Convection and Weather

In Britain, it’s hard not to head into the Great Outdoors without thinking about the weather. A favourite topic of our nation and rightly so since being an island, our weather system is very changeable so always a good filler if short of things to say! But why is it so changeable?

There are many factors involved in weather formation (and prediction) so let us focus on a key one, air temperature variation. Heating of the Earth’s surface by solar radiation is not even. At school, we all learnt about the tilt of the Earth on its axis, approximately 23˚, leading to the Equatorial Regions receiving a greater proportion of the Sun’s ray, and hence solar energy, than the Polar Regions. They are simply closer to the Sun. This results in large-scale currents forming in the air and in the oceans, carrying thermal energy from low to high latitudes (Figure 1 below).

Earth tilt and convectionFigure 1: The tilt of the Earth causes globally uneven heating from solar radiation

At the Earth’s surface, each time a ray of sunlight hits, some of the thermal energy is absorbed and some is reflected, causing localised heating from the absorbed waves. The amount of energy that is reflected depends on the brightness of the surface. This is known as the Albedo Effect. Measurements of a surfaces ‘albedo’ can vary from as much as 10% for watery surfaces to 90% for fresh snow. Another way to look at is that if only 10% is reflected by water, then 90% must be absorbed by it, causing it to heat up more than fresh snow. However, since water is also transparent, this absorbed energy is spread evenly throughout it so when water is compared to other surfaces of similar albedo, such as dark areas on land, it will heat up more slowly. This leads nicely onto explaining how sea breezes form around our coastline.Sea breezes and convectionFigure 2: Different heating rates between the land and the ocean causes convection currents, which we feel as sea breezes

Figure 2 above show the differences in heating effects of land and ocean. As the land heats up more quickly, the air above it becomes warmer than the air above the sea. Another fact that you may have remembered from school, is that warm air rises! So the warm air above the land rises upwards, leaving space for the cooler air over the ocean to move in. The beginnings of a convection current. Other examples of convection currents can be found in the ocean and within the mantle layer of the Earth. Although the movement of air is invisible, you may see the result of this convection current in the form of clouds as the warm air eventually meets cooler air at altitude, condensing into small water droplets.

Convection Pie Dish – Try this at home!

Equipment needed: Aluminium quiche or pie dish, hot plate or similar heat source with variable temperature control, 4 or 5 same denominational coins, pearlised hand soap, a cup of water, food colouring, oven mitts (for safety)

  1. Spread the coins out on the hot plate (this should be turned off), then place the quiche dish on top
  2. Add the water, so that the level is half-way up the tin, and two good squeezes of hand soap
  3. Mix gently, trying to avoid forming bubbles
  4. Add a few drops of food colouring to the mixture and again stir in gently until it is evenly spread
  5. Wait until the surface of the mixture has settled, then turn on the hot plate to a low setting and watch

The coins will conduct heat at a faster rate from the hot plate to the dish, causing localised hot spots. The soap solution above the coins will heat up more quickly, causing the solution to rise up, similar to the air above a sea shore. The cooler soap solution from other parts of the dish move into fill the spaces left behind, resulting in the beautiful patterns forming in the dish. The pearlised soap contain metallic particles that help to reflect light, making the convection currents formed easier to see. The food colouring does likewise.

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