3. Weather and Climate

Classic textbook chapters frequently feature a chapter on weather and another on climate. There are ample resources available online and offline for understanding the physical phenomena of weather and climate. This chapter focuses instead on some of the relationships between people, weather, and climate.

1. The Atmosphere

The atmosphere is a thin layer of gas surrounding Earth. The image at right (source: National Weather Service, NOAA, http://www.srh.noaa.gov/srh/jetstream/atmos/layers.htm) shows how thin this layer is over most of the United States. Phenomena we associate with outer space, like the aurora and meteors, start at about 50 miles. The layer of atmosphere in which all weather happens--the troposphere--is only about 11 miles thick at its thickest, and about half that in the mid latitudes (like where the United States and Europe are located).

The outer layers of the atmosphere serve important functions, but in this text we'll concentrate on the troposphere. The troposphere is where all humans live, with the exception of astronauts and space tourists. The troposphere is also where all weather takes place.

At its thickest the troposphere is only 0.00139 times Earth's diameter, little more than one-tenth of one percent! This nearest layer contains about 80% of the atmosphere's mass.

The atmosphere is composed mainly of four gases. From most to least concentrated, Nitrogen, oxygen, argon, and carbon dioxide together make up 99.998 percent of dry air. Thus, even though the atmosphere might seem 'big', it's relatively easy to change its composition, especially of gases that are present in only small amounts, like carbon dioxide. Carbon dioxide, the fourth most common gas in the atmosphere, had a global average concentration of 395.31 parts per million in 2013, or 0.0395 percent. Of the four most common gases, only carbon dioxide is changing in concentration significantly.

Weather is the short-term condition of the atmosphere. Weather includes temperature (hot or cold), water (fog, clouds, rain, or snow), and wind.

Weather: A rapidly developing thunderstorm blocks out the sun. Thunderstorms occur most often in areas with intense sun and sources of moisture on the ground, such as summertime fields in the American Midwest. 2011 photo by Jerry Huddleston, used under CC BY 2.0, (https://www.flickr.com/photos/huddleston/5837240053/)

2. Clouds and Precipitation: Atmospheric Lifting Mechanisms

There are many elements of weather that this text doesn't go into, but two of the most common questions about weather is "where do clouds come from?"  and "what makes it rain?" Rain falls into the more general category of precipitation, which is any form of water that falls from the sky, including snow, sleet, and hail.

To deal with the difficult problem of understanding invisible things like air, we talk about "air parcels," that is, bundles of air that move around the atmosphere. Air parcels are all around us but we focus on the interesting things that can happen when one of them changes elevation. The atmosphere is most dense at the bottom and becomes less and less dense with altitude. So when a parcel of air rises, it moves into an area of lower density. In this lower density environment, the parcel expands.

When a gas expands, it cools. So a parcel of air that rises expands and cools. Here is some evidence that an expanding gas cools. You may have encountered something similar in your own experiences:

Evidence that an expanding gas decreases in temperature: The metal fitting on a propane camp stove is cold enough to be covered in frost. As propane leaves the cylinder, it expands dramatically, cooling the fitting. Photo by author.
Air's ability to hold moisture is a function of its temperature. The warmer the air, the more invisible water vapor it can hold. But as air cools, its capacity to hold water vapor decreases, so the air becomes more and more humid. Below a certain temperature depending on how much water vapor is in the air parcel (called the dew point temperature), the water vapor in the air condenses--becomes liquid--into tiny water droplets. These tiny droplets together form a cloud. If condensation continues (that is, the cloud continues to rise), the droplets grow in size until they start to fall under gravity--we know this phenomenon as rain. Different conditions lead to the formation of snow or hail.

To review:

  1. The atmosphere is less dense at higher elevations.
  2. When a parcel of air rises through the atmosphere, it expands.
  3. When a gas expands, it cools, all other things being equal.
  4. Air's capacity for water vapor decreases with its temperature.
  5. If a parcel of air cools enough, condensation will form.
So a parcel of air lifted through the atmosphere expands and cools. If there is sufficient water vapor in the parcel, it will condense to form a cloud and eventually precipitation. The next step in our understanding is to learn the four main ways that air parcels rise--what we call atmospheric lifting mechanisms. These mechanisms are frontal lifting, orographic lifting, convective lifting, and convergence lifting.

In convection, a parcel of air next to the surface is warmed and rises. The effect of convergence may be accentuated if the heating is differential, that is, if some areas are warmed while others stay relatively cool. An example illustrated below is what might happen over a field surrounded by forest. The rising warm air expands, cools, and condenses, forming a cloud and sometimes precipitation. This phenomenon is evident on many sunny summer days in areas where sources of moisture, like fields, rivers, or lakes, are in the landscape. A day that starts clear and sunny often has many puffy white fair weather cumulus clouds by afternoon, which dissipate at night.
Atmospheric lifting from convection. Warm air rises from surface heating.
 In convergent uplift, major forces work to create areas of low surface pressure. The resultant forces create converging winds that then force air upwards. Perhaps the best example of a major convergence zone is the InterTropical Convergence Zone (ITCZ), a band of low pressure circling Earth near the Equator that moves north and south with the sun's most intense rays. The ITCZ is responsible for consistent, plentiful rainfall--over 100 inches per year in many places--that creates the lush jungles of the Amazon and Congo Rainforests as well as the rainforests of Indonesia.
Atmospheric convergence.  

A lifting mechanism common to much of North America is frontal lifting. In this process massive bodies of air, called air masses, move across the surface. If a colder air mass is moving into a warmer one, it's called a cold front and the warm air is forced up and over the colder, denser air. Other fronts also result in lifting and precipitation, namely warm fronts and occluded fronts.
Atmospheric lifting and precipitation at a cold front.
The final lifting mechanism considered here is orographic lifting, lifting caused by mountains. When the atmosphere moves across mountainous terrain, it is forced to rise. If there is a consistent source of new air, that is, a generally steady wind, and also a source of water vapor, most classically the ocean or a large lake, the orographic effect can result in some of the highest precipitation amounts in the world.
The orographic effect means that precipitation tends to fall on  the windward side of a mountain range, leaving a rain shadow on the other (leeward) side.

The orographic precipitation effect is especially evident in the northwestern United States as well as British Columbia, Canada. Examine this precipitation map of Oregon:

Precipitation map of Oregon modified from public domain on Wikimedia Commons, http://commons.wikimedia.org/wiki/File:Oregon_Average_Annual_Precipitation_(1961-1990)_Map.png. Original map from National Atlas, nationalatlas.gov.
The dark purple areas represent the areas with the most precipitation--up to 200 or more inches of water equivalent each year on average! The Oregon Coast Range is responsible for this first strip of high rainfall. Westerly winds (winds out of the west) push moist air up these continuous but not super high mountains. The Willamette Valley, immediately east of the Coast Range, has moderate precipitation, then there is another line of high precipitation corresponding to the Cascade Range. East of the Cascades, the land is much drier, an effect we call a rain shadow. Note that the northeastern part of the state there are two areas of moderate rainfall. These areas correspond with the Blue and Wallowa Mountains, indicating orographic effects again.

3. Controls on Temperature

The previous discussion was about precipitation, which we think of as a weather event. But when we start talking about annual average precipitation, it veers more into a discussion of climate. We'll formally define climate in a later section of this chapter but keep in mind that the same is true for temperature--the temperature right now, today, is part of today's weather, but as we start talking about average temperatures or annual variations in averages we're moving into a climate domain. That's OK because weather and climate are inextricably linked, but keeping in mind that they are also different is helpful to our understanding of those differences.

When we talk about what makes temperatures hot or cold, we talk about controls on temperature. It might sound a little odd when talking about temperature but we are talking about something that works like the knob you might picture when you read the word "control."

But does it go to 11? Control knobs image by Derek Jensen, public domain image via Wikimedia Commons, http://commons.wikimedia.org/wiki/File%3AKnobs-for-climate-control.jpg
Thinking about controls on temperature is similar to the knob above--a change in the level of one thing make the temperature colder or hotter. Keep in mind that as we discuss each control, it's about that control independent of the others. That is, to understand what one knob does we leave all the other knobs alone.


Your first recollection of a hike up a steep mountain might be the sweat your body perspired in an effort to cool itself. But one up at a higher elevation, you may have noticed how cool the breeze felt. The troposphere is heated from the bottom, so as we go up through it, its average temperature decreases.

The effects of altitude on temperature are most dramatic when seen close together in mountainous terrain. Tropical mountains are the most iconic of this effect, such as Mauna Kea in Hawaii, USA, which receives precipitation as snow in winter, and Kilimanjaro in Tanzania, which is just 3 degrees south of the Equator but is capped by large icefields year-round:
Sunset on Kilimanjaro. At 19,341 feet high and with more than 10,000 feet of local relief, the slopes of Kilimanjaro make a powerful demonstration of the effects of altitude on temperature.  2012 photo by Tim Scharks.
A nearly as dramatic example, consider the Ecuadorian cities Quito and Nuevo Rocafuerte. Both are located within a few degrees of the Equator, but Quito has an elevation of around 9,000 feet above sea level, while Nuevo Rocafuerte is in the upper Amazon Basin at only 700 feet above sea level. The annual average temperature in Nuevo Rocafuerte is 77.5 degrees Fahrenheit, while in Quito it's a chilly 56.3 degrees.

Latitude: from Equator to poles

Latitude is a measure of angular distance from the Equator to the North or South poles. At the Equator, the surface of Earth is on average square to the Sun's rays, so the Sun's energy is most concentrated there throughout the year. But the farther north and south we travel, the more oblique-at a shallow angle-the Sun's rays become when they strike Earth. We experience this as seeing the sun's path lower in the sky throughout the year when we move farther north.

This difference has a powerful influence on the amount of energy received at any given place, making latitude an important control on temperature. Take a look at the differences that latitude makes to temperature in these cities:

City Latitude Average temp. July January
Caracas, Venezuela 10.5° N 69.4 69.8 66.2
Miami, FL, USA  25.8° N 75.7 82.6 67.1
Wilmington, NC, USA 34.2° N 63.3 79.7 47.1
Boston, MA, USA  42.4° N 49.5 72.5 28
Sept-Iles, Canada 50.2° N 34 59.4 6.3
Iqaluit, Canada 63.7° N 15.6 45.9 -14.7

All temperatures are in degrees Fahrenheit. Note that these are average temperatures for the month and year, so reflect a melding of both daily highs and lows. On inspection, two major trends should stand out. First, annual average temperatures decrease with increases in latitude once out of the tropics (Miami and Venezuela appear to break this rule but they are only a little different). Second, the annual range in temperatures, the difference between summer and winter, increases with latitude. This is because the changes in day length from summer to winter depend on latitude. A high latitude location like Iqaluit, Canada, has very short days in the middle of winter, with only a about four hours of sunlight, but in summer the opposite is the case, with 20-hour days. The sun's low angle in the sky means the sun's rays are still not powerful at heating the surface, so temperatures do not get as high as one might expect with 20 hours of daylight.

Continentality: land-water heating differences

Heat and temperature are not the same thing. Heat is a measure of the total amount of energy in a substance, while temperature is a measure of how fast the molecules in a substance are moving. Substances can have different heat properties, which means that given the same amount of energy they change temperature more quickly or slowly than one another.

Water and land are two such substances which differ in the way they gain and retain heat energy. Water has a great ability to hold heat energy from the sun because of its physical properties. Additionally it is clear and mixes, meaning that more than the surface of a body of water can absorb heat from the sun. Finally, as water increases in temperature, the rate of evaporation from its surface increases, which cools the surface in a negative feedback. In contrast, land is opaque and does not mix. Rocks also have a lower specific heat than water, meaning they will have a hotter temperature after absorbing the same amount of heat energy.

All these differences add up to big differences in the temperature of the atmosphere. We call differences due to the proximity to large bodies of water, like oceans, continentality, or land-water heating differences. Areas near large bodies of water are slow to increase temperature in the summer and slow to decrease temperature in the winter. This is because the water effectively serves as a "heat sink," evening out the irregularities of the season. On the other hand, areas far from large bodies of water, especially near the middle of continents, become hotter in summer and colder in winter than otherwise similar places near water.

Let's look at some temperatures from two otherwise similar locations in the United States, San Francisco and Wichita:

San Francisco Wichita
Latitude 37.8° N 37.7° N
Elevation 0 feet (sea level) 1,299 feet
January temp.  48.6° F 30.4° F
July temp.  62.6° F 80.1° F
Difference 14° F 49.7° F

Latitude and elevation are similar, but San Francisco is surrounded by the Pacific Ocean on one side and the Bay of San Francisco on the other. In contrast, Wichita is near the center of the continent. Note that not only is Wichita much colder in January than San Francisco, it is also much warmer in July.

4. The Earth-Atmosphere Energy System

All weather represents transfers of energy between Earth's surface and through the atmosphere. The Sun is the essential source of energy for all processes on Earth's surface and atmosphere. The equatorial regions receive about 2-1/2 times more solar energy than the poles. Much of our weather can be explained as a evening out of energy imbalances between the surface and atmosphere and between the Equator and poles.

Thinking back to Chapter 1, we can diagram the flow of energy as a system:

This diagram is a bit simplistic to explain much, but helps us realize a few important facts about solar energy and the Earth. First, solar energy is the only source of energy driving Earth's atmosphere. Contributions from internal sources like radioactive decay and volcanoes are negligible sources of energy when we look at the heat present in the atmosphere. Second, energy comes to Earth as shortwave radiation, specifically, visible light. Some of this light is reflected back into space without becoming part of Earth's energy system, which explains why we can see Earth from space. But Earth loses energy, too. (Thinking about systems, what would happen if Earth didn't balance its energy gains with similar energy losses?) The second major point to be drawn from this diagram, and most important, is that the energy emitted by Earth to space is different from what comes in. Earth's energy lost to space is longwave, infrared radiation. This matters, a lot, to the way our atmosphere works.

Have a look at this fairly complex diagram that displays the flows of energy within the Earth system:
Earth-Atmosphere Energy Balance diagram, by National Weather Service, NOAA, accessed from http://www.srh.noaa.gov/srh/jetstream/atmos/energy_balance.htm

Whoa! That's intimidating at first but if we draw a box around the atmosphere and a box around the surface we can see a more familiar system diagram:

The larger diagram is more helpful because it shows the size of flows, but the simplified system diagram should show you that: 1) Shortwave radiation is the only system input, some of which is reflected and does not enter the system; 2) Longwave radiation is the only system output, some from the surface and some from the atmosphere, and 3) Most importantly, the largest flows of energy in this system are back and forth between the surface and the atmosphere. If we wanted to really emphasize this last point, we might re-draw our system diagram like so:

See that loop, where nearly all energy emitted from the surface is absorbed by the atmosphere, and re-emitted from the atmosphere back to the surface? That loop is commonly called the greenhouse effect. Some scientists don't like that term because Earth does not have a glass cover so the phenomenon of solar heating is different from a greenhouse, but the effect is similar enough for popular understanding that the term has become common.

Particular characteristics of the atmosphere are responsible for the greenhouse effect. Note that in the complex color diagram that shortwave solar radiation has relatively little problem getting from outer space to the surface. That's because the atmosphere is mostly transparent, or at least transparent to visible light, the main frequency of radiation emitted by the sun. But note in the same diagram that most-nearly all-of the energy emitted from Earth is absorbed by the atmosphere. So while the atmosphere is transparent to visible light, it is an efficient absorber of infrared radiation emitted from Earth. This absorption is the primary source of heat for the atmosphere, another way of saying that the atmosphere is heated from Earth's surface.

The gases in the atmosphere that absorb the Earth's infrared radiation are commonly called greenhouse gases. These gases are present in trace amounts of parts per million or even parts per billion. The most abundant greenhouse gas is carbon dioxide at about 395 parts per million. We have to thank the greenhouse effect for our life on Earth as we know it. If the atmosphere were as transparent to infrared radiation, Earth's average surface temperature would be much colder-instead of the global average surface temperature of 56 degrees F we experience today, it would be a chilly -6 degrees F! That's right, without the greenhouse effect on average the entire Earth would be frozen, not just the poles and mountain areas.

5. Human Actions in the Climate System

Most of you know what we've been working up to: yes, humans have been changing the world climate, starting fairly gradually but at an increasing pace, especially picking up since the 1950s. This human-caused change is commonly referred to as global warming. One of the arguments from people in denial of the human causes of global warming is that climate change is nothing new and Earth's climate is always changing. While it's true that Earth's climate has changed in the past, the abundant evidence indicates that past climate changes were usually slower and tended to be self-corrected by negative feedback loops. In contrast, the last century has seen something truly new under the sun: Humans have changed the composition of the atmosphere to a point where there may be positive feedback in the future, moving us farther from the "normal" climate that we know. 

We are altering Earth's climate at a speed and scale not previously evidenced in the recent geologic record through the artificial increase of greenhouse gases. There are several different classes of greenhouse gas, but the gas principally responsible for the greenhouse effect, as well as the observed changes in Earth's climate, is carbon dioxide (CO2). Humans have increased the concentration of carbon dioxide on two fronts. First, we have dug up and burnt increasing amounts of fossil fuels. Fossil fuels are carbon compounds formed deep in the Earth from plants that grew, died, and were buried hundreds of millions of years ago. The plants took carbon out of atmospheric circulation over the course of tens of millions of years that they were buried, but we have been digging it up and burning it at a rate that will consume much of it in only a few hundred years. 

Here's a graphic looking at the growth in just coal consumption from 1980-2010. In 2010 we were burning nearly 8 billion tons of coal worldwide, mostly to generate electricity. Coal is just one fossil fuel of several. Natural gas and oil products, including gasoline, are also major sources of carbon dioxide emissions. 

World coal consumption by region, 1980-2010. Source: U.S. Energy Information Administration, International Energy Statistics. Graphic accessed at http://www.eia.gov/todayinenergy/detail.cfm?id=4390.
The second way we are causing increases in carbon dioxide is by land use changes, a topic geographers are particularly interested in. All plants, including trees, remove carbon dioxide from the air, store carbon as plant tissue, and release oxygen. So forests represent a massive storage of carbon. When cut down and consumed, and especially when replaced with another use, like farmland or city, the forest stops "sequestering" more carbon dioxide. This second way is a less-appreciated way that we are making the problems created by our massive consumption of fossil fuels worse: we're altering many of the systems that might have mitigated some of the impacts from fossil fuel consumption.

The result has been an unprecedented increase in the concentration of carbon dioxide in our atmosphere. Here is the instrumental record kept at the Mauna Loa observatory in Hawaii. Its location near the center of the Pacific away from any industrial centers makes it a good place to keep tabs on global gas concentrations:

It's apparent that carbon dioxide has increased rapidly since at least the 1950s. But how new of a phenomenon is not so clear: this is the highest level of carbon dioxide our planet has seen in at least the last 420,000 years!

The link between human activities and increased carbon dioxide is clear. But how do we know Earth is warming? Just measuring one temperature wouldn't be a good indication that it's warmer. In fact, we would expect multiple types of evidence for a warming Earth:

IPCC 2013 AR5 FAQ 2.1, Figure1: Independent analyses of many components of the climate system that would be expected to change in a warming world exhibit trends consistent with warming (arrow direction denotes the sign of the change), as show in FAQ 2.1, Figure 2. Accessed from http://www.climatechange2013.org/images/report/WG1AR5_Chapter02_FINAL.pdf
In fact, all of our records confirm these expected changes:

IPCC 2013 AR5 FAQ 2.1, Figure 2: Multiple independent indicators of a changing global climate. Each line represents an independently derived estimate of change in the climate element. In each panel all data sets have been normalized to a common period of record. A full detailing of which source data sets go into which panel is given in Supplementary Material 2.SM.5. Accessed from http://www.climatechange2013.org/images/report/WG1AR5_Chapter02_FINAL.pdf.

This book doesn't have time to address fantasists and conspiracy theorists with ideas about how literally thousands of scientists have plotted for decades to lie about global warming; but a lot of ordinary people still may look at the evidence and ask, "how do we know the warming is not caused by something else, like natural sources of CO2 (volcanoes), or variations in the sun's output?" It turns out that thousands of scientists have already looked for other possible causes of the observed warming:

IPCC 2013 AR5 FAQ 5.1, Figure 1: Global surface temperature anomalies from 1870 to 2010, and the natural (solar, volcanic, and internal) and anthropogenic factors that influence them. (a) Global surface temperature record (1870-2010) relative to the average global surface temperature for 1961-1990 (black line). A model of global surface temperature change (a: red line) produced using the sum of the impacts on temperature of natural (b, c, d) and anthropogenic factors (e).(b) Estimated temperature response to solar forcing. (c) Estimated temperature response to volcanic eruptions. (d) Estimated temperature variability due to internal variability, here related to the El NiƱo-Southern Oscillation. (e) Estimated temperature response to anthropogenic forcing, consisting of a warming component from greenhouse gases, and a cooling component from most aerosols. Accessed from http://www.climatechange2013.org/images/report/WG1AR5_Chapter05_FINAL.pdf

There ARE natural sources of climate variability-but only one is a good match, therefore a good explanation for, the observed increase in temperature: human causes (labeled "Anthropogenic Component" in the graphic above).

6. Climate vs. Weather

What kind of changes in the weather can we expect from global warming? You've already learned that weather is the day-to-day condition of the atmosphere, but we haven't yet formally defined climate. Climate is the long-term average condition of the atmosphere. Climate "normals" are usually calculated using thirty years of data. As climates change from global warming, new "normals" are adopted to reflect changes.

A useful way to think about the differences between climate and weather is to realize that climate is made up of lots and lots of weather events, averaged over a long period of time. Climate is made up of weather the same way a baseball player's batting average is made up of individual at-bats. The present at-bat might tell you more about how the player is feeling that day, how he does against a left-handed pitcher, or something of that sort, but the batter's average over time is a better indicator of what kind of player he is.

People regularly commit the mistake of equating weather and climate. They are related but separate concepts. It's equally silly to wake up on an unseasonably cold morning and say "this proves climate change isn't real!" as it is to wait for an unseasonably hot day and say that it does prove climate change. Each day's weather is only one data point for the long-term climate average of many thousands of data points. If a batter strikes out, it doesn't prove he is a terrible player (the best in the game still strike out more often than not), just as a batter getting a hit doesn't prove he is great.

BUT, we can talk about the likelihood or probability of certain types of weather because of climate change, the same way we can talk about the likelihood of a particular hitter's connecting with the ball on any given at-bat. We would expect someone who bats .300 to connect twice as often, on average, as someone who bats .150. But will they get a hit THIS pitch? THIS at-bat? Will it be a home run? We don't know. But what if we put that batter on steroids? We still can't tell whether this particular at-bat will be a homer, but what happened to the likelihood of home runs? We can presume with some confidence that the overall likelihood of home runs has increased for those players on steroids.

The same is true for global warming. We can't look at an individual weather event and say, "global warming did this." But we can make statements like, "global warming makes this weather event more likely."

This graphic displays weather events that scientists have high confidence have been influenced by human-caused global warming. A few of them may seem like good news, like decreased droughts in some areas of the world. But on balance scientists agree that there are more destabilizing events than good ones made more likely by global warming.

IPCC 2013 AR5 FAQ 2.2, Figure 2: Trends in the frequency (or intensity) of various climate extremes (arrow direction denotes the sign of the change) since the middle of the 20th century (except for North Atlantic storms where the period covered is from the 1970s). Accessed from http://www.climatechange2013.org/images/report/WG1AR5_Chapter02_FINAL.pdf 
Last updated January 17, 2015