Convection

This window offers the choice of using measurements of wind speed to adjust conductance (hence, rates of heat loss) when calculating metabolic rates.   If this option is selected, a data channel for wind speed must be chosen.   Some points to consider:

  • Metabolic rate is not adjusted for convection when animals are in torpor.   The assumption is that when torpid, animals are in sheltered, wind-free microhabitats.

  • The algorithm for calculating effects of convection comes from a 1983 paper by David Goldstein (Effect of wind on avian metabolic rate with particular reference to Gambel's quail; Physiological Zoology 56: 485-492).   Goldstein (who I know from my time at UCLA in the late 1970s) complied convection data from several bird species (goldfinches to golden eagles; 13 to 3800 g) and derived an equation that predicted metabolism with high accuracy over a range of temperatures and wind speeds.   From his equation, I derived this adjustment to conductance (C):

               Cwind = C * (1 + b * wind speed0.5)

             where b = .0092 * mass0.66 * (LCT - Te)0.32

    (mass in grams; wind speed in m/s; LCT is lower critical temperature and Te is environmental temperature, both in °C)

  • Lower critical temperature is not adjusted for wind effects, although LCT will be higher (i.e., the thermal neutral zone will be reduced) as wind speed increases.   For most species, this error is probably small enough to ignore when estimating energy costs.

  • CAUTION:   Accurate determination of the effects of convection on heat loss is extremly difficult in most natural situations.   The values computed by this equation are based on data from birds in fairly laminar-flow regimes, without (many) confounding influences of turbulence or boundary layer effects.  Additionally, small adjustments to posture, plumage (or pelage), or position can have strong effects on convective exchange.
              Finally, while the Goldstein 1983 equation accurately predicted metabolic rates in wind for a number of bird species, one cannot assume it is accurate for all birds (particularly very large ones), or for mammals, or for all wind speeds (especially high ones).   Therefore, the results from these calculations should be regarded with due caution.


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