Table of Contents

Mars Weather Terms

Aerosol – Particles of matter, solid or liquid, larger than a molecule but small enough to remain suspended in the atmosphere.

Atmosphere – The mass of air surrounding the planet and kept in place by gravitational forces. The lidar on Phoenix can take measurements of activity in the atmosphere as high as 20 kilometres above the surface, depending on conditions.

Boundary layer – The layer of the atmosphere in contact with the surface of the planet. The boundary layer is often turbulent, and its top can be marked by an inversion. The top of the boundary layer can be as low as 1km or possibly as high as 10km on Mars under conditions of convection.

Calm – The absence of apparent movement of the atmosphere near the surface.

Circulation – The pattern of movement in the near-surface atmosphere.

Clouds – On Mars, these are made up of ice crystals formed from water, or, under the exceptionally cold conditions of a Martian winter, carbon dioxide. Dust storms, which can occur on Mars for several months at a time, can turn the clouds a yellow or red colour.

Convection – The vertical transfer of heat and moisture in the air, especially by updrafts and downdrafts in the unstable atmosphere produced by the heating of the surface by the sun in the hottest part of the day.

Diamond dust – This is the name given to the sparkling clouds of tiny ice crystals that appear to fall out of a cloudless sky. The ice crystals can disappear before they reach the ground through a process called sublimation.

Dust – Dust is a major factor in the climate. Dust particles are small and fine, but sharp. Dust can be carried up by winds into the atmosphere in a number of ways. See also dust devil, dust storm, and dust plume. The Canadian-built lidar aboard Phoenix can detect dust in the atmosphere.

Dust devil – A localized whirling wind, like a mini-tornado, which can carry dust up to 10 kms into the atmosphere.

Dust plume – A non-rotating wind raises the dust straight up into the atmosphere.

Dust storm – Loose dust is borne aloft into the atmosphere by high-speed winds. Dust storms can cover very large areas and have been known to cover the entire planet.

Fog – Tiny ice crystals suspended in the air near the planet's surface.

Frost – The formation of ice crystals from water vapour as it comes in contact with a surface at a temperature below the freezing point.

Gravity waves – Oscillations can be produced in the atmosphere under stable conditions (see vertical temperature gradient) when the atmosphere flows over an obstacle such as a mountain. These resemble waves in the ocean and occur in the clear atmosphere so that they are normally invisible to us. However, they can be detected by the MET lidar. Just as in the ocean, these waves can break, causing what is known as clear air turbulence.

Humidity – The amount of water vapour in the local atmosphere. Relative humidity is measured by the Phoenix's Microscopy, Electrochemistry, and Conductivity Analyzer (MECA) thermal and electrical conductivity probe (TECP). Absolute humidity will be taken by the Thermal and Evolved Gas Analyzer (TEGA) instrument.

Ice crystals – A crystalline form of ice so small that they are suspended in the atmosphere. The Meteorological Station's lidar can detect ice crystals.

Pressure – The pressure exerted by the weight of the atmosphere directly above the point being measured. On Mars, the pressure is about 1/100th of Earth's. The unit for measuring this pressure is the kilopascal. Phoenix's Meteorological Station is equipped with a pressure sensor that will measure atmospheric pressure.

Refraction – The bending of light as it passes through areas of different density, such as from air through ice crystals.

Sublimation – The term used for a change from water ice (solid) to water vapour (gas). The process is due to a combination of factors such as radiation from the sun, temperature, pressure, and other atmospheric conditions.

Sun – The planet's surface temperature is governed by heating by solar radiation, moderated by the effects of the atmosphere, which can act like a blanket trapping heat that would otherwise escape to space. Mars has a much thinner atmosphere than Earth so this "Greenhouse Effect" is much weaker. Being further away from the sun means that Mars also receives less solar radiation. These two effects combined mean that temperatures on Mars are, on average, colder than Earth.

Temperature – In the Martian arctic region of the landing site, temperatures can change by as much as 50ºC from night to day since the thin Martian atmosphere does not have the insulating properties of Earth. An average temperature for Mars is about -60ºC. Temperature will be measured by three sensors at different heights on the Meteorological Station's mast (see Vertical temperature gradient for more).

Vertical temperature gradient – The change in temperature with height. The vertical temperature gradient will be measured by the three sensors on the Meteorological Station's mast. If temperature increases with height, cold air will generally remain at the surface; the atmosphere is therefore described as stable, a condition favourable to the formation of inversions, gravity wave activity and down-slope winds. If temperature does not change with height, the atmosphere is described as neutral. If temperature decreases with height, the atmosphere is described as unstable, a condition favourable to convection and the formation of dust plumes and dust devils.

Wind – Winds are caused by the flow of the atmosphere from high pressure to low pressure. Scientists have direct information about the winds on Mars only from the Pathfinder and Viking lander missions. Winds at the Phoenix landing site will be measured by the telltale instrument, which sits at the top of the Meteorological Station's mast. Simulations of the Mars atmosphere suggest that winds here will be fairly light, rarely more than 25 kilometres per hour. However, during sudden gusts and dust storms, winds of up to 483 kilometres per hour may be possible.