Why Weather Fronts Matter
So you chanced it and left home without the umbrellar. It's not going to rain right? All of a sudden the wind picks up and the skies go dark. There's a meteorology term for that - and that's a 'weather front'. The reason behind that sudden chill is an advancing front edge of air that replaces the current mass of air where you are which results in a sudden and dramatic change in weather conditions.
That might need a bit more explanation so we thought we'd dive in and explore the concept.
At the edge of a weather front, the air has a different density from the one that's currently in a location based on it's temperature. This difference in density doesn't easily merge which forces one of the air masses up and over the other. If the air that is lifted up has water in it, then you can expect to see condensation and precipitation.
The other thing you'll notice on a weather front, other than the change in temprature and possibly a spot of rain, is the wind. As you'd expect when you bring all of these forces together - fronts are the main cause of stormy weather
Imagine standing in the middle of a field as a cold front approaches. What you'll see is a line of rain, snow, or thunderstorms with blustering winds coming towards you. With these kinds of fronts - cold dense air forces the warm air up, as that happens the air pressure decreases. If the humidity is high enough, you'll see some types of cumulus clouds growing. In a cold front, high up in the atmosphere, winds blow ice crystals from the tops of theses clouds - generating cirrostratus and cirrus clouds.
Once the front has passed over the top of you, you'll really notice the cold air mass. This mass is generally pretty dry so the downpour petters out and if you're looking up you'd expect to see weather that's cold but really only partly cloudy or even clear.
So that's what you'd generally expect to see, although it can change depending on the season. Here's some more info on what to expect by season:
- Late Spring to Summer: The air can be volatile so thunderstorms can appear along the front.
- Spring: You can expect some strong winds if the temprature gradient is high.
- Autumn: There is normally really strong rain in these months.
- Winter: Those cold air masses are likely to have formed in the frigid antarctic.
You're back out in the middle of a field during winter. This time a warm air mass slides over a cold air mass. When hot, less dense air moves over dense, frigid air, the atmosphere remains reasonably stable which means the change from cold to warm air takes place over a long distance. So unlike with a cold front, the first signs of changing weather appear long before the front is actually over you.
As you can see from the above visual, the warm air is above the cold air meaning that for you on the ground the air still feels cold. The way you can spot the transition is by looking for high cirrus clouds - marking the line between one air mass and another.
With time, as the front develops the sky will turn grey and altostratus and altocumulus clouds will appear. Nimbostratus clouds start to form as the clouds thicken and winds grow stronger as the low pressure moves in. Looking up now all you see are clouds - but as the front gets closer the warm air is not too far above that cold air.
Don't expect the weather to get better just yet - in fact it's going to get worse. As that warm air mass approaches, any falling snow will start to melt and turn into sleet, this sleet will also be accompanied by freezing cold rain. Stratus clouds and fog will also start to form at the edge of the front as the warm and cold air mix together.
One other front that is worth mentioning are strationary fronts; as the name implies these are fronts where air masses do not move. This can occur for a few different reasons but a place you're likely to often see it is when a mass of air comes up against a barrier, like a mountain range. When things stop moving you better be ready for days of fog, mist, and rain.
Because it's still a front where two air masses meet you can expect winds to blow, however these are in parallel to the front and in opposite directions. After several days of these kinds of conditions a stationary front will most likely break apart.
A Good Rule of Thumb
If colder air is replacing warmer air, it is a cold front, if warmer air is replacing cold air, then it is a warm front, and if the front's not moving it's a stationary front.
Where Weather Fronts Began
The way weather works hasn't really changed - thanks to physics - but what does change is our understanding of it's impact and flow. The varying weather fronts that we know of today can be traced back to Jacob Bjerknes who, in 1919, announced his discovery of air masses and fronts.
Before this discovery we ascribed cold weather to yesterdays air but with some of the heat rising and dissapearing into space - or if it was warmer than it was the same air but just with more heat added into the mix. What Bjerknes realised was that air is far more dynamic, flowing around the globe rather than sitting still in one spot. As the air travels around it bumps up against other which creates these fronts where two air masses interact creating unsettled weather.