Estimating cumuliform cloud bases from a 33°C air temperature and a 15°C dew point to about 7,200 ft AGL.

At what altitude above the surface would you expect the base of cumuliform clouds if the surface air temperature is 33 C and the dewpoint is 15 C?

• A. 5,200 ft AGL
• B. 7,200 ft AGL
• C. 9,200 ft AGL
• D. 11,200 ft AGL

Answer: (B)

To determine the altitude at which the base of cumuliform clouds generally forms, a common method involves calculating the height where the air temperature will cool to the dew point temperature. This is crucial for understanding cloud formation, as clouds develop when air rises and cools to its saturation point. Using the temperature and dew point provided in the question—33°C for the air temperature and 15°C for the dew point—you can find the difference between these two temperatures, which is 18°C. Since the air cools at a standard lapse rate of approximately 2°C per 1,000 feet of ascent, you can estimate how high you need to ascend for the temperature to drop to the dew point. Dividing the temperature difference by the lapse rate gives: \[ \text{Altitude} = \frac{18 \text{°C}}{2 \text{°C/1000 ft}} = 9 \text{ (thousands of feet)} = 9,000 \text{ feet} \] However, in practical meteorological calculations, the base of the cumuliform clouds can typically be found at about 1,000 feet above this figure due to variations in local atmospheric conditions and to convert from the dew point to

Cloud bases and what they tell you about the sky

Ever look up on a hot day and wonder where those puffball cumulus clouds actually start? For pilots and weather-minded folks, the base—the bottom edge—of cumuliform clouds isn’t just a pretty sight. It’s a practical clue about where flying conditions might tighten up, where you’ll lose visibility, or where you’ll need to switch from visual to instrument navigation. The basics are simple: clouds form when rising air cools to its dew point, the temperature at which air becomes saturated and moisture begins to condense. The challenge is turning that idea into a quick, usable mental check you can do in your head while planning or en route.

What the temperature difference tells you

Let’s anchor this with a real-world example. Suppose the surface air temperature is 33°C and the dew point is 15°C. That means the air’s temperature is 18°C warmer than its dew point. If you’re thinking in terms of “how high must the air rise before it cools enough to condense moisture and form a cloud base,” you’re on the right track.

In aviation, there’s a handy rule of thumb: the base of cumuliform clouds is roughly found where the air has to cool by the temperature difference (T minus Td) as it rises. A popular, practical rule uses about 400 feet of altitude per 1°C of temperature-dew point spread. Do the math, and you get:

• Temperature difference, T - Td = 18°C

• Multiply by 400 feet per degree = 18 × 400 = 7,200 feet

So, the estimated cloud base is around 7,200 feet above the surface (AGL). That’s the number many pilots carry in their heads when they’re evaluating VFR routes, potential ceilings, and the likelihood of cumulus development along the flight path.

Here’s the thing about the 7,200-foot figure: it’s a practical approximation, not a precise meteorological instrument. Real atmospheres aren’t perfectly uniform. Lapse rates (how fast air cools with height), moisture distribution, and local turbulence can nudge cloud bases higher or lower. The 400 ft/°C rule is a reliable quick-check, a way to translate “there’s a lot of moisture up there and it’s hot” into something you can act on without pulling out charts and calculators every couple of minutes.

Why that rule of thumb works (in plain terms)

To keep things grounded, picture the air parcel rising like a hot-air balloon. As it climbs, it cools. The rate at which it cools depends on the surrounding air and whether the air is dry or already moist. The dew point is the temperature at which the air becomes saturated with moisture. Until the air hits that dew point, you won’t see condensation and cloud formation.

• The air must cool by the amount T - Td to reach saturation.

• The typical cooling you’d expect in many shallow, convective lift scenarios runs around a few hundred feet per degree Celsius of T - Td, depending on conditions.

• The 400 ft/°C rule is a compact way to capture that typical behavior without getting lost in the weeds of every atmospheric subtlety.

You might hear other ways to estimate cloud base, like the classic LCL formula used in some meteorology texts. Those approaches differ in how they handle the exact physics, and they can give different numbers under the same surface conditions. The important part for everyday planning is that there’s a solid, repeatable shortcut that tends to line up with what you observe in the sky.

A quick mental-math toolkit you can use

If you want a simple, repeatable way to estimate cloud base in your head (without pulling out a calculator every time), here’s a compact method you can keep in your mental toolbox:

• Step 1: Note T and Td. In our example, T = 33°C, Td = 15°C.

• Step 2: Compute the difference. 33 - 15 = 18°C.

• Step 3: Multiply by 400 ft/°C. 18 × 400 = 7,200 ft.

• Step 4: Remember this is an estimate. Local conditions can nudge the actual base a bit higher or lower.

• Step 5: Use it as a rough guide for flight planning. If you’re cruising at, say, 6,000 ft, you’d expect cloud bases around that 7,200 ft mark to begin forming above you, with the risk of developing cumulus above that layer.

A few practical notes for pilots

• Temperature and dew point are standard data you’ll see on METARs and weather reports. If you know the surface T and Td for the area you’re flying through, you can apply this rule quickly.

• The cloud base isn’t the only factor. Wind shear, instability, and trigger mechanisms (like a warm front or a sea breeze) can change the timing and height of cloud formation.

• In very dry or very moist environments, the rule can under- or overestimate the real base a bit. Think of it as a fast, useful first guess rather than a final verdict.

• If you’re planning a flight near the cumuliform cloud deck, you’ll want to supplement this estimate with radar or satellite imagery, pilot reports, and actual weather observations along your route.

Connecting the numbers to the sky you’ll actually see

Let’s bring this into a more tangible scene. You’re evaluating your route on a hot July morning. The ground temperature is climbing into the mid-30s Celsius, the dew point sits in the mid-teens, and you can already see a few white puffs forming over distant hills. Your cloud base estimate of roughly 7,000–7,500 feet gives you a frame to think about: if you’re planning to fly at 6,000 feet, you’re well under the base—but if you climb toward 7,500 feet, you might start encountering those cumulus bases along the way. That awareness helps you decide whether you’re comfortably in the clear or if you should consider a slight altitude adjustment, a different route, or a quick check-in with the latest weather updates.

Some pilots keep a small mental cushion for safety. In practice, you might tolerate a tiny deviation in the forecast base, but a rough 7,200 ft estimate gives you a concrete reference point. It’s not about chasing perfection; it’s about giving yourself a reliable situational read so you can fly with a little more confidence.

A few digressions that stay on point

• Weather is full of little surprises. One sunny morning can turn breezy, a warm layer can loft air more vigorously, and what starts as a shallow lift becomes a strong, towering cloudscape. The base height you estimate is a snapshot, not a promise. Keeping an eye on cloud development as you move helps you stay ahead of changes.

• The dew point is more than a number you memorize. It’s a signal about moisture in the air. When Td is close to T, the air is moist, and cloud bases tend to form lower. When Td is far below T, the air is drier, and bases rise. That simple interplay—temperature versus moisture—drives a lot of the day’s flying weather.

• Regional quirks matter. Coastal air, mountain valleys, or desert plains each present their own typical ranges for cloud bases. It’s worth mapping out the local patterns so your “rule of thumb” stays accurate for the places you fly most often.

• Tools in the cockpit aren’t only charts. Modern avionics, satellite weather, and real-time reports enrich the mental model you bring to the seat. The 7,200-foot rule is a foundation you can combine with live data to stay ahead of weather changes.

A final check-in on how this helps you think like a pilot

Knowing the approximate cloud base height isn’t just about sinking another number into your head. It’s about building a mental model of the air around you. When you look at the sky and see those cumulus towers forming, the base estimate gives you a quiet, practical gauge for decision-making. It’s a way to translate the humidity and heat you feel into something actionable: “If I’m here, I might encounter clouds around that height; I can adjust my climb, route, or timing accordingly.”

So, next time you’re checking the weather or planning a flight path on a hot day, try this: take the surface temperature, subtract the dew point, multiply the difference by 400, and round to the nearest hundred. The result is a solid, functional baseline for the cloud base. It’s not a perfect forecast, but it’s a helpful compass—one that makes the sky feel a little less mysterious and a lot more navigable.

Bottom line: with a surface temperature of 33°C and a dew point of 15°C, the rough cloud base you’d expect for cumuliform development is about 7,200 feet AGL, give or take a bit. That quick calculation isn’t the entire weather story, but it’s a reliable starting point that helps you read the sky with more confidence and keep your flight planning grounded in practical science.