dagvariatie

Dagvariatie, or diurnal variation, is a term used to describe a pattern or change that repeats roughly every 24 hours. It arises from the rotation of the Earth and, in many phenomena, from daily solar heating. Diurnal variation can occur alongside longer-term trends and varies with latitude, season, and local conditions.

In meteorology and climatology, surface temperature shows a pronounced diurnal cycle: temperatures rise after sunrise, peak in the late afternoon, and fall during the night. Relative humidity often behaves inversely. Wind direction and strength may shift due to land–sea breezes and daytime heating. Solar radiation itself follows a diurnal pattern, influencing evaporation, cloud formation, and weather processes.

In geophysics, the Earth's magnetic field exhibits diurnal variation, known as Sq, caused by ionospheric currents driven by solar heating and tidal forces. The amplitudes are small but measurable, on the order of tens of nanoteslas. In hydrology and water resources, diurnal changes can appear in streams or groundwater in systems with strong pumping, transpiration cycles, or tidal effects, producing daily fluctuations in discharge or level.

Diurnal variation is important for practical purposes: forecasting solar power output, planning energy use, calibrating sensors, and interpreting time-series data. Analysts often separate the diurnal component using harmonic analysis or Fourier decomposition to study other signals.