Why Earth’s surface is so much warmer than the Moon’s – Part 1

A lot of the climate debate is highly technical, so in order to help those less tuned in to climate science jargon and the concepts of thermodynamics and radiative theory, I’ve written this intro piece on the difference in temperature on the surface of the Moon and Earth.

Why Earth’s surface is so much warmer than the Moon’s
Roger Tattersall Dec 2012

The Earth and the Moon are at the same average distance from the Sun, but their average surface temperatures are different by around 91C. Very little heat is escaping from their interiors, around 10,000 times less than the heat from a 1kw electric bar fire puts into a square metre around itself so we can discount that as a significant cause. At any one time, around 60% of the Earth’ surface is shaded from the Sun by clouds, reflecting around 20% of the Sun’s energy straight back into space. So why is Earth’s surface warmer by 91C, given that, on average over the year, only around 240 watts per square metre get past the clouds, whereas the Moon is receiving around 316W/m^2, about a third of a bar fire’s worth for every square metre rather than about a quarter?

Several factors are involved. The first thing to do to help find out what they are is to put the question the other way round: Why is the Moon’s surface so much colder than the Earth’s?

The Moon’s surface near the equator on the day side gets very hot, because there are no clouds to shade it, and no atmosphere to conduct and convect heat away to cooler areas. There is no ocean to cool the surface by evaporation, or take heat away from the equatorial area to cooler areas with currents. The only way the Moon’s surface can lose heat back to space is by radiating it from the surface. This happens quickly. As a point on the Moon’s surface near the equator moves round from the day side to the night side, it’s temperature drops rapidly from nearly as hot as it ever gets to nearly as cold as it ever gets in the space of a couple of days. This is because the dust and rocks of its surface can’t hold heat for long. The surface is also a poor conductor of heat, so heat doesn’t travel through the surface from the hot day side to the cold night side. Big temperature swings mean a lower average temperature, because the relationship between temperature and radiation is not linear. Something twice as hot as something else radiates more than twice as much heat.

The Moon’s average surface temperature, averaged over a long period is stable at around -76C. If it was able to spread the heat it receives from the Sun out evenly over its surface, it would be just below freezing at -0.5C.

This discussion tells us some of the reasons why the Earth’s surface is warmer than the Moon’s. It has oceans which absorb and retain heat much better, this means the night side stays warmer as the oceans release heat they gained during the day slower than the Moon’s surface. The oceans also shift heat from the equator towards the poles making the difference between the polar and equatorial temperature much less than on the Moon. Just because Earth has oceans, we’re much nearer to spreading the incoming solar energy evenly which as we saw with the Moon, would make it’s surface 75.5C warmer. However, Earth’s clouds are reflecting around a fifth of the sunlight back into space, so even if the energy getting past the clouds was spread evenly, Earth’s surface wouldn’t get to -0.5C but only to around -18C. So we need to look for some more factors.

The other big difference, apart from oceans heat capacity and mobility, between the Moon and Earth is that the Earth has an atmosphere. Before we start considering the more complex factors arising from its composition, lets first consider its mass. All mass is affected by Earth’s gravity, which is pulling the atmosphere downwards. Because nearly all the mass of the atmosphere is between the surface and about 20km altitude, we don’t need to consider the weakening of gravity as we get further from Earth’s core. But we do need to consider the effect of the weight of the upper parts of the atmosphere on the lower parts. As we descend from the top of the atmosphere towards the surface, there is more of the atmosphere above pressing down on us. This is why air pressure increases as we descend and is much higher near the surface than it is high up.  The increase in pressure causes an increase in density too. That means the molecules that make up the atmosphere get pushed closer together as we get nearer the surface. That’s why you have to breathe harder on top of a high mountain – there are less molecules of oxygen per lungful than at sea level. But more important than the density for our discussion of temperature is heat capacity.

Down at sea level, where the pressure is high, and the molecules are more densely packed together, more of the energy from the incoming sunlight is absorbed by each litre or pint of air. This is because empty gaps between molecules don’t absorb any energy, the sunlight just goes straight through. But if the sunlight hits a molecule of a type which can absorb energy, the molecule gets hotter. About 16% of the incoming energy from the Sun is absorbed in the atmosphere and most of that absorption will take place nearer the surface. That’s where the energy is more likely to be absorbed by molecules in the air, because that’s where they are packed more closely together. Gravity acting on the mass of the atmosphere to create a pressure gradient leading to higher density and heat capacity near the surface is another reason Earth’s near surface temperature is warmer than the Moon’s. Air also spreads heat around from hotter places to cooler places, warming them up, reducing the differential in temperature between equator and poles and damping down temperature swings by helping to keep the night side warmer, along with the ocean.

The mass of the atmosphere being acted on by gravity also helps keep the ocean warmer. This is because the higher air pressure near the surface limits the rate the ocean can evaporate at. Evaporation cools the ocean surface, just like putting a wet cloth over a bottle of milk in the sunshine keeps the milk cooler. Extra energy called the latent heat of evaporation is used up when water evaporates. Less energy left behind when the evaporated water heads upwards means a cooler ocean. So by limiting the rate of evaporation, the weight of the atmosphere makes the ocean less able to lose energy back through the atmosphere into space. This causes it to warm up because sunlight carries on putting enrgy into it. By warming up, it regains its ability to evaporate, and lose energy as fast as it is arriving from the Sun, so it can stay at a stable temperature. This is another reason the surface is warm.

Time to consider another factor: Clouds. This might seem strange at first, because clouds reflect around 20% of the incoming sunlight straight back into space, preventing it from reaching the surface. But as well as this cooling effect, there are warming effects which offset the cooling effect. Clouds absorb some of the Suns energy, and also heat from warm air rising from the surface. The exact amount isn’t known, because there are problems with the physics, but we do know that half of this absorbed energy gets radiated downwards towards the surface. This is why cloudy nights feel warmer than clear nights.

Now it’s time to consider the composition of the atmosphere rather than its mass and find out why it makes a difference to surface temperature. Here’s quick breakdown of the atmospheric composition:

• 77% Nitrogen
• 20% Oxygen
• 1% Argon
• 1-4% Water vapour and water
• 0.04% Carbon dioxide

Around 99% of the atmosphere is composed of two diatomic gases, Nitrogen and Oxygen. Nearly all the rest is the inert gas Argon. There’s some water vapour, and traces of Carbon Dioxide, Methane and other sundry gases. The Water vapour and Carbon dioxide are important, because they radiate energy much more readily than Nitrogen and Oxygen. Radiation is the only way energy can get back into space, so without them in the upper atmosphere, it would get very hot as the solar energy would have to circulate from the dayside surface through the atmosphere, back to the surface on the nightside and be radiated to space direct from the ground. In the lower atmosphere, they directly absorb some of the incoming solar energy and some of the departing long wave radiation leaving the ground and thermalise Nitrogen and Oxygen molecules in collisions. So a bit like clouds, the radiatively active gases are both part of the cooling and part of the warming factors.

Finally a small but non-zero factor is that the Moon spins only once per month, whereas the Earth spins once a day. This means the surface gets longer to cool on the Moon and bigger the swings in temperature between day and night the lower the average temperature due to the power law relating temperature to radiation.

We’ve found seven reasons why the Earth’s surface is warmer than the Moon’s.

• The oceans spread heat polewards
• The oceans retain heat overnight
• The atmosphere has a higher heat capacity near the surface due to gravity acting on its mass
• The atmosphere’s weight due to gravity restricts the oceans ability to evaporate
• Clouds reflect heat out to space but also re-radiate absorbed heat partly downwards
• Water vapour and CO2 radiate heat to space but also re-radiate absorbed heat partly downwards
• Earth spins faster than the Moon, giving less time to cool overnight, so swings in temperature are reduced

In further posts, we’ll try to quantify how much each of these factors contributes to the 91C difference between the Moon’s surface temperature and the Earth’s. There will be some surprises.