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 2024-03-08.

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% 6% 3 6% 6% 3 5% 5% 6% 8% 40%	78963
	Night	14% 6% 2 5% 5% 3 5% 5% 5% 7% 43%	80612
Transparency	Day	43% 4 11% 22% 18% 3	78686
	Night	41% 3 10% 22% 19% 4	80408
Seeing	Day	18% 10% 22% 23% 18% 9%	13792
	Night	16% 9% 23% 29% 19% 3	23941

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% 12% 3 6% 6% 3 4 4 5% 5% 36%	2836
	11th to 20th	Day	19% 9% 5% 6% 5% 3 3 3 4 5% 39%	2840
	21st to end	Day	23% 11% 3 6% 4 2 3 4 3 5% 37%	3282
February	1st to 10th	Day	19% 8% 3 6% 5% 3 3 4 4 6% 37%	1900
	11th to 20th	Day	21% 8% 4 6% 5% 3 4 4 4 6% 35%	1848
	21st to end	Day	21% 9% 3 7% 5% 3 3 3 4 5% 36%	1570
March	1st to 10th	Day	17% 6% 3 5% 4 3 5% 3 5% 8% 41%	1877
	11th to 20th	Day	18% 8% 3 7% 5% 3 4 5% 5% 6% 37%	1980
	21st to end	Day	15% 7% 3 6% 5% 4 6% 5% 5% 7% 37%	2178
April	1st to 10th	Day	12% 4 2 5% 5% 4 6% 5% 6% 7% 43%	2110
	11th to 20th	Day	13% 6% 3 6% 6% 3 4 4 4 6% 46%	2134
	21st to end	Day	11% 5% 3 5% 6% 4 5% 4 6% 8% 44%	2279
May	1st to 10th	Day	11% 5% 3 6% 5% 3 5% 5% 6% 8% 45%	2340
	11th to 20th	Day	11% 5% 3 7% 7% 4 5% 6% 6% 7% 39%	2301
	21st to end	Day	9% 5% 2 6% 6% 4 5% 5% 7% 10% 41%	2531
June	1st to 10th	Day	9% 6% 2 5% 6% 4 6% 7% 8% 10% 37%	2340
	11th to 20th	Day	5% 4 2 5% 5% 4 7% 7% 8% 11% 41%	2340
	21st to end	Day	7% 4 3 6% 6% 4 7% 7% 8% 11% 37%	2340
July	1st to 10th	Day	10% 5% 3 6% 6% 5% 8% 8% 8% 11% 31%	2340
	11th to 20th	Day	9% 5% 3 5% 7% 4 8% 8% 9% 11% 31%	2340
	21st to end	Day	8% 6% 3 7% 7% 5% 8% 8% 9% 10% 30%	2547
August	1st to 10th	Day	7% 4 2 6% 8% 5% 8% 7% 9% 11% 32%	2327
	11th to 20th	Day	10% 6% 3 7% 8% 4 6% 6% 8% 10% 34%	2132
	21st to end	Day	13% 6% 2 7% 7% 5% 6% 7% 7% 9% 32%	2376
September	1st to 10th	Day	10% 5% 3 6% 6% 4 5% 6% 6% 10% 39%	2034
	11th to 20th	Day	14% 6% 2 5% 5% 4 5% 4 6% 7% 42%	1980
	21st to end	Day	12% 4 2 5% 6% 3 4 4 5% 7% 48%	1980
October	1st to 10th	Day	11% 5% 1 4 4 3 5% 4 4 5% 54%	1980
	11th to 20th	Day	11% 5% 2 4 4 2 3 4 5% 6% 54%	1908
	21st to end	Day	14% 4 2 4 4 3 4 3 5% 6% 51%	1914
November	1st to 10th	Day	12% 6% 1 3 4 2 3 3 4 5% 56%	1672
	11th to 20th	Day	13% 6% 2 5% 4 2 4 4 3 6% 51%	1710
	21st to end	Day	15% 6% 3 5% 5% 3 4 3 4 7% 45%	1710
December	1st to 10th	Day	21% 9% 4 6% 5% 3 4 4 4 5% 35%	1710
	11th to 20th	Day	16% 8% 3 6% 6% 4 5% 5% 4 6% 40%	2153
	21st to end	Day	19% 10% 3 6% 6% 3 4 4 5% 8% 33%	3124
January	1st to 10th	Night	18% 8% 2 6% 6% 3 5% 4 4 5% 39%	4109
	11th to 20th	Night	19% 9% 3 5% 5% 2 5% 4 4 5% 41%	4120
	21st to end	Night	22% 12% 3 5% 5% 3 4 3 4 4 36%	4372
February	1st to 10th	Night	16% 8% 3 6% 4 3 5% 4 5% 6% 43%	2280
	11th to 20th	Night	21% 6% 3 6% 6% 3 5% 5% 3 4 38%	2218
	21st to end	Night	21% 7% 3 6% 5% 3 5% 4 5% 6% 35%	1884
March	1st to 10th	Night	15% 7% 3 5% 5% 2 4 4 5% 7% 43%	2243
	11th to 20th	Night	16% 6% 3 6% 5% 4 4 4 6% 6% 40%	1980
	21st to end	Night	13% 7% 3 7% 6% 3 4 5% 5% 5% 41%	2178
April	1st to 10th	Night	12% 4 2 4 4 2 5% 4 5% 9% 50%	1864
	11th to 20th	Night	11% 6% 2 4 4 3 4 3 5% 7% 50%	1775
	21st to end	Night	10% 4 2 5% 5% 3 5% 4 5% 8% 49%	1679
May	1st to 10th	Night	9% 4 3 6% 4 3 4 5% 5% 8% 49%	1620
	11th to 20th	Night	10% 6% 2 5% 5% 3 5% 6% 6% 8% 42%	1592
	21st to end	Night	8% 4 2 5% 5% 3 7% 6% 6% 11% 42%	1744
June	1st to 10th	Night	6% 5% 2 7% 7% 4 7% 6% 8% 11% 39%	1620
	11th to 20th	Night	5% 3 2 4 4 3 5% 6% 7% 14% 49%	1620
	21st to end	Night	5% 4 2 5% 5% 4 5% 6% 7% 12% 45%	1620
July	1st to 10th	Night	10% 4 2 5% 6% 4 7% 7% 8% 11% 36%	1620
	11th to 20th	Night	8% 5% 3 6% 5% 3 5% 7% 8% 13% 39%	1620
	21st to end	Night	7% 4 3 5% 6% 5% 7% 6% 8% 13% 37%	1771
August	1st to 10th	Night	7% 3 1 5% 6% 4 6% 8% 8% 12% 40%	1611
	11th to 20th	Night	8% 6% 2 5% 5% 4 5% 7% 7% 9% 41%	1766
	21st to end	Night	12% 5% 3 5% 5% 4 6% 5% 8% 10% 38%	1980
September	1st to 10th	Night	9% 5% 3 5% 7% 4 5% 6% 7% 8% 41%	1908
	11th to 20th	Night	11% 5% 3 5% 5% 3 5% 6% 6% 8% 43%	1980
	21st to end	Night	10% 5% 2 5% 6% 3 4 4 5% 7% 50%	1980
October	1st to 10th	Night	11% 5% 2 3 4 3 3 3 4 6% 57%	1980
	11th to 20th	Night	11% 6% 2 6% 5% 3 3 3 3 6% 53%	2050
	21st to end	Night	13% 5% 3 4 4 3 3 4 4 5% 53%	2438
November	1st to 10th	Night	12% 5% 2 4 4 2 3 3 4 5% 55%	2406
	11th to 20th	Night	13% 6% 3 5% 4 2 3 3 4 6% 52%	2470
	21st to end	Night	16% 6% 2 5% 5% 3 5% 4 4 5% 46%	2470
December	1st to 10th	Night	20% 8% 2 6% 5% 3 5% 5% 5% 6% 36%	2470
	11th to 20th	Night	17% 8% 3 4 5% 3 3 4 4 5% 45%	3042
	21st to end	Night	19% 9% 3 6% 6% 4 5% 5% 5% 7% 32%	4532

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	51% 2 5% 19% 21% 2	2837
	11th to 20th	Day	50% 1 6% 18% 23% 2	2840
	21st to end	Day	53% 1 6% 21% 18% 2	3282
February	1st to 10th	Day	49% 2 6% 21% 18% 3	1900
	11th to 20th	Day	52% 3 5% 19% 17% 4	1858
	21st to end	Day	52% 1 6% 15% 21% 5%	1570
March	1st to 10th	Day	44% 2 8% 18% 21% 7%	1877
	11th to 20th	Day	49% 3 10% 19% 16% 3	1980
	21st to end	Day	47% 3 10% 25% 14% 2	2178
April	1st to 10th	Day	39% 4 10% 25% 18% 4	2110
	11th to 20th	Day	41% 2 6% 21% 24% 6%	2134
	21st to end	Day	39% 3 9% 30% 17% 3	2279
May	1st to 10th	Day	38% 4 12% 26% 16% 4	2340
	11th to 20th	Day	43% 4 15% 22% 15% 1	2305
	21st to end	Day	39% 5% 13% 26% 16% 3	2531
June	1st to 10th	Day	40% 5% 16% 24% 13% 1	2340
	11th to 20th	Day	33% 5% 18% 27% 13% 2	2340
	21st to end	Day	38% 6% 18% 21% 14% 3	2340
July	1st to 10th	Day	44% 7% 17% 20% 10% 2	2275
	11th to 20th	Day	42% 8% 20% 17% 10% 4	2271
	21st to end	Day	44% 7% 18% 21% 10%	2405
August	1st to 10th	Day	42% 8% 19% 23% 8% 1	2314
	11th to 20th	Day	44% 6% 15% 20% 14% 1	2132
	21st to end	Day	47% 6% 15% 17% 12% 3	2376
September	1st to 10th	Day	40% 6% 14% 19% 16% 6%	2034
	11th to 20th	Day	41% 5% 11% 21% 20% 2	1980
	21st to end	Day	37% 3 10% 19% 23% 7%	1980
October	1st to 10th	Day	33% 3 8% 21% 26% 8%	1980
	11th to 20th	Day	32% 2 8% 25% 24% 8%	1908
	21st to end	Day	35% 1 5% 25% 28% 6%	1914
November	1st to 10th	Day	33% 2 7% 19% 31% 8%	1672
	11th to 20th	Day	37% 2 5% 23% 27% 7%	1710
	21st to end	Day	42% 2 6% 22% 23% 4	1710
December	1st to 10th	Day	52% 2 6% 20% 16% 3	1710
	11th to 20th	Day	48% 2 6% 20% 21% 3	2150
	21st to end	Day	52% 1 6% 24% 15% 1	3124
January	1st to 10th	Night	50% 1 4 22% 21% 2	4109
	11th to 20th	Night	47% 2 6% 21% 18% 5%	4120
	21st to end	Night	55% 1 4 18% 20% 2	4372
February	1st to 10th	Night	44% 2 8% 21% 21% 4	2280
	11th to 20th	Night	52% 2 5% 17% 18% 6%	2219
	21st to end	Night	51% 2 7% 17% 18% 5%	1884
March	1st to 10th	Night	42% 2 7% 18% 22% 8%	2243
	11th to 20th	Night	45% 3 9% 21% 18% 4	1980
	21st to end	Night	45% 3 8% 25% 17% 3	2178
April	1st to 10th	Night	33% 3 11% 25% 23% 5%	1864
	11th to 20th	Night	35% 2 7% 24% 22% 9%	1775
	21st to end	Night	36% 3 11% 28% 20% 2	1679
May	1st to 10th	Night	33% 4 14% 22% 23% 3	1620
	11th to 20th	Night	38% 5% 19% 24% 14% 1	1593
	21st to end	Night	37% 5% 15% 25% 17% 2	1745
June	1st to 10th	Night	37% 6% 17% 23% 14% 3	1620
	11th to 20th	Night	25% 6% 23% 30% 14% 2	1620
	21st to end	Night	31% 7% 21% 21% 15% 5%	1620
July	1st to 10th	Night	39% 7% 18% 21% 13% 2	1575
	11th to 20th	Night	34% 7% 23% 20% 12% 5%	1566
	21st to end	Night	37% 6% 20% 24% 11% 2	1672
August	1st to 10th	Night	35% 8% 20% 26% 10% 1	1600
	11th to 20th	Night	37% 6% 21% 20% 14% 2	1766
	21st to end	Night	39% 7% 18% 20% 13% 3	1980
September	1st to 10th	Night	39% 6% 15% 19% 15% 5%	1908
	11th to 20th	Night	37% 6% 13% 21% 21% 2	1980
	21st to end	Night	34% 4 9% 22% 23% 8%	1980
October	1st to 10th	Night	30% 2 8% 22% 29% 8%	1980
	11th to 20th	Night	36% 2 6% 22% 25% 9%	2050
	21st to end	Night	36% 1 3 23% 27% 9%	2438
November	1st to 10th	Night	33% 3 6% 23% 25% 10%	2406
	11th to 20th	Night	36% 1 5% 22% 27% 9%	2470
	21st to end	Night	42% 2 5% 23% 23% 4	2470
December	1st to 10th	Night	50% 3 8% 19% 17% 3	2470
	11th to 20th	Night	44% 2 6% 19% 26% 2	3044
	21st to end	Night	53% 1 7% 23% 15% 2	4532

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	30% 20% 31% 14% 4	270
	11th to 20th	Day	33% 19% 31% 13% 4 1	270
	21st to end	Day	25% 24% 37% 12% 2	297
February	1st to 10th	Day	31% 20% 34% 9% 6%	270
	11th to 20th	Day	37% 23% 27% 11% 1	270
	21st to end	Day	40% 22% 30% 8%	225
March	1st to 10th	Day	27% 20% 31% 15% 6% 1	268
	11th to 20th	Day	25% 20% 30% 16% 7% 1	420
	21st to end	Day	23% 17% 36% 15% 7% 1	462
April	1st to 10th	Day	20% 17% 29% 18% 13% 3	420
	11th to 20th	Day	18% 15% 30% 23% 13% 2	412
	21st to end	Day	20% 7% 32% 25% 12% 3	420
May	1st to 10th	Day	15% 10% 30% 28% 12% 5%	423
	11th to 20th	Day	13% 6% 25% 30% 17% 10%	447
	21st to end	Day	12% 5% 16% 32% 21% 13%	485
June	1st to 10th	Day	12% 3 18% 30% 23% 14%	450
	11th to 20th	Day	7% 3 12% 32% 26% 19%	450
	21st to end	Day	12% 3 15% 27% 25% 17%	450
July	1st to 10th	Day	12% 4 9% 24% 27% 24%	445
	11th to 20th	Day	10% 2 8% 26% 33% 21%	444
	21st to end	Day	12% 2 6% 23% 32% 24%	488
August	1st to 10th	Day	11% 3 9% 27% 30% 20%	446
	11th to 20th	Day	11% 3 9% 32% 31% 15%	439
	21st to end	Day	17% 2 7% 26% 31% 17%	495
September	1st to 10th	Day	10% 3 9% 28% 33% 17%	450
	11th to 20th	Day	16% 2 12% 25% 30% 15%	450
	21st to end	Day	13% 5% 13% 31% 30% 8%	450
October	1st to 10th	Day	15% 4 20% 30% 22% 9%	450
	11th to 20th	Day	14% 9% 30% 26% 19% 3	450
	21st to end	Day	19% 9% 26% 25% 17% 3	429
November	1st to 10th	Day	22% 11% 28% 23% 12% 4	270
	11th to 20th	Day	17% 12% 37% 24% 9% 1	270
	21st to end	Day	16% 19% 34% 22% 8% 2	270
December	1st to 10th	Day	30% 14% 34% 16% 4 1	270
	11th to 20th	Day	23% 16% 37% 17% 7% 1	270
	21st to end	Day	27% 19% 33% 16% 5%	297
January	1st to 10th	Night	24% 20% 31% 20% 4	760
	11th to 20th	Night	23% 17% 31% 22% 6%	760
	21st to end	Night	20% 17% 35% 26% 2	836
February	1st to 10th	Night	19% 14% 36% 26% 4	760
	11th to 20th	Night	27% 18% 30% 23% 2	743
	21st to end	Night	29% 18% 33% 18% 3	628
March	1st to 10th	Night	20% 13% 35% 27% 5%	751
	11th to 20th	Night	19% 18% 31% 22% 9% 1	720
	21st to end	Night	18% 13% 33% 27% 9%	792
April	1st to 10th	Night	14% 12% 32% 31% 10% 1	604
	11th to 20th	Night	15% 11% 33% 30% 10%	533
	21st to end	Night	12% 9% 30% 36% 13% 1	540
May	1st to 10th	Night	11% 6% 31% 32% 20% 1	540
	11th to 20th	Night	13% 4 23% 34% 22% 5%	531
	21st to end	Night	9% 3 13% 36% 33% 6%	584
June	1st to 10th	Night	8% 1 16% 38% 32% 5%	540
	11th to 20th	Night	5% 2 10% 35% 37% 12%	540
	21st to end	Night	6% 2 8% 33% 40% 11%	540
July	1st to 10th	Night	13% 1 6% 26% 42% 11%	525
	11th to 20th	Night	9% 3 5% 28% 42% 13%	522
	21st to end	Night	9% 1 7% 26% 45% 11%	591
August	1st to 10th	Night	9% 1 5% 27% 45% 13%	537
	11th to 20th	Night	12% 2 6% 34% 38% 9%	534
	21st to end	Night	13% 3 8% 29% 38% 9%	594
September	1st to 10th	Night	11% 3 9% 30% 40% 8%	648
	11th to 20th	Night	14% 2 12% 31% 36% 5%	720
	21st to end	Night	12% 2 14% 34% 34% 3	720
October	1st to 10th	Night	13% 4 18% 30% 27% 7%	720
	11th to 20th	Night	15% 6% 28% 32% 18% 1	720
	21st to end	Night	16% 7% 24% 35% 16% 2	792
November	1st to 10th	Night	15% 7% 22% 34% 19% 3	740
	11th to 20th	Night	16% 12% 31% 29% 12% 1	760
	21st to end	Night	21% 10% 30% 28% 10% 1	760
December	1st to 10th	Night	27% 15% 25% 24% 8%	760
	11th to 20th	Night	23% 12% 31% 26% 7%	760
	21st to end	Night	26% 12% 30% 26% 5%	836

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