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 2026-10-14.

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% 8% 39%	148772
	Night	14% 7% 2 6% 4 4 4 4 6% 7% 41%	142978
Transparency	Day	42% 5% 12% 22% 16% 4	152534
	Night	41% 4 11% 22% 18% 4	147287
Seeing	Day	17% 10% 23% 23% 18% 8%	17323
	Night	17% 9% 23% 29% 19% 3	27278

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	19% 10% 3 7% 4 3 3 2 3 4 40%	5258
	11th to 20th	Day	12% 8% 4 5% 3 3 3 2 3 4 53%	5280
	21st to end	Day	21% 10% 3 8% 4 3 3 3 4 5% 35%	6077
February	1st to 10th	Day	18% 9% 3 7% 5% 3 4 4 5% 7% 37%	2100
	11th to 20th	Day	21% 9% 3 7% 4 4 4 3 4 6% 35%	2048
	21st to end	Day	20% 9% 3 7% 4 4 3 3 4 5% 38%	1730
March	1st to 10th	Day	19% 9% 3 7% 5% 5% 5% 3 6% 6% 33%	4615
	11th to 20th	Day	12% 6% 2 6% 4 4 3 3 5% 6% 50%	5090
	21st to end	Day	15% 7% 2 6% 4 5% 4 3 4 6% 44%	5586
April	1st to 10th	Day	12% 5% 2 5% 5% 5% 6% 4 6% 7% 42%	2462
	11th to 20th	Day	12% 6% 2 6% 5% 4 4 3 4 7% 48%	2494
	21st to end	Day	11% 6% 3 6% 6% 5% 5% 4 6% 7% 42%	2660
May	1st to 10th	Day	16% 8% 3 7% 4 5% 5% 4 7% 8% 33%	7616
	11th to 20th	Day	14% 7% 3 8% 6% 5% 5% 4 7% 8% 33%	7626
	21st to end	Day	18% 9% 3 9% 5% 5% 5% 4 5% 6% 30%	8391
June	1st to 10th	Day	10% 6% 2 6% 5% 5% 5% 5% 8% 10% 37%	2730
	11th to 20th	Day	5% 5% 2 6% 5% 6% 7% 6% 8% 11% 40%	2730
	21st to end	Day	6% 4 2 6% 6% 6% 7% 5% 8% 11% 38%	2730
July	1st to 10th	Day	6% 5% 2 7% 6% 6% 7% 6% 9% 12% 34%	7680
	11th to 20th	Day	9% 6% 3 7% 6% 6% 7% 6% 9% 12% 28%	7680
	21st to end	Day	12% 8% 3 9% 6% 6% 7% 6% 8% 11% 25%	8340
August	1st to 10th	Day	8% 5% 3 7% 6% 6% 7% 6% 9% 11% 33%	6014
	11th to 20th	Day	5% 4 2 5% 4 4 5% 4 8% 12% 45%	5493
	21st to end	Day	6% 4 1 5% 4 4 5% 4 6% 9% 51%	6080
September	1st to 10th	Day	9% 5% 2 7% 5% 5% 5% 5% 7% 9% 41%	2373
	11th to 20th	Day	13% 6% 2 6% 4 5% 5% 4 6% 7% 43%	2310
	21st to end	Day	13% 4 2 6% 5% 4 4 3 5% 7% 46%	2310
October	1st to 10th	Day	11% 5% 1 5% 4 5% 4 3 4 6% 53%	2310
	11th to 20th	Day	10% 5% 2 4 3 3 3 4 5% 6% 57%	2153
	21st to end	Day	14% 5% 2 5% 3 4 3 3 5% 6% 52%	2126
November	1st to 10th	Day	12% 7% 2 4 4 3 3 3 4 5% 54%	1852
	11th to 20th	Day	12% 6% 2 5% 4 4 3 3 3 7% 50%	1890
	21st to end	Day	15% 7% 3 6% 4 4 4 3 4 7% 45%	1890
December	1st to 10th	Day	16% 8% 3 5% 4 4 4 3 4 4 45%	3020
	11th to 20th	Day	16% 11% 4 7% 5% 5% 5% 3 3 5% 35%	3463
	21st to end	Day	18% 10% 3 6% 4 4 4 3 5% 8% 36%	4565
January	1st to 10th	Night	17% 8% 3 7% 4 4 4 3 4 4 41%	7635
	11th to 20th	Night	16% 7% 2 6% 3 3 4 2 3 4 49%	7680
	21st to end	Night	20% 10% 3 5% 4 4 3 3 4 5% 40%	8088
February	1st to 10th	Night	15% 8% 2 7% 4 4 4 3 5% 6% 43%	2520
	11th to 20th	Night	22% 7% 3 7% 5% 4 4 4 3 4 37%	2458
	21st to end	Night	20% 8% 3 7% 4 4 5% 3 5% 5% 36%	2076
March	1st to 10th	Night	21% 8% 3 7% 5% 4 5% 3 5% 7% 32%	5457
	11th to 20th	Night	12% 5% 2 5% 3 4 4 4 6% 7% 49%	5090
	21st to end	Night	13% 7% 3 7% 5% 4 4 3 5% 5% 43%	5588
April	1st to 10th	Night	12% 4 2 4 4 4 5% 3 5% 9% 48%	2174
	11th to 20th	Night	11% 6% 2 5% 3 4 3 3 5% 8% 51%	2075
	21st to end	Night	11% 6% 2 7% 4 4 4 3 5% 8% 47%	1958
May	1st to 10th	Night	14% 8% 3 8% 5% 5% 6% 4 6% 8% 33%	5237
	11th to 20th	Night	15% 8% 3 8% 5% 5% 5% 4 6% 7% 35%	5234
	21st to end	Night	17% 9% 3 8% 5% 5% 5% 4 6% 8% 30%	5761
June	1st to 10th	Night	7% 5% 2 8% 6% 5% 6% 4 8% 11% 38%	1890
	11th to 20th	Night	5% 4 1 4 4 4 5% 5% 7% 13% 47%	1890
	21st to end	Night	5% 5% 2 6% 4 5% 5% 5% 8% 11% 45%	1890
July	1st to 10th	Night	7% 6% 2 6% 6% 5% 6% 5% 9% 12% 36%	5278
	11th to 20th	Night	8% 6% 2 8% 6% 6% 6% 6% 8% 12% 32%	5277
	21st to end	Night	10% 7% 3 8% 6% 5% 6% 5% 8% 12% 29%	5736
August	1st to 10th	Night	7% 5% 2 7% 5% 5% 6% 6% 8% 12% 36%	4135
	11th to 20th	Night	5% 4 2 5% 4 5% 5% 5% 7% 11% 47%	4536
	21st to end	Night	6% 3 2 4 3 4 5% 4 7% 9% 52%	5046
September	1st to 10th	Night	9% 5% 3 7% 6% 4 6% 5% 8% 8% 41%	2226
	11th to 20th	Night	11% 5% 2 5% 4 4 5% 4 6% 8% 44%	2310
	21st to end	Night	11% 5% 2 6% 5% 5% 4 3 6% 7% 47%	2310
October	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 4 6% 55%	2310
	11th to 20th	Night	10% 5% 2 5% 4 3 3 3 3 5% 56%	2312
	21st to end	Night	13% 5% 2 5% 3 3 3 3 4 4 54%	2710
November	1st to 10th	Night	12% 6% 2 5% 4 3 3 2 4 5% 54%	2666
	11th to 20th	Night	13% 6% 2 5% 3 3 3 2 5% 6% 52%	2730
	21st to end	Night	15% 6% 2 5% 4 4 4 3 4 5% 46%	2730
December	1st to 10th	Night	19% 9% 2 5% 3 3 3 3 4 5% 44%	4380
	11th to 20th	Night	19% 10% 3 6% 4 3 4 3 4 4 39%	4952
	21st to end	Night	19% 8% 2 6% 4 5% 5% 4 5% 6% 36%	6633

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	50% 2 7% 23% 17% 1	5259
	11th to 20th	Day	38% 2 4 18% 33% 6%	5280
	21st to end	Day	53% 1 6% 20% 16% 5%	6077
February	1st to 10th	Day	48% 3 6% 22% 18% 3	2100
	11th to 20th	Day	52% 3 5% 20% 17% 4	2058
	21st to end	Day	50% 2 6% 15% 21% 5%	1730
March	1st to 10th	Day	52% 2 7% 14% 18% 8%	4615
	11th to 20th	Day	37% 2 6% 19% 24% 12%	5090
	21st to end	Day	44% 2 7% 29% 17% 2	5586
April	1st to 10th	Day	39% 4 10% 25% 17% 4	2462
	11th to 20th	Day	38% 2 6% 23% 25% 6%	2494
	21st to end	Day	40% 3 9% 29% 16% 3	2660
May	1st to 10th	Day	48% 4 12% 21% 13% 2	7651
	11th to 20th	Day	48% 5% 15% 21% 10% 1	7630
	21st to end	Day	55% 4 8% 18% 13% 2	8405
June	1st to 10th	Day	40% 5% 16% 24% 13% 2	2730
	11th to 20th	Day	35% 6% 18% 27% 13% 2	2730
	21st to end	Day	37% 6% 18% 22% 14% 3	2730
July	1st to 10th	Day	37% 11% 24% 19% 9% 1	7615
	11th to 20th	Day	43% 12% 25% 15% 4 1	7611
	21st to end	Day	49% 9% 18% 17% 7%	8156
August	1st to 10th	Day	40% 11% 16% 26% 7%	6001
	11th to 20th	Day	28% 6% 22% 28% 14% 1	5529
	21st to end	Day	28% 5% 14% 34% 18% 1	6079
September	1st to 10th	Day	38% 6% 14% 20% 17% 5%	2373
	11th to 20th	Day	39% 5% 11% 23% 19% 2	2310
	21st to end	Day	38% 4 12% 18% 22% 7%	2310
October	1st to 10th	Day	27% 3 5% 17% 36% 12%	3700
	11th to 20th	Day	21% 2 5% 21% 33% 17%	3471
	21st to end	Day	29% 2 5% 27% 27% 9%	3415
November	1st to 10th	Day	33% 3 8% 19% 29% 7%	1852
	11th to 20th	Day	36% 2 6% 24% 26% 6%	1890
	21st to end	Day	42% 2 6% 23% 23% 4	1890
December	1st to 10th	Day	44% 2 5% 20% 24% 4	3020
	11th to 20th	Day	54% 2 6% 19% 15% 3	3460
	21st to end	Day	49% 2 9% 27% 12% 1	4565
January	1st to 10th	Night	48% 2 10% 23% 16% 2	7636
	11th to 20th	Night	41% 2 5% 21% 28% 4	7680
	21st to end	Night	49% 2 5% 20% 18% 6%	8088
February	1st to 10th	Night	44% 2 8% 22% 20% 4	2520
	11th to 20th	Night	53% 2 6% 18% 17% 5%	2459
	21st to end	Night	50% 2 7% 17% 19% 5%	2076
March	1st to 10th	Night	53% 2 7% 16% 19% 4	5457
	11th to 20th	Night	35% 2 7% 20% 23% 13%	5090
	21st to end	Night	44% 2 6% 26% 19% 4	5588
April	1st to 10th	Night	34% 3 12% 24% 22% 5%	2174
	11th to 20th	Night	34% 2 7% 25% 23% 9%	2075
	21st to end	Night	37% 3 11% 27% 19% 2	1958
May	1st to 10th	Night	48% 4 13% 20% 13% 2	5264
	11th to 20th	Night	48% 4 14% 20% 13%	5235
	21st to end	Night	53% 5% 11% 19% 12% 1	5771
June	1st to 10th	Night	38% 6% 17% 22% 15% 3	1890
	11th to 20th	Night	27% 7% 21% 28% 15% 2	1890
	21st to end	Night	31% 7% 20% 22% 15% 5%	1890
July	1st to 10th	Night	37% 12% 25% 20% 6% 1	5235
	11th to 20th	Night	40% 13% 25% 16% 4 1	5226
	21st to end	Night	47% 10% 20% 15% 7% 1	5606
August	1st to 10th	Night	37% 11% 17% 26% 9%	4124
	11th to 20th	Night	28% 7% 25% 27% 12% 1	4566
	21st to end	Night	26% 6% 15% 31% 21% 1	5046
September	1st to 10th	Night	39% 6% 15% 19% 16% 5%	2226
	11th to 20th	Night	37% 6% 13% 23% 20% 2	2310
	21st to end	Night	37% 5% 10% 20% 21% 7%	2310
October	1st to 10th	Night	25% 3 7% 20% 31% 15%	3700
	11th to 20th	Night	21% 1 4 19% 35% 19%	3733
	21st to end	Night	25% 2 3 28% 29% 12%	4371
November	1st to 10th	Night	33% 4 7% 23% 24% 9%	2666
	11th to 20th	Night	36% 1 5% 22% 27% 9%	2730
	21st to end	Night	42% 2 5% 23% 23% 5%	2730
December	1st to 10th	Night	45% 2 6% 20% 25% 2	4380
	11th to 20th	Night	50% 2 7% 18% 21% 2	4954
	21st to end	Night	49% 2 8% 26% 13% 1	6633

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% 21% 30% 14% 4	330
	11th to 20th	Day	31% 21% 32% 13% 3 1	330
	21st to end	Day	25% 25% 36% 12% 2	363
February	1st to 10th	Day	27% 21% 37% 8% 5%	330
	11th to 20th	Day	38% 24% 26% 10% 1	330
	21st to end	Day	36% 21% 30% 10% 3	273
March	1st to 10th	Day	29% 22% 31% 13% 4 1	339
	11th to 20th	Day	25% 21% 30% 17% 6% 1	540
	21st to end	Day	21% 18% 39% 16% 6% 1	594
April	1st to 10th	Day	20% 19% 31% 16% 12% 3	540
	11th to 20th	Day	18% 16% 31% 21% 12% 2	532
	21st to end	Day	20% 8% 33% 25% 11% 4	540
May	1st to 10th	Day	17% 10% 31% 25% 10% 5%	543
	11th to 20th	Day	13% 8% 26% 29% 16% 8%	567
	21st to end	Day	14% 8% 19% 30% 18% 11%	617
June	1st to 10th	Day	13% 4 19% 31% 21% 12%	570
	11th to 20th	Day	7% 4 15% 33% 26% 15%	570
	21st to end	Day	10% 4 14% 28% 26% 18%	570
July	1st to 10th	Day	10% 4 11% 23% 28% 23%	565
	11th to 20th	Day	10% 3 8% 24% 33% 21%	564
	21st to end	Day	13% 3 9% 25% 30% 20%	620
August	1st to 10th	Day	11% 4 11% 26% 31% 17%	566
	11th to 20th	Day	9% 4 10% 31% 30% 16%	559
	21st to end	Day	15% 2 10% 24% 30% 19%	627
September	1st to 10th	Day	9% 4 10% 28% 32% 17%	570
	11th to 20th	Day	15% 2 11% 24% 32% 15%	570
	21st to end	Day	14% 6% 14% 30% 28% 7%	570
October	1st to 10th	Day	14% 4 21% 30% 22% 8%	570
	11th to 20th	Day	13% 8% 29% 28% 19% 3	542
	21st to end	Day	19% 10% 27% 26% 17% 3	509
November	1st to 10th	Day	21% 11% 29% 25% 10% 3	330
	11th to 20th	Day	16% 13% 37% 24% 8% 1	330
	21st to end	Day	16% 21% 34% 20% 7% 2	330
December	1st to 10th	Day	31% 16% 34% 15% 3 1	330
	11th to 20th	Day	24% 15% 37% 18% 6% 1	330
	21st to end	Day	27% 19% 32% 17% 6%	363
January	1st to 10th	Night	25% 20% 30% 20% 4	840
	11th to 20th	Night	23% 17% 32% 23% 6%	840
	21st to end	Night	20% 17% 36% 25% 2	924
February	1st to 10th	Night	19% 15% 35% 26% 4	840
	11th to 20th	Night	28% 20% 29% 21% 2	823
	21st to end	Night	28% 18% 33% 18% 3	692
March	1st to 10th	Night	21% 14% 34% 27% 5%	840
	11th to 20th	Night	19% 17% 31% 24% 8% 1	840
	21st to end	Night	18% 13% 35% 26% 7%	924
April	1st to 10th	Night	14% 13% 33% 30% 9% 1	704
	11th to 20th	Night	15% 11% 33% 31% 10%	623
	21st to end	Night	13% 9% 29% 35% 13% 1	630
May	1st to 10th	Night	12% 6% 30% 32% 18% 2	630
	11th to 20th	Night	14% 5% 23% 35% 19% 4	621
	21st to end	Night	11% 4 15% 35% 29% 5%	683
June	1st to 10th	Night	9% 2 17% 38% 30% 5%	630
	11th to 20th	Night	6% 2 12% 35% 35% 10%	630
	21st to end	Night	6% 2 8% 32% 41% 10%	630
July	1st to 10th	Night	13% 1 7% 26% 42% 11%	615
	11th to 20th	Night	10% 3 5% 27% 43% 12%	612
	21st to end	Night	11% 2 8% 26% 42% 11%	690
August	1st to 10th	Night	10% 1 6% 28% 44% 11%	627
	11th to 20th	Night	11% 2 7% 32% 39% 9%	624
	21st to end	Night	12% 3 9% 31% 36% 9%	693
September	1st to 10th	Night	10% 3 10% 30% 38% 8%	756
	11th to 20th	Night	13% 2 11% 30% 38% 6%	840
	21st to end	Night	14% 2 15% 34% 33% 3	840
October	1st to 10th	Night	13% 4 19% 31% 27% 6%	840
	11th to 20th	Night	14% 6% 27% 34% 18% 1	813
	21st to end	Night	15% 7% 25% 36% 15% 2	880
November	1st to 10th	Night	15% 8% 23% 34% 17% 2	820
	11th to 20th	Night	16% 12% 31% 29% 11% 1	840
	21st to end	Night	21% 13% 30% 27% 10%	840
December	1st to 10th	Night	27% 16% 25% 25% 7%	840
	11th to 20th	Night	23% 13% 30% 27% 7%	840
	21st to end	Night	25% 13% 29% 27% 6%	924

Page generated at 2026-10-11 05:06:29. Version ac07

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