Source: https://guthlabs.ai/docs/instruments/cloud-base-height-from-spread-quick-field-rule/mechanism/

instrument mechanism / verified-clean

# Cloud Base Height from Spread (quick field rule) mechanism

The inspectable method record for Cloud Base Height from Spread (quick field rule), kept separate from implementation and qualification.

Status: **VERIFIED_CLEAN**Lifecycle: **verified-clean**

## Model scope

The existing honest label 'rule-of-thumb approximation' is appropriate, but the computes description must not state that this is exactly the LCL. Require temperature and dew point in the same unit and require temperature to be at least as high as dew point.

- The formula h(ft) ≈ 400 × (T−Td in °F) is a standard rule-of-thumb estimate for cloud-base height above ground level, but it is not exactly the same as the thermodynamic LCL calculation. The claim 'same as LCL' is therefore potentially misleading and should be changed to 'rough LCL/cloud-base approximation.'

- The formula does not state the required physical constraint T ≥ Td; when Td > T it produces a negative, physically invalid cloud-base height.

## Units and conventions

- T and Td are both actual temperatures in °F; the difference must be calculated in Fahrenheit degrees.

- The output is an approximate height in feet above ground level (AGL), not feet above mean sea level.

- The 400 ft/°F coefficient assumes near-surface atmospheric mixing and typical conditions; it is not a universal exact LCL coefficient.

## Inspectable vector

One of 4 source-bound review vectors:

````
{
  "inputs": {
    "temperature_f": 70,
    "dew_point_f": 60
  },
  "expected_outputs": {
    "estimated_cloud_base_ft_agl": 4000
  },
  "working": "Spread = T − Td = 70 − 60 = 10 °F. Height ≈ 400 × 10 = 4,000 ft AGL."
}
````

## Failure boundary

**BREAK_CLAIMED** · SELF_FLAGGED

The formula produces a negative raw height when dew point exceeds temperature and overflows to Infinity for opposite extreme finite inputs. Both cases should be rejected or explicitly flagged rather than silently returned as ordinary estimates.

## Other records

[01**Overview**](<https://guthlabs.ai/docs/instruments/cloud-base-height-from-spread-quick-field-rule/>)[03**Implementation**](<https://guthlabs.ai/docs/instruments/cloud-base-height-from-spread-quick-field-rule/implementation/>)[04**Qualification**](<https://guthlabs.ai/docs/instruments/cloud-base-height-from-spread-quick-field-rule/qualification/>)

## Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

- `VERIFIED_LEDGER.json`

- `evidence-tranche5/tranche5-results.json`

- `BOUNDARY_HAZARD_CATALOG.json`