman Empire (5th century AD), several variants of the cursive minuscule developed through Western Europe.

Among these were the semicursive minuscule of Italy, the Merovingian script in France, the Visigothic script in Spain, and the Insular or Anglo-Irish semi-uncial or Anglo-Saxon majuscule of Great Britain.

By the 9th century, the Caroline script, which was very similar to the present-day form, was the principal form used in book-making, before the advent of the printing press.

This form was derived through a combining of prior forms. 15th-century Italy saw the formation of the two main variants that are known today.

These variants, the Italic and Roman forms, were derived from the Caroline Script version.

The Italic form, also called script a, is used in most current handwriting; it consists of a circle and vertical stroke on the right ("ɑ").

This slowly developed from the fifth-century form resembling the Greek letter tau in the hands of medieval Irish and English writers.

The Roman form is used in most printed material; it consists of a small loop with an arc over it ("a").

Both derive from the majuscule (capital) form.

In Greek handwriting, it was common to join the left leg and horizontal stroke into a single loop, as demonstrated by the uncial version shown.

Many fonts then made the right leg vertical.

In some of these, the serif that began the right leg stroke developed into an arc, resulting in the printed form, while in others it was dropped, resulting in the modern handwritten form.

Graphic designers refer to the Italic and Roman forms as "single decker a" and "double decker a" respectively.

Italic type is commonly used to mark emphasis or more generally to distinguish one part of a text from the rest (set in Roman type).

There are some other cases aside from italic type where script a ("ɑ"), also called Latin alpha, is used in contrast with Latin "a" (such as in the International Phonetic Alphabet).

Use in writing systems English In modern English orthography, the letter represents at least seven different vowel sounds: the near-open front unrounded vowel as in pad; the open back unrounded vowel as in father, which is closer to its original Latin and Greek sound; the diphthong as in ace and major (usually when is followed by one, or occasionally two, consonants and then another vowel letter) – this results from Middle English lengthening followed by the Great Vowel Shift; the modified form of the above sound that occurs before , as in square and Mary; the rounded vowel of water; the shorter rounded vowel (not present in General American) in was and what; a schwa, in many unstressed syllables, as in about, comma, solar.

The double sequence does not occur in native English words, but is found in some words derived from foreign languages such as Aaron and aardvark.

However, occurs in many common digraphs, all with their own sound or sounds, particularly , , , , and . is the third-most-commonly used letter in English (after and ) and French, the second most common in Spanish, and the most common in Portuguese.

About 8.167% of letters used in English texts tend to be ; the number is around 7.636% in French, 11.525% in Spanish, and 14.634% for Portuguese.

Other languages In most languages that use the Latin alphabet, denotes an open unrounded vowel, such as , , or .

An exception is Saanich, in which (and the glyph Á) stands for a close-mid front unrounded vowel .

Other systems In phonetic and phonemic notation: in the International Phonetic Alphabet, is used for the open front unrounded vowel, is used for the open central unrounded vowel, and is used for the open back unrounded vowel. in X-SAMPA, is used for the open front unrounded vowel and is used for the open back unrounded vowel.

Other uses In algebra, the letter a along with various other letters of the alphabet is often used to denote a variable, with various conventional meanings in different areas of mathematics.

Moreover, in 1637, René Descartes "invented the convention of representing unknowns in equations by x, y, and z, and knowns by a, b, and c", and this convention is still often followed, especially in elementary algebra.

In geometry, capital A, B, C etc. are used to denote segments, lines, rays, etc.

A capital A is also typically used as one of the letters to represent an angle in a triangle, the lowercase a representing the side opposite angle A.

"A" is often used to denote something or someone of a better or more prestigious quality or status: A-, A or A+, the best grade that can be assigned by teachers for students' schoolwork; "A grade" for clean restaurants; A-list celebrities, etc.

Such associations can have a motivating effect, as exposure to the letter A has been found to improve performance, when compared with other letters.

"A" is used as a prefix on some words, such as asymmetry, to mean "not" or "without" (from Greek).

In English grammar, "a", and its variant "an", is an indefinite article, used to introduce noun phrases.

Finally, the letter A is used to denote size, as in a narrow size shoe, or a small cup size in a brassiere.

Albedo (; ) is the measure of the diffuse reflection of solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body that reflects all incident radiation.

Surface albedo is defined as the ratio of radiosity Je to the irradiance Ee (flux per unit area) received by a surface.

The proportion reflected is not only determined by properties of the surface itself, but also by the spectral and angular distribution of solar radiation reaching the Earth's surface.

These factors vary with atmospheric composition, geographic location, and time (see position of the Sun).

While bi-hemispherical reflectance is calculated for a single angle of incidence (i.e., for a given position of the Sun), albedo is the directional integration of reflectance over all solar angles in a given period.

The temporal resolution may range from seconds (as obtained from flux measurements) to daily, monthly, or annual averages.

Unless given for a specific wavelength (spectral albedo), albedo refers to the entire spectrum of solar radiation.

Due to measurement constraints, it is often given for the spectrum in which most solar energy reaches the surface (between 0.3 and 3 μm).

This spectrum includes visible light (0.4–0.7 μm), which explains why surfaces with a low albedo appear dark (e.g., trees absorb most radiation), whereas surfaces with a high albedo appear bright (e.g., snow reflects most radiation).

Albedo is an important concept in climatology, astronomy, and environmental management (e.g., as part of the Leadership in Energy and Environmental Design (LEED) program for sustainable rating of buildings).

The average albedo of the Earth from the upper atmosphere, its planetary albedo, is 30–35% because of cloud cover, but widely varies locally across the surface because of different geological and environmental features.

The term albedo was introduced into optics by Johann Heinrich Lambert in his 1760 work Photometria.

Terrestrial albedo Any albedo in visible light falls within a range of about 0.9 for fresh snow to about 0.04 for charcoal, one of the darkest substances.

Deeply shadowed cavities can achieve an effective albedo approaching the zero of a black body.

When seen from a distance, the ocean surface has a low albedo, as do most forests, whereas desert areas have some of the highest albedos among landforms.

Most land areas are in an albedo range of 0.1 to 0.4.

The average albedo of Earth is about 0.3.

This is far higher than for the ocean primarily because of the contribution of clouds.

Earth's surface albedo is regularly estimated via Earth observation satellite sensors such as NASA's MODIS instruments on board the Terra and Aqua satellites, and the CERES instrument on the Suomi NPP and JPSS.

As the amount of reflected radiation is only measured for a single direction by satellite, not all directions, a mathematical model is used to translate a sample set of satellite reflectance measurements into estimates of directional-hemispherical reflectance and bi-hemispherical reflectance (e.g.,).

These calculations are based on the bidirectional reflectance distribution function (BRDF), which describes how the reflectance of a given surface depends on the view angle of the observer and the solar angle.

BDRF can facilitate translations of observations of reflectance into albedo.

Earth's average surface temperature due to its albedo and the greenhouse effect is currently about .

If Earth were frozen entirely (and hence be more reflective), the average temperature of the planet would drop below .

If only the continental land masses became covered by glaciers, the mean temperature of the planet would drop to about .

In contrast, if the entire Earth was covered by water – a so-called ocean planet – the average temperature on the planet would rise to almost .

In 2021, scientists reported that Earth dimmed by ~0.5% over two decades (1998-2017) as measured by earthshine using modern photometric techniques.

This may have both been co-caused by climate change as well as a substantial increase in global warming.

However, the link to climate change has not been explored to date and it is unclear whether or not this represents an ongoing trend.

White-sky, black-sky, and blue-sky albedo For land surfaces, it has been shown that the albedo at a particular solar zenith angle θi can be approximated by the proportionate sum of two terms: the directional-hemispherical reflectance at that solar zenith angle, , sometimes referred to as black-sky albedo, and the bi-hemispherical reflectance, , sometimes referred to as white-sky albedo. with being the proportion of direct radiation from a given solar angle, and being the proportion of diffuse illumination, the actual albedo (also called blue-sky albedo) can then be given as: This formula is important because it allows the albedo to be calculated for any given illumination conditions from a knowledge of the intrinsic properties of the surface.

Examples of terrestrial albedo effects Illumination Albedo is not directly dependent on illumination because changing the amount of incoming light proportionally changes the amount of reflected light, except in circumstances where a change in illumination induces a change in the Earth's surface at that location (e.g. through melting of reflective ice).

That said, albedo and illumination both vary by latitude.

Albedo is highest near the poles and lowest in the subtropics, with a local maximum in the tropics.

Insolation effects The intensity of albedo temperature effects depends on the amount of albedo and the level of local insolation (solar irradiance); high albedo areas in the Arctic and Antarctic regions are cold due to low insolation, whereas areas such as the Sahara Desert, which also have a relatively high albedo, will be hotter due to high insolation.

Tropical and sub-tropical rainforest areas have low albedo, and are much hotter than their temperate forest counterparts, which have lower insolation.

Because insolation plays such a big role in the heating and cooling effects of albedo, high insolation areas like the tropics will tend to show a more pronounced fluctuation in local temperature when local albedo changes.

Arctic regions notably release more heat back into space than what they absorb, effectively cooling the Earth.

This has been a concern since arctic ice and snow has been melting at higher rates due to higher temperatures, creating regions in the arctic that are notably darker (being water or ground which is darker color) and reflects less heat back into space.

This feedback loop results in a reduced albedo effect.

Climate and weather Albedo affects climate by determining how much radiation a planet absorbs.

The uneven heating of Earth from albedo variations between land, ice, or ocean surfaces can drive weather.

Albedo–temperature feedback When an area's albedo changes due to snowfall, a snow–temperature feedback results.

A layer of snowfall increases local albedo, reflecting away sunlight, leading to local cooling.

In principle, if no outside temperature change affects this area (e.g., a warm air mass), the raised albedo and lower temperature would maintain the current snow and invite further snowfall, deepening the snow–temperature feedback.

However, because local weather is dynamic due to the change of seasons, eventually warm air masses and a more direct angle of sunlight (higher insolation) cause melting.

When the melted area reveals surfaces with lower albedo, such as grass, soil, or ocean, the effect is reversed: the darkening surface lowers albedo, increasing local temperatures, which induces more melting and thus reducing the albedo further, resulting in still more heating.

Snow Snow albedo is highly variable, ranging from as high as 0.9 for freshly fallen snow, to about 0.4 for melting snow, and as low as 0.2 for dirty snow.

Over Antarctica snow albedo averages a little more than 0.8.

If a marginally snow-covered area warms, snow tends to melt, lowering the albedo, and hence leading to more snowmelt because more radiation is being absorbed by the snowpack (the ice–albedo positive feedback).

Just as fresh snow has a higher albedo than does dirty snow, the albedo of snow-covered sea ice is far higher than that of sea water.