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Summer Science: Ten cool things about summer heat

With the days getting longer and the temperatures climbing, there's a lot of cool science going on, from the motion of our planet around the sun to the effects of the tiniest bits of matter on the world around us.

Cool Thing #1: Canadians are actually further away from the Sun in summer than we are in winter

Just from common sense, everyone knows that if you want to be warmer, you need to get closer to a source of heat. However, in the case of our planet and its seasons, that isn't necessarily the case, and for anyone living north of the equator, it's exactly the opposite, in fact — when it's summer in the northern hemisphere, the Earth is actually around 5 million kilometres further away from the Sun than it is in winter.

It's common to say that 'the Earth circles the Sun', but in actual fact we 'ellipse' the Sun... or in other words, our orbit is more egg-shaped (it's pretty exaggerated in the diagram, though). Right now, as we northerners approach the first day of summer, the planet is approaching its furthest distance from the Sun — called aphelion (or more generally apoapsis). By the time winter rolls around, the Earth will be approaching its closest distance to the Sun — called perihelion (or periapsis). It's opposite in the southern hemisphere, of course. They're approaching winter now and it'll be summer when the Earth is closest to the Sun.

Two other neat things about this are that the Earth is traveling slower along its orbit near aphelion and faster near perihelion, and due to something called 'precession', the date of our aphelion and perihelion are changing. So, even though the seasons on the calendar are always roughly the same length, the actual length of them right now is slightly longer when we're at aphelion and slightly shorter at perihelion, but that's going to change over time. In fact, the solstices are slightly off from perihelion and aphelion right now, and 10,000 years from now they'll be opposite, so we'll be closest to the Sun in summer, and furthest in winter.

Cool Thing #2: The reason for the seasons is the tilt of Earth's axis

If you look at any globe, it's always mounted on its base at an angle. That's not just for artistic reasons (although it does look nicer that way, I think), but because our planet is tilted on its axis. This tilt is the real reason for our seasons, as it means that a different hemisphere of the planet is getting more sunlight during different parts of the year.

Even though the seasons change, our tilt always 'points' in the same direction. Check out the image from the section above again, and see how the tilted axis always points off to the upper right, no matter where we are in our orbit. At aphelion and perihelion, the imaginary line of our axis matches up perfectly to another imaginary line running through the centre of the Sun, so the northern hemisphere is getting its most direct exposure to the Sun's light at aphelion and the southern hemisphere is getting its most direct exposure to the Sun's light at perihelion.

Cool Thing #3: 'The Dog Days of Summer'

Although summer is a great time to be out with your dog, the 'Dog Days of Summer' actually have nothing to do with our canine companions (not directly, anyway). They refer to the hottest days of the summer, and the name comes from Sirius, the 'Dog Star'. Sirius is the brightest star in our sky, but it's actually two stars — a blue-white star about 1.7 times the size of the Sun and a white dwarf star only about 1% the size of the Sun — and it's located about 8.7 light years away from here.

Because of Sirius' position in the sky, it's visible all year long, although it's a little harder to spot from May through August, since it's roughly in the same part of the sky as the Sun. There's a chance to spot it near sunset in May and June, but it's much easier to spot after dawn in July and August. Conditions have to be ideal to spot it, and you need to know where to look (to the south of the Sun), but it's this dawn or 'heliacal' rising of Sirius that marks the 'Dog Days'.

Cool Thing #4: The longest day is not the hottest day

Since June 21st is the longest day of year (at least this year), it follows that it should be the hottest day, but it's not. June 21st is considered the start of summer, rather than the middle of it, because the hottest temperatures typically happen after that day. Why is that? Basically it's because heating things up and cooling them down takes time, so even on a barren, airless rock of a planet, the hottest day might not be exactly on the summer solstice, but the fact that Earth has vast oceans and a moderately thick atmosphere means that the effect is dragged out even longer.

For the first part of the year, a lot of the energy that we get from the Sun goes into melting the snow and ice from winter, and once most of that is gone, the energy gets more directly invested in the ground and in the ocean water. The oceans, in particular, cling on to that heat for a long time, accumulating it and only releasing it slowly. So, the heat builds and builds, they get their most direct amount of sunlight during the summer solstice, but they keep on accumulating more heat on top of that as the summer progresses, reaching a peak later in the season, when the amount of sunlight they're receiving matches how much they're releasing into the atmosphere, and then the oceans start to cool down, cooling down the weather along with them. This is called the 'seasonal lag'.

Cool Thing #5: The sunniest days are the smoggiest days

Okay, I'll admit that this one is more 'interesting' and 'informative' than it is 'cool', because there's really nothing cool about smog. Smog, which also goes by the more technical name of 'photochemical smog', can contain a lot of different pollutants, but the two main ones are 'ground-level ozone' and 'fine particulate matter'. Ground-level ozone is the same as the stuff in the ozone layer, which is high above our heads, absorbing harmful rays from the Sun, but instead it's down here where we can breathe it in, and it mainly comes from car exhaust and power-plant emissions reacting with oxygen in the presence of sunlight. Fine particulate matter lumps together all the tiny particles of ash, smoke, chemicals and even metals, that are floating around in the air from industrial pollution, car exhaust and power-plant exhaust (to name a few).

There can still be days during the rest of the year when we have bad air quality days, but in summer they're usually worse because the rays of light from the Sun stream in from nearly directly above us, so there's less atmosphere to pass through and they still have plenty of their energy when they reach the ground. In other times of the year, the angle is more indirect and there's lots more atmosphere for the rays to pass through, so they lose more energy on their way to us. The chemical reactions that make ozone and other photochemicals work better with higher energy light, so a lot more of these reactions happen in the summer and concentrations of the chemicals can climb pretty high.

If it's a fairly nice day with a good wind, any pollution in the air may just blow and mix around and not end up being much of a problem for most of us, but if it's a hot, sticky, muggy day with little to no wind, that ozone and any other pollution in the air will linger, making it harder for people to breathe. If that muggy weather persists for a few days, the mixture will cook, day after day, until it's a harmful brew that can make breathing difficult for even the healthiest of us, and send some people to the emergency room.

Cool Thing #6: Frying an egg on the sidewalk

This has to be the biggest cliché about summer.

Personally, I wouldn't suggest eating any fried egg that was cooked anywhere but in a frying pan, on a grill, or in the microwave (most preferably clean in all cases), but this is actually possible.

One of my heroes, Bill Nye, talked about this a few years ago. He found that an egg cooks in the pan at around 125 degrees C, but he could get it down to about 55°C and the egg would still cook — it just took around 20 minutes (not the typical time you'd leave an egg in the pan). So, depending on what ground you were standing on — rock, concrete, asphalt — it is possible. Since transfer of the heat from the surface to the egg will depend on 'conduction', you can even speed that up a bit by using oil or butter on your cooking surface, so that more heat passes through to the egg.

Again, though, I still wouldn't recommend eating it.

Cool Thing #7: Beware of hot cars on a summer's day

Again, maybe not so cool, as noone likes sitting in a hot car and it's very dangerous to leave pets and/or children alone in the car when the Sun is out. This is mostly due to the glass of the windows. We can see through glass because it lets most wavelengths of light through without having any effect on them.

However, for infrared wavelengths, glass windows may as well be solid walls, as they let little to none of it through. When the stuff inside of your car absorbs the sunlight coming in through the windows, it radiates a bunch of it back out. Plenty of this radiation passes back out through the windows (which is why we can see the inside of the car from the outside, of course), but any of it at infrared wavelengths just bounces back off the windows and is reabsorbed by the stuff in the car. This continues, constantly, as long as the sunlight is streaming in through the windows, and the temperature of everything in the car — the surfaces, the objects, the air, and anyone or anything living — goes up and up. The results can, unfortunately, be tragic.

Cool Thing #8: Cool down on the hot beach

Anyone walking on the beach in bare feet knows how hot the sand can get, and the sand can stay warm well into the night. Obviously it gets that heat from the Sun, but if you've ever wondered how it gets as hot as it does, and why it can stay warm well into the night, to make those late-night beach parties so comfortable, it's actually due to air.

Pack a jar up to the rim with sand and it looks pretty full, but you can actually pour quite a bit of water into that jar. This is because the tiny irregularly-shaped crystals of quarts that make up the sand can't pack together tightly enough to pinch out all the space between them. If the sand is dry, there is a lot of air trapped between the crystals, and as the crystals absorb sunlight, they radiate heat outwards, which is absorbed by the air. As long as the sunlight is hitting the sand, this will continue, and the only way the heat can escape the air is if the sand gets turned up and that air can mix with the air above the sand, or it can escape by being absorbed by your skin.

When the Sun goes down, the sand and air no longer absorb heat, but because air is a good insulator (remember those tips about digging into a snowbank if you're ever trapped in a blizzard?) it hangs on to that heat. The sand grains at the surface radiate their heat into the air above, then absorb more heat from the air and grains underneath and transfer this to the air above as well. This continues, very slowly, all night long until the sun comes up again in the morning.

This may seem like just common sense, but if you want to cool your feet down on that hot beach... walk on the wet sand. The water between the grains not only needs more energy to heat it up than air does (so it does so more slowly), but as the waves move in and out, they circulate the water between the grains of sand, meaning that any water there that does get warmer is likely replaced by cooler water.

Cool Thing #9: Remember to wear your sunblock

One of the iconic parts of summer now is ads for, and warnings about, using sunblock. Sunblocks work by having chemicals mixed into them that actually physically absorb the harmful ultraviolet rays from the Sun.

Any sunblock you find should have an SPF rating (Sun Protection Factor). Anything below SPF 15 isn't going to be enough protection, and anything above 50 probably is just a waste of time and money, but values of 15, 30 and 50 are the most common. The number represents the fraction of the 'medium range' ultraviolet rays (UVB) that the product blocks, so SPF 15 lets through 1/15th of the UVB, SPF 30 lets through 1/30th of the UVB and SPF 50 lets through 1/50th of the UVB. That translates to blocking 95%, 97% and 98% of the UVB, respectively (it seems there isn't a rating for UVA).

Why is it important? These ranges of ultraviolet radiation can cause damage to our skin, resulting in the painful 'sunburn' that we probably have all suffered from at some point in our lives, but they can also cause damage to our DNA, which can result in skin cancer.

Cool Thing #10: Making your own ice cream with science

Want a way to cool down from the heat? A trip to the ice cream parlour is a fun outing, but using a bit of science you can actually make your own. All you need is milk, cream, sugar, flavouring, ice and salt. The first four of those will mix together to become the ice cream. The last two are what mix together to actually make the ice cream. This guide says to use a 1/2 cup of milk, a 1/2 cup of cream, a 1/4 cup of sugar and a 1/2 teaspoon of flavouring (vanilla or chocolate or whatever). Mix those in a bowl and then pour it into a freezer bag and seal it. Mix together about 2 cups of ice and a half cup of salt (they suggest rock salt but you can use table salt instead) in a larger freezer bag and put the smaller bag with the ice cream ingredients in as well. Seal it and shake for about five minutes, then let it sit.

What's the key to this? The temperature of what's around the ice cream ingredients while you shake them up. Ice is pretty cold, for sure, but when it's in any solid form, its surface area (the area that it can transfer heat across) becomes smaller. If you could have water that is as cold as the ice is, but still in liquid form, that would give a lot more surface area to transfer heat, and that's where the salt comes in. Adding salt to water lowers the temperature at which the water will freeze. Add salt to ice and the ice will melt, but its temperature doesn't change, so it's still just as cold as it was when it was in solid form. That gives you the right combination of cold and surface area so that when you shake up the ice cream mixture, you'll cool it down very quickly and very consistently. The only difference in using rock salt over table salt is that the rock salt won't melt the ice quite as fast, so the ice cream will freeze a bit slower and it'll be smoother.

After waiting a few minutes for your mixture to settle, open the bag, scoop out the ice cream and enjoy!

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(Images courtesy: Wikimedia Commons, Getty Images)

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