Clarkson Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Clarkson Observatory clear sky chart from 2005-11-06 to 2026-06-16.

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	18% 12% 4 9% 5% 5% 4 3 5% 6% 28%	137998
	Night	18% 15% 5% 10% 5% 4 4 3 4 5% 26%	134026
Transparency	Day	55% 6% 11% 19% 8% 1	141452
	Night	60% 5% 10% 16% 8% 1	138661
Seeing	Day	28% 17% 29% 18% 6% 2	16562
	Night	27% 16% 29% 20% 6% 1	25665

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	14% 22% 8% 18% 7% 6% 4 3 3 4 12%	4661
	11th to 20th	Day	10% 21% 9% 21% 9% 6% 4 2 3 4 11%	4650
	21st to end	Day	11% 18% 8% 15% 9% 11% 5% 3 3 3 13%	5286
February	1st to 10th	Day	16% 18% 5% 15% 7% 6% 4 3 5% 4 18%	1890
	11th to 20th	Day	14% 17% 7% 13% 7% 5% 4 3 4 5% 20%	1907
	21st to end	Day	18% 20% 6% 10% 7% 5% 4 3 4 5% 18%	1730
March	1st to 10th	Day	20% 15% 6% 12% 5% 4 5% 3 4 5% 22%	4576
	11th to 20th	Day	11% 17% 4 10% 6% 5% 5% 4 5% 5% 27%	4985
	21st to end	Day	20% 12% 4 10% 5% 5% 3 2 3 4 31%	5979
April	1st to 10th	Day	21% 15% 4 8% 4 3 4 3 4 4 29%	2520
	11th to 20th	Day	22% 12% 3 7% 4 4 4 2 4 4 34%	2494
	21st to end	Day	23% 11% 4 6% 4 4 4 3 4 5% 32%	2611
May	1st to 10th	Day	25% 12% 4 8% 5% 5% 4 3 5% 6% 24%	8237
	11th to 20th	Day	19% 9% 3 8% 5% 5% 4 4 5% 7% 30%	8221
	21st to end	Day	24% 9% 3 6% 4 4 4 4 5% 6% 31%	9060
June	1st to 10th	Day	24% 7% 2 7% 5% 5% 4 5% 6% 7% 30%	2940
	11th to 20th	Day	18% 6% 2 6% 5% 3 5% 5% 6% 9% 33%	2870
	21st to end	Day	22% 8% 3 6% 5% 5% 4 4 5% 7% 31%	2800
July	1st to 10th	Day	14% 6% 3 7% 5% 5% 5% 5% 7% 9% 35%	6360
	11th to 20th	Day	12% 6% 3 6% 5% 5% 5% 4 6% 9% 39%	6360
	21st to end	Day	13% 6% 2 5% 4 4 4 4 6% 10% 42%	6926
August	1st to 10th	Day	17% 7% 3 7% 5% 4 5% 4 6% 8% 34%	4540
	11th to 20th	Day	17% 7% 3 7% 4 4 5% 4 6% 8% 36%	4022
	21st to end	Day	17% 6% 3 6% 4 4 4 4 6% 8% 38%	4312
September	1st to 10th	Day	16% 6% 2 6% 4 4 4 3 6% 7% 42%	2400
	11th to 20th	Day	14% 6% 2 5% 3 4 4 3 5% 7% 46%	2365
	21st to end	Day	19% 5% 2 6% 5% 5% 4 4 6% 6% 38%	2192
October	1st to 10th	Day	21% 8% 3 8% 4 4 4 3 5% 6% 34%	2000
	11th to 20th	Day	20% 12% 4 8% 5% 5% 5% 4 4 6% 27%	2000
	21st to end	Day	27% 14% 3 7% 4 4 4 2 5% 5% 26%	2010
November	1st to 10th	Day	17% 14% 4 9% 6% 5% 4 3 5% 5% 28%	1851
	11th to 20th	Day	15% 17% 5% 9% 5% 5% 5% 4 4 4 27%	1769
	21st to end	Day	19% 20% 5% 12% 5% 4 4 3 3 5% 18%	1680
December	1st to 10th	Day	19% 22% 7% 17% 6% 4 3 4 3 3 10%	2680
	11th to 20th	Day	18% 19% 7% 12% 5% 3 4 3 4 4 20%	3066
	21st to end	Day	20% 20% 5% 13% 5% 4 4 3 3 4 20%	4048
January	1st to 10th	Night	14% 24% 9% 18% 6% 4 4 3 3 3 14%	8234
	11th to 20th	Night	13% 22% 10% 19% 8% 5% 4 2 3 3 10%	8235
	21st to end	Night	13% 17% 7% 16% 9% 9% 5% 3 3 4 15%	8880
February	1st to 10th	Night	16% 19% 7% 14% 8% 5% 3 3 4 4 17%	2730
	11th to 20th	Night	15% 17% 7% 12% 7% 5% 5% 3 4 4 22%	2597
	21st to end	Night	16% 18% 5% 13% 6% 3 3 3 5% 5% 23%	2076
March	1st to 10th	Night	19% 16% 5% 10% 6% 4 3 2 3 4 27%	5484
	11th to 20th	Night	15% 17% 5% 7% 4 4 4 4 5% 7% 28%	5191
	21st to end	Night	19% 16% 5% 11% 5% 4 3 2 3 4 28%	5195
April	1st to 10th	Night	21% 14% 4 9% 5% 3 3 2 4 6% 29%	2100
	11th to 20th	Night	23% 12% 3 7% 5% 3 3 3 3 5% 32%	2075
	21st to end	Night	23% 12% 3 7% 4 4 3 3 4 5% 34%	2016
May	1st to 10th	Night	26% 12% 3 8% 4 4 5% 3 6% 7% 22%	4684
	11th to 20th	Night	21% 10% 3 7% 4 4 4 4 5% 7% 29%	4639
	21st to end	Night	21% 9% 3 6% 4 4 4 4 6% 8% 31%	5114
June	1st to 10th	Night	22% 7% 3 6% 6% 5% 5% 4 6% 7% 31%	1680
	11th to 20th	Night	18% 6% 3 6% 4 4 4 4 5% 9% 37%	1640
	21st to end	Night	20% 7% 3 6% 6% 5% 6% 5% 5% 7% 30%	1600
July	1st to 10th	Night	16% 5% 2 6% 5% 5% 4 4 7% 8% 37%	3600
	11th to 20th	Night	16% 7% 2 6% 4 4 4 3 5% 8% 41%	3600
	21st to end	Night	12% 7% 2 7% 5% 4 5% 4 5% 8% 42%	3926
August	1st to 10th	Night	17% 9% 3 7% 4 4 4 3 5% 7% 37%	2606
	11th to 20th	Night	18% 8% 3 8% 4 4 4 3 5% 7% 37%	2993
	21st to end	Night	15% 6% 3 6% 4 4 4 4 6% 7% 40%	3586
September	1st to 10th	Night	15% 7% 2 6% 4 4 4 3 5% 6% 43%	2000
	11th to 20th	Night	15% 6% 2 4 3 4 3 3 4 7% 50%	2035
	21st to end	Night	17% 8% 3 6% 4 3 4 4 5% 7% 38%	2205
October	1st to 10th	Night	21% 8% 3 6% 4 4 4 3 5% 6% 35%	2400
	11th to 20th	Night	20% 10% 2 7% 5% 4 5% 3 5% 6% 33%	2400
	21st to end	Night	27% 12% 3 8% 5% 4 4 3 4 6% 25%	2830
November	1st to 10th	Night	21% 13% 4 7% 4 4 4 3 5% 5% 31%	2667
	11th to 20th	Night	18% 15% 4 8% 5% 5% 4 3 4 5% 29%	2850
	21st to end	Night	22% 18% 5% 10% 5% 4 4 3 4 5% 20%	2940
December	1st to 10th	Night	18% 27% 8% 14% 6% 4 3 3 3 4 10%	4720
	11th to 20th	Night	21% 20% 6% 12% 5% 4 3 2 3 3 20%	5348
	21st to end	Night	22% 19% 5% 13% 5% 5% 4 2 3 4 18%	7150

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	76% 6% 7% 9% 2	4662
	11th to 20th	Day	78% 5% 6% 5% 6% 1	4680
	21st to end	Day	76% 4 7% 7% 6%	5286
February	1st to 10th	Day	70% 5% 6% 15% 3 1	1890
	11th to 20th	Day	68% 4 8% 13% 6% 1	1916
	21st to end	Day	68% 5% 7% 11% 6% 1	1730
March	1st to 10th	Day	64% 5% 9% 13% 7% 2	4579
	11th to 20th	Day	57% 7% 14% 13% 8% 1	4985
	21st to end	Day	56% 5% 6% 15% 16% 2	5979
April	1st to 10th	Day	59% 4 7% 18% 10% 2	2520
	11th to 20th	Day	54% 4 7% 18% 14% 2	2494
	21st to end	Day	55% 4 9% 16% 13% 3	2611
May	1st to 10th	Day	59% 6% 8% 18% 8% 1	8249
	11th to 20th	Day	53% 8% 11% 20% 8% 1	8225
	21st to end	Day	53% 6% 9% 23% 9% 1	9061
June	1st to 10th	Day	52% 7% 10% 18% 12% 1	2940
	11th to 20th	Day	46% 7% 12% 24% 10% 2	2870
	21st to end	Day	52% 8% 10% 20% 10%	2800
July	1st to 10th	Day	43% 7% 14% 26% 10%	6290
	11th to 20th	Day	40% 7% 15% 27% 12%	6285
	21st to end	Day	37% 7% 13% 37% 7%	6728
August	1st to 10th	Day	47% 6% 15% 24% 9%	4526
	11th to 20th	Day	45% 7% 16% 25% 6%	4061
	21st to end	Day	43% 6% 17% 24% 10%	4312
September	1st to 10th	Day	41% 6% 14% 28% 12% 1	2400
	11th to 20th	Day	37% 5% 12% 31% 14% 1	2365
	21st to end	Day	45% 7% 13% 24% 10%	2192
October	1st to 10th	Day	46% 5% 12% 27% 10%	3260
	11th to 20th	Day	51% 4 14% 23% 7% 1	3240
	21st to end	Day	50% 5% 17% 23% 6% 1	3222
November	1st to 10th	Day	57% 6% 11% 20% 6%	1851
	11th to 20th	Day	60% 6% 10% 14% 8% 2	1769
	21st to end	Day	70% 6% 11% 9% 4	1680
December	1st to 10th	Day	79% 4 9% 6% 1	2680
	11th to 20th	Day	69% 6% 8% 13% 5%	3066
	21st to end	Day	69% 4 7% 14% 5%	4048
January	1st to 10th	Night	76% 5% 9% 8% 2	8234
	11th to 20th	Night	80% 5% 5% 6% 4	8280
	21st to end	Night	74% 4 7% 8% 6%	8880
February	1st to 10th	Night	72% 4 8% 10% 5%	2730
	11th to 20th	Night	66% 5% 8% 15% 6%	2599
	21st to end	Night	65% 5% 8% 15% 8%	2076
March	1st to 10th	Night	62% 4 6% 16% 10% 1	5494
	11th to 20th	Night	55% 7% 16% 13% 7% 2	5192
	21st to end	Night	61% 4 5% 19% 10% 2	5195
April	1st to 10th	Night	58% 5% 8% 17% 11% 1	2100
	11th to 20th	Night	57% 5% 8% 17% 12% 1	2075
	21st to end	Night	53% 4 9% 18% 13% 3	2016
May	1st to 10th	Night	60% 6% 12% 15% 7% 1	4690
	11th to 20th	Night	53% 7% 11% 19% 10% 1	4640
	21st to end	Night	51% 6% 11% 19% 13% 1	5115
June	1st to 10th	Night	50% 8% 12% 18% 11% 1	1680
	11th to 20th	Night	44% 7% 14% 19% 16% 1	1640
	21st to end	Night	51% 8% 12% 19% 10%	1600
July	1st to 10th	Night	44% 5% 17% 22% 11%	3560
	11th to 20th	Night	40% 6% 15% 26% 11% 1	3552
	21st to end	Night	40% 7% 12% 29% 12%	3809
August	1st to 10th	Night	47% 6% 14% 23% 9%	2596
	11th to 20th	Night	46% 6% 16% 22% 8% 1	3020
	21st to end	Night	41% 6% 15% 26% 11% 1	3586
September	1st to 10th	Night	41% 6% 12% 28% 12% 1	2000
	11th to 20th	Night	35% 6% 14% 31% 13%	2035
	21st to end	Night	44% 6% 11% 25% 13% 1	2205
October	1st to 10th	Night	47% 5% 12% 25% 10% 1	3920
	11th to 20th	Night	48% 5% 14% 21% 11% 1	3899
	21st to end	Night	54% 5% 16% 19% 6% 1	4568
November	1st to 10th	Night	56% 6% 12% 17% 8% 1	2667
	11th to 20th	Night	59% 4 12% 15% 7% 3	2850
	21st to end	Night	68% 5% 11% 11% 4	2940
December	1st to 10th	Night	80% 5% 7% 7% 1	4720
	11th to 20th	Night	71% 5% 6% 12% 5%	5348
	21st to end	Night	71% 5% 6% 10% 6% 1	7150

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	31% 17% 35% 12% 4	220
	11th to 20th	Day	33% 26% 29% 10% 2	220
	21st to end	Day	29% 23% 32% 15% 1	272
February	1st to 10th	Day	37% 27% 28% 7% 1	330
	11th to 20th	Day	31% 30% 32% 6%	394
	21st to end	Day	37% 24% 31% 8% 1	446
March	1st to 10th	Day	33% 22% 30% 12% 3	540
	11th to 20th	Day	37% 22% 27% 11% 2 1	540
	21st to end	Day	39% 20% 25% 10% 5%	594
April	1st to 10th	Day	39% 20% 26% 11% 3	540
	11th to 20th	Day	36% 20% 24% 14% 4 2	532
	21st to end	Day	36% 19% 28% 10% 5% 1	540
May	1st to 10th	Day	32% 17% 29% 15% 6% 1	543
	11th to 20th	Day	24% 20% 30% 18% 8% 1	567
	21st to end	Day	24% 17% 28% 22% 8%	617
June	1st to 10th	Day	30% 15% 29% 18% 6% 1	570
	11th to 20th	Day	19% 17% 35% 20% 6% 2	550
	21st to end	Day	27% 15% 34% 15% 8% 1	530
July	1st to 10th	Day	17% 11% 34% 26% 10% 1	525
	11th to 20th	Day	15% 13% 34% 29% 7% 2	524
	21st to end	Day	18% 11% 34% 29% 8% 1	576
August	1st to 10th	Day	17% 11% 28% 32% 10% 2	526
	11th to 20th	Day	18% 10% 26% 30% 13% 4	519
	21st to end	Day	19% 12% 33% 21% 11% 4	583
September	1st to 10th	Day	19% 13% 28% 24% 11% 6%	530
	11th to 20th	Day	14% 11% 25% 26% 16% 8%	530
	21st to end	Day	20% 11% 31% 24% 11% 3	530
October	1st to 10th	Day	26% 16% 25% 19% 9% 4	530
	11th to 20th	Day	28% 20% 31% 15% 6% 1	530
	21st to end	Day	38% 15% 27% 16% 4	478
November	1st to 10th	Day	33% 18% 29% 15% 4	425
	11th to 20th	Day	31% 23% 28% 10% 6% 1	309
	21st to end	Day	35% 24% 31% 8% 2	220
December	1st to 10th	Day	38% 15% 30% 16% 1	220
	11th to 20th	Day	32% 30% 30% 6% 1	220
	21st to end	Day	37% 20% 29% 11% 2 1	242
January	1st to 10th	Night	31% 22% 32% 12% 2	840
	11th to 20th	Night	33% 22% 27% 15% 3	840
	21st to end	Night	22% 23% 32% 18% 4	924
February	1st to 10th	Night	29% 22% 33% 14% 3	840
	11th to 20th	Night	27% 22% 35% 14% 2	823
	21st to end	Night	29% 18% 35% 15% 2	692
March	1st to 10th	Night	31% 20% 30% 18% 2	840
	11th to 20th	Night	32% 20% 30% 15% 3	840
	21st to end	Night	32% 18% 26% 19% 4 1	924
April	1st to 10th	Night	33% 17% 30% 18% 3	840
	11th to 20th	Night	34% 17% 27% 16% 6%	831
	21st to end	Night	32% 15% 31% 17% 5%	756
May	1st to 10th	Night	30% 11% 33% 20% 5% 1	431
	11th to 20th	Night	23% 12% 35% 24% 6%	414
	21st to end	Night	19% 14% 33% 26% 7%	455
June	1st to 10th	Night	25% 10% 27% 31% 5% 1	420
	11th to 20th	Night	21% 10% 30% 27% 10%	410
	21st to end	Night	21% 10% 35% 27% 8% 1	400
July	1st to 10th	Night	12% 10% 32% 37% 8% 1	390
	11th to 20th	Night	13% 6% 33% 36% 12%	388
	21st to end	Night	16% 7% 34% 36% 7% 1	438
August	1st to 10th	Night	17% 6% 27% 36% 14%	405
	11th to 20th	Night	20% 6% 23% 33% 16% 1	660
	21st to end	Night	17% 9% 32% 30% 11% 2	880
September	1st to 10th	Night	18% 10% 29% 30% 13% 1	800
	11th to 20th	Night	17% 9% 23% 29% 19% 3	800
	21st to end	Night	21% 13% 25% 26% 15% 1	800
October	1st to 10th	Night	26% 12% 29% 21% 12%	800
	11th to 20th	Night	28% 15% 27% 23% 6% 1	800
	21st to end	Night	37% 15% 26% 20% 3	880
November	1st to 10th	Night	31% 17% 30% 17% 5%	820
	11th to 20th	Night	30% 19% 27% 14% 8% 1	840
	21st to end	Night	33% 22% 26% 13% 5%	840
December	1st to 10th	Night	37% 21% 25% 15% 2	840
	11th to 20th	Night	33% 23% 28% 12% 3 1	840
	21st to end	Night	37% 18% 28% 14% 3	924

Page generated at 2026-06-13 02:31:43. Version ac07

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