What influences weather?

As water evaporates from lakes, oceans, and plants, it becomes water vapor, or moisture in the air. Water vapor is invisible. The amount of water vapor in the air is called humidity. As water evaporates and becomes water vapor, the humidity of the air increases. The air’s ability to hold water vapor changes as the temperature of the air changes. One way to express humidity is through relative humidity. Relative humidity is the amount of water vapor in the air compared with the maximum amount of water vapor that the air can hold at a certain temperature. When air holds all of the water that it can at a given temperature, it is said to be saturated.

When water from the air returns to Earth’s surface, it returns as a precipitation. Precipitation is water, in solid or liquid form, which falls from the air to Earth. There are four major forms of precipitations, rain snow, sleet, and hail. Rain is the most common form of precipitation is rain. As cloud produces as rain when the water drops in the cloud would become a certain size. Sleet forms when rain falls through a layer of freezing air. The rain freezes the air, which produces falling ice. Snow forms when temperatures are so cold that water vapor changes directly to a solid. Snow can fall as a single ice crystal or can join to form snowflakes. Hail is balls or lumps of ice that fall from clouds. Hail forms in cumulonimbus clouds. When updrafts of air in the clouds carry raindrops high in the clouds, the raindrops freeze and hail forms

When air hovers for a while over a surface area with uniform humidity and temperature, it takes on the characteristics of the area below. For example, an air mass over the tropical Atlantic Ocean would become warm and humid; an air mass over the winter snow and ice of northern Canada would become cold and dry. These massive volumes of air often cover thousands of miles and reach to the stratosphere. Overtime, mid-latitude cyclonic storms and global wind patterns move them to locations far from their source regions. When two air masses meet the cold air pushes the hot air upwards, when going up the temperature drops and the air can't hold as much water when it is warm, so the cold water molecules condense and form clouds. The transition zone between two air masses of different humidity and temperature is called a front. Along a cold front, cold air displaces warm air; along a warm front, warm air displaces cold air. When neither air mass displaces the other, a stationary front develops. Intense storms may form along cold fronts, while widespread clouds and rain, snow, sleet, or drizzle may accompany warm fronts