Glossary
Short, plain definitions for terms used throughout Barograph — mostly practical vocabulary for keeping and reading a home weather log, not a full meteorology dictionary.
Aspirated shield
A housing that draws air across a thermometer with a small fan, reducing the radiant-heat error a passive shield can pick up in still air.
Barometric tendency
The change in pressure over a fixed recent period, usually three hours; the basis for a trend arrow on a home barometer.
Calibration offset
The steady difference between a home instrument and a trusted reference, recorded once and then subtracted or added to every later reading.
Dew point
The temperature at which air becomes saturated and moisture begins to condense; a steadier companion figure to humidity than relative humidity alone.
Diurnal range
The difference between the day's high and low reading; a long series of diurnal range can say more about local exposure than the average alone.
Homogeneity break
A point in a long series where a change in instrument, location, or observing time makes earlier and later data not directly comparable.
Hygrometer lag
The delay between an actual change in moisture and a hygrometer's reported reading, most noticeable during quick weather changes.
Instrument drift
A slow, gradual change in an instrument's accuracy over months or years, distinct from a sudden fault.
Isobar
A line on a pressure map connecting points of equal barometric pressure; useful context even for a single-station log.
Micro-site
The specific, small-scale environment around an instrument — nearby pavement, a wall, shade pattern — that can bias readings independent of the wider local climate.
Parallax error
A reading error caused by viewing a dial or gauge from an angle rather than straight on, common with analog barometers and rain gauges.
Reduction to sea level
A standard adjustment applied to a pressure reading to account for a station's elevation, allowing comparison between stations at different heights.
Reference instrument
A second instrument, trusted or independently checked, used specifically to test another instrument's accuracy rather than for daily readings.
Rounding convention
The fixed rule a logkeeper uses for rounding readings (for example, always to the nearest tenth), kept consistent to avoid introducing noise into a series.
Screen
A ventilated enclosure that shields a thermometer from direct sun and precipitation while still allowing free air movement around it.
Synoptic hour
One of a small set of standard times used for weather observation, chosen so readings from different stations can be compared directly.
Ten-minute average
A short averaging window used to smooth out brief gusts or spikes before logging a wind or gust reading.
Time-of-observation bias
A systematic effect on daily high/low figures caused by changing the hour a reading is taken partway through a series.
Wet-bulb depression
The difference between a dry thermometer reading and one kept wet by a wick; used as a manual method for estimating humidity.
Zero offset
The reading an instrument shows under a known, fixed condition (such as freezing water); used to check whether it has drifted from true zero.