Temperature Related

Thaw is the melting of snow or ice at the Earth’s surface due to a temperature rise above 0°C (WMO, 1992).

A cold wave is a period of marked and unusual cold weather characterised by a sharp and significant drop in air temperatures near the surface (maximum, minimum and daily average) over a large area and persisting below certain thresholds for at least two consecutive days during the cold season (WMO, 2020).

A dzud (a Mongolian term that describes ‘severe winter conditions’’, sometimes spelled zud) is a cold-season disaster in which anomalous climatic (i.e., heavy snow and severe cold) and/or land-surface (snow/ ice cover and lack of pasture) conditions lead to reduced accessibility and/or availability of forage/pastures, and ultimately to high livestock mortality during winter–spring. Severe dzuds (high mortality) result from a combination of growing-season drought and severe weather (Natsagdorj and Dulamsuren, 2001; Nandintsetseg et al., 2017, 2018a,b).

A freeze is an air temperature equal to or less than the freezing point of water (D °C) (adapted from WMO, 1992).

A hoar frost is a deposit of ice produced by the deposition of water vapour from the surrounding air and is generally crystalline in appearance (WMO, 2017).

Freezing rain is rain where the temperature of the water droplets is below 0°C. Drops of supercooled rain may freeze on impact with the ground, in-flight aircraft or other objects (WMO, 2017).

Glaze is a smooth compact deposit of ice, generally transparent, formed by the freezing of super-cooled drizzle droplets or raindrops on objects with a surface temperature below or slightly above 0°C (WMO, 2017).

Ground frost is a covering of ice, in one of its many forms, produced by the sublimation of the water vapour on objects colder than 0°C (WMO, 1992). Ground frost occurs when the temperature of the upper layer of the soil is less than 0°C (WMO, 1992).

A heatwave is a marked warming of the air, or the invasion of very warm air, over a large area; it usually lasts from a few days to a few weeks (WMO, 1992). Alternative definition: A heatwave is a marked unusual period of hot weather over a region persisting for at least two consecutive days during the hot period of the year based on local climatological conditions, with thermal conditions recorded above given thresholds (WMO, 2020).

Icing refers to any deposit or coating of ice on an object caused by the impact of liquid hydrometeors, usually supercooled (WMO, 1992).