Cold Fronts, Warm Fronts & Review of Humidity
Weather is the short term variation in atmospheric conditions. Weather occurs
when air mixes. The air in the troposphere is not all the same, it actually forms “pockets”
or, as meteorologists say, “air masses.” These air masses have similar characteristics.
For example, an air mass can carry warm dry air, or warm moist air, or cold dry air, etc.
Air masses also vary in density. When very different air masses collide it can create
some violent weather. In this mini lesson we will look at two types of weather that is
created when cold and warm air masses meet.
Cold Front
A cold front is when cold air moves into an area dominated by warm air. The warmer air
mass will keep close to the earth’s surface and as the cold air mass moves in, they ‘butt
heads.’ When a cold air mass collides with a warm air mass, they don’t mix very well
and as a result, the cold air is forced upwards quickly all along the front of the warm air
mass. When cold air rises quickly, it condenses and formed large cumulus clouds. If
there is enough uplift of the cold air, significant storms can occur all along the boundary
between the warm and cold air masses. Such weather includes, thunderstorms, strong
winds and hail. Weather systems created via cold fronts do not cover a large area of the
earth’s surface and move very quickly.
Think back to a hot humid day in the summer. Ever remember there being a
thunderstorm in the evening and then afterwards the air seemed much cooler? This is
because a cold air mass has come into the area – that is, a cold front has moved into
the area!
Cold Front Diagram
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Warm Front
A warm front is when warm air moves into an area dominated by cold air. As the warm air mass
moves in, the two air masses meet. When a warm air mass collides with a cold air mass, they
interact very differently. The warm air is forced upwards very gradually all along the front of the
warm air mass. This takes time, and as a result covers a much larger area of the earth’s
surface;(unlike cold fronts where the weather created covers a very small area of the earth’s
surface and moves quickly.) When the warm air rises it does so on a more gradual incline. As a
result, clouds will form at lower altitudes. Weather associated with warm fronts include fog,
drizzle, and rain for long durations, typically a day or more as they move slowly.
Humidity
Humidity is the amount of water vapour in the air. Colder air cannot hold the same amount
of water vapour as warm air, thus when warm air rises and becomes cooler clouds form and
often leads to precipitation.
Humidity is measured in a percentage. 100% means the air is saturated and cannot hold any
more water vapour. This is also when it rains/snows. If humidity is not at 100% then it is not
raining/snowing.
How does the air become saturated?
For air to become saturated, the air must drop in temperature. This temperature is referred to as
the dewpoint. If you visit the weather websites you will often see a current temperature
recording and a dewpoint temperature. This dewpoint temperature allows meteorologists and
the general public, to know how many degrees the air must decrease by in order for
precipitation to occur.
Why does the temperature of the air need to drop in order for it become saturated?
Warmer air can hold much more water vapour than cold air. Think of it this way, warm air is a
larger bowl than cold air; thus, it can hold more water. When warm air rises, that bowl shrinks in
size…the amount of water vapour in that bowl doesn’t. When the air gets to the dewpoint
temperature the bowl can no longer hold the water vapour and it begins to overflow…this is the
rain/snow!
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