Deer Run Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Deer Run Observatory clear sky chart from 2005-11-06 to 2025-06-10.

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	12% 6% 2 6% 5% 5% 5% 4 6% 8% 42%	119680
	Night	12% 6% 2 6% 4 4 4 4 6% 8% 44%	118551
Transparency	Day	38% 7% 11% 18% 19% 8%	123534
	Night	36% 6% 10% 18% 20% 9%	122834
Seeing	Day	14% 9% 24% 25% 19% 9%	15668
	Night	14% 8% 24% 30% 21% 4	25716

By Month

Percentage Cloud Cover SE (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	14% 6% 2 5% 3 3 3 2 4 5% 52%	4056
	11th to 20th	Day	14% 6% 2 4 3 3 3 3 5% 6% 53%	4060
	21st to end	Day	18% 7% 3 6% 4 4 4 3 4 6% 42%	4857
February	1st to 10th	Day	14% 8% 2 7% 4 4 5% 3 5% 7% 40%	2000
	11th to 20th	Day	18% 7% 2 6% 3 3 3 3 5% 6% 43%	1948
	21st to end	Day	15% 7% 2 6% 5% 4 4 3 5% 7% 43%	1650
March	1st to 10th	Day	19% 8% 2 7% 4 4 4 4 6% 6% 37%	4615
	11th to 20th	Day	8% 5% 2 5% 4 4 4 3 5% 6% 53%	4980
	21st to end	Day	16% 7% 2 6% 4 4 3 3 5% 6% 45%	5465
April	1st to 10th	Day	11% 6% 2 6% 5% 4 5% 4 6% 6% 46%	2242
	11th to 20th	Day	11% 5% 2 4 4 3 4 3 5% 7% 51%	2374
	21st to end	Day	7% 4 1 5% 4 4 4 4 5% 7% 54%	2500
May	1st to 10th	Day	11% 6% 2 7% 5% 5% 5% 4 6% 8% 40%	5862
	11th to 20th	Day	14% 7% 2 7% 4 4 5% 4 7% 9% 36%	5861
	21st to end	Day	6% 5% 2 6% 5% 5% 6% 4 6% 10% 44%	6433
June	1st to 10th	Day	10% 6% 3 7% 6% 7% 7% 6% 9% 9% 31%	2587
	11th to 20th	Day	5% 5% 2 6% 5% 5% 6% 6% 8% 12% 40%	2470
	21st to end	Day	5% 5% 2 7% 7% 8% 8% 6% 10% 9% 33%	2470
July	1st to 10th	Day	11% 8% 4 9% 8% 8% 8% 7% 10% 10% 19%	4120
	11th to 20th	Day	6% 5% 3 8% 8% 8% 7% 6% 8% 10% 31%	4120
	21st to end	Day	6% 7% 3 10% 8% 9% 9% 7% 10% 12% 20%	4466
August	1st to 10th	Day	9% 5% 3 7% 6% 6% 7% 6% 8% 13% 31%	3990
	11th to 20th	Day	6% 4 2 6% 5% 5% 6% 4 6% 10% 47%	3729
	21st to end	Day	8% 5% 2 5% 5% 6% 6% 5% 8% 10% 40%	4163
September	1st to 10th	Day	9% 8% 3 7% 6% 6% 7% 5% 7% 9% 34%	2090
	11th to 20th	Day	14% 6% 2 7% 5% 5% 4 4 7% 8% 38%	2090
	21st to end	Day	12% 6% 2 7% 5% 4 4 4 5% 6% 45%	2090
October	1st to 10th	Day	11% 6% 2 6% 4 3 3 3 4 6% 52%	2090
	11th to 20th	Day	10% 4 2 4 3 3 3 2 5% 7% 57%	2024
	21st to end	Day	10% 5% 1 6% 3 4 3 3 4 6% 56%	2090
November	1st to 10th	Day	11% 5% 2 5% 4 3 4 3 4 5% 54%	1819
	11th to 20th	Day	13% 5% 2 4 3 3 3 3 4 6% 55%	1800
	21st to end	Day	14% 6% 3 5% 3 4 3 3 5% 7% 47%	1800
December	1st to 10th	Day	15% 6% 2 4 4 3 4 3 4 6% 49%	2930
	11th to 20th	Day	14% 7% 3 6% 4 3 4 4 5% 6% 46%	3373
	21st to end	Day	19% 8% 3 6% 3 3 3 2 3 5% 43%	4466
January	1st to 10th	Night	14% 6% 2 4 3 3 4 3 5% 6% 51%	5889
	11th to 20th	Night	12% 7% 3 7% 4 4 4 3 4 6% 45%	5900
	21st to end	Night	18% 8% 3 6% 3 3 3 3 5% 7% 42%	6096
February	1st to 10th	Night	14% 7% 3 6% 4 4 4 3 5% 6% 44%	2400
	11th to 20th	Night	19% 7% 2 5% 4 3 3 2 4 6% 45%	2338
	21st to end	Night	15% 7% 2 5% 3 3 4 4 5% 7% 44%	1980
March	1st to 10th	Night	20% 9% 2 7% 4 4 3 3 4 6% 38%	5235
	11th to 20th	Night	11% 5% 2 5% 4 4 4 4 6% 7% 50%	4980
	21st to end	Night	13% 6% 2 5% 4 4 5% 4 5% 5% 47%	5467
April	1st to 10th	Night	10% 6% 2 5% 4 4 4 3 7% 7% 48%	2173
	11th to 20th	Night	11% 5% 1 4 4 3 4 3 5% 8% 51%	1975
	21st to end	Night	7% 4 2 6% 4 3 4 3 5% 7% 55%	1898
May	1st to 10th	Night	9% 6% 2 7% 5% 5% 4 4 6% 11% 42%	4033
	11th to 20th	Night	16% 5% 2 6% 4 4 4 5% 7% 9% 38%	4032
	21st to end	Night	7% 4 2 5% 5% 4 5% 4 7% 12% 45%	4421
June	1st to 10th	Night	8% 6% 3 7% 6% 7% 6% 5% 7% 11% 33%	1797
	11th to 20th	Night	3 3 2 5% 5% 5% 6% 6% 8% 14% 42%	1710
	21st to end	Night	5% 5% 2 7% 6% 6% 6% 6% 8% 12% 36%	1710
July	1st to 10th	Night	10% 8% 3 9% 7% 7% 8% 7% 9% 11% 21%	2840
	11th to 20th	Night	4 4 2 8% 6% 6% 7% 6% 11% 13% 34%	2840
	21st to end	Night	8% 8% 3 10% 6% 7% 7% 7% 9% 11% 25%	3087
August	1st to 10th	Night	9% 6% 3 8% 5% 7% 6% 5% 9% 12% 30%	2934
	11th to 20th	Night	6% 3 1 6% 4 4 6% 5% 8% 12% 46%	3100
	21st to end	Night	7% 5% 2 5% 5% 5% 5% 4 8% 10% 45%	3476
September	1st to 10th	Night	11% 7% 3 7% 5% 6% 5% 4 5% 12% 37%	2072
	11th to 20th	Night	13% 7% 3 6% 4 4 4 4 5% 8% 40%	2090
	21st to end	Night	9% 5% 2 7% 4 5% 5% 4 6% 8% 45%	2090
October	1st to 10th	Night	10% 5% 2 5% 4 3 4 3 5% 7% 51%	2090
	11th to 20th	Night	10% 3 2 4 3 3 3 3 5% 5% 59%	2154
	21st to end	Night	10% 5% 1 5% 3 4 3 3 4 5% 57%	2508
November	1st to 10th	Night	11% 6% 1 4 3 4 4 3 5% 5% 55%	2474
	11th to 20th	Night	14% 5% 2 4 3 3 3 2 5% 5% 55%	2600
	21st to end	Night	13% 6% 2 5% 4 3 4 3 4 6% 49%	2600
December	1st to 10th	Night	13% 6% 2 4 3 3 2 3 4 6% 54%	4250
	11th to 20th	Night	15% 5% 2 5% 3 4 3 3 4 6% 50%	4822
	21st to end	Night	19% 8% 3 6% 4 3 3 2 4 5% 43%	6490

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	35% 2 4 17% 33% 9%	4057
	11th to 20th	Day	32% 3 4 16% 32% 13%	4060
	21st to end	Day	44% 4 7% 14% 19% 13%	4857
February	1st to 10th	Day	44% 4 7% 16% 20% 9%	2000
	11th to 20th	Day	42% 5% 5% 15% 24% 9%	1958
	21st to end	Day	42% 4 6% 17% 20% 11%	1650
March	1st to 10th	Day	46% 5% 7% 13% 19% 9%	4615
	11th to 20th	Day	30% 5% 8% 13% 21% 22%	4980
	21st to end	Day	40% 4 8% 18% 18% 11%	5465
April	1st to 10th	Day	37% 6% 8% 21% 17% 11%	2242
	11th to 20th	Day	33% 4 8% 20% 24% 11%	2374
	21st to end	Day	28% 5% 9% 26% 24% 7%	2500
May	1st to 10th	Day	40% 6% 13% 22% 16% 3	5897
	11th to 20th	Day	41% 8% 17% 22% 11% 1	5865
	21st to end	Day	33% 8% 15% 24% 19% 1	6447
June	1st to 10th	Day	44% 9% 16% 19% 11% 2	2587
	11th to 20th	Day	32% 10% 17% 23% 16% 2	2470
	21st to end	Day	40% 13% 16% 17% 12% 2	2470
July	1st to 10th	Day	50% 16% 21% 9% 3 1	4055
	11th to 20th	Day	41% 14% 21% 19% 4 1	4051
	21st to end	Day	50% 16% 18% 12% 4	4324
August	1st to 10th	Day	40% 11% 19% 23% 7%	3977
	11th to 20th	Day	31% 8% 15% 29% 15% 1	3765
	21st to end	Day	34% 11% 18% 25% 11% 1	4163
September	1st to 10th	Day	43% 10% 16% 14% 14% 3	2090
	11th to 20th	Day	42% 8% 9% 19% 17% 4	2090
	21st to end	Day	40% 5% 8% 20% 20% 6%	2090
October	1st to 10th	Day	33% 3 7% 12% 25% 20%	3480
	11th to 20th	Day	17% 3 4 17% 24% 35%	3342
	21st to end	Day	21% 3 4 18% 39% 16%	3428
November	1st to 10th	Day	33% 5% 7% 14% 26% 15%	1819
	11th to 20th	Day	32% 4 6% 16% 28% 15%	1800
	21st to end	Day	37% 4 7% 19% 23% 11%	1800
December	1st to 10th	Day	37% 4 7% 16% 27% 9%	2930
	11th to 20th	Day	39% 4 7% 20% 23% 8%	3370
	21st to end	Day	45% 3 4 20% 24% 3	4466
January	1st to 10th	Night	34% 4 5% 15% 32% 10%	5889
	11th to 20th	Night	40% 4 4 17% 24% 10%	5900
	21st to end	Night	43% 3 5% 16% 24% 9%	6096
February	1st to 10th	Night	41% 3 7% 16% 22% 12%	2400
	11th to 20th	Night	42% 3 5% 15% 22% 13%	2339
	21st to end	Night	38% 5% 7% 16% 17% 16%	1980
March	1st to 10th	Night	46% 5% 8% 14% 17% 10%	5235
	11th to 20th	Night	32% 5% 8% 13% 23% 19%	4980
	21st to end	Night	38% 4 7% 17% 22% 13%	5467
April	1st to 10th	Night	34% 5% 11% 18% 18% 13%	2173
	11th to 20th	Night	32% 4 8% 21% 24% 12%	1975
	21st to end	Night	28% 5% 10% 26% 23% 8%	1898
May	1st to 10th	Night	35% 9% 17% 20% 15% 4	4060
	11th to 20th	Night	39% 8% 19% 22% 9% 2	4033
	21st to end	Night	29% 8% 17% 26% 18% 2	4429
June	1st to 10th	Night	42% 10% 15% 19% 13% 1	1797
	11th to 20th	Night	28% 10% 19% 27% 13% 2	1710
	21st to end	Night	36% 12% 19% 17% 12% 3	1710
July	1st to 10th	Night	51% 17% 20% 9% 3 1	2795
	11th to 20th	Night	34% 16% 27% 17% 5% 1	2786
	21st to end	Night	49% 15% 18% 14% 4 1	2988
August	1st to 10th	Night	40% 12% 20% 22% 6%	2923
	11th to 20th	Night	28% 10% 19% 29% 13% 1	3130
	21st to end	Night	30% 10% 20% 28% 11% 1	3476
September	1st to 10th	Night	40% 9% 17% 16% 14% 5%	2072
	11th to 20th	Night	42% 8% 13% 16% 18% 4	2090
	21st to end	Night	36% 8% 11% 17% 21% 7%	2090
October	1st to 10th	Night	34% 5% 7% 13% 22% 19%	3480
	11th to 20th	Night	17% 2 5% 12% 30% 34%	3575
	21st to end	Night	20% 3 3 19% 34% 20%	4120
November	1st to 10th	Night	32% 4 8% 15% 27% 15%	2474
	11th to 20th	Night	33% 3 5% 15% 28% 16%	2600
	21st to end	Night	37% 4 7% 18% 22% 12%	2600
December	1st to 10th	Night	33% 3 6% 21% 26% 11%	4250
	11th to 20th	Night	36% 4 8% 20% 24% 8%	4824
	21st to end	Night	44% 3 5% 19% 25% 3	6490

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	20% 24% 43% 9% 3	300
	11th to 20th	Day	19% 23% 36% 14% 6% 2	300
	21st to end	Day	23% 27% 39% 8% 2	330
February	1st to 10th	Day	18% 24% 40% 13% 4	300
	11th to 20th	Day	29% 18% 38% 13% 1	300
	21st to end	Day	21% 21% 45% 11% 2	249
March	1st to 10th	Day	20% 21% 32% 21% 5% 1	357
	11th to 20th	Day	16% 17% 40% 18% 8% 1	500
	21st to end	Day	20% 15% 37% 21% 7% 1	550
April	1st to 10th	Day	17% 17% 30% 25% 10% 1	500
	11th to 20th	Day	15% 13% 32% 26% 13% 2	492
	21st to end	Day	10% 10% 29% 31% 17% 3	500
May	1st to 10th	Day	9% 12% 26% 30% 18% 5%	503
	11th to 20th	Day	11% 4 26% 30% 20% 9%	527
	21st to end	Day	11% 4 19% 32% 24% 10%	573
June	1st to 10th	Day	10% 3 16% 33% 26% 12%	526
	11th to 20th	Day	8% 3 15% 27% 27% 21%	490
	21st to end	Day	5% 2 15% 31% 30% 16%	490
July	1st to 10th	Day	8% 2 7% 24% 33% 26%	485
	11th to 20th	Day	7% 1 5% 19% 36% 31%	484
	21st to end	Day	5% 2 7% 24% 38% 24%	532
August	1st to 10th	Day	10% 1 9% 28% 33% 19%	486
	11th to 20th	Day	8% 1 7% 26% 34% 23%	479
	21st to end	Day	12% 3 9% 28% 31% 17%	539
September	1st to 10th	Day	9% 4 14% 26% 33% 14%	490
	11th to 20th	Day	16% 3 14% 34% 24% 9%	490
	21st to end	Day	16% 4 14% 27% 28% 12%	490
October	1st to 10th	Day	14% 6% 28% 28% 19% 6%	490
	11th to 20th	Day	11% 7% 23% 33% 19% 6%	490
	21st to end	Day	14% 9% 26% 29% 20% 3	539
November	1st to 10th	Day	18% 8% 24% 33% 13% 4	357
	11th to 20th	Day	14% 15% 39% 22% 7% 2	300
	21st to end	Day	15% 13% 39% 23% 9% 1	300
December	1st to 10th	Day	22% 18% 34% 21% 4 1	300
	11th to 20th	Day	24% 18% 35% 16% 7%	300
	21st to end	Day	25% 15% 32% 23% 5% 1	330
January	1st to 10th	Night	19% 17% 35% 24% 5%	800
	11th to 20th	Night	19% 14% 28% 29% 9%	800
	21st to end	Night	21% 15% 37% 23% 4	880
February	1st to 10th	Night	18% 11% 32% 31% 8%	800
	11th to 20th	Night	22% 13% 33% 26% 5%	783
	21st to end	Night	17% 14% 39% 27% 3	660
March	1st to 10th	Night	16% 14% 31% 32% 7%	800
	11th to 20th	Night	15% 13% 33% 27% 11% 1	800
	21st to end	Night	14% 14% 32% 29% 10%	880
April	1st to 10th	Night	13% 11% 32% 32% 11%	773
	11th to 20th	Night	14% 10% 28% 32% 15% 1	593
	21st to end	Night	7% 7% 29% 37% 21% 1	600
May	1st to 10th	Night	9% 7% 26% 37% 20% 2	600
	11th to 20th	Night	10% 3 25% 35% 22% 5%	591
	21st to end	Night	9% 1 17% 36% 33% 4	650
June	1st to 10th	Night	8% 3 16% 37% 33% 5%	599
	11th to 20th	Night	4 2 10% 33% 38% 13%	570
	21st to end	Night	6% 2 8% 35% 39% 10%	570
July	1st to 10th	Night	8% 2 5% 27% 41% 17%	555
	11th to 20th	Night	8% 1 3 23% 47% 17%	552
	21st to end	Night	4 2 6% 28% 48% 12%	624
August	1st to 10th	Night	8% 2 9% 27% 41% 14%	567
	11th to 20th	Night	10% 1 5% 25% 46% 12%	564
	21st to end	Night	14% 2 12% 23% 36% 14%	627
September	1st to 10th	Night	12% 2 12% 27% 38% 9%	742
	11th to 20th	Night	15% 2 14% 33% 30% 5%	760
	21st to end	Night	11% 2 15% 33% 35% 4	760
October	1st to 10th	Night	13% 3 23% 35% 22% 4	760
	11th to 20th	Night	12% 4 24% 31% 26% 3	760
	21st to end	Night	14% 8% 21% 33% 23% 1	836
November	1st to 10th	Night	14% 8% 24% 33% 20% 2	780
	11th to 20th	Night	16% 10% 30% 31% 13% 1	800
	21st to end	Night	16% 11% 29% 30% 13% 1	800
December	1st to 10th	Night	17% 10% 32% 29% 11% 1	800
	11th to 20th	Night	19% 10% 32% 30% 9% 1	800
	21st to end	Night	25% 12% 27% 28% 8%	880

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