University of Connecticut Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the University of Connecticut Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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	17% 7% 2 6% 4 4 4 4 5% 7% 38%	131082
	Night	18% 8% 2 6% 4 4 4 3 5% 6% 40%	131064
Transparency	Day	44% 5% 11% 21% 15% 3	134615
	Night	44% 5% 11% 21% 16% 3	135618
Seeing	Day	26% 17% 31% 18% 7% 2	16822
	Night	24% 15% 32% 22% 7% 1	25920

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	11% 8% 3 8% 4 4 5% 3 5% 6% 43%	4661
	11th to 20th	Day	15% 8% 3 6% 4 4 4 3 5% 5% 44%	4650
	21st to end	Day	15% 10% 2 4 3 3 4 4 5% 6% 44%	5124
February	1st to 10th	Day	17% 7% 2 6% 5% 5% 6% 4 5% 7% 36%	1995
	11th to 20th	Day	15% 8% 2 7% 5% 5% 4 4 5% 7% 39%	2048
	21st to end	Day	19% 11% 2 6% 4 2 4 3 5% 5% 37%	1730
March	1st to 10th	Day	22% 10% 2 6% 4 3 3 2 4 5% 38%	4576
	11th to 20th	Day	15% 8% 2 6% 4 5% 4 4 6% 6% 39%	4779
	21st to end	Day	22% 9% 2 6% 4 4 5% 4 6% 7% 33%	6050
April	1st to 10th	Day	20% 9% 2 5% 5% 4 4 4 5% 6% 37%	2520
	11th to 20th	Day	18% 7% 2 5% 4 4 4 4 5% 6% 41%	2446
	21st to end	Day	20% 6% 2 6% 5% 4 4 3 5% 6% 38%	2552
May	1st to 10th	Day	31% 8% 3 6% 4 4 4 3 5% 8% 25%	5886
	11th to 20th	Day	21% 9% 3 7% 4 5% 4 3 5% 6% 32%	6060
	21st to end	Day	20% 7% 2 6% 4 5% 4 4 6% 8% 34%	6949
June	1st to 10th	Day	18% 6% 2 6% 5% 4 5% 4 7% 8% 34%	2800
	11th to 20th	Day	17% 6% 2 6% 5% 5% 5% 4 5% 9% 36%	2800
	21st to end	Day	15% 6% 3 6% 5% 5% 5% 4 7% 9% 36%	2800
July	1st to 10th	Day	17% 7% 2 7% 5% 5% 5% 5% 6% 9% 33%	6360
	11th to 20th	Day	13% 6% 2 7% 5% 4 5% 5% 7% 10% 35%	6360
	21st to end	Day	17% 6% 2 5% 4 4 4 3 5% 6% 42%	6926
August	1st to 10th	Day	18% 7% 3 8% 5% 5% 5% 4 7% 8% 30%	4344
	11th to 20th	Day	12% 6% 2 7% 5% 5% 5% 4 6% 8% 40%	3961
	21st to end	Day	7% 5% 3 7% 5% 5% 5% 4 6% 9% 44%	4312
September	1st to 10th	Day	15% 6% 2 6% 5% 4 4 3 5% 6% 45%	2400
	11th to 20th	Day	13% 6% 2 6% 3 3 4 3 5% 6% 51%	2235
	21st to end	Day	17% 7% 3 5% 4 4 4 3 4 7% 41%	2200
October	1st to 10th	Day	21% 8% 2 4 3 3 3 3 5% 5% 43%	2200
	11th to 20th	Day	15% 7% 2 5% 3 3 3 3 5% 7% 47%	1955
	21st to end	Day	18% 8% 2 7% 3 3 3 3 4 6% 41%	1980
November	1st to 10th	Day	13% 7% 2 6% 4 4 4 2 4 5% 48%	1851
	11th to 20th	Day	18% 5% 2 4 3 3 4 4 5% 6% 47%	1890
	21st to end	Day	17% 8% 2 5% 4 4 3 4 5% 6% 42%	1888
December	1st to 10th	Day	14% 8% 2 6% 4 4 3 3 4 6% 45%	2680
	11th to 20th	Day	20% 7% 2 4 3 3 3 3 4 5% 46%	3066
	21st to end	Day	20% 10% 2 7% 4 3 3 3 4 5% 39%	4048
January	1st to 10th	Night	14% 7% 2 6% 3 4 4 4 5% 7% 44%	8234
	11th to 20th	Night	16% 9% 3 8% 4 4 4 3 4 5% 40%	8235
	21st to end	Night	15% 9% 2 6% 4 5% 5% 3 5% 6% 41%	9042
February	1st to 10th	Night	16% 9% 3 7% 4 5% 3 4 5% 6% 39%	2624
	11th to 20th	Night	16% 7% 3 6% 5% 4 4 4 6% 7% 39%	2458
	21st to end	Night	18% 10% 3 6% 5% 3 4 3 5% 6% 38%	2076
March	1st to 10th	Night	22% 10% 2 4 4 3 4 4 4 5% 39%	5484
	11th to 20th	Night	16% 7% 3 6% 5% 4 4 4 5% 7% 39%	5400
	21st to end	Night	22% 9% 3 6% 3 4 4 4 5% 5% 36%	5121
April	1st to 10th	Night	20% 7% 2 6% 5% 4 5% 4 6% 6% 35%	2100
	11th to 20th	Night	19% 7% 2 7% 4 3 4 4 6% 6% 37%	2036
	21st to end	Night	22% 7% 2 7% 4 3 3 3 5% 6% 39%	1846
May	1st to 10th	Night	29% 9% 3 7% 4 4 3 3 5% 6% 26%	4054
	11th to 20th	Night	20% 10% 3 8% 5% 5% 5% 4 5% 7% 29%	3862
	21st to end	Night	22% 8% 3 6% 3 4 5% 4 5% 7% 33%	3926
June	1st to 10th	Night	17% 7% 2 7% 4 4 5% 4 5% 8% 38%	1600
	11th to 20th	Night	18% 7% 3 7% 5% 3 4 3 6% 8% 37%	1600
	21st to end	Night	14% 6% 2 7% 5% 3 4 3 6% 10% 41%	1600
July	1st to 10th	Night	17% 6% 3 8% 4 4 4 4 6% 9% 36%	3600
	11th to 20th	Night	18% 8% 3 6% 4 4 4 4 6% 8% 35%	3600
	21st to end	Night	15% 7% 3 6% 4 4 4 3 5% 5% 44%	3926
August	1st to 10th	Night	23% 7% 2 7% 4 5% 5% 4 6% 7% 31%	2772
	11th to 20th	Night	12% 9% 3 7% 5% 4 6% 4 5% 7% 37%	3069
	21st to end	Night	9% 4 2 4 3 3 3 3 5% 8% 56%	3586
September	1st to 10th	Night	14% 7% 2 7% 4 4 4 4 6% 6% 44%	2000
	11th to 20th	Night	13% 6% 2 5% 3 3 3 3 5% 6% 53%	2165
	21st to end	Night	18% 7% 2 6% 4 3 4 4 5% 6% 42%	2200
October	1st to 10th	Night	19% 7% 2 6% 3 4 3 3 4 6% 43%	2200
	11th to 20th	Night	16% 5% 1 6% 3 3 4 2 6% 6% 47%	2443
	21st to end	Night	20% 8% 2 6% 3 3 3 3 5% 6% 40%	2860
November	1st to 10th	Night	13% 6% 2 6% 3 3 3 3 6% 6% 48%	2667
	11th to 20th	Night	17% 7% 2 5% 4 4 3 3 5% 5% 45%	2730
	21st to end	Night	16% 7% 2 5% 4 4 4 3 4 7% 44%	2730
December	1st to 10th	Night	18% 7% 2 5% 3 3 3 3 4 5% 47%	4720
	11th to 20th	Night	24% 8% 3 5% 3 3 4 3 4 4 41%	5348
	21st to end	Night	18% 10% 3 7% 4 3 3 3 4 4 42%	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	43% 4 10% 25% 15% 2	4662
	11th to 20th	Day	42% 4 9% 21% 16% 7%	4680
	21st to end	Day	40% 4 11% 24% 19% 1	5124
February	1st to 10th	Day	46% 6% 12% 19% 15% 2	1995
	11th to 20th	Day	45% 6% 10% 22% 15% 3	2057
	21st to end	Day	48% 5% 10% 20% 15% 2	1730
March	1st to 10th	Day	49% 3 10% 15% 13% 9%	4579
	11th to 20th	Day	42% 7% 12% 27% 11% 1	4779
	21st to end	Day	50% 4 8% 16% 17% 6%	6050
April	1st to 10th	Day	47% 6% 11% 18% 15% 4	2520
	11th to 20th	Day	42% 6% 11% 20% 18% 3	2446
	21st to end	Day	47% 5% 9% 20% 16% 4	2552
May	1st to 10th	Day	58% 6% 14% 13% 8% 1	5897
	11th to 20th	Day	51% 6% 13% 17% 12% 1	6064
	21st to end	Day	47% 6% 12% 21% 13% 1	6949
June	1st to 10th	Day	46% 7% 12% 22% 12% 1	2800
	11th to 20th	Day	44% 7% 13% 21% 13% 2	2800
	21st to end	Day	43% 9% 12% 20% 15% 1	2800
July	1st to 10th	Day	48% 7% 11% 20% 13% 1	6290
	11th to 20th	Day	43% 7% 17% 22% 10% 1	6285
	21st to end	Day	42% 5% 11% 23% 18% 1	6728
August	1st to 10th	Day	50% 7% 15% 19% 8% 1	4330
	11th to 20th	Day	41% 6% 13% 29% 10% 1	4000
	21st to end	Day	35% 6% 12% 28% 17% 2	4312
September	1st to 10th	Day	41% 6% 10% 23% 19% 2	2400
	11th to 20th	Day	34% 5% 11% 23% 24% 3	2235
	21st to end	Day	43% 5% 9% 21% 20% 2	2200
October	1st to 10th	Day	38% 5% 9% 20% 24% 4	3590
	11th to 20th	Day	31% 4 8% 25% 29% 4	3158
	21st to end	Day	34% 4 12% 26% 21% 3	3180
November	1st to 10th	Day	39% 4 12% 22% 20% 2	1851
	11th to 20th	Day	37% 5% 11% 22% 20% 4	1890
	21st to end	Day	43% 5% 10% 27% 12% 3	1888
December	1st to 10th	Day	40% 4 14% 28% 13% 1	2680
	11th to 20th	Day	41% 3 14% 18% 20% 3	3066
	21st to end	Day	49% 4 8% 20% 15% 5%	4048
January	1st to 10th	Night	40% 3 11% 28% 15% 2	8234
	11th to 20th	Night	46% 4 11% 19% 15% 6%	8280
	21st to end	Night	44% 4 9% 19% 20% 4	9042
February	1st to 10th	Night	46% 5% 12% 23% 13% 2	2624
	11th to 20th	Night	45% 5% 9% 21% 17% 4	2460
	21st to end	Night	47% 5% 8% 22% 14% 4	2076
March	1st to 10th	Night	47% 4 10% 18% 15% 6%	5494
	11th to 20th	Night	44% 6% 15% 21% 11% 3	5401
	21st to end	Night	50% 3 7% 12% 23% 5%	5121
April	1st to 10th	Night	48% 5% 11% 18% 15% 3	2100
	11th to 20th	Night	47% 6% 10% 18% 18% 2	2036
	21st to end	Night	46% 5% 9% 21% 14% 4	1846
May	1st to 10th	Night	58% 6% 15% 11% 8% 1	4060
	11th to 20th	Night	53% 8% 12% 15% 9% 3	3863
	21st to end	Night	50% 7% 15% 19% 10%	3927
June	1st to 10th	Night	45% 8% 11% 21% 15% 1	1600
	11th to 20th	Night	44% 7% 14% 23% 11% 1	1600
	21st to end	Night	39% 8% 15% 24% 14% 1	1600
July	1st to 10th	Night	46% 6% 13% 22% 12% 1	3560
	11th to 20th	Night	46% 6% 17% 21% 9% 1	3552
	21st to end	Night	41% 5% 11% 24% 17% 1	3809
August	1st to 10th	Night	53% 7% 12% 18% 9% 1	2762
	11th to 20th	Night	45% 6% 15% 26% 8%	3096
	21st to end	Night	27% 6% 16% 31% 19% 1	3586
September	1st to 10th	Night	40% 7% 10% 25% 18% 1	2000
	11th to 20th	Night	34% 5% 10% 23% 26% 2	2165
	21st to end	Night	44% 5% 9% 20% 19% 3	2200
October	1st to 10th	Night	39% 5% 12% 23% 19% 2	3590
	11th to 20th	Night	28% 4 9% 26% 31% 2	3979
	21st to end	Night	33% 4 12% 29% 19% 2	4610
November	1st to 10th	Night	37% 5% 11% 23% 20% 4	2667
	11th to 20th	Night	40% 4 10% 21% 20% 4	2730
	21st to end	Night	40% 4 14% 28% 13% 1	2730
December	1st to 10th	Night	41% 3 12% 30% 13% 1	4720
	11th to 20th	Night	48% 3 12% 15% 19% 2	5348
	21st to end	Night	47% 3 8% 22% 17% 4	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	35% 24% 26% 13% 2	220
	11th to 20th	Day	36% 26% 28% 6% 3	220
	21st to end	Day	35% 25% 29% 8% 2 1	242
February	1st to 10th	Day	34% 26% 27% 12% 1	435
	11th to 20th	Day	35% 26% 31% 7% 1	536
	21st to end	Day	32% 26% 29% 9% 4	446
March	1st to 10th	Day	34% 21% 30% 13% 2	540
	11th to 20th	Day	33% 23% 30% 11% 2	540
	21st to end	Day	34% 23% 30% 9% 3 2	594
April	1st to 10th	Day	38% 22% 26% 11% 3 1	540
	11th to 20th	Day	33% 22% 29% 11% 4	516
	21st to end	Day	30% 21% 30% 14% 3 2	500
May	1st to 10th	Day	36% 19% 27% 12% 5% 1	503
	11th to 20th	Day	26% 15% 37% 15% 5% 1	527
	21st to end	Day	23% 12% 32% 23% 6% 3	573
June	1st to 10th	Day	22% 13% 32% 22% 9% 2	530
	11th to 20th	Day	19% 15% 36% 21% 8% 1	530
	21st to end	Day	18% 15% 35% 22% 8% 2	530
July	1st to 10th	Day	19% 9% 30% 26% 13% 2	525
	11th to 20th	Day	15% 8% 32% 29% 12% 3	524
	21st to end	Day	16% 10% 36% 28% 9% 1	576
August	1st to 10th	Day	16% 7% 33% 29% 12% 3	526
	11th to 20th	Day	16% 9% 30% 27% 13% 5%	519
	21st to end	Day	10% 14% 33% 23% 14% 6%	583
September	1st to 10th	Day	21% 9% 30% 22% 12% 6%	530
	11th to 20th	Day	16% 11% 29% 22% 12% 9%	530
	21st to end	Day	20% 13% 34% 20% 9% 5%	530
October	1st to 10th	Day	28% 14% 23% 20% 12% 5%	530
	11th to 20th	Day	23% 19% 29% 20% 7% 3	500
	21st to end	Day	29% 15% 30% 17% 7% 2	462
November	1st to 10th	Day	20% 19% 33% 17% 8% 1	425
	11th to 20th	Day	30% 22% 28% 15% 5%	430
	21st to end	Day	30% 22% 27% 17% 4	428
December	1st to 10th	Day	26% 28% 29% 14% 4	220
	11th to 20th	Day	39% 24% 29% 7% 1	220
	21st to end	Day	33% 24% 28% 10% 3 2	242
January	1st to 10th	Night	25% 22% 37% 14% 3	840
	11th to 20th	Night	30% 20% 32% 15% 2	840
	21st to end	Night	24% 22% 33% 18% 2	924
February	1st to 10th	Night	25% 20% 35% 18% 2	840
	11th to 20th	Night	28% 23% 36% 12% 1	823
	21st to end	Night	30% 20% 33% 14% 2	692
March	1st to 10th	Night	28% 20% 34% 15% 2	840
	11th to 20th	Night	25% 23% 33% 17% 3	840
	21st to end	Night	27% 22% 34% 15% 3	924
April	1st to 10th	Night	28% 19% 33% 17% 2	840
	11th to 20th	Night	24% 23% 31% 19% 3	816
	21st to end	Night	27% 19% 35% 15% 4	646
May	1st to 10th	Night	30% 15% 30% 20% 5%	600
	11th to 20th	Night	22% 16% 37% 21% 4	525
	21st to end	Night	22% 10% 33% 29% 6% 1	433
June	1st to 10th	Night	19% 8% 35% 29% 8% 1	400
	11th to 20th	Night	20% 8% 36% 31% 6%	400
	21st to end	Night	18% 6% 34% 33% 9%	400
July	1st to 10th	Night	15% 4 29% 36% 15%	390
	11th to 20th	Night	16% 3 28% 37% 16%	388
	21st to end	Night	15% 6% 32% 37% 9% 1	438
August	1st to 10th	Night	14% 5% 31% 38% 11% 1	506
	11th to 20th	Night	14% 5% 27% 35% 17% 1	711
	21st to end	Night	14% 6% 26% 33% 18% 1	880
September	1st to 10th	Night	16% 8% 32% 30% 14% 1	800
	11th to 20th	Night	16% 7% 27% 31% 16% 3	800
	21st to end	Night	21% 7% 32% 25% 13% 2	800
October	1st to 10th	Night	24% 10% 28% 23% 12% 3	800
	11th to 20th	Night	21% 15% 32% 23% 9% 1	800
	21st to end	Night	28% 16% 31% 19% 6%	880
November	1st to 10th	Night	20% 15% 34% 24% 7% 1	820
	11th to 20th	Night	26% 19% 33% 16% 6% 1	840
	21st to end	Night	25% 18% 32% 20% 5%	840
December	1st to 10th	Night	30% 17% 32% 16% 5%	840
	11th to 20th	Night	28% 21% 31% 17% 2	840
	21st to end	Night	28% 19% 35% 16% 2	924

Page generated at 2026-04-14 02:44:25. Version ac07

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