Photosynthesis occurs in two stages. In the first phase, light-dependent reactions or photosynthetic reactions (also called the Light reactions) capture the energy of light and use it to make high-energy molecules. During the second phase, the light-independent reactions (also called the Calvin-Benson Cycle, and formerly known as the Dark Reactions) use the high-energy molecules to capture carbon dioxide (CO2) and make the precursors of carbohydrates.
The process only takes a couple seconds to take place but both of the stages require the molecules to have a certain energy level.
When the air temperature is low, fewer molecules have the proper energy level and so only a small amount of photosynthesis takes place. Warm temperatures allow a lot more molecular conversions.