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 2024-12-24.

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	18% 7% 2 6% 4 4 4 4 5% 7% 38%	101641
	Night	18% 8% 2 6% 4 4 4 3 5% 6% 39%	98483
Transparency	Day	44% 6% 11% 22% 15% 2	105192
	Night	44% 6% 12% 22% 15% 2	103021
Seeing	Day	25% 17% 31% 18% 7% 2	15162
	Night	23% 15% 32% 22% 7% 1	24200

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	15% 8% 3 6% 4 4 5% 3 4 5% 45%	2517
	11th to 20th	Day	17% 8% 3 7% 5% 5% 5% 3 5% 5% 36%	2520
	21st to end	Day	22% 13% 4 6% 3 3 4 3 5% 5% 33%	2772
February	1st to 10th	Day	18% 8% 2 6% 5% 4 6% 4 5% 7% 36%	1805
	11th to 20th	Day	14% 8% 2 6% 5% 5% 4 4 5% 7% 40%	1848
	21st to end	Day	20% 11% 2 6% 4 2 4 3 6% 5% 36%	1570
March	1st to 10th	Day	24% 12% 3 8% 4 2 3 3 3 4 33%	3116
	11th to 20th	Day	12% 6% 2 6% 5% 5% 5% 4 7% 7% 39%	3274
	21st to end	Day	21% 8% 3 6% 4 3 4 4 5% 7% 35%	4129
April	1st to 10th	Day	19% 9% 2 5% 5% 4 4 4 5% 6% 37%	2280
	11th to 20th	Day	18% 7% 2 5% 4 4 4 4 5% 6% 41%	2254
	21st to end	Day	21% 6% 2 6% 5% 4 4 3 4 6% 38%	2424
May	1st to 10th	Day	28% 8% 3 6% 4 4 4 3 5% 7% 28%	4106
	11th to 20th	Day	23% 8% 3 6% 4 4 4 3 5% 6% 33%	4224
	21st to end	Day	15% 6% 2 5% 4 4 4 3 6% 8% 44%	4837
June	1st to 10th	Day	18% 7% 2 6% 5% 4 5% 4 7% 8% 35%	2660
	11th to 20th	Day	16% 6% 2 6% 5% 5% 5% 5% 6% 9% 36%	2660
	21st to end	Day	15% 6% 3 6% 5% 5% 5% 4 7% 9% 35%	2660
July	1st to 10th	Day	14% 6% 2 7% 6% 5% 6% 5% 6% 9% 34%	4440
	11th to 20th	Day	14% 5% 2 5% 5% 4 5% 5% 6% 10% 38%	4440
	21st to end	Day	19% 8% 3 7% 5% 5% 4 4 6% 6% 34%	4814
August	1st to 10th	Day	19% 7% 3 8% 5% 5% 5% 4 7% 8% 29%	4211
	11th to 20th	Day	12% 6% 2 7% 5% 5% 5% 4 6% 8% 40%	3837
	21st to end	Day	8% 5% 3 7% 5% 5% 5% 4 6% 9% 44%	4180
September	1st to 10th	Day	14% 7% 2 6% 5% 4 4 3 5% 5% 45%	2280
	11th to 20th	Day	13% 6% 2 6% 3 3 3 3 4 6% 50%	2123
	21st to end	Day	17% 7% 3 5% 4 4 4 3 4 7% 41%	2090
October	1st to 10th	Day	22% 8% 2 5% 3 3 3 3 5% 5% 41%	2090
	11th to 20th	Day	14% 7% 2 5% 3 3 3 3 5% 6% 48%	1858
	21st to end	Day	18% 8% 2 7% 4 3 3 4 4 6% 41%	1881
November	1st to 10th	Day	13% 7% 2 6% 4 4 4 2 4 5% 49%	1761
	11th to 20th	Day	19% 5% 1 4 3 3 4 4 5% 6% 47%	1800
	21st to end	Day	17% 7% 2 5% 4 4 4 4 5% 5% 43%	1798
December	1st to 10th	Day	19% 7% 3 7% 4 4 4 4 5% 6% 37%	1600
	11th to 20th	Day	22% 7% 2 4 4 3 4 2 3 5% 44%	1986
	21st to end	Day	20% 10% 3 7% 5% 4 4 3 4 6% 34%	2796
January	1st to 10th	Night	17% 7% 2 5% 3 3 3 3 4 7% 45%	4429
	11th to 20th	Night	17% 9% 3 8% 4 4 3 3 4 5% 40%	4440
	21st to end	Night	23% 12% 3 6% 4 4 4 3 4 5% 31%	4883
February	1st to 10th	Night	16% 9% 3 7% 4 4 3 4 4 6% 39%	2375
	11th to 20th	Night	15% 7% 3 6% 5% 4 4 3 6% 7% 39%	2218
	21st to end	Night	19% 10% 2 6% 4 3 4 3 5% 6% 38%	1884
March	1st to 10th	Night	26% 12% 2 5% 4 3 3 3 4 5% 33%	3724
	11th to 20th	Night	11% 7% 3 6% 5% 5% 5% 4 6% 7% 42%	3685
	21st to end	Night	23% 9% 2 6% 3 3 4 4 5% 5% 36%	3500
April	1st to 10th	Night	19% 7% 2 6% 5% 4 5% 4 6% 6% 35%	1900
	11th to 20th	Night	19% 7% 2 7% 4 3 4 4 5% 5% 38%	1875
	21st to end	Night	22% 7% 2 7% 4 3 3 2 5% 6% 39%	1754
May	1st to 10th	Night	24% 9% 4 8% 5% 3 4 3 5% 7% 28%	2834
	11th to 20th	Night	22% 9% 3 8% 5% 5% 5% 4 5% 7% 28%	2698
	21st to end	Night	18% 6% 2 5% 3 4 4 4 5% 8% 41%	2738
June	1st to 10th	Night	17% 7% 2 7% 4 4 5% 4 5% 8% 38%	1520
	11th to 20th	Night	18% 6% 3 6% 5% 3 4 3 5% 8% 38%	1520
	21st to end	Night	14% 6% 2 7% 5% 3 4 3 6% 10% 40%	1520
July	1st to 10th	Night	14% 6% 2 8% 4 5% 5% 5% 7% 9% 34%	2520
	11th to 20th	Night	18% 6% 2 5% 4 4 4 4 6% 9% 38%	2520
	21st to end	Night	19% 8% 3 7% 4 4 4 4 5% 5% 34%	2739
August	1st to 10th	Night	23% 7% 2 7% 4 5% 5% 4 6% 6% 30%	2686
	11th to 20th	Night	13% 9% 3 7% 6% 4 6% 4 5% 7% 37%	2973
	21st to end	Night	8% 4 2 4 3 3 3 3 5% 8% 56%	3476
September	1st to 10th	Night	14% 7% 2 7% 4 4 4 4 6% 6% 43%	1900
	11th to 20th	Night	13% 6% 2 5% 3 3 3 3 5% 6% 53%	2057
	21st to end	Night	18% 6% 2 6% 4 3 4 4 5% 6% 42%	2090
October	1st to 10th	Night	19% 7% 2 7% 4 4 3 3 5% 6% 41%	2090
	11th to 20th	Night	16% 5% 1 6% 3 3 4 2 6% 6% 47%	2320
	21st to end	Night	20% 8% 3 6% 3 3 3 3 5% 6% 40%	2717
November	1st to 10th	Night	13% 6% 2 6% 3 3 4 3 5% 6% 49%	2537
	11th to 20th	Night	18% 7% 2 5% 4 4 3 3 5% 5% 45%	2600
	21st to end	Night	16% 7% 2 5% 4 4 4 3 4 7% 44%	2600
December	1st to 10th	Night	20% 8% 3 5% 4 4 3 3 5% 5% 41%	2800
	11th to 20th	Night	20% 7% 2 5% 4 4 4 3 5% 5% 40%	3428
	21st to end	Night	21% 11% 3 6% 4 3 3 3 5% 5% 38%	4933

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	41% 4 10% 31% 11% 2	2518
	11th to 20th	Day	48% 6% 10% 24% 11% 1	2520
	21st to end	Day	53% 5% 9% 21% 10% 2	2772
February	1st to 10th	Day	47% 6% 12% 19% 14% 2	1805
	11th to 20th	Day	44% 6% 10% 22% 15% 3	1857
	21st to end	Day	49% 6% 10% 20% 14% 2	1570
March	1st to 10th	Day	56% 3 11% 16% 12% 2	3119
	11th to 20th	Day	38% 9% 12% 28% 12% 2	3274
	21st to end	Day	49% 3 8% 17% 18% 5%	4129
April	1st to 10th	Day	47% 6% 11% 19% 14% 3	2280
	11th to 20th	Day	42% 6% 11% 20% 18% 3	2254
	21st to end	Day	48% 5% 9% 19% 16% 4	2424
May	1st to 10th	Day	55% 7% 13% 14% 11% 1	4120
	11th to 20th	Day	51% 7% 14% 17% 10% 1	4228
	21st to end	Day	37% 7% 13% 24% 18% 2	4837
June	1st to 10th	Day	46% 7% 12% 23% 12% 1	2660
	11th to 20th	Day	43% 8% 13% 22% 13% 1	2660
	21st to end	Day	43% 10% 12% 20% 14% 1	2660
July	1st to 10th	Day	46% 7% 11% 20% 15% 1	4370
	11th to 20th	Day	41% 7% 15% 23% 12% 2	4365
	21st to end	Day	50% 6% 11% 18% 14% 2	4661
August	1st to 10th	Day	51% 7% 15% 19% 8%	4197
	11th to 20th	Day	41% 6% 13% 29% 10% 1	3876
	21st to end	Day	35% 6% 12% 28% 16% 2	4180
September	1st to 10th	Day	41% 6% 11% 22% 19% 2	2280
	11th to 20th	Day	35% 5% 11% 23% 24% 3	2123
	21st to end	Day	43% 5% 9% 21% 20% 2	2090
October	1st to 10th	Day	39% 5% 9% 20% 24% 4	3480
	11th to 20th	Day	30% 4 8% 25% 30% 4	3061
	21st to end	Day	34% 4 13% 26% 21% 3	3081
November	1st to 10th	Day	39% 4 12% 22% 20% 2	1761
	11th to 20th	Day	37% 5% 11% 21% 21% 4	1800
	21st to end	Day	43% 5% 11% 26% 13% 3	1798
December	1st to 10th	Day	46% 6% 13% 18% 16% 1	1600
	11th to 20th	Day	44% 4 14% 18% 19% 2	1986
	21st to end	Day	51% 5% 11% 22% 11% 1	2796
January	1st to 10th	Night	39% 4 11% 31% 12% 3	4429
	11th to 20th	Night	47% 4 11% 23% 11% 3	4440
	21st to end	Night	56% 5% 9% 18% 11% 2	4883
February	1st to 10th	Night	46% 5% 12% 23% 12% 2	2375
	11th to 20th	Night	45% 5% 9% 20% 17% 4	2220
	21st to end	Night	47% 5% 8% 22% 14% 4	1884
March	1st to 10th	Night	53% 5% 9% 20% 11% 2	3734
	11th to 20th	Night	38% 8% 14% 23% 15% 2	3686
	21st to end	Night	49% 3 8% 12% 22% 5%	3500
April	1st to 10th	Night	47% 6% 11% 19% 14% 3	1900
	11th to 20th	Night	46% 6% 9% 18% 18% 2	1875
	21st to end	Night	46% 5% 9% 21% 13% 5%	1754
May	1st to 10th	Night	54% 7% 14% 14% 8% 2	2840
	11th to 20th	Night	54% 10% 12% 14% 8% 2	2699
	21st to end	Night	39% 8% 17% 22% 13% 1	2739
June	1st to 10th	Night	44% 8% 11% 21% 14% 1	1520
	11th to 20th	Night	43% 8% 14% 23% 11% 1	1520
	21st to end	Night	39% 8% 15% 24% 13%	1520
July	1st to 10th	Night	45% 8% 13% 21% 12% 1	2480
	11th to 20th	Night	43% 7% 17% 23% 9% 1	2472
	21st to end	Night	50% 6% 12% 15% 16% 1	2651
August	1st to 10th	Night	54% 7% 12% 17% 8% 1	2676
	11th to 20th	Night	45% 6% 15% 25% 8%	3000
	21st to end	Night	27% 6% 17% 31% 18% 1	3476
September	1st to 10th	Night	41% 7% 10% 24% 17% 1	1900
	11th to 20th	Night	34% 5% 11% 22% 26% 2	2057
	21st to end	Night	44% 5% 9% 20% 20% 3	2090
October	1st to 10th	Night	40% 5% 13% 22% 19% 2	3480
	11th to 20th	Night	27% 4 9% 26% 32% 2	3856
	21st to end	Night	33% 4 12% 29% 19% 2	4467
November	1st to 10th	Night	37% 5% 12% 22% 21% 4	2537
	11th to 20th	Night	41% 4 10% 21% 20% 4	2600
	21st to end	Night	40% 4 14% 28% 12% 1	2600
December	1st to 10th	Night	46% 5% 13% 22% 14% 1	2800
	11th to 20th	Night	46% 5% 13% 18% 17% 2	3428
	21st to end	Night	50% 4 9% 24% 12% 1	4933

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	33% 22% 27% 16% 2	180
	11th to 20th	Day	34% 27% 28% 7% 4	180
	21st to end	Day	32% 24% 32% 9% 3 1	198
February	1st to 10th	Day	35% 26% 25% 13% 1	365
	11th to 20th	Day	33% 27% 31% 7% 2	456
	21st to end	Day	33% 24% 29% 9% 4	382
March	1st to 10th	Day	33% 20% 32% 13% 2	460
	11th to 20th	Day	32% 23% 30% 11% 3	460
	21st to end	Day	32% 22% 31% 10% 3 2	506
April	1st to 10th	Day	35% 22% 27% 12% 4 1	460
	11th to 20th	Day	33% 19% 31% 13% 4	452
	21st to end	Day	30% 21% 30% 14% 3 2	460
May	1st to 10th	Day	35% 19% 27% 13% 5% 1	463
	11th to 20th	Day	26% 15% 37% 16% 5% 1	487
	21st to end	Day	22% 12% 32% 24% 6% 3	529
June	1st to 10th	Day	21% 14% 31% 22% 9% 2	490
	11th to 20th	Day	18% 15% 37% 22% 8% 1	490
	21st to end	Day	17% 15% 36% 21% 8% 2	490
July	1st to 10th	Day	20% 8% 29% 27% 14% 3	485
	11th to 20th	Day	15% 8% 32% 29% 12% 4	484
	21st to end	Day	16% 10% 36% 27% 9% 2	532
August	1st to 10th	Day	17% 8% 34% 28% 11% 2	486
	11th to 20th	Day	16% 9% 31% 27% 12% 5%	479
	21st to end	Day	11% 12% 32% 24% 14% 7%	539
September	1st to 10th	Day	20% 9% 30% 22% 13% 6%	490
	11th to 20th	Day	18% 12% 29% 20% 12% 10%	490
	21st to end	Day	21% 14% 34% 19% 8% 5%	490
October	1st to 10th	Day	29% 14% 23% 20% 10% 4	490
	11th to 20th	Day	21% 19% 30% 21% 7% 3	463
	21st to end	Day	29% 15% 30% 17% 7% 2	429
November	1st to 10th	Day	20% 17% 34% 18% 9% 2	395
	11th to 20th	Day	31% 22% 28% 14% 5%	400
	21st to end	Day	29% 21% 28% 17% 5% 1	398
December	1st to 10th	Day	25% 28% 29% 15% 4	200
	11th to 20th	Day	38% 25% 29% 8% 1	200
	21st to end	Day	30% 27% 28% 9% 3 2	204
January	1st to 10th	Night	24% 21% 38% 15% 3	760
	11th to 20th	Night	30% 20% 33% 15% 2	760
	21st to end	Night	24% 22% 33% 18% 3	836
February	1st to 10th	Night	25% 20% 35% 18% 2	760
	11th to 20th	Night	27% 24% 37% 11% 1	743
	21st to end	Night	32% 20% 32% 14% 2	628
March	1st to 10th	Night	28% 21% 34% 15% 2	760
	11th to 20th	Night	23% 24% 34% 16% 2	760
	21st to end	Night	27% 21% 33% 15% 3	836
April	1st to 10th	Night	27% 19% 34% 18% 2	760
	11th to 20th	Night	24% 22% 31% 20% 3	751
	21st to end	Night	27% 18% 35% 15% 4	614
May	1st to 10th	Night	30% 16% 29% 20% 5%	570
	11th to 20th	Night	22% 16% 37% 21% 4	499
	21st to end	Night	21% 10% 32% 29% 6% 1	411
June	1st to 10th	Night	19% 8% 36% 28% 8% 1	380
	11th to 20th	Night	19% 8% 36% 31% 6%	380
	21st to end	Night	18% 6% 34% 34% 8%	380
July	1st to 10th	Night	15% 4 29% 36% 15%	370
	11th to 20th	Night	16% 2 28% 38% 16%	368
	21st to end	Night	16% 6% 32% 36% 9% 1	416
August	1st to 10th	Night	14% 5% 32% 38% 11% 1	480
	11th to 20th	Night	15% 5% 28% 34% 17% 1	675
	21st to end	Night	14% 6% 25% 34% 19% 1	836
September	1st to 10th	Night	16% 8% 32% 30% 13% 1	760
	11th to 20th	Night	17% 8% 28% 29% 16% 3	760
	21st to end	Night	21% 7% 32% 25% 13% 2	760
October	1st to 10th	Night	25% 10% 28% 23% 11% 3	760
	11th to 20th	Night	20% 14% 32% 24% 9% 1	760
	21st to end	Night	28% 15% 31% 18% 7%	836
November	1st to 10th	Night	18% 14% 34% 24% 8% 1	780
	11th to 20th	Night	26% 19% 33% 17% 6% 1	800
	21st to end	Night	25% 18% 33% 19% 5%	800
December	1st to 10th	Night	30% 17% 32% 16% 5%	800
	11th to 20th	Night	28% 21% 31% 18% 2	800
	21st to end	Night	28% 19% 36% 15% 2	851

Page generated at 2024-12-21 04:13:49. Version ac07

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