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 2025-01-07.

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 3 5% 7% 41%	105012
	Night	13% 9% 3 7% 4 4 4 3 5% 6% 42%	104119
Transparency	Day	41% 4 12% 25% 16% 2	108647
	Night	41% 4 11% 25% 17% 2	108625
Seeing	Day	16% 8% 27% 28% 17% 5%	15930
	Night	16% 8% 29% 32% 14% 1	25852

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% 12% 5% 13% 6% 4 5% 4 5% 7% 29%	2575
	11th to 20th	Day	12% 12% 4 11% 6% 6% 6% 3 7% 5% 28%	2736
	21st to end	Day	16% 20% 8% 10% 5% 3 2 3 3 4 24%	3135
February	1st to 10th	Day	16% 10% 5% 8% 5% 5% 4 4 5% 7% 32%	1710
	11th to 20th	Day	13% 11% 3 7% 6% 5% 4 3 6% 7% 36%	1733
	21st to end	Day	14% 11% 4 8% 4 4 3 4 5% 5% 37%	1570
March	1st to 10th	Day	16% 12% 3 6% 4 3 3 3 4 5% 41%	3275
	11th to 20th	Day	10% 6% 3 5% 4 4 4 3 6% 7% 47%	3480
	21st to end	Day	18% 12% 4 8% 5% 4 4 3 5% 6% 30%	3939
April	1st to 10th	Day	15% 9% 2 7% 4 4 3 4 5% 7% 39%	2280
	11th to 20th	Day	15% 9% 3 7% 5% 4 4 3 4 6% 41%	2254
	21st to end	Day	20% 7% 3 7% 6% 3 4 3 5% 6% 36%	2280
May	1st to 10th	Day	13% 9% 3 7% 6% 4 5% 4 7% 9% 34%	4033
	11th to 20th	Day	17% 7% 3 6% 5% 4 4 3 4 7% 40%	4399
	21st to end	Day	12% 7% 2 6% 4 4 4 4 5% 6% 46%	4824
June	1st to 10th	Day	6% 4 2 7% 5% 4 5% 5% 7% 10% 46%	2660
	11th to 20th	Day	7% 5% 2 6% 5% 5% 5% 4 6% 8% 47%	2660
	21st to end	Day	9% 6% 2 6% 5% 6% 5% 5% 6% 8% 43%	2660
July	1st to 10th	Day	11% 6% 2 7% 5% 5% 5% 5% 7% 9% 39%	4440
	11th to 20th	Day	6% 4 2 5% 4 5% 5% 4 7% 10% 49%	4440
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 4 8% 11% 41%	4546
August	1st to 10th	Day	5% 4 1 6% 4 4 5% 5% 7% 10% 47%	4104
	11th to 20th	Day	13% 5% 2 6% 4 4 5% 4 6% 9% 43%	3884
	21st to end	Day	6% 4 2 6% 4 4 4 3 5% 10% 53%	4180
September	1st to 10th	Day	12% 5% 1 4 4 4 4 3 5% 7% 51%	2280
	11th to 20th	Day	10% 5% 2 4 3 3 4 3 5% 7% 53%	2280
	21st to end	Day	8% 4 2 4 5% 4 4 4 6% 7% 52%	2018
October	1st to 10th	Day	12% 5% 2 6% 4 4 4 3 3 6% 52%	1900
	11th to 20th	Day	16% 8% 2 5% 3 3 3 2 4 6% 48%	1900
	21st to end	Day	17% 9% 3 6% 4 3 3 3 4 5% 43%	2090
November	1st to 10th	Day	11% 8% 2 5% 4 3 4 3 4 7% 50%	1772
	11th to 20th	Day	15% 10% 3 7% 4 3 3 2 4 6% 43%	1775
	21st to end	Day	16% 13% 4 7% 4 4 3 2 4 5% 37%	1600
December	1st to 10th	Day	7% 9% 3 7% 4 4 3 3 5% 5% 51%	2610
	11th to 20th	Day	14% 9% 6% 11% 4 4 4 3 5% 6% 33%	3008
	21st to end	Day	23% 20% 6% 11% 4 4 4 3 3 4 18%	3982
January	1st to 10th	Night	13% 12% 7% 12% 5% 5% 4 2 4 5% 31%	4513
	11th to 20th	Night	13% 17% 6% 9% 4 5% 6% 6% 5% 4 25%	4219
	21st to end	Night	19% 18% 5% 12% 4 4 3 3 4 8% 22%	4532
February	1st to 10th	Night	15% 12% 4 8% 5% 4 5% 3 4 6% 34%	2470
	11th to 20th	Night	14% 10% 3 7% 5% 4 4 3 5% 6% 40%	2332
	21st to end	Night	15% 9% 3 8% 4 3 3 2 4 4 44%	1884
March	1st to 10th	Night	18% 10% 2 6% 4 4 3 3 5% 4 41%	3606
	11th to 20th	Night	14% 7% 2 6% 3 3 3 3 4 5% 51%	3480
	21st to end	Night	20% 12% 4 8% 5% 4 4 4 6% 6% 27%	3691
April	1st to 10th	Night	12% 8% 3 6% 4 3 4 4 5% 6% 45%	1900
	11th to 20th	Night	14% 8% 2 6% 4 4 4 3 5% 6% 44%	1875
	21st to end	Night	18% 7% 2 7% 4 4 5% 4 5% 6% 40%	1900
May	1st to 10th	Night	17% 9% 3 7% 4 4 4 4 6% 7% 34%	2864
	11th to 20th	Night	12% 6% 2 6% 5% 4 4 3 5% 8% 44%	2506
	21st to end	Night	8% 5% 3 5% 4 5% 4 3 5% 8% 50%	2743
June	1st to 10th	Night	7% 5% 1 6% 2 4 3 4 6% 8% 53%	1520
	11th to 20th	Night	7% 4 2 6% 4 3 5% 5% 7% 9% 48%	1520
	21st to end	Night	6% 5% 2 4 5% 6% 4 5% 7% 8% 48%	1520
July	1st to 10th	Night	13% 5% 2 6% 4 4 4 3 6% 9% 44%	2530
	11th to 20th	Night	5% 3 1 4 3 4 4 3 6% 7% 60%	2530
	21st to end	Night	8% 5% 2 5% 4 4 3 4 7% 9% 48%	2999
August	1st to 10th	Night	5% 2 1 4 3 3 4 4 6% 9% 58%	2836
	11th to 20th	Night	10% 5% 2 5% 4 4 4 4 5% 6% 53%	2947
	21st to end	Night	4 3 1 4 4 3 5% 3 6% 8% 58%	3476
September	1st to 10th	Night	11% 4 1 4 4 3 3 3 5% 6% 56%	1900
	11th to 20th	Night	8% 4 2 6% 3 3 4 3 6% 6% 54%	1900
	21st to end	Night	9% 4 1 5% 3 5% 4 4 5% 6% 54%	2137
October	1st to 10th	Night	9% 5% 2 5% 3 3 4 3 4 7% 55%	2280
	11th to 20th	Night	14% 5% 2 5% 4 3 3 3 4 5% 53%	2280
	21st to end	Night	15% 8% 3 6% 4 4 4 4 3 5% 44%	2508
November	1st to 10th	Night	11% 6% 2 4 4 3 3 3 5% 5% 54%	2526
	11th to 20th	Night	14% 7% 3 6% 3 4 4 4 5% 6% 45%	2620
	21st to end	Night	17% 10% 3 6% 4 3 3 3 4 5% 41%	2800
December	1st to 10th	Night	10% 8% 3 7% 4 3 3 3 5% 6% 48%	4580
	11th to 20th	Night	14% 10% 4 8% 4 4 4 3 5% 5% 37%	5199
	21st to end	Night	23% 18% 6% 10% 5% 4 4 3 3 4 20%	6996

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	54% 5% 11% 22% 9%	2576
	11th to 20th	Day	56% 3 13% 20% 8%	2736
	21st to end	Day	64% 3 10% 14% 8% 1	3135
February	1st to 10th	Day	52% 3 13% 20% 10% 2	1710
	11th to 20th	Day	48% 4 11% 23% 13% 1	1742
	21st to end	Day	47% 3 10% 25% 12% 2	1570
March	1st to 10th	Day	47% 3 10% 22% 17% 1	3275
	11th to 20th	Day	36% 4 13% 27% 18% 2	3480
	21st to end	Day	55% 3 16% 20% 5% 1	3939
April	1st to 10th	Day	44% 3 13% 26% 14% 1	2280
	11th to 20th	Day	45% 3 9% 25% 15% 2	2254
	21st to end	Day	49% 4 11% 21% 14% 1	2280
May	1st to 10th	Day	44% 6% 13% 23% 14% 1	4069
	11th to 20th	Day	46% 3 11% 30% 9%	4403
	21st to end	Day	38% 5% 9% 25% 22% 1	4837
June	1st to 10th	Day	32% 6% 15% 30% 15% 3	2660
	11th to 20th	Day	33% 6% 12% 27% 20% 4	2660
	21st to end	Day	36% 6% 16% 23% 17% 3	2660
July	1st to 10th	Day	41% 5% 12% 26% 13% 2	4371
	11th to 20th	Day	30% 5% 15% 30% 19% 2	4365
	21st to end	Day	35% 6% 19% 26% 14% 1	4403
August	1st to 10th	Day	29% 6% 15% 36% 13% 1	4090
	11th to 20th	Day	38% 5% 13% 28% 16% 1	3923
	21st to end	Day	29% 5% 16% 31% 14% 5%	4179
September	1st to 10th	Day	32% 4 10% 27% 20% 6%	2280
	11th to 20th	Day	30% 4 10% 31% 21% 4	2280
	21st to end	Day	31% 5% 11% 29% 22% 2	2018
October	1st to 10th	Day	23% 2 6% 17% 47% 5%	3160
	11th to 20th	Day	26% 2 5% 22% 36% 9%	3140
	21st to end	Day	41% 3 7% 25% 23% 2	3428
November	1st to 10th	Day	35% 3 13% 25% 20% 4	1772
	11th to 20th	Day	42% 2 10% 25% 17% 3	1775
	21st to end	Day	49% 4 11% 24% 11% 1	1600
December	1st to 10th	Day	36% 2 17% 31% 14%	2610
	11th to 20th	Day	50% 4 12% 22% 11% 1	3005
	21st to end	Day	70% 4 11% 11% 5%	3982
January	1st to 10th	Night	57% 3 9% 22% 9% 1	4513
	11th to 20th	Night	60% 3 10% 17% 9% 1	4219
	21st to end	Night	63% 3 10% 17% 6% 1	4532
February	1st to 10th	Night	52% 3 10% 22% 11% 2	2470
	11th to 20th	Night	44% 3 12% 25% 15% 2	2333
	21st to end	Night	45% 3 9% 24% 15% 4	1884
March	1st to 10th	Night	47% 3 8% 26% 16% 1	3606
	11th to 20th	Night	38% 3 9% 28% 18% 5%	3480
	21st to end	Night	55% 5% 13% 21% 5%	3691
April	1st to 10th	Night	38% 4 12% 26% 17% 3	1900
	11th to 20th	Night	41% 3 9% 26% 17% 4	1875
	21st to end	Night	45% 3 11% 22% 17% 2	1900
May	1st to 10th	Night	47% 5% 12% 23% 12% 1	2893
	11th to 20th	Night	41% 5% 14% 28% 12% 1	2507
	21st to end	Night	31% 5% 13% 23% 25% 3	2751
June	1st to 10th	Night	28% 5% 16% 31% 17% 3	1520
	11th to 20th	Night	32% 4 11% 26% 22% 5%	1520
	21st to end	Night	31% 5% 16% 22% 22% 3	1520
July	1st to 10th	Night	38% 4 13% 27% 15% 3	2490
	11th to 20th	Night	22% 4 13% 34% 25% 2	2482
	21st to end	Night	30% 6% 18% 28% 17% 1	2904
August	1st to 10th	Night	21% 5% 18% 36% 18% 2	2826
	11th to 20th	Night	32% 4 11% 31% 21% 1	2974
	21st to end	Night	23% 5% 19% 32% 17% 4	3476
September	1st to 10th	Night	29% 3 11% 23% 27% 7%	1900
	11th to 20th	Night	30% 4 11% 25% 25% 5%	1900
	21st to end	Night	30% 4 8% 28% 26% 3	2137
October	1st to 10th	Night	20% 2 6% 22% 45% 4	3800
	11th to 20th	Night	26% 2 8% 22% 35% 8%	3779
	21st to end	Night	39% 3 6% 25% 23% 4	4120
November	1st to 10th	Night	32% 4 13% 27% 20% 5%	2526
	11th to 20th	Night	40% 2 9% 27% 20% 2	2620
	21st to end	Night	46% 3 9% 27% 14% 1	2800
December	1st to 10th	Night	38% 3 11% 36% 12% 1	4580
	11th to 20th	Night	46% 5% 12% 23% 13% 1	5201
	21st to end	Night	68% 4 10% 13% 5% 1	6996

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% 13% 33% 24% 8%	288
	11th to 20th	Day	25% 15% 38% 19% 3	270
	21st to end	Day	20% 14% 37% 24% 5%	297
February	1st to 10th	Day	24% 13% 41% 18% 4	270
	11th to 20th	Day	20% 18% 33% 20% 8% 1	270
	21st to end	Day	20% 12% 45% 18% 4	225
March	1st to 10th	Day	19% 15% 46% 17% 3	270
	11th to 20th	Day	24% 13% 35% 21% 6% 1	270
	21st to end	Day	32% 13% 33% 16% 5% 1	371
April	1st to 10th	Day	22% 12% 34% 22% 8% 1	460
	11th to 20th	Day	21% 10% 34% 24% 9% 2	452
	21st to end	Day	22% 7% 35% 26% 9% 1	460
May	1st to 10th	Day	19% 8% 34% 27% 10% 2	582
	11th to 20th	Day	21% 8% 27% 27% 15% 3	675
	21st to end	Day	16% 6% 25% 29% 17% 7%	734
June	1st to 10th	Day	6% 4 22% 35% 25% 8%	680
	11th to 20th	Day	8% 3 23% 38% 21% 7%	680
	21st to end	Day	11% 6% 28% 33% 19% 4	680
July	1st to 10th	Day	11% 1 14% 32% 27% 15%	671
	11th to 20th	Day	8% 3 19% 37% 25% 8%	668
	21st to end	Day	9% 1 16% 34% 32% 8%	739
August	1st to 10th	Day	9% 2 12% 41% 27% 9%	676
	11th to 20th	Day	8% 2 17% 33% 29% 10%	573
	21st to end	Day	8% 2 12% 32% 33% 14%	539
September	1st to 10th	Day	13% 3 13% 32% 29% 10%	490
	11th to 20th	Day	9% 1 16% 32% 29% 13%	490
	21st to end	Day	11% 7% 26% 23% 21% 11%	390
October	1st to 10th	Day	15% 6% 29% 27% 16% 8%	300
	11th to 20th	Day	15% 18% 33% 23% 10% 1	300
	21st to end	Day	25% 13% 35% 21% 6% 1	330
November	1st to 10th	Day	13% 10% 35% 28% 11% 3	300
	11th to 20th	Day	20% 16% 32% 21% 8% 2	300
	21st to end	Day	26% 12% 31% 22% 8%	300
December	1st to 10th	Day	15% 12% 40% 23% 6% 4	300
	11th to 20th	Day	22% 14% 36% 23% 5%	300
	21st to end	Day	27% 17% 36% 18% 2	330
January	1st to 10th	Night	18% 12% 39% 24% 7%	981
	11th to 20th	Night	19% 13% 38% 24% 6% 1	950
	21st to end	Night	19% 11% 37% 29% 3	1045
February	1st to 10th	Night	21% 11% 39% 25% 5%	950
	11th to 20th	Night	19% 12% 36% 29% 4	858
	21st to end	Night	21% 13% 39% 23% 4	628
March	1st to 10th	Night	18% 11% 41% 25% 5%	685
	11th to 20th	Night	22% 8% 35% 29% 6%	570
	21st to end	Night	26% 10% 33% 24% 6%	627
April	1st to 10th	Night	16% 13% 37% 30% 5%	570
	11th to 20th	Night	17% 10% 33% 30% 9% 1	563
	21st to end	Night	20% 5% 35% 32% 8%	570
May	1st to 10th	Night	16% 5% 31% 37% 9% 1	570
	11th to 20th	Night	17% 4 26% 38% 14% 1	561
	21st to end	Night	13% 3 25% 36% 20% 3	617
June	1st to 10th	Night	9% 2 18% 45% 24% 2	570
	11th to 20th	Night	10% 2 20% 44% 22% 2	570
	21st to end	Night	8% 3 25% 43% 20% 1	570
July	1st to 10th	Night	9% 3 9% 38% 36% 4	555
	11th to 20th	Night	8% 2 14% 44% 29% 4	552
	21st to end	Night	9% 2 9% 44% 34% 2	624
August	1st to 10th	Night	6% 2 10% 45% 34% 3	567
	11th to 20th	Night	9% 1 10% 41% 34% 5%	564
	21st to end	Night	6% 1 12% 39% 39% 3	627
September	1st to 10th	Night	13% 1 15% 38% 29% 4	570
	11th to 20th	Night	9% 2 16% 39% 29% 4	570
	21st to end	Night	10% 5% 21% 37% 22% 5%	712
October	1st to 10th	Night	12% 4 27% 31% 24% 2	760
	11th to 20th	Night	17% 9% 31% 32% 11% 1	760
	21st to end	Night	21% 9% 32% 29% 8%	836
November	1st to 10th	Night	14% 8% 35% 33% 9% 1	780
	11th to 20th	Night	17% 12% 36% 27% 7%	820
	21st to end	Night	23% 10% 34% 26% 7%	1000
December	1st to 10th	Night	20% 9% 37% 28% 6% 1	1000
	11th to 20th	Night	23% 8% 33% 30% 7%	1000
	21st to end	Night	26% 13% 32% 25% 3	1100

Page generated at 2025-01-04 02:23:58. Version ac07

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