Green River Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Green River Observatory clear sky chart from 2005-11-06 to 2025-09-19.

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	3 5% 2 6% 4 5% 5% 4 7% 8% 52%	135330
	Night	2 5% 2 6% 4 4 5% 4 7% 9% 50%	131254
Transparency	Day	26% 7% 14% 23% 21% 10%	130090
	Night	25% 8% 15% 25% 19% 8%	127497
Seeing	Day	6% 7% 23% 34% 23% 7%	16823
	Night	4 4 24% 46% 19% 2	26904

By Month

Percentage Cloud Cover SW (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	4 3 9% 6% 7% 6% 6% 10% 11% 38%	3795
	11th to 20th	Day	1 4 3 8% 4 6% 6% 4 6% 10% 48%	4100
	21st to end	Day	1 4 3 8% 6% 5% 5% 5% 8% 11% 45%	4510
February	1st to 10th	Day	2 6% 3 8% 6% 5% 5% 4 7% 9% 45%	1800
	11th to 20th	Day	1 8% 4 8% 5% 6% 6% 5% 7% 9% 40%	1755
	21st to end	Day	1 6% 3 8% 6% 4 4 6% 7% 9% 45%	1571
March	1st to 10th	Day	2 7% 3 7% 5% 5% 7% 6% 7% 9% 42%	4954
	11th to 20th	Day	1 5% 3 8% 6% 6% 6% 5% 7% 9% 45%	5000
	21st to end	Day	3 10% 4 11% 7% 6% 5% 5% 7% 9% 32%	5486
April	1st to 10th	Day	2 7% 4 8% 6% 6% 6% 5% 7% 8% 42%	2325
	11th to 20th	Day	3 8% 4 9% 6% 5% 5% 4 7% 9% 40%	2440
	21st to end	Day	5% 8% 4 8% 6% 6% 6% 4 7% 9% 38%	2600
May	1st to 10th	Day	4 6% 3 8% 6% 5% 5% 5% 6% 8% 45%	5880
	11th to 20th	Day	5% 6% 2 5% 5% 5% 5% 5% 6% 8% 47%	6016
	21st to end	Day	4 6% 2 6% 5% 5% 5% 5% 8% 9% 46%	6925
June	1st to 10th	Day	4 4 2 6% 4 5% 5% 5% 7% 9% 49%	2800
	11th to 20th	Day	4 4 1 5% 4 4 5% 4 6% 8% 56%	2800
	21st to end	Day	3 2 1 4 3 3 4 3 6% 7% 65%	2800
July	1st to 10th	Day	2 2 1 3 2 3 4 3 6% 6% 67%	6339
	11th to 20th	Day	2 2 1 4 3 4 4 4 6% 8% 62%	6069
	21st to end	Day	3 3 1 4 3 4 5% 4 6% 9% 57%	6401
August	1st to 10th	Day	2 3 1 3 3 4 4 4 6% 8% 61%	5779
	11th to 20th	Day	2 2 1 4 4 4 5% 4 6% 8% 60%	5798
	21st to end	Day	3 3 1 5% 4 4 4 4 6% 7% 58%	6293
September	1st to 10th	Day	3 3 2 3 3 4 3 3 6% 7% 64%	2400
	11th to 20th	Day	4 4 1 4 3 3 4 3 6% 7% 61%	2202
	21st to end	Day	4 4 1 4 3 3 4 3 3 5% 66%	2090
October	1st to 10th	Day	4 4 1 3 2 2 2 2 4 5% 69%	3180
	11th to 20th	Day	4 5% 2 4 3 4 3 3 5% 7% 60%	3150
	21st to end	Day	3 6% 2 6% 3 3 4 3 5% 7% 59%	3438
November	1st to 10th	Day	4 5% 3 5% 5% 4 5% 4 5% 8% 52%	1784
	11th to 20th	Day	1 6% 2 5% 5% 4 6% 4 7% 9% 51%	1600
	21st to end	Day	1 5% 2 6% 4 4 5% 3 6% 7% 55%	1600
December	1st to 10th	Day	1 3 2 4 3 3 3 4 5% 8% 64%	2620
	11th to 20th	Day	1 6% 3 6% 5% 5% 6% 6% 11% 13% 38%	3026
	21st to end	Day	1 7% 3 9% 7% 6% 7% 6% 9% 9% 37%	4004
January	1st to 10th	Night	4 3 8% 6% 6% 8% 6% 9% 12% 36%	6212
	11th to 20th	Night	1 5% 2 7% 5% 6% 8% 5% 9% 12% 41%	5880
	21st to end	Night	1 6% 3 11% 10% 7% 5% 4 7% 11% 35%	6468
February	1st to 10th	Night	1 5% 2 7% 5% 5% 6% 4 8% 12% 46%	2600
	11th to 20th	Night	2 6% 3 7% 5% 5% 6% 5% 8% 10% 42%	2532
	21st to end	Night	1 5% 3 9% 6% 6% 6% 4 6% 9% 45%	2054
March	1st to 10th	Night	2 9% 3 8% 5% 6% 8% 5% 7% 10% 37%	4940
	11th to 20th	Night	1 6% 4 8% 6% 6% 8% 7% 9% 10% 35%	5000
	21st to end	Night	3 9% 3 8% 5% 5% 5% 4 6% 8% 43%	5489
April	1st to 10th	Night	3 7% 3 7% 4 4 6% 4 6% 9% 47%	2075
	11th to 20th	Night	4 8% 2 7% 6% 5% 5% 5% 6% 8% 43%	1905
	21st to end	Night	5% 7% 3 7% 4 4 4 3 6% 8% 48%	1800
May	1st to 10th	Night	5% 6% 3 7% 5% 4 4 3 5% 7% 50%	4100
	11th to 20th	Night	5% 5% 2 5% 3 4 4 3 5% 7% 57%	3896
	21st to end	Night	4 5% 2 5% 4 3 4 4 6% 8% 56%	3972
June	1st to 10th	Night	4 4 2 4 3 3 4 3 7% 11% 56%	1600
	11th to 20th	Night	5% 4 1 6% 3 2 4 3 5% 8% 59%	1600
	21st to end	Night	3 2 1 2 2 2 3 2 5% 7% 69%	1600
July	1st to 10th	Night	3 3 1 4 3 3 3 3 5% 6% 66%	3640
	11th to 20th	Night	1 2 1 3 2 2 4 3 5% 8% 69%	3906
	21st to end	Night	1 1 1 3 3 3 4 4 5% 8% 67%	4472
August	1st to 10th	Night	2 3 1 3 3 3 3 4 6% 7% 66%	4033
	11th to 20th	Night	3 3 1 5% 4 4 4 4 6% 7% 61%	4051
	21st to end	Night	4 3 2 4 3 3 4 3 5% 6% 63%	4683
September	1st to 10th	Night	2 3 1 3 3 3 3 3 5% 6% 68%	2000
	11th to 20th	Night	5% 3 2 3 3 3 3 3 5% 7% 62%	2151
	21st to end	Night	4 4 2 3 3 3 2 2 4 5% 68%	2090
October	1st to 10th	Night	4 4 1 3 2 2 2 2 3 4 73%	3790
	11th to 20th	Night	4 6% 3 6% 3 3 4 3 4 6% 57%	3778
	21st to end	Night	4 7% 2 5% 3 4 3 3 5% 8% 54%	4120
November	1st to 10th	Night	4 5% 2 5% 4 4 4 4 6% 9% 53%	2492
	11th to 20th	Night	1 4 2 5% 4 4 4 5% 7% 8% 56%	2800
	21st to end	Night	2 5% 2 6% 4 3 4 4 6% 8% 55%	2800
December	1st to 10th	Night	1 4 2 5% 3 3 5% 5% 9% 16% 49%	4570
	11th to 20th	Night	1 6% 3 7% 5% 5% 6% 6% 10% 15% 36%	5181
	21st to end	Night	2 7% 3 9% 6% 7% 8% 7% 11% 10% 29%	6974

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	30% 10% 24% 21% 15% 1	3796
	11th to 20th	Day	24% 10% 19% 21% 21% 5%	4100
	21st to end	Day	26% 13% 20% 25% 14% 3	4510
February	1st to 10th	Day	32% 10% 22% 21% 12% 2	1800
	11th to 20th	Day	33% 9% 23% 23% 10% 2	1764
	21st to end	Day	30% 10% 22% 22% 12% 4	1571
March	1st to 10th	Day	29% 10% 20% 24% 13% 4	4954
	11th to 20th	Day	30% 8% 18% 28% 16%	5000
	21st to end	Day	40% 12% 19% 24% 5%	5486
April	1st to 10th	Day	35% 11% 17% 23% 12% 2	2325
	11th to 20th	Day	35% 9% 18% 26% 12% 1	2443
	21st to end	Day	37% 10% 15% 26% 12%	2600
May	1st to 10th	Day	31% 6% 16% 29% 14% 4	5880
	11th to 20th	Day	29% 7% 14% 25% 20% 4	6019
	21st to end	Day	28% 7% 14% 30% 20% 2	6926
June	1st to 10th	Day	27% 10% 16% 26% 18% 3	2800
	11th to 20th	Day	23% 6% 12% 19% 27% 12%	2800
	21st to end	Day	17% 4 8% 17% 36% 17%	2800
July	1st to 10th	Day	16% 3 5% 15% 25% 36%	6271
	11th to 20th	Day	17% 5% 4 15% 29% 30%	5995
	21st to end	Day	21% 5% 5% 14% 34% 21%	6217
August	1st to 10th	Day	19% 6% 7% 20% 27% 21%	4208
	11th to 20th	Day	21% 5% 8% 21% 29% 16%	4197
	21st to end	Day	17% 4 7% 16% 35% 21%	4536
September	1st to 10th	Day	19% 4 10% 17% 31% 20%	2400
	11th to 20th	Day	21% 5% 10% 23% 28% 12%	2202
	21st to end	Day	21% 4 9% 25% 33% 7%	2090
October	1st to 10th	Day	19% 4 7% 21% 31% 17%	3180
	11th to 20th	Day	24% 5% 10% 30% 22% 9%	3150
	21st to end	Day	24% 5% 15% 37% 17% 2	3438
November	1st to 10th	Day	28% 9% 18% 25% 18% 2	1784
	11th to 20th	Day	25% 9% 24% 25% 15% 2	1600
	21st to end	Day	25% 6% 22% 28% 17% 2	1600
December	1st to 10th	Day	17% 8% 23% 32% 17% 3	2620
	11th to 20th	Day	30% 12% 24% 25% 8% 1	3024
	21st to end	Day	33% 10% 18% 20% 13% 6%	4004
January	1st to 10th	Night	29% 10% 18% 26% 15% 1	6212
	11th to 20th	Night	26% 11% 20% 22% 18% 3	5880
	21st to end	Night	31% 10% 21% 19% 17% 2	6468
February	1st to 10th	Night	27% 11% 23% 26% 11% 2	2600
	11th to 20th	Night	29% 10% 23% 23% 13% 2	2533
	21st to end	Night	30% 10% 20% 23% 14% 3	2054
March	1st to 10th	Night	32% 7% 18% 23% 14% 6%	4940
	11th to 20th	Night	30% 11% 16% 26% 15% 1	5000
	21st to end	Night	33% 11% 19% 31% 6% 1	5489
April	1st to 10th	Night	31% 8% 20% 23% 13% 4	2075
	11th to 20th	Night	34% 7% 16% 26% 15% 1	1905
	21st to end	Night	31% 8% 18% 29% 15%	1800
May	1st to 10th	Night	29% 7% 14% 24% 23% 3	4097
	11th to 20th	Night	23% 6% 13% 35% 17% 6%	3897
	21st to end	Night	23% 6% 15% 36% 17% 3	3972
June	1st to 10th	Night	22% 8% 20% 27% 20% 3	1600
	11th to 20th	Night	22% 6% 11% 22% 26% 14%	1600
	21st to end	Night	14% 4 7% 18% 35% 22%	1600
July	1st to 10th	Night	18% 5% 6% 14% 23% 35%	3600
	11th to 20th	Night	13% 3 3 18% 30% 32%	3857
	21st to end	Night	14% 4 6% 16% 37% 23%	4344
August	1st to 10th	Night	19% 5% 6% 19% 30% 21%	2928
	11th to 20th	Night	22% 4 8% 20% 29% 17%	2928
	21st to end	Night	14% 4 7% 19% 28% 28%	3371
September	1st to 10th	Night	16% 4 9% 19% 32% 21%	2000
	11th to 20th	Night	21% 5% 10% 24% 28% 13%	2151
	21st to end	Night	19% 4 9% 27% 33% 8%	2090
October	1st to 10th	Night	17% 3 7% 22% 39% 11%	3790
	11th to 20th	Night	28% 4 9% 29% 22% 8%	3778
	21st to end	Night	25% 7% 11% 36% 17% 3	4120
November	1st to 10th	Night	25% 9% 16% 27% 20% 3	2492
	11th to 20th	Night	22% 8% 23% 32% 14% 2	2800
	21st to end	Night	26% 6% 20% 29% 16% 3	2800
December	1st to 10th	Night	19% 9% 21% 31% 17% 2	4570
	11th to 20th	Night	30% 11% 25% 25% 10%	5182
	21st to end	Night	36% 11% 16% 21% 11% 6%	6974

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	4 5% 20% 46% 19% 7%	300
	11th to 20th	Day	3 6% 21% 43% 22% 5%	300
	21st to end	Day	3 4 18% 51% 20% 4	330
February	1st to 10th	Day	4 14% 37% 31% 12% 2	300
	11th to 20th	Day	5% 6% 28% 43% 15% 3	300
	21st to end	Day	4 11% 25% 41% 16% 3	330
March	1st to 10th	Day	5% 8% 32% 38% 14% 2	500
	11th to 20th	Day	4 8% 30% 36% 20% 1	500
	21st to end	Day	7% 13% 29% 33% 16% 2	550
April	1st to 10th	Day	7% 14% 31% 26% 15% 7%	500
	11th to 20th	Day	8% 19% 30% 24% 15% 4	492
	21st to end	Day	10% 14% 28% 27% 15% 5%	500
May	1st to 10th	Day	9% 16% 27% 24% 16% 8%	503
	11th to 20th	Day	13% 10% 21% 30% 21% 5%	599
	21st to end	Day	11% 7% 20% 28% 25% 8%	789
June	1st to 10th	Day	6% 5% 22% 32% 28% 8%	730
	11th to 20th	Day	5% 9% 25% 37% 21% 3	730
	21st to end	Day	4 6% 20% 37% 27% 7%	730
July	1st to 10th	Day	3 3 18% 35% 32% 8%	721
	11th to 20th	Day	4 2 20% 37% 30% 7%	604
	21st to end	Day	4 2 19% 36% 29% 10%	575
August	1st to 10th	Day	6% 2 21% 33% 28% 10%	527
	11th to 20th	Day	2 2 18% 30% 35% 12%	517
	21st to end	Day	4 5% 19% 32% 28% 12%	583
September	1st to 10th	Day	3 4 21% 30% 28% 14%	530
	11th to 20th	Day	6% 7% 26% 27% 22% 12%	523
	21st to end	Day	6% 8% 21% 28% 27% 10%	490
October	1st to 10th	Day	7% 4 17% 30% 25% 17%	310
	11th to 20th	Day	5% 8% 17% 33% 26% 11%	300
	21st to end	Day	5% 10% 26% 35% 23% 2	330
November	1st to 10th	Day	5% 10% 23% 34% 23% 5%	300
	11th to 20th	Day	3 8% 21% 47% 18% 3	300
	21st to end	Day	3 6% 16% 52% 18% 5%	300
December	1st to 10th	Day	3 5% 20% 44% 23% 5%	300
	11th to 20th	Day	4 4 21% 51% 16% 4	300
	21st to end	Day	6% 6% 24% 42% 15% 7%	330
January	1st to 10th	Night	2 4 28% 47% 15% 3	950
	11th to 20th	Night	3 5% 33% 39% 17% 4	800
	21st to end	Night	4 1 27% 51% 14% 3	880
February	1st to 10th	Night	2 5% 37% 43% 11% 2	800
	11th to 20th	Night	4 3 28% 49% 13% 2	784
	21st to end	Night	2 3 32% 51% 10% 1	660
March	1st to 10th	Night	5% 3 31% 48% 12% 2	800
	11th to 20th	Night	3 3 26% 52% 16% 1	800
	21st to end	Night	6% 3 29% 44% 17% 1	880
April	1st to 10th	Night	6% 4 30% 44% 16% 1	675
	11th to 20th	Night	7% 5% 33% 43% 11% 1	593
	21st to end	Night	7% 8% 27% 39% 18% 1	600
May	1st to 10th	Night	8% 7% 28% 37% 19% 1	600
	11th to 20th	Night	7% 4 21% 42% 25% 2	591
	21st to end	Night	9% 5% 21% 38% 25% 2	650
June	1st to 10th	Night	6% 4 19% 42% 27% 2	600
	11th to 20th	Night	6% 6% 27% 45% 16% 1	600
	21st to end	Night	3 2 17% 54% 24% 1	600
July	1st to 10th	Night	2 1 12% 49% 34% 2	585
	11th to 20th	Night	2 1 11% 55% 30% 1	582
	21st to end	Night	2 8% 52% 37% 1	657
August	1st to 10th	Night	3 2 10% 52% 32% 1	597
	11th to 20th	Night	3 1 8% 52% 34% 2	594
	21st to end	Night	4 1 11% 45% 35% 3	660
September	1st to 10th	Night	3 2 13% 49% 32% 3	600
	11th to 20th	Night	5% 2 13% 46% 28% 5%	765
	21st to end	Night	5% 2 16% 43% 28% 7%	760
October	1st to 10th	Night	8% 1 21% 41% 25% 4	760
	11th to 20th	Night	6% 4 22% 42% 23% 3	760
	21st to end	Night	5% 4 25% 48% 15% 3	836
November	1st to 10th	Night	4 4 26% 49% 15% 2	785
	11th to 20th	Night	2 7% 31% 48% 11% 1	1000
	21st to end	Night	4 4 29% 48% 13% 3	1000
December	1st to 10th	Night	3 5% 30% 48% 14% 1	1000
	11th to 20th	Night	3 3 30% 53% 10% 1	1000
	21st to end	Night	4 5% 31% 44% 13% 3	1100

Page generated at 2025-09-15 23:49:12. Version ac07

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