1. Water in liquid, solid & gaseous states is constantly being recycled through the water cycle.
The water cycle is the continuous movement of water from sources on Earth’s surface (lakes, oceans & plants) into the air, onto and over land, into the ground, and back to the surface.
Humidity refers to the amount of water vapor in the air.
Relative humidity refers to 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. This is given as a %.

2. Air Masses
An air mass is a large body of air of relatively similar temperature and humidity characteristics covering thousands of square kilometers. Typically, air masses are classified according to the characteristics of their source region or area of formation. A source region can have one of four temperature attributes: equatorial, tropical, polar or arctic. Air masses are also classified as being either continental or maritime in terms of moisture characteristics. Combining these two categories, several possibilities are commonly found associated with North America: maritime polar (mP), continental polar (cP), maritime tropical (mT), continental tropical (cT), and continental arctic (A).
Once formed, air masses can move out of their source regions bringing cold, warm, wet, or dry conditions to other parts of the world.

3. Fronts Air masses that form from different areas often don’t mix
Different densities
When 2 types of air masses meet, it’s called a FRONT.
4 types
Cold
Warm
Occluded
Stationary
Air masses that form from different areas often do not mix because they have different densities. When a warm air mass meets a cold air mass it’s called a FRONT.

4. Cold Front –
Cold air replaces warm air.
Cold front is moving faster than warm front.
Cold air moves under warm air and pushes it up.
Cold fronts move quickly & bring thunderstorms, heavy rain or snow.
Steep slope of front causes warm air to rise quickly. Both cold & warm air masses touch the ground.
Cooler weather usually follows a cold front b/c the air mass behind it is cooler and drier than the air mass that it is replacing. (See graphic)
Warm Front –
Warm air replaces cold air.
Both warm and cold air masses are moving slowly.
Gradual slope of front causes air to rise gradually bringing precipitation for longer periods. Both cold & warm air masses touch the ground.
Slow movement in warm fronts generally bring drizzly rain and are followed by clear & warm weather.

5. Occluded Front –
Forms when a slow moving warm air mass is caught between 2 faster moving colder air masses. (The 2 cold air masses are not the same temperature..one is a colder cold)
The coldest air mass moves under and pushes up the warm air mass.
The coldest air mass then moves forward until it meets a cold air mass that is warmer & less dense.
The colder of these two air masses moves under and pushes up the warmer (cold) air mass. The warm air mass does not touch the ground. It is occluded.
The occluded front has cool temperatures and large amounts of rain & snow.
Stationary Front –
Forms when equally weak cold air mass meets warm air masses.
Not enough force is available to lift the warm air over the cold air mass.
The two air masses remain separated. This may happen b/c there isn’t enough wind to keep the air masses pushing against each other. Both air masses are touching the ground.
Stationary fronts often bring many days of cloudy weather with rain or snow depending on temperature and moisture level.