1. Air masses/Atmospheric conditions

2. What is an air mass? -wikipedia.
In meteorology, an air mass is a volume of air defined by its temperature and water vapor content. Air masses cover many hundreds or thousands of square miles, and adopt the characteristics of the surface below them
-wikipedia.

3. What happens when air masses collide?
From sciences360.com
When two air masses collide, they tend to stay separate in the first instance, as oil remains separate from water. What happens when two masses collide relates to the differing characteristics of each mass.

4. In other words, warm air rises!
Warm air is lighter than cool air, therefore when a mass of warm air meets a mass of cooler air, it tends to rise up over it. How quickly this rising takes place will be affected by the type of collision between the masses.
In other words, warm air rises!

5. If a warm air mass collides with a cool or cold mass, it will ride gradually up and over it. The line which separates the warmer air from the cooler air is called a front and the front will rise from the ground at an angle with the warmer air above the cooler air. Normally, if it is warm air which approaches the cooler air the front will form a gentle angle and the warmer air will consequently rise gently upwards. However, if cooler air approaches and collides with warmer air, the heavy, denser cool air will tend to undercut the warmer air, pushing it rather like a bulldozer, forming a steeply rising front.

6. The name of each front is derived from the type of air which follows behind it at ground level. For example when warm air is approaching cool air, the passing of the front will presage a rise in temperature as an observer passes from the cool mass to the warm mass, and consequently this is a warm front, marked with red semi-circles on a weather map.

7. On the other hand, if cool air is approaching a warmer air mass the front will mark a fall in temperature and be called a cold front, marked with blue triangles on a weather map.

8. How clouds and rain form
In each case, with warm air rising over cool air, the air will expand and cool as it rises. If the rising air contains a large amount of moisture, this moisture will tend to condense into water droplets when the temperature of the rising air reaches dew point. This is not a fixed temperature, but varies due to different meteorological characteristics.
The formation of water droplets will cause clouds to develop, and if the clouds grow to a sufficient extent, rain will fall. In general terms, the faster the air rises, the taller will be the clouds, and the heavier will be the rain.

10. Atmospheric circulation
The large-scale movement of air, and the means (together with the smaller ocean circulation) by which thermal energy is distributed on the surface of the Earth.

11. Factors that affect atmospheric circulation
There are 5 major factors affecting global air circulation
- uneven heating of earth's surface
- seasonal changes in temperature and precipitation
- rotating of earth on its axis
- properties of air and water
- long term variation in the amount of solar energy striking the earth

12. The coriolis effect

13. Anticyclones and depressions
Most air masses move horizontally, however it is also possible that they move vertically.
When air cools, the particles do not collide as frequently, and the pressure tends to fall. To compensate, the particles move together and the air density increases. The air mass thus becomes heavier and sinks toward the ground, compressing the particles beneath it and creating an area of HIGH PRESSURE, called an ANTICYCLONE.

15. Depressions
when air warms, its density decreases, the air mass becomes lighter and rises, leaving an empty space beneath it. The space becomes an area of low pressure called a...

16. Cyclones.
When strong depressions develop over over warm waters of tropical oceans, a huge spiral forms stretching as far as 800 km in diameter, inside the wind blows at speeds up to 360 km. These storms are called cyclones, hurricanes or typhoons.

20. How hurricanes are formed