Most weather patterns are created by air masses, so what happens when two air masses meet?
Air masses don’t merge when they come into contact but push against the other. This occurs when warm air masses meet cold air masses.
Since it is lighter, warm air rises, spilling water vapor and condensed water into the cold air mass above. When warm air rises, it creates a low-pressure area called a thermal low.
As colder air moves into the area, it spirals around the thermal low in a counter-clockwise direction, creating an effect called winds.
When warm, moist air meets cold, dry air, the boundary between the two is called a weather front.
Warm air then rises over the cold air and creates a low-pressure zone, or trough, with downward-moving air that can cause stormy weather.
There are four types of fronts formed when air masses meet:
- Cold Front
- Stationary Front
- Warm Front
- Occluded Front
Since air masses are composed primarily of the temperature and humidity characteristics of the region they are in, they tend to stay together.
If one part emerges from a low-pressure region, it will become an area of high pressure somewhere else.
Convection and Weather Fronts
Whenever moving heat sources such as hot air or fluids touch something that is cooler, they can transfer their heat to the object.
Therefore, the heated object will become warmer and should move away from the original heat source as a result.
This change in position is known as convection, which is also a heat transfer mechanism.
An everyday example of this process is when warm moist air rises to form clouds that will condense for us to experience rainfall.
Fact: If two air masses meet to form intense enough winds and central pressure is low enough, it becomes a hurricane or tornado.
Cold fronts are a widespread occurrence and crucial to understanding what happens when two different air masses meet.
When cold air masses replace warm air masses and push the warm air upwards, it creates clouds, then precipitation, and in some cases, very winds.
Sometimes, there is such a change in pressure that the warm air mass retreats and cold air move in to replace it, causing a cold front.
Cold Fronts and Rain Clouds
Weather along cold fronts can vary with its vertically uplifted air mass structure.
Generally, it’s characterized by a narrow band of cloudiness which will then lead to precipitation from many types of clouds, especially those with a dark appearance (nimbostratus), which can produce rain.
Another common occurrence during the cold front’s approach is increased wind activity in warm areas and the appearance of cirrus clouds.
What follows is usually altostratus and alto-cumulous, which are often less dense.
Fact: Cold fronts have the most dramatic weather changes associated with them, but the weather along cold front usually fades off quickly and is sometimes violent.
In a nutshell, a warm front is a slope of warm air over cold air along a gradient.
On a warm front, there is often a gradient variation of between 1:100 and 1:200.
As the warm wind moves up the slope, it condenses and precipitates; this causes temperature and wind direction to change gradually.
Warm Fronts and Rain Clouds
A warm front commonly brings nimbus and cirrostratus clouds.
When cirrostratus clouds appear before the warm front, a halo effect around the sun and moon may be observed, usually in the afternoon.
A warm front usually brings a large area of precipitation, lasting several hours.
As the warm front passes, the temperature rises, and pressure increases.
Note: Unlike in cold fronts, temperature and wind speed direction changes in a warm front aren't abrupt.
A warm front is also rarely accompanied by thunderstorms or severe weather.
Another interesting phenomenon that happens when two different air masses meet is a stationary front.
It’s a front that forms if the surface position of a front doesn’t change. A stationary front is where the wind speed is equal on both sides of the front.
If warm air from the South runs into colder air from the North, precipitation occurs because the warm air has a higher moisture content than the cold air.
Stationary Front, Cold Front, and Warm Front
Generally, all front systems tend to exhibit a stationary phase due to the presence of a warm front.
When warm air overruns a cold air mass on its “warm” side, precipitation water will be released in the form of rain and snow.
Sometimes the precipitation release is prevented when the air is rapidly cooled down; in this case, the process is called dewpoint depression.
On weather maps, a stationary front is represented by alternating red and blue lines, with red triangles pointing to the region of colder air and blue semi-circles showing the area of warmer air.
An occluded front develops when a cold air mass overtakes a warm air mass and slides under it.
The warm sector weakens, and the cold air mass increases, gaining the warm sector ground.
The warm sector of the atmosphere diminishes, and the cold air takes over the whole front of the warm sector.
Occlusion fronts are known for their complex weather, a mix of cold-front weather with lethargic activity and warm-front weather with quick-moving storms.
Fact: Occluded fronts typically stretch from the Arctic to the subtropics and are a common occurrence in Europe, Northern states of the US, and Southern Canada
Characteristics of Air Masses
As the temperature difference between the cooler and warmer air masses in a front rises, the thickness of the frontal zone also increases.
When temperature and pressure isobars are smooth curves, they can be seen to curve towards low pressure.
Pressure also increases with a drop in temperature, and it’s a primary occurrence of what happens when two air masses meet.
Pressure causes the atmosphere to crest into an accordion shape. The change in pressure is reflected in an upwards motion of the air mass front.
Coriolis Force and Wind Motion
The form of the front is also affected by a sudden temperature change.
Frontal systems are low-pressure areas that experience a wind shift; this means weather changes happen when the fronts are present.
The show commonly brings a difference in the wind due to pressure changes and the Coriolis force (a function of earth’s rotation) pushing on the air.
This explains why cold fronts usually occur in low-pressure troughs in the Northern Hemisphere.
Rainfall and Air Masses
The formation of atmospheric clouds is associated with frontal activity, which occurs when warm air ascends and cools.
The amount of rain depends on how fast the air rises and how much water vapor it contains.
Fact: The weather at the boundary between air masses is referred to as the boundary layer.
It’s safe to say whenever there is a weather change in your region. It means two air masses have met.
However, it’s worth noting that other factors can also affect weather.
Weather can also be influenced by solar radiation, the earth’s tilt, its distance from the sun, its latitude, and the temperature, air pressure, and water supply of any particular region.
Read Next: Stable Air vs Unstable Air?
One crucial fact to consider about what happens when two air masses meet is when an air mass from one region moves over a second region, these two areas have different conditions.
The movement of air masses carries their characteristics to new locations.
This can change the temperature and humidity of the second region since different air masses are constantly moving around the world, so their other characteristics affect other areas.
For example, when warm air comes into contact with the cold ground, it can cause a phenomenon called inversion over a region.