No matter if you're VFR or IFR, clouds affect how you operate. Knowing how clouds develop helps you predict conditions in real-time, and it helps you make smart flying decisions. So how do they form?

What Do Clouds Need To Form?

If you want to make clouds, you need the right recipe. There are two ingredients to form a cloud: moisture (water vapor), and condensation nuclei.

You know what water vapor is, so we won't spend much time on that. But condensation nuclei might be a new term. Condensation nuclei are what water molecules bond to in order to form cloud droplets. They are often particles like smoke, dust, or dirt.

Generally, condensation nuclei are 100x larger than water vapor molecules. This means they have more surface area (and surface tension), allowing enough water vapor to bond to form a droplet.

Once you have condensation nuclei and water vapor, you need to heat, and then cool, a parcel of air.

The Dew Point

Before we jump into the process of cloud formation, let's dive into how air and water vapor relate.

Warm air can hold more water than cold air, meaning if you cool an air parcel enough, the air becomes saturated (assuming a constant pressure). This is called the dew point, and it's the temperature where visible moisture can start to form.

The altitude where clouds form is highly dependent on the dewpoint, but more on that in a bit.

The Process Of Cloud Formation

Generally, cloud formation is started by the sun warming the earth's surface. This heating causes an air parcel to rise. As the air parcel rises, it cools, eventually to its dew point.

Cooling a parcel to its dew point doesn't guarantee a cloud formation. Otherwise, every time the air outside hit 100% humidity you'd have visible moisture (clouds). But, if you combine air that has been cooled to its dew point with condensation particles like smoke, dust, or dirt a cloud is more likely to form.

Lapse Rate To Find The Cloud Bases

Earth's atmosphere can be split into layers, with the lowest being the troposphere. The troposphere is where most of the earth's weather processes happen. On average as you go up in the atmosphere, you'll see a decrease of ~2* C (3.5F) per thousand feet (PHAK 4-3).

Knowing the lapse rate can be useful for more than your enroute performance planning. You can use the current temperature, dew point, and lapse rate to help determine what altitude clouds will start to form.

Here's the formula:

Remember, cloud bases in both your calculation and METAR will be in AGL, not MSL. Let's use a real-world example, from Denver International's METAR:

Subtract 28C (temperature) - 03C (dew point) = 25C. Now divide this by 2.5 to get 10, and then multiply by 1,000. This leaves you with 10,000 feet, which matches Denver's METAR report of the first level of clouds, which are scattered at 10,000' AGL.

What Clouds Tell You

Learning to interpret the clouds along your next flight can help you understand the current and future atmospheric conditions you'll be flying in.

It also makes your flying safer and more comfortable.

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