Farmers and Graziers Warnings

From September to April, the Bureau of Meteorology in Victoria can issue a farmers and graziers warning when the weather poses a potential threat to livestock.

How does the Bureau decide when to issue a warning?

Warnings are issued at the discretion of senior forecasters, but there are various guidance tools at their disposal, such as the chart shown at right.

The chart - known as a nomogram - takes into account temperature, wind speed and rainfall during the critical weather period.

The example (red arrows) shows that with an estimated lowest temperature of 15 degrees Celsius, an estimated highest average wind speed of 20 kilometres an hour, and an estimated rainfall of 25 millimetres during the critical period, the forecaster should consider issuing a warning.

Another factor taken into account is that the most significant livestock losses occur when a spell of warm, dry weather is followed by the outbreak of cold, wet and windy conditions with a chill factor of 230 or more.

The following combinations of wind, rain, and temperature produce a chill factor of 230:

Temperature (degrees C)	10	10	15	15	20
Wind (knots)	10	15	10	15	15
Rainfall (mm)	10	8	20	17	30

However when conditions are already cool, in early spring for example, a brief cold outbreak does not pose a serious threat. A warning would only be justified with prolonged periods of severe weather.