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Predicting Weather

Weather affects everyone. The clothes we wear, our travel plans, our daily activities—all of these depend on the weather. Predicting weather is the work of special scientists known as meteorologists. How do meteorologists do their work? How do they predict the weather?

Predicting weather is complex and not precise.To make an accurate weather forecast, meteorologists need to know what has happened before and what is occurring right now.

Meteorologists create the weather mapsseen in newspapers and on television.

meteorologist – scientist who studies weather

Let’s use baseball as an example. We want to predict if a batter will hit or strike out. Suppose we know that the batter has a low batting average and has struck out both previous times at bat. Suppose we also know the pitcher strikes out seven out of every 10 left-handed batters and that the batter is left-handed. We might predict the batter will strike out, but will he? Maybe yes or maybe no. Many other things might affect the outcome. Predicting the weather is like predicting the outcome of the baseball player at the plate.

What do you think meteorologists need to know to predict the weather? The current air temperature? How much water vapor is in the air (what is the humidity)? Is there precipitation and cloud cover, or is it sunny? What is the air pressure? Is there wind? Knowing the answers to these questions and others will help predict the weather. It also helps to know what kinds of weather has happened in the past when conditions were similar.

weather – condition of the atmosphere

Weather is influenced by interactions involving sunlight, the ocean, the atmosphere, ice, landforms, and living things. These interactions vary with latitude, altitude, and local and regional geography, all of which can affect oceanic and atmospheric flow patterns. Because these patterns are so complex, weather is predicted based on a percentage that an event will occur.

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Predicting Weather

Why Weather Predictions FailThe size and scale of weather predictions are much larger than for a single event, like a batter at bat. There are many conditions to take into consideration, and they all interact with one another in complex ways. In addition, we usually need to know conditions over a very large area. Just trying to collect all the information we need is a difficult task. After collecting huge amounts of data from sources such as satellites and weather balloons, meteorologists plug the information into formulas, called algorithms, that help them predict future weather conditions.

algorithm – a formula for solving a problem

Even with lots of data and very good algorithms, however, the number of conditions or factors, the complex ways they interact with one another, and the large areas covered make it impossible for meteorologists to say exactly what will happen with 100% certainty. They can only tell us what is likely to happen.

Because meteorologists can’t tell us for certain what will happen, they often provide information, including numbers, that help us put the information in context. Meteorologists often report the probability of precipitation. One common example of this is percent chance of rain. You might see a weather prediction that says we have a 50% chance of rain today. What does this mean? Many people are confused about this.

It does not mean that rain will fall on half (50%) of the area. It also does not mean that rain will fall for half of the time. It is the chance of precipitation at any point in the area that day. It takes into account both the likelihood that it will rain somewhere in the area and how much of the area will get some of the rain.

probability – the likelihood or chance of something happening

Air masses and pressure systems drive how weather changes. Air masses flow from regions of high pressure (H, or sinking air) to areas of low pressure (L, or rising air), causing weather at a fixed location to change over time. Sudden changes in weather can result when different air masses collide. The uneven heating of Earth’s surface and the transfer of that heat to the the lower atmosphere cause air masses to have different temperatures. In general, weather will improve when pressure increases and worsen when pressure decreases. Pressure is measured with a barometer.

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Predicting Weather

Weather depends on heat from the sun and water.The condition of air and how it acts to create weather is affected by two conditions: heat from the sun and water. The sun heats Earth, which warms our atmosphere and causes water to evaporate into the air. Rising air cools and condenses into clouds that can produce rain, hail, sleet, or snow. These air masses drive how weather changes as they move across the planet.

A rising barometer indicates high pressure. Air sinks, and good weather moves in. A falling barometer indicates low pressure. Air rises, and bad weather moves in.

Polar air masses (P) are cold air masses that form in polar regions.Tropical air masses (T) are warm air masses of subtropical or tropical origins.Continental air masses (c) form over continents.Maritime air masses (m) form over water and are moister than continental air masses at the same latitude.

Air masses bring different types of weather, depending on their source.The motion of air overhead affects the weather we have at the surface. Warm air cools as it rises, which causes water vapor in the air to condense into liquid water droplets that form clouds and precipitation. In contrast, sinking air usually accompanies good weather and dry conditions. Warmer air masses are less dense and have less pressure than colder air masses. Remember, rising air = moistening; sinking air = drying.

Meteorologists identify air masses by the regions from which they originate.

Predicting Weather

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As you can see from the air mass map above, the distribution and movement of air masses depends on their origin, the landform and region they form over (polar, tropical, or continental), ocean temperatures (cold or warm), and the ocean currents (surface or deep; global or local); the interactions of all of these factors create the weather that occurs.

Fronts are boundaries between air masses.Air masses give us information to predict temperature, humidity (moisture level), and air pressure for the area to which they are moving. When different air masses collide, a boundary forms between them called a front. Stormy weather is associated with these fronts.

● Cold front: A cold front (shown as a line of blue triangles on a weather map) forms where a cold air mass moves at a sharp angle under a warm air mass because the cold air is denser and heavier than warm air. The warm air mass is pushed upward, causing the moisture to condense into precipitation rapidly. Storm clouds form and thunderstorms may be scattered along cold fronts. As the front passes, the wind changes direction. Skies begin to clear, and the temperature usually drops.

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● Warm front: A warm front forms where a warm air mass moves up and over a cold air mass at a less steep angle than a cold front. The less dense warm air rises and cools to form wispy cirrus clouds, followed by layered stratus clouds that can bring precipitation. When the front passes, the clouds clear, and the air pressure and temperature increase.

Predicting Weather

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● Stationary front: A stationary front occurs when two air fronts do not move. Sometimes precipitation and minor winds result.

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Predicting Weather

We can try to make our own weather predictions by using historical weather data and comparing it with current weather data. Below is a table of various weather conditions for the same location over many days, including specific weather that resulted (did it rain that day or not?). Use the historical weather data below to assign a probability of rain.

Day 1Winds: Easterly

Air Pressure: High

Clouds: None

No rain

Day 2Winds: Easterly

Air Pressure: High

Clouds: None

No rain

Day 3Winds: Easterly

Air Pressure: High

Clouds: None

No rain

Day 4Winds: Westerly

Air Pressure: Falling

Clouds: Cumulus

No rain

Day 5Winds: Westerly

Air Pressure: Low

Clouds: Cumulus towers

Rain

TodayWinds: Westerly

Air Pressure: Low

Clouds: Cumulus towers

What is your best prediction for the probability of rain today? Explain.