What’s With the Weather

For many of us, looking at the weather forecast is part of our morning routine because the weather determines what activities we’re able to do and where we’re able to go. Everyone on Earth experiences the effects of weather in some way or another, hot, cold, dry, humid, snowy, or sunny. So its worth considering why our planet has the weather that it does.

What Is The Weather?

Weather is broadly defined as the short-term state of a region’s atmosphere. Changes in the atmosphere are influenced by interactions between air temperature, wind, cloud coverage, precipitation, air humidity, and air pressure.

The weather we experience on the planet’s surface has much to do with heating by radiation from the sun. The sun is the primary source of energy for Earth’s weather.

Albedo

Only a fraction of the energy radiated from the sun is absorbed by the Earth. Some of the sun’s energy is reflected back into space due to the Albedo effect. Albedo refers to the world’s reflectivity of sunlight (heat from the sun). Surfaces that appear white and light-colored reflect much more than those that are darkly colored. So the Earth’s albedo is positively enhanced by ice, snow and clouds. Earth is estimated to reflect about 30 percent of incoming solar energy. As cloud cover and the total amount of ice and snow change, so too does the planet’s average albedo and temperature.

albedo diagram: sunlight being reflected off snow and cloud
Albedo effect diagram: Sunlight being reflected off clouds and snow

Decreases in snow and ice cover result in decreased average Albedo for Earth and increased global surface temperature.

Greenhouse

Our atmosphere is made up of a thin layer of mixed gases loosely connected to Earth. The most abundant of these gases are nitrogen and oxygen, which are about 99% of the atmosphere’s gases. So-called greenhouse gases, however, are much less present and only appear in trace amounts. Greenhouse gases, including water vapor, methane, carbon dioxide, nitrous oxide, ozone and chlorofluorocarbons, are molecules that absorb and emit heat radiation.

It’s important to note that the greenhouse gases in the atmosphere are mostly heated by the Earth, not the sun. This is because solar radiation interacts with greenhouse gases differently than terrestrial radiation.

The sun’s energy contains visible, shortwave radiation that mostly passes through Earth’s atmosphere because greenhouse gases do not absorb shortwave radiation very well. Shortwave radiation that reaches Earth’s surface is then reemitted by Earth as infrared, longwave radiation. Greenhouse gases are highly effective at absorbing longwave radiation. Some of the heat absorbed by greenhouse gases radiates out into space and some of it returns to further heat the Earth.

Of all greenhouse gases emitted by human activity, methane and carbon dioxide contribute most to global warming. Methane has the greatest warming potential, carbon dioxide stays in the atmosphere the longest (for an estimated 100 years). Greenhouse, including those produced from burning fossil fuels, reinforce the warming effect that greenhouse gases have on Earth’s average surface temperature.

Wind

The wind is the movement of air and other particles in the atmosphere. The wind is caused by differences in air pressure. Air pressure, or air density, is the measure of force with which air molecules push on a surface.

Air pressure is closely correlated with temperature (elevation and air moisture are also relevant). Generally, warm air is less dense than cold air. As molecules of air are heated, the space between them expands and creates lower density. Inversely, as air molecules are cooled, they group together more tightly and exert more force on whatever is beneath. If one area heats up more than another, the warmer air will expand and rise, and cooler, more dense air, will rush in to take its place. The speed of the wind is largely determined by the differences between air pressures. Wind flow patterns form circular loops over land and water as temperatures fluctuate continuously between night and day and between seasons.

Precipitation

Precipitation is any water that is pulled down from clouds by gravity. Precipitation may fall as a liquid or a solid. Rain is an example of liquid water fall; hail and snow are examples of solid. Precipitation is a facet of the water cycle, which includes evaporation, condensation, and transpiration.

Storm cloud formation over field
Storm cloud formation over field

Standing water on the Earth evaporates because of solar heat and becomes water vapor. In most cases, water vapor in the air is invisible to us. Sometimes, however, we’re able to see air moisture in the form of mist or fog. At this level, water molecules are gaseous. Water molecules do not transition to liquids unless they accumulate in greater numbers on the surfaces of larger particles, such as dust or smoke. This is why dust and smoke particles are examples of cloud condensation nuclei, they allow for a sufficient build up of water molecules for cloud formation. Depending on the temperature of the cloud, precipitation may fall as a frozen solid, or liquid water, or a combination of both.

Liquid water also moves along the ground in rivers and run offs, which gets absorbed by plants and eventually passes back into the air as water vapor.

Temperature

Among other factors, a region’s temperature depends on its elevation and distance from the equator (the imaginary line that divides the Earth into northern hemisphere and southern hemisphere). Both of Earth’s poles (marked with red leading lines on the diagram below) are the furthest distances from the equator that one can go. These two extremes, each of the planet’s poles and the equator are the coldest and warmest places on Earth respectively.

The equatorial region is consistently hot year-round because the sun’s rays always impinge on it from overhead. In other words, sunlight strikes Earth most directly at the equator. At the poles, however, the sun’s ray strike Earth at more acute angles, therefore sunlight is spread over a larger distance, which lessens its heating effect. The greater the surface area energy is spread across, the lower the energy per unit area. This is why the sun’s heat is not fully felt at sunrise or sunset, but rather when the sun is most directly overhead.

Earth is vertically tilted 23.5 degrees relative to its plane of orbit around the sun. So for half the year, the northern hemisphere is tilted away from the sun, while the southern hemisphere is pointed toward the sun. For the remainder of the year, the reverse is true. The polar regions then, experience less direct sunlight due to their 6-month periods of “away-tilt”. The equator has no away tilt periods and instead is exposed to more direct sunlight year-round.

Rotating Earth diagram with marked poles

Temperature is also greatly influenced by the presence of vegetation, cloud cover, wind patterns, distance from the sea, and so on.

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