Final answer:
Water enters the atmosphere through evaporation, which is accelerated by increased air temperatures and positive feedbacks like melted pools of water on ice.
Step-by-step explanation:
Water enters the atmosphere through a process called evaporation. Evaporation occurs when heat from the Sun causes water to change from a liquid to a gas, forming water vapor in the atmosphere. Factors that can speed up this process include increased air temperatures, which result in faster ice melting, and positive feedbacks like melted pools of water on ice, which absorb more solar energy and increase evaporation rates.