- 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 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 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
- 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.
- 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 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.
- 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.
- 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 occurs when a vapor is cooled and transformed to a
more dense liquid state.
- 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
- 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 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.
- 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 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.
- 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, 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 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
- 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
- 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 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
- 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%.
- 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.
- 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 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!!!
- 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 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.
- 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.
- Outgassing occurs when gases are released from materials in the
surface of a planet or other body into the atmosphere of that planet
- 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.
- 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
- 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.
- 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
- 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.
- 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.
- 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
- 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 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 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.
- 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.
- 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 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.
- Vaporization occurs when liquids or solids are heated and converted
to the gaseous state.
- 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.
- The weather is the condition of the atmosphere at a particular time
Last updated: March 11, 1998
Joe Twicken /