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 2024-01-17.

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	13% 8% 3 6% 5% 3 4 5% 5% 7% 41%	74765
	Night	13% 8% 3 6% 5% 3 4 4 5% 6% 43%	75082
Transparency	Day	29% 36% 8% 16% 10% 1	74474
	Night	29% 36% 7% 16% 10% 2	74894
Seeing	Day	16% 7% 26% 28% 17% 6%	14577
	Night	16% 7% 29% 32% 14% 1	24552

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 9% 6% 3 5% 4 5% 8% 32%	1517
	11th to 20th	Day	15% 14% 7% 10% 7% 4 4 4 5% 4 28%	1612
	21st to end	Day	15% 14% 5% 9% 7% 3 4 4 4 6% 30%	1782
February	1st to 10th	Day	17% 10% 5% 7% 6% 3 5% 4 5% 7% 31%	1620
	11th to 20th	Day	14% 10% 4 6% 7% 4 5% 4 6% 7% 34%	1640
	21st to end	Day	15% 11% 4 8% 5% 3 3 4 5% 6% 36%	1480
March	1st to 10th	Day	14% 10% 6% 7% 5% 4 4 4 4 5% 36%	1885
	11th to 20th	Day	15% 9% 4 5% 7% 3 4 4 5% 7% 37%	1980
	21st to end	Day	19% 11% 5% 7% 5% 3 5% 5% 5% 6% 30%	2248
April	1st to 10th	Day	15% 8% 3 6% 5% 3 4 4 5% 8% 40%	2160
	11th to 20th	Day	16% 9% 3 6% 5% 3 4 3 4 6% 41%	2134
	21st to end	Day	20% 7% 3 6% 6% 3 4 5% 5% 6% 36%	2160
May	1st to 10th	Day	16% 9% 3 7% 6% 4 5% 5% 6% 8% 32%	2303
	11th to 20th	Day	17% 7% 3 5% 6% 4 5% 4 5% 7% 38%	2479
	21st to end	Day	14% 7% 3 6% 6% 4 5% 6% 6% 8% 36%	2725
June	1st to 10th	Day	7% 4 3 6% 6% 3 5% 6% 6% 11% 45%	2520
	11th to 20th	Day	7% 4 2 5% 6% 4 5% 5% 6% 8% 47%	2520
	21st to end	Day	9% 5% 3 5% 5% 4 5% 6% 6% 9% 43%	2520
July	1st to 10th	Day	7% 3 2 5% 5% 2 5% 6% 7% 10% 47%	2520
	11th to 20th	Day	6% 4 2 4 6% 3 5% 6% 7% 9% 48%	2520
	21st to end	Day	8% 4 2 4 5% 3 5% 5% 6% 9% 47%	2599
August	1st to 10th	Day	7% 5% 2 6% 5% 3 6% 6% 7% 10% 42%	2328
	11th to 20th	Day	8% 4 2 5% 5% 3 5% 5% 6% 9% 46%	2214
	21st to end	Day	7% 4 2 5% 5% 3 4 4 5% 9% 51%	2376
September	1st to 10th	Day	12% 4 2 4 5% 3 5% 4 5% 7% 49%	2160
	11th to 20th	Day	10% 4 2 3 4 2 4 4 5% 7% 54%	2160
	21st to end	Day	7% 3 2 4 5% 3 4 5% 6% 7% 53%	1913
October	1st to 10th	Day	12% 4 2 6% 5% 3 5% 4 3 6% 51%	1800
	11th to 20th	Day	17% 8% 3 5% 4 3 3 3 4 6% 46%	1800
	21st to end	Day	18% 9% 3 6% 4 2 3 3 4 5% 42%	1980
November	1st to 10th	Day	10% 6% 3 5% 4 2 3 4 4 7% 51%	1682
	11th to 20th	Day	16% 8% 3 6% 4 2 4 3 4 6% 43%	1687
	21st to end	Day	16% 12% 5% 6% 5% 3 4 3 4 6% 38%	1520
December	1st to 10th	Day	13% 12% 4 8% 5% 3 5% 4 4 5% 37%	1520
	11th to 20th	Day	15% 9% 5% 6% 5% 3 4 4 5% 7% 37%	1918
	21st to end	Day	22% 16% 7% 9% 5% 3 4 4 3 5% 21%	2783
January	1st to 10th	Night	12% 11% 4 6% 6% 4 5% 4 4 5% 40%	2648
	11th to 20th	Night	13% 12% 7% 8% 6% 3 4 4 4 5% 33%	2491
	21st to end	Night	13% 11% 5% 9% 6% 3 4 4 5% 7% 32%	2574
February	1st to 10th	Night	16% 12% 4 8% 5% 3 5% 4 4 6% 33%	2340
	11th to 20th	Night	14% 9% 4 6% 5% 3 4 4 5% 7% 40%	2205
	21st to end	Night	16% 9% 4 7% 5% 2 3 3 4 4 42%	1776
March	1st to 10th	Night	14% 9% 3 5% 5% 3 3 4 4 6% 44%	2089
	11th to 20th	Night	17% 8% 4 6% 5% 2 5% 4 4 5% 38%	1980
	21st to end	Night	22% 11% 4 7% 6% 3 5% 5% 6% 6% 27%	2107
April	1st to 10th	Night	12% 7% 3 5% 5% 3 4 4 5% 6% 45%	1800
	11th to 20th	Night	15% 7% 3 5% 4 3 5% 3 5% 6% 44%	1775
	21st to end	Night	18% 6% 3 6% 5% 3 5% 5% 5% 6% 40%	1800
May	1st to 10th	Night	15% 7% 3 6% 5% 3 5% 5% 6% 8% 38%	1652
	11th to 20th	Night	14% 6% 2 5% 6% 4 4 4 5% 7% 43%	1416
	21st to end	Night	11% 5% 4 5% 6% 3 5% 5% 5% 9% 42%	1551
June	1st to 10th	Night	7% 4 2 6% 3 3 3 5% 5% 9% 55%	1440
	11th to 20th	Night	8% 3 2 5% 5% 2 5% 5% 7% 9% 48%	1440
	21st to end	Night	6% 4 2 4 5% 4 6% 6% 7% 8% 48%	1440
July	1st to 10th	Night	8% 3 2 4 5% 3 4 5% 6% 10% 51%	1440
	11th to 20th	Night	5% 3 1 3 3 3 4 5% 7% 8% 57%	1440
	21st to end	Night	7% 4 2 3 4 3 3 5% 5% 9% 54%	1715
August	1st to 10th	Night	6% 3 2 4 5% 2 5% 6% 6% 10% 51%	1611
	11th to 20th	Night	8% 4 1 4 4 3 5% 4 5% 5% 55%	1684
	21st to end	Night	6% 3 1 3 4 2 4 4 5% 8% 61%	1980
September	1st to 10th	Night	12% 4 2 3 4 2 3 4 5% 6% 55%	1800
	11th to 20th	Night	8% 4 2 5% 4 2 4 5% 6% 6% 54%	1800
	21st to end	Night	8% 3 1 4 4 4 4 5% 5% 6% 56%	2024
October	1st to 10th	Night	10% 5% 2 4 4 2 4 4 4 7% 54%	2160
	11th to 20th	Night	14% 5% 2 4 4 2 3 3 4 5% 52%	2160
	21st to end	Night	16% 8% 3 6% 4 3 4 5% 3 5% 44%	2376
November	1st to 10th	Night	11% 5% 2 4 4 2 3 4 5% 6% 56%	2396
	11th to 20th	Night	14% 6% 3 6% 4 3 4 5% 5% 7% 45%	2489
	21st to end	Night	17% 9% 3 5% 4 2 4 4 4 5% 42%	2660
December	1st to 10th	Night	14% 10% 4 7% 5% 3 4 5% 5% 6% 37%	2660
	11th to 20th	Night	15% 8% 4 7% 5% 3 4 4 4 5% 41%	3279
	21st to end	Night	21% 17% 7% 9% 6% 3 4 3 3 4 21%	4884

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	32% 39% 8% 13% 8%	1518
	11th to 20th	Day	36% 39% 7% 13% 5%	1612
	21st to end	Day	39% 35% 9% 12% 4 1	1782
February	1st to 10th	Day	34% 36% 10% 13% 6% 1	1620
	11th to 20th	Day	32% 37% 8% 14% 8% 1	1649
	21st to end	Day	32% 36% 8% 17% 6% 1	1480
March	1st to 10th	Day	32% 35% 7% 17% 8%	1885
	11th to 20th	Day	34% 36% 9% 14% 5%	1980
	21st to end	Day	38% 36% 8% 13% 5% 1	2248
April	1st to 10th	Day	27% 35% 9% 20% 9% 1	2160
	11th to 20th	Day	32% 34% 6% 17% 10% 1	2134
	21st to end	Day	33% 36% 9% 14% 8% 1	2160
May	1st to 10th	Day	34% 37% 8% 14% 7% 1	2303
	11th to 20th	Day	32% 36% 7% 15% 9% 1	2483
	21st to end	Day	31% 38% 7% 12% 11% 1	2725
June	1st to 10th	Day	25% 36% 10% 19% 9%	2520
	11th to 20th	Day	21% 37% 8% 17% 14% 2	2520
	21st to end	Day	25% 37% 8% 15% 13% 2	2520
July	1st to 10th	Day	23% 38% 9% 18% 10% 2	2451
	11th to 20th	Day	21% 38% 9% 17% 12% 2	2445
	21st to end	Day	25% 37% 8% 19% 10% 1	2455
August	1st to 10th	Day	23% 37% 11% 19% 9% 1	2314
	11th to 20th	Day	22% 36% 8% 21% 11% 1	2214
	21st to end	Day	17% 36% 7% 19% 16% 5%	2376
September	1st to 10th	Day	24% 36% 5% 17% 13% 5%	2160
	11th to 20th	Day	22% 35% 6% 22% 15% 1	2160
	21st to end	Day	19% 36% 7% 20% 15% 2	1913
October	1st to 10th	Day	19% 35% 7% 20% 18% 2	1800
	11th to 20th	Day	31% 35% 5% 16% 12% 2	1800
	21st to end	Day	29% 36% 6% 17% 10% 2	1980
November	1st to 10th	Day	21% 39% 9% 17% 12% 2	1682
	11th to 20th	Day	28% 37% 7% 17% 9% 2	1687
	21st to end	Day	28% 39% 9% 16% 8%	1520
December	1st to 10th	Day	31% 39% 10% 14% 6%	1520
	11th to 20th	Day	32% 31% 9% 19% 7% 1	1915
	21st to end	Day	55% 24% 7% 10% 3	2783
January	1st to 10th	Night	31% 38% 7% 16% 7%	2648
	11th to 20th	Night	33% 39% 8% 13% 7%	2491
	21st to end	Night	36% 37% 10% 11% 6% 1	2574
February	1st to 10th	Night	35% 35% 6% 14% 7% 1	2340
	11th to 20th	Night	29% 37% 7% 16% 9% 1	2206
	21st to end	Night	31% 35% 7% 17% 8% 2	1776
March	1st to 10th	Night	27% 35% 7% 20% 11%	2089
	11th to 20th	Night	34% 35% 9% 14% 7% 1	1980
	21st to end	Night	41% 36% 8% 11% 4	2107
April	1st to 10th	Night	24% 35% 8% 18% 12% 2	1800
	11th to 20th	Night	30% 34% 4 17% 11% 4	1775
	21st to end	Night	30% 35% 8% 15% 10% 1	1800
May	1st to 10th	Night	29% 37% 9% 16% 8% 1	1652
	11th to 20th	Night	29% 37% 6% 13% 15% 1	1417
	21st to end	Night	26% 38% 9% 14% 11% 2	1552
June	1st to 10th	Night	20% 36% 11% 21% 10% 1	1440
	11th to 20th	Night	21% 36% 8% 15% 16% 4	1440
	21st to end	Night	23% 37% 9% 12% 16% 3	1440
July	1st to 10th	Night	21% 38% 7% 20% 11% 2	1400
	11th to 20th	Night	18% 38% 7% 20% 15% 3	1392
	21st to end	Night	20% 38% 8% 22% 10% 2	1620
August	1st to 10th	Night	17% 37% 10% 22% 11% 2	1601
	11th to 20th	Night	20% 35% 7% 22% 15% 1	1684
	21st to end	Night	15% 36% 7% 19% 18% 7%	1980
September	1st to 10th	Night	21% 35% 6% 14% 18% 6%	1800
	11th to 20th	Night	22% 36% 6% 17% 16% 2	1800
	21st to end	Night	19% 35% 6% 20% 17% 3	2024
October	1st to 10th	Night	15% 35% 7% 20% 21% 3	2160
	11th to 20th	Night	27% 35% 6% 17% 11% 4	2160
	21st to end	Night	30% 36% 5% 15% 13% 2	2376
November	1st to 10th	Night	17% 40% 9% 19% 13% 3	2396
	11th to 20th	Night	25% 38% 7% 17% 11% 1	2489
	21st to end	Night	29% 38% 6% 18% 8% 1	2660
December	1st to 10th	Night	32% 38% 8% 17% 4	2660
	11th to 20th	Night	33% 32% 8% 18% 9%	3281
	21st to end	Night	57% 24% 7% 9% 3	4884

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% 33% 25% 9%	270
	11th to 20th	Day	27% 15% 38% 17% 3	258
	21st to end	Day	19% 16% 38% 22% 5%	264
February	1st to 10th	Day	27% 13% 41% 16% 3	240
	11th to 20th	Day	22% 20% 33% 18% 7% 1	240
	21st to end	Day	22% 13% 43% 18% 4 1	198
March	1st to 10th	Day	18% 14% 48% 16% 3	240
	11th to 20th	Day	26% 10% 35% 22% 6% 1	240
	21st to end	Day	33% 12% 33% 16% 6% 1	334
April	1st to 10th	Day	21% 11% 34% 23% 9% 1	420
	11th to 20th	Day	21% 8% 34% 24% 10% 3	412
	21st to end	Day	22% 7% 33% 28% 9% 1	420
May	1st to 10th	Day	20% 8% 33% 27% 10% 2	535
	11th to 20th	Day	21% 8% 28% 26% 15% 3	625
	21st to end	Day	16% 5% 24% 29% 18% 8%	679
June	1st to 10th	Day	6% 3 20% 36% 27% 9%	630
	11th to 20th	Day	9% 3 22% 37% 21% 8%	630
	21st to end	Day	11% 5% 26% 34% 20% 4	630
July	1st to 10th	Day	11% 1 11% 32% 29% 16%	621
	11th to 20th	Day	9% 3 19% 37% 26% 7%	618
	21st to end	Day	10% 1 15% 34% 33% 7%	684
August	1st to 10th	Day	10% 1 11% 42% 27% 10%	626
	11th to 20th	Day	7% 2 16% 33% 31% 11%	528
	21st to end	Day	8% 2 12% 31% 33% 14%	495
September	1st to 10th	Day	14% 3 13% 32% 28% 10%	450
	11th to 20th	Day	9% 1 16% 33% 28% 13%	450
	21st to end	Day	10% 6% 27% 23% 23% 11%	356
October	1st to 10th	Day	17% 6% 29% 27% 14% 7%	270
	11th to 20th	Day	17% 18% 34% 21% 8% 1	270
	21st to end	Day	27% 11% 34% 21% 6% 1	297
November	1st to 10th	Day	12% 9% 37% 28% 11% 4	270
	11th to 20th	Day	20% 16% 31% 22% 9% 3	270
	21st to end	Day	26% 11% 31% 23% 9%	270
December	1st to 10th	Day	15% 11% 38% 25% 7% 4	270
	11th to 20th	Day	21% 12% 36% 25% 5%	270
	21st to end	Day	27% 16% 36% 19% 2	297
January	1st to 10th	Night	18% 12% 39% 24% 7%	950
	11th to 20th	Night	19% 12% 38% 24% 6% 1	935
	21st to end	Night	18% 12% 38% 29% 3	990
February	1st to 10th	Night	22% 11% 40% 23% 4	900
	11th to 20th	Night	20% 12% 35% 29% 4	811
	21st to end	Night	21% 14% 39% 22% 4 1	592
March	1st to 10th	Night	18% 11% 41% 25% 5%	649
	11th to 20th	Night	22% 8% 34% 29% 6%	540
	21st to end	Night	27% 10% 33% 24% 6% 1	594
April	1st to 10th	Night	16% 12% 36% 30% 5%	540
	11th to 20th	Night	18% 10% 32% 30% 10% 1	533
	21st to end	Night	20% 5% 35% 33% 8%	540
May	1st to 10th	Night	17% 5% 31% 37% 10% 1	540
	11th to 20th	Night	17% 5% 26% 37% 14% 1	531
	21st to end	Night	13% 3 24% 36% 21% 3	584
June	1st to 10th	Night	8% 2 17% 46% 25% 2	540
	11th to 20th	Night	10% 2 19% 44% 23% 2	540
	21st to end	Night	8% 2 24% 45% 20% 1	540
July	1st to 10th	Night	8% 2 9% 38% 38% 5%	525
	11th to 20th	Night	8% 1 14% 44% 29% 4	522
	21st to end	Night	9% 1 9% 45% 34% 2	591
August	1st to 10th	Night	7% 1 9% 46% 34% 4	537
	11th to 20th	Night	9% 1 9% 41% 35% 5%	534
	21st to end	Night	6% 1 12% 38% 39% 3	594
September	1st to 10th	Night	14% 1 15% 38% 28% 4	540
	11th to 20th	Night	9% 2 17% 38% 29% 4	540
	21st to end	Night	10% 4 21% 37% 23% 5%	674
October	1st to 10th	Night	12% 4 28% 30% 23% 2	720
	11th to 20th	Night	18% 9% 31% 32% 10% 1	720
	21st to end	Night	22% 9% 33% 28% 9%	792
November	1st to 10th	Night	13% 8% 35% 34% 10% 1	740
	11th to 20th	Night	17% 11% 37% 27% 7% 1	779
	21st to end	Night	23% 9% 34% 26% 7%	950
December	1st to 10th	Night	21% 9% 37% 27% 6% 1	950
	11th to 20th	Night	23% 8% 32% 30% 7%	950
	21st to end	Night	26% 13% 32% 25% 4	1045

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