What if there were no clouds?

Look up! Some days the sky is full of fluffy clouds. Some days it is clear and blue.

What if clouds never came back? Would we miss them?

Clouds can seem like the least important part of the sky. They block the sunshine at the beach and bring rain on the day of a picnic.

But clouds do some big jobs for our planet. What would happen if there were no clouds anywhere, ever? In this thought experiment you can follow three paths: what happens to temperature, what happens to rain, and what happens to the colors of the sky.

Clouds are a central part of Earth's energy budget and water cycle. NASA notes that low, thick clouds mainly reflect sunlight and cool the surface, while high, thin clouds trap outgoing infrared and warm it.

This thought experiment removes clouds entirely and asks what might follow, while keeping clear which parts are established science and which are reasoned ideas no model here can quantify.

Clouds are made of tiny drops of water. The drops are so small that they float.

Clouds give shade on hot days. And rain falls from clouds.

Clouds are made of tiny water droplets or ice crystals. USGS explains that cloud droplets are far too small to fall; it takes millions of them joining together to make a single raindrop. Rain, snow and hail all come from clouds.

Clouds also change the temperature. Thick clouds bounce sunlight back to space, keeping the ground cooler by day. At night, clouds send some of the ground's heat back down, so cloudy nights are usually milder than clear ones.

USGS describes precipitation as water released from clouds as rain, freezing rain, sleet, snow or hail, and the main way atmospheric water returns to the surface. Cloud droplets form on dust, salt or smoke particles; millions are needed for one raindrop.

On radiation, NASA explains that a cloud usually has a higher albedo than the surface beneath, so it reflects more sunlight, while its cold top reduces infrared emission to space. Penn State's meteorology course notes that a clear, calm night is colder than a cloudy one, because clouds are sources of downwelling infrared.

Let's pretend all the clouds are gone. The sky is always clear.

Days might get hotter in the sunshine. Nights might get colder. And where would rain come from?

Now imagine every cloud gone. With nothing to bounce sunlight away, days in the sun might get hotter. With nothing sending heat back down, nights might get colder. The Met Office says that is already what happens in deserts, where clear skies lead to big temperature swings.

The biggest change might be rain. Rain falls from clouds, so with no clouds there might be no rain or snow at all. These are ideas: no one can switch clouds off, so we reason from what clouds do today.

Remove clouds and two opposing effects vanish: their shading (cloud albedo forcing) and their downward infrared (cloud greenhouse forcing). Since NASA reports that clouds overall leave the surface cooler than a cloudless atmosphere would, a cloud-free Earth might be warmer on average, with larger day–night swings like those of clear-sky deserts.

The water cycle might change most: without condensation into clouds there might be no precipitation. These consequences are ideas; a real atmosphere would respond in ways a thought experiment cannot predict.

Clouds are like shade umbrellas in the sky. They also help keep nights from getting too cold.

And clouds carry the water that falls as rain.

Clouds do two opposite things to temperature. NASA explains that thick, low clouds reflect sunlight and cool the ground, while high, thin clouds trap some of Earth's heat and warm it. Added up over the whole planet, clouds cool Earth's surface overall.

People often say clouds are like a blanket at night, but Penn State meteorologists say that isn't quite right. Clouds don't hold warm air in like a blanket; instead they send heat energy back down to the ground, which keeps it from cooling as much.

NASA: low stratocumulus clouds have a large albedo forcing and cool; high cirrus are transparent to sunlight but absorb outgoing infrared and warm; deep convective clouds are roughly neutral. Globally, cooling predominates.

Penn State's METEO 3 course argues that 'clouds act like blankets' is a misconception: blankets suppress convection, whereas clouds act as sources of downwelling infrared radiation that offset the ground's nighttime radiation deficit. NSIDC adds that which effect wins depends on cloud type and thickness, sunlight and the surface below.

In deserts the sky is often clear. Days are hot, and nights can be very cold.

Clouds are a very important part of our weather!

Some places on Earth are already almost cloud-free for most of the year: deserts. The Met Office explains that sinking air near the subtropics brings almost cloudless skies, giving rise to deserts. These places are very dry, and their clear skies bring big temperature swings: some of the hottest days on Earth, yet many deserts drop below freezing at night.

So deserts give us a small glimpse of a cloudless world. Elsewhere, clouds bring the rain and snow that return water from the air to the ground.

The Met Office links subtropical deserts to descending air and near-permanent high pressure that produce almost cloud-free skies; those skies drive large diurnal temperature swings, so many hot deserts fall below freezing at night.

Clear-sky deserts are a partial analog for a cloudless planet, not a model of one: deserts are also dry, so they lack the water vapor that would still absorb infrared in a moist cloud-free atmosphere. UCAR notes that 99 percent of atmospheric water vapor, and most clouds, are in the troposphere.