Worlds Without End Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Worlds Without End Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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	12% 7% 3 6% 4 4 4 4 5% 7% 43%	131041
	Night	13% 8% 3 7% 4 4 4 3 5% 6% 44%	130385
Transparency	Day	38% 6% 11% 25% 17% 3	134637
	Night	39% 5% 11% 24% 18% 3	134867
Seeing	Day	18% 13% 28% 24% 13% 3	17309
	Night	18% 13% 30% 25% 11% 1	27268

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	12% 11% 4 11% 4 4 3 3 4 6% 38%	4807
	11th to 20th	Day	7% 10% 3 8% 4 5% 3 3 4 6% 47%	5310
	21st to end	Day	16% 8% 4 8% 4 3 3 3 4 5% 42%	5812
February	1st to 10th	Day	15% 10% 3 9% 5% 4 3 3 4 6% 36%	1890
	11th to 20th	Day	14% 10% 3 7% 4 4 4 3 5% 6% 38%	1991
	21st to end	Day	12% 10% 3 7% 5% 4 4 3 6% 6% 40%	1730
March	1st to 10th	Day	15% 11% 3 6% 3 3 3 2 3 4 46%	4747
	11th to 20th	Day	10% 6% 2 4 3 3 3 3 5% 6% 53%	4980
	21st to end	Day	13% 9% 2 7% 5% 5% 6% 4 6% 7% 36%	5645
April	1st to 10th	Day	12% 7% 2 6% 5% 4 4 4 6% 8% 45%	2400
	11th to 20th	Day	17% 8% 2 6% 5% 4 4 4 5% 5% 40%	2374
	21st to end	Day	19% 9% 3 6% 5% 5% 4 3 5% 6% 35%	2400
May	1st to 10th	Day	15% 8% 3 7% 6% 5% 6% 4 7% 8% 30%	5422
	11th to 20th	Day	15% 7% 3 7% 5% 5% 4 4 6% 8% 35%	6217
	21st to end	Day	17% 7% 2 6% 4 4 4 3 5% 6% 40%	6935
June	1st to 10th	Day	9% 5% 3 6% 4 4 5% 5% 7% 8% 44%	2800
	11th to 20th	Day	7% 4 1 6% 4 5% 5% 4 6% 9% 49%	2800
	21st to end	Day	5% 4 2 5% 5% 5% 6% 4 7% 9% 49%	2800
July	1st to 10th	Day	10% 5% 2 6% 4 4 5% 4 6% 9% 45%	6360
	11th to 20th	Day	5% 5% 2 7% 5% 5% 5% 5% 7% 11% 42%	6189
	21st to end	Day	7% 4 2 6% 5% 5% 6% 5% 8% 10% 41%	6422
August	1st to 10th	Day	6% 4 2 4 3 4 5% 4 6% 10% 53%	4234
	11th to 20th	Day	11% 5% 2 5% 4 4 5% 4 6% 7% 45%	3883
	21st to end	Day	7% 5% 2 5% 4 4 5% 4 7% 8% 51%	4312
September	1st to 10th	Day	12% 5% 2 4 4 4 4 3 6% 8% 49%	2400
	11th to 20th	Day	10% 6% 2 7% 4 4 4 3 5% 7% 50%	2400
	21st to end	Day	9% 5% 2 6% 4 4 4 3 6% 7% 51%	2140
October	1st to 10th	Day	9% 4 2 5% 3 4 4 3 4 6% 57%	2000
	11th to 20th	Day	13% 7% 2 5% 3 2 3 2 4 5% 54%	2000
	21st to end	Day	17% 9% 3 5% 3 3 3 2 4 5% 47%	2200
November	1st to 10th	Day	12% 6% 1 6% 4 3 4 3 3 5% 54%	1918
	11th to 20th	Day	13% 8% 3 4 4 4 3 3 4 7% 48%	1890
	21st to end	Day	15% 11% 2 5% 4 3 4 3 4 7% 44%	1785
December	1st to 10th	Day	9% 10% 2 6% 4 3 3 3 5% 6% 50%	2690
	11th to 20th	Day	12% 12% 4 8% 4 3 3 3 4 6% 42%	3088
	21st to end	Day	18% 19% 5% 9% 4 4 4 3 3 5% 27%	4070
January	1st to 10th	Night	13% 11% 5% 10% 5% 4 3 3 4 4 38%	8112
	11th to 20th	Night	9% 9% 4 11% 5% 4 4 3 3 4 44%	7680
	21st to end	Night	15% 11% 4 8% 5% 4 3 3 4 4 39%	8402
February	1st to 10th	Night	16% 11% 3 8% 4 3 3 3 4 7% 38%	2730
	11th to 20th	Night	14% 9% 3 8% 4 3 3 3 4 6% 43%	2515
	21st to end	Night	15% 9% 3 8% 4 3 3 3 5% 5% 42%	2076
March	1st to 10th	Night	16% 12% 2 6% 3 3 4 3 4 5% 42%	5181
	11th to 20th	Night	12% 8% 2 6% 3 3 3 3 4 5% 49%	4980
	21st to end	Night	14% 10% 3 7% 5% 4 5% 4 5% 6% 35%	5284
April	1st to 10th	Night	10% 6% 2 7% 3 4 3 3 6% 7% 49%	2000
	11th to 20th	Night	14% 8% 2 5% 4 3 3 3 5% 6% 46%	1975
	21st to end	Night	19% 7% 2 6% 4 3 4 3 5% 5% 40%	2000
May	1st to 10th	Night	18% 9% 3 7% 4 4 4 3 5% 7% 36%	4457
	11th to 20th	Night	13% 7% 3 6% 4 4 3 4 6% 7% 42%	3695
	21st to end	Night	18% 8% 2 7% 5% 4 4 3 6% 7% 37%	3942
June	1st to 10th	Night	8% 5% 2 5% 4 3 4 5% 7% 8% 49%	1600
	11th to 20th	Night	7% 4 1 4 4 3 3 3 7% 11% 54%	1600
	21st to end	Night	4 2 1 4 3 4 5% 4 6% 9% 59%	1600
July	1st to 10th	Night	11% 5% 2 5% 4 4 4 4 6% 9% 46%	3618
	11th to 20th	Night	5% 4 2 5% 4 4 4 4 6% 9% 53%	3770
	21st to end	Night	6% 5% 2 7% 4 4 5% 4 7% 10% 47%	4449
August	1st to 10th	Night	5% 2 1 5% 3 3 4 4 5% 8% 59%	2926
	11th to 20th	Night	10% 4 2 6% 4 4 4 4 6% 8% 49%	3165
	21st to end	Night	5% 4 1 5% 3 3 3 4 5% 9% 58%	3586
September	1st to 10th	Night	11% 5% 2 5% 4 4 3 4 4 6% 52%	2000
	11th to 20th	Night	9% 5% 1 5% 3 3 4 3 5% 8% 52%	2000
	21st to end	Night	9% 5% 2 4 3 3 4 3 5% 6% 55%	2235
October	1st to 10th	Night	7% 3 2 4 2 3 3 3 5% 7% 63%	2400
	11th to 20th	Night	15% 5% 2 4 3 2 3 3 4 5% 55%	2400
	21st to end	Night	20% 7% 2 4 3 3 3 2 4 5% 48%	2640
November	1st to 10th	Night	11% 6% 1 5% 3 3 3 2 4 6% 56%	2598
	11th to 20th	Night	14% 6% 2 5% 4 3 3 3 4 6% 50%	2730
	21st to end	Night	16% 10% 3 6% 3 3 3 2 4 6% 44%	2830
December	1st to 10th	Night	12% 9% 3 8% 4 3 3 3 4 6% 47%	4720
	11th to 20th	Night	14% 10% 3 7% 5% 4 4 2 4 5% 44%	5339
	21st to end	Night	21% 17% 5% 8% 4 4 4 3 4 5% 25%	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	47% 4 11% 24% 12% 2	4808
	11th to 20th	Day	38% 4 7% 27% 20% 5%	5310
	21st to end	Day	45% 3 9% 18% 16% 9%	5812
February	1st to 10th	Day	50% 4 11% 22% 12% 1	1890
	11th to 20th	Day	47% 5% 10% 19% 15% 4	2001
	21st to end	Day	43% 5% 9% 23% 15% 5%	1730
March	1st to 10th	Day	43% 4 5% 22% 17% 10%	4747
	11th to 20th	Day	31% 5% 8% 24% 23% 9%	4980
	21st to end	Day	43% 8% 14% 26% 9%	5647
April	1st to 10th	Day	37% 6% 12% 23% 19% 2	2400
	11th to 20th	Day	45% 5% 11% 21% 16% 3	2374
	21st to end	Day	49% 6% 11% 20% 13% 1	2400
May	1st to 10th	Day	47% 9% 12% 20% 11% 1	5457
	11th to 20th	Day	45% 8% 11% 21% 13% 1	6221
	21st to end	Day	43% 7% 9% 23% 16% 2	6949
June	1st to 10th	Day	35% 9% 14% 26% 15% 2	2800
	11th to 20th	Day	30% 8% 13% 27% 18% 3	2800
	21st to end	Day	28% 9% 18% 25% 16% 5%	2800
July	1st to 10th	Day	35% 7% 11% 28% 18% 2	6291
	11th to 20th	Day	33% 9% 15% 30% 11% 2	6114
	21st to end	Day	33% 10% 15% 30% 11% 1	6240
August	1st to 10th	Day	26% 6% 17% 32% 18%	4220
	11th to 20th	Day	37% 7% 13% 26% 16% 2	3919
	21st to end	Day	29% 6% 14% 35% 12% 3	4311
September	1st to 10th	Day	32% 7% 12% 26% 18% 4	2400
	11th to 20th	Day	34% 6% 9% 26% 24% 1	2400
	21st to end	Day	32% 7% 7% 26% 25% 3	2140
October	1st to 10th	Day	18% 3 6% 19% 45% 10%	3260
	11th to 20th	Day	23% 2 6% 21% 36% 11%	3240
	21st to end	Day	31% 4 7% 30% 25% 4	3538
November	1st to 10th	Day	34% 4 10% 24% 22% 6%	1918
	11th to 20th	Day	38% 4 8% 23% 20% 6%	1890
	21st to end	Day	43% 4 10% 26% 16% 2	1785
December	1st to 10th	Day	37% 4 8% 35% 15% 1	2690
	11th to 20th	Day	45% 5% 8% 23% 19% 1	3085
	21st to end	Day	61% 5% 10% 13% 9% 1	4070
January	1st to 10th	Night	50% 4 11% 22% 12% 1	8113
	11th to 20th	Night	44% 4 6% 23% 21% 3	7680
	21st to end	Night	49% 4 9% 19% 16% 3	8402
February	1st to 10th	Night	48% 4 10% 22% 14% 2	2730
	11th to 20th	Night	43% 4 11% 23% 16% 3	2516
	21st to end	Night	44% 5% 8% 22% 17% 4	2076
March	1st to 10th	Night	46% 4 7% 21% 18% 5%	5181
	11th to 20th	Night	37% 5% 9% 22% 19% 9%	4980
	21st to end	Night	45% 9% 13% 22% 11% 1	5284
April	1st to 10th	Night	33% 5% 13% 27% 19% 3	2000
	11th to 20th	Night	39% 6% 11% 23% 16% 5%	1975
	21st to end	Night	45% 6% 9% 23% 16% 1	2000
May	1st to 10th	Night	46% 8% 15% 18% 12% 1	4487
	11th to 20th	Night	41% 7% 12% 24% 14% 3	3696
	21st to end	Night	46% 8% 11% 22% 12% 2	3950
June	1st to 10th	Night	30% 9% 16% 25% 17% 2	1600
	11th to 20th	Night	24% 7% 14% 30% 21% 4	1600
	21st to end	Night	19% 9% 18% 28% 19% 7%	1600
July	1st to 10th	Night	34% 7% 13% 25% 20% 2	3580
	11th to 20th	Night	27% 8% 15% 34% 15% 2	3725
	21st to end	Night	30% 8% 17% 30% 14% 1	4320
August	1st to 10th	Night	21% 7% 20% 30% 21% 1	2916
	11th to 20th	Night	32% 7% 17% 26% 15% 3	3195
	21st to end	Night	22% 6% 16% 37% 17% 3	3586
September	1st to 10th	Night	32% 5% 10% 26% 21% 5%	2000
	11th to 20th	Night	30% 6% 11% 26% 26% 1	2000
	21st to end	Night	29% 6% 10% 23% 28% 3	2235
October	1st to 10th	Night	14% 3 6% 16% 49% 11%	3920
	11th to 20th	Night	23% 4 6% 21% 32% 14%	3899
	21st to end	Night	31% 4 7% 27% 27% 4	4252
November	1st to 10th	Night	31% 5% 11% 27% 21% 5%	2598
	11th to 20th	Night	36% 4 7% 24% 23% 6%	2730
	21st to end	Night	43% 4 9% 24% 18% 2	2830
December	1st to 10th	Night	40% 4 9% 30% 16% 1	4720
	11th to 20th	Night	44% 4 10% 23% 19% 1	5341
	21st to end	Night	61% 6% 10% 15% 6% 1	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	27% 26% 34% 10% 2	330
	11th to 20th	Day	30% 27% 36% 7%	330
	21st to end	Day	23% 32% 34% 10% 1	363
February	1st to 10th	Day	27% 24% 34% 14% 1	330
	11th to 20th	Day	30% 30% 30% 8% 2 1	330
	21st to end	Day	31% 25% 33% 10%	273
March	1st to 10th	Day	24% 26% 39% 10% 2	306
	11th to 20th	Day	31% 24% 33% 9% 3 1	300
	21st to end	Day	34% 23% 32% 10% 2	410
April	1st to 10th	Day	24% 20% 35% 16% 5%	500
	11th to 20th	Day	27% 20% 30% 17% 5% 1	492
	21st to end	Day	27% 18% 32% 17% 5% 1	500
May	1st to 10th	Day	22% 21% 32% 20% 6%	507
	11th to 20th	Day	20% 13% 31% 24% 10% 2	725
	21st to end	Day	14% 11% 36% 24% 12% 4	789
June	1st to 10th	Day	12% 7% 27% 28% 21% 5%	730
	11th to 20th	Day	9% 5% 26% 38% 19% 3	730
	21st to end	Day	6% 6% 31% 38% 15% 3	730
July	1st to 10th	Day	12% 4 18% 30% 29% 8%	721
	11th to 20th	Day	3 6% 23% 37% 26% 5%	718
	21st to end	Day	9% 4 18% 35% 27% 6%	794
August	1st to 10th	Day	10% 3 18% 42% 22% 5%	726
	11th to 20th	Day	11% 4 22% 30% 25% 8%	541
	21st to end	Day	11% 2 17% 31% 30% 8%	583
September	1st to 10th	Day	12% 5% 17% 35% 23% 7%	530
	11th to 20th	Day	7% 4 20% 34% 24% 11%	530
	21st to end	Day	12% 11% 24% 30% 17% 7%	455
October	1st to 10th	Day	11% 12% 32% 29% 12% 4	330
	11th to 20th	Day	23% 15% 33% 22% 6% 2	330
	21st to end	Day	33% 19% 29% 15% 3	363
November	1st to 10th	Day	16% 17% 40% 18% 8% 2	330
	11th to 20th	Day	24% 19% 34% 16% 5% 1	330
	21st to end	Day	32% 16% 31% 15% 5%	330
December	1st to 10th	Day	23% 23% 35% 13% 5% 2	330
	11th to 20th	Day	28% 20% 36% 12% 3	330
	21st to end	Day	29% 23% 36% 9% 2	363
January	1st to 10th	Night	23% 23% 34% 17% 3	1050
	11th to 20th	Night	24% 28% 32% 14% 3	1050
	21st to end	Night	20% 23% 38% 18% 1	1155
February	1st to 10th	Night	24% 20% 37% 16% 2	1050
	11th to 20th	Night	25% 20% 38% 16% 2	880
	21st to end	Night	25% 21% 35% 17% 1	692
March	1st to 10th	Night	27% 18% 36% 17% 2	722
	11th to 20th	Night	26% 15% 36% 20% 3	600
	21st to end	Night	26% 16% 35% 19% 4	660
April	1st to 10th	Night	17% 19% 39% 22% 4	600
	11th to 20th	Night	21% 15% 32% 27% 4	593
	21st to end	Night	23% 14% 39% 18% 6%	600
May	1st to 10th	Night	18% 11% 35% 29% 7%	600
	11th to 20th	Night	19% 8% 32% 26% 14% 1	591
	21st to end	Night	13% 8% 31% 30% 15% 3	650
June	1st to 10th	Night	10% 5% 23% 35% 26% 2	600
	11th to 20th	Night	7% 4 26% 40% 21% 2	600
	21st to end	Night	3 3 29% 43% 21% 2	600
July	1st to 10th	Night	12% 2 12% 36% 30% 7%	585
	11th to 20th	Night	6% 2 18% 40% 31% 4	582
	21st to end	Night	6% 3 15% 45% 28% 3	657
August	1st to 10th	Night	7% 2 15% 42% 31% 3	597
	11th to 20th	Night	7% 2 17% 40% 31% 3	594
	21st to end	Night	7% 2 16% 35% 35% 6%	660
September	1st to 10th	Night	14% 3 15% 36% 28% 5%	600
	11th to 20th	Night	13% 2 12% 39% 29% 6%	600
	21st to end	Night	12% 7% 24% 28% 24% 5%	765
October	1st to 10th	Night	8% 5% 30% 32% 23% 3	800
	11th to 20th	Night	17% 14% 33% 26% 11%	800
	21st to end	Night	24% 15% 33% 22% 6%	880
November	1st to 10th	Night	14% 12% 39% 27% 8%	820
	11th to 20th	Night	20% 15% 34% 23% 7%	840
	21st to end	Night	24% 19% 33% 19% 5%	940
December	1st to 10th	Night	23% 18% 37% 19% 4	1050
	11th to 20th	Night	24% 17% 36% 20% 3	1050
	21st to end	Night	25% 24% 32% 17% 2	1155

Page generated at 2026-02-28 00:40:20. Version ac07

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