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Ancient history is the aggregate of past events from the beginning of writing and recorded human history and extending as far as post-classical history. The phrase may be used either to refer to the period of time or the academic discipline.

The span of recorded history is roughly 5,000 years, beginning with the Sumerian cuneiform script, with the oldest coherent texts from about 2600 BC. Ancient history covers all continents inhabited by humans in the period 3000 BC – AD 500.

The broad term "ancient history" is not to be confused with "classical antiquity". The term classical antiquity is often used to refer to Western history in the Ancient Mediterranean from the beginning of recorded Greek history in 776 BC (first Olympiad). This roughly coincides with the traditional date of the founding of Rome in 753 BC, the beginning of the history of ancient Rome, and the beginning of the Archaic period in Ancient Greece.

The academic term "history" is not to be confused with colloquial references to times past. History is fundamentally the study of the past, and can be either scientific (archaeology, with the examination of physical evidence) or humanistic (the study of history through texts, poetry, and linguistics).

Although the ending date of ancient history is disputed, some Western scholars use the fall of the Western Roman Empire in 476 AD (the most used), the closure of the Platonic Academy in 529 AD, the death of the emperor Justinian I in 565 AD, the coming of Islam, or the rise of Charlemagne as the end of ancient and Classical European history. Outside of Europe, there have been difficulties with the 450–500 time frame for the transition from ancient to post-classical times.

During the time period of ancient history (starting roughly from 3000 BC), the world population was already exponentially increasing due to the Neolithic Revolution, which was in full progress. According to HYDE estimates from the Netherlands, world population increased exponentially in this period. In 10,000 BC in prehistory, the world population had stood at 2 million, rising to 45 million by 3,000 BC. By the rise of the Iron Age in 1,000 BC, the population had risen to 72 million. By the end of the period in 500 AD, the world population is thought to have stood at 209 million. In 3,500 years, the world population increased by 100 times.

INFO: Full form for AH is Asteroid Hunt in Academic & Science category
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INFO: Full form for AH is Auditory Hallucinations in Academic & Science category
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INFO: Full form for AH is Air Handler in Academic & Science category
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INFO: Full form for AH is Agricultural Holding in Academic & Science category
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INFO: Full form for AH is algebra AH in Academic & Science category
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Humidity is the concentration of water vapour present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present.

Humidity depends on the temperature and pressure of the system of interest. The same amount of water vapor results in higher humidity in cool air than warm air. A related parameter is the dew point. The amount of water vapor needed to achieve saturation increases as the temperature increases. As the temperature of a parcel of air decreases it will eventually reach the saturation point without adding or losing water mass. The amount of water vapor contained within a parcel of air can vary significantly. For example, a parcel of air near saturation may contain 28 g (0.99 oz) of water per cubic metre of air at 30 °C (86 °F), but only 8 g (0.28 oz) of water per cubic metre of air at 8 °C (46 °F).

Three primary measurements of humidity are widely employed: absolute, relative, and specific. Absolute humidity is expressed as either mass of water vapor per volume of moist air (in grams per cubic metre) or as mass of water vapor per mass of dry air (usually in grams per kilogram). Relative humidity, often expressed as a percentage, indicates a present state of absolute humidity relative to a maximum humidity given the same temperature. Specific humidity is the ratio of water vapor mass to total moist air parcel mass.

Humidity plays an important role for surface life. For animal life dependent on perspiration (sweating) to regulate internal body temperature, high humidity impairs heat exchange efficiency by reducing the rate of moisture evaporation from skin surfaces. This effect can be calculated using a heat index table, also known as a humidex.

The notion of air "holding" water vapor or being "saturated" by it is often mentioned in connection with the concept of relative humidity. This, however, is misleading—the amount of water vapor that enters (or can enter) a given space at a given temperature is almost independent of the amount of air (nitrogen, oxygen, etc.) that is present. Indeed, a vacuum has approximately the same equilibrium capacity to hold water vapor as the same volume filled with air; both are given by the equilibrium vapor pressure of water at the given temperature. There is a very small difference described under "Enhancement factor" below, which can be neglected in many calculations unless high accuracy is required.

INFO: Full form for AH is Aunts Husband in Academic & Science category
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The history of the United States was preceded by the arrival of Native Americans in North America around 15,000 BC. Numerous indigenous cultures formed, and many disappeared in the 16th century. The arrival of Christopher Columbus in 1492 started the European colonization of the Americas. Most colonies were formed after 1600, and the United States was the first nation whose most distant origins are fully recorded. By the 1760s, the thirteen British colonies contained 2.5 million people and were established along the Atlantic Coast east of the Appalachian Mountains. After defeating France, the British government imposed a series of taxes, including the Stamp Act of 1765, rejecting the colonists' constitutional argument that new taxes needed their approval. Resistance to these taxes, especially the Boston Tea Party in 1773, led to Parliament issuing punitive laws designed to end self-government. Armed conflict began in Massachusetts in 1775.

In 1776, in Philadelphia, the Second Continental Congress declared the independence of the colonies as the "United States". Led by General George Washington, it won the Revolutionary War. The peace treaty of 1783 established the borders of the new nation. The Articles of Confederation established a central government, but it was ineffectual at providing stability as it could not collect taxes and had no executive officer. A convention wrote a new Constitution that was adopted in 1789 and a Bill of Rights was added in 1791 to guarantee inalienable rights. With Washington as the first president and Alexander Hamilton his chief adviser, a strong central government was created. Purchase of the Louisiana Territory from France in 1803 doubled the size of the United States.

Encouraged by the notion of manifest destiny, United States expanded to the Pacific Coast. While the nation was large in terms of area, its population in 1790 was only 4 million. The population grew rapidly and economic growth was even greater. Compared to European powers, the nation's military strength was relatively limited in peacetime before 1940. Westward expansion was driven by a quest for inexpensive land for yeoman farmers and slave owners. The expansion of slavery was increasingly controversial and fueled political and constitutional battles, which were resolved by compromises. Slavery was abolished in all states north of the Mason–Dixon line by 1804, but the South continued the institution, mostly for the production of cotton. Abraham Lincoln was elected president in 1860 on a platform of halting the expansion of slavery. Seven Southern slave states rebelled and created the foundation of the Confederacy. Its 1861 attack on a federal fort started a Civil War. The defeat of the Confederates in 1865 led to the abolition of slavery. In the Reconstruction era following the war, legal and voting rights were extended to freed slaves. The national government emerged much stronger, and gained explicit duty to protect individual rights. However, when white southerners regained their power in the South in 1877, often by paramilitary suppression of voting, they passed Jim Crow laws to maintain white supremacy, as well as new disenfranchising state constitutions that prevented most African Americans and many Poor Whites from voting.

The United States became the world's leading industrial power at the turn of the 20th century, due to an outburst of entrepreneurship and industrialization and the arrival of millions of immigrant workers and farmers. A national railroad network was completed and large-scale mines and factories were established. Mass dissatisfaction with corruption, inefficiency, and traditional politics stimulated the Progressive movement, from the 1890s to the 1920s, leading to reforms, including the federal income tax, direct election of Senators, alcohol prohibition, and women's suffrage. Initially neutral during World War I, the United States declared war on Germany in 1917 and funded the Allied victory the following year. After the prosperous Roaring Twenties, the Wall Street Crash of 1929 marked the onset of the decade-long worldwide Great Depression. President Franklin D. Roosevelt implemented his New Deal programs, including relief for the unemployed, support for farmers, Social Security, and a minimum wage. The New Deal defined modern American liberalism. Following the Japanese attack on Pearl Harbor in 1941, the United States entered World War II and financed the Allied war effort, and helped defeat Nazi Germany and Fascist Italy in the European theater. Its involvement later culminated in using newly invented nuclear weapons on two Japanese cities to defeat Imperial Japan in the Pacific War.

The United States and the Soviet Union emerged as rival superpowers in the aftermath of World War II. During the Cold War, the two countries confronted each other indirectly in th reference

INFO: Full form for AH is After Humanity in Academic & Science category
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