Walker Cell

Closed direct zonal tropical circulation of the atmosphere, confined to equatorial regions, mostly driven by oceanic temperature gradient. In the Pacific Ocean, air flows westward from the colder eastern area, to the warm western ocean, where it gains in warmth and moisture, and therefore, rises. Return flow aloft and subsidence over eastern ocean complete the cell.

Walker Circulation

Term used to refer to the cell induced by the contrast between warm and cool waters on the western and eastern sides of the Pacific Ocean, respectively. Sometimes, the term is used to denote the entire chain of east-west equatorial circulation cells that stretches around the globe. The variability in the Walker circulation is associated with the Southern Oscillation.

Warm front

Non-occluded front, which moves in such way that warm air replaces cold air.

Warm occlusion / Warm occluded front

Occlusion in which the cold air behind the occluded front, is warmer than the cold air in advance of it.


Information issued when a hazardous weather or hydrologic event, that represents a threat to life or property, is occurring, is about to occur, or has high probability of occurring.


Information issued when there a significantly increased risk of occurrence of hazardous weather or hydrological event, but its actual occurrence, location and timing remain uncertain.

Water born disease

Disease caused in humans or animals by ingestion or contact with unsafe water, that is where pathogen agents live.

Water cycle

Succession of phases through which water passes in the atmosphere: water evaporates from oceans, seas or inland waters as water vapor, it condensates to form clouds, precipitates again as rain or snow, accumulates in the soil in bodies of water, to then re-start the cycle.


Any tornado (spout), usually a non-supercell one, over a body of water. It has the tendency to dissipate upon reaching shore.


Distance between periodic spatial repetitions of an electromagnetic wave at a given instant of time. Its extensive use relates to the classification of the nature of the radiation.


State of the atmosphere at a particular time, as defined by various meteorological elements, mainly with respect to its influence on life and human activities.

Weather chart / Synoptic chart

Geographical map on which meteorological data, at a specified time, is displayed to describe the atmospheric conditions at the synoptic scale.

Weather extreme

Highest and lowest values of a climatic element, observed during a given time interval or during a given month or season of that period.

Weather forecast

Assessment of expected meteorological conditions (state of the atmosphere regarding precipitation, clouds, winds and temperature) for a specific period, and for a specific area or portion of space.

Weather map

Any graphical representation of meteorological data over a given area of the terrestrial surface.

Weather modification

Generally, intentional or unintentional change in the weather produced by human activities. Usually, it refers to any effort made to artificially alter the natural atmospheric phenomena. Cloud seeding is a typical example of weather modification.

Weather station

Location where meteorological observations are taken, that is, surface, upper air and climatological observations.Offices that prepare weather maps, charts, forecasts, warnings, and climatological information.


Zone situated between latitudes 35° and 65° of each hemisphere, where the west-to-east motion of the atmosphere is dominant, especially in the high troposphere and low stratosphere. Its equatorward boundary is rather well defined by the subtropical high pressure belt; its poleward boundary is quite diffuse and variable.

Widespread ascent


Air motion relative to the Earth´s surface. Generally, it denotes only the horizontal component of wind, since its vertical components are relatively small.

Wind chill

Portion of the cooling of a human body caused by air motion, as it accelerates heat transfer from a human body to the surrounding atmosphere. It comes from the combination of wind speed and air temperature, incident on the human body. That heat loss is expressed in watts per square meter (of skin).

Wind direction

Direction from which the wind blows.

Wind shear

Local variation of the wind vector or any of its components in a given direction. It is the difference in wind speed and/or direction between two points in the atmosphere. Shear can be vertical or horizontal, depending on the location of these two points.

Wind speed / Wind velocity

Ratio of the distance covered by the air to the time taken to cover it.

Wind vane

Instrument or device used to indicate the direction from which the wind is blowing.