Three College Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Three College Observatory 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	13% 6% 2 6% 4 4 5% 4 6% 8% 43%	126606
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 45%	123311
Transparency	Day	38% 7% 12% 21% 18% 5%	130212
	Night	38% 6% 10% 20% 21% 6%	127774
Seeing	Day	17% 10% 24% 24% 18% 6%	16777
	Night	17% 9% 25% 31% 17% 2	26887

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	16% 6% 2 5% 3 2 2 2 3 4 56%	4056
	11th to 20th	Day	11% 6% 2 5% 3 2 3 2 3 4 58%	4060
	21st to end	Day	15% 7% 3 6% 4 4 4 3 4 4 47%	4466
February	1st to 10th	Day	14% 6% 3 5% 4 4 4 4 6% 8% 43%	1800
	11th to 20th	Day	21% 9% 1 5% 3 3 4 3 3 4 44%	1779
	21st to end	Day	18% 8% 3 6% 4 3 4 3 5% 6% 41%	1790
March	1st to 10th	Day	18% 7% 2 6% 4 3 4 3 5% 6% 41%	4935
	11th to 20th	Day	9% 5% 2 5% 3 4 4 4 4 6% 53%	4980
	21st to end	Day	15% 6% 2 6% 4 4 4 3 5% 5% 47%	5465
April	1st to 10th	Day	10% 4 1 4 3 3 4 3 5% 8% 54%	2295
	11th to 20th	Day	11% 5% 1 5% 4 3 5% 3 6% 7% 49%	2374
	21st to end	Day	13% 5% 2 6% 5% 4 4 4 5% 7% 45%	2429
May	1st to 10th	Day	9% 7% 3 8% 5% 5% 6% 5% 7% 10% 35%	5862
	11th to 20th	Day	19% 7% 2 8% 5% 5% 5% 5% 7% 8% 29%	5861
	21st to end	Day	12% 5% 2 6% 4 4 5% 4 6% 9% 42%	6433
June	1st to 10th	Day	8% 6% 2 6% 5% 5% 5% 5% 8% 10% 40%	2600
	11th to 20th	Day	8% 5% 2 6% 4 4 5% 4 7% 11% 44%	2600
	21st to end	Day	6% 6% 2 6% 5% 5% 6% 5% 8% 11% 41%	2600
July	1st to 10th	Day	9% 6% 3 7% 5% 6% 6% 5% 8% 10% 36%	5900
	11th to 20th	Day	8% 6% 2 7% 5% 6% 7% 6% 9% 15% 29%	5900
	21st to end	Day	9% 6% 3 7% 5% 6% 6% 6% 8% 11% 33%	6424
August	1st to 10th	Day	18% 8% 3 8% 5% 5% 6% 5% 7% 10% 23%	4234
	11th to 20th	Day	8% 6% 3 8% 6% 6% 6% 5% 8% 11% 35%	4108
	21st to end	Day	6% 4 2 5% 4 4 5% 4 7% 10% 49%	4312
September	1st to 10th	Day	9% 5% 2 5% 4 5% 5% 4 6% 8% 47%	2400
	11th to 20th	Day	14% 7% 2 6% 4 4 4 5% 6% 8% 40%	2210
	21st to end	Day	21% 7% 3 6% 4 4 4 3 5% 7% 38%	2185
October	1st to 10th	Day	14% 6% 2 5% 3 4 3 2 5% 5% 51%	2000
	11th to 20th	Day	13% 4 1 4 3 3 3 2 3 5% 57%	2000
	21st to end	Day	11% 5% 2 4 4 3 3 2 3 5% 57%	2200
November	1st to 10th	Day	14% 5% 2 5% 3 3 3 2 4 5% 54%	1977
	11th to 20th	Day	13% 5% 1 5% 3 3 3 3 5% 6% 52%	1945
	21st to end	Day	13% 5% 2 4 4 3 4 3 6% 6% 50%	1800
December	1st to 10th	Day	12% 6% 3 5% 3 3 3 3 4 5% 53%	2787
	11th to 20th	Day	17% 7% 2 7% 4 4 3 3 5% 5% 45%	3373
	21st to end	Day	18% 6% 2 5% 3 3 4 4 5% 7% 44%	4466
January	1st to 10th	Night	13% 5% 2 4 3 3 3 2 4 5% 55%	5889
	11th to 20th	Night	12% 6% 2 4 3 2 3 2 4 4 57%	5900
	21st to end	Night	15% 7% 3 5% 3 3 3 3 4 6% 48%	6490
February	1st to 10th	Night	17% 7% 2 6% 4 3 4 3 5% 7% 44%	2600
	11th to 20th	Night	20% 7% 2 4 3 4 3 3 5% 6% 43%	2503
	21st to end	Night	15% 5% 2 5% 4 3 4 3 5% 7% 47%	1839
March	1st to 10th	Night	18% 7% 2 6% 4 3 4 3 4 4 44%	4918
	11th to 20th	Night	11% 5% 2 4 3 3 3 3 4 6% 55%	4980
	21st to end	Night	14% 4 2 5% 4 4 4 3 5% 7% 48%	5467
April	1st to 10th	Night	14% 5% 2 4 4 3 5% 3 4 6% 50%	2105
	11th to 20th	Night	12% 6% 2 5% 4 3 4 3 5% 7% 50%	1975
	21st to end	Night	12% 5% 2 5% 5% 4 4 4 5% 7% 47%	1988
May	1st to 10th	Night	12% 7% 3 7% 5% 4 5% 4 6% 9% 37%	4033
	11th to 20th	Night	17% 8% 3 8% 4 3 4 3 6% 8% 36%	4032
	21st to end	Night	15% 5% 2 6% 3 4 3 3 6% 9% 44%	4421
June	1st to 10th	Night	10% 6% 2 5% 4 5% 7% 5% 7% 10% 38%	1800
	11th to 20th	Night	8% 7% 3 6% 5% 4 5% 4 7% 10% 40%	1800
	21st to end	Night	7% 5% 2 5% 5% 5% 6% 5% 8% 11% 40%	1800
July	1st to 10th	Night	12% 6% 3 7% 6% 6% 6% 5% 8% 10% 31%	4058
	11th to 20th	Night	11% 8% 3 9% 5% 5% 7% 5% 9% 10% 27%	4057
	21st to end	Night	11% 8% 3 7% 6% 5% 5% 5% 8% 11% 31%	4427
August	1st to 10th	Night	22% 7% 3 7% 5% 5% 6% 5% 8% 9% 23%	2915
	11th to 20th	Night	7% 5% 2 7% 5% 5% 4 5% 8% 11% 39%	2908
	21st to end	Night	6% 4 2 6% 3 4 4 4 6% 9% 51%	3586
September	1st to 10th	Night	9% 6% 2 6% 4 3 4 4 6% 8% 48%	2000
	11th to 20th	Night	14% 6% 2 6% 3 4 4 4 6% 7% 43%	2187
	21st to end	Night	18% 8% 2 7% 5% 4 4 3 6% 5% 38%	2215
October	1st to 10th	Night	14% 6% 2 5% 4 3 3 3 4 6% 50%	2400
	11th to 20th	Night	13% 7% 2 5% 3 2 4 2 4 4 55%	2400
	21st to end	Night	13% 5% 2 4 3 3 3 3 3 5% 56%	2640
November	1st to 10th	Night	13% 6% 2 5% 3 3 3 3 4 5% 53%	2366
	11th to 20th	Night	12% 6% 2 6% 4 3 4 3 4 5% 50%	2450
	21st to end	Night	13% 4 1 5% 3 3 3 4 6% 7% 51%	2600
December	1st to 10th	Night	12% 6% 2 6% 4 4 4 2 4 5% 52%	4250
	11th to 20th	Night	18% 8% 3 7% 4 4 4 3 4 5% 41%	4822
	21st to end	Night	18% 5% 2 5% 3 3 3 3 5% 7% 47%	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 21% 32% 7%	4057
	11th to 20th	Day	32% 3 4 16% 36% 9%	4060
	21st to end	Day	42% 3 5% 22% 22% 7%	4466
February	1st to 10th	Day	39% 5% 10% 21% 20% 5%	1800
	11th to 20th	Day	44% 4 6% 19% 20% 7%	1788
	21st to end	Day	44% 5% 10% 15% 20% 6%	1790
March	1st to 10th	Day	44% 4 6% 17% 18% 12%	4935
	11th to 20th	Day	31% 5% 7% 18% 23% 17%	4980
	21st to end	Day	40% 4 8% 21% 20% 6%	5465
April	1st to 10th	Day	29% 4 11% 24% 24% 8%	2295
	11th to 20th	Day	33% 5% 11% 23% 20% 7%	2374
	21st to end	Day	38% 7% 11% 23% 18% 4	2429
May	1st to 10th	Day	40% 9% 17% 21% 11% 2	5897
	11th to 20th	Day	50% 8% 12% 21% 8% 1	5865
	21st to end	Day	37% 8% 13% 21% 18% 3	6447
June	1st to 10th	Day	36% 8% 16% 24% 15% 1	2600
	11th to 20th	Day	33% 7% 19% 23% 16% 2	2600
	21st to end	Day	34% 10% 17% 23% 15% 1	2600
July	1st to 10th	Day	38% 11% 18% 19% 12% 2	5835
	11th to 20th	Day	40% 15% 27% 15% 4 1	5831
	21st to end	Day	41% 10% 16% 23% 9%	6240
August	1st to 10th	Day	53% 9% 17% 16% 5%	4221
	11th to 20th	Day	39% 11% 20% 20% 8% 1	4147
	21st to end	Day	28% 7% 14% 33% 15% 2	4312
September	1st to 10th	Day	34% 8% 15% 20% 19% 5%	2400
	11th to 20th	Day	40% 7% 11% 18% 21% 3	2210
	21st to end	Day	47% 5% 10% 19% 15% 4	2185
October	1st to 10th	Day	33% 4 8% 19% 26% 10%	3260
	11th to 20th	Day	21% 2 4 22% 26% 25%	3240
	21st to end	Day	24% 3 8% 25% 24% 16%	3538
November	1st to 10th	Day	34% 3 7% 22% 26% 7%	1977
	11th to 20th	Day	33% 4 7% 22% 26% 7%	1945
	21st to end	Day	35% 3 9% 22% 26% 5%	1800
December	1st to 10th	Day	35% 4 7% 20% 27% 7%	2787
	11th to 20th	Day	42% 4 9% 17% 22% 6%	3370
	21st to end	Day	39% 4 11% 26% 18% 2	4466
January	1st to 10th	Night	32% 3 6% 17% 32% 10%	5889
	11th to 20th	Night	33% 2 5% 17% 35% 7%	5900
	21st to end	Night	39% 3 5% 20% 27% 5%	6490
February	1st to 10th	Night	41% 4 9% 21% 20% 5%	2600
	11th to 20th	Night	43% 4 8% 16% 21% 7%	2505
	21st to end	Night	37% 5% 9% 19% 22% 8%	1839
March	1st to 10th	Night	43% 4 6% 14% 23% 10%	4918
	11th to 20th	Night	31% 4 6% 15% 25% 19%	4980
	21st to end	Night	37% 4 7% 24% 23% 5%	5467
April	1st to 10th	Night	36% 5% 9% 19% 25% 6%	2105
	11th to 20th	Night	34% 4 10% 23% 22% 8%	1975
	21st to end	Night	36% 6% 12% 24% 18% 4	1988
May	1st to 10th	Night	42% 9% 15% 19% 14% 1	4060
	11th to 20th	Night	47% 7% 12% 19% 14% 1	4033
	21st to end	Night	36% 7% 14% 26% 15% 3	4429
June	1st to 10th	Night	37% 9% 16% 20% 16% 2	1800
	11th to 20th	Night	38% 8% 18% 21% 14% 2	1800
	21st to end	Night	33% 12% 16% 22% 14% 3	1800
July	1st to 10th	Night	43% 12% 15% 16% 12% 1	4015
	11th to 20th	Night	48% 14% 21% 12% 4 1	4006
	21st to end	Night	43% 10% 17% 21% 9%	4297
August	1st to 10th	Night	54% 10% 16% 14% 5% 1	2904
	11th to 20th	Night	35% 11% 20% 24% 8% 1	2935
	21st to end	Night	29% 7% 13% 29% 20% 2	3586
September	1st to 10th	Night	33% 8% 14% 23% 17% 5%	2000
	11th to 20th	Night	39% 7% 11% 18% 22% 3	2187
	21st to end	Night	47% 6% 11% 17% 14% 4	2215
October	1st to 10th	Night	37% 5% 8% 19% 25% 6%	3920
	11th to 20th	Night	25% 3 5% 21% 28% 19%	3899
	21st to end	Night	25% 3 6% 25% 24% 16%	4252
November	1st to 10th	Night	35% 3 7% 20% 25% 9%	2366
	11th to 20th	Night	36% 4 6% 16% 26% 11%	2450
	21st to end	Night	32% 4 6% 24% 26% 6%	2600
December	1st to 10th	Night	35% 3 6% 20% 29% 6%	4250
	11th to 20th	Night	47% 4 8% 17% 16% 8%	4824
	21st to end	Night	37% 4 12% 23% 22% 2	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	32% 19% 33% 14% 2	300
	11th to 20th	Day	28% 23% 31% 14% 3 1	300
	21st to end	Day	21% 21% 43% 12% 3	330
February	1st to 10th	Day	20% 19% 42% 15% 4	300
	11th to 20th	Day	38% 22% 32% 6% 2	300
	21st to end	Day	28% 18% 34% 18% 2 1	389
March	1st to 10th	Day	24% 17% 33% 20% 6%	500
	11th to 20th	Day	19% 19% 41% 16% 5% 1	500
	21st to end	Day	23% 19% 35% 18% 5% 1	550
April	1st to 10th	Day	21% 16% 35% 18% 8% 1	500
	11th to 20th	Day	16% 18% 26% 25% 12% 3	492
	21st to end	Day	19% 10% 26% 29% 13% 3	500
May	1st to 10th	Day	17% 15% 28% 24% 12% 4	503
	11th to 20th	Day	17% 7% 22% 26% 22% 5%	527
	21st to end	Day	17% 7% 19% 26% 23% 8%	573
June	1st to 10th	Day	12% 4 22% 30% 25% 7%	530
	11th to 20th	Day	7% 5% 20% 29% 28% 11%	530
	21st to end	Day	8% 3 16% 33% 31% 9%	530
July	1st to 10th	Day	12% 2 9% 28% 32% 16%	525
	11th to 20th	Day	6% 2 13% 27% 37% 16%	524
	21st to end	Day	9% 2 12% 32% 32% 13%	576
August	1st to 10th	Day	16% 4 15% 26% 28% 11%	526
	11th to 20th	Day	11% 4 13% 27% 30% 15%	519
	21st to end	Day	10% 2 11% 33% 25% 19%	583
September	1st to 10th	Day	13% 3 15% 25% 27% 16%	530
	11th to 20th	Day	17% 2 13% 26% 29% 12%	530
	21st to end	Day	24% 8% 19% 29% 16% 4	530
October	1st to 10th	Day	15% 6% 22% 24% 23% 9%	530
	11th to 20th	Day	14% 11% 23% 27% 21% 4	530
	21st to end	Day	16% 10% 26% 29% 16% 3	583
November	1st to 10th	Day	19% 11% 27% 27% 13% 3	503
	11th to 20th	Day	18% 14% 30% 24% 13% 1	445
	21st to end	Day	15% 18% 31% 22% 12% 2	300
December	1st to 10th	Day	22% 17% 40% 16% 5%	259
	11th to 20th	Day	20% 18% 32% 22% 7% 1	300
	21st to end	Day	27% 15% 29% 21% 7% 1	330
January	1st to 10th	Night	22% 15% 36% 22% 5%	800
	11th to 20th	Night	20% 16% 34% 25% 5%	800
	21st to end	Night	20% 14% 39% 24% 2	880
February	1st to 10th	Night	23% 13% 35% 26% 3	800
	11th to 20th	Night	26% 19% 34% 20% 1	783
	21st to end	Night	18% 17% 37% 25% 3	660
March	1st to 10th	Night	19% 14% 35% 27% 5%	800
	11th to 20th	Night	20% 16% 34% 23% 8%	800
	21st to end	Night	19% 13% 38% 26% 5%	880
April	1st to 10th	Night	20% 16% 33% 26% 6%	800
	11th to 20th	Night	15% 12% 33% 28% 10% 1	791
	21st to end	Night	16% 6% 31% 38% 10%	788
May	1st to 10th	Night	12% 10% 31% 37% 10% 1	600
	11th to 20th	Night	15% 6% 21% 37% 19% 2	591
	21st to end	Night	14% 4 14% 37% 27% 4	650
June	1st to 10th	Night	13% 3 16% 39% 27% 2	600
	11th to 20th	Night	12% 4 14% 36% 31% 4	600
	21st to end	Night	9% 1 13% 40% 35% 3	600
July	1st to 10th	Night	12% 2 8% 34% 38% 6%	585
	11th to 20th	Night	8% 1 6% 34% 43% 7%	582
	21st to end	Night	7% 2 10% 38% 37% 7%	657
August	1st to 10th	Night	16% 3 7% 35% 35% 5%	597
	11th to 20th	Night	11% 3 9% 33% 38% 6%	614
	21st to end	Night	11% 3 9% 34% 37% 6%	880
September	1st to 10th	Night	12% 3 9% 35% 35% 6%	800
	11th to 20th	Night	17% 3 11% 32% 32% 6%	800
	21st to end	Night	23% 4 14% 34% 23% 2	800
October	1st to 10th	Night	16% 4 17% 30% 27% 6%	800
	11th to 20th	Night	17% 9% 23% 32% 18% 2	800
	21st to end	Night	15% 10% 25% 30% 18% 2	880
November	1st to 10th	Night	16% 8% 25% 34% 15% 2	789
	11th to 20th	Night	15% 11% 32% 32% 10% 1	800
	21st to end	Night	15% 12% 29% 31% 12% 1	800
December	1st to 10th	Night	20% 11% 33% 30% 6%	800
	11th to 20th	Night	19% 9% 34% 32% 7%	800
	21st to end	Night	23% 13% 28% 29% 7%	880

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