Weather & Climate
The average person often misuses the terms weather and climate. Think of weather as what it’s like outside today, whereas climate is the “average weather” or what we should expect the weather to be like today based on years and years of data. It might be cold one day and hot the next, but that’s not necessarily climate. That’s just weather.

In this course, we are going to focus much more on climate, although severe weather will be discussed in certain regions/chapters. Temperature and precipitation are the main two ways to measure climate and are crucial for distinguishing between the five, main climate types we will use this semester. Before I discuss the five climate types, let’s first cover the basics about temperature and precipitation.

Why does temperature vary across Earth’s surface?
There are many different reasons for temperature variation, but we’ll just focus on three key reasons here.

First, the most obvious is the role of latitude. The closer you are to the Equator, the more powerful and consistent the sun’s rays are and, therefore, the warmer the temperatures. As you go farther away from the Equator towards either the North Pole or South Pole, temperature on average declines with less and less incoming solar radiation/energy. The North America isotherm map for the month of March below clearly shows the latitudinal pattern with yellows/greens (warm temperatures) found horizontally towards the bottom of the map and blue/white/purple (cold/cooler temperatures) colors found towards the top of the map (to the north).

March Temperature-1.jpg

Second, altitude or the distance or elevation above mean sea level, can also explain temperature variation as temperature decreases with increasing altitude. On average, temperatures decline about 2.5 degrees Fahrenheit for every 1000 ft rise in elevation. This rate of temperature change is called the normal lapse rate. If you’ve ever climbed to a top of a mountain, even if it was the Smokey Mountains, you should have noticed the temperature goes down as you go up. In the image below, I zoomed into the western half of North America portion of an isotherm map for the month of August. Places with warmer temperatures are orange or yellow and cooler places are green and blue.
Isotherm Map August.png
Although this map only measures the temperature of the air at the Earth's surface, you can see the rugged terrain and mountainous areas (as we discussed on a previous page in this chapter) are clearly evident. Along the coast of Washington and Oregon, there is a north-south line of "green" in the map. This is where the Coast Ranges are located. Then, there is a low-lying, yellow-colored valley that is evident to the east (or right) of the green temepratures along the coast. Then, as you go east, the Cascades are evident by another linear belt of cooler temperatures that runs all the way down the eastern side of California as the Sierra Nevadas. Even farther inland, the Rockies have really cool August temperatures as there are areas of blue (cool) temperatures in Colorado and Wyoming.

Two cities that are both located at 39 degrees north latitude are Indianapolis and Aspen, Colorado. In August, the average high temperature in Indy is 84 degrees. In Aspen, it's only 72 degrees. What's the difference between these two cities? Indianapolis is located at 715 feet above sea level and Aspen's up at 8,000 feet above sea level.

Third, where you are located on a continent can also influence the average temperatures. Places near a large water body are called “maritime” locations. What is a large water body? It’s not the White River or Lake Monroe, but oceans, seas, and gulfs and occasionally, big lakes like the Great Lakes that can influence the temperatures around them. Places that are deep within the continent and far from a major water body are very “continental” and have high “continentality”. Those are essentially big words that mean that you aren’t anywhere close to a beach (no, Indiana Beach doesn’t count).

In essence, maritime locations have moderated temperature patterns with milder winters and summers as the summer period doesn’t get too hot, but the winter isn’t too cold. Highly continental places have more extreme average temperatures throughout the year, with colder winters and hotter summers. For example, San Francisco and Indianapolis are also basically located at the same latitude (39 degrees North) but have much different temperature patterns throughout the year. During winter, it rarely is cold enough in San Francisco for significant snowfall and freezing temperatures to occur. If you’ve lived a winter here in Indy, you know a snow-less winter is a fantasy. That said, a summer in San Francisco can be surprisingly cool. One time I visited there during the summer and learned this the hard way. As I packed for my trip, I brought shorts and t-shirts because, after all, I was going to sunny California. One hour after arriving in San Francisco, I was buying a stupid, $60 souvenir sweatshirt because it was only 48 degrees!

How do you make it rain? (lol)
It’s very simple. If you can get a large, warm mass of moist air to rise, it will eventually cool because temperature declines with altitude. Eventually, the tiny droplets of moisture are going to cool enough that they condense and become ice crystals and drops of water. Eventually, the weight of the ice crystals and water drops can’t be held aloft and gravity causes the moisture to fall back down to the surface as rain.

There are four different ways to make it rain (or, more professionally, we would say that there are four different ways to lift a moist air mass (evaporation) to the point of condensation and then precipitation). These four mechanisms are convergence, orographic, frontal, and convection. For the purposes of this discussion, we will not cover “convection”.

Convergence
This section might be complicated, so I will try to keep it as simple as possible. Across earth, the amount of moisture in the air varies. You already know this as what else would explain why the desert regions of the world are so dry while the rainforests are so wet.

To better understand this, take a look at the model of global pressure belts below. The arrows below denote the general direction of wind patterns and the temperature of the moving air (red = hot, blue = cold, purple = warm). Key to understanding these pressure areas is the difference between a high and low pressure. Simply speaking, where a high pressure (H) is located (Polar and Subtropical), expect dry weather and low pressure (L) areas (Equatorial and Midlatitudes) see steady precipitation and rainfall.
Global Pressure Belts.jpg
Global Pressure Belts (a model)

Let's begin in the middle of the map where you see the "Equatorial Low Pressure - ITCZ". First, notice the location of this predominant belt of pressure surrounding the Equator. Second, notice the arrows are coming in to each other. What that means is that air is converging from both sides (north and south) and meeting near the Equator. This is why this area is also called the "Inter-Tropical Convergence Zone (ITCZ)". It is important to note that convergence only occurs along and near the Equator. Considering not much of North America is tropical or near the Equator, there is not a lot of influence from the ITCZ in this region. That said, the hurricanes that hit the Southeast United States each year originate from this convergence zone.

The outcome of the Equatorial Low Pressure dominating near the Equator throughout the year is noticed in the precipitation map below. Horizontally across the map, you should see a dense, green band. For this map, green equals precipitation. Areas shown in white are where we see predominately dry conditions. These large swathes of dry areas (located approximately at 20 degrees to 30 degrees North and South) are especially noticeable on both sides of the dense, green rain belt in the Americas and Africa. These areas are influenced by the Subtropical High Pressure found throughout a good portion of the year. Subsequently, some of the world's most well-known deserts (Sahara Desert, Arabian Desert, Kalahari Desert) are found in these areas on both sides of the Equatorial Low Pressure.

World Precipitation - Convergence.jpg
Annual Precipitation Map (green = rain; white = no rain, blue = some rain)

Orographic
Now, what if an air mass is forced to rise because it encounters an upland or mountain? This is called orographic precipitation and results in some pretty dramatic precipitation differences on one side of a mountain versus the other. In the video that follows, I provide an explanation of orographic precipitation, some key characteristics of it, and then present a case study of the precipitation differences on two sides of the same mountain.
Video - Orographic Precipitation Explained

Frontal
The last way to make it rain is called frontal (warm and cold fronts). Once again, for the purposes of this course, we will keep it very simple. Frontal precipitation is most likely found in the Midlatitudes, which are anywhere between about 30 degrees and 60 degrees latitude in both the northern and southern hemispheres and also were we find most of the world's "C" climate regions (see the section that follows for description of "C" climates). These areas are noted by having changing weather from one day to the next, with not too much rain, but plenty enough for farming. This is because in the Midlatitudes there are low pressure areas carrying precipitation that migrate through bringing constantly changing weather. Go back up to the Global Pressure Belts image and you will see the Midlatitudes containing an "L" next to it. What does "L" stand for? Low pressure. In the Midlatitudes, as you go closer to the Equator, you are likely to see more influence from the very dry Subtropical High Pressure. Towards the poles, you'll find the Polar High Pressure, another area of predominately dry conditions.

Köppen Climate Regions
At the global-level, there are five main climate regions (A, B, C, D, E) that are classified based on the average temperature and precipitation characteristics found there. The Koppen-Geiger climate map below maps out each of the five major region types. For the purposes of this course, we will only focus on the big categories (i.e. A or B) instead of the sub-categories (i.e. Bsk or Dwd).
Koppen Climate Map.jpg
World Climate Map

“A” Climate Regions
Humid, tropical areas often near the Equator with consistently high temperatures and precipitation. In short, it’s hot and wet in an “A” Climate. Temperatures are consistently high here because the sun’s incoming solar energy is much, much higher here than in areas towards the poles. Precipitation is high as a result from the constant presence of the Equatorial Low Pressure belt and convergence (Intertropical Convergence Zone). Here you will find tropical rainforests, monsoons, and savannas, all of which we will discuss more in-depth later in the semester.

“B” Climate Regions
Climates that are warm, but very, very dry (arid) are called “B” Climates. Places with a “B” climate have a high number of sunny weather and not that many clouds. With no clouds above helping to block the sun’s incoming energy, these places can have extremely hot temperatures during the day. Conversely, temperatures can be very cold during the night because there are no clouds to help keep the heat in from the Earth’s surface. There are two main reasons precipitation is lacking in “B” climates. First, the constant presence of the Subtropical High Pressure helps to keep these places persistently dry. Second, warmer places located in a rain shadow. In a “B” climate, you will of course find deserts, but also semi-arid steppes, which is a transition area from deserts to grasslands.

“C” Climate Regions
Think of “C” climates as being “comfortable” and are referred to as temperate. These are places in the Midlatitudes with much more seasonality than A or B climates. In an “A” climate, it’s almost always summer, but in a “C” climate, there are more changes in temperature and precipitation throughout the year. Essentially, “C” climates are where it’s not too hot, not too dry, not too cold, and not too wet, which helps to explain why most of the world’s most modern economies and highest levels of development are found in “C” climate regions. Often, but not always, “C” climates are located near coasts.

“D” Climate Regions
Also found in the Midlatitudes, “D” climates are similar to “C” climates with changing seasons and milder temperatures. The main difference between the two is that “D” climates are more continental, with very, very cold winters. Also, total precipitation isn’t as high as a “C” climate. One key characteristic of “D” climates across the world are that they are very agriculturally productive and, on average, have the best soils in the world.

“E” Climate Regions
The coldest (most miserable?) places on Earth are “E” climates. These are places where there is less than four months each year when the temperature even gets above freezing. Subsequently, vegetation is minimal/limited here. These are places dominated by the Polar High Pressure and are extremely dry. In fact, Antarctica, not the Sahara Desert or Death Valley, is the driest place on Earth. You can also find “E” climates in high, mountainous areas, too.