In the first two years of the Italian art high school (liceo artistico), natural science gets about 2 hours a week, and almost all the time that is left goes to the coursework of the specialist subjects: plates, drawing, the workshop. Atmosphere and hydrosphere are studied between October and November, and the test falls four to seven weeks from today. With so few hours the teacher moves fast, and the test usually asks for definitions, a couple of numbers to remember and a few conversions. Here you will find the concepts in the order they are asked, with the essential data and two short worked exercises.
The atmosphere: what it is made of
The atmosphere is the layer of gas that surrounds the Earth, held around the planet by gravity. Dry air is made up of:
| Gas | Percentage by volume |
|---|---|
| Nitrogen () | about 78% |
| Oxygen () | about 21% |
| Argon and other gases | about 1% |
| Carbon dioxide () | about 0.04% |
Nitrogen and oxygen therefore make up 99% of the air. Carbon dioxide is very little, but it matters enormously for the climate, as we will see. Water vapour does not appear in the table because it varies from place to place and from moment to moment: it is enough to know that it is there, in a small but variable amount, and that it is what gives clouds and rain.
The layers of the atmosphere
They are told apart by the way the temperature behaves. From the bottom upwards:
- Troposphere. This is the layer where we live and where weather happens. It reaches about 8-16 km, lower at the poles, higher at the equator. The temperature falls as you go up, on average by about 6.5 °C for every kilometre. Almost all the water vapour is here.
- Stratosphere. Up to about 50 km. It contains the ozone layer, which absorbs a large part of the Sun’s ultraviolet rays: for this reason the temperature here rises with height.
- Mesosphere. Up to about 80-85 km: the temperature goes down again and this is the coldest point of the atmosphere.
- Thermosphere. Beyond the mesosphere, with extremely thin air and very high temperatures. The auroras form here.
- Exosphere. The outermost layer, which fades gradually into space. The transition has no sharp boundary.
To remember the order, a simple rule: troposphere, stratosphere, mesosphere, thermosphere, exosphere, always from the ground towards space. If a test asks you “in which layer does the weather happen”, the answer is the troposphere.
Atmospheric pressure
Air has weight, and the weight of the column of air above us presses on every surface. This is atmospheric pressure. At sea level the average value is:
The hectopascal is one hundred pascals: . As you climb, the pressure decreases, because there is less air above you; and it decreases fast: at 5-6 km it is already about half of that at the ground. This is why it is harder to breathe in the mountains: the air contains the same percentage of oxygen, but less of it arrives with every breath.
To move from one unit to another you need the table and a little attention to the direction of the conversion. If you need to refresh unit equivalences and scientific notation, you will find the method in unit conversions and scientific notation.
| To convert | Operation |
|---|---|
| hPa to atm | divide by 1013 |
| atm to hPa | multiply by 1013 |
| hPa to Pa | multiply by 100 |
| mmHg to hPa | multiply by 1.333 |
Exercise 1. The weather map says 980 hPa. How many atm is that? And how many pascals?
Check: 980 is a little less than 1013, so the result in atm must be just under 1. It is. A pressure this low is typical of a weather disturbance.
Exercise 2. A barometer reads 1.05 atm. How many hPa is that?
Check: the value is above 1013, so we are in a high-pressure area, usually with settled weather. If you had divided instead of multiplying, you would have found a tiny number, which the “above or below the average” check immediately flags as wrong.
How winds form
Wind is moving air, and the engine is the difference in pressure: air moves from high-pressure areas towards low-pressure ones, like water running from a higher level to a lower one. The bigger the difference between two nearby areas, the stronger the wind.
But why does the pressure change from one place to another? Because of the uneven heating of the surface. Where the ground heats up, the air above it warms, becomes less dense and rises: at ground level a low-pressure area is left behind, a cyclone (or depression), often associated with clouds and rain. Where the air cools, it becomes denser and sinks: at ground level a high-pressure area forms, an anticyclone, usually with a clear sky.
An example you can see at the seaside is the sea breeze. In the daytime the sand heats up more than the water, the air above the land rises and the pressure drops there; the cooler air from the sea moves towards the land and you feel the breeze from the sea. At night the opposite happens, because the land cools down faster.
On a planetary scale, the rotation of the Earth deflects the winds: to the right in the northern hemisphere, to the left in the southern one (the Coriolis force). If you need to revise why the Earth moves as it does, the post on the motions of the Earth and the seasons explains rotation and revolution.
The greenhouse effect
The Sun heats the surface of the Earth, which in turn gives heat back towards space. Some gases in the atmosphere, the greenhouse gases (water vapour, carbon dioxide, methane), hold back part of this heat. This is the natural greenhouse effect, and it is a good thing: without it the average temperature of the planet would be about −18 °C instead of the present roughly 15 °C, and the water would be largely frozen.
The problem is the increase in the greenhouse effect. By burning coal, oil and gas, and through deforestation, human activity has released a great deal more carbon dioxide: from about 280 parts per million before the Industrial Revolution to over 420 today. More greenhouse gas means more heat trapped and therefore global warming. In an open question the distinction to write is this: the natural greenhouse effect makes the Earth habitable, the increase due to human activity alters the climate.
The hydrosphere and the water cycle
The hydrosphere is all the water on the planet: oceans, seas, lakes, rivers, glaciers, groundwater and atmospheric vapour. The oceans cover about 71% of the Earth’s surface and hold about 97% of the water. The rest is fresh water, and of that small fresh part most is locked up as ice, in glaciers and polar ice caps.
The water cycle is the continuous movement of water between these reservoirs, powered by the energy of the Sun. The stages, in order:
- Evaporation: water from seas, lakes and rivers, heated by the Sun, turns into vapour. Plants also release some, through transpiration.
- Condensation: as it rises, the vapour cools and becomes droplets: clouds form.
- Precipitation: rain, snow or hail bring the water back to the ground.
- Runoff and infiltration: part of it flows over the surface towards rivers and seas, another part soaks into the ground and feeds the aquifers.
Then it starts again: water is neither created nor destroyed, it is moved around.
Sea waters and continental waters
Sea waters are salty: salinity is the amount of dissolved salts, on average about 35‰, that is, 35 grams of salts in every kilogram of water. It varies from sea to sea: it is a little higher in the Mediterranean (about 38‰) because evaporation is strong.
Exercise 3. How much salt do you get by completely evaporating 2 kg of seawater with a salinity of 35‰?
35‰ means 35 g for every 1000 g. For 2000 g:
With 500 g of water, a quarter of 2000 g, you would have a quarter of 70 g: 17.5 g. The symbol ‰ is read “per thousand”, not “per cent”: confusing them gives a result that is ten times wrong, and if you have doubts about percentages and proportions, revise proportions and percentages in Year 1.
Continental waters are the fresh waters of the mainland: rivers, lakes, glaciers and groundwater. Rivers begin from springs, glaciers or rain and reach the sea; lakes are basins that collect water; aquifers are water accumulated underground, and springs are where an aquifer comes to the surface.
How to prepare for the test
To study efficiently it is worth making a diagram to draw by hand, which art-school students tend to do well: the layers of the atmosphere in a column with temperature and phenomena, and the water cycle as a ring with arrows. Then redo the conversions without looking. If the syllabus also includes the soil and earthquakes, you will need the post on plate tectonics, margins, volcanoes and earthquakes, and for the overall picture of the subject there is the science page. If instead the maths of the first two years trips you up in the calculations of these exercises, take a look at numbers, fractions and powers in Year 1.
When you feel you have understood, try the free practice test: it tells you at once what you remember and what you do not. If you would like to work on it with me, write to me: the first introductory call is free and lasts 30 minutes.
