Anyone growing seedlings on a windowsill or running a year-round greenhouse faces the same illusion. In June at noon, the sun floods the room, and on a clear day in December, it seems there is plenty of light too. The human eye easily adapts to brightness variations, but for plants, reality looks completely different. The difference between a summer and winter solar noon can be six to ten times.
To stop guessing by eye, we created an interactive simulator widget. With it, you can move the time slider and see how the real physics of light change at your latitude: the instantaneous PPFD level, the solar elevation angle, daylight duration, and the daily light integral (DLI). Just switch the season from summer to winter, and it becomes mathematically clear why a December windowsill is a survival zone for seedlings.
The Physics of the Process: Why the Winter Sun Warms and Shines Weaker
It is commonly believed that plants lack light in winter purely due to short days. However, this is only the tip of the iceberg. Even if a December day were artificially stretched to 16 hours, light would still be critically low due to two fundamental atmospheric factors.
Factor 1: Ray Geometry and the Angle of Incidence
In the mid-latitudes in mid-summer, the noon sun rises to 60–63° above the horizon. In December, this figure drops to a modest 13–16°. When a beam of light falls vertically, all its energy is concentrated in a small spot. With a shallow winter incidence, the same photons are "smeared" over a huge area. The illuminance of a horizontal surface decreases in proportion to the sine of the solar elevation angle, which causes a winter noon to immediately lose about 70% of its power.
Factor 2: Optical Thickness of the Atmosphere (Air Mass)
The second mechanism is even harsher. At its zenith, a sunbeam punches through the atmosphere via the shortest path — this reference distance is called a unit of Air Mass (Air Mass = 1.0). When the sun hangs low over the horizon, the beam travels through the ocean of air at an acute angle, and its path length increases significantly. At an angle of 30°, it is already two air masses; at 15°, nearly four; and at sunset, up to forty.
On this long journey, photons collide with gas molecules, dust, water vapor, and ozone. The shortwave (blue) part of the spectrum is actively scattered — this is exactly why the setting sun appears red and doesn't blind the eyes. For plants, this means a double blow: not only does the number of light quanta drop in winter, but their spectral composition is also distorted.
Combined, both effects mean that a cloudless summer noon in Kyiv or Dublin outputs about 1800 µmol/m²·s, while a winter noon barely reaches 300 µmol/m²·s.
What PPFD and DLI Are in Practice
PPFD (Photosynthetic Photon Flux Density) is the flux density of photosynthetically active photons. Simply put, it is the number of useful light quanta falling on one square meter of surface per second (measured in µmol/m²·s). This is a "snapshot" of light intensity. The maximum PPFD occurs at noon and approaches zero in the morning and evening. You can read more about how photons are distributed over an area in our guide on photosynthetic photon flux density ↗.
DLI (Daily Light Integral) is the daily dose of light that a crop accumulates over the day (measured in mol/m²·day). If PPFD is a car's speed on a speedometer at a given second, then DLI is the total distance traveled in a day.
For agronomy, DLI is the main benchmark. A sprout generally doesn't care whether it gets its dose from 6 hours of aggressive sun or 14 hours of soft supplemental lighting. In summer, nature provides a luxurious 50–60 mol/m²·day, while in winter, this figure drops to 4–6 mol/m²·day, sending plants into stagnation. For detailed tables of DLI requirements for different crops, see our article: DLI in Crop Production ↗.
How to Use the Widget: Instructions
Powered by SpecLED Sun Path & Natural PPFD Simulator
Step 1. Setting the Geographical Latitude
Sun paths are tied to parallels, not countries. Move the latitude slider or click on a preset for the desired city (Dublin, Kyiv, Berlin, Madrid, Oslo, or the Equator). Note: the further north you go, the closer the winter sun "presses" to the ground. In Oslo (60° N), a winter day lasts 5.5 hours, and the sun barely exceeds 7° in elevation.
Step 2. Choosing the Season (Date)
Season buttons are tied to key astronomical points—equinoxes and solstices (March 21, June 21, September 23, December 21). Try switching from "Summer" to "Winter" at the same latitude: the dashed arc of the sun's path will shrink both in height and width.
Step 3. Accounting for Cloud Cover
Three states are available: clear, partly cloudy, and overcast. Sunlight is divided into direct and diffuse. A clear sky provides a powerful direct beam. Clouds block it but cause the entire sky dome to glow (diffuse light). Overcast weather cuts summer light by about 60%, and winter light by 40%. Because of this, only 150–200 µmol/m²·s remains on a windowsill on a gloomy December day.
Step 4. Timeline Slider
Move the slider from sunrise to sunset. You will see how PPFD smoothly rises, peaks at solar noon, and drops towards evening. The main insight of the winter mode is that the productive "light window," when plants actually photosynthesize, narrows down to just 3–4 hours a day.
Step 5. Evaluating Crop Requirements
At the bottom of the widget, there is a switch for plant groups:
- Greens and seedlings (lettuce, herbs) — norm 10–17 mol/m²·day;
- Fruiting crops (tomatoes, peppers, cucumbers) — norm 20–30 mol/m²·day;
- Light-demanding plants (including medical tomatoes) — norm 30–45 mol/m²·day.
The tool will automatically compare the calculated solar DLI with the crop's appetite and issue a verdict: light deficit, norm, or surplus.
Full article on our website - https://specled.com/ru/knowledge/how-much-light-does-the-sun-give-ppfd-dli/
Practical Benefits for the Grower
The widget allows you to translate a subjective "it feels dark" into an exact calculation for buying or setting up grow lights. For example, if you select your latitude in December under an overcast sky and see a solar DLI of 3 mol/m²·day, while your peppers need 20 mol/m²·day, the deficit is exactly 17 moles.
To compensate for these 17 moles over 16 hours of light operation, you need supplemental lighting with an intensity of about 295 µmol/m²·s at the canopy level. No magic—just pure math.
Model Limitations and Real Measurements
The widget is based on strict astronomical formulas and the Kasten-Young atmospheric model. However, the calculation is made for clear air at 100 meters above sea level. It does not account for urban smog, window glass, shadows from neighboring buildings, or the bonus reflected light from snow.
The model shows the ideal upper limit of what you can expect from the sun. Of course, the most accurate way is to measure the light physically, but quantum meters (PAR meters) cost hundreds of dollars and not everyone has one. For household measurements, I previously created a lux to PPFD conversion calculator ↗. With a standard lux meter (or smartphone), you can select your grow light's preset and get accurate micromoles. And to convert lux under open sunlight, select the "Full Spectrum" preset and add about 10% to the PPFD result.
Summary
In winter, plants starve not only because of short daylight hours. The low sun and the long path of the ray through the atmosphere cut the midday brightness by 5–6 times, and clouds increase this gap up to 10 times.
For a full harvest, the total daily dose, DLI, is crucial. The difference between a meager winter 4–6 mol/m²·day and the 25–30 mol/m²·day a plant needs is exactly the job your LED grow lights must do.