Many of us joke that it would be great to have a job where you could be wrong most of the time, and still considered to be good at what you do. We are often times referring to the local weather forecaster when we say this.

So is forecasting the weather a science or an art? Wikipedia, the internet’s free encyclopedia, defines weather forecasting as the application of science and technology to predict the state of the atmosphere for a given location. Human beings have attempted to predict the weather informally for millennia, and formally since the nineteenth century. Weather forecasts are made by collecting quantitative data about the current state of the atmosphere and using scientific understanding of atmospheric processes to project how the atmosphere will evolve.

So, why is forecasting the weather always a topic of discussion and so important to everyone? There are a variety of uses for weather forecasts. Weather warnings are important forecasts because they are used to protect life and property. Forecasts based on temperature and precipitation are important to agriculture, and therefore to traders within commodity markets. Temperature forecasts are used by utility companies to estimate demand over coming days. On an everyday basis, people use weather forecasts to determine what to wear on a given day. Since outdoor activities are severely curtailed by heavy rain, snow and the wind chill, forecasts can be used to plan activities around these events.

In the past, the human forecaster was responsible for generating the entire weather forecast based upon available observations. Today, human input is generally confined to choosing a model based on various parameters. Using a consensus of forecast models, can help reduce forecast error. However, regardless how small the average error becomes with any individual system, large errors within any particular piece of guidance are still possible on any given model run. Humans are required to interpret the model data into weather forecasts that are understandable to the end user. While increasing accuracy of forecast models implies that humans may no longer be needed in the forecast process at some point in the future, there is currently still a need for human intervention.

So what does it mean when the forecaster predicts a 40 percent chance of precipitation? The National Weather Service (NWS) is the federal agency charged with providing weather services to the nation. It is the official source of watches, warnings and advisories for hazardous weather.

The basic weather forecast includes the following weather elements: precipitation, probability of precipitation, sky condition, temperature and wind. Forecasts describe the weather in 12-hour increments such as “today”, “tonight” and “tomorrow”. The probability of precipitation forecast is one of the most misunderstood elements of the weather forecast. The probability of precipitation has the following features:

To summarize, the probability of precipitation is simply a statistical probability of 0.01” inch of more of precipitation at a given area in the given forecast area in the time period specified. Using a 40 percent probability of rain as an example, it does not mean that 40 percent of the area will be covered by precipitation at given time in the given forecast area or that you will be seeing precipitation 40 percent of the time in the given forecast area, for the given forecast time period. It means that if a forecast for a given county says that there is a 40 percent chance of rain this afternoon, then there is a 40 percent chance of rain at any point in the county from noon to 6 p.m. local time.

This point probability of precipitation is predetermined and arrived at by the forecaster by multiplying two factors: (Forecaster certainty that precipitation will form or move into the area) time the (Area coverage of precipitation that is expected) and then expressed as a percentage.

For example: If the forecaster was 80 percent certain that rain would develop but only expected to cover 50 percent of the forecast area, then the forecast would read “a 40 percent chance of rain” (0.80 x 0.50 = 0.40 or 40 percent) for any given location. If the forecaster expected a widespread area of precipitation with 100 percent coverage to approach, but he/she was only 40 percent certain that it would reach the forecast area, this would, as well, result in a “40 percent chance of rain” at any given location in the forecast area.

Hopefully, now you have a better understanding of what the forecaster is telling you about the weather, and whether or not to pack your rain coat for your next picnic.