Chemistry 2nd Paper · Classes 11-12 · Chapter 1 · 1.1 to 1.2
Structure and Composition of the Atmosphere; Cyclone and Storm-Surge.
Look up and you see nothing, yet over your head a blanket of gas stretches past 500 km. It gives you breath and weather, and it shields you from the sun's UV rays. Now and then it also breeds a storm like Sidr. Let us look inside the invisible blanket.
1. What the atmosphere is made of
The invisible envelope of gases that wraps the earth and reaches to more than about 500 km above the surface is the atmosphere. The main gases of dry air are nitrogen N₂ at 78.09%, oxygen O₂ at 20.94% and carbon dioxide CO₂ at 0.033%. The rest is water vapour, the inert gases, and assorted gases of nitrogen and sulfur.
The gas is not spread evenly. About 75% of it lies within 11 km of the surface and 99% within 30 km. At the surface the blanket presses with 14.7 lb per square inch, or 1.033 kg per square centimetre. At sea level that is 760 mm(Hg), or 1 atm.
The higher you go, the thinner the air: density and pressure both fall. At 15 km the pressure is about 100 mm(Hg), and above 100 km only 3.0×10⁻⁷ atm.
CO₂ and water vapour are few, but they matter. They hold the heat of the sun's rays and keep the earth's mean temperature near 15°C. Without them it would be −30°C.
2. Four layers, four stories
Take a balloon from the ground up to 500 km. Watch the layer, the temperature and the pressure change, then take the ozone layer away.
Which layer, how cold, how much pressure?
The heights are not to scale. The dots are the book's values; the dashed line between them is drawn by hand.
The atmosphere is divided into four layers by how pressure and temperature change. From the ground up: troposphere, stratosphere, mesosphere and thermosphere.
The troposphere runs from the surface to 15 km. Pressure falls from 760 to about 100 mm(Hg), and temperature drops 7°C for every km, reaching about −55°C (218 K) at 12 km. Storms happen here and almost all aircraft fly here, so it is also called the turbulent sphere.
The stratosphere runs from 15 km to 50 km, 35 km thick. Pressure falls from 10 to 1 mm(Hg), but temperature goes the other way: from −55°C up to 2°C (275 K) at 50 km. The reason is that the ozone layer absorbs UV rays here. There are no storms, the weather is dry and calm, so it is the calm sphere, and jet planes fly through it.
The mesosphere runs from 50 km to 85 km. Temperature falls again, because UV rays and heat-producing chemicals are absent. At 83 km it drops to −93°C (180 K), the coldest part of the atmosphere.
The thermosphere runs from 85 km to 500 km. Temperature climbs from −93°C to between 427°C and 1727°C (700 K to 2000 K). High-energy UV ionises the gases here, so it is also called the ionosphere. Now match the features to the four layers.
Which layer?
The thin layer between the troposphere and the stratosphere is the tropopause. The change of temperature with height is the lapse rate. Where temperature falls (troposphere and mesosphere) it is positive, and where it rises (stratosphere) it is negative. Drag the handle and climb for yourself.
What does temperature do as you climb?
Only the points the book gives are marked; the dashed line between them is drawn by hand, not to scale.
3. The ozone layer: a shield against UV
The ozone layer sits in the stratosphere, about 20 to 40 km above the surface. By absorbing the sun's UV rays it protects living things.
Ozone is made and broken all the time. First UV splits O₂: O₂(g) --UV--> 2O(g). Then an O atom joins another O₂ to make ozone: M + O(g) + O₂(g) → O₃(g) + M, where M is any particle that carries away the leftover energy. And ozone itself absorbs UV and breaks up: O₃(g) --UV--> O₂(g) + O(g). Turn over the three cards.
The life of ozone
Without the ozone shield, UV rays break chemical bonds, damage biological processes, cause cataract in the eye and raise skin cancer. Strangely, move ozone and its role changes: low down in the troposphere, near the ground, it acts as an air pollutant and as a greenhouse gas.
4. Another split, by composition
By the make-up of the gas, the atmosphere has two zones. The homosphere runs to about 85 km: troposphere, stratosphere and mesosphere, where the composition of the gas is nearly steady. Above 85 km is the heterosphere: thermosphere and exosphere.
In the heterosphere the gases sit in layers by mass: O₂ and N₂ at the bottom, then O atoms, then He, and free H at the very top. The ionosphere holds the ions O⁺, NO⁺, O₂⁺ and N₂⁺.
5. Cyclone and storm-surge
Temperature, pressure, air density and the state of marine water vapour all influence cyclones and storm-surges. At root, heat from the sun lowers air density and makes a low pressure, and a low pressure opens the way to disasters like cyclones and storm-surges.
A small sample happens every day. By day the land heats fast and makes a low pressure, so sea breeze blows from the water onto land. At night it reverses: land breeze blows towards the sea.
A cyclone has two root causes: high sea-water temperature in the deep sea (above 27°C or 80°F) and a low pressure over the sea. It is born over the sea between 5° and 20° north of the equator.
Watch a cyclone being born over a warm sea, step by step.
From a low to a whirl
The drawing is a sketch, not to scale.
Fix the order of formation in your head. Put the steps below in sequence.
Order of cyclone formation
A cyclone is a circular sea storm moving toward land with a speed of at least 63 km/h (40 mi/h). At sea it is a cyclone, hurricane or typhoon, and on land a tornado or kalbaishakhi. The word cyclone comes from the Greek Kyklos, meaning a coil of snakes.
A cyclone lasts 2 to 6 days. The cyclone that hit Bangladesh in 1991 blew at 225 km/h.
The storm-surge is the deadly side of a cyclone. A surge raised by an earthquake, landslide or eruption on the sea floor is a tsunami. One struck the Indian Ocean near Sumatra on 26 December 2004.
A tornado is far smaller than a cyclone: a few metres wide and 5 to 30 km long. In Bangladesh, tornadoes and kalbaishakhi strike on land in Chaitra and Baishakh.
The named storms in the book's table: cyclone 1970 (Bhola) and 1991 (southern coast); Katrina (hurricane) 2005, United States; Sidr (cyclone) 2007, Bangladesh; Sandy (hurricane) 2012, New York; Mahasen (surge) 2013, Bangladesh and India.
6. Check yourself
Three questions in the exam's style. Start with the first.
Question 1
Which is the coldest region of the atmosphere?
The second is about the layers.
Question 2
Which property of the atmosphere keeps falling as one rises above the surface?
Read the last one slowly.
Question 3
On a summer afternoon on a coast, wind blows from the sea towards the land. What is the reason?
The whole note on seven cards.
Seven cards
Source: Chemistry 2nd Paper, Classes 11-12 (NCTB), Chapter 1 “Environmental Chemistry”, section 1.1 to 1.2.