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 2024-11-06.

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%	101116
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 44%	97836
Transparency	Day	38% 7% 12% 21% 18% 5%	104766
	Night	38% 6% 10% 20% 20% 6%	102325
Seeing	Day	17% 10% 24% 24% 18% 6%	15431
	Night	17% 9% 24% 31% 17% 2	25554

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% 8% 3 8% 5% 5% 6% 5% 7% 9% 35%	4082
	11th to 20th	Day	18% 6% 2 8% 5% 4 4 4 7% 7% 35%	4081
	21st to end	Day	7% 4 2 5% 4 5% 5% 4 7% 9% 48%	4476
June	1st to 10th	Day	8% 6% 2 6% 5% 5% 5% 4 8% 10% 40%	2470
	11th to 20th	Day	8% 5% 2 6% 4 4 5% 4 7% 10% 45%	2470
	21st to end	Day	6% 6% 2 6% 5% 6% 6% 5% 7% 10% 40%	2470
July	1st to 10th	Day	7% 6% 2 7% 5% 6% 7% 5% 8% 10% 36%	4120
	11th to 20th	Day	10% 6% 2 7% 5% 6% 6% 5% 8% 12% 32%	4120
	21st to end	Day	11% 8% 3 8% 6% 7% 6% 6% 8% 9% 27%	4466
August	1st to 10th	Day	18% 8% 3 8% 5% 6% 6% 5% 7% 10% 24%	4104
	11th to 20th	Day	8% 6% 3 8% 6% 6% 6% 5% 8% 11% 35%	3980
	21st to end	Day	6% 4 2 5% 4 4 5% 4 7% 10% 49%	4180
September	1st to 10th	Day	9% 5% 2 6% 4 5% 5% 4 6% 8% 47%	2280
	11th to 20th	Day	15% 7% 2 6% 4 4 4 5% 6% 8% 39%	2100
	21st to end	Day	21% 6% 3 6% 4 4 4 3 5% 6% 38%	2076
October	1st to 10th	Day	14% 6% 2 5% 3 3 3 2 5% 5% 51%	1900
	11th to 20th	Day	12% 4 1 4 4 3 3 2 3 5% 57%	1900
	21st to end	Day	11% 5% 2 4 4 3 3 2 3 5% 57%	2090
November	1st to 10th	Day	13% 5% 2 5% 3 3 3 2 4 5% 55%	1908
	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	12% 8% 3 7% 4 4 5% 4 5% 8% 41%	2813
	11th to 20th	Night	17% 8% 3 7% 5% 3 4 3 4 6% 40%	2812
	21st to end	Night	9% 4 1 6% 3 4 4 4 7% 11% 46%	3079
June	1st to 10th	Night	10% 6% 2 5% 4 5% 7% 5% 7% 10% 38%	1710
	11th to 20th	Night	8% 7% 3 6% 5% 4 5% 4 7% 10% 41%	1710
	21st to end	Night	7% 5% 2 6% 5% 5% 5% 5% 7% 11% 40%	1710
July	1st to 10th	Night	8% 6% 3 7% 6% 6% 6% 5% 8% 10% 33%	2840
	11th to 20th	Night	11% 8% 3 7% 5% 4 6% 5% 8% 9% 34%	2840
	21st to end	Night	14% 10% 3 8% 6% 5% 5% 5% 7% 9% 29%	3087
August	1st to 10th	Night	21% 7% 3 7% 5% 5% 6% 5% 8% 9% 23%	2825
	11th to 20th	Night	7% 5% 2 7% 5% 5% 4 5% 8% 11% 39%	2817
	21st to end	Night	6% 4 2 6% 3 4 4 4 6% 9% 51%	3476
September	1st to 10th	Night	9% 6% 2 6% 4 4 4 4 6% 8% 48%	1900
	11th to 20th	Night	14% 6% 2 7% 3 4 4 4 6% 7% 43%	2077
	21st to end	Night	18% 8% 2 7% 5% 4 4 3 6% 5% 38%	2104
October	1st to 10th	Night	14% 6% 2 5% 4 3 3 2 4 6% 49%	2280
	11th to 20th	Night	12% 7% 2 5% 4 2 3 2 4 4 55%	2280
	21st to end	Night	12% 5% 2 4 3 3 3 3 3 5% 57%	2508
November	1st to 10th	Night	14% 6% 2 5% 3 3 3 2 4 5% 53%	2292
	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	43% 9% 15% 19% 12% 2	4120
	11th to 20th	Day	46% 8% 11% 24% 10% 1	4085
	21st to end	Day	30% 9% 16% 23% 20% 4	4489
June	1st to 10th	Day	36% 8% 16% 24% 15% 1	2470
	11th to 20th	Day	33% 7% 18% 24% 16% 2	2470
	21st to end	Day	35% 10% 17% 22% 15% 1	2470
July	1st to 10th	Day	38% 13% 18% 17% 12% 3	4055
	11th to 20th	Day	41% 11% 23% 19% 5% 1	4051
	21st to end	Day	49% 11% 13% 17% 9%	4324
August	1st to 10th	Day	53% 9% 17% 16% 5%	4091
	11th to 20th	Day	39% 11% 20% 21% 8% 1	4019
	21st to end	Day	28% 7% 14% 33% 15% 2	4180
September	1st to 10th	Day	34% 8% 14% 19% 19% 5%	2280
	11th to 20th	Day	40% 8% 11% 17% 21% 3	2100
	21st to end	Day	46% 5% 10% 19% 16% 4	2076
October	1st to 10th	Day	33% 4 8% 18% 26% 10%	3160
	11th to 20th	Day	20% 2 5% 22% 26% 25%	3140
	21st to end	Day	24% 3 8% 25% 24% 16%	3428
November	1st to 10th	Day	32% 4 7% 22% 27% 7%	1908
	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	40% 8% 15% 22% 13% 2	2840
	11th to 20th	Night	45% 6% 10% 21% 16% 1	2813
	21st to end	Night	29% 8% 18% 27% 16% 2	3087
June	1st to 10th	Night	37% 10% 16% 21% 15% 1	1710
	11th to 20th	Night	37% 7% 18% 21% 15% 2	1710
	21st to end	Night	34% 12% 16% 22% 14% 3	1710
July	1st to 10th	Night	42% 13% 15% 15% 13% 2	2795
	11th to 20th	Night	43% 12% 22% 16% 5% 2	2786
	21st to end	Night	50% 10% 13% 17% 9%	2988
August	1st to 10th	Night	53% 10% 17% 14% 5% 1	2814
	11th to 20th	Night	35% 11% 20% 24% 8% 1	2844
	21st to end	Night	29% 7% 13% 29% 20% 2	3476
September	1st to 10th	Night	33% 8% 14% 22% 17% 6%	1900
	11th to 20th	Night	40% 7% 11% 18% 22% 3	2077
	21st to end	Night	46% 6% 11% 17% 15% 4	2104
October	1st to 10th	Night	37% 5% 8% 19% 25% 6%	3800
	11th to 20th	Night	24% 3 5% 21% 28% 19%	3779
	21st to end	Night	25% 3 6% 26% 24% 16%	4120
November	1st to 10th	Night	35% 3 7% 20% 25% 9%	2292
	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% 14% 27% 24% 12% 4	463
	11th to 20th	Day	17% 7% 21% 27% 24% 5%	487
	21st to end	Day	16% 5% 18% 28% 24% 9%	529
June	1st to 10th	Day	11% 4 21% 31% 26% 7%	490
	11th to 20th	Day	7% 6% 20% 28% 28% 11%	490
	21st to end	Day	8% 4 17% 33% 30% 8%	490
July	1st to 10th	Day	12% 2 8% 28% 32% 17%	485
	11th to 20th	Day	6% 2 13% 27% 37% 14%	484
	21st to end	Day	9% 2 13% 32% 31% 12%	532
August	1st to 10th	Day	14% 4 16% 26% 28% 12%	486
	11th to 20th	Day	10% 4 14% 27% 30% 15%	479
	21st to end	Day	11% 2 10% 31% 26% 20%	539
September	1st to 10th	Day	14% 3 15% 22% 29% 17%	490
	11th to 20th	Day	19% 2 12% 24% 30% 12%	490
	21st to end	Day	24% 7% 19% 29% 17% 4	490
October	1st to 10th	Day	16% 6% 22% 23% 23% 9%	490
	11th to 20th	Day	13% 11% 23% 26% 22% 4	490
	21st to end	Day	15% 9% 26% 30% 17% 4	539
November	1st to 10th	Day	19% 11% 27% 27% 14% 3	476
	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% 6% 21% 37% 20% 2	561
	21st to end	Night	13% 4 13% 38% 28% 4	617
June	1st to 10th	Night	13% 3 16% 39% 28% 2	570
	11th to 20th	Night	12% 3 14% 35% 31% 4	570
	21st to end	Night	9% 1 13% 40% 33% 2	570
July	1st to 10th	Night	11% 2 8% 33% 39% 6%	555
	11th to 20th	Night	9% 1 6% 34% 43% 7%	552
	21st to end	Night	7% 2 10% 39% 36% 6%	624
August	1st to 10th	Night	15% 3 7% 35% 36% 5%	567
	11th to 20th	Night	11% 3 9% 35% 37% 6%	583
	21st to end	Night	12% 3 8% 33% 38% 7%	836
September	1st to 10th	Night	13% 3 9% 33% 36% 6%	760
	11th to 20th	Night	18% 3 11% 30% 32% 6%	760
	21st to end	Night	22% 4 15% 33% 23% 2	760
October	1st to 10th	Night	17% 4 17% 31% 26% 6%	760
	11th to 20th	Night	16% 9% 23% 31% 18% 2	760
	21st to end	Night	14% 9% 25% 31% 19% 2	836
November	1st to 10th	Night	16% 8% 24% 33% 16% 2	764
	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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