Surface tension is created from the cohesive forces between the molecules of a liquid. Surface tension allows the liquid’s surface to resist certain external forces. Cohesion is what attracts water molecules to one another. Adhesion is what attracts water molecules to other types of molecules. The cohesive forces between the water molecules at the surface of a cup of water are stronger than the adhesive forces between the water molecules and the air above the water. The surface molecules do not have any water molecules above them, so they cohere more strongly to the water molecules next to and below them. The result is what appears to be a thin “skin or membrane” keeping the surface of the water from spilling over the edges of the cup.


The importance of cohesion and surface tension can be observed in a number of ways. For example, surface tension is why bubbles have a round shape and why water striders and other small insects are able to walk on water. Surface tension also plays a role in consumer products. Soaps and detergents help lower the surface tension of water, allowing it to better soak and clean clothes, and fabrics in rain coats and tents use surface tension to be more water resistant.


Lightning strikes an estimated 50 to 100 times per second across the globe. However, it remains one of the least understood weather phenomena. To better understand the stages of thunderstorm development, including lightning strikes, scientists use high-speed cameras and other remote sensing technologies. Thunderstorms develop as clouds form ice crystals that move and collide, causing them to become charged. Lighter crystals become positively charged and move to the upper part of the cloud. Heavier crystals become negatively charged and move to the middle and lower parts of the cloud. Storm clouds then become anvil-shaped. On the ground, a small positive charge develops underneath the main part of the storm cloud and a small negative charge develops underneath the overhanging anvil portion of the cloud.


When the positive and negative charges grow large enough, lightning is released, or conducted between the oppositely charged particles. This results in three different types of lightning strikes: positive cloud-to-ground flashes, negative cloud-to-ground flashes, and intra-cloud flashes. Most lightning takes place as intra-cloud flashes. Lightning can heat the air around it to temperatures of 27,760 degrees Celsius (50,000 degrees Fahrenheit). This is five times hotter than the sun. The excessive heat causes the air to expand very quickly, breaking the sound barrier and producing a sound wave heard as thunder.

  • High-speed cameras capture thousands of frames per second. When played back in slow motion, the captured frames can be viewed in detail. This is possible because one second of recorded video can be viewed over a period of two or three minutes. 
  • Globally, the highest number of lightning strikes occurs in Central Africa, the Himalayas, and South America. In the United States, Florida has the highest rate of lightning strikes. 
  • Thunder travels as sound waves through the atmosphere at a rate of about one mile every five seconds. To calculate a storm’s distance, simply count the seconds between a flash of lightning and its thunder, then divide by five. For example, 20 seconds is four miles away. 

the act of sticking or attaching to something.


the molecular force between particles within a substance that acts to unite them.


sudden electrical discharge from clouds.

surface tension

property of the surface of a liquid where the molecules act like a thin, elastic film, allowing it to resist external forces.


the sound that follows a flash of lightning and is caused by sudden expansion of the air in the path of the electrical discharge.


This material is based in part upon work supported by the National Science Foundation under Grant No. DRL-0840250. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation.


Made Possible in Part By