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.

English Transcript:

- [Narrator] When you see drops hit the water, here's what you don't see. Every drop bounces, like a ball. Held together by surface tension, it continues to get smaller and smaller. This happens every time a raindrop hits a puddle. 100 times every second, lightning strikes somewhere on earth. Little was known about lightning until high speed cameras turned the research upside down, literally. What our eyes see is energy flowing downward from the clouds. Now we can see that electricity also moves upward from the ground. If we can see lightning bolts, we can see almost anything that's lightning fast.

Spanish Transcript:

- [Narrador] Cuando ves gotas golpear el agua, esto es lo que no ves. Cada gota rebota, como una pelota. Ligada por la tensión superficial, continúa haciéndose más y más pequeña. Esto sucede cada vez que una gota de lluvia golpea un charco. 100 veces cada segundo, un rayo cae en algún lugar de la Tierra. Poco se sabía sobre los rayos hasta que las cámaras de alta velocidad dieron vuelta la investigación, literalmente. Lo que nuestros ojos ven es energía fluyendo hacia abajo desde las nubes. Ahora, también podemos ver que la electricidad se mueve hacia arriba desde el suelo. Si podemos ver los rayos, podemos ver casi cualquier cosa que tenga la velocidad de un rayo.