McKenzie Minimum Temperature Toolv0.1

Night-time minimum air temperature from maximum temperature, dew point, overnight wind and cloud · McKenzie's method, Met Office Forecasters' Reference Book §2.4.2.1, Table 2.8, with a 1000-member Monte Carlo uncertainty cloud

Station & month

Radiation night

Afternoon

Cannot exceed the maximum temperature.

Overnight

Input uncertainty

Gaussian perturbation. The ± value below is treated as:

Forecast

    Uncertainty cloud · 1000 members

    Frequency Individual members Unperturbed forecast 5th–95th percentile Interquartile range (25th–75th)

    Local constant K

    Edit any value. The highlighted cell is the one used for your inputs. Changes apply to the working copy until you save.
    Saved stations are stored in this browser only.

    Monthly corrections to K

    Added to K. A = good radiation nights, B = poor radiation nights. These apply at all stations.
    How it works

    Tmin = 0.5 (Tmax + Td) − K, where Tmax is the representative maximum temperature, Td the air-mass dew point at the time of Tmax, and K a local constant depending on forecast mean overnight surface wind and average low cloud.

    K is read from the station table (default: Birmingham Airport, Table 2.8(a)) and then corrected for the month and for good (A) or poor (B) radiation nights (Table 2.8(b)). The correction is added to K, so a negative correction raises Tmin.

    The uncertainty cloud draws 1000 perturbed sets of inputs from independent Gaussians centred on your values. Wind and cloud are rounded to whole knots/oktas, clamped (wind ≥ 0, above 21 kn uses the 17–21 kn row; cloud 0–8) and binned. Perturbed dew point is capped at the perturbed Tmax. The random draws are seeded so the cloud stays stable until you press Resample.

    K values are specific to a site. Use your local forecast office's table for other stations and save it with a name.