A home weather diary: what a garden or balcony sensor records and why it differs from the official station

What it is

A weather diary is a daily record of what a household's own instruments and eyes report: maximum and minimum temperature since the last reset, rain if a gauge exists, sky state, wind by observation, and notable events. The Met Office page on how it measures temperature states that at its stations a platinum resistance thermometer is exposed in a Stevenson screen at a height of 1.25 m above the ground, ventilated only naturally through the side louvers, and that the ground minimum is a separate measurement taken by a thermometer exposed to the open sky over grass. A consumer sensor screwed to a house wall, hung under an eave or standing on a balcony has none of that exposure, so its readings describe that spot and not the air the station describes.

Why it matters

A diary without an exposure description cannot be compared with a neighbour's, with a station, or with itself after the sensor was moved. With the exposure written down, the same numbers become usable: the difference from the station is then a property of the site that can be looked at by time of day and sky state.

How to apply

• Write an exposure description once: height above ground, compass orientation, distance from the nearest wall and window, cover overhead, hours of direct sun in summer and winter, and the ground surface below.
• State the reset time for maximum and minimum and keep it; a diary that resets at 07:00 and one that resets at midnight report different "days".
• Record sky state and wind in a fixed vocabulary (clear, part cloud, overcast, rain; calm, light, strong) at a fixed time.
• Add a reference column with the nearest official station's daily high and low and its distance; compute home minus station as its own column. The National Weather Service page on degree days shows the common convention of a daily mean as the high plus the low divided by two; if a mean is wanted, state that this is how it was made.
• Log every change of sensor, battery or position as an event, and start a new series after a move.

Pitfalls

A wall-mounted sensor partly reads the wall, and a spot in morning sun gives a high that the station never sees. A gauge under cover under-reads rain. Low batteries in wireless sensors are commonly reported to produce gaps and odd values; the gap itself should be logged. The difference series is not an error of the home sensor, and it is not an error of the station either; it is the siting.