I just saw the Museum of Science of Boston’s presentation on weather for 5th graders today and I remembered how difficult it was for me to help my fifth grader figure out how to study for her weather unit. I spent hours googling weather terminology to help her with flashcards but then, trying to put together all the casual relationships was confusing, even to me.

While that weather presentation was fresh in my head, I thought I’d capture it and have also added links and video to help kids really understand these concepts.

p.s. I was pre-med in college so I never actually studied the weather. Please leave a comment to add to this or to make corrections. Especially you, George, from the Museum of Science. Thank you! Also leave questions if anything is confusing and I’ll keep adding to this.

p.p.s. I am adding a great post by Doodles and Jots about Clouds. She has cloud photos, cloud types diagram, & companion art project.

What Causes Weather: Heat, Air Pressure, Wind and Moisture

Heat: is from our sun. The light energy from the sun converts to heat energy when it hits the earth.

The tilt of the planet affects how much heat an area on the planet receives and this is also why we have seasons. Because the earth is sphere, when it is tilted the light energy in the middle of the planet is spread out a smaller distance than the top or the bottom (depending on angle of the tilt). The same light energy which converts to heat energy upon hitting the earth means less heat for a larger area versus a smaller area. [see link for explantory video]

Air pressure: is the weight of the Earth’s atmosphere pressing down on everything at the surface. Explaining Air pressure video from The Weather Channel. Watch to learn how air pressure is caused by heat (more heat is less pressure because molecules spread out so air rises, and less heat makes molecules squish together so pressure falls.)

The relationship between air pressure and heat. Cold air is more dense (has a greater weight per unit volume) than warm air. If the air is unconfined and heated — as would be the circumstances in the atmosphere — the density of the gas decreases (i.e. gets lighter). This is because the molecules in the air spread out when heated, causing air pressure to decrease. Think of a hot air balloon. Heating the air causes the air balloon rise because the heated air is lighter than the colder air around it.

The relationship between air pressure and weather. Air has weight, and a barometer measures the changes in air pressure above. When a high-pressure area is in control, the air sinks. Sinking air inhibits the development of clouds. When the air sinks, more force pushes down toward the ground, so the barometric pressure increases. Conversely, when a low-pressure area moves in, the air rises, cools and condenses out moisture, which forms clouds and precipitation. Since a rising column of air above weighs less, the air pressure falls.

As the sun heats the ground or ocean, warning them, the air near the ground or ocean warms and becomes less dense. As this happens the air begins rising, which lowers the air pressure at the Earth’s surface.

Very cold air, on the other hand, can create large areas of high pressure because cold air is more dense (heavier) than warm air. The Earth’s highest surface air pressures are found in masses of very cold air over places such as Siberia.

The relationship between air pressure and wind. Air pressure and wind speed are related: as air pressure drops, wind speed increases.

Wind: is air in motion. It is produced by the uneven heating of the earth’s surface by the sun. See video below to learn how changes in air pressure from high to low pressure cause wind.

Moisture: water can exist in three different states: solid, liquid, and vapor.

Water droplets form from warm air. As the warm air rises in the sky it cools. Water vapor (invisible water in the air) always exists in our air. Warm air holds quite a bit of water. For example, in the summer it is usually very humid. When enough of these droplets collect together, we see them as clouds. If the clouds are big enough and have enough water droplets, the droplets bang together and form even bigger drops. When the drops get heavy, they fall because of gravity, and you see and feel rain.

A cloud is a large collection of very tiny droplets of water or ice crystals. The droplets are so small and light that they can float in the air.

How are Clouds Formed? All air contains water, but near the ground it is usually in the form of an invisible gas called water vapor. When warm air rises, it expands and cools. Cool air can’t hold as much water vapor as warm air, so some of the vapor condenses onto tiny pieces of dust that are floating in the air and forms a tiny droplet around each dust particle. When billions of these droplets come together they become a visible cloud.

Cloud Types

Latin Root                   Translation                 Example

cumulus                           heap                                   fair weather cumulus
stratus                              layer                                  altostratus
cirrus                                curl of hair                      cirrus
nimbus                             rain                                    cumulonimbus

Humidity: is the amount of moisture in the air. The relative humidity is the amount of moisture in the air as a percentage of the most moisture that could be in the air at a certain temperature. If the air has half the amount of moisture it could have then the relative humidity is 50%.

Dewpoint: Another popular expression of humidity is the dewpoint. This is the temperature that the relative humidity will be 100% when air is cooled. When air cools the relative humidity will increase. It will continue to increase as the air cools until it reaching the saturation point. Morning dew is common in humid places in the morning. This occurs from air cooling to the dewpoint and further cooling results in condensation.

Climate versus Weather

Mountains and Climate: The temperature on mountains becomes colder the higher the altitude gets. Mountains tend to have much wetter climates than the surrounding flat land.

Oceans and Climate: the ocean responds very slowly to changes in the seasons, causing it to have a moderating effect on climate (i.e. water is harder to heat or cool than land so it affects the weather around it). The sea makes winters in coastal regions a bit warmer and summers near the coast a bit cooler than they are farther inland. . Notice how coastal cities have milder winters and more pleasant summers.

On a smaller scale, the sun heats up land faster than nearby water, causing the air over land to begin rising sooner than air over the ocean. As rising air over the land creates lower air pressure, cooler air from over the ocean flows in to replace it, creating a sea breeze.

Climate versus Weather: The difference between weather and climate is a measure of time. Weather is what conditions of the atmosphere are over a short period of time, and climate is how the atmosphere “behaves” over relatively long periods of time.

More Terms

Barometer: A barometer measures atmospheric pressure.

Barometric Pressure: another term for air pressure or atmospheric pressure. Air pressure is also called barometric pressure because barometers are used to measure it.

Atmospheric Pressure: or air pressure is the force exerted on you by the weight of tiny particles of air (air molecules).

Molecule: Molecules are small particles that make up all living and non-living things. They are made up of even tinier particles called atoms.

Temperature: Temperature is a degree of hotness or coldness the can be measured using a thermometer. It’s also a measure of how fast the atoms and molecules of a substance are moving. Temperature is measured in degrees on the Fahrenheit, Celsius, and Kelvin scales.

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