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Methods & Principles of Weather Forecasting

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

Knowing what the weather is going to be like is very helpful. But how do we know? Learn what methods and principles do meteorologists use to forecast the weather. Take a quiz to see what you know

What Is a Weather Forecast?

When you outside, you have to decide what to wear. Is it shorts and a t-shirt weather? Or sweater and raincoat weather? To figure this out you need to look at the weather forecast.

A weather forecast is a prediction of what the state of the atmosphere will be at a particular time and place. This includes temperature, rain, cloudiness, wind speed, and humidity.

Predicting the weather can be useful
Predicting the weather can be useful

But how can meteorologists (weather experts) predict something that hasn't happened yet?

These days, weather reporters usually just say what they're told to say. But not that long ago, most weather reporters were experts in weather. They were real meteorologists. In the hours leading up to their report, what would they be doing? How did they get their information?

Their job wasn't easy, but weather forecasters are still right more often than they're wrong. Today we're going to talk about the methods and principles they use in predicting and forecasting the weather.

Methods & Principles

The first step in predicting the weather is to collect data. We need to know the history of weather in the area because one of the key principles of weather forecasting is that past patterns can indicate future events.

Thankfully a weather reporter doesn't need to do this work all by themselves: they have satellites in space collecting data, and weather stations all over the world. Various instruments are used: weather vanes to measure speed and direction, barometers to measure air pressure, and thermometers to measure temperature among other things. In older cities like New York and Boston, we have weather data doing back over 100 years!

Current Atmospheric Conditions

Once you have historical data, the next step is to look at current atmospheric conditions. What's the weather like right now? And what happened when there were similar conditions in the past?

There are a few factors to consider when taking in the current atmospheric conditions.

Air pressure matters because it causes air motion: high-pressure air is attracted to low-pressure areas. And once you have air moving, different air masses can collide with each other, usually causing hot air to rise over cold. The hot air that's forced upwards eventually cools, forming clouds that can cause rain. Cold air displacing warm air is called a cold front, and warm air displacing cold air is a warm front. Both can cause sudden changes in temperature, and both can cause rain, though cold fronts tend to have the most severe rain.

We know what causes rain, and can use that to make predictions
We know what causes rain, and can use that to make predictions

The humidity of the air is an additional factor, because if there's no moisture in the air then there can't be much rain in the first place. Moisture is related to temperature, because higher temperatures mean more evaporation, causing greater humidity.

A weather balloon is just one of many pieces of equipment used to collect data about the weather
A weather balloon is just one of many pieces of equipment used to collect data about the weather

A meteorologist will be looking at all this data before the weather report. Next, they have to find similar days in the past to see what happened. But the truth is that with experienced meteorologists, looking to the past isn't necessary every time. After a while, they learn to see the patterns and they know what to look for: What pressure systems are around? Where are they moving? What's the wind speed? How much water are the clouds carrying? What usually happens at this time of year?

And these days, even when they don't know, they have computer programs that help them figure it out.

Current Land Conditions

But it isn't just about temperature, wind, and pressure. It's also about the lay of the land. This is where local knowledge comes into play. A weather reporter who knows how air currents tend to move across their local mountains, seas, and lakes can do a better job of making predictions.

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