A source of error is any factor that may affect the outcome of an experiment. There are countless conceivable sources of error in any experiment; you want to focus on the factors that matter most. Identify each source of error specifically and then explain how that source of error would have affected the results. Keep in mind that an "error" to a scientist does not mean "mistake"; it more closely means "uncertainty". Many students are tempted to say "human error", but this term is vague and lazy; any decent teacher will not accept it. Instead, think about specific things that happened during the lab exercise where the end results may have been affected. To give an example one might find in a bio lab: perhaps a water bath's temperature was not monitored very carefully and you found that an enzyme's activity was greater than you expected. In that case, you could write something like, "The temperature of the water bath during this exercise was not monitored carefully. It is possible that it was warmer or cooler than intended, and this would have affected the enzyme activity accordingly. The fact that our enzyme activity was found to be higher than expected leads me to believe that perhaps the water bath was too warm."
Calorimetry experiment refers to the measurement of change in energy of a system by measuring the heat exchanged by the system with its surroundings. A potential source of error in this experiment is the unwanted heat loss to the environment which can reduced by adding lid and insulating its sides. Also, the experiment must be repeated multiple times in order to obtain reliable results.
Basically when frozen water/ice crystals high in the atmosphere collect water vapor molecules they grow. They are sometimes supplied by microscopic cloud droplets.
