Vocabulary List

Absolute zero
Absolute zero is 0 degrees on the Kelvin scale. It is the temperature at which all molecular motion due to thermal energy ceases. It is the lowest possible temperature.
Absorption
Absorption occurs when radiation loses energy as it passes through a medium like an atmosphere. The energy which is lost from the radiation is transfered to the medium.
Acceleration
Acceleration is the rate at which the velocity of an object changes. All objects will be accelerated if a force is applied to them. Acceleration is usually measured in units such as meters per second per second or feet per second per second. These units are equivalent to meters per second2 and feet per second2, where second2 is seconds to the 2nd power. The acceleration due to gravity at the surface of the Earth is about 9.8 meters per second per second.
Atmosphere
An atmosphere is a layer of gases around a planet or natural satellite. The atmospheric gases are held in place by the force of gravity. The large gas giant planets are composed only of gases and do not have a distinct surface. For these planets, the atmosphere is considered to be the outermost layer of gases.
Barometer
A barometer is an instrument used to measure atmospheric pressure. Barometers are often constructed with a glass tube immersed in a pool of Mercury, and they measure the weight of the column of Mercury in the tube which can be supported by atmospheric pressure. The average pressure at the surface of the Earth is approximately 1 bar.
Circulation
Circulation of the atmosphere refers to the circular flow of winds and motion of air masses across a globe. Global circulation takes place in response to uneven heating by the Sun, and it transfers heat energy from lower latitudes to higher latitudes.
Climate
The climate in a particular place is the typical weather observed over a long period of time in that location. Climate includes seasonal weather patterns and also weather extremes.
Composition
The composition of an atmosphere is the makeup of that atmosphere. Usually, the composition is expressed as a list of the prominent gases in the atmosphere and the fraction of the mass or volume of the atmosphere that each gas represents.
Condensation
Condensation occurs when a vapor is cooled and transformed to a more dense liquid state.
Convection
Convective heating takes place when a warm gas rises and carries heat energy away from a heat source. As is rises, the warm gas is replaced by cooler gas from above. This cool gas is then heated and the cycle continues. The warm gas rises because it is less dense than the cooler gas which sinks below it. Earth's troposphere (lowest atmospheric layer) is heated by convection from the warm surface below it.
Coriolis
Objects which are moving on a rotating disk or sphere tend to be deflected from motion in a straight line by the Coriolis force. For instance, objects which move northward or southward from the equator of the Earth tend to be deflected eastward by the Coriolis force. The force can generate very complex circulation patterns in atmospheres of planets which rotate very quickly. On the other hand, the Coriolis force does not complicate the atmospheric circulation of planets like Venus which rotate very slowly.
Density
Density relates the mass of a substance to its volume. If you have a fixed volume of two substances, then the substance which is more dense will be heavier and the substance which is less dense will be lighter. To calulate the density of a substance, divide the mass of the substance by its volume. Density is usually measured in units such as grams per cubic centimeter and kilograms per cubic meter. The density of water is 1 gram per cubic centimeter. The average density of the Earth is about 5.5 grams per cubic centimeter.
Dew Point
The saturation limit of air decreases as the temperature decreases. If moist air is cooled, eventually a temperature is reached where the air becomes saturated and condensation begins to take place. The temperature where the air becomes saturated is called the dew point.
Electromagnetic radiation
Electromagnetic radiation is energy which is transported in waves by changing electric and magnetic fields. Electromagnetic radiation covers a complete spectrum of wavelengths. The longest electromagnetic waves are radio waves, followed by infrared waves, visible light waves, ultraviolet and extreme ultraviolet waves, and X-rays. The shortest electromagnetic waves are gamma rays. All electromagnetic waves, regardless of wavelength, travel at the speed of light, which (in a vacuum) is 186,000 miles per second. As the wavelengths get shorter, the energy carried by electromagnetic waves increases.
Ellipse
An ellipse is a closed curve similar to an oval. If you have an two points called foci, then an ellipse is determined by all points whose total distance from the two foci is a constant. This is in contrast to a circle which is determined by all points whose distance from a single focal point is constant and equal to the radius of the circle. If you compress an ellipse so that the foci come together then a circle results, so a circle is a special case of an ellipse. Planets orbit the Sun in the shape of ellipses, with the Sun at one of the foci of the ellipse. This means that the distance of planets from the Sun changes as they orbit the Sun. The speed at which the planets orbit also changes over the course of the orbit. Some planetary orbits are more elliptical than others. The orbits of Mercury and Venus are almost perfectly circular.
Escape velocity
If an object near the surface of a planet or other body has a sufficiently large velocity, then it will be able to overcome the gravitational attraction of the planet or body and escape to space. The minimum velocity required to escape the gravity of a planet is called the escape velocity of that planet. Near the equator of the Earth, the escape velocity is 11.2 kilometers per second.
Evaporation
Evaporation is a process where a liquid is converted to a less dense vapor state. Heat is removed from the surroundings when a liquid evaporates which reduces the temperature there. When the vapor condenses, the heat lost during evaporation is released.
Fluid
A fluid is a substance which is free to flow such as a liquid or a gas. These substances do not have to flow very quickly, they just have to be free to flow. Water is obviously a fluid, as is the atmosphere. Toothpaste is a fluid too!!!
Flux
Flux, or flux density, is the rate at which radiation reaches a surface of unit size. If you want to know how much radiated power is incident on a given surface, then you multiply the flux density of the radiation on the surface times the area of the surface (which is perpendicular to the direction of the incoming radiation). Flux density is measured in units such as Watts per meter2 or Watts per square meter.
Force
Force is a push or pull applied to an object. When a force is applied, a free object will be accelerated in the direction of the force. Force is usually measured in units of Newtons, but can also be measured in units of pounds. When a person steps on a scale, the force of gravity pulling that person toward the center of the Earth is equal to the weight of the person measured by the scale.
Friction
Friction is a force which exists between two objects or materials which are in contact with one another. The force of friction tends to reduce any relative motion between the objects or materials in contact. For example, when you give a push to an object along the ground, it eventually stops due to friction (unless it hits something first!). If you throw a ball, it slows down due to wind resistance which is actually friction in the atmosphere. Wind speeds in the atmosphere tend to decrease near the surface due to friction. Also, wind which blows across water tends to generate waves and currents due to friction.
Gas
A gas is a collection of molecules which have been heated sufficiently that the molecules are free to wander and expand to fill whatever volume is available to them. When cooled, gases will change their state to liquids and solids.
Gravitational constant
A gravitational force exists between any massive objects. The amount of the force is proportional to the product of the two masses and is inversely proportional to the square of the distance between the objects. The constant of proportionality is called the gravitational constant. It is usually denoted with the symbol G. Its value is 6.67 * 10-11 Newton-meter2 per kilogram2. If you want to calculate the gravitational force between two objects, you need to know this number. Otherwise, you don't need to know it!!! The value means that if there are two 1 kilogram masses and they are 1 meter apart, then the gravitational force between them is equal to 6.67 * 10-11 Newtons.
Gravity
Gravity is a force which attracts massive objects toward each other. The amount of the force depends on the mass of each of the objects and on the distance between them. If the mass of either object increases while the distance between them remains the same, then the gravitational attraction will increase. If the masses remain the same but the distance between the objects increases, then the force of gravity between the objects will decrease.
Greenhouse effect
The Greenhouse effect describes the heating of the atmosphere which takes place when infrared radiation emitted by a warm planetary surface is trapped and absorbed by gases and vapor in the atmosphere. The planetary surface itself is heated by shorter wavelength radiation from the Sun which is free to propagate through the atmosphere. The most common Greenhouse gases are carbon dioxide and water vapor. Some planets have only a modest Greenhouse effect, but it is possible for the trapped infrared radiation to substantially increase the temperature of the atmosphere and surface above what they would otherwise be without the Greenhouse effect.
Humidity
The relative humidity is the ratio of the actual moisture content of the air to the moisture content of saturated air at the same temperature. The relative humidity is usually expressed as a percentage. For example, if the air at some time contains half of the amount of water vapor which could be contained by saturated air at the same temperature, then the relative humidity at that time is 50%. When the air is saturated, then the relative humidity is 100%.
Inversion
Atmospheric layers which are heated by convection from a warm surface or from a warm layer below have a temperature profile where the temperature of the layer decreases as height increases. We know that this is the case in our troposphere because we see snow and ice at high altitudes in the mountains while temperatures may be quite comfortable below. Under certain conditions, a temperature inversion takes place and the temperature increases as height increases. This takes place on Mars, for example, during dust storms where the dust absorbs the incoming Solar radiation and prevents it from reaching low in the atmosphere and to the surface. Temperature inversions can also occur near the surface of a planet at night time when the surface and atmosphere near it are cooled when heat is radiated away to space.
Latitude
Lines of latitude circle a globe parallel to its equator. Latitude is measured in units of degrees. Latitudes above the equator are said to be North latitudes, and those below the equator are said to be South latitudes. In both the North and the South, latitudes extend from 0 degrees at the equator to 90 degrees at the pole. It is possible to locate any point on the surface of a planet uniquely by its latitude and longitude.
Luminosity
Luminosity is the rate at which energy is radiated. It is measured in units such as Watts, although the luminosity of stars is a very large number of Watts!!!
Mass
The mass of an object is the quantity of matter that it contains. The mass of an object does not depend on where it is located, but the weight of an object does matter. The same object weighs more on Earth than it does on Mars even though it has the same mass. Mass is usually expressed in units of grams or kilograms. Mass is occasionally measured in units of pounds.
Meteorology
Meteorology is the study of atmospheres and atmospheric phenomena. The name dates back to the Greek philosopher Aristotle. He wrote a book about atmospheres and weather called Meteorologica.
Molecule
A molecule is the smallest bit of a gas or other compound which can exist and still maintain the characteristics of the gas or substance. Molecules are usually composed of small number of atoms bonded together. When water is referred to as H2O it is because water molecules consist of two atoms of hydrogen (H) bonded with one atom of oxygen (O). Likewise, carbon dioxide is called CO2, and its molecules consist of one atom of carbon (C) bonded with two atoms of oxygen.
Nuclear Fusion
Nuclear fusion is the process of conversion of lighter atoms into heavier atoms. Mass which is lost in the conversion is released as energy. In the core of the Sun, hydrogen nuclei are fused into helium nuclei, and tremendous amounts of energy are released. Great heat is required for nuclear fusion to take place, and the temperature in the core of the Sun is believed to be 15 million Kelvin degrees!!! The radiation produced by fusion in the core of Sun prevents the Sun from collapsing on itself. Once the hygrogen in the core has all been converted into helium, the temperature will not be sufficient for the helium to be fused into more massive nuclei, and the interior of the Sun will collapse.
Outgas
Outgassing occurs when gases are released from materials in the surface of a planet or other body into the atmosphere of that planet or body.
Period
Events which occur with a constant regularity are said to be periodic. Seasonal effects occur yearly so they have a period of one year. The tides occur slightly less often than twice per day so they have a period of about 12 hours. Some phenomena have more than one period. Temperatures fluctuate on a daily basis and also on a yearly basis, so they have periodic components of one day and one year.
Plane
A plane is a flat surface. A single circle or two (distinct) lines or three (distinct) points determine a plane. The Earth orbits the Sun in a plane, called the plane of the ecliptic. The equator of a planet also lies in a plane, typically called the equatorial plane.
Pressure
Pressure is defined as the force acting on an object divided by the area upon which the force is acting. Pressure is expressed in units of Pascals, where 1 Pascal = 1 Newton per square meter. Atmospheric pressure is also expressed in units of bars where 1 bar = 100,000 Pascals. Air pressure can also be measured in units such as psi where 1 psi = 1 pound per square inch.
Psychrometer
A psychrometer is an instrument which uses wet and dry thermometers to determine the the relative humidity of the air. The temperature of wet and dry thermometer bulbs is determined and then the relative humidity is found from a table or chart. The wet bulb temperature is lower than the dry bulb temperature because evaporation cools the wet bulb.
Radius
The radius of a circle is the distance from the center to any point on the circle. The radius of a sphere is the distance from the center of the sphere to any point on the surface.
Rotation axis
Planets spin about a line which extends through their North and South Poles. This line is called the rotation axis of the body. It takes one day on the planet for a rotation to be completed with respect to the Sun.
Satellite
A satellite is an object which orbits a celestial body. If the satellite is not man-made, it is considered to be a natural satellite. The Moon is a satellite of the Earth. Likewise, the Earth can be considered to be a satellite of the Sun.
Saturation
There is a limit to the amount of water vapor which the air can contain. Once this limit is reached, the air is said to be saturated and the relative humidity is 100%. The attempt to add additional vapor to saturated air will result in condensation. The saturation limit increases as the temperature increases, so warm air can contain more water vapor than cold air. Dew generally forms over night when moist air cools and becomes saturated.
Stratification
Objects which form in layers, or strata, are said to be stratified. Stratification is very evident, for example, in the Grand Canyon of Arizona. Many geologic ages are represented in the many layers of rock and sediment there. Atmospheres tend to be stratified as well. They lowest layer of Earth's atmosphere is called the troposphere, followed by the stratosphere, the mesosphere and the thermosphere.
Sublimation
Sublimation occurs when a solid is heated and converted directly to a gaseous state without passing through a liquid phase. On Mars, the carbon dioxide ice in the Poles sublimates into the atmosphere during the northern and southern Springs. The process where a gas is cooled and converted directly to a solid is called deposition.
Surface gravity
When a force acts on an object, that object is accelerated in the direction of the force. The force of gravity accelerates free objects toward the center of the attracting body. Surface gravity is the acceleration due to gravity at the surface of a body. The surface gravity of the Earth is about 9.8 meters per second per second. This means that the velocity of an object dropped near the surface of the Earth will increase by 9.8 meters per second for every second that the object is falling.
Temperature
Temperature is a measure of the average velocity of molecules or atoms in a gas or other substance. If the average velocity of the molecules or atoms increases, then the temperature is said to increase. If the average velocity decreases, then the temperature is said to decrease. Temperature is commonly measured in degrees on the Fahrenheit, Celsius and Kelvin scales.
Terrestrial
The terrestrial planets are the inner planets of the Solar System. These include Mercury, Venus, Earth and Mars. The terrestrial planets are made of rocky material and have iron cores. They have no rings and few (if any) moons. The word terrestrial means of the Earth.
Thermometer
A thermometer is a device used for measuring temperature. In common thermometers, a liquid such as mercury is placed in a glass tube, and the liquid rises and falls due to changes in temperature. The temperature can be read in one of a number of temperature scales, such as Fahrenheit, Celsius and Kelvin.
Transpiration
Transpiration is a complicated process where water from the leaves of plants is lost to the atmosphere as water vapor. Most water vapor enters the atmosphere by evaporation but some also enters the atmosphere through transpiration. Water is returned from the atmosphere by condensation of water vapor and precipitation.
Vaporize
Vaporization occurs when liquids or solids are heated and converted to the gaseous state.
Velocity
The velocity of an object is the speed at which it is moving. Velocity is often measured in units such as meters per second, feet per second and miles per hour.
Weather
The weather is the condition of the atmosphere at a particular time and place.

Last updated: March 11, 1998
Joe Twicken / joe@nova.stanford.edu