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 2024-11-02.

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% 2 7% 5% 5% 5% 4 6% 8% 40%	100914
	Night	13% 7% 2 6% 4 4 4 4 6% 7% 42%	98034
Transparency	Day	43% 4 11% 23% 17% 3	100723
	Night	42% 3 10% 22% 18% 4	97894
Seeing	Day	18% 10% 23% 24% 18% 9%	14741
	Night	16% 9% 23% 29% 19% 3	24744

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 7% 4 4 4 3 5% 5% 36%	2836
	11th to 20th	Day	19% 9% 5% 6% 4 4 3 3 4 5% 39%	2840
	21st to end	Day	23% 11% 3 6% 4 3 3 3 3 5% 37%	3282
February	1st to 10th	Day	19% 9% 3 7% 5% 3 4 3 4 6% 37%	1900
	11th to 20th	Day	21% 8% 3 7% 4 4 4 3 4 6% 35%	1848
	21st to end	Day	21% 9% 3 7% 4 4 3 2 4 5% 36%	1570
March	1st to 10th	Day	23% 10% 3 7% 4 4 4 3 5% 6% 31%	3141
	11th to 20th	Day	14% 7% 2 6% 4 3 3 3 4 5% 49%	3480
	21st to end	Day	17% 8% 2 7% 5% 5% 4 3 5% 7% 39%	3815
April	1st to 10th	Day	12% 5% 2 6% 5% 6% 5% 4 6% 7% 43%	2228
	11th to 20th	Day	13% 7% 2 6% 5% 4 4 3 4 6% 46%	2254
	21st to end	Day	11% 6% 3 5% 6% 5% 5% 3 6% 7% 44%	2406
May	1st to 10th	Day	11% 7% 3 7% 5% 4 5% 4 6% 8% 39%	4082
	11th to 20th	Day	15% 8% 3 7% 5% 5% 5% 4 6% 7% 34%	4081
	21st to end	Day	9% 5% 2 7% 5% 5% 5% 4 7% 9% 42%	4476
June	1st to 10th	Day	10% 6% 2 7% 5% 5% 5% 5% 8% 9% 37%	2470
	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% 56%	1681
	11th to 20th	Day	13% 6% 2 5% 4 4 3 3 3 7% 50%	1710
	21st to end	Day	15% 7% 2 5% 4 4 4 3 4 7% 45%	1710
December	1st to 10th	Day	21% 10% 3 7% 4 4 4 4 4 5% 35%	1710
	11th to 20th	Day	15% 8% 3 7% 5% 5% 5% 4 4 6% 40%	2153
	21st to end	Day	19% 10% 3 7% 4 4 4 3 5% 8% 33%	3124
January	1st to 10th	Night	18% 9% 2 7% 5% 4 4 3 4 5% 39%	4109
	11th to 20th	Night	19% 9% 3 5% 4 3 4 3 4 5% 41%	4120
	21st to end	Night	22% 12% 3 6% 5% 3 3 3 4 3 36%	4372
February	1st to 10th	Night	15% 8% 2 6% 4 4 4 3 5% 6% 42%	2280
	11th to 20th	Night	21% 7% 3 7% 6% 4 4 4 3 4 38%	2218
	21st to end	Night	21% 8% 2 7% 4 4 5% 3 5% 6% 35%	1884
March	1st to 10th	Night	21% 9% 3 8% 5% 4 4 3 5% 6% 32%	3711
	11th to 20th	Night	12% 5% 2 5% 3 4 4 4 6% 7% 48%	3480
	21st to end	Night	14% 8% 3 8% 5% 4 4 3 5% 5% 41%	3817
April	1st to 10th	Night	12% 4 2 4 4 4 4 3 4 9% 49%	1967
	11th to 20th	Night	12% 7% 2 5% 3 4 3 3 5% 7% 49%	1875
	21st to end	Night	10% 5% 2 6% 4 4 4 3 5% 8% 49%	1772
May	1st to 10th	Night	12% 7% 3 7% 5% 5% 4 4 6% 8% 39%	2813
	11th to 20th	Night	15% 7% 3 7% 5% 4 4 4 6% 7% 38%	2812
	21st to end	Night	7% 5% 2 6% 4 5% 5% 5% 8% 11% 42%	3079
June	1st to 10th	Night	7% 5% 2 8% 6% 6% 6% 4 8% 11% 37%	1710
	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 4 4 3 3 2 4 5% 55%	2431
	11th to 20th	Night	13% 7% 2 5% 3 3 3 2 5% 6% 51%	2470
	21st to end	Night	15% 6% 2 5% 4 4 5% 3 4 5% 46%	2470
December	1st to 10th	Night	20% 8% 2 7% 4 4 4 4 5% 6% 36%	2470
	11th to 20th	Night	17% 8% 2 5% 4 4 3 3 4 5% 44%	3042
	21st to end	Night	19% 10% 3 7% 5% 5% 5% 4 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	55% 1 8% 14% 13% 9%	3141
	11th to 20th	Day	40% 2 6% 23% 22% 7%	3480
	21st to end	Day	47% 2 8% 25% 15% 3	3815
April	1st to 10th	Day	40% 4 10% 24% 18% 4	2228
	11th to 20th	Day	41% 2 6% 20% 25% 6%	2254
	21st to end	Day	39% 3 9% 30% 16% 3	2406
May	1st to 10th	Day	44% 3 12% 23% 15% 2	4120
	11th to 20th	Day	49% 2 12% 24% 12% 1	4085
	21st to end	Day	39% 3 11% 25% 20% 2	4489
June	1st to 10th	Day	41% 5% 16% 24% 13% 2	2470
	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	33% 3 8% 21% 27% 8%	2090
	11th to 20th	Day	31% 3 8% 24% 26% 9%	2014
	21st to end	Day	35% 1 5% 25% 27% 7%	2020
November	1st to 10th	Day	33% 2 7% 19% 31% 8%	1681
	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	55% 2 8% 14% 16% 5%	3711
	11th to 20th	Night	36% 2 8% 23% 22% 10%	3480
	21st to end	Night	47% 1 6% 24% 19% 3	3817
April	1st to 10th	Night	34% 2 11% 24% 23% 5%	1967
	11th to 20th	Night	36% 2 7% 24% 22% 9%	1875
	21st to end	Night	36% 3 10% 29% 20% 2	1772
May	1st to 10th	Night	45% 3 14% 22% 15% 2	2840
	11th to 20th	Night	45% 3 14% 25% 13%	2813
	21st to end	Night	36% 3 15% 26% 19% 1	3087
June	1st to 10th	Night	39% 6% 16% 22% 14% 2	1710
	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	30% 2 8% 21% 28% 9%	2090
	11th to 20th	Night	34% 2 6% 22% 26% 10%	2164
	21st to end	Night	35% 2 3 24% 27% 10%	2574
November	1st to 10th	Night	33% 3 6% 23% 24% 10%	2431
	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	29% 20% 31% 14% 5% 1	278
	11th to 20th	Day	24% 20% 30% 18% 7% 1	460
	21st to end	Day	23% 17% 38% 15% 7% 1	506
April	1st to 10th	Day	20% 18% 29% 17% 12% 3	460
	11th to 20th	Day	20% 15% 29% 22% 12% 2	452
	21st to end	Day	20% 7% 34% 25% 11% 3	460
May	1st to 10th	Day	15% 10% 30% 27% 12% 6%	463
	11th to 20th	Day	14% 6% 25% 29% 17% 9%	487
	21st to end	Day	12% 5% 18% 32% 20% 12%	529
June	1st to 10th	Day	13% 3 19% 30% 22% 13%	490
	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% 26% 18% 3	469
November	1st to 10th	Day	22% 11% 28% 23% 12% 4	273
	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	21% 13% 34% 27% 5%	760
	11th to 20th	Night	19% 18% 31% 23% 9% 1	760
	21st to end	Night	19% 13% 33% 27% 8%	836
April	1st to 10th	Night	15% 13% 33% 30% 10% 1	637
	11th to 20th	Night	16% 11% 33% 30% 9%	563
	21st to end	Night	12% 9% 31% 35% 13% 1	570
May	1st to 10th	Night	11% 6% 29% 32% 19% 2	570
	11th to 20th	Night	14% 4 22% 35% 21% 4	561
	21st to end	Night	9% 3 14% 37% 31% 6%	617
June	1st to 10th	Night	9% 1 16% 38% 30% 5%	570
	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% 7% 22% 34% 18% 3	748
	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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