Zenith - The imaginary point on the dome-shaped sky that is directly above a place on the Earth's ground surface.

Zibar - A type of sand dune that has a low profile and a poorly formed slip face. These features seem to be associated with coarse-textured sand.

Zonal - The movement of wind or ocean waters in a direction roughly parallel to the lines of latitude.

Zonal Circulation - Any circulation of the atmosphere or ocean that is roughly parallel to the lines of latitude. 

Zonal Index - The strength of the surface westerlies and connected upper air geostrophic wind as measured by determining the surface atmospheric pressure difference between 35° and 55° N latitude. A zonal index of over 8 millibars indicates a strong westerly component in the direction of the geostrophic wind. A zonal index of less than 3 millibars produces a weak geostrophic wind that wants to flow meridionally. See index cycle.

Zonal Soil - A soil that has developed its particular characteristics primarily from the pedogenic influence of climate and vegetation. Compare with an azonal soil and an intrazonal soil.

Zonation - A term frequently used in Physical Geography to describe a pattern in some phenomenon that occurs in regions correlated to another environmental variable. For example, a zonal distribution pattern is often observed in plant communities along an altitudinal gradient.

Zone of Ablation - The area on a glacier where ice losses from melting, evaporation, and sublimation exceed additions of snow annually. The ablation zone is often defined by elevation because air temperature decreases with height.

Zone of Accumulation - The area on a glacier where additions of snow exceed losses of ice from melting, evaporation, and sublimation. The zone of accumulation is often defined by elevation because air temperature decreases with height.

Zone of Aeration - Horizontal zone that extends from the top of the water table to the ground surface. Soil and rock pore spaces in this zone may or may not store water. In this zone, water entering the ground surface percolates downward under gravity and temporarily fills soil pore space. Water leaves the pore space in this zone when gravity moves it into the zone of saturation, by plant root water consumption and transpiration, and by evaporation from the soil surface. Also called an aeration zone.

Zone of Saturation - A groundwater zone within the Earth's bedrock where all available pore spaces are filled by water. Found beneath the water table. Also called the saturation zone.

Zoogeography - The scientific study of animals in relation to geography. Zoogeography examines the spatial distribution of animals over the Earth's surface.

Zooplankton - Small heterotrophic organisms found inhabiting aquatic ecosystems. Also see plankton and phytoplankton.