Our weather consists of many powerful forces – strong winds and hurricanes, drenching rain, blanketing snow or simply the stifling heat of summer. The power of a single hurricane can be more than 200 times the capacity of the world’s energy capacity. It is clear that the weather is driven by tremendous energy. But where does this energy come from? How does this energy power the weather we experience every day?
Forms of energy
Before we look at the way that energy powers the weather, we must remind ourselves about energy itself and the forms energy can take. The energy of an object is the capacity of that object to do work. ‘Work’ can be thought of as different forms of action on an object such as moving the object or heating an object. So for example, when we throw a ball we transfer the energy of motion (‘kinetic energy’) from our body. Our body itself is also transferring energy in the process. Our arm is also accelerated with kinetic energy and that kinetic energy is itself from chemical energy of the body.
Energy is never created or destroyed. It merely changes form from form to another. In the above example, chemical energy was changed to kinetic energy. When the ball is thrown through the air we observe that it slows down. This slowing is due to friction either with the air or the ground so another energy transform is taking place – from kinetic energy to heat (or thermal) energy. For the ball is heating is imperceptible, but we see it much more vividly when our car brakes heat up when descending a big hill. Another example of energy transfer occurs when lighting our rooms. The electrical energy that powers the light bulb is converted to light energy, in the form of radiant energy. Heat energy is also produced (in large amounts for older style filament bulbs).
There are a number of different forms of energy – kinetic energy (the energy of motion), heat or thermal energy (which is the kinetic energy of the molecules within a substance) and multiple types of potential energy. Potential energy represents the ability for an object to perform work. Potential energy exists in multiple forms including gravitational potential energy (the energy that will be available when an object drops), chemical potential energy (the energy of chemical reactions), electromagnetic potential energy (the energy of electromagnetic radiation including light and electricity) and many more.
Let’s now consider the energy in transfers within our atmosphere.
Energy flow in the atmosphere
So what kind of energy do we see in the weather around us? We see the energy of movement, kinetic energy, when the wind blows, and we see electrical potential energy being converted to heat and kinetic energy in a lightning strike. One of the simplest forms of energy that we feel on a summer’s day is the radiant energy of the sun in the form of visible light (and other electromagnetic radiation). When the bright summer sun shines on our faces, we immediately feel the radiant energy being transferred to heat energy within our own bodies. If we touch a parking lot, the heat of the asphalt immediately illustrates that the energy of the sun is capable of tremendous capacity to deliver heat to our atmosphere.
It is the sun that powers the weather we experience. Other factors such as energy from geothermal sources are virtually zero compared to the power of the sun.
How the sun warms the Earth
As we have seen, the sun is the power source of the weather. In its simplest form, we can consider that the earth is a ball in space being heated by the sun. The energy of sunlight, solar energy , will fall on the earth, be absorbed by the planet and increase the kinetic energy of the molecules. Increased thermal energy in the molecules increases the thermal energy and so the planet warms.
Anything that is greater than absolute zero temperature (the temperature when molecules stop moving), will itself radiate energy. So as the earth gets hotter, it radiates more heat. When the amount of energy that is supplied to the earth from the sun equals the amount that the earth radiates out then it will stop heating and reach a ‘temperature equilibrium’. The equilibrium temperature is the mean temperature of the earth – the temperature where incoming solar radiation is balanced by the outgoing emitted radiation. We can see this effect in a pan of water on a very low flame in a stove. Initially cold water in the pan absorbs more energy from the stove than the energy emitted from the water in the pan. When the temperature of the water warms to the point where the emitted energy equals that provided by the pan flame, the water warms no more. Providing the pan is left on the stove, the water stays the same temperature. If the stove is turned off, the pan of water will cool. If the stove temperature increases, the pan of water will warm up again. At some point the stove may provide enough energy to the molecules in the water to break away from the surrounding liquid molecules and it forms gas bubbles. This is the boiling point and is related to the concept of evaporation, a topic for a later discussion.
Why is the Earth so warm?
We can see therefore that the solar energy of the sun is heating the earth in space. The Earth heats us to an equilibrium temperature – the temperature where the sun’s energy falling on the earth equals that of the energy being emitted from the earth. If we apply the physics simplification of a ‘blackbody’ then we can calculate the expected temperature of the earth because we know the incoming solar energy from the sun. We can use Stefan–Boltzmann law to calculate that the expected temperature of the Earth should be -23C (-9.4F). However we can measure the temperature near the surface to be on average 15C (59F). How is this possible?
The answer lies within the atmosphere. Radiation arriving from the sun with a short wavelength during the day passes through the atmosphere without interruption and so warms the ground. The ground is warmed and emits radiation itself but much of this radiation is a longer wavelength known as infrared radiation than the incoming solar radiation. This outgoing infrared radiation does not all pass through the atmosphere in the same way the incoming solar radiation. Some of the longer wavelengths are suitable for being absorbed by the gases found in the atmosphere such as water vapor, carbon dioxide and and methane. By absorbing the outgoing radiation, these gases are heated and so the atmosphere is heated. This effect raises the average temperature of the atmosphere above the expected -23C to 15C. This effect is known as the “Greenhouse Effect” where naturally occuring gases within the atmosphere cause act like a blanket, keeping the energy of the sun trapped in the atmosphere to keep us warm.
Because it is a warm body, the earth emits energy whether or not the sun is shining, both day and night. Whilst clouds may keep the daytime temperatures on the surface lower by reflecting much of the incoming solar radiation, clouds at night do the opposite. Clouds at night reflect the outgoing radiation emitted from the Earth back to the ground. Therefore clouds during the day lower the temperature by blocking incoming solar energy but clouds at night will keeps the surface warmer by preventing the outgoing radiation escaping.
Impact of changes in the atmospheric gases
Much of the greenhouse effect warming we have discussed comes from a fairly small proportion of the atmospheric gas composition. Therefore small changes in the composition have a disproportionately high impact in changing the amount of energy that escapes the atmosphere. Most scientists now believe that an increase in carbon dioxide, water vapor and other gases, may well be causing a slight imbalance in the incoming vs outgoing energy. This imbalance will cause an increase the overall temperature of the atmosphere as slightly more energy will be absorbed by the atmosphere so the temperature needs to rise to restore the temperature equilibrium. This effect is known is man made greenhouse effect. The changes to the weather patterns that would be caused by such a warming are commonly known as climate change. It’s worth noting that the Greenhouse Effect is not a solely human caused behavior. The Greenhouse effect is a requirement for life as we know it to exist as it keeps water in liquid form on the Earth rather than the earth being an icy world.
How does the energy power the weather?
Now we know that the sun is the energy source for the weather, how does this energy cause the weather? Our experience with the weather suggests that energy flow is much more complex than a simple ball being heated in space – we see vast differences in temperature throughout the world and even day by day for the same place. This is caused by a number of factors. The first factor is the uneven heating of the atmosphere. A much larger amount of energy arrives at the equator than at the poles. When a gas or liquid is heated unevenly, the gas temperature will attempt to even out the temperature. In the atmosphere, the primary means of moving the energy from the hotter to colder areas is through the process of convection. Convection is that act of warmer gas molecules with more energy taking up more space, causing a reduction in weight and therefore rising. As the warmer air rises, colder air can fill in behind.
This movement of air is the wind we experience every day. On a global scale the imbalance between heating at the equator and the poles cause large scale winds. We also see more localized imbalances. For example land warms more quickly than the sea. Therefore on a sunny, summer’s day the air above the land will warm more quickly than the air over the sea. The air over the land rises and the cooler air over the sea rushes in. We feel a sea breeze as the air moves from the sea to the land. At night, the land cools faster and so the effect is reversed with the air flowing from the land.
In a simple version of the Earth, we would therefore expect huge convection currents to balance the warm air at the equator with the cool air at the poles. The stronger sun at the equator warms the air at the equator relative to the polar air causing the equatorial air to rise, the cold air at the poles flows south to take up the space left by the warmed air and therefore a large convection current would be created. However it is is clear that this is not the full explanation as it would create a continual northerly wind (in the northern hemisphere) as the cooler polar air flowed south towards the equator. What else could be causing the similar convection currents caused by sunlight warming the atmosphere to be different? In a future edition of Weather 101, we will discover that the rotation of the earth and the interaction of the moving air with the oceans and lands disrupts the simple movement of warm equatorial air to the colder polar regions.
The sun’s energy another part of the weather. The solar energy shining on the world’s oceans, lakes rivers and wet land causes the molecules within the water to break free and enter the atmosphere through evaporation. This water vapor will then be available to form clouds and if those clouds grow, they will in turn form precipitation such as rain, snow and hail. Water vapor in the air also changes the properties of the atmosphere itself which leads to other weather behaviors we will investigate in a future edition of Weather 101. Finally, as we saw earlier, water vapor is a power greenhouse gas which keeps the planet warm (and for this reason there is a possible positive feedback loop where a warmer planet produces more water vapor and therefore more greenhouse effect heating).
What did we learn?
We have learned that the sun is the primary driver for the earth’s weather. We have also found that if it weren’t for the greenhouse effect, the Earth would be a cold, frozen planet. As the energy source for the weather, the sun drives the wind, the water cycle and everything that happens within our atmosphere. However finding the energy source is just the beginning. The gases within the atmosphere use this energy in a series of complex interactions to form the weather we see around us. We will start investigating those interactions in the next edition in How does the wind blow?.
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