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 2024-05-13.

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 4 4 6% 7% 44%	83944
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 45%	85504
Transparency	Day	38% 6% 11% 21% 18% 5%	83666
	Night	38% 6% 10% 20% 20% 6%	85301
Seeing	Day	17% 10% 24% 24% 18% 6%	14723
	Night	17% 9% 25% 31% 17% 2	24944

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	19% 5% 2 5% 3 2 3 2 4 5% 52%	2836
	11th to 20th	Day	13% 6% 3 5% 4 3 3 2 4 5% 52%	2840
	21st to end	Day	21% 9% 2 6% 4 3 3 2 4 4 41%	3124
February	1st to 10th	Day	14% 6% 3 5% 4 4 4 4 6% 8% 43%	1710
	11th to 20th	Day	19% 9% 1 5% 3 3 4 3 3 5% 45%	1687
	21st to end	Day	18% 8% 3 7% 4 3 4 3 5% 6% 39%	1703
March	1st to 10th	Day	22% 7% 3 6% 3 3 4 3 5% 7% 38%	3435
	11th to 20th	Day	10% 5% 1 5% 3 3 4 4 5% 6% 55%	3480
	21st to end	Day	20% 7% 2 6% 4 3 3 3 4 5% 43%	3815
April	1st to 10th	Day	10% 4 1 4 3 3 4 3 5% 7% 55%	2180
	11th to 20th	Day	11% 5% 1 5% 4 4 4 4 6% 7% 49%	2254
	21st to end	Day	13% 5% 2 6% 5% 4 4 4 5% 7% 46%	2307
May	1st to 10th	Day	11% 7% 2 6% 5% 4 5% 5% 6% 8% 42%	2470
	11th to 20th	Day	13% 6% 2 7% 5% 5% 4 5% 8% 7% 39%	2329
	21st to end	Day	11% 5% 2 6% 5% 6% 5% 5% 7% 9% 38%	2531
June	1st to 10th	Day	8% 6% 2 6% 5% 5% 5% 4 8% 10% 40%	2340
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 4 7% 10% 44%	2340
	21st to end	Day	6% 6% 2 6% 5% 6% 7% 5% 7% 10% 39%	2340
July	1st to 10th	Day	8% 5% 2 6% 5% 7% 7% 5% 8% 10% 36%	2340
	11th to 20th	Day	7% 5% 2 7% 5% 6% 6% 6% 9% 13% 34%	2340
	21st to end	Day	6% 5% 3 7% 6% 7% 7% 6% 8% 10% 33%	2547
August	1st to 10th	Day	9% 5% 3 7% 6% 6% 7% 5% 8% 10% 32%	2327
	11th to 20th	Day	10% 6% 3 8% 7% 6% 7% 5% 8% 9% 30%	2267
	21st to end	Day	10% 6% 3 7% 5% 5% 6% 5% 8% 9% 36%	2376
September	1st to 10th	Day	10% 5% 2 5% 4 5% 5% 4 7% 8% 46%	2160
	11th to 20th	Day	14% 7% 2 6% 4 4 4 5% 6% 8% 41%	1990
	21st to end	Day	20% 6% 2 6% 4 4 4 3 5% 6% 39%	1967
October	1st to 10th	Day	15% 6% 2 5% 3 3 3 2 5% 5% 51%	1800
	11th to 20th	Day	13% 5% 2 4 4 3 3 2 3 5% 55%	1800
	21st to end	Day	11% 6% 2 4 4 3 3 3 3 5% 56%	1980
November	1st to 10th	Day	14% 5% 2 4 3 3 3 2 4 5% 56%	1857
	11th to 20th	Day	13% 5% 1 5% 3 3 3 3 5% 6% 52%	1848
	21st to end	Day	14% 5% 2 4 4 2 4 3 6% 6% 50%	1710
December	1st to 10th	Day	16% 7% 3 5% 4 4 4 4 5% 5% 41%	1637
	11th to 20th	Day	16% 5% 1 4 2 2 3 2 5% 5% 55%	2153
	21st to end	Day	17% 6% 2 5% 3 4 5% 4 6% 7% 41%	3124
January	1st to 10th	Night	15% 5% 2 5% 4 3 4 3 5% 5% 47%	4109
	11th to 20th	Night	14% 8% 2 5% 4 3 4 3 4 5% 48%	4120
	21st to end	Night	20% 7% 3 6% 3 3 3 3 4 5% 44%	4532
February	1st to 10th	Night	16% 7% 2 6% 4 3 4 3 5% 7% 44%	2470
	11th to 20th	Night	20% 7% 2 4 4 4 3 2 5% 6% 44%	2375
	21st to end	Night	15% 6% 2 5% 4 4 4 4 5% 7% 45%	1750
March	1st to 10th	Night	22% 9% 3 7% 5% 4 4 3 5% 4 35%	3418
	11th to 20th	Night	13% 6% 2 5% 3 4 3 3 4 6% 53%	3480
	21st to end	Night	18% 5% 2 5% 3 4 4 3 4 6% 48%	3817
April	1st to 10th	Night	14% 5% 2 4 4 3 5% 3 4 6% 50%	2000
	11th to 20th	Night	12% 6% 2 5% 4 3 4 3 5% 7% 49%	1875
	21st to end	Night	13% 5% 2 5% 4 4 4 4 4 7% 48%	1889
May	1st to 10th	Night	10% 6% 2 7% 4 5% 5% 5% 5% 8% 44%	1710
	11th to 20th	Night	13% 6% 2 7% 5% 4 4 3 6% 9% 40%	1619
	21st to end	Night	11% 6% 2 7% 4 5% 5% 4 8% 10% 40%	1744
June	1st to 10th	Night	10% 6% 2 5% 4 5% 7% 5% 7% 11% 38%	1620
	11th to 20th	Night	9% 7% 3 6% 5% 4 5% 5% 7% 10% 40%	1620
	21st to end	Night	8% 5% 2 6% 6% 5% 5% 5% 7% 12% 40%	1620
July	1st to 10th	Night	8% 5% 2 6% 6% 5% 5% 5% 9% 11% 37%	1620
	11th to 20th	Night	5% 6% 2 6% 6% 5% 6% 7% 9% 10% 38%	1620
	21st to end	Night	5% 6% 2 6% 5% 6% 5% 6% 8% 12% 39%	1771
August	1st to 10th	Night	10% 7% 2 8% 5% 5% 6% 6% 9% 9% 33%	1611
	11th to 20th	Night	10% 7% 3 7% 6% 6% 5% 5% 8% 9% 35%	1616
	21st to end	Night	10% 6% 2 7% 3 5% 5% 4 8% 11% 38%	1980
September	1st to 10th	Night	9% 5% 2 6% 4 3 4 4 6% 8% 48%	1800
	11th to 20th	Night	13% 6% 2 6% 3 4 4 4 6% 7% 45%	1967
	21st to end	Night	17% 8% 2 7% 5% 4 4 3 6% 5% 38%	1993
October	1st to 10th	Night	14% 6% 2 5% 4 3 3 2 4 6% 50%	2160
	11th to 20th	Night	13% 7% 2 5% 4 3 3 2 4 5% 54%	2160
	21st to end	Night	12% 5% 2 4 3 3 3 3 3 5% 55%	2376
November	1st to 10th	Night	14% 6% 2 5% 3 3 3 2 4 5% 53%	2221
	11th to 20th	Night	12% 6% 2 6% 4 4 4 3 4 5% 51%	2327
	21st to end	Night	13% 4 1 5% 3 3 3 4 6% 7% 50%	2470
December	1st to 10th	Night	17% 7% 3 7% 4 4 4 3 4 5% 42%	2470
	11th to 20th	Night	16% 6% 2 5% 3 3 3 3 4 5% 49%	3042
	21st to end	Night	18% 6% 2 5% 3 3 4 3 5% 7% 44%	4532

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	37% 3 5% 27% 23% 6%	2837
	11th to 20th	Day	36% 4 6% 18% 32% 6%	2840
	21st to end	Day	47% 3 5% 17% 21% 6%	3124
February	1st to 10th	Day	40% 5% 10% 21% 21% 4	1710
	11th to 20th	Day	43% 4 6% 20% 21% 7%	1696
	21st to end	Day	45% 5% 11% 15% 18% 6%	1703
March	1st to 10th	Day	46% 4 7% 20% 14% 8%	3435
	11th to 20th	Day	29% 5% 9% 21% 24% 13%	3480
	21st to end	Day	45% 4 8% 20% 20% 4	3815
April	1st to 10th	Day	29% 4 11% 24% 24% 8%	2180
	11th to 20th	Day	34% 5% 11% 22% 21% 8%	2254
	21st to end	Day	37% 6% 11% 23% 19% 4	2307
May	1st to 10th	Day	38% 7% 16% 20% 16% 3	2470
	11th to 20th	Day	41% 9% 14% 23% 11% 2	2333
	21st to end	Day	40% 9% 14% 24% 13% 1	2531
June	1st to 10th	Day	36% 8% 16% 24% 15% 1	2340
	11th to 20th	Day	34% 7% 19% 23% 16% 2	2340
	21st to end	Day	36% 10% 17% 21% 14% 1	2340
July	1st to 10th	Day	38% 12% 16% 18% 13% 3	2275
	11th to 20th	Day	36% 12% 21% 20% 9% 2	2271
	21st to end	Day	39% 11% 19% 22% 9%	2405
August	1st to 10th	Day	43% 10% 16% 21% 9%	2314
	11th to 20th	Day	46% 11% 16% 17% 9% 1	2267
	21st to end	Day	40% 9% 16% 20% 11% 3	2376
September	1st to 10th	Day	34% 9% 15% 20% 17% 5%	2160
	11th to 20th	Day	39% 8% 11% 18% 22% 3	1990
	21st to end	Day	46% 5% 9% 20% 17% 4	1967
October	1st to 10th	Day	36% 4 10% 21% 24% 4	1800
	11th to 20th	Day	32% 4 7% 28% 21% 8%	1800
	21st to end	Day	32% 3 8% 24% 25% 8%	1980
November	1st to 10th	Day	32% 4 7% 22% 27% 8%	1857
	11th to 20th	Day	33% 4 8% 22% 26% 7%	1848
	21st to end	Day	35% 3 9% 22% 25% 6%	1710
December	1st to 10th	Day	43% 6% 10% 19% 19% 4	1637
	11th to 20th	Day	32% 3 7% 24% 25% 9%	2150
	21st to end	Day	40% 5% 10% 23% 20% 2	3124
January	1st to 10th	Night	38% 3 7% 20% 26% 6%	4109
	11th to 20th	Night	39% 3 7% 18% 27% 6%	4120
	21st to end	Night	44% 4 5% 20% 21% 7%	4532
February	1st to 10th	Night	40% 4 9% 21% 21% 5%	2470
	11th to 20th	Night	43% 4 8% 16% 22% 8%	2377
	21st to end	Night	38% 5% 9% 20% 20% 7%	1750
March	1st to 10th	Night	51% 5% 8% 15% 16% 5%	3418
	11th to 20th	Night	34% 4 6% 15% 24% 16%	3480
	21st to end	Night	39% 4 7% 23% 22% 6%	3817
April	1st to 10th	Night	35% 5% 9% 19% 26% 6%	2000
	11th to 20th	Night	34% 4 9% 22% 22% 8%	1875
	21st to end	Night	36% 6% 11% 24% 18% 4	1889
May	1st to 10th	Night	36% 8% 13% 23% 17% 3	1710
	11th to 20th	Night	40% 8% 13% 22% 15% 2	1620
	21st to end	Night	37% 9% 15% 26% 13% 1	1745
June	1st to 10th	Night	37% 10% 16% 21% 14% 1	1620
	11th to 20th	Night	38% 7% 19% 20% 14% 2	1620
	21st to end	Night	35% 12% 16% 21% 14% 3	1620
July	1st to 10th	Night	37% 11% 16% 19% 14% 3	1575
	11th to 20th	Night	35% 13% 20% 20% 9% 3	1566
	21st to end	Night	35% 11% 20% 23% 12%	1672
August	1st to 10th	Night	43% 12% 18% 17% 9% 1	1600
	11th to 20th	Night	41% 11% 17% 18% 11% 2	1616
	21st to end	Night	38% 10% 18% 23% 8% 3	1980
September	1st to 10th	Night	33% 8% 14% 23% 16% 6%	1800
	11th to 20th	Night	38% 7% 11% 18% 23% 3	1967
	21st to end	Night	46% 6% 11% 18% 15% 4	1993
October	1st to 10th	Night	37% 4 9% 22% 23% 4	2160
	11th to 20th	Night	34% 5% 6% 24% 23% 8%	2160
	21st to end	Night	32% 3 6% 22% 26% 10%	2376
November	1st to 10th	Night	35% 3 7% 19% 26% 10%	2221
	11th to 20th	Night	36% 4 6% 17% 26% 11%	2327
	21st to end	Night	33% 4 6% 24% 26% 6%	2470
December	1st to 10th	Night	45% 4 8% 19% 19% 5%	2470
	11th to 20th	Night	38% 4 7% 20% 22% 9%	3044
	21st to end	Night	39% 4 8% 23% 23% 2	4532

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% 19% 35% 15% 2	270
	11th to 20th	Day	29% 22% 31% 15% 3 1	270
	21st to end	Day	23% 20% 42% 11% 3	297
February	1st to 10th	Day	21% 17% 41% 16% 4	270
	11th to 20th	Day	36% 23% 34% 6% 2	270
	21st to end	Day	29% 18% 33% 16% 3 1	358
March	1st to 10th	Day	24% 17% 33% 20% 6%	460
	11th to 20th	Day	18% 20% 42% 15% 5% 1	460
	21st to end	Day	24% 18% 35% 18% 5% 1	506
April	1st to 10th	Day	22% 16% 34% 19% 8% 1	460
	11th to 20th	Day	16% 19% 24% 25% 13% 3	452
	21st to end	Day	20% 10% 27% 28% 12% 3	460
May	1st to 10th	Day	18% 15% 27% 24% 12% 4	463
	11th to 20th	Day	16% 7% 21% 26% 24% 6%	456
	21st to end	Day	17% 5% 16% 28% 26% 8%	485
June	1st to 10th	Day	12% 4 20% 30% 27% 7%	450
	11th to 20th	Day	8% 6% 20% 28% 27% 11%	450
	21st to end	Day	8% 4 18% 33% 29% 9%	450
July	1st to 10th	Day	12% 2 9% 29% 32% 17%	445
	11th to 20th	Day	6% 2 14% 27% 37% 15%	444
	21st to end	Day	9% 2 13% 31% 33% 13%	488
August	1st to 10th	Day	13% 4 15% 26% 29% 13%	446
	11th to 20th	Day	11% 4 11% 26% 32% 16%	439
	21st to end	Day	12% 2 11% 30% 25% 20%	495
September	1st to 10th	Day	14% 3 14% 22% 29% 18%	450
	11th to 20th	Day	17% 2 11% 25% 31% 13%	450
	21st to end	Day	24% 7% 19% 29% 18% 4	450
October	1st to 10th	Day	17% 6% 21% 22% 23% 10%	450
	11th to 20th	Day	14% 9% 24% 25% 23% 4	450
	21st to end	Day	16% 9% 26% 31% 15% 3	495
November	1st to 10th	Day	19% 11% 26% 26% 14% 3	455
	11th to 20th	Day	18% 14% 29% 24% 13% 1	408
	21st to end	Day	14% 17% 30% 23% 13% 3	270
December	1st to 10th	Day	23% 15% 39% 17% 6%	234
	11th to 20th	Day	20% 19% 32% 22% 7% 1	270
	21st to end	Day	27% 15% 30% 20% 7% 1	297
January	1st to 10th	Night	23% 14% 35% 23% 5%	760
	11th to 20th	Night	21% 15% 34% 25% 5%	760
	21st to end	Night	20% 14% 38% 25% 3	836
February	1st to 10th	Night	22% 13% 34% 27% 3	760
	11th to 20th	Night	26% 19% 34% 20% 1	743
	21st to end	Night	19% 16% 38% 24% 3	628
March	1st to 10th	Night	19% 14% 34% 28% 5%	760
	11th to 20th	Night	20% 16% 33% 23% 8%	760
	21st to end	Night	19% 12% 36% 27% 5%	836
April	1st to 10th	Night	19% 16% 32% 26% 6%	760
	11th to 20th	Night	15% 12% 32% 29% 11% 1	751
	21st to end	Night	17% 6% 31% 37% 9%	749
May	1st to 10th	Night	12% 10% 31% 36% 11% 1	570
	11th to 20th	Night	15% 5% 21% 37% 20% 2	540
	21st to end	Night	14% 3 12% 38% 29% 4	584
June	1st to 10th	Night	13% 3 16% 39% 28% 2	540
	11th to 20th	Night	13% 3 14% 34% 31% 5%	540
	21st to end	Night	10% 1 13% 40% 33% 3	540
July	1st to 10th	Night	11% 2 8% 33% 40% 6%	525
	11th to 20th	Night	8% 1 6% 35% 43% 7%	522
	21st to end	Night	6% 1 10% 39% 37% 6%	591
August	1st to 10th	Night	13% 3 7% 36% 36% 5%	537
	11th to 20th	Night	12% 3 8% 34% 38% 6%	552
	21st to end	Night	12% 3 8% 32% 38% 7%	792
September	1st to 10th	Night	13% 3 8% 33% 37% 7%	720
	11th to 20th	Night	17% 3 11% 31% 33% 6%	720
	21st to end	Night	22% 4 15% 33% 24% 2	720
October	1st to 10th	Night	18% 4 17% 30% 26% 6%	720
	11th to 20th	Night	17% 9% 23% 30% 19% 2	720
	21st to end	Night	15% 9% 26% 31% 17% 2	792
November	1st to 10th	Night	16% 8% 25% 33% 16% 2	740
	11th to 20th	Night	14% 11% 33% 32% 10% 1	760
	21st to end	Night	15% 12% 28% 32% 13% 1	760
December	1st to 10th	Night	20% 11% 32% 31% 6%	760
	11th to 20th	Night	19% 8% 34% 31% 7%	760
	21st to end	Night	23% 13% 28% 27% 8%	836

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