Why do Clouds Float and Trees Grow?

How do clouds stay in the sky if gravity is down? Why do trees grow up if gravity is down?
- Necole (age 12)
Texas

Hi Necole,

Thanks for your interesting question!

To answer your question about why clouds are in the sky, first let's talk about how clouds form. As the height above Earth's surface increases, the temperature decreases. This means that in the atmosphere there is cold air above the warmer air near the ground. But you may know that hot air rises above cold air, because it's less dense. (It's similar to the way helium balloons float because the gas they're filled with is less dense than air.) The warm air near the surface to rises up into the atmosphere and cool as it ascends. Eventually the air gets cold enough that the water vapor it contains condenses into water droplets. These water droplets scatter light from the sun, making the clouds visible. Why don't the droplets fall because of gravity? They're not the same as the droplets you would normally experience, say from a leaky faucet. They're much tinier, so the upward force from the rising air is enough to keep them floating against the force of gravity. If there are enough tiny droplets in the cloud that they start to stick together, becoming larger and heavier, then gravity wins and they fall to the ground - as rain!

Trees too have to overcome the effects of gravity. They need to grow upward, so that they can catch more sunlight with their leaves. But plants get their nutrients and water from the soil, so they've evolved ways to raise them from the ground up. Trees have different types of "veins" called xylem and phloem to move water and nutrients throughout their "bodies". Xylem is made of narrow tubes of dead cells that move sap from the roots upward when water evaporates from the leaves and drags along the sap - the same as when water is sucked through a straw. Phloem is also made of narrow tubes, but are composed of live cells. Phloem can move material up or down the tree by creating a difference in concentration across it's cell wall. This is called "osmotic pressure", and it causes molecules to shift from areas of high concentration to areas of lower concentration.

- Courtney K

(published on 01/25/2016)