13: Weather, Water and Winds

Weather is weird right? The north being colder than the south makes sense (well, in the northern hemisphere, flip that sentence around if you live in the south), but as recent weather has shown, that logic can sometimes be thrown out the window. Why?? Well, there are many things that affect the weather, like how close you are to the sea and mountains. However, on a planet wide scale wind, water and air pressure have a huge effect on weather everywhere. This post is all about what makes weather, including things like hurricanes, how dams make rain, and how Scotland might soon be as chilly as Alaska!

A weather vane showing the changing winds (Picture: Arne Koehler/Wikimedia Commons/ CC BY-SA 3.0)

Jet stream

Weather is the state of the atmosphere at a particular place and time including temperature, cloudiness, dryness, wind, rain etc… Part of our atmosphere is the jet stream, which is a band of strong winds around 5 to 7 miles above the Earth’s surface, blowing from west to east. Wind speeds can be over 200 mph but we don’t see or feel this at ground level. This part of the atmosphere helps develop and steer weather around the globe, sometimes bringing storms, and sometimes bringing calm and settled days. How does it do this? Well, let’s look even further into the basics.

So the jet stream is a band of winds, north of which it is high pressure, and south of which is low pressure. Hot air rises, cold air falls, and in high pressure areas, air moves downwards making it dry, leaving the sky clear and generally creating nice weather. In low pressure areas, as the air is hot, evaporation happens causing liquid in the ground to form clouds and rain or snow. The jet stream is a wobbly band of winds (see photo below) so depending on how it’s wobbling you’ll have high or low pressure. What happens if you’re at the point where it’s switching from high to low? Well, if a cold front (cold air arriving at warm air) arrives then you get big rainstorms and thunder, while a warm front (warm air arriving at cold) creates more steady and less dramatic rain.

The weather around a changing jet stream (purple line)

Arctic Oscillation

So weather depends on air pressure and temperature, and this changes as the wobbly jet stream moves. The jet stream which affects the northern hemisphere is called the Arctic Oscillation. It can have a negative phase, where it’s super wobbly and areas parallel to each other have very different weather, and a positive phase where the jet stream is steadier and weather is more consistent. The Arctic Oscillation switches between these on a daily, monthly, seasonally and yearly basis, which is why it can be tricky to forecast the weather. Extremely negative phases can lead to extreme weather like in 2010 which saw massive snowstorms in Maryland, New York and Catalonia.

The Arctic Oscillation in positive and negative phase on the planet. Picture: NOAA/Public Domain

Effects of water

Of course there are many more things that affect the weather than just a band of wind. Mountains force air upwards and can cause it to drop all its moisture meaning only dry air reaches the other side, which is why often one side of a large mountain range is very wet whilst the other side is really dry.

Water also has a huge effect. Warm water evaporates easier than cold water, and when it does this over a large area, say an ocean, it can create a build-up of energy. This is when you get hurricanes, and is why they usually happen at the equator where water is warmest. Near large lakes more water can be taken up into the clouds and dropped as rain or snow on nearby communities, and this isn’t just limited to natural lakes. It can also happen at large man-made dam reservoirs, so building a big dam can actually alter the weather in an area.

Water also takes longer to warm up and cool down than air does, which is why areas near water tend to have more stable temperatures than areas inland. The reason why northern Europe is far warmer on average than areas at the same latitude in Russia and North America is because of the Gulf Stream, a warm water current in the Atlantic. This current is pretty permanent, but on the west coast of the U.S the temperature of the Pacific Ocean can change. Sometimes it becomes warmer than usual causing ‘El Niño’, where the air above the water changes temperature too splitting the jet stream. This creates milder temperatures in the northern US, and a wetter winter in the south. When the opposite happens and the Pacific is cool, it creates La Niña, with mild weather in the south and cold air moving to the New England region (Eastern US).

A hurricane as seen from space (Picture: NASA/Public Domain)

Potential future changes

All of this is, as mentioned, a very simplified overview of weather, and even professional meteorologists can’t predict everything about what it will do. Weather changes, and that’s natural, but it is likely to become even more changeable and unpredictable in the future. Average warming of the atmosphere because of greenhouse gases might change the balance of hot and cold airs, and so when and where fronts happen. Melting ice caps means that there’s more cold water in the oceans which could also knock things out of balance. In fact there are some concerns that if too much cold water enters the oceans from ice caps, the Gulf Stream might stop bringing warm water near Europe, which would mean places like Lisbon would have the climate of New York, and Edinburgh may become similar in climate to parts of Alaska. Chilly…

All this is a good example of how climate change isn’t just warming, it’s a change in the balance of weather systems too and so in some places might mean cooling even if overall average temperatures are on the rise. No one knows exactly what’s going to happen, so maybe keep a hold of your puffy jackets, welly boots and bikinis as you may need them all within days of each other in the coming years.

For more info:

Another article about weather

Links between climate change and more extreme weather

Music: kongano.com

Title Image: Mathias Krumbholz/Wikimedia Commons/ CC BY-SA 3.0

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