UFOWorks Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the UFOWorks Observatory clear sky chart from 2005-11-06 to 2026-09-14.

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	16% 7% 2 6% 4 4 4 4 5% 7% 38%	148771
	Night	16% 8% 3 6% 4 4 4 3 5% 6% 40%	142307
Transparency	Day	45% 3 11% 23% 15% 2	152293
	Night	45% 3 11% 23% 16% 2	146861
Seeing	Day	26% 16% 32% 18% 7% 2	17398
	Night	24% 16% 33% 21% 6% 1	26281

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	10% 8% 3 9% 5% 4 5% 4 5% 6% 41%	4661
	11th to 20th	Day	13% 9% 3 7% 4 4 4 4 4 5% 44%	4650
	21st to end	Day	14% 10% 2 5% 3 3 4 3 5% 6% 45%	5124
February	1st to 10th	Day	16% 7% 3 6% 6% 5% 6% 5% 5% 6% 36%	1974
	11th to 20th	Day	13% 7% 3 7% 6% 4 5% 4 6% 6% 38%	2048
	21st to end	Day	18% 11% 2 8% 4 3 4 3 4 5% 37%	1730
March	1st to 10th	Day	19% 12% 3 7% 4 3 4 3 4 5% 36%	4576
	11th to 20th	Day	15% 7% 2 6% 5% 5% 5% 4 6% 7% 37%	4798
	21st to end	Day	21% 11% 3 5% 4 4 4 4 6% 7% 33%	6025
April	1st to 10th	Day	18% 10% 3 6% 5% 4 4 3 5% 6% 36%	2520
	11th to 20th	Day	17% 8% 2 5% 4 4 5% 4 6% 6% 40%	2494
	21st to end	Day	20% 6% 2 7% 4 4 4 4 5% 6% 38%	2697
May	1st to 10th	Day	27% 9% 2 6% 4 3 4 3 5% 7% 28%	7639
	11th to 20th	Day	20% 8% 2 8% 5% 4 4 3 5% 7% 36%	7896
	21st to end	Day	21% 8% 3 7% 4 4 5% 4 6% 7% 31%	9060
June	1st to 10th	Day	17% 7% 2 5% 5% 4 6% 4 6% 9% 35%	2940
	11th to 20th	Day	16% 6% 2 6% 4 4 5% 5% 6% 9% 37%	2940
	21st to end	Day	17% 6% 2 6% 5% 5% 5% 5% 7% 8% 35%	2940
July	1st to 10th	Day	17% 6% 2 6% 5% 5% 5% 4 7% 9% 34%	8280
	11th to 20th	Day	9% 5% 2 5% 5% 4 5% 4 7% 11% 42%	8280
	21st to end	Day	17% 7% 2 5% 4 4 4 3 5% 6% 43%	8993
August	1st to 10th	Day	14% 6% 3 7% 6% 5% 6% 5% 8% 9% 30%	6180
	11th to 20th	Day	12% 5% 2 6% 5% 4 5% 4 6% 8% 44%	5719
	21st to end	Day	11% 5% 2 6% 4 5% 5% 4 6% 9% 42%	6080
September	1st to 10th	Day	13% 7% 2 5% 4 4 5% 4 5% 5% 45%	2520
	11th to 20th	Day	14% 6% 2 5% 2 3 3 3 4 7% 51%	2263
	21st to end	Day	18% 6% 2 6% 4 4 4 3 5% 7% 42%	2200
October	1st to 10th	Day	21% 6% 3 5% 3 4 4 3 4 6% 41%	2200
	11th to 20th	Day	15% 7% 2 6% 3 3 4 3 5% 7% 46%	1945
	21st to end	Day	19% 7% 3 6% 3 4 4 3 4 5% 41%	1980
November	1st to 10th	Day	12% 8% 2 6% 5% 4 3 2 4 5% 48%	1851
	11th to 20th	Day	16% 6% 2 5% 4 3 3 3 5% 6% 47%	1890
	21st to end	Day	17% 8% 2 6% 4 4 4 3 5% 6% 41%	1884
December	1st to 10th	Day	14% 9% 2 5% 4 3 4 3 4 6% 45%	2680
	11th to 20th	Day	18% 7% 2 5% 3 3 3 3 5% 5% 45%	3066
	21st to end	Day	18% 11% 3 6% 4 4 3 3 4 6% 38%	4048
January	1st to 10th	Night	13% 8% 2 6% 4 4 4 4 6% 7% 42%	8234
	11th to 20th	Night	14% 10% 3 8% 5% 4 4 3 5% 6% 38%	8235
	21st to end	Night	14% 9% 3 6% 4 4 4 4 5% 5% 42%	9057
February	1st to 10th	Night	14% 8% 3 7% 4 5% 4 4 5% 6% 39%	2643
	11th to 20th	Night	13% 8% 3 7% 4 5% 5% 4 6% 7% 38%	2458
	21st to end	Night	16% 9% 3 7% 5% 5% 4 3 5% 5% 37%	2076
March	1st to 10th	Night	21% 10% 2 4 3 4 4 3 5% 6% 38%	5484
	11th to 20th	Night	14% 8% 3 6% 5% 5% 4 3 5% 7% 40%	5379
	21st to end	Night	22% 11% 2 6% 3 3 4 4 5% 6% 34%	5133
April	1st to 10th	Night	18% 9% 3 7% 6% 4 4 4 5% 6% 35%	2100
	11th to 20th	Night	18% 8% 3 7% 4 4 4 4 4 6% 38%	2075
	21st to end	Night	20% 8% 3 6% 4 3 4 3 4 6% 40%	1922
May	1st to 10th	Night	26% 9% 3 6% 4 3 4 4 5% 6% 30%	5259
	11th to 20th	Night	17% 9% 3 8% 5% 5% 5% 4 5% 7% 32%	5006
	21st to end	Night	21% 8% 3 7% 5% 5% 4 4 5% 7% 31%	5114
June	1st to 10th	Night	16% 8% 2 7% 5% 3 4 4 4 7% 40%	1680
	11th to 20th	Night	17% 7% 3 6% 4 3 5% 4 5% 8% 37%	1680
	21st to end	Night	14% 6% 3 5% 4 4 4 4 6% 8% 41%	1680
July	1st to 10th	Night	17% 7% 2 6% 5% 4 4 4 6% 9% 36%	4680
	11th to 20th	Night	12% 7% 2 6% 4 4 4 4 6% 8% 44%	4680
	21st to end	Night	14% 6% 3 6% 4 3 4 3 4 6% 45%	5084
August	1st to 10th	Night	16% 6% 2 8% 5% 5% 5% 5% 6% 8% 33%	3921
	11th to 20th	Night	12% 7% 3 8% 5% 5% 5% 4 6% 8% 39%	4322
	21st to end	Night	11% 6% 2 4 3 4 4 3 5% 7% 50%	5045
September	1st to 10th	Night	13% 6% 2 7% 3 4 5% 4 5% 5% 45%	2100
	11th to 20th	Night	12% 6% 2 5% 4 3 3 3 5% 6% 52%	2203
	21st to end	Night	17% 8% 2 6% 4 3 3 3 5% 6% 43%	2200
October	1st to 10th	Night	19% 7% 2 5% 3 4 4 3 5% 6% 43%	2200
	11th to 20th	Night	15% 6% 2 5% 3 4 4 4 5% 6% 46%	2448
	21st to end	Night	20% 7% 3 5% 4 4 3 3 5% 5% 40%	2860
November	1st to 10th	Night	13% 6% 2 6% 4 4 3 4 5% 6% 47%	2667
	11th to 20th	Night	16% 7% 1 6% 4 3 3 3 5% 5% 45%	2730
	21st to end	Night	15% 7% 2 5% 4 4 4 4 5% 7% 43%	2734
December	1st to 10th	Night	16% 9% 2 5% 4 3 3 3 5% 6% 44%	4720
	11th to 20th	Night	22% 9% 3 5% 3 3 4 3 4 4 40%	5348
	21st to end	Night	16% 11% 3 7% 4 3 3 3 4 5% 40%	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	45% 2 9% 25% 16% 2	4662
	11th to 20th	Day	44% 2 9% 21% 17% 7%	4680
	21st to end	Day	42% 2 9% 24% 21% 1	5124
February	1st to 10th	Day	48% 3 12% 21% 15% 2	1974
	11th to 20th	Day	47% 3 9% 23% 15% 3	2057
	21st to end	Day	51% 2 9% 20% 16% 2	1730
March	1st to 10th	Day	51% 2 10% 15% 13% 9%	4579
	11th to 20th	Day	46% 2 11% 29% 10% 2	4798
	21st to end	Day	51% 2 4 17% 18% 6%	6025
April	1st to 10th	Day	49% 2 8% 21% 15% 4	2520
	11th to 20th	Day	45% 3 11% 20% 18% 3	2494
	21st to end	Day	48% 3 7% 21% 18% 3	2697
May	1st to 10th	Day	57% 3 12% 16% 11% 1	7651
	11th to 20th	Day	51% 2 13% 20% 13% 1	7900
	21st to end	Day	53% 3 9% 22% 12% 1	9061
June	1st to 10th	Day	46% 4 11% 26% 13%	2940
	11th to 20th	Day	44% 4 13% 23% 14% 2	2940
	21st to end	Day	45% 5% 12% 23% 14% 1	2940
July	1st to 10th	Day	47% 5% 13% 22% 12% 1	8210
	11th to 20th	Day	36% 4 16% 29% 14% 1	8205
	21st to end	Day	42% 3 10% 26% 17% 1	8795
August	1st to 10th	Day	48% 4 17% 20% 9% 1	6166
	11th to 20th	Day	39% 3 15% 34% 8% 1	5758
	21st to end	Day	38% 4 13% 28% 15% 2	6080
September	1st to 10th	Day	41% 4 9% 26% 19% 2	2520
	11th to 20th	Day	35% 2 8% 27% 24% 3	2263
	21st to end	Day	43% 2 8% 24% 20% 2	2200
October	1st to 10th	Day	40% 4 8% 23% 22% 4	3590
	11th to 20th	Day	33% 2 8% 27% 27% 4	3135
	21st to end	Day	34% 3 12% 29% 18% 3	3180
November	1st to 10th	Day	40% 2 12% 25% 19% 2	1851
	11th to 20th	Day	39% 3 8% 25% 22% 3	1890
	21st to end	Day	44% 3 11% 28% 13% 2	1884
December	1st to 10th	Day	41% 2 14% 28% 13% 1	2680
	11th to 20th	Day	42% 2 13% 18% 21% 4	3066
	21st to end	Day	50% 2 7% 21% 15% 5%	4048
January	1st to 10th	Night	42% 2 10% 29% 15% 2	8234
	11th to 20th	Night	49% 2 9% 20% 15% 5%	8280
	21st to end	Night	45% 1 9% 20% 21% 4	9057
February	1st to 10th	Night	46% 3 11% 24% 15% 2	2643
	11th to 20th	Night	45% 3 9% 22% 17% 4	2460
	21st to end	Night	49% 2 7% 24% 14% 4	2076
March	1st to 10th	Night	48% 2 8% 18% 18% 6%	5494
	11th to 20th	Night	47% 1 16% 20% 12% 3	5380
	21st to end	Night	52% 2 4 13% 23% 5%	5133
April	1st to 10th	Night	51% 2 9% 20% 16% 3	2100
	11th to 20th	Night	48% 3 8% 21% 19% 2	2075
	21st to end	Night	47% 2 9% 21% 16% 4	1922
May	1st to 10th	Night	56% 2 14% 15% 12% 1	5265
	11th to 20th	Night	52% 3 13% 20% 10% 2	5007
	21st to end	Night	54% 2 12% 22% 9%	5115
June	1st to 10th	Night	45% 4 11% 25% 14% 1	1680
	11th to 20th	Night	47% 3 13% 24% 12% 1	1680
	21st to end	Night	41% 5% 15% 25% 15%	1680
July	1st to 10th	Night	46% 4 15% 21% 13%	4640
	11th to 20th	Night	39% 3 16% 26% 15% 1	4632
	21st to end	Night	41% 3 11% 28% 16% 1	4967
August	1st to 10th	Night	50% 5% 16% 18% 11% 1	3911
	11th to 20th	Night	44% 5% 16% 28% 8%	4349
	21st to end	Night	35% 3 15% 31% 15% 1	5045
September	1st to 10th	Night	41% 4 9% 26% 18% 1	2100
	11th to 20th	Night	35% 2 9% 25% 26% 2	2203
	21st to end	Night	43% 3 8% 24% 19% 3	2200
October	1st to 10th	Night	39% 4 13% 25% 18% 3	3590
	11th to 20th	Night	28% 2 10% 27% 29% 3	3984
	21st to end	Night	35% 2 11% 31% 18% 2	4610
November	1st to 10th	Night	38% 4 11% 24% 21% 4	2667
	11th to 20th	Night	41% 2 9% 22% 22% 4	2730
	21st to end	Night	41% 3 14% 28% 13% 1	2734
December	1st to 10th	Night	42% 2 12% 31% 13% 1	4720
	11th to 20th	Night	48% 2 13% 15% 20% 2	5348
	21st to end	Night	48% 2 6% 23% 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	30% 26% 28% 14% 3	220
	11th to 20th	Day	35% 26% 28% 8% 2	220
	21st to end	Day	30% 27% 32% 9% 2	242
February	1st to 10th	Day	33% 26% 30% 10% 1	424
	11th to 20th	Day	34% 27% 30% 8% 1	536
	21st to end	Day	34% 23% 31% 9% 3	446
March	1st to 10th	Day	33% 22% 33% 10% 2	540
	11th to 20th	Day	34% 24% 29% 11% 2	540
	21st to end	Day	33% 26% 28% 9% 3 2	594
April	1st to 10th	Day	36% 20% 29% 9% 5% 1	540
	11th to 20th	Day	34% 22% 30% 11% 3 1	532
	21st to end	Day	31% 16% 34% 14% 3 1	540
May	1st to 10th	Day	35% 18% 29% 13% 4 1	543
	11th to 20th	Day	26% 16% 36% 17% 4 1	567
	21st to end	Day	22% 14% 35% 21% 7% 2	617
June	1st to 10th	Day	20% 11% 36% 24% 8% 2	570
	11th to 20th	Day	20% 13% 37% 23% 6% 2	570
	21st to end	Day	19% 13% 35% 21% 9% 2	570
July	1st to 10th	Day	19% 9% 30% 28% 12% 3	565
	11th to 20th	Day	16% 6% 32% 32% 12% 2	564
	21st to end	Day	16% 7% 37% 29% 8% 2	620
August	1st to 10th	Day	15% 6% 30% 35% 11% 4	566
	11th to 20th	Day	15% 7% 35% 27% 13% 4	559
	21st to end	Day	13% 11% 34% 24% 13% 4	627
September	1st to 10th	Day	19% 10% 32% 21% 12% 5%	570
	11th to 20th	Day	19% 8% 27% 24% 13% 10%	542
	21st to end	Day	22% 11% 32% 22% 9% 4	530
October	1st to 10th	Day	26% 13% 25% 20% 11% 4	530
	11th to 20th	Day	23% 17% 32% 19% 6% 2	491
	21st to end	Day	28% 18% 31% 16% 6% 1	462
November	1st to 10th	Day	22% 19% 34% 15% 8% 1	425
	11th to 20th	Day	30% 21% 29% 15% 4 1	430
	21st to end	Day	33% 21% 28% 14% 4	424
December	1st to 10th	Day	28% 26% 29% 12% 5%	220
	11th to 20th	Day	42% 22% 28% 5% 3	220
	21st to end	Day	33% 23% 30% 10% 3 1	242
January	1st to 10th	Night	26% 22% 37% 13% 2	840
	11th to 20th	Night	32% 22% 31% 14% 2	840
	21st to end	Night	24% 24% 33% 16% 2	924
February	1st to 10th	Night	25% 22% 35% 16% 2	840
	11th to 20th	Night	27% 25% 36% 11% 1	823
	21st to end	Night	30% 22% 34% 13% 2	692
March	1st to 10th	Night	28% 21% 34% 14% 2	840
	11th to 20th	Night	25% 22% 36% 15% 2	840
	21st to end	Night	29% 22% 31% 15% 3	924
April	1st to 10th	Night	28% 17% 36% 15% 3	840
	11th to 20th	Night	25% 23% 31% 18% 3	831
	21st to end	Night	25% 20% 35% 17% 3	662
May	1st to 10th	Night	31% 16% 32% 18% 3	630
	11th to 20th	Night	21% 15% 39% 21% 4	549
	21st to end	Night	22% 9% 38% 25% 5%	455
June	1st to 10th	Night	19% 6% 37% 30% 8%	420
	11th to 20th	Night	20% 9% 38% 27% 5%	420
	21st to end	Night	15% 5% 38% 36% 7%	420
July	1st to 10th	Night	18% 5% 29% 37% 10% 1	410
	11th to 20th	Night	19% 2 28% 38% 13%	408
	21st to end	Night	13% 4 35% 39% 10%	460
August	1st to 10th	Night	13% 4 33% 39% 10% 1	530
	11th to 20th	Night	15% 4 32% 34% 14% 1	722
	21st to end	Night	15% 5% 28% 35% 15% 2	924
September	1st to 10th	Night	18% 6% 33% 31% 11% 1	840
	11th to 20th	Night	16% 8% 26% 30% 17% 2	813
	21st to end	Night	20% 8% 31% 28% 12% 2	800
October	1st to 10th	Night	25% 10% 28% 24% 12% 2	800
	11th to 20th	Night	21% 14% 34% 20% 10% 1	800
	21st to end	Night	28% 17% 29% 19% 6%	880
November	1st to 10th	Night	20% 16% 35% 21% 7% 1	820
	11th to 20th	Night	24% 23% 30% 17% 6% 1	840
	21st to end	Night	25% 20% 34% 17% 4	840
December	1st to 10th	Night	28% 20% 32% 15% 4	840
	11th to 20th	Night	28% 23% 31% 16% 2	840
	21st to end	Night	27% 22% 34% 14% 2	924

Page generated at 2026-09-11 04:11:51. Version ac07

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