1. 气象条件 (Meteorological Conditions):

Snow forms when the temperature in the atmosphere is at or below freezing (0°C or 32°F). Therefore, for snow to reach the ground, the air temperature near the surface must also be at or below freezing.

雪的形成需要大气温度在冰点（0°C 或 32°F）或以下。因此，雪要落到地面，地表附近的空气温度也必须达到或低于冰点。

2. 能量交换 (Energy Exchange):

When snow falls, it absorbs heat from the surrounding environment as it melts. This process, known as the heat of fusion¹, cools the air around it. Additionally, snow reflects sunlight, preventing the ground from absorbing heat and further contributing to the cold sensation².

下雪时，雪花融化会吸收周围环境的热量，这个过程称为熔化热，会使周围空气变冷。此外，雪会反射阳光，阻止地面吸收热量，进一步加剧寒冷感。

It's cold when it snows because the air temperature is already at or below freezing, and the snow itself absorbs heat and reflects sunlight, making it feel even colder.

下雪时冷是因为气温本身已经达到或低于冰点，而雪又会吸收热量并反射阳光，使人感觉更加寒冷。