Final answer:
The 6.4°C/1000 m (3.5°F/1000 ft.) refers to the normal lapse rate, which is the average rate at which atmospheric temperature decreases with an increase in altitude in the troposphere.
Step-by-step explanation:
6.4°C/1000 m (3.5°F/1000 ft.) refers to the normal lapse rate, which is the rate at which the temperature decreases with an increase in altitude within the lower atmosphere under normal conditions. This rate is averaged to be about 6.5 K/km (or roughly equivalent to 3.5°F/1000 ft) for dry air.
It's important to distinguish this from an environmental lapse rate, which is the actual observed rate of temperature decrease with altitude in the current state of the atmosphere and can vary due to local weather conditions. The normal lapse rate is a useful concept in understanding the temperature profile of the atmosphere, which in turn affects weather patterns, climate, and aviation.