Union College Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Union College Observatory clear sky chart from 2005-11-06 to 2026-02-16.

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	15% 8% 3 8% 5% 5% 5% 4 6% 7% 33%	130840
	Night	16% 9% 3 8% 5% 5% 5% 4 5% 7% 33%	129991
Transparency	Day	47% 7% 12% 21% 11% 1	134316
	Night	49% 7% 11% 20% 12% 1	134589
Seeing	Day	21% 16% 28% 22% 11% 3	16333
	Night	20% 13% 31% 27% 8% 1	25523

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	10% 10% 5% 15% 7% 7% 6% 5% 6% 6% 23%	4661
	11th to 20th	Day	13% 11% 4 11% 6% 6% 7% 5% 7% 7% 24%	4650
	21st to end	Day	13% 10% 3 8% 5% 5% 5% 4 6% 7% 33%	5322
February	1st to 10th	Day	14% 9% 3 9% 8% 6% 7% 4 6% 6% 27%	1890
	11th to 20th	Day	13% 10% 5% 9% 6% 6% 7% 5% 6% 7% 28%	1958
	21st to end	Day	17% 11% 3 9% 7% 5% 4 4 5% 6% 30%	1650
March	1st to 10th	Day	19% 10% 4 8% 5% 5% 5% 4 5% 5% 30%	4476
	11th to 20th	Day	15% 9% 3 7% 5% 4 4 3 5% 7% 37%	4740
	21st to end	Day	18% 8% 2 7% 6% 5% 4 3 5% 6% 34%	5957
April	1st to 10th	Day	18% 8% 3 8% 6% 4 5% 4 5% 6% 34%	2400
	11th to 20th	Day	17% 7% 2 7% 5% 5% 6% 4 5% 6% 36%	2374
	21st to end	Day	18% 7% 3 7% 4 5% 5% 5% 5% 7% 33%	2485
May	1st to 10th	Day	30% 8% 3 8% 4 4 4 3 5% 6% 24%	6024
	11th to 20th	Day	15% 9% 3 8% 5% 5% 6% 4 6% 7% 32%	6318
	21st to end	Day	17% 7% 3 7% 5% 5% 5% 4 7% 9% 31%	6949
June	1st to 10th	Day	14% 6% 3 7% 6% 5% 6% 5% 7% 8% 34%	2800
	11th to 20th	Day	14% 6% 2 6% 4 5% 6% 5% 8% 10% 34%	2800
	21st to end	Day	14% 7% 3 7% 5% 5% 5% 4 6% 10% 36%	2800
July	1st to 10th	Day	8% 6% 3 8% 5% 6% 6% 5% 7% 9% 36%	6360
	11th to 20th	Day	8% 6% 2 7% 5% 6% 6% 6% 8% 10% 37%	6360
	21st to end	Day	12% 5% 2 6% 4 5% 5% 4 6% 8% 42%	6926
August	1st to 10th	Day	14% 6% 2 7% 5% 5% 6% 4 7% 8% 36%	4539
	11th to 20th	Day	12% 6% 3 8% 5% 6% 6% 4 7% 9% 34%	3857
	21st to end	Day	10% 4 2 6% 5% 5% 5% 5% 7% 10% 40%	4312
September	1st to 10th	Day	11% 5% 2 5% 4 4 6% 4 7% 8% 44%	2400
	11th to 20th	Day	11% 5% 2 4 4 3 4 3 6% 8% 49%	2315
	21st to end	Day	15% 5% 2 6% 4 4 4 5% 6% 8% 41%	2200
October	1st to 10th	Day	17% 8% 2 6% 5% 5% 5% 4 5% 6% 36%	2077
	11th to 20th	Day	14% 7% 3 7% 4 5% 5% 4 6% 7% 39%	2000
	21st to end	Day	22% 7% 3 6% 5% 5% 4 4 5% 6% 32%	1994
November	1st to 10th	Day	13% 7% 2 6% 5% 4 4 4 7% 7% 42%	1851
	11th to 20th	Day	14% 8% 3 7% 5% 5% 5% 4 5% 7% 37%	1890
	21st to end	Day	18% 8% 2 7% 6% 5% 5% 5% 7% 6% 32%	1711
December	1st to 10th	Day	15% 14% 4 11% 6% 5% 5% 4 6% 7% 23%	2680
	11th to 20th	Day	20% 10% 3 6% 5% 5% 5% 4 5% 6% 31%	3066
	21st to end	Day	21% 13% 4 9% 5% 4 5% 4 4 6% 24%	4048
January	1st to 10th	Night	13% 10% 4 13% 7% 6% 6% 4 6% 7% 24%	8234
	11th to 20th	Night	11% 15% 5% 12% 7% 5% 5% 4 6% 7% 24%	8235
	21st to end	Night	14% 9% 3 9% 6% 6% 6% 4 6% 7% 31%	8842
February	1st to 10th	Night	13% 11% 3 11% 7% 6% 5% 4 6% 7% 28%	2730
	11th to 20th	Night	11% 9% 4 11% 6% 6% 6% 4 6% 7% 30%	2437
	21st to end	Night	16% 10% 3 9% 7% 4 4 4 5% 6% 30%	1980
March	1st to 10th	Night	18% 9% 3 8% 5% 5% 5% 5% 6% 6% 28%	5364
	11th to 20th	Night	15% 9% 3 7% 4 4 5% 4 5% 7% 37%	5215
	21st to end	Night	20% 9% 3 9% 5% 5% 5% 3 5% 5% 32%	4974
April	1st to 10th	Night	19% 8% 3 7% 6% 5% 4 5% 5% 6% 32%	2000
	11th to 20th	Night	19% 8% 3 7% 5% 4 5% 3 5% 6% 35%	1975
	21st to end	Night	19% 6% 2 7% 4 4 5% 4 5% 6% 39%	1914
May	1st to 10th	Night	29% 7% 3 8% 5% 4 4 3 5% 6% 26%	3944
	11th to 20th	Night	14% 10% 3 10% 5% 5% 5% 5% 6% 8% 29%	3575
	21st to end	Night	17% 8% 3 8% 5% 5% 5% 4 6% 8% 31%	3926
June	1st to 10th	Night	17% 6% 2 7% 4 5% 4 4 5% 7% 39%	1600
	11th to 20th	Night	14% 6% 2 5% 5% 4 4 5% 7% 8% 40%	1600
	21st to end	Night	12% 5% 2 6% 5% 4 6% 4 6% 8% 43%	1600
July	1st to 10th	Night	8% 5% 2 8% 6% 5% 5% 5% 6% 9% 40%	3600
	11th to 20th	Night	11% 7% 3 8% 5% 6% 5% 4 7% 8% 37%	3600
	21st to end	Night	9% 6% 2 6% 5% 4 4 3 5% 7% 47%	3926
August	1st to 10th	Night	18% 6% 2 6% 4 4 5% 5% 6% 7% 36%	2607
	11th to 20th	Night	15% 7% 2 7% 4 5% 5% 4 6% 7% 38%	3157
	21st to end	Night	12% 6% 2 5% 4 4 4 4 6% 9% 45%	3586
September	1st to 10th	Night	12% 6% 3 6% 4 4 4 4 5% 8% 45%	2000
	11th to 20th	Night	13% 4 2 5% 4 3 4 3 4 8% 52%	2085
	21st to end	Night	14% 6% 2 6% 4 4 4 4 5% 7% 44%	2200
October	1st to 10th	Night	18% 7% 3 7% 4 4 5% 4 4 6% 39%	2320
	11th to 20th	Night	15% 6% 2 7% 5% 4 5% 4 5% 8% 40%	2400
	21st to end	Night	21% 7% 2 7% 5% 4 4 4 5% 7% 33%	2845
November	1st to 10th	Night	14% 8% 2 7% 4 4 3 3 5% 6% 42%	2667
	11th to 20th	Night	16% 7% 2 6% 6% 4 6% 4 5% 7% 37%	2730
	21st to end	Night	15% 9% 2 7% 5% 5% 5% 4 6% 7% 36%	2905
December	1st to 10th	Night	15% 11% 4 9% 6% 5% 5% 4 6% 8% 27%	4720
	11th to 20th	Night	23% 9% 3 6% 4 4 5% 4 5% 6% 31%	5348
	21st to end	Night	23% 14% 5% 8% 4 4 4 3 4 5% 26%	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	57% 8% 11% 19% 6% 1	4662
	11th to 20th	Day	55% 8% 9% 13% 13% 2	4680
	21st to end	Day	49% 6% 10% 23% 12% 1	5322
February	1st to 10th	Day	55% 7% 12% 16% 9% 1	1890
	11th to 20th	Day	54% 7% 11% 18% 8% 2	1967
	21st to end	Day	54% 7% 10% 17% 11% 1	1650
March	1st to 10th	Day	54% 6% 10% 15% 11% 4	4479
	11th to 20th	Day	47% 6% 17% 19% 8% 3	4740
	21st to end	Day	51% 6% 6% 15% 19% 3	5957
April	1st to 10th	Day	51% 6% 9% 20% 12% 2	2400
	11th to 20th	Day	47% 7% 10% 20% 14% 2	2374
	21st to end	Day	49% 7% 10% 18% 14% 3	2485
May	1st to 10th	Day	60% 7% 8% 15% 9% 1	6035
	11th to 20th	Day	49% 9% 12% 20% 9% 1	6322
	21st to end	Day	47% 9% 12% 22% 10% 1	6949
June	1st to 10th	Day	45% 9% 14% 20% 11% 1	2800
	11th to 20th	Day	43% 10% 14% 19% 13% 1	2800
	21st to end	Day	43% 8% 14% 22% 12% 1	2800
July	1st to 10th	Day	42% 8% 14% 22% 13% 1	6290
	11th to 20th	Day	38% 10% 15% 24% 12% 1	6285
	21st to end	Day	38% 7% 12% 30% 13%	6728
August	1st to 10th	Day	44% 9% 15% 21% 10% 1	4525
	11th to 20th	Day	43% 8% 16% 26% 7%	3893
	21st to end	Day	36% 9% 15% 29% 11% 1	4312
September	1st to 10th	Day	36% 8% 13% 28% 14% 1	2400
	11th to 20th	Day	34% 7% 11% 27% 20% 1	2315
	21st to end	Day	39% 9% 11% 25% 14% 2	2200
October	1st to 10th	Day	42% 7% 8% 24% 17% 1	3376
	11th to 20th	Day	38% 6% 12% 29% 16% 1	3240
	21st to end	Day	36% 7% 14% 28% 13% 1	3194
November	1st to 10th	Day	40% 8% 14% 24% 13% 1	1851
	11th to 20th	Day	46% 7% 11% 21% 14% 2	1890
	21st to end	Day	51% 7% 16% 18% 7% 1	1711
December	1st to 10th	Day	60% 7% 12% 16% 5%	2680
	11th to 20th	Day	54% 6% 12% 17% 10% 1	3066
	21st to end	Day	61% 5% 9% 15% 10%	4048
January	1st to 10th	Night	56% 8% 12% 14% 9% 1	8234
	11th to 20th	Night	58% 7% 9% 12% 11% 2	8280
	21st to end	Night	50% 6% 10% 16% 15% 3	8842
February	1st to 10th	Night	54% 8% 11% 18% 7% 1	2730
	11th to 20th	Night	52% 8% 12% 18% 9% 1	2439
	21st to end	Night	52% 8% 13% 17% 8% 2	1980
March	1st to 10th	Night	53% 7% 10% 15% 12% 2	5374
	11th to 20th	Night	47% 7% 16% 20% 8% 2	5216
	21st to end	Night	54% 6% 6% 17% 16% 2	4974
April	1st to 10th	Night	52% 6% 11% 19% 11% 2	2000
	11th to 20th	Night	50% 6% 10% 20% 13% 1	1975
	21st to end	Night	46% 7% 9% 19% 16% 3	1914
May	1st to 10th	Night	59% 8% 11% 13% 9% 1	3950
	11th to 20th	Night	51% 10% 11% 20% 8% 1	3576
	21st to end	Night	51% 8% 12% 17% 11% 1	3927
June	1st to 10th	Night	43% 8% 11% 23% 14% 1	1600
	11th to 20th	Night	40% 8% 15% 23% 12% 1	1600
	21st to end	Night	38% 9% 13% 25% 14% 1	1600
July	1st to 10th	Night	39% 8% 14% 23% 15% 1	3560
	11th to 20th	Night	44% 8% 13% 20% 15%	3552
	21st to end	Night	35% 6% 11% 34% 13% 1	3809
August	1st to 10th	Night	45% 9% 14% 23% 9% 1	2597
	11th to 20th	Night	44% 8% 16% 22% 10%	3187
	21st to end	Night	35% 7% 13% 33% 11% 1	3586
September	1st to 10th	Night	38% 8% 11% 27% 16% 1	2000
	11th to 20th	Night	33% 5% 11% 27% 21% 1	2085
	21st to end	Night	40% 8% 10% 24% 17% 1	2200
October	1st to 10th	Night	46% 5% 11% 23% 14% 1	3788
	11th to 20th	Night	33% 6% 12% 28% 20%	3899
	21st to end	Night	41% 7% 13% 29% 10% 1	4595
November	1st to 10th	Night	42% 7% 10% 22% 16% 2	2667
	11th to 20th	Night	46% 7% 10% 22% 13% 2	2730
	21st to end	Night	46% 8% 16% 21% 8%	2905
December	1st to 10th	Night	54% 8% 12% 21% 5%	4720
	11th to 20th	Night	54% 7% 12% 14% 12% 1	5348
	21st to end	Night	61% 6% 9% 14% 10% 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	25% 29% 24% 18% 4	220
	11th to 20th	Day	30% 24% 27% 16% 2	220
	21st to end	Day	23% 26% 31% 14% 5%	278
February	1st to 10th	Day	28% 26% 33% 11% 1	330
	11th to 20th	Day	27% 25% 34% 12% 2	476
	21st to end	Day	26% 21% 32% 19% 3	414
March	1st to 10th	Day	29% 18% 32% 17% 3	500
	11th to 20th	Day	27% 22% 30% 16% 5%	500
	21st to end	Day	30% 23% 25% 14% 5% 2	550
April	1st to 10th	Day	31% 22% 24% 16% 6% 1	500
	11th to 20th	Day	26% 25% 24% 16% 7% 1	492
	21st to end	Day	28% 18% 31% 16% 6% 2	500
May	1st to 10th	Day	30% 14% 30% 16% 8% 2	503
	11th to 20th	Day	18% 15% 35% 23% 7% 2	527
	21st to end	Day	18% 9% 31% 26% 13% 3	573
June	1st to 10th	Day	16% 16% 28% 25% 13% 2	530
	11th to 20th	Day	16% 14% 27% 25% 16% 2	530
	21st to end	Day	17% 11% 33% 24% 13% 2	530
July	1st to 10th	Day	12% 9% 29% 28% 18% 5%	525
	11th to 20th	Day	11% 8% 28% 32% 18% 3	524
	21st to end	Day	13% 7% 31% 30% 16% 4	576
August	1st to 10th	Day	12% 7% 24% 31% 19% 7%	526
	11th to 20th	Day	13% 6% 26% 30% 18% 7%	519
	21st to end	Day	12% 11% 27% 28% 17% 5%	583
September	1st to 10th	Day	15% 8% 24% 24% 18% 10%	530
	11th to 20th	Day	14% 7% 23% 22% 20% 14%	530
	21st to end	Day	19% 10% 23% 27% 14% 8%	530
October	1st to 10th	Day	22% 10% 27% 20% 14% 7%	530
	11th to 20th	Day	17% 18% 29% 22% 11% 2	530
	21st to end	Day	27% 15% 28% 23% 7% 1	469
November	1st to 10th	Day	24% 16% 27% 18% 13% 2	425
	11th to 20th	Day	23% 21% 27% 20% 7% 1	430
	21st to end	Day	25% 24% 27% 20% 4	251
December	1st to 10th	Day	27% 18% 34% 16% 5%	220
	11th to 20th	Day	27% 24% 33% 13% 4	220
	21st to end	Day	26% 22% 30% 19% 2 1	242
January	1st to 10th	Night	22% 17% 38% 21% 2	840
	11th to 20th	Night	24% 21% 31% 22% 3	840
	21st to end	Night	19% 17% 39% 23% 2	924
February	1st to 10th	Night	22% 17% 39% 20% 2	840
	11th to 20th	Night	20% 20% 40% 19% 1	804
	21st to end	Night	22% 20% 35% 20% 3	660
March	1st to 10th	Night	24% 19% 36% 19% 2	800
	11th to 20th	Night	24% 19% 34% 20% 3	800
	21st to end	Night	23% 20% 32% 20% 5%	880
April	1st to 10th	Night	25% 18% 33% 22% 3	800
	11th to 20th	Night	25% 16% 34% 20% 5%	791
	21st to end	Night	22% 18% 32% 24% 4	714
May	1st to 10th	Night	25% 15% 30% 23% 7% 1	555
	11th to 20th	Night	19% 10% 35% 29% 7% 1	394
	21st to end	Night	13% 8% 30% 38% 9% 1	433
June	1st to 10th	Night	18% 6% 32% 34% 10% 1	400
	11th to 20th	Night	17% 8% 27% 38% 11%	400
	21st to end	Night	13% 6% 30% 38% 13%	400
July	1st to 10th	Night	12% 5% 25% 43% 14% 1	390
	11th to 20th	Night	13% 5% 22% 43% 16% 1	388
	21st to end	Night	11% 4 24% 47% 13% 1	438
August	1st to 10th	Night	13% 3 22% 43% 18% 1	406
	11th to 20th	Night	13% 4 24% 36% 20% 3	762
	21st to end	Night	16% 5% 23% 35% 19% 1	880
September	1st to 10th	Night	13% 6% 24% 38% 18% 1	800
	11th to 20th	Night	15% 5% 22% 33% 22% 3	800
	21st to end	Night	17% 6% 26% 34% 16% 2	800
October	1st to 10th	Night	21% 9% 26% 29% 14% 2	800
	11th to 20th	Night	17% 11% 29% 33% 9%	800
	21st to end	Night	26% 10% 32% 26% 5%	880
November	1st to 10th	Night	20% 13% 33% 27% 7%	820
	11th to 20th	Night	20% 15% 34% 23% 6% 1	840
	21st to end	Night	20% 13% 35% 26% 5%	840
December	1st to 10th	Night	24% 17% 37% 19% 3	840
	11th to 20th	Night	25% 18% 31% 23% 3	840
	21st to end	Night	30% 15% 33% 19% 3	924

Page generated at 2026-02-13 01:42:13. Version ac07

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