calculating cloud bases chart
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How to Calculate Cloud Bases

ByBriana Ear
Aviation Weather

How to Calculate Cloud Bases: The Formula Every Pilot Should Know

Weather and clouds play a big role in aviation, and one of the most useful skills a pilot can learn is how to quickly calculate cloud bases. Here is the formula, where its numbers come from, and the one change Transport Canada makes on the PPAER.

Jump to the Formula View PPL Ground School

By Brianna, Canadian Flight Trainers · September 2026

2.5°Cper 1,000 ft: the rate the temperature and dew point close on each other in rising air
3°Cper 1,000 ft: the simplified lapse rate Transport Canada uses on the PPAER
AGLthe formula gives height above ground level, so add field elevation for an altitude ASL
Key takeaways:
  • Cloud base (AGL) = (temperature minus dew point) ÷ 2.5 × 1,000. Both temperatures in degrees Celsius.
  • The 2.5 is the dry adiabatic lapse rate (3°C/1,000 ft) minus the rate the dew point falls in rising air (0.5°C/1,000 ft).
  • On a Transport Canada exam, use 3°C/1,000 ft unless the question tells you otherwise.
Why Bother?

The Official Products Already Tell You the Cloud Base. So Why Learn This?

“But why do we have to know this if it already says what the cloud bases are on the METARs, TAFs and GFAs?” you may ask.

Good question, and you are right. Official weather products such as the ones listed above should always be used for flight planning purposes, but understanding how to calculate an estimated cloud base can help you better visualize the atmosphere. This calculation can help predict ceilings, cloud development and overall flying conditions.

The Basics

What Is a Cloud Base?

A cloud base is the lowest visible part of the cloud. It is where rising air cools to its dew point temperature and becomes saturated. This causes water vapour to condense and form visible cloud droplets. The altitude at which clouds form depends on two big factors: the current air temperature in the area, and the current dew point.

Small spread

The closer the temperature and dew point are together, the lower the cloud base is likely to be.

Large spread

The bigger the difference between the two numbers, the higher the cloud base is likely to be.

The Formula

Estimating the Cloud Base

What you have all been looking for. The formula used for estimating cloud bases is:

Cloud Base (AGL) = (Temperature − Dew Point) ÷ 2.5 × 1,000

Temperature and dew point in degrees Celsius. Result in feet above ground level.

Where:

  • Temperature is in degrees Celsius
  • Dew point is in degrees Celsius
  • The cloud base is above ground level (AGL)

Remember: if you want the cloud base as an altitude above sea level, you will have to add the resulting value to the field elevation. See our glossary entry on indicated altitude for how altitude references differ.

Behind the Formula

Where Do These Numbers Come From?

Although it’s good to have the formula memorized, it’s even better to know where we get these numbers. Mainly the “2.5.” And no, it’s not just a number that somebody found to work for this calculation.

As unsaturated air rises through the atmosphere, it cools at the Dry Adiabatic Lapse Rate, or the DALR, of 3°C per 1,000 ft. You can read more about how lapse rates affect the air around you in our glossary entry on atmospheric stability.

It’s important to consider that the dew point also decreases as the air rises, at 0.5°C per 1,000 ft.

To determine how many thousands of feet the air must rise before saturation occurs, we take the difference: 3°C − 0.5°C. This means the temperature and dew point move closer together at a rate of 2.5°C per 1,000 ft.

3°C Dry Adiabatic Lapse Rate

How fast the rising air cools, per 1,000 ft.

0.5°C Dew point lapse rate

How fast the dew point of that rising air falls, per 1,000 ft.

3°C − 0.5°C = 2.5°C per 1,000 ft. That is the whole story behind the number.

Why the spread closes at 2.5°C for every 1,000 ft of lift
Worked Example

Example Calculation

Given a temperature of 22°C and a dew point of 9°C:

1

Calculate the temperature and dew point spread

22 − 9 = 13°C

2

Divide by 2.5

13 ÷ 2.5 = 5.2

3

Multiply by 1,000

5.2 × 1,000 = 5,200

Therefore the estimated cloud base for this example is 5,200 feet AGL.

Important Note

What Transport Canada Expects on the PPAER

Knowing how to estimate cloud bases allows pilots to better understand the weather and anticipate conditions before arriving at a destination, especially considering some remote aerodromes may not have their own aviation weather forecasts. This is a great skill to have when anticipating weather. Not to mention, it will help you when writing your PPAER.

If you’re studying for the Transport Canada PPAER exam, a critical thing to remember is that TC likes to use the simplified version of this formula to calculate cloud base. Meaning instead of using 2.5°C per 1,000 ft, they use 3°C per 1,000 ft. The use of the formula stays the same.

Cloud Base (AGL) = (Temperature − Dew Point) ÷ 3 × 1,000

The Transport Canada exam version. Same formula, simplified lapse rate.

For example, if the temperature is 22°C and the dew point is 9°C, like in the previous example:

  • The temperature and dew point spread is still 13°C
  • The spread gets divided by the lapse rate (3°C): 13 ÷ 3 = 4.33
  • 4.33 × 1,000 = 4,330 feet

Therefore the cloud base in this scenario is approximately 4,300 feet AGL.

Exam tip: although the dew point lapse rate should be taken into consideration when calculating cloud base in order to give an accurate prediction, be sure to use 3°C per 1,000 ft when answering PPAER exam questions unless other instructions are provided.

The next time you read a METAR, take some time to use this formula to calculate the cloud base. This can help paint a clearer picture of what you might be flying yourself into.

FAQ

Cloud Base Questions

What is the formula for calculating cloud bases in Canada?

The common formula is: Cloud Base (AGL) = (Temperature − Dew Point) ÷ 2.5 × 1,000. Temperature and dew point are both measured in degrees Celsius, and the result gives an estimated cloud base in feet above ground level.

What is the formula for calculating cloud bases when writing a Transport Canada exam?

The formula to use when writing TC exams is: Cloud Base (AGL) = (Temperature − Dew Point) ÷ 3 × 1,000. Instead of using 2.5°C per 1,000 ft, Transport Canada uses 3°C per 1,000 ft. The use of the formula stays the same.

Where does the “2.5” in the formula come from?

As unsaturated air rises through the atmosphere, it cools at the Dry Adiabatic Lapse Rate (DALR) of 3°C per 1,000 ft. The dew point of that rising air also decreases, at 0.5°C per 1,000 ft. The difference, 3.0 − 0.5, gives 2.5°C per 1,000 ft.

Why is this important to know?

Knowing how to estimate cloud bases helps you predict the kind of weather you will encounter during your flight. This increases situational awareness and can help decision making. Not to mention, you may be asked to calculate cloud bases in your written exams.

Keep Reading

More Aviation Weather

GFA: how to read a GFA like a professional → Where to view live GFAs now that AWWS is retired → Glossary: ceiling → Glossary: standard atmosphere → CX-3 flight computer for the TC exam →

Meteorology Is a Big Part of the PPAER

Cloud base calculations are one small piece of the weather knowledge Transport Canada expects on the exam. Our Private Pilot Ground School covers meteorology, and every other PPAER subject, with instructor support when the material stops making sense.

View PPL Ground School

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