Meteorology and Climate Change

Introduction to Meteorology

The science of meteorology

Meteorology is the study of the weather and climate of the Earth’s atmosphere. The atmosphere is a relative thin layer (about 1% of the Earth’s radius) of air that blankets the surface of our planet. Air is a mixture of numerous gases, consisting mostly of molecular nitrogen (N2) and oxygen (O2). Note the distinction between weather and climate:

Topics in Meteorology

There are number of key concepts that we will explore as we consider the behavior of the Earth’s atmosphere. This includes

The atmosphere in context

It is important to recognize that the atmosphere is not isolated from other Earth systems, and we can consider interactions between the and geosphere, hydrosphere, atmosphere, biosphere.

The geosphere is the solid rocky mass of the Earth and provides the lower boundary to the atmosphere and oceans (which cover 71% of the area of the Earth). As a whole, planet Earth has a average radius of 6371 km and a mass of 6x1024 kg. This provides clues to its bulk composition: an iron core surrounded by a large silicate (rocky) mantle.

The hydrosphere consists of the water inventory of the Earth, making up about 0.03% of the Earth’s mass. The water of the Earth is distributed into 97% oceans and 3% freshwater; only about 0.001% of the Earth’s water is present in the atmosphere. Of the freshwater inventory, less than 1% is found in surface lakes and rivers; most freshwater is present in glaciers (70%) or groundwater (30%).

The atmosphere is a thin layer of air (a mixture of different gases) above the Earth’s surface. The mass of atmosphere is about one-millionth of the total mass of the Earth. The atmosphere is also relatively thin, and over 99% of the atmosphere’s mass is found below an altitude of 30 km (19 miles)

The biosphere consists of all plant and animal life on the planet. The biosphere is closely connected to the atmosphere through the processes of photosynthesis and respiration.

System Mass (kg)
Geosphere 6.0x1024
Hydrosphere 1.8x1023
Atmosphere 5x1015
Biosphere 1x10^12

Note that we can weigh the atmosphere very accurately from measurements of surface pressure: the pressure generated at the surface by the weight of overlying air. An expression we can use is:


where P = 1 atm = 101325 Pa, A=5.1x10^14 m^2 and g=9.8 m/s^2

solving for m = 5x10^18 kg (about 1 millionth of Earth mass)

Composition of the atmosphere

Although hundreds of species are present in the Earth’s atmosphere, it mostly consists of nitrogen and oxygen, along with argon, water, and carbon dioxide.

Component Formula Abundance
Nitrogen N2 78.1%
Oxygen O2 20.9%
Argon Ar 0.9%
Water H2O up to 4%
Carbon Dioxide CO2 400 ppm
Methane CH4 2 ppm
Nitrous Oxide N2O 300 ppb
Ozone O3 10-100 ppb
Halocarbons CFCs, etc 2 ppb total

Aerosols - suspended particles in the atmosphere - also play an important role for assessing air quality and act as condensation nuclei in the formation of clouds.

Evolution of the atmosphere

Theres is evidence that the atmosphere was not always this composition, but has evolved over geological timescales. Most significant is the appearance of oxygen as a major atmospheric component.

\(6\textrm{CO}_2+6\textrm{H}_2\textrm{O}\xrightarrow{h\nu} \textrm{C}_{6}\textrm{H}_{12}\textrm{O}_6+6\textrm{O}_2\)

\(\textrm{C}_{6}\textrm{H}_{12}\textrm{O}_6+6\textrm{O}_2\longrightarrow 6\textrm{CO}_2+6\textrm{H}_2\textrm{O}\)

These reactions can be used to summarize the main interaction between the atmosphere and the biosphere.

The structure of the atmosphere

Atmospheric Pressure and Density

The atmosphere is bound to the Earth by gravity. As a result, atmospheric density and pressure are highest at surface and decreases (exponentially) with increasing altitude. The rate of change depends upon the force of gravity, temperature, the molecular weight of the gas(es).

Planetary atmospheres are in hydrostatic equilibrium. This means that gravity and pressure are in balance (in other words, the atmosphere does not escape into space nor does it collapse onto the surface of the planet).

Assuming hydrostatic equilibrium, we can determine the pressure at some altitude \(z\) by the relationship:

\(P(z)=P_o e^{(-z/H)}\)

where \(P_o\) is the initial pressure, \(z\) is the altitude, and \(H\) is the scale height defined as

\(H={RT}/{\mu g}\)

where \(R=8.314\) J/K-mol, \(g=9.8\) m/s^2, \(T=273\), and \(\mu=28.9\). This yields a scale height of about 8 km in the troposphere. The scale height is a useful term in atmospheric science because it tells us how rapidly the pressure decreases with changes in altitude.

This pressure-altitude relation can also be used to estimate the fraction of the atmosphere above/below a certain level. For example, Denver has an elevation of 5280 ft above sea level (1.609 km), so we can estimate the average pressure at this elevation:

\(P(1.609 \textrm{ km})=(1 \textrm{ atm})(e^{(-1.609\textrm{ km}/8 \textrm{ km})}=0.82\) atm

How high would we have to go for 50% of the atmosphere to be below us? About 5.5 km or 3.4 miles.

What is the estimated pressure on the summit of Mt. Everest? (8.8 km) About 0.3 atm.

Note that different units are used for pressure.

1 atm = 101325 Pa = 1.01325 bar = 760 mm Hg = 29.92 inches Hg = 10.3 m water = 33.9 feet water. This depends on what instrument - known as a barometer - is used to measure the pressure.

Pressure or temperature can be used to divide the atmosphere into layers.

Near the surface, the temperature also decreases with increasing altitude in the atmosphere. The rate of change is known as the lapse rate, defined as:

\(\Gamma=-\frac{\Delta T}{\Delta z}\)

A typical lapse rate is 6.5 K/km (3 F/1000 ft) but depends upon the presence of water (and latent heat released from condensation); a typical dry lapse rate is about 10 K/km (5 F/1000 ft).

Troposphere: closest to the earth, where the temperature generally decreases with altitude, from 288 K at surface to 217 K at tropopause

Why does temperature decrease with height?

Stratosphere: where temperature increases with altitude

Mesosphere: where temperature decreases with altitude; the mesopause separates the mesosphere from the thermosphere, where temperature increases with altitude again; the tropopause to thermosphere is sometimes called the middle atmosphere, 100 km and above is upper atmosphere.

At these upper layers, important distinction between temperature (average kinetic energy of gas particles) and heat (total thermal energy of the atmospheric gas)

Above 100 km, cosmic radiation, solar X-rays and ultraviolet radiation increasingly affect the atmosphere, which cause ionization (and hence ionosphere)

The atmosphere is a mixture of gases with constant proportions up to 80 km or more. The exceptions are ozone, which is concentrated in the lower stratosphere, and water vapor in the lower troposphere. The principal greenhouse gas is water vapor. Carbon dioxide, methane and other trace gases have increased since the Industrial Revolution, especially in the twentieth century due to the combustion of fossil fuels, industrial processes and other anthropogenic effects.

Interlude: weather maps and weather phenomena

Energy, Heat, Work Chapter 2 (AK), Chapter 3 (BC)

energy is the capacity to do work (when a force acting on an object causes it to move), and can be further divided into kinetic and potential energy.

Considering a parcel of air in the atmosphere, its temperature would describe the average kinetic energy of its atoms and molecules. Expressed in F, C, and K, where


Heat refers to the transfer of energy from warmer objects to colder objects and represents the total thermal energy of a system. It is distinct from temperature!

The amount of heat depends upon (\(q=mC_s\Delta T\)):

In atmospheric systems, energy is transferred by

Cloud formation (for example) will release heat to the surrounding atmosphere. This slows the rate of cooling with altitude (i.e., the lapse rate decreases).

The energy balance of the planet as a whole depends on radiation: how the Earth exchanges energy with space.

Radiation Laws

The Sun is the main source of energy for the weather and climate, and reaches the Earth via radiation

Energy Budget of the Earth

Starting with 1368 W/m^2 at Earth’s orbit (incoming flux at top of atmosphere): averaged over the entire surface area of the Earth we have (area of disk)/(area of sphere) = 1/4 of this value incident, on average, at the top of the Earth’s atmosphere: 342 W/^2.

The radiation transfers are via shortwave (visible) or longwave (infrared) radiation. Why these two? Because of their temperatures, the Sun is emitting in the visible and Earth materials are emitting in the infrared.

Energy Budget Diagram

All fluxes eventually reach equilibrium - if an imbalance, then temperature will change in response (to restore equilibrium):

We see this even during the course of a day, as the temperature changes in response to incoming and outgoing radiation


The difference in the energy budget as a function of latitude shows the importance of

  1. the incidence angle for the Sun, and
  2. the length of a day

The angle is important because higher solar altitudes mean greater intensity; at lower angles the sunlight is spread out over a larger area.

Because of the tilt of the Earth’s axis, this changes throughout the year and gives rise to seasonal effects.

Radiative Properties of the Atmosphere

As with the radiative budget, we also need to think about what passage through the atmosphere does to the incoming and outgoing radiation, as different molecules in the atmosphere absorb different wavelengths of light.

The atmosphere is mostly transparent to visible wavelengths. By time sunlight reaches the surface, most UV and much IR light has been absorbed.

Selective absorption leads to the greenhouse effect: the surface absorbs shortwave radiation but emits at longwave (infrared) wavelengths which are absorbed by gases in the overlying atmosphere. The atmosphere thus re-radiates this energy in all directions including toward the surface.

Temperature Cycles

Effects on Temperature (both annually and daily):

We can also explore interannual variations in temperature. These allow us to establish climate normals. Climatological temperatures refer to average temperatures over a 30-year period. This is often calculated for 1951-1980 or 1981-2010.

The difference between the temperature at a given time for a particular location and the climatological normal is the temperature anomaly.

Derived indexes are used to provide useful information about how the weather feels or to plan and monitor crop growth.

Water in the Atmosphere

Different ways of expressing water content

Coupling the ideal gas law with dew-point vapor pressure allows you to determine mixing ratio and absolute humidity:

The behavior of water with changing temperature is key to understanding cloud formation:

Droplets form around particles via nucleation.

Note that fog can be thought of as a surface cloud, formed by condensation out of saturated atmosphere. There are different ways this can occur:

But what about cloud formation? Generally need a mechanism for air to ascend. There are different ways this might occur:

The extent of lifting and the possibility of cloud formation depend upon the temperature and relative humidity of the rising air, and upon the environmental conditions of the atmosphere for that time and location.

Atmospheric Stability

Warm parcels of air will ascend through the atmosphere, but cool as they ascend. Whether they will continue to rise or subside depends upon the stability of the atmosphere.

In other words, the SALR is lower than the DALR because the condensation of water releases latent heat to the surroundings.

Whether or not the atmosphere is stable depends upon the relative values of ELR, DALR, and SALR. The atmosphere is

In general, the steps for cloud formation are:

Formation of Precipitation

Most condensed droplets & ice crystals in a cloud are too small to fall through the atmosphere (and would rapidly evaporate): condensed droplets are typically on the order of 10-20 microns.

Rain droplets form primarily through collision-coalescence where droplets collide and merge until heavy enough to fall through the atmosphere (100s microns)

Ice crystals can grow by accretion around nuclei or aggregation of ice crystals.

The Bergeron-Wegener Process can occur if a cloud contains both liquid water and ice. Because the vapor pressure is greater over water than ice (that is, it is easier for water molecules to evaporate from the liquid), the ice can grow at the expense of the liquid. This can lead to rapid growth of ice crystals in the cloud.

There are different types of precipitation:

Cloud Types

The classification scheme first developed by Luke Howard in 1803 is still generally applied today. Clouds are classified by their form (vertical extent) and by their altitude.



altitude vertical layered
high cirrocumulus cirrostratus
middle altocumulus altostratus
low cumulus stratus

Atmospheric Forces, Pressure, and Wind

Wind: horizontal movement of air; characterized by both a direction and a magnitude; a wind is labeled by the direction from which it is traveling (e.g. a NW wind is coming from the NW)

This can be expressed using windpoles, which shows the direction and magnitude of the wind (in knots or nautical miles per hour); 1 knot = 6076 ft or 1.15 miles or 1/60th a degree of latitude

Forces that influence wind

Pressure Gradient Force

pressure gradient is a change in pressure over a distance; the pressure gradient force over some distance \(n\) is thus

\(PGF = \frac{\Delta P}{\Delta n}\)

The force always points from areas of higher pressure to areas of lower pressure. The steeper the pressure gradient, the stronger the PGF and the stronger the winds.

Mapping variations in pressure can thus be used to indicate the PGF. There are two main ways to show this:

Can again think of how pressure reflects the weight of the air above: areas of high pressure have higher isobaric altitudes than areas of low pressure; this will influence air movement and subsequent weather.

If PGF always points from high pressure to low pressure, and pressure decreases with altitude, why doesn’t the atmosphere just fly away? The answer is that there is a hydrostatic balance between the PGF and gravity.

Coriolis Force

Acceleration caused by the Earth’s rotation:

This is also the source of how we describe cyclonic rotation (counter-clockwise around a low) and anticyclonic rotation (clockwise around a high)

\(CF = 2fV\)

where \(V\) is the wind speed and \(f=2\Omega sin \phi\); the Coriolis force increases with wind speed and increases with increasing latitude.

Centrifugal Force

The centrifugal force ie caused by centripetal acceleration (toward the center of the rotation); for wind the centrifugal force points outward from the center of rotation (that is, it is the tendency of wind to want to travel in a straight line)


where \(R\) is positive for cyclonic flow and negative for anticyclonic flow: works against PGF around lows and with PGF around highs; this force also gets very strong for small turning radii.


Friction with the surface will decelerate the wind; this force thus always moves in the direction opposite the wind. It is described by


where \(V\) is the wind speed and \(k\) describes the roughness of the surface. Trees and tall buildings have much more “roughness” whereas sand/snow/water have very low roughness.

Combining Forces

Buys-Ballots Law:

The forces will combine to yield the wind behavior. This will tend to produce a geostrophic wind: in which there is a geostrophic balance between the Coriolis force and the PGF (\(PGF+CF=0\)).

This will cause the wind to tend to move parallel with the isobars. Again, we find that a tighter pressure gradient will yield a stronger geostrophic wind.

If all of the effects are combined, we will see geostrophic balance. Often, however, we will see subgeostrophic flow around low pressure systems and supergeostrophic flow around high pressure systems. This will accomplish gradual movement toward the low pressure center and away from the high pressure center.

To summarize the balances:

balance forces expressions
hydrostatic vertical PGF, gravity \(dP/dz=-\rho g\)
geostrophic PGF, Coriolis \(dP/dn = -2\Omega \sin \phi v\)
gradient PGF, Coriolis, centrifual \(\frac{v^2}{r}+2\Omega \sin \phi v+\frac{dP}{dn}=0\)
Guldberg-Mohns PGF, Coriolis, friction \(-kv+2\Omega \sin \phi v+\frac{dP}{dn}=0\)

Air Masses

Characterized by temperature and humidity, which are largely inherited from their source region

Humidity: Continental, Maritime

Temperature: Arctic, Polar, Tropical

This yields: cP, mP, cT, mT, and cA   (Arctic air masses are always dry, and are also very stable)

arctic polar tropical
v. cold cold warm
continental v cold, dry (cA) cold, dry (cP) hot, dry (cT)
marine v cold, dry (cA) cool, humid (mP) warm, humid (mT)

Air masses can undergo air mass modification as they travel (advect) over the surface, via heat/moisture exchange and/or lifting.

A prime example of modification is lake-effect snow, during which a cold air masses moves over (relatively) warmer water.

The boundaries between air masses are known as fronts. There are different types of fronts:

The movement of fronts is closely tied to general cyclonic and anti-cyclonic circulation patterns. The low-pressure systems in particular can develop into large, strong storms known as…

Mid-Latitude Cyclone

How do fronts move? Often connected to pressure lows known as midlatitude cyclones (or extratropical cyclone);

Benjamin Franklin correctly determined that the surface winds of a storm system were only incidental to the forward movement of the storm;

Norwegian Cyclone Model:

First proposed by Jack Bjerknes in the 1920s (Bergen, Norway).

Note: we now understand that upper-air conditions must be favorable; this is more likely to develop just east of a trough (where we have diffluence).

All of this takes place over the course of several days as the entire cyclone moves eastward

Some Examples of common types…

Thunderstorm development

Single-cell (local) thunderstorm (or “air-mass” thunderstorm)


mature stage

dissipating stage


electrical discharged caused by thunderstorm motions

probably about 40 lightning strikes per second on the surface of the Earth (100 is a commonly cited value)

Cloud Electrification

Field Generation


Preliminary breakdown: change in field about 30 msec before first stepped leaders emerge from the cloud base.

An electric spark is an abrupt electrical discharge that occurs when a sufficiently high electric field creates an ionized, electrically conductive channel through a normally-insulating medium, often air or other gases or gas mixtures.

Local discharge between + region near base and - regions above, freeing more electrons in - region

What’s happening? if the electric field is strong enough, it exceeds the dielectric strength of the gas (which is normally a pretty good conductor), and the molecules in the air will begin to partially ionize and conduct electricity (by movement of electrons).

Multi-cell Thunderstorm

Mesoscale Convective Systems

Squall Line

Mesoscale Convective Complex

Supercell Thunderstorm

Conditions that favor violent thunderstorms…

Supercell Thunderstorms

Supercell Structure



formation & organization



Radar Signatures

Tornado Strength

Why so many tornadoes in the US?

Hail Formation


Planetary Circulation

Planetary Scales

Circulation Models

Should be able to explain:

Global Circulation on Non-Rotating Earth

single cell circulation (Hadley model)

Global Circulation on Rotating Earth

Global Circulation Model: Observation

Idealized zonal pressure belts:


Cloud and Precipitation patterns also show clear trends related to planetary circulation

Jet Stream

Seasonal Variations

Rossby Waves

Upper level wave patterns

Open wave pattern

Positive tilted trough

Negative tilted trough

Zonal flow pattern

Cut-off low (weatherman’s woe)

Blocking high

Omega block

Rex block

Global Circulation Model: Seasonal Variations

Again, all of this is driven by changes in insolation.

The Earth


From a meteorological perspective: * main source of atmospheric water vapor * exchange energy with the atmosphere * transfer heat poleward

Global water cycle

Oceanic Provinces


Sea surface temperature (SST)

Sea Surface Temperature Anomaly

The SST anomaly field (degrees C): the difference between nighttime-only SST and the nighttime-only monthly mean SST climatology (which is based on nighttime observations from 1984-1993, with the years 1991 and 1992 omitted due to aerosol contamination from the eruption Mt. Pinatubo in June of 1991)

Higher temperatures can lead to reef thermal stress and coral bleaching events

SST patterns:

These can be explained by ocean circulation:

Ekman Spiral

These currents can act to push surface water toward or away from the coast, which may lead to downwelling or upwelling


El-Nino Southern Oscillation

Normal (“cold”) phase (+SOI)

El-Nino (“warm”) phase (-SOI)

These changes yield teleconnection: climate connections over large distances in oceanic and atmospheric conditions. El Nino tends to give

Other oscillations include…

connected changes in zonal flow patterns (zonal index)




A specified interval:

Averages and extremes of variables:

determined by

Koppen-Geiger Classification scheme

Five main groups (A–E)

note relation to geography and global circulation of atmosphere

Climate History

Have the climates always been the same?

Reconstructing Past Climates: Sources

Dendrochronology (tree-ring data)

Pollen Records

Isotopic Applications


Stable Isotopes (D and O-18)

Radiometric Age Dating

Putting these proxies together to reconstruct paleoclimate

More recently,

Little Ice Age

Large-Scale Causes of Climate Change

Milankovitch Cycles

Causes of Glaciation

positive feedback (increase glaciation)

negative feedback (decrease glaciation)

positive feedback: enhance or accelerate change

negative feedback: disrupt or mitigate change

Anthropogenic Effects

Acid Rain

Stratospheric Ozone

Climate Change: Main Indicators

note: land temperatures have generally increased more than ocean temperatures because of differences in their thermal properties; so global average temperature must consider both contributions

Much of this change appears to be driven by increases in greenhouse gas concentrations:

What about water?

Why the focus on carbon (carbon dioxide)?

How do we know it is our carbon?

Anthropogenic Forcing

Forcing describes differences between solar insolation and how much energy is radiated back into space.

Climate Models


Different types of models

Model Resolution (spatial and temporal)

Climate model inputs

Model Assessment and Validation

Climate Model Intercomparison Project (CMIP): ensemble modeling

Different emissions scenarios are described by Representative Concentration Pathways (RCPs)

Climate Sensitivity: the temperature change in response to changes in radiative forcing