Freezing of Lakes - Freezing of Lakes and the Evolution of their Ice Cover - page 32

2.2.4 Air Temperature, Humidity and Wind

Air temperature is a widely used characteristic in geoscience and ecology. It is a quite
stable quantity, its measurement is easy to perform and can be made accurately, and there
are long-term data available for 5 many observation stations around the world. Humidity is
normally expressed as the relative humidity, and it is a key factor in evaporation/con-
densation and sublimation/deposition, which are important in the mass and heat budgets
of lakes. Temperature and humidity are recorded in all weather stations, and their refer-
ence altitude has been de

ned as 2 m above the surface. Wind, measured in most weather
stations, refers to two-dimensional, horizontal air ow and is expressed by its speed and

direction at the standard altitude of 10 m above the surface. 6
The annual cycle of the air temperature largely follows the solar radiation (Table 2.7 ).
In summer, the Sun and warm air heat lakes and the surface temperature reaches maxi-
mum, while in fall the situation is reversed, as lake waters are warmer than the cooling
atmosphere. In general, the amplitude of the annual cycle of air temperature is larger in
land areas than in lake districts. Apart from geothermal and deep lakes, the difference
between the air temperature and surface temperature is small except for transient events.
Thus, for a lake to freeze, in practice the air temperature cannot be much above the
freezing point. As long as the drainage basin is snow-covered, the air temperature can be
signi

fl

C only due to advection of warm air.
In cold climate the absolute humidity of air is small, and therefore mass and heat

cantly above 0 °

uxes
due to evaporation/sublimation or condensation/deposition are much smaller than in warm
climate, order of 1 mm water equivalent 7 per day. But due to the large latent heat of phase
transitions of water, these

fl

cant in the heat balance. In dry and cold
climate regions, also mass losses due to sublimation can be signi

fl fluxes are always signi

cant. Deposition of

water vapour onto ice surface is sometimes observed as beautiful ' frost fl owers ' . How-

ever, it takes place rarely so that it is insigni cant in the heat and mass balance.

Wind velocity shows large local and regional variations depending on the surface
topography, vegetation and strati

cation of the atmospheric surface layer. Therefore wind
velocity is sensitive to local conditions at an observation station, and the difference in
wind between a lake and nearby land station can be large. Sometimes correction factors
are employed to estimate the wind velocity on a lake from land station measurements.

5 Celcius ( ° C) is a convenient temperature unit in investigations of freezing lakes and mostly used in
this topic. The absolute temperature (Kelvin) is the natural scale for thermodynamics and used here
where considered preferable. These units transform by 273.15 K = 0 ° C.
6 Wind direction (WD, degrees) tells from where the wind blows in the compass angle, zero toward
north and turning clockwise. Mathematical right-hand coordinate system has zero direction toward
east (x-axis) and turns counter-clockwise. Wind vector direction (to where the wind blows) is in the
right-hand system 270 ° - WD.
7 Water equivalent refers to the thickness of a layer of liquid water with corresponding volume.