Pleiades Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Pleiades Observatory clear sky chart from 2005-11-06 to 2026-05-02.

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% 8% 3 7% 5% 4 4 4 5% 7% 41%	132007
	Night	13% 8% 3 7% 4 4 4 3 5% 6% 42%	130756
Transparency	Day	41% 4 12% 25% 16% 2	135603
	Night	42% 4 11% 25% 17% 2	135238
Seeing	Day	16% 8% 27% 27% 16% 5%	17690
	Night	16% 8% 29% 32% 13% 1	27645

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	11% 11% 5% 12% 6% 5% 5% 3 5% 7% 30%	4691
	11th to 20th	Day	7% 9% 3 8% 6% 6% 7% 4 6% 6% 38%	5112
	21st to end	Day	13% 16% 6% 9% 5% 4 3 3 3 4 33%	5812
February	1st to 10th	Day	16% 9% 5% 8% 5% 5% 4 4 6% 8% 32%	1890
	11th to 20th	Day	13% 11% 3 8% 6% 5% 4 3 6% 6% 36%	1921
	21st to end	Day	13% 11% 4 8% 4 3 3 4 5% 5% 40%	1730
March	1st to 10th	Day	17% 10% 3 5% 3 3 3 2 3 4 46%	4810
	11th to 20th	Day	8% 6% 2 5% 4 4 4 4 6% 8% 48%	5090
	21st to end	Day	17% 10% 3 8% 5% 4 5% 4 5% 7% 32%	5768
April	1st to 10th	Day	16% 9% 3 8% 4 4 3 4 5% 7% 38%	2520
	11th to 20th	Day	15% 9% 3 7% 5% 4 4 3 4 6% 40%	2494
	21st to end	Day	19% 7% 3 6% 6% 3 5% 3 6% 6% 36%	2520
May	1st to 10th	Day	13% 8% 3 7% 6% 4 5% 4 6% 8% 36%	5797
	11th to 20th	Day	15% 6% 2 6% 4 4 4 3 5% 7% 44%	6319
	21st to end	Day	15% 7% 3 7% 5% 5% 4 4 5% 7% 39%	6935
June	1st to 10th	Day	7% 4 2 7% 5% 4 5% 4 7% 9% 46%	2800
	11th to 20th	Day	7% 5% 2 6% 5% 5% 5% 4 6% 9% 46%	2800
	21st to end	Day	9% 6% 2 6% 5% 6% 5% 5% 6% 8% 44%	2800
July	1st to 10th	Day	10% 5% 2 7% 5% 4 5% 4 7% 9% 43%	6360
	11th to 20th	Day	9% 6% 2 7% 5% 5% 6% 4 7% 9% 40%	6360
	21st to end	Day	10% 5% 2 7% 5% 5% 5% 4 7% 10% 39%	6545
August	1st to 10th	Day	5% 4 1 6% 5% 4 5% 5% 7% 10% 47%	4234
	11th to 20th	Day	12% 5% 2 6% 4 4 5% 4 6% 9% 43%	4009
	21st to end	Day	6% 4 2 6% 4 4 4 3 5% 10% 53%	4312
September	1st to 10th	Day	11% 5% 1 4 4 4 4 3 5% 7% 51%	2400
	11th to 20th	Day	10% 5% 2 4 3 3 4 3 6% 7% 53%	2400
	21st to end	Day	9% 4 2 4 4 4 4 4 6% 8% 52%	2124
October	1st to 10th	Day	12% 5% 2 6% 4 4 4 3 3 6% 52%	2000
	11th to 20th	Day	15% 7% 3 6% 4 4 3 2 4 5% 48%	2000
	21st to end	Day	16% 9% 3 7% 4 3 3 3 5% 5% 42%	2200
November	1st to 10th	Day	10% 8% 3 5% 4 3 4 3 4 7% 50%	1862
	11th to 20th	Day	15% 10% 3 7% 4 3 3 3 4 6% 43%	1864
	21st to end	Day	16% 13% 4 7% 4 4 3 2 4 5% 38%	1680
December	1st to 10th	Day	7% 9% 3 7% 4 4 3 3 5% 5% 50%	2690
	11th to 20th	Day	15% 9% 6% 11% 4 4 4 3 5% 6% 33%	3088
	21st to end	Day	23% 20% 6% 11% 4 4 4 3 3 4 18%	4070
January	1st to 10th	Night	12% 12% 6% 12% 5% 4 4 3 4 5% 32%	8232
	11th to 20th	Night	10% 11% 4 10% 6% 5% 6% 4 5% 6% 34%	7863
	21st to end	Night	13% 14% 4 11% 5% 4 3 3 4 7% 32%	8402
February	1st to 10th	Night	15% 12% 4 8% 5% 4 5% 3 4 6% 35%	2730
	11th to 20th	Night	14% 10% 3 7% 4 4 4 3 5% 6% 39%	2584
	21st to end	Night	15% 9% 3 8% 4 3 3 2 4 5% 44%	2076
March	1st to 10th	Night	19% 9% 3 6% 4 3 3 3 4 4 42%	5306
	11th to 20th	Night	12% 7% 2 6% 3 3 3 3 4 6% 51%	5090
	21st to end	Night	16% 10% 4 8% 6% 4 4 4 6% 7% 32%	5402
April	1st to 10th	Night	14% 7% 2 6% 4 3 4 4 5% 6% 44%	2100
	11th to 20th	Night	14% 7% 2 6% 4 4 4 3 5% 7% 43%	2075
	21st to end	Night	17% 7% 3 7% 4 4 5% 4 4 6% 39%	2100
May	1st to 10th	Night	17% 8% 3 7% 4 4 4 4 6% 8% 36%	4115
	11th to 20th	Night	12% 5% 2 5% 4 4 4 4 6% 8% 46%	3596
	21st to end	Night	13% 6% 3 6% 4 4 4 3 5% 7% 45%	3942
June	1st to 10th	Night	7% 4 1 6% 3 3 3 4 6% 8% 54%	1600
	11th to 20th	Night	7% 4 2 6% 4 3 5% 5% 7% 9% 49%	1600
	21st to end	Night	6% 5% 2 4 5% 6% 5% 5% 7% 8% 49%	1600
July	1st to 10th	Night	10% 5% 2 6% 4 4 4 4 6% 9% 47%	3618
	11th to 20th	Night	7% 4 2 6% 5% 5% 5% 4 6% 8% 50%	3617
	21st to end	Night	11% 5% 2 6% 4 4 4 4 6% 9% 44%	4308
August	1st to 10th	Night	5% 3 1 4 3 3 4 4 6% 10% 57%	2926
	11th to 20th	Night	10% 4 2 5% 4 4 4 4 5% 6% 53%	3042
	21st to end	Night	4 3 1 4 4 3 5% 3 7% 8% 58%	3586
September	1st to 10th	Night	11% 4 1 4 4 3 3 3 5% 6% 57%	2000
	11th to 20th	Night	8% 4 2 6% 4 4 5% 3 6% 7% 53%	2000
	21st to end	Night	9% 4 1 5% 3 5% 4 4 5% 6% 55%	2250
October	1st to 10th	Night	9% 5% 2 5% 3 3 4 3 4 7% 56%	2400
	11th to 20th	Night	14% 5% 2 5% 4 3 3 3 4 5% 53%	2400
	21st to end	Night	15% 9% 3 6% 4 4 4 4 3 5% 44%	2640
November	1st to 10th	Night	11% 6% 2 4 4 3 3 3 5% 5% 54%	2656
	11th to 20th	Night	14% 7% 3 6% 3 4 4 4 5% 6% 45%	2751
	21st to end	Night	17% 10% 3 6% 4 3 3 3 4 5% 41%	2940
December	1st to 10th	Night	10% 8% 3 7% 4 3 3 3 5% 6% 47%	4720
	11th to 20th	Night	14% 10% 4 8% 4 4 4 3 5% 5% 38%	5339
	21st to end	Night	23% 18% 6% 10% 5% 4 4 3 3 4 21%	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	52% 4 11% 21% 10% 1	4692
	11th to 20th	Day	44% 3 11% 22% 15% 4	5112
	21st to end	Day	56% 2 8% 23% 9% 2	5812
February	1st to 10th	Day	51% 3 14% 21% 9% 2	1890
	11th to 20th	Day	49% 4 10% 22% 13% 2	1930
	21st to end	Day	45% 3 11% 26% 13% 2	1730
March	1st to 10th	Day	43% 2 8% 24% 21% 2	4810
	11th to 20th	Day	33% 3 9% 31% 21% 3	5090
	21st to end	Day	51% 4 15% 23% 6% 1	5770
April	1st to 10th	Day	46% 3 12% 24% 13% 1	2520
	11th to 20th	Day	46% 3 10% 25% 15% 2	2494
	21st to end	Day	48% 4 12% 21% 13% 1	2520
May	1st to 10th	Day	44% 5% 13% 21% 16% 1	5833
	11th to 20th	Day	41% 4 10% 28% 14% 3	6323
	21st to end	Day	43% 5% 9% 24% 18% 1	6949
June	1st to 10th	Day	33% 6% 15% 30% 14% 2	2800
	11th to 20th	Day	34% 6% 12% 27% 19% 3	2800
	21st to end	Day	35% 6% 16% 23% 17% 3	2800
July	1st to 10th	Day	37% 5% 13% 28% 16% 1	6291
	11th to 20th	Day	40% 5% 14% 26% 14% 1	6285
	21st to end	Day	39% 7% 17% 27% 11% 1	6360
August	1st to 10th	Day	29% 6% 16% 36% 12% 1	4220
	11th to 20th	Day	37% 5% 13% 29% 15% 1	4048
	21st to end	Day	29% 5% 16% 30% 14% 5%	4311
September	1st to 10th	Day	32% 4 10% 28% 20% 6%	2400
	11th to 20th	Day	30% 4 10% 31% 20% 4	2400
	21st to end	Day	30% 5% 11% 29% 23% 2	2124
October	1st to 10th	Day	23% 2 6% 17% 47% 5%	3260
	11th to 20th	Day	27% 2 5% 22% 35% 9%	3240
	21st to end	Day	41% 3 7% 24% 22% 2	3538
November	1st to 10th	Day	35% 3 13% 26% 20% 4	1862
	11th to 20th	Day	43% 2 10% 26% 17% 3	1864
	21st to end	Day	49% 4 11% 23% 12% 1	1680
December	1st to 10th	Day	37% 2 16% 30% 14%	2690
	11th to 20th	Day	50% 4 12% 23% 10% 1	3085
	21st to end	Day	70% 4 10% 11% 5%	4070
January	1st to 10th	Night	55% 4 13% 21% 7%	8233
	11th to 20th	Night	50% 3 8% 20% 16% 3	7863
	21st to end	Night	54% 3 11% 20% 11% 2	8402
February	1st to 10th	Night	51% 3 11% 23% 11% 2	2730
	11th to 20th	Night	45% 3 12% 24% 14% 2	2585
	21st to end	Night	43% 3 10% 25% 16% 4	2076
March	1st to 10th	Night	47% 2 6% 24% 20% 2	5306
	11th to 20th	Night	36% 2 7% 29% 21% 5%	5090
	21st to end	Night	50% 5% 12% 23% 7% 2	5402
April	1st to 10th	Night	39% 4 11% 26% 17% 3	2100
	11th to 20th	Night	42% 3 9% 24% 17% 4	2075
	21st to end	Night	45% 3 11% 22% 16% 2	2100
May	1st to 10th	Night	47% 5% 11% 24% 11% 2	4144
	11th to 20th	Night	38% 5% 13% 27% 13% 4	3597
	21st to end	Night	38% 5% 12% 21% 22% 2	3950
June	1st to 10th	Night	27% 5% 16% 32% 17% 3	1600
	11th to 20th	Night	32% 4 11% 27% 22% 5%	1600
	21st to end	Night	30% 5% 17% 22% 22% 3	1600
July	1st to 10th	Night	35% 4 12% 29% 18% 2	3580
	11th to 20th	Night	31% 6% 13% 29% 20% 1	3572
	21st to end	Night	34% 7% 17% 29% 13% 1	4183
August	1st to 10th	Night	22% 5% 18% 36% 18% 2	2916
	11th to 20th	Night	32% 4 12% 31% 21% 1	3069
	21st to end	Night	23% 5% 19% 32% 17% 4	3586
September	1st to 10th	Night	29% 3 11% 23% 27% 6%	2000
	11th to 20th	Night	30% 4 11% 27% 24% 5%	2000
	21st to end	Night	30% 3 8% 27% 27% 3	2250
October	1st to 10th	Night	20% 2 6% 22% 45% 4	3920
	11th to 20th	Night	26% 2 8% 22% 34% 7%	3899
	21st to end	Night	40% 3 6% 25% 23% 4	4252
November	1st to 10th	Night	32% 4 12% 27% 20% 4	2656
	11th to 20th	Night	39% 2 9% 28% 20% 2	2751
	21st to end	Night	46% 3 10% 26% 14% 1	2940
December	1st to 10th	Night	39% 3 11% 35% 12% 1	4720
	11th to 20th	Night	46% 5% 11% 23% 14% 1	5341
	21st to end	Night	67% 4 10% 13% 5% 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	21% 12% 36% 24% 7%	330
	11th to 20th	Day	21% 19% 41% 17% 2	330
	21st to end	Day	18% 16% 40% 22% 4	363
February	1st to 10th	Day	23% 13% 41% 19% 4	330
	11th to 20th	Day	21% 19% 35% 17% 7% 1	330
	21st to end	Day	19% 14% 47% 17% 3	273
March	1st to 10th	Day	21% 16% 45% 16% 2	330
	11th to 20th	Day	25% 15% 34% 20% 5% 1	330
	21st to end	Day	30% 14% 36% 15% 4	445
April	1st to 10th	Day	25% 13% 33% 20% 8% 1	540
	11th to 20th	Day	22% 13% 34% 21% 8% 2	532
	21st to end	Day	21% 8% 36% 26% 9% 1	540
May	1st to 10th	Day	18% 8% 35% 27% 10% 2	632
	11th to 20th	Day	20% 9% 26% 27% 15% 3	725
	21st to end	Day	17% 7% 25% 29% 16% 7%	789
June	1st to 10th	Day	7% 4 21% 35% 24% 8%	730
	11th to 20th	Day	9% 3 23% 37% 20% 8%	730
	21st to end	Day	11% 5% 27% 34% 19% 4	730
July	1st to 10th	Day	11% 1 14% 32% 27% 14%	721
	11th to 20th	Day	9% 3 20% 36% 25% 8%	718
	21st to end	Day	10% 2 16% 33% 32% 8%	794
August	1st to 10th	Day	9% 2 13% 41% 26% 9%	726
	11th to 20th	Day	7% 2 16% 32% 30% 11%	618
	21st to end	Day	8% 2 12% 32% 33% 13%	583
September	1st to 10th	Day	12% 3 14% 32% 29% 10%	530
	11th to 20th	Day	9% 1 15% 32% 30% 13%	530
	21st to end	Day	11% 7% 24% 24% 22% 12%	425
October	1st to 10th	Day	15% 6% 28% 27% 16% 8%	330
	11th to 20th	Day	15% 18% 33% 24% 10% 1	330
	21st to end	Day	25% 14% 35% 20% 5% 1	363
November	1st to 10th	Day	13% 12% 35% 27% 10% 3	330
	11th to 20th	Day	20% 16% 32% 21% 8% 2	330
	21st to end	Day	27% 13% 30% 22% 8%	330
December	1st to 10th	Day	15% 12% 41% 23% 5% 3	330
	11th to 20th	Day	22% 14% 37% 22% 5%	330
	21st to end	Day	27% 17% 36% 18% 2	363
January	1st to 10th	Night	18% 12% 39% 24% 7%	1050
	11th to 20th	Night	18% 15% 38% 23% 5%	1050
	21st to end	Night	18% 13% 38% 28% 3	1155
February	1st to 10th	Night	20% 12% 39% 25% 4	1050
	11th to 20th	Night	19% 14% 35% 28% 4	950
	21st to end	Night	20% 14% 39% 23% 3	692
March	1st to 10th	Night	19% 12% 40% 24% 5%	757
	11th to 20th	Night	22% 9% 36% 28% 5%	630
	21st to end	Night	25% 11% 35% 24% 5%	693
April	1st to 10th	Night	17% 13% 37% 28% 5%	630
	11th to 20th	Night	17% 11% 33% 29% 9%	623
	21st to end	Night	19% 6% 35% 32% 7%	630
May	1st to 10th	Night	17% 5% 32% 37% 9% 1	603
	11th to 20th	Night	17% 5% 26% 38% 14% 1	591
	21st to end	Night	13% 4 25% 37% 19% 2	650
June	1st to 10th	Night	9% 2 19% 45% 24% 2	600
	11th to 20th	Night	10% 2 20% 45% 22% 2	600
	21st to end	Night	8% 3 25% 44% 20% 1	600
July	1st to 10th	Night	9% 3 9% 39% 36% 4	585
	11th to 20th	Night	8% 2 13% 44% 29% 4	582
	21st to end	Night	10% 2 9% 44% 35% 2	657
August	1st to 10th	Night	6% 2 10% 45% 33% 3	597
	11th to 20th	Night	9% 1 10% 41% 34% 5%	594
	21st to end	Night	6% 1 12% 39% 38% 3	660
September	1st to 10th	Night	13% 2 15% 38% 29% 4	600
	11th to 20th	Night	9% 2 16% 39% 31% 4	600
	21st to end	Night	10% 5% 20% 36% 23% 5%	750
October	1st to 10th	Night	11% 4 27% 32% 24% 2	800
	11th to 20th	Night	17% 9% 30% 33% 11% 1	800
	21st to end	Night	21% 10% 32% 28% 8%	880
November	1st to 10th	Night	14% 9% 35% 33% 9% 1	820
	11th to 20th	Night	17% 12% 37% 27% 7%	861
	21st to end	Night	23% 10% 34% 26% 7%	1050
December	1st to 10th	Night	20% 9% 37% 27% 5% 1	1050
	11th to 20th	Night	22% 9% 33% 29% 6%	1050
	21st to end	Night	26% 13% 33% 25% 4	1155

Page generated at 2026-04-29 01:21:42. Version ac07

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