# How, at 93 million miles away, does the sun feel so warm, yet when a simple cloud passes over it the warmth is incredibly dampened?

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How, at 93 million miles away, does the sun feel so warm, yet when a simple cloud passes over it the warmth is incredibly dampened?

In: Physics

So to start of with, you don’t actually sense temperature. What you feel is heat flux, which is the amount of energy that flows threw an area in a given time. If you walk into a room that’s been kept at 70°F overnight, the air will be pleasant, but any metal will feel cool despite being the same temperature. This is because metal is a much better at moving thermal energy (heat) than air.

So there are 4 main ways to move thermal energy. Mass loss/evaporative cooling by sweating. Conduction is when heat flows from one object to the other by the objects touching (holding an ice cube in your hand). Covection is when heat flows from an object to a moving fluid (or vise versa). And radiation is when the object throws away some of that heat energy as a photon (light).

So when your outside on a sunny day, there all 3 modes of heat transfer are in effect. Conduction, convection are most likely cooling you off due to being warmer than the environment, while radiation is heating you up. Your body likes to stay at a constant temperature, so it sweats to the point where you maintain it, thus you have no net heat flux.

When a cloud moves in front of the sun, the amount of heat added by radiation drops drastically. The air temperature didn’t change, so the heat transfer by conduction stays the same. The wind speed (along with temperature) didn’t change, so the heat transfer by convection stays the same. Your body takes some time to adjust, so for short time scales, heat tranfer by mass loss/evaporative cooling stays the same.

What this means is suddenly your operating at a net loss of heat, and that’s what you feel.

Light transfers energy. When light hits your skin it transfers energy to your skin. The amount of energy transferred determines how hot it feels as your skin heats up. Light can be scattered or focused to cause the amount of energy transferred to be more or less.

If you are in space, nothing is between you and the the sun, so you would be exposed to the entire amount of energy transferred by the light. That’s why astronauts have to wear visors. To not get an instant sunburn.

When you are outside on a bright sunny day, the upper atmosphere is scattering and absorbing most of the light and its energy. That’s why the entire sky is lit up, and the temperature in the sun and shade aren’t that different. Energy is more evenly distributed.

On a cloudy day, there is significantly more in the way of the light, and so the light is scattered even more and the energy that gets to your skin is even less.

At the same time, a magnifying glass can focus light into a smaller area, causing the energy that hits a particular part of your skin to be even higher than the surrounding area which is why it heats up so fast.

Soooo. In space nothing scatters the light. On earth, the atmosphere scatters the light. On a cloudy day, the clouds scatter it even more. And a magnifying glass can refocus that light into a smaller area.

I wonder how a magnifying glass in space would work. Bet it would make a pretty hot focus.

It’s not super easy to explain all of it like your 5. So we will ignore the fact that the atmosphere bounces light around and traps heat. And we will ignore how heat actually works for this explanation.

Let’s go; light is energy. It’s also made up of loads of colour. Light hits your skin, and depending on the tone of your skin, a lot of thos colours get reflected. The ones that don’t, are absorbed, and the light turns from light energy, into kinetic energy. This means that the stuff that makes you up, starts to vibrate very quickly causing ‘warmth’. Genuinely serious note, people with darker skin get hotter than people with light skin. People who have tattoos getter hotter on the tattoos than the rest of their skin. This is because more light is absorbed than is reflected.

When a cloud comes over, it does two things. Scatters light (because its made of water vapour) and reflects light (because of its colour) this means that less light hits you, which means the atoms in your body don’t vibrate as much. Less ‘warmth’.

I have a follow-up question: Why, when approaching the sun, like being on top of a mountain, is it usually much cooler compared to sealevel? 😄

Because thick clouds are misleading, they are very dense and the light that comes from the sun is traveling through space and has nothing to stop it during the eight minutes that takes to get here, clouds however can be like a wall.

For all those 93 million miles, the sunlight has had no stuff to interact with. Clouds are very *very* dense compared to space. A lot of the sunlight interacts with the clouds before it can reach you on the surface, leaving less sunlight to warm you up.

So when you feel the sun’s warmth, you’re not feeling heat coming from the sun. You’re instead feeling heat created *on your skin* by the sun’s light.

Light carries energy. Things with colour, like your skin, absorb light. When they do, the atoms that make them up get ‘excited’. Depending on the atom, and what state its in, a few things can happen. If the atom is part of a molecule that energy can go to work breaking it out of the molecule. If the atom or molecule is on the surface of a solid or liquid, the energy can go towards flinging it off, into the air, turning into a gas.

If there’s not enough energy to do either of those things, then the atom will just release the energy to its surroundings. Most of the time, most of the energy is released as heat. This is what you feel when the sun feels warm. Sometimes the energy can be released as light. This is how glow-in-the-dark things work.

A cloud doesn’t block all the light from the sun, but it does ~~absorb~~ *scatter* a lot of it. *Think of water droplets in a cloud like a million tiny disco balls*. The light that gets through is either too sparce to be noticeable, or high-enough energy that it causes damage instead (ie. UV light).