Shop Windows to the Universe

Please help support Windows to the Universe, and our activities to help Earth and space science teachers, with a tax-exempt donation today!
The gas inside this balloon pushes outward in all directions. That push is called pressure. The pressure makes the rubber stretch, causing the balloon to inflate.
Click on image for full size
Image courtesy of the University Corporation for Atmospheric Research.


When you "blow up" a balloon, you are raising the pressure on the inside of the balloon. That makes the rubber in the balloon stretch, and the balloon gets bigger. Pressure is an idea scientists use to describe how gases and liquids "push" on things. Air has pressure. When you pump up a tire, you measure the air in it with a pressure gage. When you go to see a doctor, the doctor or a nurse checks your blood pressure. If you dive down under water when swimming in a pool, you can feel pressure from the water pressing on your ears. Pressure is all around us!

Pressure is a force that is spread out over an area. A submarine that is deep under the ocean must have a very strong hull. The pressure of the sea water pushes in on the submarine from every side. The force of that pressure is spread out over the whole surface of the sub.

Gases (like air) and liquids (like water) have pressure. The atoms and molecules in a gas or a liquid are moving very, very fast. They are constantly bouncing off each other like tiny rubber balls, and also off the sides of whatever container they are in. Whenever an atom or molecule hits something, it gives the thing it collides with a tiny push. Those small pushes may not seem like they would do much. However, when you add up all the pushes from millions of atoms and molecules, they can really make a difference. All of those pushes combine to make pressure.

The atmosphere has pressure. The air pressure in Earth's atmosphere is pretty strong when you are near sea level. When you go higher up, in an airplane or to the top of a mountain, there is less pressure. There is also pressure under water. You can feel water pressure if you dive down to the bottom of a pool. Deep in the ocean, the pressure is really, really strong.

The pressure in the atmosphere isn't the same everywhere. When you watch a weather forecast on TV, they may say that there is a low pressure system coming through; or that there is a high pressure system nearby. Winds usually blow from a place where pressure is high towards a place where pressure is low.

There are many different units for measuring pressure. In the English system, one unit is "pounds per square inch" (or p.s.i. for short). Notice how this is a force (pounds) spread out over an area (square inches). In the metric system, force (newtons) over area (square meters) is given a special name - "pascals". A pascal is actually a pretty small unit; the pressure of Earth's atmosphere at sea level is 101,325 pascals! That's why you'll often see the kilopascal (a thousand pascals) used instead. Some other units for pressure are the bar and millibar, millimeters of mercury (mm of Hg) which is also called a "torr", and an "atmosphere". If you want to know more about these, check out the advanced level version of this page.

Last modified May 21, 2008 by Jennifer Bergman.

Shop Windows to the Universe Science Store!

Our online store includes books on science education, classroom activities in The Earth Scientist, mineral and fossil specimens, and educational games!

Windows to the Universe Community



You might also be interested in:

Cool It! Game

Check out our online store - minerals, fossils, books, activities, jewelry, and household items!...more


Most things around us are made of groups of atoms bonded together into packages called molecules. The atoms in a molecule are held together because they share or exchange electrons. Molecules are made...more

Measuring Atmospheric Pressure

Even though we can't see air, it is real and has pressure. The pressure of the atmosphere changes. It is higher at sea level, and lessens as you go higher up in the atmosphere. Some weather systems have...more


A barometer is a weather instrument used to measure atmospheric pressure. The first barometer was invented in 1643 by Evangelista Torricelli, one of Galileo's assistants. This first barometer used mercury...more

Can there be Life in the Environment of Jupiter?

Jupiter's atmospheric environment is one of strong gravity, high pressure, strong winds, from 225 miles per hour to 1000 miles per hour, and cold temperatures of -270 degrees to +32 degrees (freezing temperature)....more


Chemistry is the study of matter, energy, and their interactions. Chemists study the composition of substances, their properties, and how they react with each other under varying circumstances. Indeed,...more

Earthquakes Under Pacific Ocean Floor Reveal Unexpected Circulation System

The Earth has a large system of ridges along the ocean floor that play a big part in the geology of the planet. A team of seismologists (geologists who study earthquakes) has been studying a place called...more

Extreme Environments - Acid, Radiation, and More!

This page describes extreme environments that are filled with acids, are blasted with radiation, are under high pressure, or are tough places for most living things in various other ways. Extreme environments...more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA