Why Morning PAR Rises Faster Than It Falls in the Evening

When I first started measuring light in my garden, I thought light intensity would rise and fall in a simple mirror pattern throughout the day. I expected the curve of PAR values to look symmetrical — as if morning and evening were two sides of the same hill.

My measurements showed otherwise.

Over many days of recording PAR values from before sunrise to after sunset, I noticed a clear pattern: PAR rises faster in the morning than it falls in the evening. In other words, light intensity increases sharply after sunrise but declines more slowly near sunset.

At first I wondered if this was a quirk of the meter, or a measurement error on my part. After several weeks of consistent data collection under different weather conditions, I concluded that this was a real pattern in natural light behavior — and understanding it made a difference in how I interpreted light for my plants.

This article shares what I observed, how I measured this pattern, and what it means for everyday gardeners trying to understand daily light exposure.

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How I Measured Light Throughout the Day

I set up my meter and recorded PAR values at regular intervals — roughly every 30 minutes — from before sunrise until after sunset for several clear days. I chose a location in my garden that was unobstructed by trees or structures so that the sunlight curve would reflect natural conditions.

Here is a simplified example of what I recorded on a clear summer day:

Time	PAR (µmol/m²/s)
05:45	0
06:30	120
07:15	340
08:00	580
09:00	800
10:00	900
11:00	950
12:00	920
15:00	700
17:00	420
18:30	140
19:30	20
20:00	0

From the data, two things stood out:

• In the morning, PAR increased rapidly between sunrise and mid-morning.
• In the evening, PAR declined gradually over a longer period.

This pattern repeated on multiple clear days and even on partly cloudy days, though clouds introduced more noise into the numbers.

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Why PAR Increases Quickly in the Morning

Light intensity rises quickly in the morning because of the sun angle and atmospheric conditions.

After sunrise, as the sun climbs higher in the sky, the atmosphere absorbs and scatters less of the incoming light. The path through the atmosphere shortens quickly, and the sun’s rays become more direct in a relatively short time.

This causes PAR values to jump sharply over a few hours as sunrise transitions to mid-morning.

When I watched the raw data curve on the meter, the slope from sunrise to mid-morning looked like the steep ascent of a hill — sudden and strong.

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Why PAR Decreases Slowly in the Evening

In contrast, when the sun begins its descent in the afternoon, it loses elevation gradually. The sun’s angle to the horizon changes slowly, and light has to pass through more atmosphere as it gets lower.

This longer atmospheric path spreads the transition over a longer period. As a result, the decrease in usable light intensity is more gradual.

In the data above, the period from mid-afternoon to sunset spans almost three times as long as the rapid rise in the morning. This means plants experience a long tail of moderate light in the evening, rather than a sharp drop-off.

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What This Means for Everyday Gardeners

For gardeners, understanding this pattern helps put daily light exposure in context.

A single PAR measurement at noon gives a snapshot of peak intensity, but plants use light throughout the entire day. The rapid rise in the morning and slow decline in the evening means that:

• Plants receive usable light long before midday
• Evening light contributes meaningfully to daily totals even when its intensity is lower
• Daily Light Integral (DLI) is influenced not just by peak values but by the shape of the entire light curve

In practical terms, this means that two days with the same peak PAR at noon might still have very different daily totals if one day has a longer tail of usable light in the morning and evening.

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Observations on Cloudy Days

On partly cloudy days, the pattern still holds, though with more variation. Clouds can suppress peak values in the late morning or midday, but I consistently saw that morning rises were steeper and evening declines more gradual than I originally expected.

For example, on a day with scattered clouds, the midday peak might be lower, but usable light often continued into the evening at moderate levels, contributing to the overall daily light plants received.

This helped me realize that even when peak numbers seem lower, usable light across the day can still be substantial.

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Final Reflection

The way light changes throughout the day is not symmetrical. Morning rises are steep, while evening declines are more gradual. This is a real pattern in natural light that I confirmed with repeated measurements in my own garden.

Understanding this helped me move beyond single snapshot readings and think in terms of how plants experience light over the entire day. It reminded me that usable light at lower intensities still matters, and that light exposure in the early morning and late afternoon contributes to plant growth more than I had initially assumed.

When you measure light throughout the day, you begin to see the full story — not just peak numbers, but the shape of the curve that represents how usable light unfolds from sunrise to sunset.