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.   However, for non-torpid animals in the inactive (rest) phase of the daily cycle, you can select whether they are sheltered from wind or not (by default, wind is assumed to be absent).

  • 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 calculated thermoregulatory costs:

               costwind = costno wind * (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)

  • Caveat 1:  Lower critical temperature is not adjusted for wind effects, although LCT will be somewhat higher as wind speed increases.   For most species, the induced error is probably small enough to safely ignore when estimating energy costs.

  • Caveat 2:  This conversion, like the rest of the thermoregulatory cost procedure, works for Te below body temperature.   At present the algorithms cannot compute costs when Te is higher than Tb (in these conditions the costs will be elevated metabolic rates to support panting, sweating, or other evaporative cooling mechanisms).

  • 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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