astronomy · planet
Solar geometry
Seasonal differences begin with Earth’s tilted axis and its orbit around the Sun. “Spring” therefore has opposite timing in the two hemispheres.
Source: NASA Science · supported referenceFlagship / explore
Spring: Field Atlas
Browse seasonal signals without treating them as one universal clock. Filter by domain, scale, or evidence posture, then follow each signal back to its claim boundary.
astronomy · planet
Equinox marker
Astronomical spring is conventionally anchored to an equinox, but “equal night” is an approximation because daylight length depends on geometry and atmospheric refraction.
Source: NOAA / National Weather Service · supported referenceclimate convention · region
Meteorological spring
Meteorological seasons use fixed calendar-month groupings tied to the annual temperature cycle rather than equinox moments.
Source: NASA · supported referencephenology · region
Leaf-out index
USA-NPN’s First Leaf Index is a synthetic model of early-season plant activity based primarily on temperature conditions; it is not direct observation of every species.
Source: USA National Phenology Network · supported referencephenology · organism
Bloom sequence
Different species begin activity at different times, so biological spring is better treated as a sequence of events than one universal start date.
Source: USA National Phenology Network · supported referencehabitat · habitat
Vernal pool
Some shallow seasonal pools fill during spring conditions and provide breeding habitat for amphibians before drying later in the warm season.
Source: U.S. National Park Service · supported referenceobservation · site
Local field note
A dated note about a bud, flower, animal, water body, or weather event can be direct observation at one place and time. It should not be generalized automatically.
Editorial guidance — not an external factual claim.representation · organism
Botanical illustration
Scientific botanical illustration emphasizes accurate diagnostic detail, while botanical art may prioritize expression.
Source: Smithsonian Gardens · supported referenceNo signals match those filters.
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