Cloud Base Calculator
Convective cloud base from spread.
What it does
Clouds start where rising air cools to its dew point. Surface spread forecasts that lift: wide spreads mean high bases while narrow spreads sit low. Glider pilots, photographers, and hikers all read the same gap.
The tool above multiplies spread into height. With 25 C over 15 C dew, the 10 deg spread lifts base near 1247 m (4,090 ft). With 20 over 18, the 2 deg spread gives only 249 m. The imperial mirror at 77 over 59 F gives 4,091 ft. Small gaps warn of fog before any cloud forms.
Rules stay rules of thumb. Inversions cap lift below the estimate, fronts force air up mechanically, and terrain triggers earlier on windward slopes. Official aviation ceilings always overrule mental math.
This page estimates convective bases only. It does not model fog, stratus decks, or storm tops. For flight, use official observations and forecasts.
How to use
- Pick metric (C) or imperial (F).
- Enter the air temperature.
- Enter the dew point at or below it.
- Read cloud base first.
- Read the converted base.
- Read spread for fog risk.
Formula
- T
- Air temperature in C or F.
- Td
- Dew point in the same scale, at or below T.
- spread
- T minus Td in matching degrees.
Dew point spread rule of thumb for convective lift. At zero spread the base sits at the ground (fog risk). Inversions and mechanical lift sit outside this line.
Worked example
Example: 25 over 15 C
Defaults: metric 25 C with 15 C dew. Imperial mirror is 77 F with 59 F dew.
- Inputs (metric default): 25 C air with 15 C dew.
- Spread: 25 minus 15 is 10 C spread.
- Base: 10 times 124.66 is 1247 m base.
- Feet: 1247 / 0.3048 is 4,090 ft.
- Imperial mirror (77 over 59 F) gives 4,091 ft. Tool shows the same.
Reference table
| Spread | Base | Note |
|---|---|---|
| 10 C spread | 1247 m base | 4,090 ft |
| 2 C spread | 249 m base | 818 ft |
| 18 F spread | 4,091 ft base | 1247 m |
| 0 C spread | 0 m base | fog risk |
| 9 F spread | 2,045 ft base | 623 m |
Limits
- Rule of thumb. Inversions excluded.
- Convective bases only. Fog and stratus differ.
- Surface parcel assumed. Profiles vary.
- No aviation clearance.
Common mistakes
- Mixing C spread with F divisor.
- Reading fog as cloud base.
- Ignoring inversions that cap lift.
- Using one station for distant hills.
- Flying on mental math alone.
Try it yourself
Quick checks. Answers sit tucked under each question.
20 C with 18 C dew?
Spread is 2 C deg. Base is 249 m with 818 ft. Fog risk is high.
68 F with 50 F dew?
Spread is 18 F deg. Base is 4,091 ft with 1247 m.
Questions
How is cloud base figured?
Base near spread_F/4.4x1000 ft (spread_C x124.66 m). Here 1247 m base.
What is dew point spread?
Air minus dew point in the same scale. Here 10 C spread.
Which clouds does this fit?
Lifting condensation level estimate for convective cloud. Not fog or stratus decks.
Does small spread mean low cloud?
Yes. Narrow spreads mean low bases and possible fog near 0 to 2 deg.
Is this exact?
No. Inversions, fronts, and terrain shift real bases from the rule.
Can pilots use this alone?
No. Aviation needs official ceilings and outlooks.