Gradients in air velocity may go unnoticed in growth
chambers, although they can affect evaporative demand, as
demonstrated by the rate of evaporation from glass beakers
(Fig. 2c). Variation in air circulation may be especially large
when plant density is high or plants are placed in trays, which
may block air circulation around the plants. Both too high and
too low wind speeds are undesirable. A factor that may strongly
vary in a temporal manner is the local atmospheric CO2
concentration: generally, CO2 levels in a building are higher
than outside and, in addition, concentrations will peak strongly
when humans are breathing freely during their presence in a
growth chamber (see section 4C).