Why do Clouds Look Black?

Why do clouds sometimes look black?

Learn about scattering!

Equipment

 2 transparent dishes

 water

 skimmed milk

 black ink

 an overhead projectordark clouds

Method

1. Put some water in each dish and place them on the overhead projector. They should look the same.

2. Add a little milk to one dish, and a little ink to the other. If you look at the dishes directly, what do they look like? If you look at the projected images, what do they look like?

If you look at the dishes directly, the difference between them is obvious, but the projected images are still identical. Looking at the dishes directly, the milky dish appears white because a lot of the incident light is reflected (scattered). The inky dish appears dark because incident light is being absorbed (and re-radiated at a different wavelength).

The image on the screen is identical but the darkness is caused by different mechanisms. Light incident on the inky water is not transmitted to the screen mostly because of absorption, whereas light incident on the milky screen is not transmitted mostly because of scattering.

N.B., if you put only a very little milk or ink in the dishes, you might see a reddish-brown image, because not all wavelengths are scattered an equal amount by milk, and not all wavelengths are absorbed an equal amount by ink.

So how does this relate to the Atmosphere?

If you look at a cloud from above, you see it mainly by the sunlight rejected off it (scattered by it), whereas if you see it from below, a lot of the incident sunlight has been scattered, making the cloud seem darker than the surrounding sky.

If you could only see the cloud from below, you would not know whether the darkness was due to absorption or scattering.

Where can I find more information?

Craig Bohren’s book ‘clouds in a glass of beer’ has some more in chapter 11.

More experiments and demonstrations

Clouds in a Glass

Make a cloud in a glass

Clouds in the real world form in warm, moist air. You can make something similar happen using things you can find around your home.

What you will need:

A clear glass or similar see-through container
Warm water
Ice
Metal or foil dish

What to do:

Place the ice into the metal dish.
Pour a small amount of warm water into the bottom of the glass.
Wait until the dish is really cold. Then place it on top of the glass.
Watch the inside carefully. You should see a ‘cloud’ form near the top of the glass.
This is the way clouds form in the real world. Warm, moist air like that in your glass, is cooled (your ice). When it is cooled it condenses into tiny water droplets, which appear as clouds.

You will needglass and ice

Web page reproduced with the kind permission of the Met Office

Alternative Cloud in a Bottle

Alternative Equipment

  • A large, clean, plastic, fizzy drink bottle. Make sure it isn’t cracked or damaged.
  • Warm water
  • Matches
  • Optional thermometer
  • Bike pump
  • Bike pump attachment, e.g. rubber bung with inner tube valve (you can buy a rubber bung from our shop)

Cloud in a bottle

Alternative Method

1. Pour a few cm of warm water into the bottle, and swill it round to wet as much of the surface as possible.

2. Firmly push the pump attachment into the neck of the bottle, and pump air into the bottle. If you have a thermometer, you can watch the temperature rise a couple
of degrees as the pressure increases. This encourages the water to evaporate.

3. Release the bottle suddenly. You may see a faint cloud appear. When you release the bottle, the pressure, and temperature in the bottle drop suddenly. However, you will not see many cloud droplets forming despite the fact that the air in the bottle is almost certainly saturated with water vapour (if you do, its probably because your bottle wasn’t very clean!).

4. Now open the bottle, and light a match (get an adult to help with this step). Hold the match near the mouth of the bottle, then blow it out, gently blowing some smoke into the bottle for a few seconds before quickly attaching the bike pump.

5. Now pump air into the bottle again and release it suddenly. You should see a much clearer cloud forming in the bottle when you release it, as the water vapour now has small particles (smoke, soot, ash), known as Cloud Condensation Nuclei, to condense onto.

N.B., you can repeat this experiment using ethanol (or surgical spirit, or alcohol based hand cleansing gels) rather than warm water. In this case, no smoke is required. Ethanol has a substantially greater vapour pressure than water, and such high levels of supersaturation can be achieved as to make nucleation possible without any CCN present.

Where can I find more information?

More information about Clouds

Cloud seeding to cool the Great Barrier Reef

Indonesia uses cloud seeding to reduce forest fires

More Experiments and Demonstrations

Cloud in a Bottle

Make your own Cloud in a Bottle

To see how clouds form

Equipment

  • A one litre plastic water bottle (with a lid)
  • Warm water
  • Matches
  • Optional thermometer (small enough to fit into the bottle)

Method

1. If you have a thermometer, put it in the bottle and close the lid tightly. Write down the temperature. See what happens to it as you squeeze the bottle. You should see the temperature increase as the bottle is squeezed and the air inside it is compressed (squashed).

2. Fill the bottle with a few drops of warm water, and put the lid on again. Squeeze the bottle many times and watch what happens.

3. As the warm water evaporates, the amount of water vapour in the air in the bottle increases. When you release the bottle, the pressure, and temperature in the bottle drops. BUT you will not see any clouds forming despite the fact that the air in the bottle is almost certainly saturated with water vapour.

4. Now open the bottle, and light a match (get an adult to help with this step). Hold the match near the mouth of the bottle, then blow it out, holding the smoking end of the match in the bottle for a few seconds before quickly closing the lid tightly.

5. Squeeze and release the sides of the bottle a few times. Now what happens when you squeeze the bottle hard for a few seconds and then suddenly release the bottle? You should see a cloud forming in the bottle when you release it, as the water vapour now has small particles (smoke, soot, ash), known as Cloud Condensation Nuclei, to condense on.

How does this relate to the atmosphere?

Clouds can only form when the air is saturated with water vapour and when there are condensation nuclei present. Cloud seeding experiments introduce extra Cloud Condensation Nuclei to the atmosphere, to influence the number and size of raindrops in a cloud.


Where can I find more information?

More information about Clouds

Cloud seeding to cool the Great Barrier Reef

Indonesia uses cloud seeding to reduce forest fires

Clouds in a Fizzy Drink

Clouds in a fizzy drink

Learn about how clouds form

Equipment

 A large glass of freshly poured fizzy drink

 Saltfizzy drink in a glass

Method

1. Although a fizzy drink is supersaturated with carbon dioxide all the gas does not escape when you open the can or bottle or pour the drink into a glass. If you look closely you can see bubbles can only form in places with imperfections on the glass or dirt.

2. Now add the salt to the drink.salt in a hand

3. Adding the salt increases the number of nucleation sites and allows more bubbles to form. This creates a cloud of carbon dioxide in the liquid drink.clouds in a fizzy drink

How does this relate to the atmosphere?clouds in the sky

Clouds can form where the temperature is low enough for water to condense at a rate faster than which it evaporates. In practise most raindrops or ice crystals form where there are small particles such as dust, soot or pollution to act as cloud condensation nuclei. These particles are usually approximately 0.0001mm in diameter. It is this process which allows pollutants to become incorporated into clouds and be washed out of the atmosphere in the rain.

Where can I find more information?

Find out more information about clouds

Extension: You can also put some sultanas in the drink. As they fall through the drink, bubbles form on the sultanas, changing the balance of forces on the sultana. Eventually, the sultana rises. As it rises, the pressure falls and the bubbles grow. When the sultana reaches the surface, the bubbles pop and the whole cycle starts again.

More experiments and demonstrations

Supercooled Water

Supercooled Water

Learn about ice and clouds!

Equipment

◊ Salt

◊ Water (bottled or distilled is best)

◊ A tub (such as a plant holder)

◊ A very clean, thin disposable beaker

Method

1. Mix enough water and salt to half fill the tub you are using. Mix in as much salt as the water will take. This solution is now called brine.

2. Pour the solution into the tub and put in the freezer for a few hours.

3. After a few hours take the ice out of the freezer and break it up a bit. The slush will be at about -12°C to -15°C!

4. Put the disposable beaker containing 2cm of bottled water taken from the fridge into the icy slush.

5. Leave for 20 minutes.

6. Take out the beaker and pour the supercooled water into an empty beaker – watch what happens….It freezes into slush on impact!!Supercooled water

So why does it happen?

Ice can not form in supercooled water unless there are particles such as dust, ash or soot for it to form on. These particles are known as freezing nuclei. In the glass the supercooled water was poured into there will be some particles, such as dust, present which allow the ice particles to form.

How does this affect the atmosphere?

In a similar way to in this experiment, cloud droplets do not form even if the water is supercooled, if there are no condensation nuclei present. Quite simply, water needs something solid around which it can change from a gas (vapour) to a liquid form. Condensation nuclei for clouds usually takes the form of dust, ash, pollution particles or soot. A cloud condensation nucleus is usually 0.0001 mm in size.

In December 2010, an ice storm closed Moscow’s airports. Rain falling from a warm layer of air through a much colder layer became supercooled. As soon as it hit the ground, it froze, turning instantly into clear, ‘glaze’ ice.

More detail on how to carry out the experiment, the science behind it and a video 

More experiments and demonstrations

Cloud in a Glass

Make a cloud in a glass

Clouds in the real world form in warm, moist air. You can make something similar happen using things you can find around your home.

What you will need:

glass, ice and foil dish

 

 

 

 

A clear glass or similar see-through container
Warm water
Ice
Metal or foil dish

What to do:

  1. Place the ice into the metal dish.
  2. Pour a small amount of warm water into the bottom of the glass.
  3. Wait until the dish is really cold. Then place it on top of the glass.
  4. Watch the inside carefully. You should see a ‘cloud’ form near the top of the glass.

This is the way clouds form in the real world. Warm, moist air like that in your glass, is cooled (your ice). When it is cooled it condenses into tiny water droplets, which appear as clouds.

Web page reproduced with the kind permission of the Met Office

More experiments

More simple experiments and demonstrations from around the web: