Answer:
The Arctic tundra is characterized by long, cold winters and short, cool summers. However, the defining feature of the Arctic tundra is not its winter lows, but its cool summer temperatures. This is because the Arctic tundra experiences a phenomenon called a "temperature inversion" during the summer months.
A temperature inversion occurs when the air close to the ground is cooler than the air above it. This is the opposite of the normal situation, where the air gets cooler as you move higher in the atmosphere. In the Arctic tundra, the ground is covered by snow and ice for much of the year, which reflects sunlight and keeps the ground cold. During the summer months, the sun shines for 24 hours a day, warming the air above the ground. However, the cold, dense air near the ground does not rise and instead stays trapped near the surface. This creates a layer of cool air that blankets the tundra, keeping temperatures low even during the summer.
Therefore, the Arctic tundra is more strongly defined by its cool summer temperatures because this is the time when the temperature inversion occurs, and when the cool air near the ground has the greatest impact on the environment.