Daily Light Integral can vary dramatically across the United States.
A greenhouse in Arizona does not receive the same natural-light environment as one in Washington State.
And the same greenhouse can receive very different natural DLI in:
January
and:
July.
The main reasons include:
latitude,
day length,
solar elevation,
cloud patterns,
season,
and local shading.
This is why national DLI maps are useful.
But a DLI map should be interpreted correctly:
It estimates the typical outdoor light climate of a location. It does not replace a sensor at the crop canopy.
Quick Answer
Across the United States, outdoor DLI generally varies with both geography and season.
Broadly:
- sunnier regions of the Southwest often receive higher outdoor DLI,
- northern and frequently cloudy regions can experience much lower winter DLI,
- summer DLI rises across much of the country because days are longer and solar elevation is higher,
- greenhouse crop-level DLI is usually lower than outdoor DLI because the structure reduces incoming light.
Purdue Extension notes that monthly outdoor DLI across the United States can span approximately:
5 to 60 mol/m²/day
depending on location and season.
That range alone shows why one national “normal DLI” value does not exist.
What Is DLI?
DLI stands for:
Daily Light Integral
and is expressed in:
mol/m²/day.
It represents the quantity of photosynthetic photons accumulated over a square meter during one day.
PPFD, by comparison, is expressed in:
µmol/m²/s
and describes instantaneous photon flux.
So:
PPFD = light now
while:
DLI = accumulated light through the day.
This distinction becomes especially important when comparing locations because two places can have similar midday PPFD but very different:
day length,
cloud cover,
and total daily photon exposure.
Why Geography Changes DLI
The United States covers a large range of:
latitudes,
climates,
elevations,
and cloud regimes.
Those differences affect the solar radiation reaching the surface.
Latitude influences:
solar elevation
and:
seasonal day length.
Regional climate influences:
cloud frequency,
atmospheric transmission,
and weather patterns.
The resulting daily photon supply is therefore not uniform across the country.
The Southwest Often Has a High Solar Resource
The southwestern United States is widely known for high solar-resource availability.
NREL’s solar-resource maps show high annual average global horizontal irradiance across much of the Southwest compared with many other U.S. regions.
This generally aligns with higher potential outdoor DLI because both are driven by incoming solar radiation.
However:
GHI and DLI are not the same measurement.
GHI is an energy-based solar-radiation quantity.
DLI is a photon-based horticultural quantity.
For horticultural work, dedicated DLI maps are more directly relevant.
The American Floral Endowment specifically notes that DLI maps were created for horticultural applications and that photovoltaic solar maps are not directly interchangeable with horticultural DLI maps.
Northern Locations Can Have Very Low Winter DLI
Winter creates a very different situation.
At northern latitudes:
days become shorter,
the sun remains lower in the sky,
and cloudy weather may further reduce incoming radiation.
Michigan State University notes that outdoor DLI can fall below:
5 mol/m²/day
during a dark, cloudy, short winter day in northern parts of the United States or Canada.
That does not mean every northern winter day has a DLI below 5.
It illustrates the magnitude of seasonal limitation that can occur.
Summer Changes the Picture
During summer:
day length increases,
solar elevation becomes higher,
and many locations receive substantially greater daily photon totals.
Under long, cloudless summer conditions, outdoor DLI can approach approximately:
60 mol/m²/day.
That upper range is cited by Michigan State and appears in Purdue’s U.S. DLI guidance.
Again, this is not a universal summer value.
Clouds, latitude, local weather and shading still matter.
The Same City Can Have Very Different DLI in January and July
This is one of the most important lessons from U.S. DLI maps.
Location is only half of the question.
You also need:
month or season.
The interactive U.S. maps were specifically designed as monthly DLI maps because annual averages can hide large seasonal variation. The high-resolution maps developed by James Faust and Joanne Logan use historical solar-radiation data to estimate monthly average outdoor DLI.
So asking:
“What is the DLI in Chicago?”
is incomplete.
A better question is:
“What is the typical outdoor DLI near Chicago in January?”
or:
“…in July?”
What the U.S. DLI Maps Actually Represent
This point deserves special attention.
The widely used high-resolution U.S. DLI maps are based on historical solar-radiation data, not live readings.
MSU describes the newer maps as being generated using data from:
1998–2012
with much higher spatial resolution than the original generation of maps.
Therefore, a map value should be interpreted as something like:
Typical historical monthly outdoor DLI for this geographic area
not:
The DLI my plants received today.
A Map Value Is an Average
Suppose the map indicates a monthly average near:
20 mol/m²/day.
Individual days might be:
much higher,
much lower,
or close to 20.
A clear day and a stormy day can differ dramatically.
The average helps describe the regional light climate.
It does not remove day-to-day variability.
DLI Maps Do Not Know About Your Tree
A national map also cannot know whether your actual crop is beside:
a house,
a mountain,
a fence,
a tree,
or:
another greenhouse.
It cannot know whether the sensor plane is shaded every afternoon.
This is why national maps should be treated as a baseline, not a replacement for site measurement.
Even recent work in complex terrain has highlighted that local mountains and valleys can create conditions that differ from broad mapped estimates.
DLI Maps Describe Outdoor Conditions
This is especially important for greenhouse growers.
If the outdoor monthly DLI is:
20 mol/m²/day
the crop inside the greenhouse does not automatically receive 20.
Incoming photons can be reduced by:
glazing,
structural framing,
shade curtains,
hanging equipment,
lighting fixtures,
dirt,
condensation,
and crop architecture.
University of Georgia’s horticultural-lighting guidance explicitly notes that U.S. DLI map values are outdoor DLI and that greenhouse transmission must be considered separately.
Greenhouse Transmission Can Be a Major Loss
Purdue explains that greenhouse structures and glazing can reduce incoming natural light substantially.
Its DLI guidance notes that greenhouse conditions are influenced by:
structure,
season,
sun angle,
cloud cover,
day length,
shading,
and overhead obstructions.
Michigan State likewise notes that greenhouse structure and glazing commonly reduce the photon supply reaching the crop.
Therefore, outdoor-map DLI and crop-level DLI should not be treated as equivalent.
Measure at the Crop When Crop DLI Matters
If your actual question is:
How much light did my lettuce receive today?
the best reference point is not a national map.
It is the crop environment.
Place the sensor at an appropriate crop reference plane and measure or log:
PPFD through time
and:
daily DLI.
Maps help with planning.
Sensors help characterize the actual site.
Why DLI Maps Are Still Extremely Useful
None of this makes the maps unhelpful.
They are useful precisely because they give growers a broader context.
For example, they can help answer:
Is my region naturally light-limited in winter?
When does seasonal DLI usually begin increasing?
Does my greenhouse location have a natural solar advantage?
When might supplemental lighting become more important?
When might shading become necessary?
The American Floral Endowment states that these maps can help growers estimate when supplemental lighting or shading may be useful during the year.
Using a DLI Map for Supplemental-Lighting Planning
Imagine a greenhouse crop requires more daily photon exposure than natural sunlight typically supplies during part of winter.
The map can help identify the broad seasonal period when low natural-light conditions are likely.
But the actual lighting decision should then use:
crop-level measurements,
greenhouse transmission,
crop requirements,
and:
economic considerations.
A regional map is useful for planning.
It is not a lighting controller.
Purdue’s Indiana Example Shows Why Location and Season Matter
A recent Purdue publication shows natural greenhouse DLI measured in West Lafayette, Indiana over the year.
It reports that greenhouse sunlight DLI is usually below:
10 mol/m²/day
from approximately November through February at that location.
That is much more useful than saying:
“Indiana has low DLI.”
The important information is seasonal.
The same greenhouse receives much more natural light during other parts of the year.
There Is No Single “U.S. DLI”
Just as the United States has no single:
temperature,
rainfall,
or:
day length,
it has no single DLI.
Outdoor DLI varies simultaneously in:
space
and:
time.
A useful DLI description therefore needs at least:
location + date or month
and ideally:
actual site measurement.
Why Cloud Climate Matters
Latitude alone does not explain everything.
Two places at similar latitude can have different solar resources because their typical cloud conditions differ.
NREL’s solar database models both clear-sky and cloudy-sky conditions when estimating surface solar radiation.
For horticulture, cloudiness affects not only the amount of light but also the balance between:
direct
and:
diffuse radiation.
So a geographic comparison should not be reduced to:
farther south = always more DLI.
Regional climate matters too.
Elevation and Terrain Can Matter
Local geography can further modify light availability.
Mountains can:
block low-angle sun,
create local cloud patterns,
and generate earlier shadowing.
Valleys may experience different conditions from nearby open plains.
The national maps are valuable regional tools, but they cannot resolve every local microclimate or obstruction.
For a specific farm or greenhouse:
measure the actual location.
U.S. DLI Map vs NREL Solar Map
These tools answer related but different questions.
| Tool | Main quantity | Typical use |
|---|---|---|
| U.S. DLI map | mol/m²/day | Plant and greenhouse light planning |
| NREL solar map | solar energy, such as kWh/m²/day | Solar-energy and irradiance applications |
| PAR/DLI logger | Actual PPFD and DLI at the sensor | Site-specific horticultural measurement |
NREL provides excellent solar-resource data, including monthly and annual GHI maps.
But for plant-light decisions, the horticultural DLI map provides a more direct photon-based reference.
Do Not Convert Solar-Energy Maps Carelessly
Solar irradiance can be converted approximately into photosynthetic photon quantities using assumptions about:
the solar spectrum
and:
the fraction of energy within PAR.
But these conversions contain assumptions.
Michigan State describes one practical conversion approach between broadband solar radiation and estimated DLI, but also recommends direct crop-level measurement when greater accuracy is needed.
If a real DLI map or quantum sensor is available, those are usually clearer options for horticultural use.
A Better Way to Use U.S. DLI Data
Use national DLI information in three stages.
Stage 1 — Regional planning
Check the historical monthly DLI for your geographic location.
Stage 2 — Site adjustment
Consider:
greenhouse transmission,
trees,
buildings,
terrain,
shade cloth,
and other obstructions.
Stage 3 — Actual measurement
Measure PPFD and DLI at the crop when decisions depend on the real photon environment.
This prevents a regional climate map from being treated as a crop-level measurement.
Comparing Two U.S. Locations
Suppose a grower is comparing greenhouse locations in two states.
The DLI map can help identify broad differences in:
winter natural light,
summer natural light,
and:
seasonal variability.
But light should not be the only site-selection variable.
Commercial decisions may also depend on:
energy cost,
temperature,
cooling demand,
humidity,
transportation,
labor,
water,
and market access.
A high-light region may reduce supplemental-light demand while increasing cooling or shading requirements.
So DLI is one environmental variable—not a complete site-selection score.
High DLI Can Also Create Management Challenges
High natural DLI is not automatically an advantage under every condition.
During intense summer conditions, crops may also experience:
high temperature,
high leaf temperature,
greater transpiration,
and excessive radiation for some species.
Shade curtains, coatings or other light-management strategies may then become useful.
The DLI map helps show potential light supply.
It does not automatically tell you whether more photons are desirable.
Crop Requirements Must Stay Crop-Specific
Avoid statements such as:
“Plants need 20 mol/m²/day.”
Different crops have different responses to DLI.
Useful ranges can also depend on:
growth stage,
temperature,
CO₂,
cultivar,
production objective,
and photoperiod.
Even the authors of the U.S. DLI-map work caution that DLI recommendations are situational rather than universal biological requirements.
So geographic DLI should always be interpreted against the specific crop system.
DLI Is Especially Useful for Greenhouse Production
Regional DLI information is particularly valuable for greenhouses because growers can actively modify light.
When natural DLI is low:
supplemental lighting may add photons.
When natural radiation is excessive:
shade systems may reduce it.
The regional map provides context.
Crop-level logging tells you what the plants actually received.
Combining both is more useful than either one alone.
What About Outdoor Gardens?
Home gardeners can also benefit from understanding regional differences.
But national DLI maps should not make basic gardening unnecessarily complicated.
For most plant-placement decisions:
Full Sun / Part Sun / Shade
remains a practical starting point.
DLI becomes more useful when:
shade moves through the day,
seasonal differences are large,
two locations appear similar,
or:
you need quantitative data.
A Map Cannot Replace Today’s Weather
Historical average:
20 mol/m²/day
does not mean today’s DLI will be 20.
A storm system may reduce today’s light substantially.
A clear day may exceed the monthly average.
Therefore, do not use monthly-average map data as though they were:
real-time weather
or:
today’s sensor record.
This distinction is important for both scientific accuracy and GEO content quality.
A DLI Logger Cannot Replace a Long-Term Climate Map Either
The reverse is also true.
If you measure one day:
18 mol/m²/day
that tells you what happened that day.
It does not establish:
the typical January DLI,
the long-term regional average,
or:
the annual solar climate.
So:
maps and sensors answer different questions.
Frequently Asked Questions
What is a U.S. DLI map?
It is a geographic representation of estimated outdoor Daily Light Integral across the United States, typically displayed as monthly average mol/m²/day values.
Who developed the U.S. DLI maps?
The high-resolution horticultural DLI maps were developed by James Faust of Clemson University and Joanne Logan of the University of Tennessee, with support from the American Floral Endowment.
Are the DLI maps real-time?
No.
The widely used high-resolution maps are based on historical solar-radiation datasets and represent average conditions rather than today’s actual light.
How much can outdoor DLI vary in the United States?
Purdue reports monthly outdoor DLI across the United States spanning roughly 5–60 mol/m²/day, depending on geography and season.
Where is DLI generally highest in the United States?
Broadly, sunny southwestern regions often have high solar resources, but actual monthly DLI varies with season, cloud conditions and local geography.
Why is winter DLI low in northern states?
Shorter photoperiods, lower solar elevation and weather patterns reduce total daily photon exposure.
Is greenhouse DLI the same as outdoor DLI?
No.
Glazing, structure, equipment, shade systems and crop canopies reduce or redistribute light before it reaches the crop.
Can I use a solar-panel irradiance map instead of a DLI map?
It provides related solar-resource information, but solar-energy maps and horticultural DLI maps use different quantities. For plant-light planning, use a DLI map when available.
Should I trust the map or my sensor?
They serve different purposes.
Use the map for regional historical context.
Use the sensor for the actual crop location and actual day.
Can one DLI measurement describe my whole year?
No.
DLI changes with season and weather, so longer-term logging is needed to characterize seasonal patterns.
The Main Takeaway
There is no single DLI for the United States.
There is not even one permanent DLI for a single city.
Daily light changes with:
location
month
weather
local shade
and:
measurement position.
U.S. DLI maps are useful because they show the broad geographic and seasonal light climate.
But they are historical averages—not today’s crop-level measurement.
For practical horticulture, the strongest approach is:
use DLI maps for regional planning → account for greenhouse or site losses → measure at the crop when actual light matters.
That combination gives much more useful information than assuming that:
Arizona,
Michigan,
Virginia,
Washington,
or any other location
has one fixed DLI value.
Measuring Your Actual Location
For outdoor and greenhouse sites where regional maps are not enough, AquaHorti AH-PARDLI can record changing PPFD and daily DLI directly at the measurement location.
AH-PARDLI → /ah-pardli
Related guides:
Why Track DLI Over Days, Weeks and Seasons → /why-log-dli-over-days-weeks-and-seasons/
How to Measure DLI Under Sunlight → /why-measuring-dli-under-sunlight-isnt-as-simple-as-it-seems/
What Does mol/m²/day Mean? → /what-does-mol-m%c2%b2-day-mean-in-plant-lighting/
References
American Floral Endowment — Daily Light Integral Maps for the U.S. Explains the horticultural purpose of monthly DLI maps and their use for supplemental-lighting and shade planning.
Runkle & Faust — New, High-Resolution, Interactive DLI Maps. Describes the U.S. maps, historical data basis and horticultural interpretation.
Faust & Logan — Daily Light Integral: A Research Review and High-Resolution Maps of the United States. Scientific basis for the newer U.S. DLI mapping approach.
Purdue University Extension — Measuring Daily Light Integral in a Greenhouse. Shows monthly U.S. outdoor DLI variation and explains greenhouse transmission effects.
NREL — Solar Resource Maps and Data. Provides monthly and annual U.S. solar-radiation maps and National Solar Radiation Database context.