Sparks are a little more complicated than just a positive and negative charge meeting. If that were the case, salt water would be filled with sparks from all the positively charged sodium ions and negatively charged chlorine ions casually meeting up. While glowing drinks would be AWESOME (see: click here) we know this is not the case.

In this case, there are a few things you need to know. First, air in an insulator. It does not conduct electricity the way metal does; air has a high resistance, metal a low resistance. Secondly, air is made of molecules, and these molecules normally have one or two atoms with small positively charged nuclei and wispy but localized electron clouds. The electrons are married to their nuclei, so to speak, and can't jump from one molecule to another. That's why air doesn't conduct electricity.

When you set up a large electric field, say from a large positive charge on your finger and a large negative charge on your doorknob*, the air molecules feel the force from that electric field. The electrons are pulled slightly toward the positive side, the nuclei toward the negative side. A spark happens when the field becomes so strong that it rips the electrons from the nuclei of gas molecules and causes what is called "dielectric breakdown". Dielectric breakdown (sometimes called electrical breakdown) is when strong electric fields turn insulators into conductors. When the electrons jump from one molecule to the next, light is emitted. The exact reason light is emitted is complicated, but the general reason is a conservation of energy, when an electron jumps from far away from an atom into a snug orbital, it loses energy. The extra energy is emitted as light.**

Something to keep in mind, is that there isn't one caravan of electrons that decides to trek all the way from your finger to the doorknob or from a thundercloud to the ground. It's more like a domino effect. Some atoms break down and balance charge on your finger. Then those atoms are now charged, so the air a little further away breaks down to compensate and so on until everything balances. When filmed on high speed cameras, you can actually see lightning move. What's crazier is that lightning can "feel out" different pathways until one back to earth is found!

Hope that helps!
*Electric fields are a measurement of the "slope" between two voltages. It's common to visualize voltages as analogous to "height" and electric fields as the slope of the hills connecting two voltage heights. Therefore you can increase electric fields by either increasing voltage or decreasing the distance between two charges. That's why you need to get two charged items close enough together to create a spark.

**If you're a chemistry nerd like I am, you'll appreciate that because the light generated from a spark (or similarly from a fluorescent light) is produced from electrons falling into orbitals, the wavelengths of light produced are quantized. This means that if you passed spark light through a prism (or if you were wearing diffraction glasses [see: geen vizions and tumblr]) you would see specific individual lines of color rather than a spectrum of color like a rainbow. What's more, the scientific field of Spectroscopy uses this principal to determine what atoms something is made of. Every element has its own spectroscopic "fingerprint", so ionizing a gas into a plasma gives us information about what the gas is made of. This principal is used for everything from finding trace elements in food products to determining what atoms stars are made of.

Thanks for the question!

I am going to talk about one specific spark, lightning! You are right when you were thinking it was when negative and positive charges meet. So first you have a cloud with electric charge in it, there are a lot of positive charges on the top of the cloud and negative charges on the bottom. Opposites attract so a positive charge builds up on the ground underneath the cloud. The ground's electrical charges concentrate on anything that sticks up such as...a tree, mountains, etc. The charge coming up from these points eventually connects with the bottom of a cloud and then you get a spark - lightning! A spark is basically when a sudden jump of electrons cross a space to a higher/lower charged object, which I explained with the cloud and tree.

All substances have what's called a resistance that prevents the flow of electric current. This resistance is reduced when the material is ionized to become a plasma, allowing electricity to flow.

Bringing positive and negative electric charges together releases energy. If the amount of energy that can be released by bringing the charges together is greater than the amount of energy needed to ionize the resistant substance between them, then an ionized channel will form through which the current can flow and bring the charges together. Merely bringing charges together without any intervening resistant material doesn't make a spark.

Also note that the quantity of resistant material (and thus the distance) that the charge needs to go through increases the necessary energy. A spark generated by static electricity from shuffling your feet and connecting your finger to a piece of metal through a millimeter of air takes quite little energy. A bolt of lightning connecting a thundercloud to the ground arcing through several thousand feet of air requires a great deal of energy, which is why lightning is so powerful (and so dangerous).

Sparks are caused by positive and negative charges meeting, and the way that sparks form is very interesting! Sparks always form near the surface of objects that are electrically charged. A good example is a plug that is being pulled out of an electrical outlet.

Even though you cannot see them, there are a very large number of electrically charged objects in the air surrounding you, including: free electrons and positive and negative ionized air molecules (such as oxygen, and nitrogen). These charged objects are accelerated towards or away from the surfaces of charged objects, depending on the charges of the surface and particles. Positive surfaces attract negative particles (like electrons) and repel positive ions, and the reverse is true for negative surfaces. This acceleration of charges results in an "avalanche" of charged objects away from surfaces, and many charged objects meet and recombine as a result of this effect.

For example, if an electron collides with a positive ionized air molecule, then the electron and ion combine to give a neutral air molecule. This combination gives off light, which accounts for the fact that sparks are seen as blue light. Interestingly, sparks cannot form in vacuum (like outer space) where there are no air molecules around to ionize and recombine.

Thanks for the great question! Please feel free to send us more!