Wallace's Line - A boundary that exists between Asia and Australia related to the geographical distribution of animal and plant species originating from the two areas. The geographical distribution of most species from each area is limited to either side of Wallace's line. This relationship is strong for most animal species. Many exceptions exist for plant species. First suggested by Alfred Russel Wallace in 1859.

Wallace's Realms - The division of the terrestrial surface of the Earth into six geographical regions (Nearctic, Neotropic, Afrotropic, Paletropic, Indomalaya, and  Australasia) that are the result of geologic processes and that share species with a similar evolutionary history. It was first proposed by Alfred Russel Wallace in 1876. Wallace's work has been updated and improved over the years and is now referred to as faunal realms. The concept of faunal realms has recently been updated by a study (2013) that analyzed the geographical distributions and phylogenetic relationships of 21,037 species of amphibians, birds, and mammals. In this work, eleven zoogeographical realms were recognized. See http://www.sciencemag.org/content/339/6115/74.abstract.

Warm Desert - A type of desert found in the subtropics or interiors of continents at the mid-latitudes where precipitation is low, and surface air temperatures can become extremely hot.

Warm Front - A transition zone in the atmosphere where an advancing warm air mass displaces a cold air mass. A wide band of cloud development and light precipitation usually occurs ahead of the front. Warm fronts are associated with mid-latitude cyclones. Compare with a cold front and an occluded front.

Warm Low - A cyclonic low pressure system that has a central pool of warm air that extends upward from the ground surface. Also called a warm-core low or a warm-core cyclone. Compare with a cold low.

Warm Sector - Is the area found between the cold front and warm front of a mid-latitude cyclone. The leading edge of this air forms the warm front. Winds in this region are generally from south to southwest in the Northern Hemisphere and from the north to the northwest in the Southern Hemisphere.

Warping - The deformation of the Earth's crust without the occurrence of folding and faulting.

Wash - (1) Coarse alluvial sediments. (2) The downslope movement of small particles of soil by overland flow. Also called sheetwash. (3) A term used in the United States for a shallow intermittent stream channel found in arid and semi-arid regions.

Washboard Moraine - A landform feature created by glaciers that consists of a series of successive ridges composed of till that are several meters (feet) tall. The ridges run at right angles to the direction of glacial ice movement.

Water Balance - An equation or table that accounts for the various components of water inputs, outputs, and storage for a location or region over an interval of time. Some common components of a water balance include precipitation, evaporation, transpiration, runoff, soil moisture, groundwater, and groundwater flow. 

Water Consumption - The complete removal of water from a source, like groundwater, for use by humans. This water is not returned to the source. Compare with water withdrawal.

Water Mass - A region of water found within an ocean with similar temperature, density, salinity, and other physical and chemical characteristics.

Water Table - The top surface of groundwater.

Water Withdrawal - The removal of water from some type of source, like groundwater, for some use by humans. This type of water is returned after use. Often, the condition of the returned water differs from when it was originally removed. Returned water may be changed in terms of temperature or chemical quality. Compare with water consumption.

Waterfall - (1) A location in the long profile of a stream where water flows vertically. A nickpoint. (2) A drop in elevation that causes a stream's discharge to flow vertically.

Watershed - (1) In Canada and the USA, this term often means drainage basin. (2) In the United Kingdom, this phrase describes the elevational boundary between adjacent drainage basins (see drainage divide).

Waterspout - A vortex of rapidly moving air that forms over a body of water and is associated with a strong updraft created by a thunderstorm. Waterspouts have the following differences when compared to their land counterparts, the tornado: 1) they tend to form mainly in the tropics and subtropics; 2) they are weaker in terms of wind speed; and 3) they can form even under a simple cumulus-type cloud. 

Watt (W) - A metric unit of power measurement.  This unit is defined as one joule per second and is used to measure energy transfer or conversion. Often used to measure the intensity of electromagnetic radiation over a two-dimensional surface. A surface area of 1 square meter is commonly used for this measurement (W/m² or Wm^-2). 

Wave - (1) A moving swell or ridge on a solid or liquid surface or within the medium of a gas. (2) A periodic disturbance of particles found in a substance that occurs without any net movement of the particles. For example, the movement of sound waves. (3) A pattern of travel exhibited in electromagnetic radiation. 

Wave Action - The energy and force imparted by the motion of waves.

Wave Clouds - A type of repetitive cloud pattern that forms on the lee side of some large topographical obstructions to airflow. Their formation is caused by an oscillating internal wave in the atmosphere.

Wave Crest - The curved tops or ridges of an oscillating wave.

Wave Cut Notch - A rock recess at the foot of a sea cliff where the energy of water waves is concentrated. 

Wave Cut Platform - A flat or slightly sloping bedrock surface that forms in the tidal zone. This feature is produced by wave erosion. Also called a shore platform.

Wave Cyclone - See mid-latitude cyclone.

Wave Height - Vertical distance between a wave's trough and crest.

Wave Period - The time elapsed for a wave to travel the distance of one wavelength.

Wave Refraction - The re-orientation of a wave so that it approaches a shoreline at a more perpendicular angle. This process is caused by the differential reduction of water depth as a linear wave approaches a curved shoreline. A reduction in water depth slows the waves, which in turn causes them to approach a nonlinear shoreline and bend with the shore's shape.

Wave Trough - Area between wave crests. 

Wavelength - Distance between two successive wave crests or troughs.

Weather - The state of the atmosphere as measured by meteorological variables and observed atmospheric phenomena for a particular place or region, for a moment or period in time.

Weather Forecast - The science of predicting the future state of the weather for a particular place or region, for a moment or period in time. Today, this task is accomplished using measurements from weather stations, satellite and radar imagery, and advanced mathematical models that simulate the processes operating in Earth's climate on powerful computers.

Weather Map - A type of map that displays the condition of the physical state of the atmosphere and its circulation at a specific time over a region of the Earth. Weather maps can display the values of many standard meteorological variables measured at weather stations. 

Weather Station - A facility stocked with meteorological instruments and data recording equipment used to measure and monitor weather and climate conditions in the atmosphere. Many weather stations are part of a network of facilities across a country, and the data they collect is used for local, regional, and international weather forecasting. Typical instruments found at a weather station include a maximum thermometer, minimum thermometer, barometer, hygrometer, anemometer, and a standard rain gauge. 

Weather Station Buoy - A portable weather station that floats on the ocean surface and records weather and climate data. Some of the weather factors measured include air temperature above the ocean surface, air pressure, wind direction, and wind speed. They may also measure ocean surface temperature (up to a depth of 3 meters or 9.8 feet), wave period, and wave height. Data collected by these weather stations is transmitted by radio, cellular, or satellite transmissions to land-based receiving stations.

Weathering - The physical, chemical, or biological breakdown of rocks and minerals into smaller-sized particles.

Weathering Landform - A type of common landform created by the physical or chemical decomposition of rock through weathering. Weathering produces landforms by decomposing and disintegrating rocks and sediments. This includes landforms with some of the following geomorphic features: karst, patterned ground, and soil profiles. 

Weathering Rind - A chemically altered surface zone found on a usually discrete piece of rock formed by weathering. Sometimes this zone will have a different color from the rock's interior mass.

Weight - A measure of the force of gravity acting on an object with mass.

Weir - A human-made barrier placed across a stream for the purpose of modifying its water flow characteristics. Changes in flow characteristics are made to lessen flooding, create reservoirs of water, and improve boat navigation. 

Well - A vertical hole humans bore into the ground to access groundwater.

Wentworth Scale - Is a system of classification used in soil science, geology, and sedimentology to describe the sizes of mineral particles found in sedimentary rocks, sediment, and soils. It was first suggested by Chester K. Wentworth in 1922. Standard particle diameter sizes in the Wentworth Scale are clay - less than 0.0039 millimeters; silt - 0.0039 to 0.0625 millimeters; sand - 0.0625 to 2.0 millimeters; granule - 2.0 to 4.0 millimeters; pebble - 4.0 to 64 millimeters; cobble - 64 to 256 millimeters; and boulder - greater than 256 millimeters.

Westerlies - Dominant surface winds of the mid-latitudes. These winds move from the subtropical highs to the subpolar lows from west to east.

Western Boundary Current - A boundary ocean current found along the eastern margin of Earth's major continental masses. Subtropical western boundary currents flow from the mid-latitudes to the equator and transport relatively warm seawater. Examples of such currents include the Gulf Stream and the Brazil Current. Polar western boundary currents flow from the mid-latitudes to one of the poles and transport relatively cold seawater. Compare with the eastern boundary current.

Wet Adiabatic Lapse Rate - See saturated adiabatic lapse rate. 

Wet Deposition - The transport of gases and minute liquid and solid particles from the atmosphere to the ground surface with the aid of precipitation or fog. Compare with dry deposition.

Wet-Bulb Depression - The value calculated by subtracting a wet-bulb thermometer reading from a dry-bulb thermometer reading. Used to determine the air's relative humidity or dew point from a psychrometric table.

Wet-Bulb Thermometer - One of two thermometers on a sling psychrometer that has a moistened cotton wick on its reservoir bulb. When ventilated, this thermometer records a temperature that is modified by the cooling effects of evaporation. This measurement and the temperature reading from a dry-bulb thermometer are then used to determine the air's relative humidity or dew point from a psychrometric table.

Wetland - A common natural land-cover type that is covered by seawater or freshwater for some time. This land type can be identified by the presence of particular plant species or characteristic conditions.

Wetted Perimeter - The total distance from one bank along the stream bed to the opposite bank that is in contact with water held in the stream channel.

Wetting and Drying - A physical weathering process where rocks are mechanically disintegrated by accumulating successive layers of water molecules in between the mineral grains of a rock. Sometimes called slaking.

Wetting Front - A zone within a soil saturated with water, and where the water originated from surface infiltration input after a precipitation event. Wetting fronts are mobile and generally move downward through the soil profile over time.  

Whirlpool - Describes a vortex of water or air rotating inward to some central point. This term is often applied to vortices that develop in water bodies. In water bodies, whirlpools are often formed by the meeting of two currents flowing in opposite directions, or by water flowing downward into a space that creates flow. Shown is a whirlpool that developed in a pond. Image Source: Wikimedia Commons, photo by Daniel Gran. This image is licensed under the Creative Commons Attribution 3.0 Unported license.

Wien's Law - This radiation law suggests that the wavelength of maximum emission of a body is inversely proportional to its absolute temperature. The following equation mathematically describes this law: λmax = C/T, where λmax is the body's maximum emitted wavelength of radiation in micrometers (µm), C is a constant equal to 0.2897, and T is the body’s temperature in Kelvins.

Wilderness - A term that refers to a place or region on our planet that has not been obviously altered by human activity.

Wilting Point - The point at which the rate of water leaving a plant's leaves is greater than the water uptake by the roots. At this point, the plant will fail to recover its turgidity.

Wind - A mass of air moving horizontally and/or vertically.

Wind Chill - The human-perceived decrease in air temperature due to the effects of wind. Wind chill is always colder than the actual temperature and is calculated in several different ways. All of these methods assume that wind removes heat from a body.  

Wind Direction - The direction from which a wind blows. Wind direction is usually measured in cardinal and intercardinal directions or degrees azimuth.

Wind Gust - A sudden and brief increase in wind speed.

Wind Ripples - Wind ripples are miniature sand dunes between 50 millimeters (2.0 inches) and 2 meters (6.6 feet) in length and 1 to 50 millimeters (0.04 to 2.0 inches) in height. They are created by saltation when the sand grains are of similar size, and the wind has a constant speed. Also called sand ripples.

Wind Rose - A type of graph used by meteorologists and climatologists to quantify wind direction patterns and wind speed for a specific location. The graph consists of a series of circles of different sizes, aligned around a central point. The distance between circles is used for proportional or absolute measurement. Bars extending from the center represent a direction being measured. 

Wind Shadow - The area located downwind of an object that can block and modify wind flow. Humans have used this fact to reduce erosion caused by wind. In agricultural settings, hedgerows and rows of trees have been used as windbreaks to protect soil from the erosive effects of prevailing winds.

Wind Shear - Is a significant horizontal and/or vertical variation in wind speed and/or direction over a distance. The distance over which wind shear can occur varies from a few meters (feet) to many kilometers (miles). Some common locations in the atmosphere where wind shear occurs include air mass fronts, jet streams, mountains, temperature inversions, and thunderstorms. Also called wind gradient.

Wind Speed - The speed at which the wind is blowing. Wind speed is usually measured in meters per second, kilometers per hour, feet per second, or miles per hour.

Wind Vane - A mechanical device used to measure the direction of wind flow. It usually consists of a horizontal bar with a fin at one end and an aerodynamic pointer at the other end. The center of the horizontal bar is attached to a vertical spindle, which is connected to a mechanical device that records direction.

Windstorm - A surface disturbance or storm in the atmosphere that generates fast-moving winds, usually with little or no precipitation.

Windward - The upwind side of an object or the side directly influenced by the direction of wind flow. The opposite of leeward.

Winter - The season between fall and spring. Astronomically, it is the period from the December Solstice to the March Equinox in the Northern Hemisphere and from the June Solstice to the September Equinox in the Southern Hemisphere. The season between fall and spring. 

Winter Solstice - The winter solstice denotes the first day of the winter season. In the Northern Hemisphere, the winter solstice falls on either December 21 or 22 (depending on the year). June 21 or 22 is the date of the winter solstice in the Southern Hemisphere. During the winter solstice, locations in their respective hemispheres experience the shortest day of the year.

Work - In physics, work, in its simplest sense, occurs when a force acts on a body, causing it to be displaced in the same direction as the applied force.

Wrench Fault - A type of strike-slip fault where the fault surface is vertical, and the fault blocks are traveling past each other.