PSU-Greenbush Astrophysical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the PSU-Greenbush Astrophysical Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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% 7% 2 6% 4 4 4 3 5% 7% 46%	125948
	Night	11% 6% 2 5% 4 3 3 3 5% 7% 51%	123675
Transparency	Day	36% 5% 12% 25% 18% 4	129560
	Night	33% 5% 11% 26% 21% 5%	128126
Seeing	Day	13% 9% 27% 29% 18% 5%	16842
	Night	14% 8% 31% 33% 13% 1	26590

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	18% 8% 3 7% 5% 4 3 2 4 6% 40%	4077
	11th to 20th	Day	8% 10% 3 8% 6% 3 4 2 4 6% 46%	4080
	21st to end	Day	15% 10% 3 6% 4 3 3 2 3 5% 45%	4488
February	1st to 10th	Day	17% 10% 3 7% 4 3 3 3 4 5% 41%	1800
	11th to 20th	Day	14% 9% 2 8% 4 3 3 2 4 5% 45%	1789
	21st to end	Day	13% 8% 3 6% 4 4 5% 5% 4 5% 42%	1728
March	1st to 10th	Day	12% 7% 3 5% 3 3 3 3 4 6% 51%	4936
	11th to 20th	Day	10% 6% 1 5% 3 3 3 3 5% 7% 54%	4980
	21st to end	Day	13% 9% 4 7% 5% 5% 5% 3 6% 8% 36%	5462
April	1st to 10th	Day	13% 7% 2 6% 4 3 4 4 5% 6% 45%	2280
	11th to 20th	Day	15% 7% 2 6% 4 5% 4 4 5% 5% 42%	2374
	21st to end	Day	20% 8% 3 6% 4 3 4 3 5% 6% 37%	2520
May	1st to 10th	Day	19% 7% 3 7% 5% 3 4 3 5% 7% 39%	5862
	11th to 20th	Day	13% 6% 2 6% 4 4 4 4 6% 7% 45%	5862
	21st to end	Day	20% 6% 2 5% 4 4 4 4 5% 7% 38%	6434
June	1st to 10th	Day	7% 4 2 6% 4 4 4 4 7% 9% 49%	2600
	11th to 20th	Day	7% 6% 2 5% 5% 4 5% 4 6% 8% 48%	2600
	21st to end	Day	5% 3 1 4 3 4 4 3 7% 9% 57%	2600
July	1st to 10th	Day	7% 5% 2 6% 4 5% 6% 4 6% 9% 45%	5900
	11th to 20th	Day	7% 4 2 6% 4 4 5% 4 6% 10% 48%	5900
	21st to end	Day	5% 4 2 5% 5% 5% 6% 6% 7% 11% 45%	6422
August	1st to 10th	Day	5% 3 2 5% 4 4 5% 4 6% 9% 54%	4234
	11th to 20th	Day	7% 5% 2 7% 4 5% 5% 4 6% 8% 46%	4171
	21st to end	Day	6% 4 2 5% 3 4 4 4 5% 7% 57%	4386
September	1st to 10th	Day	11% 6% 2 5% 4 4 4 3 4 6% 53%	2370
	11th to 20th	Day	10% 5% 2 5% 4 4 5% 4 6% 7% 48%	2200
	21st to end	Day	10% 4 2 5% 4 3 4 3 4 6% 55%	2200
October	1st to 10th	Day	11% 4 2 6% 3 3 3 3 5% 6% 55%	1939
	11th to 20th	Day	12% 5% 2 3 3 2 3 2 4 6% 59%	1900
	21st to end	Day	18% 7% 2 4 3 3 3 2 3 4 51%	2090
November	1st to 10th	Day	11% 6% 2 4 2 2 2 3 4 6% 58%	1958
	11th to 20th	Day	14% 6% 2 4 3 2 3 3 3 5% 54%	1835
	21st to end	Day	15% 7% 3 4 3 3 2 2 4 6% 52%	1700
December	1st to 10th	Day	9% 10% 2 5% 3 3 3 2 4 5% 56%	2610
	11th to 20th	Day	11% 8% 3 5% 4 4 3 2 3 5% 52%	3173
	21st to end	Day	20% 17% 6% 8% 4 3 3 3 3 5% 27%	4488
January	1st to 10th	Night	14% 8% 3 8% 4 4 3 3 4 5% 44%	5888
	11th to 20th	Night	8% 6% 3 7% 5% 5% 4 4 4 6% 48%	5900
	21st to end	Night	16% 8% 4 6% 3 2 3 2 4 5% 47%	6490
February	1st to 10th	Night	14% 11% 3 5% 4 4 4 3 4 6% 42%	2600
	11th to 20th	Night	14% 8% 2 7% 4 3 3 3 4 5% 47%	2513
	21st to end	Night	14% 6% 3 5% 4 3 3 3 6% 6% 48%	1900
March	1st to 10th	Night	13% 7% 3 6% 3 3 3 3 4 4 51%	4919
	11th to 20th	Night	8% 5% 2 4 3 3 4 3 5% 6% 58%	4980
	21st to end	Night	12% 5% 3 6% 4 4 4 4 6% 6% 46%	5467
April	1st to 10th	Night	12% 7% 2 5% 4 3 3 3 4 7% 50%	2120
	11th to 20th	Night	14% 6% 2 5% 4 3 3 2 5% 5% 50%	1975
	21st to end	Night	15% 7% 2 7% 4 3 4 3 5% 7% 43%	1876
May	1st to 10th	Night	17% 7% 2 6% 4 3 3 2 4 7% 45%	4053
	11th to 20th	Night	13% 6% 2 6% 3 3 3 3 5% 8% 46%	4053
	21st to end	Night	17% 6% 2 4 3 3 3 3 4 7% 48%	4443
June	1st to 10th	Night	6% 4 1 6% 3 4 4 4 6% 9% 53%	1800
	11th to 20th	Night	5% 4 2 4 4 3 4 4 7% 10% 53%	1800
	21st to end	Night	5% 3 2 4 4 4 4 4 5% 8% 59%	1800
July	1st to 10th	Night	8% 5% 2 6% 4 4 4 4 6% 9% 50%	4078
	11th to 20th	Night	4 3 1 5% 4 3 5% 4 6% 10% 55%	4077
	21st to end	Night	4 3 2 5% 4 4 4 5% 8% 12% 49%	4449
August	1st to 10th	Night	3 2 1 4 3 4 4 3 6% 10% 63%	2926
	11th to 20th	Night	7% 5% 2 5% 4 4 4 4 6% 8% 52%	2888
	21st to end	Night	3 2 1 4 3 3 4 3 5% 8% 64%	3512
September	1st to 10th	Night	9% 4 2 5% 4 4 3 3 4 6% 57%	2025
	11th to 20th	Night	8% 5% 2 4 4 4 5% 3 6% 7% 51%	2200
	21st to end	Night	7% 4 2 5% 3 4 3 3 4 6% 59%	2200
October	1st to 10th	Night	8% 3 1 4 2 3 4 3 5% 6% 61%	2287
	11th to 20th	Night	12% 5% 2 5% 2 3 2 2 4 5% 58%	2280
	21st to end	Night	14% 6% 2 5% 3 3 3 2 4 5% 53%	2508
November	1st to 10th	Night	11% 5% 1 4 2 2 3 2 4 5% 61%	2340
	11th to 20th	Night	10% 7% 2 5% 3 3 3 3 4 5% 55%	2563
	21st to end	Night	15% 7% 2 4 3 2 2 3 4 5% 54%	2692
December	1st to 10th	Night	8% 7% 2 4 3 3 3 3 4 6% 58%	4580
	11th to 20th	Night	14% 7% 3 5% 3 3 3 2 4 5% 51%	5003
	21st to end	Night	19% 13% 5% 8% 4 4 4 3 4 5% 32%	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	47% 3 11% 24% 14% 2	4078
	11th to 20th	Day	39% 4 11% 25% 17% 4	4080
	21st to end	Day	44% 3 9% 19% 20% 5%	4488
February	1st to 10th	Day	47% 4 10% 24% 12% 2	1800
	11th to 20th	Day	44% 3 11% 23% 16% 3	1798
	21st to end	Day	42% 5% 8% 21% 17% 7%	1728
March	1st to 10th	Day	36% 3 7% 22% 21% 11%	4936
	11th to 20th	Day	29% 4 9% 22% 25% 11%	4980
	21st to end	Day	45% 7% 15% 24% 8% 1	5464
April	1st to 10th	Day	38% 7% 13% 22% 16% 3	2280
	11th to 20th	Day	42% 6% 13% 21% 16% 3	2374
	21st to end	Day	49% 5% 11% 23% 11% 1	2520
May	1st to 10th	Day	45% 6% 11% 20% 16% 2	5900
	11th to 20th	Day	37% 6% 12% 25% 17% 3	5866
	21st to end	Day	44% 8% 14% 19% 15% 1	6447
June	1st to 10th	Day	30% 9% 18% 25% 17% 2	2600
	11th to 20th	Day	32% 7% 13% 31% 16% 2	2600
	21st to end	Day	22% 7% 17% 31% 19% 4	2600
July	1st to 10th	Day	34% 8% 12% 23% 20% 3	5836
	11th to 20th	Day	31% 7% 12% 34% 14% 2	5831
	21st to end	Day	29% 8% 20% 32% 10%	6240
August	1st to 10th	Day	25% 6% 15% 25% 24% 4	4220
	11th to 20th	Day	35% 7% 16% 25% 17% 1	4210
	21st to end	Day	26% 7% 12% 38% 13% 4	4385
September	1st to 10th	Day	33% 5% 11% 25% 18% 8%	2370
	11th to 20th	Day	33% 6% 12% 26% 22% 2	2200
	21st to end	Day	30% 6% 10% 25% 25% 5%	2200
October	1st to 10th	Day	19% 4 7% 18% 37% 15%	3199
	11th to 20th	Day	17% 2 5% 29% 36% 11%	3140
	21st to end	Day	31% 3 7% 27% 27% 5%	3428
November	1st to 10th	Day	29% 4 9% 30% 22% 5%	1958
	11th to 20th	Day	34% 3 8% 25% 25% 6%	1835
	21st to end	Day	35% 3 10% 28% 22% 1	1700
December	1st to 10th	Day	33% 3 8% 37% 19%	2610
	11th to 20th	Day	37% 3 6% 31% 21% 2	3171
	21st to end	Day	60% 4 9% 19% 7% 1	4488
January	1st to 10th	Night	43% 4 12% 26% 13% 2	5888
	11th to 20th	Night	36% 4 11% 24% 20% 4	5900
	21st to end	Night	41% 2 10% 19% 21% 6%	6490
February	1st to 10th	Night	44% 4 11% 24% 13% 3	2600
	11th to 20th	Night	40% 4 11% 24% 18% 3	2514
	21st to end	Night	37% 3 9% 22% 22% 7%	1900
March	1st to 10th	Night	37% 3 8% 21% 25% 6%	4919
	11th to 20th	Night	27% 4 8% 21% 32% 8%	4980
	21st to end	Night	37% 6% 14% 28% 14% 2	5467
April	1st to 10th	Night	34% 5% 11% 22% 22% 6%	2120
	11th to 20th	Night	37% 5% 9% 24% 19% 5%	1975
	21st to end	Night	40% 5% 12% 24% 16% 3	1876
May	1st to 10th	Night	40% 6% 13% 23% 15% 3	4080
	11th to 20th	Night	36% 5% 12% 24% 16% 6%	4054
	21st to end	Night	36% 6% 12% 26% 17% 3	4452
June	1st to 10th	Night	26% 9% 16% 28% 18% 3	1800
	11th to 20th	Night	24% 7% 17% 30% 19% 2	1800
	21st to end	Night	23% 5% 16% 31% 21% 4	1800
July	1st to 10th	Night	30% 6% 15% 26% 20% 3	4035
	11th to 20th	Night	24% 6% 11% 40% 17% 2	4026
	21st to end	Night	26% 7% 19% 35% 13%	4320
August	1st to 10th	Night	18% 5% 15% 35% 20% 7%	2916
	11th to 20th	Night	29% 7% 14% 28% 20% 2	2915
	21st to end	Night	19% 5% 13% 40% 18% 5%	3512
September	1st to 10th	Night	29% 4 11% 26% 21% 9%	2025
	11th to 20th	Night	30% 6% 10% 24% 24% 5%	2200
	21st to end	Night	26% 4 9% 26% 30% 4	2200
October	1st to 10th	Night	15% 3 6% 16% 44% 16%	3794
	11th to 20th	Night	21% 3 5% 20% 35% 17%	3779
	21st to end	Night	30% 3 7% 28% 26% 6%	4120
November	1st to 10th	Night	27% 4 10% 27% 25% 7%	2340
	11th to 20th	Night	32% 3 8% 23% 28% 6%	2563
	21st to end	Night	35% 4 9% 25% 24% 4	2692
December	1st to 10th	Night	29% 4 11% 37% 18% 1	4580
	11th to 20th	Night	37% 3 9% 25% 23% 3	5004
	21st to end	Night	54% 4 9% 23% 9% 2	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	20% 13% 36% 25% 6%	300
	11th to 20th	Day	20% 19% 38% 21% 2	300
	21st to end	Day	14% 12% 48% 20% 6%	330
February	1st to 10th	Day	24% 18% 34% 21% 3	300
	11th to 20th	Day	19% 18% 41% 17% 4 1	300
	21st to end	Day	19% 20% 41% 16% 3	249
March	1st to 10th	Day	19% 17% 37% 21% 5%	300
	11th to 20th	Day	24% 16% 34% 22% 4	300
	21st to end	Day	24% 18% 38% 16% 3 1	330
April	1st to 10th	Day	18% 17% 38% 21% 6% 1	380
	11th to 20th	Day	19% 14% 40% 21% 5%	492
	21st to end	Day	23% 14% 32% 22% 8% 1	620
May	1st to 10th	Day	16% 12% 35% 28% 8% 1	703
	11th to 20th	Day	16% 10% 31% 26% 14% 4	725
	21st to end	Day	16% 8% 29% 24% 16% 7%	789
June	1st to 10th	Day	6% 4 19% 34% 28% 9%	730
	11th to 20th	Day	7% 4 22% 39% 23% 6%	730
	21st to end	Day	5% 4 26% 41% 21% 3	730
July	1st to 10th	Day	9% 3 14% 32% 30% 12%	721
	11th to 20th	Day	5% 2 14% 38% 33% 8%	718
	21st to end	Day	7% 2 10% 36% 33% 12%	794
August	1st to 10th	Day	8% 1 14% 40% 31% 6%	726
	11th to 20th	Day	8% 2 15% 34% 32% 8%	715
	21st to end	Day	10% 1 10% 30% 38% 10%	631
September	1st to 10th	Day	11% 2 13% 32% 32% 9%	500
	11th to 20th	Day	5% 2 15% 39% 28% 11%	330
	21st to end	Day	12% 5% 19% 35% 22% 8%	330
October	1st to 10th	Day	11% 6% 26% 25% 20% 11%	309
	11th to 20th	Day	10% 12% 41% 26% 10% 2	300
	21st to end	Day	25% 11% 32% 23% 7% 1	330
November	1st to 10th	Day	16% 10% 36% 24% 12% 2	300
	11th to 20th	Day	17% 16% 33% 23% 10% 2	300
	21st to end	Day	17% 14% 35% 24% 7% 2	300
December	1st to 10th	Day	13% 12% 46% 19% 7% 3	300
	11th to 20th	Day	17% 14% 38% 23% 7% 1	300
	21st to end	Day	20% 22% 39% 17% 2	330
January	1st to 10th	Night	16% 12% 40% 27% 5%	1000
	11th to 20th	Night	15% 14% 42% 24% 5%	1000
	21st to end	Night	14% 12% 41% 28% 5%	1100
February	1st to 10th	Night	20% 13% 41% 24% 4	1000
	11th to 20th	Night	19% 15% 38% 25% 3	959
	21st to end	Night	20% 12% 39% 26% 3	580
March	1st to 10th	Night	15% 13% 39% 30% 4	600
	11th to 20th	Night	21% 12% 37% 25% 6%	600
	21st to end	Night	20% 13% 41% 21% 5%	660
April	1st to 10th	Night	14% 13% 40% 28% 5%	600
	11th to 20th	Night	18% 13% 36% 28% 6%	593
	21st to end	Night	18% 10% 38% 27% 7%	600
May	1st to 10th	Night	15% 6% 36% 34% 8% 1	600
	11th to 20th	Night	14% 7% 28% 34% 17% 1	591
	21st to end	Night	15% 4 28% 33% 19% 1	650
June	1st to 10th	Night	7% 3 16% 44% 28% 2	600
	11th to 20th	Night	6% 1 24% 47% 21% 1	600
	21st to end	Night	6% 2 25% 49% 19% 1	600
July	1st to 10th	Night	9% 1 12% 41% 35% 3	585
	11th to 20th	Night	4 1 11% 48% 34% 3	582
	21st to end	Night	7% 1 13% 40% 35% 3	657
August	1st to 10th	Night	4 2 14% 47% 32% 1	597
	11th to 20th	Night	8% 1 9% 51% 28% 2	594
	21st to end	Night	5% 1 12% 38% 41% 3	660
September	1st to 10th	Night	10% 3 14% 44% 28% 2	600
	11th to 20th	Night	11% 2 15% 43% 26% 4	600
	21st to end	Night	7% 3 19% 44% 25% 3	600
October	1st to 10th	Night	9% 3 25% 37% 24% 2	743
	11th to 20th	Night	14% 7% 39% 32% 8% 1	760
	21st to end	Night	16% 11% 33% 29% 10% 1	836
November	1st to 10th	Night	13% 7% 38% 31% 10% 1	780
	11th to 20th	Night	13% 12% 37% 29% 9%	963
	21st to end	Night	18% 12% 35% 28% 7%	1000
December	1st to 10th	Night	16% 10% 39% 29% 6%	1000
	11th to 20th	Night	19% 10% 39% 29% 4	1000
	21st to end	Night	18% 14% 37% 27% 3	1100

Page generated at 2025-10-01 00:39:05. Version ac07

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