Why Morning PAR Rises Faster Than It Falls in the Evening

After collecting long-term light data using the AH-PARDLI PAR & DLI Logger, an interesting pattern becomes clear:
PAR levels increase more quickly in the morning than they decrease in the evening.

This daily asymmetry might seem subtle, but it reveals a lot about how natural sunlight behaves — and why understanding it matters for plant growth.

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The Science Behind the Morning “Climb”

In most climates, once the sun rises above the horizon, the angle of sunlight changes rapidly. Combined with typically clearer skies in the morning, this causes PAR (photosynthetically active radiation) to rise sharply in the first few hours after sunrise.

• Low atmospheric scattering: In the early morning, the air often contains less dust and water vapor, allowing photons to reach the ground more efficiently.
• Faster solar angle gain: The sun’s angle relative to the plant canopy increases quickly after dawn, boosting the intensity of incoming light.
• Cooler temperatures: Plants and sensors experience less thermal turbulence early in the day, resulting in more stable photon flux.

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Why Evening PAR Declines More Slowly

In contrast, the drop in PAR during late afternoon and evening is usually more gradual. Several environmental and geometric factors are responsible:

• Shallower solar angle: As the sun approaches the horizon, its angle changes more slowly, causing a gentler decline in PAR.
• Atmospheric scattering increases: Dust, humidity, and aerosols accumulated throughout the day scatter more light, slightly extending useful radiation.
• Thermal lag: Surfaces and the surrounding air remain warm, which can prolong scattered and diffuse light even after direct light weakens.

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What This Means for Growers

This natural asymmetry has important implications:

• DLI planning: A large portion of the day’s light dose occurs before midday. Knowing this helps with scheduling supplemental lighting.
• Irrigation and nutrition: Photosynthesis — and therefore water and nutrient demand — often peaks earlier than you might expect.
• Sensor placement: Long-term PAR logging captures daily light dynamics that single measurements miss.

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Measured by AH-PARDLI

This kind of insight is only possible with continuous data.
The AH-PARDLI logger automatically records PAR levels throughout the day, building a complete light curve that shows exactly how quickly light rises and falls — helping growers make better, data-driven decisions.