EG4508: Issues in environmental science Meteorology and Climate Dr Mark Cresswell The Atmosphere 1

Suggested References #1 Ahrens, C. Donald. (2000) Meteorology today : an introduction to weather, climate, and the environment. Harvey, Danny. (2000) Climate and global environmental change Burroughs, William James. (2001) Climate change : a multidisciplinary approach. Climate change 2001 : The scientific basis / edited by J.T. Houghton McGuffie K and Henderson-Sellers A. (1997). A climate modelling primer. Published by John Wiley, England. Text Books:

Suggested References #2 Quarterly Journal of the Royal Meteorological Society Monthly Weather Review Meteorological Applications Journal of Climatology Scientific Journals: SEE UKSCIENCE METEOROLOGY PAGES FOR MORE INFO

Suggested References #3 KNMI climate explorer: – Royal Meteorological Society: – The Met. Office: – NOAA-ENSO: – Internet:

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General Points The atmosphere behaves like a fluid The atmosphere is a mixture of different gases, aerosols and particles The atmosphere remains around the earth as an envelope because of gravity Much of the observed motion in the atmosphere results from solar radiation

Basic Astronomy For most of the Earth, energy varies on daily (diurnal) and seasonal (annual) time-scales. Changes from daytime to night and progression through the four seasons depends on the configuration of the Earth-Sun orbit

Quantity of solar radiation may vary as a result of solar activity

Solar wind increases in magnitude at times of high sunspot activity

Basic Astronomy The Earth completes a single rotation about its axis in approx 24 hours ( hours!) - this period is known as a day

Basic Astronomy The Earth completes a single revolution around the Sun in approx 365 days ( days) - period is known as a year

Basic Astronomy Energy received at different points on the earth’s surface is not constant As we move from the equator to the poles the quantity of energy decreases This is due to Earth curvature The same amount of energy is spread over a greater area and has to pass through a thicker layer of the atmosphere

Basic Astronomy Axis about which the earth rotates tilts Spring Summer Autumn Winter

Basic Astronomy SUMMER (N. Hemisphere) WINTER (N. Hemisphere)

Basic Astronomy The Earth does not spin perfectly about its axis - but tilts to trace a cone in space - caused by the combined Sun and Moon’s gravitational pull and is called precession This tilt angle varies - between about 22º and 25º - it is currently 23.5º

Basic Astronomy In addition to tilt - the elliptical orbit the Earth takes around the Sun varies also Mean distance between the Earth and the Sun is 1AU (1.496 x 10 8 km). Minimum distance is 0.983AU and the maximum distance is 1.017AU

Basic Astronomy In addition, the Earth’s path along its ellipse will vary slightly due to differential gravitation pull. This is known as the eccentricity of the orbit.

Basic Astronomy The combination of factors such as orbital eccentricity, precession, tilt angle, distance from the Sun etc greatly affect our climate by varying the quantity of solar energy received Collective term is Orbital Forcing May have influenced the magnitude and period of past ice ages

Basic Astronomy The gravitational pull of the moon affects our tides and also moderates energy levels in the oceans Ocean dynamics greatly influences our Earth’s climate system

Composition of the atmosphere

Vertical structure of the atmosphere Weight is the mass of an object multiplied by the acceleration of gravity Weight = mass x gravity An object’s mass is the quantity of matter in the object

Vertical structure of the atmosphere The density of air is determined by the mass of molecules and the amount of space between them Density = mass/volume Density tells us how much matter is in a given space (or volume)

Vertical structure of the atmosphere Each time an air molecule bounces against an object it gives a tiny push This small pushing force divided by the area on which it pushes is called pressure Pressure = force/area

Vertical structure of the atmosphere In meteorology we discuss air pressure in units of hectopascals (hPa) (previously called millibars mb) The average atmospheric pressure at the Earth surface is hPa We can sense sudden changes in pressure when our ears ‘pop’ such as that experienced in old aircraft

Relationship between pressure and height As we climb in elevation (up a mountain or in a hot air balloon) fewer air molecules are above us: atmospheric pressure always decreases with increasing height

Energy: basic laws and theory Energy is the ability or capacity to do work on some form of matter Energy is transformed when it interacts with matter - e.g. potential energy is transformed into kinetic energy when a brick falls to the ground Matter can neither be created nor destroyed - only change form

Energy: basic laws and theory The energy stored in an object determines how much work it can do (e.g. water in a dam). This is potential energy PE = potential energy m = mass of the objectg = acceleration of gravity h = object’s height above the ground PE = mgh

Energy: basic laws and theory A volume of air aloft has more potential energy than the same volume of air above the surface The air aloft has the potential to sink and warm through a greater depth of the atmosphere Any moving object possesses energy of motion or kinetic energy

Energy: basic laws and theory The kinetic energy of an object is equal to half its mass multiplied by its velocity squared: KE = ½ mv 2 The faster something moves, the greater its kinetic energy. A strong wind has more kinetic energy than a light breeze

Energy: basic laws and theory Temperature is a measure of the average speed of the atoms and molecules, where higher temperatures correspond to faster average speeds If a volume of air within a balloon were heated the molecules would move faster and slightly further apart - making the air less dense Cooling air slows molecules down and so they crowd together becoming more dense

Energy: basic laws and theory Heat is energy in the process of being transferred from one object to another because of the temperature difference between them

Temperature scales Hypothetically, the lowest temperature attainable is absolute zero Absolute zero is ºC Absolute zero has a value of 0 on a temperature scale called the Kelvin scale - after Lord Kelvin ( ) The Kelvin scale has no negative numbers

Temperature scales The Celsius scale was introduced in the 18th century. The value of 0 is assigned to the freezing point of water and the value 100 when water boils at sea-level An increasing temperature of 1 ºC equals an increase of 1.8 ºF

Specific heat and latent heat Liquids such as water require a relatively large amount of heat energy to bring about just a small temperature change The heat capacity of a substance is the ratio of the amount of heat energy absorbed by that substance to its corresponding temperature rise

Specific heat and latent heat The heat capacity of a substance per unit mass is called specific heat Specific heat is the amount of heat needed to raise the temperature of one gram (g) of a substance by one degree Celsius 1g of liquid water on a stove would need 1 calorie (cal) to raise its temperature by 1 ºC

Specific heat and latent heat When water changes its state (solid to liquid, liquid to gas etc) heat energy will be exchanged The heat energy required to change a substance from one state to another is called latent heat Evaporation is a cooling process Condensation is a warming process

Energy transfer in the atmosphere Conduction: transfer of heat from molecule to molecule (hot spoon) Convection: transfer of heat by the mass movement of a fluid (such as water and air) Radiation: Movement of energy as waves - the electromagnetic spectrum

Electromagnetic spectrum with enhanced detail for visible region of the spectrum Note the large range of wavelengths encompassed in the spectrum - it is over twenty orders of magnitude!

EMR and the Sun-atmosphere system About 50% of incoming solar radiation is lost by the atmosphere: scattered (30%) and absorbed (20%) Scattering involves the absorption and re-emission of energy by particles Absorption (unlike scattering) involves energy exchange

EMR and the Sun-atmosphere system Wavelengths less than and greater than 0.8µm (800nm) are often referred to as shortwave and longwave radiation respectively The shortwave solar radiation consists of ultraviolet and visible The terrestrial longwave component is known as infrared

EMR and the Sun-atmosphere system Just under 50% of the radiation reaching the Earth’s surface is in the visible range Components of visible light are referred to as colours Each colour behaves differently and white light can be separated out by use of a prism

EMR and the Sun-atmosphere system The human eye cannot see infrared radiation Infrared radiation is absorbed by water vapour and carbon dioxide in the troposphere The atmosphere’s relative transparency to incoming solar (SW) radiation, and ability to absorb/re-emit outgoing infrared (LW) radiation is the natural greenhouse effect

The Earth’s energy balance Incoming solar (shortwave) energy should be balanced by outgoing terrestrial (longwave) energy Without a balance the Earth would heat up or cool down uncontrollably Energy may take a tortuous path from Sun to ground and back to space.

Greenhouse effect The natural greenhouse effect maintains a stable climate for life on earth Outgoing radiation (longwave) is absorbed by molecules such as water vapour and carbon dioxide Energy is then re-emitted in all directions - forming a blanket

Greenhouse Effect