At the very least, we want to know how hot or cold it will get, or whether there will be rain or snow. Some of us live in places where severe weather can produce dangerous conditions flooding, tornados, blizzards or fogs, so we pay attention to weather related warnings. And these days, weather reports are getting more sophisticated, with pollen counts and particulates for allergy sufferers, and UP indices to tell you whether you need sunscreen. Since weather affects us so constantly, I put together this page to help understand he science of weather.
I am not much interested In weather forecasting, which Is a very technical subject. This information is much more basic, about why weather “happens”, what’s going on in the atmosphere, what weather-related terms we see on TV really mean, how to read weather maps. It’s mostly practical Information, from a not very technical perspective. Weather and Climate Before we begin, let’s differentiate between weather and climate. Weather is the state of your local atmosphere at any given time, in terms of such measurements as temperature. Wind speed, alarm pressure. Reciprocation, etc. Weather Is very specific – it’s about a particular place at a particular time. It varies on a relatively small scale – for example, it could be raining in your area, while it’s dry 10 miles away. It could be 72 degrees near your home, but only 65 degrees a few miles away. You could have a thunderstorm at 6 p. M. And have the sky clear by midnight. So when we’re talking about weather. We are talking about a relatively small area and a very specific time. Moving to a different area, or going forward in time quickly changes the weather.
On the other hand, climate is about long term averages. It concerns the same things as weather measurements like temperature, pressure, rainfall, precipitation but these measurements are averaged over a long period. If you say “the average high temperature for Boston in April is 56 degrees”, then you are talking about climate. In order to report that average temperature, someone must have measured the high temperature each day in April, and then averaged those highs. Further, it’s not enough to do that for one year, because any given year could be hotter or colder than average.
So they must have measured high temperatures ACH day in April for several years, in order to calculate a multi-year average. In fact, in many places, such temperature records go back a century or more. These 100+ year records are used to calculate averages for temperatures, rainfall, weather patterns, etc. , and these long terms averages constitute the climate. It’s Important to remember that weather can be very variable, but climate Is not. You could hit a high of 80 degrees on April 4th in Chicago one year, but in another year, the high on the same date might barely reach the freezing point at 32 degrees. There than a year-to-year variability. It doesn’t even mean that the whole month of April was hotter, or the whole year was hotter. In order to make any long term comparisons, in order to show any trends, you absolutely need multi-year climate data. The Earth’s Atmosphere Since weather is the condition of the atmosphere above a certain location, at a certain time of day or night, let’s consider the atmosphere in more detail for a bit. The Earth’s atmosphere extends from ground surface to the edge of interplanetary space.
Most of this atmosphere is contained in a narrow band, about 7-10 kilometers high, which is known as the troposphere. About
Much of it is even visible to the naked eye. Astronauts in space can see the Corcoran, which looks like a hazy band surrounding the Earth, extending to about 100,000 km above the Earth. Of course, the upper atmosphere hundreds of miles above the Earth is unbreakable and almost empty. In fact, anything over 100 km is considered space, and if you go there, you are technically considered an astronaut by the World Air Sports Federation (this was the definition of space used for the X-Prize). The International Space Station (SIS) orbits at about 350 km.
Low earth orbit, used by a huge number of satellites, extends to about 2000 km at most. These regions are commonly referred to as “space” by most people, but they are still part of the Earth’s atmosphere. There is enough air up there that satellites slow down over time due to air friction, their orbits decay, and they ultimately fall back to the Earth. The SIS needs to be boosted every few months to a higher orbit, or it would also fall back to Earth. The Hubble telescope orbits at 595 km, and although it is more stable than the SIS, its orbit will also decay and fall back to Earth eventually.
You have to go as far as geostationary or geosynchronous orbits (about 35,000 km) before the friction of the atmosphere (communications satellites are often in such orbits) becomes a smaller concern than gravitational perturbations. But this is still within the atmosphere. For the discussion of weather, however, we do not usually need to consider such high altitudes. 80% of the Earth’s atmosphere is contained within the troposphere, a zone which extends from the surface of the Earth to about 10 km (it varies with latitude about 7 km over the poles, and about 17 km over the equator). 9% of the atmosphere is contained within the troposphere and the next zone outward – the stratosphere. The stratosphere extends from the boundary of the troposphere (known as the troposphere) to about 50 – 55 km from the surface of the Earth. Since the stratosphere), this is where weather happens. Outside this region, the air is not dense enough to display phenomena which are energetic enough to affect the weather. If the Earth’s atmosphere were in complete equilibrium, we would have no “weather”. Conditions would be unchanging – there would be no day or night, no seasons, no rainfall, nothing.
The source of changes in weather is changes in some other condition, some other variable. That variable is largely the Sun, although other factors also play a role. The spin of the Earth about its axis produces day and night, which means that energy input from the Sun on any given area of the Earth’s surface varies cyclically reaching a peak during the day, and falling at night. This day-night cycle is a major source of weather, but it is modified and added to by many other cycles and factors as well. Some of these other factors vary by time of day or year, while others vary by location on the Earth’s surface.
In either case, variations are what fuel changes in the atmosphere, which we call weather. What factors cause the energy input of some local area on the Earth to change? Here are some: Latitude How far you are from the equator determines the angle of incidence of the Sun’s rays at your location. This is extremely important in determining how much energy you receive from the Sun. The diagram at the right explains how this happens. Note that the diagram is not to scale, it shows the Sun as much smaller than the Earth, but that makes no difference to the explanation. The Sun is roughly spherical.
It radiates energy in all directions. A very small portion of this energy is intercepted by the Earth. If we assume that the Sun radiates energy equally in all directions, we can imagine its surface (which radiates the energy) as Ewing divided into patches, measured by degrees of solid angle (usually expressed in stranding). Since the Earth is very far away from the Sun, and very small, it intercepts direct light from a very small patch of Sun. Notice the qualifier “direct”, as in “direct light”. This is important because the situation described is a simplification.
In reality, the surface of the Sun emits light in all directions; therefore the Earth receives light from all parts of the Sun that are facing the Earth at a given time, not Just a single patch which is closest to the Earth. However, the density or intensity of this light is greatest when it is direct, that is, when a ray of light perpendicular to the Sun’s surface intersects the Earth. So the relationship still holds – the more the direct sunlight falls upon some area of the Earth, the greater is the energy that area receives. He equator get progressively colder, because they get less direct sunlight. This creates bands or zones on the Earth’s surface, with the hottest zones at the equator and the coldest zones at the poles. A temperature gradient is thus created, with high temperatures near the equator and cold temperatures at the poles. This temperature gradient drives the movement of air, which we perceive as winds. This variation is constant in time, meaning it does not change by time of the year. Latitude 50 North will always receive less installation than latitude 5 North, no matter what season of the year.
It is simply a variation by location, that is, dependent upon the latitude location on Earth. Latitude is very important in setting up the permanent winds on Earth. We can divide the Earth (from North to South) into several well-marked zones. The band near the equator (about 5 ON to 5 so) is called the doldrums. It’s the hottest part of the Earth, since the equator receives the most direct sunlight every year. On both sides of the equator are the tropics. These stretch roughly from the doldrums to the Tropic of Cancer (23. 5 ON) in the northern hemisphere, and to the Tropic of Capricorn (23. so) in the southern hemisphere. The tropics have a “tropical” climate – hot in the summers, mild in the winters. Beyond the tropics are the sub-tropical zones, which stretch from the Tropic of Cancer (23. 5 ON) to the Arctic Circle (66. 6 ON) in the northern hemisphere, and from the Tropic of Capricorn (23. 5 so) to the Antarctic Circle (66. 6 so) in the southern hemisphere. The subtropics usually have mild summers and cold winters. Beyond the subtropics lie the polar zones, from the Arctic Circle (66. 6 ON) to the North Pole (90 ON) in the northern hemisphere, and from the Antarctic Circle (66. so) to the South Pole (90 so) in the southern hemisphere. These are the coldest regions on Earth. Although there are many variations between different locations within the same zone (due to other differences, such as altitude, nearness to the sea, etc. Which are described below), the zones do broadly reflect the kind of climates found within. As mentioned earlier, they set up the patterns of the permanent winds – the trade winds, westerly’s, polar winds. These permanent winds have a very strong effect on climate, and you can read about them in more detail on this page.
Season The Earth’s axis is not perpendicular to the plane of the Earth’s orbit around the Sun; it is in fact tilted. The angle of tilt varies over time, but at present it is approximately 23. 5 degrees. Because the Earth revolves around the Sun, during the course of a full orbit around the Sun, each of Earth’s hemispheres is at times tilted towards the Sun summer) and at other times tilted away from the Sun (Winter). The periods of maximum tilt are the solstices. In the year 2010, Summer solstice is on towards the Sun, which corresponds to summer and the longest day of the year in the northern hemisphere.
Winter solstice in 2010 will be on Deck 21st at 1 1 PM (GMT), which corresponds to winter and the shortest day of the year for the northern hemisphere. As can be seen in the accompanying diagram, a similar effect to the latitude differential described above happens during summer and winter. During summers, since the northern hemisphere is tilted towards the Sun, it receives more direct unlighted, leading to higher temperatures. During winters, since the northern hemisphere is tilted away from the Sun, it receives less direct sunlight, leading to colder temperatures. The effect is reversed in the southern hemisphere.
Summer solstice in the northern hemisphere corresponds to winter solstice in the southern hemisphere, and vice versa. This seasonal effect can dramatically change weather patterns, and not Just in terms of temperatures. The change in temperature patterns across the globe shifts the high and low pressure areas of the atmosphere, which can lead to seasonal changes in winds. Indirectly, they can also affect precipitation, if for example, a winter wind which blows from land to land switches to a summer wind, which blows from sea to land. Wind blowing from the sea contains more moisture, which can lead to rain or snow.
It’s important to remember that while we think of seasons as a yearly phenomena, these changes are gradual and are happening constantly. Between the extremes of summer and winter solstice, each day the pattern changes gradually, the day becomes shorter or longer, depending upon whether the area is approaching summer or winter. While such small daily changes may seem miniscule when noninsured in terms of degrees of inclination or tilt, over the large surface of the Earth they correspond to significant shifts in the temperature zones. It’s easy to calculate the magnitude of these daily changes. Since the Earth’s axis is inclined at 23. Degrees, on summer solstice, latitude 23. 5 North (the Tropic of Cancer) is directly underneath the Sun (meaning, the Sun is directly overhead at noon on summer solstice day, if you happen to be at latitude 23. 5 North on that day). Similarly, on winter solstice day, latitude 23. 5 South (the Tropic of Capricorn) is directly underneath the Sun. So in the 6 months between the summer and winter solstices, the Sun changes its apparent position by 23. 5 + 23. 5 = 47 degrees in the sky. If we assume the Earth’s radius to be 6400 km, then 47 degrees of latitude correspond to 47/360 = 5350 km of the Earth’s surface.
This means that the Earth’s sun-directly-overhead-at-noon point migrates 5350 km north and south every 6 months. This is approximately 5350/180 = 29 km per day, or about 18 miles. As you can see, while it didn’t seem much when we were simply looking at angles, if you warm front moving 18 miles in a day would definitely be noticed by us. So these hinges are important not Just on a seasonal basis, but also in affecting our day-to- day weather. Altitude The higher you go, the thinner the air gets. Dense air has a greater capacity to absorb and retain heat than thin air, so this is one reason why the temperature is colder at higher altitudes.
However, this is insignificant compared to another effect, which is the cooling of air as it expands. According to the ideal gas law, the temperature of air is inversely proportional to its temperature, all else being the same. This is because as air expands under low pressure, it does work in expanding, and loses energy as ark done. Since the thermal conductivity of air is very low, it doesn’t gain much heat from its surroundings, so the cooling is mostly diabetic, and well approximated by the gas law. The presence of water vapor upsets this relationship a bit, but not by a whole lot.
This is the main reason why it’s much colder at higher altitudes than it is at sea level. Therefore places which are near sea level and have thick, dense air are hotter than places at the same latitude which are at higher elevations. This is why the summit of Mount Kilimanjaro is covered with ice, even though it’s located almost directly on the equator (about 3 so). There is a separate section here which talks about altitude-dependent atmospheric pressure changes in more detail. These changes are very important in determining the local climate of an area.
Land and Oceans Land and oceans are heated differentially by the Sun. Land has a smaller thermal capacity than water. This has several interesting effects. First, it means that the same amount of solar heat will raise the temperature of land much more than it will raise the temperature of water. Therefore, during a given day land at the same latitude as water will become much hotter than the water. Since they are at the same latitude, they have received roughly the same amount of solar energy, and absorbed roughly the same amount of energy (actually, the water absorbs a bit more).
But because of the difference in thermal capacities, land becomes much hotter than water with the same amount of energy. In terms of local winds, this might mean that the wind direction is from the land towards the water during the day (since air moves from higher temperature and low pressures towards colder temperatures and high pressures). Secondly, the greater heating or cooling of land leads to greater temperature preferential. The rate of heat gain or loss of an object depends upon the temperature differential between that object and its environment.
For example, if you heat a pot of water to boiling (100 co), and then remove it from the stove and let it 10 co. If room temperature is 20 co, then the water will drop from 100 co to 90 co very quickly, but it will go from 30 co to 20 co much more slowly. This is because the temperature differential between the water and room temperature is much higher when the water is at 100 co than when it is at 30 co. Since land heats up more during he day, the temperature differential is higher, therefore land cools very rapidly as well. Water cools much more slowly, because the temperature differential is lower.
We can think of it this way: land has rapid heating/cooling cycles with each day/night cycle. A large body of water, on the other hand, has much slower cycles. In fact, the water cannot lose all the heat it acquired during a summer day overnight, so it starts the next day slightly warmer than it was the previous morning. So as summer progresses, large bodies of water get progressively warmer, and they maintain this eat through the night hours, when the land cools down. For this reason, oceans don’t have diurnal peaks and troughs in their temperature like the land; instead, they have seasonal peaks and troughs in their temperature.
These things produce very significant effects on weather patterns. The general direction of the effect is towards the moderation of temperatures. Since the water heats more slowly but retains heat longer than land (and cools more slowly but retains coldness longer than land), the presence of oceans tends to moderate the climate of nearby land masses. At the same latitude, an area will be much hotter in he summer and colder in the winter if it’s far away from the sea. Nearness to the sea will moderate temperatures, making it both less hot in the summer and less cold in the winter.
Even smaller bodies of water such as lakes can have a moderating effect on temperatures. Check the weather map of the Midwest US, and on many days you’ll see that the temperature at the lake front in Chicago is higher or lower than out in the suburbs (by a few degrees), simply because Lake Michigan cools the lake shore during the summers, and warms it during the winters. Smaller bodies of water can also produce local diurnal winds, such as a breeze from lake to shore in the mornings, and a breeze from shore to lake in the evenings.
Again, this has to do with the differential heating of land as compared to water during the day. Topography The physical relief of land areas has much to do with weather. There can be many reasons for this. One is simply altitude – mountainous areas will be cooler than areas at the same latitude which are nearer to sea level. But in addition, variation of the terrain can influence wind patterns and therefore the weather. One example is mountains as a barrier to wind flow. If a mountain range interrupts revealing winds, air is forced upwards to pass over the mountains.
As it moves upward, it cools down. Since the water carrying capacity of air diminishes as it cools, this results in precipitation on the windward side of the mountains. Conversely, once Therefore, the leeward side of the mountains will be in “shadow” and receive much less rainfall than if there had been no mountains along the way. This effect can be seen almost anywhere in the world where there are mountains that interrupt some seasonal wind flow. It is very dramatic in the Himalayas in India, where the monsoon winds from the south meet the Himalayas.
On the windward side, in the foothills of the Terra, there is very heavy rainfall. Chirruping in the Indian state of Megalith has historically been the wettest place on Earth (450 inches of rain on average per year), as the monsoon winds from the Bay of Bengal hit the Kiosk hills and are forced to rise and shed water. Conversely, the Tibetan plateau, on the leeward side of the Himalayas is very dry, with less than 18 inches of rain/ snow per year. There are other effects of topography as well. Flat land which is uninterrupted by hills or mountains allows wind to build up over long stretches.
This is why the Midwest and plains states in the US are generally quite windy. Land which is more uneven breaks up lower level winds, so wind speeds are slower and winds are not as sustained. If a large area of flat lands then borders a hill or mountain range, these high winds can get channeled into valleys between the hills, and reach even higher velocities. You can see this effect on a much smaller scale even with man-made structures. Streets form canyons between skyscrapers in downtown areas of major cities, and wind is channeled through these “canyons”, reaching much higher speeds Han out in the suburbs.
If you’ve walked through downtown Chicago or downtown Manhattan, you may have experienced this yourself. Low lying troughs, on the other hand, may have days when the air stagnates and does not move, since it is blocked by higher elevations surrounding the trough. Ocean Currents Water, like air, is a fluid medium, which can move from one place to another under temperature differentials. Just as there are winds in the atmosphere, there are water currents in the oceans, which carry warm water or cold water from one place to another, sometimes for thousands of miles.
One well-known example of such a current is the Gulf Stream, which carries warm water from the Caribbean to near the shores of northern Europe. The Gulf Stream is largely responsible for the migration of populations into Europe after the last ice age. Without the Gulf Stream, Europe would probably be a sparsely populated wasteland. Consider London, which in terms of latitude is slightly farther north than Calgary in Canada. The average January low temperature of Calgary is 8 OF, but the average January low temperature of London is 41 OF. This is a huge difference, and the Gulf Stream is responsible.
While latitudes comparable to England and northern Europe are almost tundra-like across Canada or Asia, they are quite warm and habitable in cultures traditionally depend upon hunting, since agriculture is insufficient to provide the necessary calories. But in Europe, there is extensive farming, which can support much larger population densities. The Gulf Stream has made it possible; it is a critical part of Rupee’s habitability. Ocean currents are one of the most important contributors to climate, but the topic is fairly complex. I have written a brief explanation here, which you should really read before going ahead.