The Big Picture

Read through this page. I don't expect you to understand everything here, so don't panic. We will cover many of these topics during the semester.

Overview

The driving force for weather is the Sun. In physics terms, the sun is the source of energy for most atmospheric circulations. The energy from the Sun comes in the form of radiation. The largest portion of this radiation is visible radiation (light we can see with our eyes). The energy is delivered to the Earth when the radiation is absorbed, mostly by the Earth's surface (land and ocean) and less so by the atmosphere. Thus, the largest portion of the radiation energy from the Sun passes through the gases in the atmosphere (like a window) and is absorbed by the Earth's surface. Fortunately, not all of the radiation from the Sun passes directly through the atmosphere. High energy ultraviolet radiation, which damages the DNA in living cells, is absorbed by ozone in the stratosphere and does not reach the Earth's surface; otherwise, most life as we know it could not exist on Earth.

Because the Earth is a sphere, at any one instant in time radiation from the Sun is most intense at a single point, the point on the Earth where the Sun is directly overhead. The further away from this point, the less intense is the radiation from the Sun. Over the course of a day as the Earth rotates, the points of most intense radiation trace out a circle of latitude. Over the course of a year as the Earth orbits the Sun, the circle of most intense radiation moves. The movement of this area of most intense radiation during the year is what gives rise to the seasons on Earth. The zone of most intense radiation stays near the Equator, never moving more than 23.5 degrees of latitude away from the Equator. This region of Earth is called the tropics. Thus, the radiation energy from the Sun is most intense near the Equator (in the tropics) and is weakest near the north and south poles (polar regions). (Season link 1) (Season link 2) For this reason, warmer climate zones are located in the tropics and colder climate regions are located in the polar regions. This uneven heating of the Earth's surface results in an energy flow from warmer to the colder regions. Energy is transferred from the tropics to the polar regions by atmospheric circulations and ocean currents. In a sense, large-scale weather systems are powered by the temperature differences between the tropics and the poles. In the middle latitudes (30 to 60 degrees latitude, which includes most of the continental U.S), where the warm and cold air meet, large-scale storm systems develop. The characteristic cyclonic or counter-clockwise swirling of these storm systems is due to the rotation of the Earth.

The Earth's surface and the atmosphere emit (or give off) radiation energy in the form of infrared radiation that we cannot see with our eyes. The fact that the gases in the atmosphere are transparent to radiation from the Sun, but interact with infrared radiation is the basis for what is called the "atmospheric greenhouse effect". Without the atmospheric greenhouse effect, the Earth's surface would be much colder (in fact mostly frozen over), and life as we know it could not exist.

Because the majority of the Sun's energy is absorbed at the surface of the Earth, the atmosphere is primarily heated by the underlying ocean and land surfaces. Heating a fluid (like the atmosphere or water) from below causes warm fluid to rise upward and cooler fluid to move down to take its place. In other words, warm air rises. This is obvious to see by observing a campfire - rising warm air currents carry ashes upward. In the atmosphere of Earth, water in the form of the gas water vapor that has evaporated from the ocean and land surface rises upward as part of the rising air currents. It takes a lot of energy to evaporate water (convert it from a liquid to a gas). The rising air cools and the water vapor it contains condenses to form clouds (the opposite of evaporate, now water changes from a gas [water vapor] to a liquid [tiny cloud droplets]). Condensation in clouds releases the energy that was once used to evaporate the water from the surface. The energy release within developing clouds can power severe thunderstorms, hurricanes, and tornadoes.

The importance of the hydrological cycle cannot be overstated. This is the cycle of liquid water evaporating from the Earth's surface to become water vapor in the atmosphere, then condensing back to liquid (and ice) during cloud formation, and finally returning back to the Earth's surface as precipitation. Obviously life on land would not be able to exist without this hydrologic cycle. In addition, clouds strongly modulate the climate of the Earth by reflecting visible radiation from the Sun and by absorbing and emitting infrared radiation.

Plan for this course

In the first part of this course we will describe the atmosphere of Earth and discuss some of the ways in which it impacts our existence. In order to do this, we must cover some of the physics that explains why things operate the way in which they do. We will then cover some weather-related topics. Please realize that we cannot possibly cover all weather topics in part of a single semester. The second part of the course will deal more with climate and climate change. Before beginning the first section, let us first distinguish between weather and climate.