What is a weather forecaster?

Answer 1

Break it down. An "index" is defined as:

• a number or ratio (a value on a scale of measurement) derived from a series of observed facts; can reveal relative changes as a function of time

We observe our environment using barometric pressure, relative humidity, and temperature. Then it is possible to take an educated guess what the weather is going to do next. This is forecasting.

But how accurate is the forecast? It is important to have a number (or ratio) that tells us how likely the weatherman is to be right. This is where the "index" comes in. One can say "chattanooga will be cold" with a forecast index of 50%. However if you look outside and it is pouring rain then turn around and say "it is raining now" your forecast index would be 100%.