3 Its structure composition and dynamics In last weeks lecture we focussed on how weather is generated The key ingredients are energy and gradients Last Weeks Lecture Incoming energy from the Sun is balanced with outgoing radiative cooling of the Earth resulting in an equilibrium tem ID: 1025869
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1. All About the Atmosphere3Its structure, composition and dynamics
2. In last week’s lecture, we focussed on how weather is generated.The key ingredients are energy and gradients.Last Week’s LectureIncoming energy from the Sun is balanced with outgoing radiative cooling of the Earth, resulting in an equilibrium temperature of about 15 °C. We discussed the important role played by the atmosphere in heating the Earth via the greenhouse effect.
3. But the Earth is not heated uniformly.For a start, there is variation over the course of a day: sunlight only falls on a point on Earth during daylight hours.The Earth’s axis is tilted with respect to its orbital plane, giving rise to the seasons.There is also variation in the amount of energy we receive from the Sun throughout the year – the Earth’s orbit is not perfectly circular.Last Week’s Lecture
4. Incoming sunlight is more intense over the equator and sparser at high latitudes.There is an energy surplus at the equator and a deficit at the poles.Last Week’s LectureThis implies that the tropics should get continually warmer and the poles continually cooler. But this is not the case. As the atmosphere is a fluid, it can carry heat around, redistributing the surplus in the tropics towards the poles.
5. Heat is transferred poleward in vast global circulations of the atmosphere. These large circulations contain smaller circulations, and those contain even smaller ones.Last Week...Also, the ocean is acting to redistribute the energy, but on a slower time scale.This leads to the redistribution of energy, which creates the weather that we experience.
6. This week, we look at the atmosphere in a little more detail.We will begin with the composition of the atmosphere: its gases and its many layers.Then, we will have a look at some of the meteorological variables that describe the atmosphere, and how they interact.We conclude with global climatologies of some of these variables and attempt to explain the features on these maps.This Week’s Lecture
7. Talking about the WeatherEarth’s Weather MachineAll About the AtmosphereMid-Latitude Weather Clouds, Rain and MoistureMeasuring the WeatherWeather ForecastingThe Changing ClimateExtreme WeatherWeather in the UKLayout of the Course
8. Last time, we introduced some of the dimensions of the atmosphere.It contains a large amount of air.Its depth is about 100 km, although the exact edge of the atmosphere is tricky to define.It becomes less and less dense the further up into the atmosphere you go.In comparison to the diameter of the Earth, is depth is actually very small.The Atmosphere
9. How much air is above our heads?Pressure is a force per unit area.Surface pressure is about 1,000 hPa, which is 100,000 pascals.For each square metre there is 100,000 newtons of force.Force is equal to mass times acceleration.The acceleration under gravity (g) is nearly 10 metres per second per second.This gives a mass of air of 10,000 kg.Surface area of the Earth is about 500,000,000,000,000 m2. Total mass of the atmosphere is about 5,000,000,000,000,000,000 kg.The Atmosphere
10. The atmosphere is made of air. Air is a mixture of a number of different gases, in various quantities.The pie charts on the cards show the distribution of the 11 most abundant gases in the atmosphere.Can you match the gas to its concentration?Also, which of the gases are greenhouse gases?Composition of the Atmosphere
11. This is the distribution of gases in the atmosphere.The atmosphere is mostly made of nitrogen and oxygen and a number of trace gases. Several of these are greenhouse gases, as we saw last week.Note that ozone concentrations are much greater in the ozone layer.Also note that this is the composition of dry air.Composition of the Atmosphere
12. The atmosphere can be divided into many layers based on different criteria.First, we have the homosphere and the heterosphere. In the homosphere, the gases are “well-mixed” and exist in the quantities on the previous slide.The weather we experience is almost entirely constrained to the troposphere.The ozone layer is found in the stratosphere. Its warming effect keeps the stratopause the warmest point in the middle atmosphere.Layers of the Atmosphere
13. Space weather is more relevant when looking at the upper layers of the atmosphere.Above 100 km, the heterosphere contains molecules that are no longer well-mixed, but stratified by molecular size.High up, temperatures become very high, although the air is very sparse up here.The ionosphere, a critical component of the Earth’s electric circuit, spans the upper mesosphere and thermosphere, generated by ionisation from solar radiation.Layers of the Atmosphere
14. The reason we know all of this is because we have been making measurements of the atmosphere for a long time (although measurements of the upper atmosphere have only been made relatively recently).We monitor many aspects of the weather routinely every day (more about this in lecture 6).What properties of the atmosphere might we want to measure?Properties of the Atmosphere
15. Here are a few properties of the atmosphere that may be of interest to us – properties that we may refer to as “weather”.Temperature.Air pressure.Wind speed and direction.Hours of sunshine.Precipitation (rain, snow, hail), and evaporation.Cloud cover and cloud type.Humidity.Radiation, albedo.Magnetic field. Properties of the Atmosphere
16. Pressure is effectively the force applied by the weight of the air above the surface.PressureIt’s measured in millibars or hectopascals; at the surface, mean pressure is 1013.25 hPa.Weather charts with isobars show the horizontal distribution of mean sea-level surface pressure.
17. As pressure is related to the amount of air above you, the higher up in the atmosphere you go, the less air there is above you, so the lower the pressure falls. Indeed, pressure decreases exponentially with height.PressureSea level (0 m): 1,013 hPaReading (60 m): 1,005 hPaTop of the Shard (308 m): 975 hPaTop of the Petronas Towers (452 m): 962 hPaTop of Burj Khalifa (828 m): 922 hPaSummit of Mount Everest (8,848 m): 340 hPa
18. Note that vertical variability of pressure is much greater than horizontal variability. For a pressure change of 100 hPa, even under the most extreme weather conditions, you would need to travel hundreds of kilometres horizontally. To get the same change vertically, you would only need to ascend 1 km.Pressure
19. Temperature is a measure of how warm something is.Usually temperature is stated in °C, where the freezing point of water is the reference (0 °C). Alternatively, it can be measured in kelvin (K), where the interval of 1 K is equal to 1 °C. Zero on the Kelvin scale is absolute zero.As we saw last week, sunlight heats the Earth’s surface which, in turn, heats the atmosphere. As the surface heats during the day and cools during the night, the air above the Earth’s surface is correspondingly heated.Changes in solar heating over the diurnal and annual cycles lead to variations in temperature.Temperature
20. In the troposphere, this leads to a temperature profile that is warmest near the surface and coldest at the top (the tropopause).TemperatureThe rate of change of temperature with height is called the lapse rate.It has a typical value of 6 °C per km, and is usually between 5 °C and 10 °C.The rate of change of temperature with height is an indication of the stability of the air layer.More about this in lecture 5.
21. A third relevant variable is density. This is the measure of the mass of matter per unit volume. Water has a density of 1,000 kg/m3; air has a density of about 1.2 kg/m3 near the Earth’s surface.Density is not measured during atmospheric observations, although is strongly related to pressure and temperature, as we will shortly see.In the atmosphere, density tends to decay exponentially with height like pressure.Density
22. Together, these three quantities are pivotal in the behaviour and thermodynamics of the atmosphere.To understand their effects on the atmosphere, we must consider the atmosphere on a molecular level..The atmosphere is a jumble of molecules of different sizes and in different concentrations, all moving around in random directions.Pressure, Temperature and Density
23. So how do these three variables contribute to weather?Gradients of pressure are what set weather systems in motion.Regions of ascent give rise to low pressure; regions of descent give rise to high pressure, which leads to circulations.Pressure and Circulations
24. Say we have a column of air at high pressure next to a column of air at lower pressure.There is a tendency for the molecules in the high pressure column to move towards the low pressure column.This is a force on the molecules known as a pressure gradient force.Pressure and Circulations
25. Temperature also has an impact on circulations. Say we have two identical columns of air. Then we heat one and cool the other. The density of the air changes, as do the pressure levels.Temperature-Driven Circulations
26. At a given height, there is now a horizontal pressure gradient, which will set the air in motion.But the transfer of air between columns will then alter the surface pressure.Temperature-Driven Circulations
27. This simple model gives us an idea of how a circulation can be set up by gradients in heating. If we consider the pink column to be the tropics and the green column the poles, we get a simple one-cell circulation pattern.Temperature-Driven CirculationsThe pattern agrees with the pan-of-peas idea we mentioned in the previous lecture – ascent in the warm region and descent in the cool region.
28. Wind is moving air.It can be stated in a number of units. Standard units are metres per second, knots (nautical miles per hour) or (statute) miles per hour. Beaufort scale values can also be used.Winds are a result of the movements of air driven by gradients in temperature and pressure.Although horizontal wind speed is usually of the most interest, vertical wind is also important. However, bearing in mind the difference in horizontal and vertical scales of the atmosphere, they are of different orders of magnitude – horizontal winds are typically tens of m/s, while vertical winds rarely exceed a few cm /s.Wind
29. Air flows from high pressure to low pressure. However, we know from looking at weather maps that wind actually circulates around the pressure centres.WindIndeed, we often think of wind as moving parallel to isobars as opposed to perpendicular to them.The reason for this is the Coriolis force – a facet of the fact that we live on a rotating planet.
30. The Coriolis force is a “fictional force” that acts to deflect wind to the right in the northern hemisphere and to the left in the southern hemisphereIt is caused by the rotation of the Earth acting upon atmospheric and oceanic motions.The Coriolis Force
31. Coriolis is one of three main forces that determine the horizontal movement of air in the atmosphere.In combination, the forces keep the air moving around the weather systems, but directed from high pressure to low pressure.The Coriolis force affects movements not just in the atmosphere – it also affects ocean currents.
32. AnimationReturning to the global circulation diagram from the start of the lecture, we can now explain why the winds move in the directions they do.The Global CirculationThe flow towards the equator in the Tropics at the surface is deflected to be easterly – the “trade winds”.The poleward mid-latitude flow is deflected to be westerly at the surface.
33. Here is a climatology of mean sea-level pressure.Global Climatologies
34. Here is a climatology of 10-metre wind.Global Climatologies
35. Here is a climatology of 2-metre temperature.Global Climatologies
36. Here is a climatology of total precipitation (rain, snow, hail…).Global Climatologies
37. In this lecture, we have discussed the behaviour of the atmosphere and how it acts to redistribute the excess energy that the Sun provides in the tropics.The atmosphere consists of a mixture of gases and is divided into layers, each with specific properties.Gradients in pressure, temperature and density can set off circulations in the atmosphere.Air flows from high pressure to low pressure, although takes a circulating path on account of the Coriolis force.The atmosphere redistributes energy via three vast circulations.Next week, we focus in more detail on how the energy is used to generate weather in the mid-latitudes.SUMMARY