Weather Stations and Weather Forecasts: Can You Do It Yourself?

Meteorologists are scientists who study the layer of gasses surrounding Earth, called the atmosphere. The atmosphere is about 300 miles thick, and most of it is within 10 miles of Earth's surface. The weather is the reflection of what happens in this layer of air. Meteorologists help predict the weather by studying what happens in the atmosphere, more precisely, they follow changes in the air pressure, temperature, humidity, wind velocity, wind direction, etc., and apply physical and mathematical relationships to this information. They use weather connections (weather phenomena that frequently occur together) observed in the past to help predict the future. They also use weather patterns. A weather pattern is the repetition of the same type of weather on consecutive days. This works because the weather tends to repeat itself until something interrupts the pattern. For example, it can be clear and sunny for five consecutive days until a cold front brings a few days of cold, gloomy, and wet weather. A typical weather forecast is shown in Figure 1. Although these forecasts are often accurate, they remain educated guesses; these predictions are not guaranteed to occur.

A five-day weather forecast for San Francisco, CA during a week in August. The list has columns that describe the weather conditions, high and low temperatures, chance of precipitation as a percentage, wind direction/speed and humidity as a percentage for each day.

Figure 1. Five-day weather forecast for San Francisco.

Meteorologists collect a huge amount of data and use supercomputers to make their predictions. However, people have been predicting the weather for centuries without computers, and your class can too! With a knowledge of weather connections, one just needs to observe the current weather to be able to predict the future weather. Although these predictions are only valid for the near future, it is still exciting to be able to forecast the weather and it is a valuable skill to learn.

In this lesson, students collect weather-related data over a certain period of time. They set up a weather station to measure quantitative variables like temperature, precipitation, wind speed, etc., and collect qualitative data like cloud coverage. Students then graph their data and are challenged to confirm weather connections using their data. Finally, they use these connections to predict the weather.

Below is a list of weather-related variables with an indication of some global weather connections that are related to that variable. Notice that this is not an exhaustive list, and you might also find connections that are specific to your area.

Air pressure*, or how hard the air is pressing down on the Earth:

• Low air pressure indicates a change in the weather pattern.
• High air pressure indicates nice stable weather for the next 12 hours.

Sky coverage*:

• Clear skies allow for bigger temperature changes between day and night. This happens because clouds reduce the passage of light and heat.
• White flat clouds (e.g. cirrocumulus clouds) high in the sky, as shown in Figure 2, mean stable air. The weather will stay as it is.
• Fluffy clouds (e.g. cumulus clouds) indicate that the air inside the cloud is rising. When these clouds are surrounded by blue sky, the weather will be fine in the coming hours. They can also grow into thunderstorm clouds. In that case, a storm is brewing.
• Thunderstorm clouds (cumulonimbus cloud) are huge towering clouds. They can be white at first but grow darker as time passes. These clouds bring thunderstorms. The darker the cloud, the sooner the storm will hit.
• Dark, low hanging clouds that cover the sky (nimbostratus) indicate a storm is close. Notice that normal wind patterns can drop just before a storm hits.
• Gathering, darkening clouds, as shown in Figure 3, indicate bad weather is coming. Dispersing and/or lifting clouds mean the weather is clearing out.

Figure 2. High clouds predict stable weather.

Figure 3. Gathering and darkening clouds predict rain.

Relative humidity* (often referred to as humidity):

• The relative humidity usually drops when the air heats up and rises when the air cools. This is why evenings and mornings can be damp (high humidity) and why fog (a very humid condition) often burns off over the course of the morning.
• If the humidity is higher than usual for your area, a storm or rain is probably on its way.

Wind direction:

• A change in wind direction often brings a change in the weather pattern, except if that wind is light.
• The wind direction can also inform you about the type of air that is brought in. If you have the ocean to the north and a desert to the south, wind from the north will bring moist air and wind from the south will bring dry air.

Temperature*:

• Other variables can help predict if the temperature will rise or fall. One needs the current temperature to make a more exact prediction.

Wind speed*:

• Storms are accompanied by higher speed winds.

Precipitation*:

• Precipitation is a variable that indicates what happened before. It is linked with the passage of or absence of a storm, rain, snow, hail, etc., in the past.

* Links in the additional background section provide more information on these variables.

Air pressure is a powerful predictor of the weather. Air is made up of gas molecules such as nitrogen, oxygen, and carbon dioxide. All of these molecules press down on the Earth, and this pressure is called air, or atmospheric pressure. At sea level, the air pressure ranges from 800 to 1050°millibars (23.6 to 31.0 InHg). Because warm air is less dense, it rises which results in lower air pressure near Earth. While it rises, air cools and, in the process, water vapor in the air condenses into liquid. This leads to cloud formation and rain. "Low pressure," therefore, is generally associated with cloudy and rainy weather. On the other hand, high-density cold air results in increased air pressure. As cold air sinks, it dries, causing dry weather conditions in "high pressure" zones.