Final answer:
Snowflakes can melt and fall as rain if the surface temperature is above 4°C, but this depends on various factors including humidity and temperature gradients. Snow may persist on mountain slopes above freezing temperatures due to the heat required for phase changes and other environmental factors.
Step-by-step explanation:
When the surface temperature is about 4 degrees Celsius (4°C) or higher, snowflakes may still reach the ground without melting if the overall conditions allow, for instance, if there is very low humidity or the snowflakes are falling through a shallow layer of warm air. However, typically, snowflakes begin to melt as they fall through warmer layers of air and may transition into rain before reaching the ground. This melting process is not solely dependent on air temperature; it also depends on other factors such as humidity and the temperature gradient between the cloud base and the ground.
Regarding the stability of snow on mountain slopes at temperatures above freezing, snow is the solid phase of water formed from ice crystals. Enormous heat is required for phase changes, and this heat must be absorbed from the surrounding environment. If the air temperature is above 0°C, the warmer the air, the faster the heat exchange and the faster the snow melts. However, in mountainous areas, the snow may still remain due to factors such as the reflection of solar radiation, sub-surface ice layers, or periodic re-freezing that prevents rapid melting.