How does snow form?

The white fluffy snow we see in the winter does not begin in this shape. The process of forming snow begins high in the atmosphere in clouds. The amount of moisture and temperature help determine how snow forms.

Snow begins as a tiny ice crystal, or a speck of dust. When there is enough moisture in the air, condensation starts to occur. The tiny drops of condensation start to stick together. If the temperature in the cloud is cold enough, the tiny drops will freeze and form ice crystals.

As more crystals stick together, the drops get heavier and start falling toward the ground. During the fall, the snowflakes may pass through air pockets that are warmer or cooler. This melting and refreezing is what gives snowflakes their final shape, like big soft snowflakes or small fast snowflakes.

Even after the snow reaches the ground, it still changes shape as it melts or refreezes. If it is very cold, the snow forms a hard crust on top, or if it is warmer, the snow melts some and makes slush.

What causes fog?

Fog is a type of cloud that can be found at ground level. It is caused by the super saturation of the air, which means that the air cannot hold any more water. The water is forced out of the air and forms condensation. Think of a water bottle sitting on your table. The water droplets that form on the outside of the bottle are condensation. This happens in air, except that the droplets float above the ground. This is what makes the fog.

Four main types of fog exist, and each is formed in a slightly different way:

Radiation fog is formed as the ground becomes cooled by the night sky, the air above the ground also cools. If the air is below its dew point, the point where condensation occurs, fog is formed. Advection fog comes from warm air moving left to right across a cool surface, such as when air from a lake blows over the land. The land cools the air below the dew point causing fog.

Upslope fog occurs when warm air flows up over the slope of a cool mountain. Again the air cools below the dew point and forms fog. Evaporation fog is caused by water vapor entering air that is very close to being supersaturated. The additional water causes the air to reach the dew point and fog forms.