Before we dive into our next meteorology 101 class, there has been some breaking weather news. Well, by the time you read this, the news might not be so breaking but, according to the U.S. National Oceanic and Atmospheric Administration, after three years and one of the longest La Niñas on record, ocean temperatures across the equatorial Pacific have warmed to around average, officially bringing this La Niña to an end.
Current predictions are for conditions to remain neutral during the spring and summer, with El Niño, or warmer-than-average conditions, developing by the fall.
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A short review: La Niña is when we see a cooling of ocean temperatures in the Pacific, and El Niño is a warming of this same ocean area. Both events have a large impact on the atmosphere, global temperatures and rainfall or lack thereof.
La Niña tends to cool global temperatures while El Niño warms them. The disturbing part of this is that during the last La Niña we really did not see much global cooling. In fact, 2021 was the seventh warmest year on record, with 2022 coming in as the sixth warmest.
It appears this La Niña event helped make it appear that global warming has slowed, but with El Niño possibly developing by 2024, scientists predict we may see our first year with the average global temperature surpassing 1.5 C of warming. It’s definitely something to watch over the next year or two.
Scattered rays
In our last class, we looked at the composition of the atmosphere, breaking it down into a heterosphere and homosphere. Then we looked at the atmosphere from a temperature point of view and proceeded to break it down into four regions or layers: the thermosphere, mesosphere, stratosphere and troposphere.
We finished off by saying one of these layers is responsible for most, if not all, of our weather.
This week we will extend our understanding of weather and the atmosphere by beginning our look at the atmosphere and surface energy balances.
To begin to understand how solar energy is spent as it reaches Earth’s surface, and thus understand our surface energy budget, we need to look at the pathways in which solar energy can travel once it reaches Earth’s surface.
Earth receives energy from the sun in the form of shortwave radiation. When this energy is turned into heat, it takes on the form of long-wave radiation. A good portion of both of these types of radiation passes through our atmosphere in the process known as transmission.
When we look at shortwave radiation reaching Earth’s surface, we call it insolation, and this insolation is the driving force behind all weather.
Insolation is comprised of shortwave radiation that is transmitted directly to the ground, along with diffused or scattered radiation (indirect radiation). As shortwave radiation travels through our atmosphere, some of it interacts with gas, dust, pollutants, water droplets and water vapour, changing the direction of the shortwave radiation or scattering it.
This scattering is what causes the sky to be blue during the day and why sunsets and sunrises take on a reddish hue.
The principle behind why we see these colours is known as Rayleigh scattering, named after the English physicist Lord Rayleigh, who came up this principle in 1881. The principle relates wavelength to the size of the particles that are causing the scattering.
The general rule is, the shorter the wavelength, the greater the scattering and the longer the wavelength, the less the scattering. Small gas molecules will scatter shorter wavelengths. With visible light, blues and violets have the shortest wavelengths, while oranges and reds have the longest wavelengths.
Since short wavelengths are scattered the most and the molecules in our atmosphere scatter short waves, the lower atmosphere is dominated by scattered blue waves.
At sunrise and sunset, the angle of the sun is such that the insolation has to travel through much more atmosphere than during the day. The short blue wavelengths are still scattered, but now they encounter so much scattering that only the longer orange and red wavelengths are left to reach our eyes, so we tend to see these colours.
Another thing that happens to shortwave radiation as it enters the atmosphere is that it refracts. Refraction is the bending of light as it passes from one medium to the next.
In this case, it is passing from the virtual vacuum of space to our dense atmosphere. We have all seen examples of refraction. Rainbows are created when light passes through dense water droplets causing the different wavelengths of light to refract at different rates.
Mirages are another example of refraction. Most of us have experienced mirages on warm days along a highway when you see what appears to be something floating above the road. In this case, it is the hot air above the highway that causes the light to be refracted.
One interesting note about refraction is that without it, the amount of daylight we receive would be about eight minutes less each day.
When the sun sets or rises, the light refracts as it passes from space into our atmosphere. This refraction allows us to “see” the sun when it is actually below the horizon. In the morning we see the sun rise four minutes before it actually moves above the horizon and at sunset we continue to see the sun for four minutes after it has actually dropped below the horizon.
