Altitude

Altitude or height has means different things based on how it is used. Common uses include aviation, geometry, geographical survey and sports. As a general definition, altitude is a distance measurement. It is usually in the vertical or "up" direction. It is the difference between a set point (a reference datum) and a point or object. The reference datum also changed depending on how the term altitiude is being used. Altitude is commonly used to mean the height above sea level of a location but the term elevation is used more often.

Vertical distance measurements in the "down" direction are commonly referred to as depth.

Altitude in aviation and in spaceflight

Vertical Distance Comparison

In aviation, the term altitude can have several meanings. Aviation altitude is measured using either Mean Sea (MSL) or local ground level (Above Ground Level, or AGL) as the reference.

Airplanes usually use air pressure to know what their altitude is. The instrument for measuring altitude is the pressure altimeter. It is an aneroid barometer with a front face showing distance (feet or metres) instead of atmospheric pressure.

There are several types of aviation altitude.

In simple terms, these types of altitude can be explained as ways of measuring the altitude:

• Indicated altitude -- the altimeter reading
• Absolute altitude -- altitude in terms of the distance above the ground directly below it
• True altitude -- altitude in terms of elevation above sea level
• Height -- altitude in terms of the distance above a certain point
• Pressure altitude -- altitude in terms of the air pressure
• Density altitude -- altitude in terms of the density of the air

In fuller definitions:

• Indicated altitude is the reading on the altimeter when the altimeter is set to the local barometric pressure at Mean Sea Level.
• Absolute altitude is the height of the aircraft above the terrain it is flying over. ([[Above Ground Level)
• True altitude is the elevation above mean sea level.
• Height is the elevation above a set point on the ground. This point is commonly the terrain elevation.
• Pressure altitude is the elevation above a set air-pressure level. This level is normally 1013.25 millibars or 29.92" Hg and 15°C). Pressure altitude and indicated altitude are the same when the altimeter is set to 29.92" Hg or 1013.25 millibars.
• Density altitude is the altitude when changes have been made to deal with non-ISA International Standard Atmosphere atmospheric conditions. The density of the air affects how well an aircraft works. Density altitude is affected by barometric pressure, humidity and temperature. On a very hot day, density altitude at an airport (especially one at a high elevation) may be so high that aircraft can not take-off. This most often affects helicopters or an aircraft that is carrying a large amount of cargo.

Altitude regions

The Earth's atmosphere is divided into several altitude regions:

• Troposphere — surface to 8,000 metres (5.0 mi) at the north and south Poles – 18,000 metres (11 mi) at the equator.
• Stratosphere — Troposphere to 50 kilometres (31 mi)
• Mesosphere — Stratosphere to 85 kilometres (53 mi)
• Thermosphere — Mesosphere to 675 kilometres (419 mi)
• Exosphere — Thermosphere to 10,000 kilometres (6,200 mi)

High altitude and low air pressure

Regions on the surface of the Earth or in its atmosphere that are high above mean sea level are said to be high altitude. High altitude is often said to begin at 2,400 metres (8,000 ft) above sea level.

At high altitude, atmospheric pressure is lower than that at sea level. This is because of two effects. These effect s are gravity and the heat in the air. Gravity makes the air to be as close as possible to the ground. The amount of heat in the air makes the molecules move quickly. This causes the air to expand.

As the air expands, it rises to where the air pressure is less. As it rises, it begins to cool. Because of this, high altitude air is cold. This causes an alpine climate. This climate affects the ecology at high altitude.

Effects of high altitude on humans

Altitudes above 1,500 metres (4,900 ft) start to affect humans. Humans cannot live in very high altitudes above 5500–6000 metres (18,000–20,000 ft) As altitude increases, atmospheric pressure decreases. This affects humans because there is less oxygen to breathe. The low amount of oxygen above 2,400 metres (8,000 ft) can cause illnesses such as altitude sickness, High altitude pulmonary edema, and High altitude cerebral edema.

The human body can deal with high altitude by breathing faster, having a higher heart rate, and changing its blood chemistry. This can take days or weeks to happen. Above 8,000 metres (26,000 ft), the human body cannot make the needed changes and will die.

People living at higher elevations have a higher rate of suicide. The cause for this is not known so far.

Other websites

• "Altitude pressure calculator". Apex (altitude physiology expeditions). http://www.altitude.org/air_pressure.php. Retrieved 2006-08-08. 
• "The Race to the Stratosphere". U.S. Centennial of Flight Commission. http://www.centennialofflight.gov/essay/Lighter_than_air/race_to_strato/LTA11.htm. Retrieved 25 January 2006. 
• Downloadable ETOPO2 Raw Data Database (2 minute grid)
• Downloadable ETOPO5 Raw Data Database (5 minute grid)
• Calculate true altitude with these JavaScript applications
• Find the altitude of any place