Figure 6: Environmental effects on basic reproductive ratio, . (a) Seasonal change in virus inactivation rate, (Figure 1; (15), (18)), giving virus inactivation by temperature alone. is mean daily air temperature (appropriate to southern Britain) (19); is an assumed temperature threshold for virus inactivation, here equal to 13°C ((18), Table 1)); is the seasonally modulated basic reproductive ratio (20), calculated assuming temperature is constant. (b) For different values of mean annual air temperature, (19), basic reproductive ratio (20) is seasonally modulated as in (a) via the seasonal modulation of , for a constant threshold temperature for virus inactivation of = 13°C (18), giving for a maximum, minimum, annual mean, and amplitude (maximum-minimum). denotes the number of forcing days per year (a “forcing day” is a day on which air temperature is sufficiently high as to increase virus mortality).