1 00:00:00,476 --> 00:00:05,248 Here is the forecast for the next 30 minutes. It will be an 2 00:00:05,288 --> 00:00:10,044 unpredictable start, giving way to clear blue skies. Later it there 3 00:00:10,084 --> 00:00:13,796 will be wind with rain and the occasional flash of lightning. 4 00:00:13,836 --> 00:00:17,156 There is also the distinct possibility of a hurricane so 5 00:00:17,196 --> 00:00:21,028 please stay tuned for further updates. And if you do decide to 6 00:00:21,068 --> 00:00:28,436 head out, here is the important things you need to know about the 7 00:00:28,476 --> 00:00:32,748 weather. First up, why it is the weather so 8 00:00:32,788 --> 00:00:38,324 unpredictable? We have probably all been victims of bad weather 9 00:00:38,364 --> 00:00:43,120 forecasts at some point. Believe it or not, you need a degree to be a 10 00:00:43,160 --> 00:00:48,368 meteorologist. In their defence, the planet's weather is in chaos. 11 00:00:48,408 --> 00:00:51,968 It is what scientists call a chaotic dynamics system. Dynamic 12 00:00:52,008 --> 00:00:57,136 because it changes all the time, and chaotic because it doesn't 13 00:00:57,176 --> 00:01:02,808 follow any set pattern. That is because the Alf has massive 14 00:01:02,848 --> 00:01:09,360 mountains, huge oceans and a thick atmosphere -- the earth. It is 15 00:01:09,400 --> 00:01:16,448 being constantly bombarded by heat from the Sun. This combination of 16 00:01:16,488 --> 00:01:20,496 limitless energy and lopsided landscapes provide -- produces 17 00:01:20,536 --> 00:01:25,156 large and violent weather systems, and they bump into each other as 18 00:01:25,196 --> 00:01:27,936 they stomp around the planet. This complexity makes the weather in the 19 00:01:27,976 --> 00:01:32,952 earth the least predictable in the phone system, but it has not 20 00:01:32,992 --> 00:01:39,168 stopped us from trying. In olden times, we relied on guesswork and 21 00:01:39,208 --> 00:01:43,840 folklore, like interpreting animal behaviour. Or entrails. But the 22 00:01:43,880 --> 00:01:48,596 only vaguely accurate method was observation. During the Renaissance, 23 00:01:48,636 --> 00:01:53,296 we invented machines like a barometer, which measures 24 00:01:53,336 --> 00:01:57,440 variations in air pressure. This helped us with short-term weather 25 00:01:57,480 --> 00:02:05,624 changes. Then Samuel Morse patented the electromagnetic Telegraph, 26 00:02:05,664 --> 00:02:10,124 which meant we could find out what the weather was like elsewhere, but 27 00:02:10,164 --> 00:02:15,092 that was still no guarantee that this weather was on its way to us. 28 00:02:15,132 --> 00:02:19,544 We were still stuck in observation mode. A major breakthrough came in 29 00:02:19,584 --> 00:02:24,844 1922, when a scientist, Lewis Richardson, developed a 30 00:02:24,884 --> 00:02:28,080 mathematical model for forecasting future weather, using information 31 00:02:28,120 --> 00:02:32,452 gained from observations of conditions around the world. This 32 00:02:32,492 --> 00:02:38,344 was great, except it would have needed 64,000 people working flat 33 00:02:38,384 --> 00:02:46,032 out to keep the forecast up-to-date. Luckily, we then invented computers 34 00:02:46,072 --> 00:02:50,236 that did the math for us and the idea took off. But even with all 35 00:02:50,276 --> 00:02:54,328 today's modern forecasting technology, meteorologists still 36 00:02:54,368 --> 00:03:01,296 struggle to stay on top of the chaos of the world's weather, which 37 00:03:01,336 --> 00:03:04,924 is why they can still be wrong about tomorrow. So now you know why 38 00:03:04,964 --> 00:03:08,376 forecasters make bad predictions. Not that we should best as they 39 00:03:08,416 --> 00:03:13,204 really give them a break about it. It does seem that all you need to 40 00:03:13,244 --> 00:03:17,656 do is press this button and the weather changes, and all anybody 41 00:03:17,696 --> 00:03:25,596 wants his sunny days and clear skies. Except they are not really 42 00:03:25,636 --> 00:03:29,640 clear. They are blue. Why is the sky-blue? This would be a strong 43 00:03:29,680 --> 00:03:35,144 contender for signs's dirtiest trick question because really, it 44 00:03:35,184 --> 00:03:39,704 isn't blue. It isn't any colour at all. It all comes down to how the 45 00:03:39,744 --> 00:03:43,968 human brain interprets visible electromagnetic radiation. Also 46 00:03:44,008 --> 00:03:47,956 known as sunlight. Although sunlight appears as white, it is 47 00:03:47,996 --> 00:03:51,752 actually a mixture of all the magical colours of the rainbow. 48 00:03:51,792 --> 00:03:57,128 Isaac Newton demonstrated this when he used a prism to separate them 49 00:03:57,168 --> 00:04:00,848 out, a nifty little party trick known as refraction. These colours 50 00:04:00,888 --> 00:04:07,704 form the visible range of what is called the electromagnetic spectrum. 51 00:04:07,744 --> 00:04:15,096 It includes super hero powers, like the Marais and X-rays, and the less 52 00:04:15,136 --> 00:04:20,144 sexy microwaves and radio waves. Like waves on the spectrum of so 53 00:04:20,184 --> 00:04:25,504 small they are measured in nanometres, and each colour have 54 00:04:25,544 --> 00:04:29,268 its own individual wave length and frequency. But in space, they 55 00:04:29,308 --> 00:04:35,376 combine to produce white lights and they all travel at the same speed. 56 00:04:35,416 --> 00:04:39,064 The speed of light. The action doesn't really begin until the 57 00:04:39,104 --> 00:04:45,300 light reaches the Earth and crashes headlong into the oxygen and 58 00:04:45,340 --> 00:04:47,912 nitrogen molecules that make up 99% of our atmosphere. It is then that 59 00:04:47,952 --> 00:04:52,912 the colours started separate, with some getting knocked about more 60 00:04:52,952 --> 00:04:57,160 than others. An effect known as Rayleigh scattering. The shorter 61 00:04:57,200 --> 00:05:00,612 the colour's wavelength, the more it gets bounced around and the blue 62 00:05:00,652 --> 00:05:05,532 light is that the shorter end of the spectrum. This means that when 63 00:05:05,572 --> 00:05:08,880 it hits the atmosphere, the air molecules scatter the blue light 64 00:05:08,920 --> 00:05:14,240 everywhere, while other colours pass straight through with barely a 65 00:05:14,280 --> 00:05:21,272 scratch. So the colour blue appears to be coming from all over the sky 66 00:05:21,312 --> 00:05:26,080 as it makes its way to your eyes. But here is why it is a trick 67 00:05:26,120 --> 00:05:32,440 question. This guy isn't really blue, we just think it is. -- the 68 00:05:32,480 --> 00:05:38,096 sky isn't really blue. The are his only pick up the wavelength and 69 00:05:38,136 --> 00:05:44,176 send that signal to be a brain. The brain and then decides what colour 70 00:05:44,216 --> 00:05:51,456 that wavelength should be, while painting a picture in real time. It 71 00:05:51,496 --> 00:05:56,456 is the original blue-sky thinking. This guy isn't the only vast 72 00:05:56,496 --> 00:06:00,612 expanse that appears to us to be blue -- the sky. The oceans do for 73 00:06:00,652 --> 00:06:04,612 the same reasons. They scatter at the Blue Light while absorbing the 74 00:06:04,652 --> 00:06:12,708 other colours and that is probably why the Earth is referred to as the 75 00:06:12,748 --> 00:06:19,944 Blue Planet. There are 1.3 billion cubic kilometres of water out there 76 00:06:19,984 --> 00:06:25,972 and that begs the question: How do oceans affect the weather? Water, 77 00:06:26,012 --> 00:06:32,224 water, everywhere and not a drop to drink. Unless, of course, you are 78 00:06:32,264 --> 00:06:36,896 one of these. Oceans cover 71% of the Earth's surface and contain 90 79 00:06:36,936 --> 00:06:41,804 tempers cent of its water. So it is not surprising that the ocean has a 80 00:06:41,844 --> 00:06:46,400 big say in the world's weather. Their most important role is in 81 00:06:46,440 --> 00:06:51,176 storing and distributing heat. In fact, over half of the heat we get 82 00:06:51,216 --> 00:06:56,412 from the sun is stored in them. Most of its days near the surface 83 00:06:56,452 --> 00:07:00,928 in the first few metres of what is called the epic pelagic zone. Ocean 84 00:07:00,968 --> 00:07:06,916 temperatures can reach up to 36 degrees Celsius and this warm-water 85 00:07:06,956 --> 00:07:12,712 it swims around in surface currents, created by passing winds. Other 86 00:07:12,752 --> 00:07:17,384 currents move about in water as Col des minus two, and this change 87 00:07:17,424 --> 00:07:22,880 helps to keep worldwide temperatures in balance -- as cold 88 00:07:22,920 --> 00:07:28,524 as minus two. The most well-known is the Gulf Stream. It starts out 89 00:07:28,564 --> 00:07:32,992 in the Gulf of Mexico and passes the tip of Florida, where it begins 90 00:07:33,032 --> 00:07:38,848 a 2500 kilometres migration across the Atlantic. Amazingly it shifts 91 00:07:38,888 --> 00:07:48,848 over 100 times more water than all the rivers on earth. Ocean's also 92 00:07:49,160 --> 00:07:52,200 supply around 90% of the water used to create rain. They are hugely 93 00:07:52,240 --> 00:07:58,120 effective at absorbing carbon dioxide, which manages the Earth's 94 00:07:58,160 --> 00:08:03,244 climate. They are also home to more slippery things, like hurricane. 95 00:08:03,284 --> 00:08:08,996 But perhaps the most slippery of phenomenon is a mean you have. It 96 00:08:09,036 --> 00:08:18,996 is a complex phenomenon in the earth's climate -- L Lemieux. It 97 00:08:21,452 --> 00:08:24,696 appears every three-- El Nino. Its arrival causes a number of strange 98 00:08:24,736 --> 00:08:29,448 things to happen, including a dramatic increase in rainfall in 99 00:08:29,488 --> 00:08:35,180 South America, while at the same time droughts in Australia, and 100 00:08:35,220 --> 00:08:38,840 these extremes caused upset to everyone. We still don't completely 101 00:08:38,880 --> 00:08:44,104 understand what causes El Nino but did implement on the weather is 102 00:08:44,144 --> 00:08:49,588 second only to the changing of the seasons. And we are never really 103 00:08:49,628 --> 00:08:54,244 totally sure when it will return. So you can think of the oceans 104 00:08:54,284 --> 00:08:58,276 might joined factories where a lot of the weather is manufactured -- 105 00:08:58,316 --> 00:09:02,996 like joint factories. But it would not get anywhere without another 106 00:09:03,036 --> 00:09:06,776 vital component, the wind. Sometimes it is a gentle breeze, 107 00:09:06,816 --> 00:09:16,776 sometimes it is strong enough to blow you over, but how does that 108 00:09:18,064 --> 00:09:27,336 come out of thin air? Why does the wind blow? The wind is guaranteed 109 00:09:27,376 --> 00:09:32,368 to get you on the moves. The key is the Sun, which produces 386 billion 110 00:09:32,408 --> 00:09:36,416 billion megawatts of power every second. Some of this energy gets 111 00:09:36,456 --> 00:09:40,232 transferred onto the Earth's atmosphere. The equator receives 112 00:09:40,272 --> 00:09:44,400 the most direct sunlight while the North Pole and the South Pole have 113 00:09:44,440 --> 00:09:48,640 to make do with whatever they can get. To redress the imbalance, the 114 00:09:48,680 --> 00:09:54,184 warm air at the equator rises and heads off to the North Pole and the 115 00:09:54,224 --> 00:10:00,280 South Pole. Once there, it gets colder then goes back to the 116 00:10:00,320 --> 00:10:05,120 rises because the molecules get excited and begin to jump about so 117 00:10:05,160 --> 00:10:09,544 that the air expands. Colder temperatures, the molecules down, 118 00:10:09,584 --> 00:10:13,436 so the air contracts, becomes more dense and fought back to earth. 119 00:10:13,476 --> 00:10:17,232 These motions create regions of high and low air pressure and the 120 00:10:17,272 --> 00:10:25,684 air will always flow from where the pressure is high to where it is low. 121 00:10:25,724 --> 00:10:33,856 This results in wind. Added to that is friction, caused by the planets 122 00:10:33,896 --> 00:10:38,528 dips and bums. This helped to decide when speed and direction. 123 00:10:38,568 --> 00:10:42,684 Then at the Coriolis effect comes into the mix. As the Earth rotates, 124 00:10:42,724 --> 00:10:46,528 it sends the moving air into a spin, throwing it to the right in the 125 00:10:46,568 --> 00:10:51,136 northern hemisphere and to the left down under, leading to weather 126 00:10:51,176 --> 00:10:55,824 systems that rotate in opposite directions. But wind is not simply 127 00:10:55,864 --> 00:11:01,496 a global phenomenon. It also happens on a much smaller scale. 128 00:11:01,536 --> 00:11:05,560 With different surfaces, such as forests, mountains, deserts, Oceans, 129 00:11:05,600 --> 00:11:09,028 all been heated unevenly, it creates localised wind patterns. 130 00:11:09,068 --> 00:11:14,776 What you end up with are those funny circles on a weather map that 131 00:11:14,816 --> 00:11:17,720 look as though they have been drawn by a two-year-old. They represent 132 00:11:17,760 --> 00:11:24,000 regions of high pressure and low pressure, with the Arrows telling 133 00:11:24,040 --> 00:11:28,496 you which way things are going. The wind comes recommended by 134 00:11:28,536 --> 00:11:33,700 Christopher Columbus, geese and the Netherlands. But please remember to 135 00:11:33,740 --> 00:11:39,624 treat it with caution. As it can, in some cases, lead to extreme 136 00:11:39,664 --> 00:11:47,236 devastation. The wind is a fundamental part of Mother Nature's 137 00:11:47,276 --> 00:11:50,840 game plan for the weather, but occasionally she throws in a couple 138 00:11:50,880 --> 00:11:55,264 of other players as well, and almost, their least favourite 139 00:11:55,304 --> 00:12:00,312 opponent is the rain. Love it or loathe it, it is an essential part 140 00:12:00,352 --> 00:12:05,060 of the weather. The next question is: Where does all of the rain come 141 00:12:05,100 --> 00:12:10,344 from? Rain, rain, go away, come again another day. You might 142 00:12:10,384 --> 00:12:14,764 remember that one from your childhood. It is actually a good 143 00:12:14,804 --> 00:12:18,856 summary of the hydrological cycle. The never-ending journey mortar mix 144 00:12:18,896 --> 00:12:23,544 between the Earth and the atmosphere. -- water makes. It 145 00:12:23,584 --> 00:12:28,780 begins with water vapour, which is water in gas form that has been 146 00:12:28,820 --> 00:12:38,780 sweating of the planet's surface by heat from the sun. As it rises, the 147 00:12:39,368 --> 00:12:44,372 Weber calls off, ready to condense back into a liquid -- the vapour 148 00:12:44,412 --> 00:12:49,936 calls down. It uses condensation nuclei, the microscopic stuff that 149 00:12:49,976 --> 00:12:54,188 gets coughed up by dust storms, volcanoes, fire and pollution. The 150 00:12:54,228 --> 00:12:59,168 vapour condenses into these particles, forming water droplets 151 00:12:59,208 --> 00:13:03,876 or, if it has travelled where the temperature was below freezing, ice 152 00:13:03,916 --> 00:13:09,248 crystals. This process is repeated until then number in the trillions, 153 00:13:09,288 --> 00:13:13,784 forming what scientists call clouds. To fall as rain, the droplets have 154 00:13:13,824 --> 00:13:19,296 to be heavy enough to succumb to gravity and large enough to power 155 00:13:19,336 --> 00:13:23,360 through the rising warm air beneath a cloud. To do this, the droplets 156 00:13:23,400 --> 00:13:27,880 joined forces to create raindrops, which can be up to one centimetre 157 00:13:27,920 --> 00:13:32,500 in diameter. Some parts of the world are more prone to heavy rain 158 00:13:32,540 --> 00:13:37,048 and others because of the different way the climates work. India is 159 00:13:37,088 --> 00:13:42,452 famous for its monsoon season. There would monsoon actually 160 00:13:42,492 --> 00:13:47,188 referred to seasonal changes in wind direction -- the word monsoon. 161 00:13:47,228 --> 00:13:57,188 During in the year's summer, warm, wet air is drawn in from the ocean, 162 00:13:59,728 --> 00:14:07,036 In winter, the wind changes direction and blows the rain away. 163 00:14:07,076 --> 00:14:13,192 Although, like the Rhine says, it will come again another day. The 164 00:14:13,232 --> 00:14:19,892 high score for the most rain in 24 hours is held by a tiny island off 165 00:14:19,932 --> 00:14:25,424 the east coast of Madagascar. It was swamped by 73 inches in 1952. 166 00:14:25,464 --> 00:14:30,328 That is going to be tough to beat. Precipitation can come in other 167 00:14:30,368 --> 00:14:34,736 varieties besides rain. My least favourite example of this is snow. 168 00:14:34,776 --> 00:14:39,520 It always makes me think of my grandmother, who used to look out 169 00:14:39,560 --> 00:14:46,328 of the window and say things like "it is trying to snow but it is 170 00:14:46,368 --> 00:14:52,472 just too cold". I wonder if she was ever write about that. Can it be 171 00:14:52,512 --> 00:14:59,660 too cold to snow? Snow might look soft but don't be fooled, it can 172 00:14:59,700 --> 00:15:04,736 close down schools, roads, and even airport. It is hardly likely to let 173 00:15:04,776 --> 00:15:09,708 the drop in temperature get in the way. It can see no no matter how 174 00:15:09,748 --> 00:15:15,520 cold it is, but there is more to it than that. Just like rain, snow 175 00:15:15,560 --> 00:15:21,880 needs water vapour to rise up from the ground, which then freezes into 176 00:15:21,920 --> 00:15:28,488 snow crystals. However, the further the air temperature falls below 177 00:15:28,528 --> 00:15:35,864 zero, the less water vapour there is floating about. So any crystals 178 00:15:35,904 --> 00:15:42,116 that do fall will lack the moisture they need to get bigger. This makes 179 00:15:42,156 --> 00:15:48,176 it harder for snow to fall, but not impossible. It has been seen 180 00:15:48,216 --> 00:15:56,408 falling below minus 40 degrees Celsius. At these temperatures 181 00:15:56,448 --> 00:16:02,972 there is less of it about and the crystals are much smaller. Another 182 00:16:03,012 --> 00:16:08,116 commonly asked questions about snow is weather or not any two 183 00:16:08,156 --> 00:16:13,068 snowflakes can be identical. Let's find out. Snowflakes definitely 184 00:16:13,108 --> 00:16:18,788 seem to think they are too cool for school because they do look 185 00:16:18,828 --> 00:16:22,584 different when viewed close-up. The random way in which they fall as 186 00:16:22,624 --> 00:16:27,680 they fall through the atmosphere means there are more potential 187 00:16:27,720 --> 00:16:32,804 shapes than atoms in the universe, so the likelihood of two snowflakes 188 00:16:32,844 --> 00:16:38,584 being exactly alike is bordering on impossible. Apparently scientists 189 00:16:38,624 --> 00:16:42,816 came close in 1988 with two that were very much alike, but that is 190 00:16:42,856 --> 00:16:52,816 not the same as identical. Of course snow is not the only Ic 191 00:16:52,936 --> 00:16:57,536 thing falling from the sky, there is also sleet and hail. When hill 192 00:16:57,576 --> 00:17:02,208 is earthbound, you had better watch out, otherwise you might end up 193 00:17:02,248 --> 00:17:07,992 losing some of your teeth. The largest hailstone on record that 194 00:17:08,032 --> 00:17:15,772 landed in Bangladesh in 1986 weighed over one kilo. Of course it 195 00:17:15,812 --> 00:17:20,848 did not travel alone. Incredibly, cold as it is, hill has been 196 00:17:20,888 --> 00:17:24,880 identified as a major contributor to the creation of lightning. Which 197 00:17:24,920 --> 00:17:29,868 is right off the other end of the temperature scale and more 198 00:17:29,908 --> 00:17:36,352 difficult to sidestep. Maybe I can give you a few pointers. How do I 199 00:17:36,392 --> 00:17:40,412 avoid being hit by lightning? Lightning flashes up to 100 times a 200 00:17:40,452 --> 00:17:45,432 second worldwide. You would think this would be enough for us to 201 00:17:45,472 --> 00:17:50,148 understand it properly, but it isn't. We can't predict when or 202 00:17:50,188 --> 00:17:56,772 were lightning will strike. We know Ben Franklin was very lucky with 203 00:17:56,812 --> 00:18:00,448 his kite experiment because lightning has up to 1 billion 204 00:18:00,488 --> 00:18:05,384 vaults of static electricity. It mainly appears during thunderstorms, 205 00:18:05,424 --> 00:18:09,792 which are named after the sound of a lightning strike. The big issue 206 00:18:09,832 --> 00:18:15,712 is that we don't fully understand how lightning forms. One of the 207 00:18:15,752 --> 00:18:19,368 leading theories is that in a storm tiny ice crystals and lumps of 208 00:18:19,408 --> 00:18:23,120 hailstones are crashing against each other. This causes electrons 209 00:18:23,160 --> 00:18:27,480 on some of the ice crystals to break off and attach themselves to 210 00:18:27,520 --> 00:18:32,384 some of the heavier falling hailstones. Then the hailstones 211 00:18:32,424 --> 00:18:38,696 become negatively charged. We know the electricity is divided into 212 00:18:38,736 --> 00:18:42,412 positive and negative charge. It was Benjamin Franklin who 213 00:18:42,452 --> 00:18:46,808 introduced this idea. The negative charge starts to collect in the 214 00:18:46,848 --> 00:18:51,384 base of the storm cloud. The ground below contains negative and 215 00:18:51,424 --> 00:18:54,716 positive charge, but the negative charges are repulsed by the 216 00:18:54,756 --> 00:19:01,168 negativity in the cloud base, leaving the positive charges or 217 00:19:01,208 --> 00:19:06,244 loan. Just like in a bad romantic comedy, opposite always attract. 218 00:19:06,284 --> 00:19:11,680 Eventually, this attraction becomes overwhelming and all the pent-up 219 00:19:11,720 --> 00:19:15,604 energy is released with a bold five times hotter than the sum. 220 00:19:15,644 --> 00:19:24,088 Lightning comes in several additions, including four Oct, 221 00:19:24,128 --> 00:19:28,464 streaked, rocket, and the very rare ball. Not all lightning strikes the 222 00:19:28,504 --> 00:19:33,620 ground, in fact most of the time it just fires around inside the storm 223 00:19:33,660 --> 00:19:38,368 cloud. It you see this happening, count the seconds from when you see 224 00:19:38,408 --> 00:19:43,436 lightning to when you hear thunder, and then divide that number by five. 225 00:19:43,476 --> 00:19:49,356 This will tell you how many miles away the storm is. If you get 226 00:19:49,396 --> 00:19:54,384 caught out, Steer clear of any metal. Water it is also a good 227 00:19:54,424 --> 00:20:00,152 conductor of electricity, so avoid that. Try to get indoors if you can 228 00:20:00,192 --> 00:20:05,588 but don't use the telephone to let people know you are OK. That is 229 00:20:05,628 --> 00:20:10,684 wind, rain, snow and lightning taking care of. It is getting 230 00:20:10,724 --> 00:20:16,104 lively but I can't help thinking that we have not yet hit on the 231 00:20:16,144 --> 00:20:22,416 real big movers and shakers, like Hurricanes for example. They are in 232 00:20:22,456 --> 00:20:29,448 a class of their own when it comes to big weather. When is a hurricane 233 00:20:29,488 --> 00:20:34,056 not a hurricane? They are dry rating mega machines with a minimum 234 00:20:34,096 --> 00:20:38,760 speed of 119 kilometres per hour. Really, they are just very big 235 00:20:38,800 --> 00:20:43,640 storms. And they are not the only storms that operate like this, 236 00:20:43,680 --> 00:20:47,964 there are also typhoons and tropical cyclones. Want to know the 237 00:20:48,004 --> 00:20:53,232 difference? There is not one, at least not from a meteorological 238 00:20:53,272 --> 00:20:59,044 perspective. To develop, all three need the water temperature to be 239 00:20:59,084 --> 00:21:02,904 26.6 degrees and they all have the distinctive look to them known as 240 00:21:02,944 --> 00:21:08,728 closed wind circulation. Simply put, they have been given different 241 00:21:08,768 --> 00:21:15,292 names according to where in the world they form. They are called 242 00:21:15,332 --> 00:21:22,340 hurricanes in the North Atlantic and the north-east Pacific. 243 00:21:22,380 --> 00:21:25,728 Typhoons occur in the north-west Pacific, and cyclones fall in the 244 00:21:25,768 --> 00:21:32,420 south-west Pacific and the Indian Ocean. Even though they have been 245 00:21:32,460 --> 00:21:35,840 given different names, they function in the same way. It begins 246 00:21:35,880 --> 00:21:40,468 with what experts call tropical disturbances in the atmosphere. 247 00:21:40,508 --> 00:21:46,420 Warm, wet air rises into the sky and cools rapidly creating 248 00:21:46,460 --> 00:21:54,088 thunderstorms. The Coriolis effect, caused by the Earth's rotation, 249 00:21:54,128 --> 00:22:02,936 blends these storms together, spinning them around together. This 250 00:22:02,976 --> 00:22:09,560 becomes the eye of the storm where it is actually pretty calm. These 251 00:22:09,600 --> 00:22:12,328 storm systems can only get upgraded when their wind reaches that magic 252 00:22:12,368 --> 00:22:16,652 119 kilometres per hour mark. But although they are not really 253 00:22:16,692 --> 00:22:21,528 different, they are all record- holders in their own right with a 254 00:22:21,568 --> 00:22:29,748 top wind speed of 408 kilometres per hour. Tropical cyclone of 255 00:22:29,788 --> 00:22:35,624 Bolivia, which struck Australia in 1996, was the fastest. Typhoon tip 256 00:22:35,664 --> 00:22:42,800 was the largest, but hurricane Catrina, which hit the US in 2005, 257 00:22:42,840 --> 00:22:47,640 caused the most damaged - $81 billion. So a hurricane is not a 258 00:22:47,680 --> 00:22:53,484 hurricane when it is a typhoon or cyclone. It is all a roller-coaster 259 00:22:53,524 --> 00:22:58,856 ride but it is not amusing. Things don't come much bigger than a 260 00:22:58,896 --> 00:23:03,704 tropical cyclone, but there are smaller forces at work, subtle 261 00:23:03,744 --> 00:23:09,844 forces all around you that you didn't even know existed. They are 262 00:23:09,884 --> 00:23:14,264 called micro-climates. Try to imagine the important stuff that 263 00:23:14,304 --> 00:23:21,548 can affect the weather, like wind, temperature, a humidity and 264 00:23:21,588 --> 00:23:28,452 rainfall. Then imagine cramming it into a small box. You are just 265 00:23:28,492 --> 00:23:33,144 about ready to build your very own micro-climate. Essentially, micro- 266 00:23:33,184 --> 00:23:38,016 climates are small areas where the climate is different from the one 267 00:23:38,056 --> 00:23:42,688 outside it. These differences are caused by the lay of the land in 268 00:23:42,728 --> 00:23:47,408 your plot and the way it influences the weather's behaviour. For 269 00:23:47,448 --> 00:23:52,376 example, if your plot has sandy coloured soil, it will bounce more 270 00:23:52,416 --> 00:23:58,360 heat back into the atmosphere than darker soil, which absorbs it. This 271 00:23:58,400 --> 00:24:03,064 will affect how hot it is. Trees can provide shade in the summer, 272 00:24:03,104 --> 00:24:08,440 which keeps the plot cool, but deciduous trees lose their leaves 273 00:24:08,480 --> 00:24:15,676 in the winter so the sun shines through which warms it up. Trees 274 00:24:15,716 --> 00:24:20,472 their leaves, making the air more moist. If your plot has a mountain 275 00:24:20,512 --> 00:24:25,548 near it, this will affect the way the wind blows through it and will 276 00:24:25,588 --> 00:24:31,284 force moist air to rise, creating rain clouds. You also need to think 277 00:24:31,324 --> 00:24:35,564 about what latitude you want to be on, and how far above sea level 278 00:24:35,604 --> 00:24:44,348 because for every 1000 metres you go up, the temperature can drop by 279 00:24:44,388 --> 00:24:54,348 10 degrees Celsius. These features have created countless natural my - 280 00:24:54,436 --> 00:24:59,880 - micro-climate around the world. Cities can be several degrees 281 00:24:59,920 --> 00:25:05,144 warmer than rural areas. The pioneering amateur meteorologist 282 00:25:05,184 --> 00:25:10,756 Haward was the first to document this in 1818. He described London 283 00:25:10,796 --> 00:25:14,376 as having an artificial excess of heat, one of its many excesses that 284 00:25:14,416 --> 00:25:20,568 continues today, although scientists now refer to the place 285 00:25:20,608 --> 00:25:24,892 as an urban heat island. You can even change the micro-climate in 286 00:25:24,932 --> 00:25:31,584 your own garden, depending on what plants you have got, what the soil 287 00:25:31,624 --> 00:25:36,128 is like, the direction it faces and so on. Your DIY contribution to 288 00:25:36,168 --> 00:25:42,736 micro climate change. The weather works in time as well as space, and 289 00:25:42,776 --> 00:25:49,348 some aspects of it are as regular as clockwork. Four of them at any 290 00:25:49,388 --> 00:25:53,476 rate, these are the seasons. Why do we have seasons? Up close, the 291 00:25:53,516 --> 00:25:59,176 Earth looks like a well-built machine, but take a step back and 292 00:25:59,216 --> 00:26:09,176 you will realise it has a few flaws. For starters, it has a wonky 293 00:26:09,468 --> 00:26:13,040 vertical axis, off by about 23.5 degrees so that North Pole does not 294 00:26:13,080 --> 00:26:21,400 point straight up. This tool was first measured accurately over 2000 295 00:26:21,440 --> 00:26:26,740 years ago by this Greek mathematician, a very smart man. It 296 00:26:26,780 --> 00:26:33,760 always tilts in the same direction, pointing towards the North Star, no 297 00:26:33,800 --> 00:26:37,400 matter where it is on its journey around the sun. And, depending on 298 00:26:37,440 --> 00:26:42,692 where you live, the tilt affect how much sunlight you get during 299 00:26:42,732 --> 00:26:48,380 different times of the year. This results in the four very different 300 00:26:48,420 --> 00:26:57,692 seasons. Spring, summer, autumn, and winter. The people in the 301 00:26:57,732 --> 00:27:00,584 northern hemisphere get more attention during their summer 302 00:27:00,624 --> 00:27:05,536 months when the Earth's tilt is in their favour. But they get the cold 303 00:27:05,576 --> 00:27:09,760 shoulder in winter, when the Earth is tilting away and the sun is 304 00:27:09,800 --> 00:27:14,008 shining on everyone below the equator. Although seasonal weather 305 00:27:14,048 --> 00:27:19,848 changes gradually, there are days when the polls of the Earth are 306 00:27:19,888 --> 00:27:25,164 tilted as near to or as far from the sun as is possible, and we call 307 00:27:25,204 --> 00:27:31,772 these the solstices. The summer solstice is the longest day of the 308 00:27:31,812 --> 00:27:36,320 year, and the winter solstice is the shortest. You will know when 309 00:27:36,360 --> 00:27:43,868 they are because you will see people dancing at World Heritage 310 00:27:43,908 --> 00:27:48,444 sites on the news. Not everywhere has four different seasons. If you 311 00:27:48,484 --> 00:27:55,180 live near the equator, you only get wet and dry seasons because you get 312 00:27:55,220 --> 00:28:00,196 a lot of sunlight nearly all the time. The polls only have summer 313 00:28:00,236 --> 00:28:05,884 and winter, but that does not seem to bother anyone who lives there. 314 00:28:05,924 --> 00:28:09,336 But they will be concerned by the fact that they are melting and the 315 00:28:09,376 --> 00:28:14,120 long-term forecast is for global warming. That will change things in 316 00:28:14,160 --> 00:28:19,180 all manner of ways. In fact the weight of professional opinion says 317 00:28:19,220 --> 00:28:23,260 it's already happening. But let us stick with what I've told you for now 318 00:28:23,300 --> 00:28:27,760 and maybe lets reconvene in a couple of years for a re-cap.