Final answer:
The least heat flow into the atmosphere from 6pm to 6am occurs from 3am to 6am due to the Earth's surface cooling throughout the night after the sunset, resulting in the lowest temperatures just before sunrise.
Step-by-step explanation:
The time from 6pm to 6am when heat flow into the atmosphere is at its least is usually during the early morning hours, specifically from 3am to 6am. The primary reason for this is due to the Earth's surface cooling down over the course of the night after the sun has set. During the evening times, such as from 6pm to 9pm, there is still some residual warmth from the day. As the night progresses, the Earth continues to lose heat through radiation. By the time the early morning arrives, the accumulated warmth from the previous day has been mostly lost to the atmosphere, and the heat flow is at its minimum since the temperature of the Earth's surface is at its lowest before sunrise. This is further influenced by certain factors such as seasonal changes, latitude, and regional climate. For example, the Earth's tilt and orbit mean that the heat received from the sun varies throughout the year and affects the cooling rate at night.