George Landis Arboretum
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the George Landis Arboretum clear sky chart from 2005-11-06 to 2025-11-03.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	15% 10% 3 9% 6% 5% 5% 4 6% 7% 31%	126920
	Night	16% 10% 4 10% 6% 5% 5% 4 5% 7% 30%	123124
Transparency	Day	49% 8% 11% 21% 10% 1	130396
	Night	50% 8% 11% 19% 10% 1	127735
Seeing	Day	23% 19% 28% 18% 9% 3	16065
	Night	22% 18% 27% 24% 8% 1	25108

By Month

Percentage Cloud Cover NE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	11% 12% 6% 20% 10% 6% 5% 4 5% 4 18%	3597
	11th to 20th	Day	12% 15% 6% 13% 7% 6% 6% 4 6% 6% 20%	3570
	21st to end	Day	15% 14% 4 11% 6% 4 5% 4 5% 6% 25%	4163
February	1st to 10th	Day	14% 13% 4 11% 8% 7% 6% 5% 5% 6% 21%	1800
	11th to 20th	Day	10% 12% 5% 13% 8% 7% 6% 4 4 5% 25%	1892
	21st to end	Day	16% 12% 5% 12% 7% 5% 4 3 3 6% 26%	1650
March	1st to 10th	Day	19% 12% 5% 11% 5% 5% 4 3 4 5% 27%	4476
	11th to 20th	Day	14% 12% 5% 9% 5% 4 4 3 4 6% 34%	4790
	21st to end	Day	18% 10% 4 9% 5% 5% 4 3 4 5% 33%	5914
April	1st to 10th	Day	17% 12% 4 9% 6% 3 4 4 5% 5% 32%	2400
	11th to 20th	Day	16% 10% 3 8% 5% 5% 5% 3 6% 4 35%	2374
	21st to end	Day	19% 9% 3 8% 6% 4 5% 3 5% 6% 32%	2465
May	1st to 10th	Day	29% 11% 3 8% 5% 4 4 3 4 6% 23%	6088
	11th to 20th	Day	15% 10% 3 9% 5% 5% 5% 4 6% 7% 31%	6318
	21st to end	Day	17% 8% 3 8% 6% 5% 5% 4 7% 8% 29%	6949
June	1st to 10th	Day	16% 7% 3 7% 5% 5% 5% 4 7% 9% 32%	2800
	11th to 20th	Day	15% 7% 3 7% 5% 5% 5% 5% 7% 9% 32%	2800
	21st to end	Day	14% 7% 3 7% 5% 5% 6% 4 6% 10% 32%	2800
July	1st to 10th	Day	9% 7% 3 8% 6% 5% 6% 5% 8% 10% 34%	6360
	11th to 20th	Day	8% 5% 2 7% 6% 6% 6% 6% 8% 10% 37%	6360
	21st to end	Day	12% 6% 2 7% 5% 5% 5% 4 6% 8% 41%	6926
August	1st to 10th	Day	16% 6% 2 7% 5% 5% 5% 5% 6% 8% 34%	4499
	11th to 20th	Day	11% 7% 3 8% 6% 6% 6% 5% 7% 9% 31%	3886
	21st to end	Day	12% 6% 2 6% 4 5% 5% 4 7% 10% 38%	4312
September	1st to 10th	Day	12% 6% 2 6% 4 5% 5% 4 5% 7% 43%	2400
	11th to 20th	Day	12% 5% 2 6% 3 4 4 4 5% 8% 48%	2330
	21st to end	Day	14% 7% 3 6% 4 4 5% 4 7% 8% 39%	2200
October	1st to 10th	Day	17% 9% 2 7% 5% 5% 5% 4 6% 5% 35%	2054
	11th to 20th	Day	14% 7% 3 8% 4 4 4 4 6% 8% 37%	2000
	21st to end	Day	20% 10% 3 8% 5% 4 5% 4 5% 7% 29%	2012
November	1st to 10th	Day	13% 10% 3 7% 4 4 4 3 5% 7% 40%	1779
	11th to 20th	Day	15% 10% 3 8% 6% 4 4 3 5% 8% 34%	1800
	21st to end	Day	17% 12% 3 9% 6% 5% 4 4 6% 6% 29%	1610
December	1st to 10th	Day	15% 19% 6% 12% 7% 5% 5% 4 6% 5% 16%	2600
	11th to 20th	Day	17% 14% 5% 10% 6% 5% 4 4 5% 7% 24%	2986
	21st to end	Day	17% 14% 7% 9% 6% 5% 4 3 5% 7% 22%	3960
January	1st to 10th	Night	13% 11% 7% 21% 7% 5% 5% 4 5% 5% 18%	6349
	11th to 20th	Night	9% 13% 5% 17% 9% 7% 6% 4 6% 6% 17%	6315
	21st to end	Night	14% 13% 4 11% 7% 6% 6% 4 6% 7% 24%	6789
February	1st to 10th	Night	14% 13% 5% 11% 8% 5% 5% 4 5% 7% 22%	2600
	11th to 20th	Night	12% 11% 5% 13% 8% 5% 6% 5% 6% 7% 24%	2392
	21st to end	Night	15% 12% 5% 11% 6% 5% 4 4 5% 7% 25%	1980
March	1st to 10th	Night	17% 9% 4 11% 8% 6% 5% 3 5% 7% 24%	5364
	11th to 20th	Night	15% 12% 4 8% 5% 4 5% 3 5% 6% 34%	5166
	21st to end	Night	18% 12% 4 11% 6% 4 4 3 4 5% 29%	5017
April	1st to 10th	Night	18% 10% 4 8% 6% 5% 4 4 4 7% 29%	2000
	11th to 20th	Night	19% 10% 2 7% 5% 4 5% 3 4 6% 34%	1975
	21st to end	Night	19% 9% 2 6% 4 4 5% 4 5% 6% 36%	1933
May	1st to 10th	Night	32% 10% 3 7% 4 4 4 3 4 5% 23%	3882
	11th to 20th	Night	15% 12% 3 8% 5% 5% 5% 4 5% 7% 31%	3575
	21st to end	Night	19% 10% 4 8% 5% 5% 5% 4 5% 7% 29%	3926
June	1st to 10th	Night	17% 7% 2 7% 4 4 4 4 5% 7% 39%	1600
	11th to 20th	Night	15% 7% 2 5% 4 3 4 4 6% 10% 40%	1600
	21st to end	Night	12% 4 2 6% 6% 5% 4 4 6% 9% 41%	1600
July	1st to 10th	Night	9% 6% 2 9% 6% 5% 5% 4 6% 9% 39%	3600
	11th to 20th	Night	11% 8% 3 8% 4 6% 5% 4 6% 8% 37%	3600
	21st to end	Night	10% 6% 2 6% 4 4 4 4 6% 7% 46%	3926
August	1st to 10th	Night	18% 8% 2 7% 5% 4 4 3 5% 7% 36%	2646
	11th to 20th	Night	14% 7% 2 8% 5% 4 4 4 5% 7% 38%	3128
	21st to end	Night	12% 6% 2 6% 4 3 4 4 5% 9% 43%	3586
September	1st to 10th	Night	13% 5% 2 7% 3 4 4 3 6% 6% 46%	2000
	11th to 20th	Night	12% 5% 2 5% 4 3 3 2 5% 7% 52%	2070
	21st to end	Night	14% 6% 2 6% 4 4 4 4 5% 7% 43%	2200
October	1st to 10th	Night	18% 7% 3 7% 4 4 4 4 5% 6% 37%	2343
	11th to 20th	Night	15% 8% 2 7% 4 4 5% 3 5% 7% 39%	2400
	21st to end	Night	20% 10% 3 7% 5% 4 4 4 5% 8% 30%	2826
November	1st to 10th	Night	14% 8% 4 8% 5% 3 4 4 5% 6% 39%	2564
	11th to 20th	Night	16% 9% 3 9% 5% 5% 4 3 6% 6% 33%	2600
	21st to end	Night	14% 12% 4 8% 5% 5% 5% 4 5% 7% 30%	2788
December	1st to 10th	Night	15% 14% 6% 12% 6% 6% 5% 4 6% 7% 19%	4580
	11th to 20th	Night	21% 11% 4 11% 5% 5% 5% 4 4 5% 25%	5208
	21st to end	Night	20% 15% 7% 8% 5% 4 4 3 4 5% 23%	6996

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	61% 11% 8% 14% 4	3598
	11th to 20th	Day	59% 10% 9% 13% 7% 2	3600
	21st to end	Day	55% 8% 9% 18% 9% 1	4163
February	1st to 10th	Day	61% 8% 9% 14% 7% 1	1800
	11th to 20th	Day	59% 7% 8% 17% 8% 1	1901
	21st to end	Day	58% 6% 9% 15% 11% 1	1650
March	1st to 10th	Day	57% 6% 10% 15% 10% 3	4479
	11th to 20th	Day	48% 7% 15% 19% 7% 4	4790
	21st to end	Day	54% 6% 6% 14% 18% 2	5914
April	1st to 10th	Day	54% 5% 7% 21% 11% 2	2400
	11th to 20th	Day	50% 6% 10% 19% 13% 1	2374
	21st to end	Day	52% 7% 9% 18% 12% 3	2465
May	1st to 10th	Day	62% 6% 8% 15% 9% 1	6099
	11th to 20th	Day	51% 8% 12% 20% 9% 1	6322
	21st to end	Day	49% 8% 10% 23% 9% 1	6949
June	1st to 10th	Day	46% 9% 13% 21% 11% 1	2800
	11th to 20th	Day	45% 9% 13% 21% 11% 1	2800
	21st to end	Day	46% 8% 13% 22% 11% 1	2800
July	1st to 10th	Day	42% 9% 13% 24% 12%	6290
	11th to 20th	Day	37% 11% 14% 25% 12% 1	6285
	21st to end	Day	40% 7% 12% 30% 11%	6728
August	1st to 10th	Day	45% 9% 14% 21% 11% 1	4485
	11th to 20th	Day	44% 9% 16% 25% 5%	3923
	21st to end	Day	37% 9% 13% 31% 9% 1	4312
September	1st to 10th	Day	39% 8% 11% 27% 15% 1	2400
	11th to 20th	Day	34% 7% 11% 28% 19% 1	2330
	21st to end	Day	42% 8% 11% 25% 14% 2	2200
October	1st to 10th	Day	42% 8% 10% 24% 15% 1	3340
	11th to 20th	Day	39% 6% 13% 25% 15% 2	3240
	21st to end	Day	37% 8% 14% 27% 12% 2	3224
November	1st to 10th	Day	42% 7% 13% 24% 13% 1	1779
	11th to 20th	Day	48% 8% 11% 19% 14% 1	1800
	21st to end	Day	53% 7% 15% 17% 7% 1	1610
December	1st to 10th	Day	64% 9% 11% 12% 3	2600
	11th to 20th	Day	56% 9% 12% 16% 7% 1	2986
	21st to end	Day	61% 7% 8% 15% 8% 1	3960
January	1st to 10th	Night	61% 10% 9% 13% 6%	6349
	11th to 20th	Night	59% 11% 10% 12% 7% 1	6360
	21st to end	Night	55% 8% 10% 13% 12% 2	6789
February	1st to 10th	Night	59% 6% 10% 17% 7% 1	2600
	11th to 20th	Night	56% 9% 11% 15% 8% 1	2394
	21st to end	Night	56% 8% 11% 15% 8% 2	1980
March	1st to 10th	Night	55% 8% 12% 15% 9% 1	5374
	11th to 20th	Night	49% 7% 17% 18% 7% 2	5167
	21st to end	Night	56% 6% 6% 17% 14% 2	5017
April	1st to 10th	Night	54% 7% 10% 18% 10% 1	2000
	11th to 20th	Night	50% 6% 10% 19% 14% 1	1975
	21st to end	Night	48% 7% 10% 18% 15% 3	1933
May	1st to 10th	Night	62% 7% 8% 13% 9% 1	3888
	11th to 20th	Night	51% 8% 12% 21% 8% 1	3576
	21st to end	Night	52% 9% 11% 17% 10% 1	3927
June	1st to 10th	Night	44% 6% 11% 25% 13% 2	1600
	11th to 20th	Night	40% 7% 16% 24% 12% 1	1600
	21st to end	Night	37% 8% 15% 26% 13% 1	1600
July	1st to 10th	Night	40% 7% 16% 23% 14% 1	3560
	11th to 20th	Night	42% 9% 13% 22% 14%	3552
	21st to end	Night	36% 7% 12% 34% 12% 1	3809
August	1st to 10th	Night	46% 8% 13% 25% 8%	2636
	11th to 20th	Night	42% 9% 16% 24% 9%	3157
	21st to end	Night	35% 7% 13% 33% 10% 1	3586
September	1st to 10th	Night	37% 7% 11% 29% 15% 1	2000
	11th to 20th	Night	32% 5% 12% 28% 21% 1	2070
	21st to end	Night	40% 7% 11% 25% 15% 2	2200
October	1st to 10th	Night	46% 7% 11% 24% 11% 1	3837
	11th to 20th	Night	36% 7% 13% 25% 18% 1	3899
	21st to end	Night	41% 7% 13% 28% 10% 1	4564
November	1st to 10th	Night	43% 7% 10% 22% 15% 2	2564
	11th to 20th	Night	50% 7% 9% 19% 12% 2	2600
	21st to end	Night	50% 9% 14% 20% 6% 1	2788
December	1st to 10th	Night	59% 10% 12% 16% 3	4580
	11th to 20th	Night	57% 9% 9% 13% 11%	5208
	21st to end	Night	61% 8% 8% 12% 10% 1	6996

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	30% 27% 29% 12% 3 1	200
	11th to 20th	Day	34% 29% 28% 8% 2 1	200
	21st to end	Day	27% 28% 35% 6% 4	264
February	1st to 10th	Day	33% 32% 28% 7% 1	300
	11th to 20th	Day	32% 35% 23% 9% 1	440
	21st to end	Day	29% 26% 30% 12% 2	414
March	1st to 10th	Day	32% 25% 30% 11% 3	500
	11th to 20th	Day	32% 25% 29% 11% 4 1	500
	21st to end	Day	34% 27% 22% 12% 5% 1	550
April	1st to 10th	Day	35% 26% 20% 12% 6% 1	500
	11th to 20th	Day	29% 27% 25% 11% 6% 2	492
	21st to end	Day	32% 21% 28% 12% 5% 2	500
May	1st to 10th	Day	34% 21% 24% 13% 5% 3	503
	11th to 20th	Day	19% 22% 30% 20% 7% 2	527
	21st to end	Day	20% 13% 30% 23% 10% 3	573
June	1st to 10th	Day	20% 16% 32% 21% 9% 2	530
	11th to 20th	Day	19% 16% 30% 23% 11% 1	530
	21st to end	Day	18% 18% 32% 22% 9% 1	530
July	1st to 10th	Day	12% 11% 32% 26% 16% 3	525
	11th to 20th	Day	13% 7% 32% 31% 14% 3	524
	21st to end	Day	12% 11% 37% 27% 10% 3	576
August	1st to 10th	Day	10% 10% 30% 26% 18% 5%	526
	11th to 20th	Day	16% 8% 29% 25% 14% 8%	519
	21st to end	Day	13% 14% 28% 24% 16% 4	583
September	1st to 10th	Day	16% 12% 26% 23% 17% 8%	530
	11th to 20th	Day	12% 9% 26% 23% 18% 11%	530
	21st to end	Day	18% 12% 28% 24% 13% 5%	530
October	1st to 10th	Day	23% 14% 25% 18% 15% 4	530
	11th to 20th	Day	25% 16% 27% 20% 10% 1	530
	21st to end	Day	27% 19% 30% 18% 6%	478
November	1st to 10th	Day	26% 19% 23% 18% 12% 2	401
	11th to 20th	Day	27% 25% 25% 17% 6% 1	400
	21st to end	Day	31% 26% 25% 16% 1	210
December	1st to 10th	Day	29% 26% 29% 13% 4	200
	11th to 20th	Day	34% 28% 27% 11% 2	200
	21st to end	Day	25% 25% 32% 13% 4	220
January	1st to 10th	Night	23% 29% 31% 14% 2	800
	11th to 20th	Night	26% 25% 30% 17% 3	800
	21st to end	Night	19% 28% 34% 18% 2	880
February	1st to 10th	Night	25% 25% 32% 17% 2	800
	11th to 20th	Night	25% 28% 31% 15% 1	783
	21st to end	Night	28% 25% 28% 17% 2	660
March	1st to 10th	Night	26% 24% 33% 15% 3	800
	11th to 20th	Night	26% 25% 30% 17% 3	800
	21st to end	Night	25% 23% 29% 18% 5%	880
April	1st to 10th	Night	29% 22% 28% 19% 3	800
	11th to 20th	Night	26% 22% 27% 19% 6%	791
	21st to end	Night	24% 21% 26% 22% 6%	733
May	1st to 10th	Night	29% 15% 27% 20% 8% 1	532
	11th to 20th	Night	21% 13% 29% 29% 7% 1	394
	21st to end	Night	17% 10% 30% 30% 12% 1	433
June	1st to 10th	Night	19% 10% 28% 33% 11% 1	400
	11th to 20th	Night	17% 12% 23% 36% 13%	400
	21st to end	Night	14% 8% 28% 35% 14% 1	400
July	1st to 10th	Night	10% 7% 25% 39% 18% 1	390
	11th to 20th	Night	14% 5% 24% 40% 16% 1	388
	21st to end	Night	11% 6% 24% 44% 15% 1	438
August	1st to 10th	Night	12% 3 27% 35% 22%	432
	11th to 20th	Night	15% 5% 22% 36% 18% 4	745
	21st to end	Night	15% 9% 22% 33% 20% 1	880
September	1st to 10th	Night	14% 8% 25% 33% 19% 1	800
	11th to 20th	Night	13% 8% 23% 30% 21% 6%	800
	21st to end	Night	17% 10% 24% 31% 16% 2	800
October	1st to 10th	Night	21% 13% 23% 26% 15% 2	800
	11th to 20th	Night	21% 15% 27% 28% 8%	800
	21st to end	Night	27% 16% 28% 24% 5%	880
November	1st to 10th	Night	21% 19% 25% 28% 7%	789
	11th to 20th	Night	23% 22% 26% 21% 7% 2	800
	21st to end	Night	23% 23% 28% 21% 5%	800
December	1st to 10th	Night	25% 26% 30% 15% 4	800
	11th to 20th	Night	27% 25% 27% 18% 3	800
	21st to end	Night	28% 26% 25% 17% 3	880

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