PS-191 Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the PS-191 Observatory clear sky chart from 2005-11-06 to 2025-06-10.

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% 7% 3 7% 5% 5% 5% 4 6% 7% 40%	119611
	Night	14% 7% 2 6% 4 4 4 4 5% 7% 42%	118628
Transparency	Day	42% 4 10% 22% 18% 4	123415
	Night	41% 3 9% 22% 19% 5%	122960
Seeing	Day	18% 10% 23% 23% 17% 8%	15493
	Night	17% 9% 24% 29% 19% 3	25555

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	16% 11% 4 6% 4 3 3 3 4 4 43%	4056
	11th to 20th	Day	14% 9% 5% 6% 3 3 3 2 3 5% 46%	4060
	21st to end	Day	21% 9% 2 6% 4 3 3 3 4 6% 37%	4694
February	1st to 10th	Day	18% 9% 3 7% 5% 3 4 4 4 7% 37%	2000
	11th to 20th	Day	21% 9% 3 7% 4 4 4 3 4 6% 34%	1948
	21st to end	Day	20% 9% 3 7% 4 4 3 3 4 5% 38%	1650
March	1st to 10th	Day	19% 9% 3 7% 4 5% 5% 3 6% 6% 33%	4515
	11th to 20th	Day	12% 6% 2 6% 4 4 3 3 5% 6% 50%	4980
	21st to end	Day	15% 7% 2 7% 4 5% 4 3 4 6% 43%	5465
April	1st to 10th	Day	12% 5% 2 6% 5% 5% 6% 4 6% 7% 42%	2345
	11th to 20th	Day	12% 7% 2 6% 5% 4 4 3 4 6% 47%	2374
	21st to end	Day	11% 6% 3 5% 6% 5% 5% 4 6% 7% 43%	2533
May	1st to 10th	Day	16% 8% 3 7% 5% 5% 5% 4 7% 8% 32%	5862
	11th to 20th	Day	16% 8% 3 9% 6% 5% 5% 4 7% 8% 28%	5861
	21st to end	Day	14% 7% 3 7% 5% 4 5% 4 6% 7% 39%	6433
June	1st to 10th	Day	9% 6% 2 7% 5% 5% 5% 5% 8% 10% 37%	2587
	11th to 20th	Day	5% 4 2 6% 5% 5% 7% 6% 8% 11% 42%	2470
	21st to end	Day	7% 5% 2 7% 6% 6% 7% 5% 8% 10% 37%	2470
July	1st to 10th	Day	7% 5% 2 7% 6% 7% 8% 6% 9% 12% 31%	4120
	11th to 20th	Day	9% 6% 3 7% 5% 6% 6% 5% 8% 10% 36%	4120
	21st to end	Day	15% 11% 4 10% 7% 7% 7% 6% 8% 8% 19%	4466
August	1st to 10th	Day	5% 3 2 6% 5% 5% 7% 5% 8% 12% 42%	4104
	11th to 20th	Day	7% 5% 2 6% 5% 5% 6% 4 8% 11% 41%	3736
	21st to end	Day	8% 4 1 6% 4 4 5% 4 6% 8% 49%	4180
September	1st to 10th	Day	10% 5% 3 7% 4 5% 5% 5% 7% 9% 40%	2147
	11th to 20th	Day	14% 6% 2 7% 4 5% 5% 3 6% 7% 41%	2090
	21st to end	Day	13% 4 2 6% 5% 3 4 3 5% 7% 47%	2090
October	1st to 10th	Day	10% 5% 1 5% 4 5% 4 3 4 5% 55%	2090
	11th to 20th	Day	10% 5% 2 4 3 3 3 3 5% 6% 56%	2014
	21st to end	Day	13% 4 2 5% 3 4 3 3 5% 6% 52%	2020
November	1st to 10th	Day	12% 7% 2 4 4 3 3 2 4 5% 54%	1762
	11th to 20th	Day	13% 6% 2 5% 4 4 3 3 3 7% 49%	1800
	21st to end	Day	14% 7% 2 6% 4 4 4 3 4 7% 45%	1800
December	1st to 10th	Day	17% 8% 3 5% 4 4 4 3 4 4 45%	2930
	11th to 20th	Day	17% 12% 4 8% 5% 5% 5% 3 3 5% 34%	3373
	21st to end	Day	18% 10% 3 6% 4 4 4 3 5% 8% 36%	4466
January	1st to 10th	Night	15% 9% 3 8% 4 4 4 3 4 4 43%	5889
	11th to 20th	Night	18% 8% 2 6% 4 3 3 2 3 4 46%	5900
	21st to end	Night	18% 9% 2 5% 4 3 3 3 4 5% 42%	6246
February	1st to 10th	Night	15% 8% 2 6% 4 4 4 3 4 6% 42%	2400
	11th to 20th	Night	22% 7% 3 7% 6% 4 4 4 3 4 37%	2338
	21st to end	Night	20% 8% 2 6% 4 4 5% 3 5% 6% 37%	1980
March	1st to 10th	Night	21% 8% 3 7% 4 4 5% 3 5% 7% 32%	5337
	11th to 20th	Night	12% 5% 2 5% 3 4 4 4 6% 7% 49%	4980
	21st to end	Night	14% 7% 3 7% 5% 4 4 3 5% 5% 42%	5467
April	1st to 10th	Night	13% 4 2 4 4 4 4 3 5% 9% 48%	2070
	11th to 20th	Night	11% 6% 2 5% 3 4 3 3 5% 7% 50%	1975
	21st to end	Night	10% 5% 2 7% 4 4 4 3 5% 8% 48%	1865
May	1st to 10th	Night	13% 7% 3 8% 5% 5% 6% 5% 6% 9% 33%	4033
	11th to 20th	Night	19% 9% 3 8% 6% 5% 5% 4 6% 7% 29%	4032
	21st to end	Night	13% 7% 3 7% 4 4 5% 4 7% 9% 38%	4421
June	1st to 10th	Night	6% 5% 2 8% 6% 6% 7% 4 8% 11% 38%	1797
	11th to 20th	Night	4 4 1 4 4 4 5% 5% 7% 14% 49%	1710
	21st to end	Night	5% 5% 2 6% 4 5% 5% 5% 8% 10% 45%	1710
July	1st to 10th	Night	9% 6% 2 6% 7% 6% 6% 5% 9% 10% 33%	2840
	11th to 20th	Night	8% 5% 2 6% 4 5% 5% 5% 8% 11% 41%	2840
	21st to end	Night	14% 9% 4 9% 7% 6% 6% 4 7% 9% 24%	3087
August	1st to 10th	Night	5% 3 1 6% 5% 5% 5% 5% 8% 12% 45%	2825
	11th to 20th	Night	7% 5% 2 5% 4 4 4 4 6% 9% 49%	3090
	21st to end	Night	7% 3 2 4 3 4 5% 4 7% 9% 51%	3476
September	1st to 10th	Night	9% 5% 3 7% 6% 4 5% 5% 8% 8% 41%	2014
	11th to 20th	Night	12% 6% 2 5% 4 4 5% 4 6% 8% 42%	2090
	21st to end	Night	11% 5% 1 7% 5% 4 3 3 5% 7% 48%	2090
October	1st to 10th	Night	10% 5% 1 4 3 3 3 3 4 6% 57%	2090
	11th to 20th	Night	11% 6% 2 6% 4 4 3 3 3 5% 55%	2164
	21st to end	Night	12% 5% 2 5% 3 3 3 3 4 5% 54%	2574
November	1st to 10th	Night	12% 5% 2 5% 4 3 3 3 4 6% 54%	2536
	11th to 20th	Night	13% 6% 2 5% 3 3 3 2 5% 6% 51%	2600
	21st to end	Night	15% 6% 2 5% 4 4 5% 3 4 5% 46%	2600
December	1st to 10th	Night	19% 9% 2 5% 3 3 3 3 4 5% 45%	4250
	11th to 20th	Night	20% 10% 3 6% 4 3 4 3 4 4 39%	4822
	21st to end	Night	19% 8% 2 6% 4 4 5% 4 5% 6% 36%	6490

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	46% 2 6% 22% 22% 1	4057
	11th to 20th	Day	44% 2 4 20% 28% 2	4060
	21st to end	Day	50% 1 6% 21% 19% 3	4694
February	1st to 10th	Day	48% 3 7% 22% 18% 3	2000
	11th to 20th	Day	52% 3 5% 19% 17% 4	1958
	21st to end	Day	50% 2 6% 16% 22% 5%	1650
March	1st to 10th	Day	52% 2 7% 13% 19% 8%	4515
	11th to 20th	Day	37% 2 5% 19% 24% 12%	4980
	21st to end	Day	44% 2 6% 29% 17% 2	5465
April	1st to 10th	Day	40% 4 10% 24% 18% 4	2345
	11th to 20th	Day	40% 2 6% 22% 25% 6%	2374
	21st to end	Day	40% 3 9% 29% 16% 3	2533
May	1st to 10th	Day	49% 5% 13% 20% 11% 2	5897
	11th to 20th	Day	54% 5% 13% 19% 9%	5865
	21st to end	Day	45% 3 9% 23% 17% 2	6447
June	1st to 10th	Day	40% 5% 16% 25% 13% 2	2587
	11th to 20th	Day	33% 5% 18% 28% 14% 2	2470
	21st to end	Day	38% 6% 17% 21% 14% 4	2470
July	1st to 10th	Day	40% 9% 21% 20% 8% 1	4055
	11th to 20th	Day	40% 8% 22% 21% 8% 2	4051
	21st to end	Day	60% 7% 13% 13% 6%	4324
August	1st to 10th	Day	32% 7% 16% 35% 10% 1	4091
	11th to 20th	Day	35% 6% 17% 25% 16% 1	3772
	21st to end	Day	32% 5% 13% 31% 17% 2	4179
September	1st to 10th	Day	39% 6% 14% 18% 17% 6%	2147
	11th to 20th	Day	42% 5% 10% 21% 20% 2	2090
	21st to end	Day	38% 3 12% 18% 22% 7%	2090
October	1st to 10th	Day	26% 3 6% 17% 36% 12%	3480
	11th to 20th	Day	21% 2 5% 21% 33% 17%	3332
	21st to end	Day	28% 2 6% 27% 28% 10%	3309
November	1st to 10th	Day	33% 3 7% 20% 30% 7%	1762
	11th to 20th	Day	37% 2 5% 23% 27% 7%	1800
	21st to end	Day	42% 2 6% 23% 23% 4	1800
December	1st to 10th	Day	44% 2 5% 21% 25% 4	2930
	11th to 20th	Day	55% 2 6% 19% 15% 3	3370
	21st to end	Day	49% 2 9% 27% 12% 1	4466
January	1st to 10th	Night	47% 1 6% 24% 20% 2	5889
	11th to 20th	Night	44% 2 5% 21% 24% 5%	5900
	21st to end	Night	46% 1 4 23% 21% 5%	6246
February	1st to 10th	Night	44% 2 8% 21% 20% 4	2400
	11th to 20th	Night	52% 2 6% 17% 17% 5%	2339
	21st to end	Night	50% 2 7% 18% 19% 5%	1980
March	1st to 10th	Night	53% 2 6% 16% 20% 4	5337
	11th to 20th	Night	35% 2 7% 20% 23% 14%	4980
	21st to end	Night	45% 2 5% 25% 19% 4	5467
April	1st to 10th	Night	34% 3 11% 24% 23% 5%	2070
	11th to 20th	Night	35% 2 7% 25% 23% 9%	1975
	21st to end	Night	37% 3 11% 28% 19% 2	1865
May	1st to 10th	Night	48% 5% 15% 19% 12% 1	4060
	11th to 20th	Night	55% 4 14% 18% 9%	4033
	21st to end	Night	43% 4 12% 24% 16% 1	4429
June	1st to 10th	Night	38% 6% 16% 23% 14% 2	1797
	11th to 20th	Night	25% 6% 22% 29% 16% 2	1710
	21st to end	Night	31% 7% 21% 21% 15% 5%	1710
July	1st to 10th	Night	41% 10% 19% 21% 7% 1	2795
	11th to 20th	Night	34% 8% 23% 25% 8% 3	2786
	21st to end	Night	56% 8% 14% 15% 6% 1	2988
August	1st to 10th	Night	29% 7% 15% 35% 12% 1	2814
	11th to 20th	Night	31% 5% 20% 29% 14% 1	3120
	21st to end	Night	28% 6% 15% 26% 23% 2	3476
September	1st to 10th	Night	39% 6% 15% 19% 15% 6%	2014
	11th to 20th	Night	38% 6% 12% 21% 20% 2	2090
	21st to end	Night	36% 4 10% 21% 21% 8%	2090
October	1st to 10th	Night	24% 3 7% 20% 31% 16%	3480
	11th to 20th	Night	22% 1 4 19% 35% 19%	3585
	21st to end	Night	24% 2 3 29% 29% 12%	4235
November	1st to 10th	Night	33% 3 7% 23% 24% 10%	2536
	11th to 20th	Night	37% 2 4 21% 27% 9%	2600
	21st to end	Night	41% 2 6% 23% 23% 5%	2600
December	1st to 10th	Night	44% 2 6% 20% 26% 2	4250
	11th to 20th	Night	50% 2 6% 19% 20% 2	4824
	21st to end	Night	50% 2 8% 26% 14% 1	6490

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	32% 22% 29% 13% 4	300
	11th to 20th	Day	32% 19% 32% 13% 4 1	300
	21st to end	Day	24% 25% 36% 13% 2	330
February	1st to 10th	Day	29% 20% 35% 9% 6%	300
	11th to 20th	Day	39% 22% 27% 11% 1	300
	21st to end	Day	37% 22% 30% 9% 2	249
March	1st to 10th	Day	29% 22% 29% 13% 5% 1	309
	11th to 20th	Day	25% 21% 30% 18% 6% 1	500
	21st to end	Day	22% 18% 37% 15% 6% 1	550
April	1st to 10th	Day	21% 19% 30% 16% 12% 3	500
	11th to 20th	Day	19% 15% 30% 22% 12% 1	492
	21st to end	Day	19% 8% 34% 26% 11% 3	500
May	1st to 10th	Day	17% 11% 31% 26% 11% 5%	503
	11th to 20th	Day	14% 8% 25% 28% 16% 9%	527
	21st to end	Day	13% 7% 20% 31% 18% 11%	573
June	1st to 10th	Day	12% 3 20% 30% 22% 12%	526
	11th to 20th	Day	7% 3 13% 33% 27% 18%	490
	21st to end	Day	11% 4 15% 28% 24% 17%	490
July	1st to 10th	Day	12% 4 11% 24% 27% 22%	485
	11th to 20th	Day	10% 2 8% 26% 32% 21%	484
	21st to end	Day	13% 3 7% 25% 30% 22%	532
August	1st to 10th	Day	10% 3 10% 28% 29% 18%	486
	11th to 20th	Day	10% 4 12% 32% 29% 14%	479
	21st to end	Day	16% 2 7% 24% 31% 20%	539
September	1st to 10th	Day	10% 3 10% 28% 33% 16%	490
	11th to 20th	Day	17% 2 12% 24% 30% 14%	490
	21st to end	Day	15% 6% 13% 29% 29% 8%	490
October	1st to 10th	Day	13% 4 22% 30% 22% 9%	490
	11th to 20th	Day	13% 9% 29% 27% 19% 3	490
	21st to end	Day	18% 9% 26% 27% 18% 3	469
November	1st to 10th	Day	21% 11% 29% 25% 11% 4	300
	11th to 20th	Day	18% 14% 35% 23% 9% 1	300
	21st to end	Day	16% 20% 34% 21% 7% 2	300
December	1st to 10th	Day	30% 15% 35% 15% 4 1	300
	11th to 20th	Day	25% 15% 36% 17% 6% 1	300
	21st to end	Day	28% 18% 32% 17% 5%	330
January	1st to 10th	Night	25% 21% 31% 20% 4	800
	11th to 20th	Night	23% 17% 31% 23% 6%	800
	21st to end	Night	20% 17% 35% 26% 2	880
February	1st to 10th	Night	19% 14% 36% 27% 4	800
	11th to 20th	Night	28% 19% 30% 22% 2	783
	21st to end	Night	28% 18% 33% 19% 3	660
March	1st to 10th	Night	21% 14% 34% 26% 5%	800
	11th to 20th	Night	19% 18% 31% 24% 9% 1	800
	21st to end	Night	19% 13% 34% 26% 8%	880
April	1st to 10th	Night	15% 14% 33% 29% 9% 1	670
	11th to 20th	Night	15% 11% 34% 30% 9%	593
	21st to end	Night	12% 9% 31% 35% 13% 1	600
May	1st to 10th	Night	11% 6% 30% 32% 19% 2	600
	11th to 20th	Night	14% 5% 22% 34% 20% 4	591
	21st to end	Night	10% 4 15% 36% 30% 6%	650
June	1st to 10th	Night	8% 2 16% 38% 30% 5%	599
	11th to 20th	Night	5% 2 10% 36% 36% 11%	570
	21st to end	Night	6% 2 9% 32% 40% 10%	570
July	1st to 10th	Night	13% 1 7% 26% 41% 11%	555
	11th to 20th	Night	9% 3 5% 28% 42% 13%	552
	21st to end	Night	10% 2 8% 26% 43% 11%	624
August	1st to 10th	Night	9% 1 5% 28% 45% 12%	567
	11th to 20th	Night	12% 2 7% 34% 37% 9%	564
	21st to end	Night	13% 3 8% 29% 38% 9%	627
September	1st to 10th	Night	11% 3 10% 31% 38% 7%	684
	11th to 20th	Night	14% 2 11% 31% 36% 5%	760
	21st to end	Night	13% 2 14% 34% 34% 3	760
October	1st to 10th	Night	13% 4 19% 30% 27% 7%	760
	11th to 20th	Night	15% 6% 27% 32% 18% 1	760
	21st to end	Night	15% 7% 24% 36% 16% 2	836
November	1st to 10th	Night	15% 8% 23% 34% 18% 2	780
	11th to 20th	Night	16% 12% 31% 29% 12% 1	800
	21st to end	Night	21% 11% 30% 27% 10% 1	800
December	1st to 10th	Night	27% 16% 25% 24% 8%	800
	11th to 20th	Night	24% 12% 31% 26% 7%	800
	21st to end	Night	26% 13% 30% 26% 6%	880

Page generated at 2025-06-07 10:28:14. Version ac07

Copyright © 2025 Attilla Danko.