Greenhawk Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Greenhawk Observatory clear sky chart from 2005-11-06 to 2025-04-26.

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	5% 4 1 4 3 3 3 3 4 6% 63%	117657
	Night	6% 5% 2 5% 3 3 3 3 5% 7% 58%	119853
Transparency	Day	23% 3 8% 21% 26% 20%	117415
	Night	27% 4 9% 20% 23% 17%	119685
Seeing	Day	7% 10% 24% 31% 21% 8%	15997
	Night	8% 4 15% 36% 30% 6%	25485

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	9% 6% 2 7% 4 3 4 3 5% 8% 48%	3897
	11th to 20th	Day	8% 6% 3 7% 4 4 4 3 4 4 53%	4100
	21st to end	Day	11% 8% 3 6% 5% 5% 5% 3 5% 7% 42%	4510
February	1st to 10th	Day	10% 8% 2 6% 4 4 4 3 6% 6% 45%	1800
	11th to 20th	Day	10% 6% 2 6% 5% 4 4 4 6% 8% 44%	1860
	21st to end	Day	9% 6% 2 6% 5% 5% 5% 3 5% 8% 46%	1650
March	1st to 10th	Day	9% 7% 3 7% 6% 6% 7% 5% 7% 8% 36%	4862
	11th to 20th	Day	6% 5% 2 5% 4 5% 5% 4 6% 10% 48%	5000
	21st to end	Day	7% 7% 2 6% 6% 5% 6% 5% 7% 10% 39%	5573
April	1st to 10th	Day	9% 6% 2 6% 5% 4 4 3 6% 8% 48%	2400
	11th to 20th	Day	7% 6% 2 5% 3 3 3 3 4 7% 57%	2369
	21st to end	Day	5% 5% 1 3 3 3 3 3 5% 8% 61%	2342
May	1st to 10th	Day	5% 3 1 3 2 1 2 2 3 5% 74%	3883
	11th to 20th	Day	3 2 1 2 2 1 1 1 2 5% 80%	4152
	21st to end	Day	3 4 1 3 2 2 2 2 3 5% 74%	4813
June	1st to 10th	Day	4 4 1 4 2 2 3 3 4 7% 65%	2660
	11th to 20th	Day	2 2 1 1 1 1 1 2 3 6% 80%	2660
	21st to end	Day	2 2 1 2 2 2 2 1 3 4 81%	2660
July	1st to 10th	Day	1 1 1 1 1 1 1 2 3 88%	4420
	11th to 20th	Day	1 1 1 2 2 2 2 2 3 5% 81%	4114
	21st to end	Day	1 1 1 1 1 1 1 3 5% 85%	4455
August	1st to 10th	Day	1 2 1 4 2 2 3 3 4 7% 71%	4091
	11th to 20th	Day	1 1 1 1 1 2 2 2 5% 85%	3796
	21st to end	Day	1 1 2 2 2 2 2 3 6% 79%	4180
September	1st to 10th	Day	1 2 1 2 1 1 1 1 3 4 83%	2280
	11th to 20th	Day	2 3 1 3 3 2 3 3 4 7% 69%	2280
	21st to end	Day	2 2 2 2 2 1 2 3 5% 78%	2020
October	1st to 10th	Day	2 2 1 2 2 2 3 2 4 5% 75%	3180
	11th to 20th	Day	3 3 1 3 2 2 3 2 4 6% 71%	3161
	21st to end	Day	2 3 1 3 2 3 3 3 5% 7% 67%	3448
November	1st to 10th	Day	5% 5% 2 4 4 3 5% 3 5% 7% 56%	1924
	11th to 20th	Day	6% 6% 3 5% 4 4 4 5% 6% 6% 52%	1800
	21st to end	Day	8% 5% 2 5% 4 3 4 3 5% 8% 53%	1665
December	1st to 10th	Day	5% 3 1 5% 4 4 4 4 6% 10% 54%	2620
	11th to 20th	Day	10% 7% 2 8% 6% 6% 5% 4 7% 9% 36%	3028
	21st to end	Day	9% 8% 4 9% 6% 6% 7% 5% 7% 7% 32%	4004
January	1st to 10th	Night	8% 7% 2 5% 4 4 4 3 6% 8% 50%	6042
	11th to 20th	Night	8% 8% 4 7% 4 4 4 3 4 5% 49%	5880
	21st to end	Night	10% 8% 3 7% 5% 4 5% 3 4 6% 44%	6468
February	1st to 10th	Night	9% 7% 3 6% 4 4 5% 4 4 6% 49%	2600
	11th to 20th	Night	9% 7% 2 5% 5% 4 5% 4 6% 8% 45%	2425
	21st to end	Night	8% 7% 1 5% 4 4 5% 3 4 7% 52%	1980
March	1st to 10th	Night	10% 8% 3 7% 5% 5% 5% 5% 6% 7% 39%	5029
	11th to 20th	Night	9% 6% 2 7% 4 4 4 3 4 7% 51%	5000
	21st to end	Night	9% 9% 3 6% 4 5% 5% 4 5% 7% 43%	5402
April	1st to 10th	Night	8% 6% 2 6% 5% 4 4 3 6% 8% 49%	2000
	11th to 20th	Night	7% 6% 2 4 3 3 3 3 5% 7% 56%	1979
	21st to end	Night	4 3 1 4 2 3 4 4 5% 9% 62%	1954
May	1st to 10th	Night	3 5% 2 3 2 2 2 2 4 5% 70%	3090
	11th to 20th	Night	2 3 1 3 2 2 2 2 4 5% 76%	2746
	21st to end	Night	1 3 1 3 2 2 2 2 3 6% 75%	2762
June	1st to 10th	Night	3 3 1 4 3 3 2 2 5% 8% 65%	1520
	11th to 20th	Night	2 1 2 1 2 2 1 4 7% 78%	1520
	21st to end	Night	1 3 1 2 2 2 2 2 2 4 80%	1520
July	1st to 10th	Night	2 1 1 1 1 1 1 3 89%	2540
	11th to 20th	Night	1 2 1 4 2 2 2 2 3 5% 77%	2851
	21st to end	Night	2 2 3 2 2 2 2 4 6% 75%	3109
August	1st to 10th	Night	2 4 3 6% 4 2 3 1 4 7% 63%	2848
	11th to 20th	Night	2 2 2 2 1 1 4 6% 80%	3048
	21st to end	Night	1 3 1 3 2 2 2 2 3 5% 76%	3498
September	1st to 10th	Night	1 2 1 2 2 2 1 2 3 8% 77%	1900
	11th to 20th	Night	3 5% 1 4 3 2 3 1 4 6% 69%	1900
	21st to end	Night	2 3 1 3 2 2 1 2 3 5% 76%	2155
October	1st to 10th	Night	2 2 1 2 1 2 2 2 3 6% 77%	3800
	11th to 20th	Night	4 5% 1 4 2 2 2 2 3 6% 69%	3777
	21st to end	Night	3 3 2 4 3 3 3 3 4 7% 66%	4121
November	1st to 10th	Night	5% 5% 2 5% 3 2 3 3 6% 7% 58%	2371
	11th to 20th	Night	6% 6% 2 6% 3 3 4 3 6% 8% 54%	2600
	21st to end	Night	6% 5% 2 4 4 4 4 3 5% 8% 56%	2730
December	1st to 10th	Night	4 4 2 4 3 3 4 4 5% 7% 59%	4560
	11th to 20th	Night	11% 8% 2 7% 5% 5% 5% 5% 7% 10% 35%	5176
	21st to end	Night	11% 8% 3 8% 6% 6% 6% 5% 7% 7% 33%	6952

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	36% 4 17% 23% 16% 4	3897
	11th to 20th	Day	36% 3 8% 14% 20% 19%	4100
	21st to end	Day	42% 3 7% 14% 18% 16%	4510
February	1st to 10th	Day	40% 4 6% 22% 18% 11%	1800
	11th to 20th	Day	39% 3 10% 20% 21% 7%	1869
	21st to end	Day	40% 4 7% 24% 19% 7%	1650
March	1st to 10th	Day	46% 3 10% 19% 20% 2	4862
	11th to 20th	Day	30% 5% 14% 31% 13% 7%	5000
	21st to end	Day	39% 6% 16% 27% 11% 2	5573
April	1st to 10th	Day	36% 4 12% 23% 21% 4	2400
	11th to 20th	Day	29% 3 14% 27% 22% 6%	2372
	21st to end	Day	24% 3 12% 31% 24% 5%	2342
May	1st to 10th	Day	17% 2 8% 32% 29% 13%	3883
	11th to 20th	Day	11% 1 5% 37% 32% 14%	4155
	21st to end	Day	17% 1 8% 35% 34% 5%	4814
June	1st to 10th	Day	21% 2 11% 28% 28% 10%	2660
	11th to 20th	Day	10% 2 7% 24% 40% 18%	2660
	21st to end	Day	10% 2 3 14% 33% 38%	2660
July	1st to 10th	Day	4 1 1 6% 16% 71%	4351
	11th to 20th	Day	7% 2 3 8% 19% 61%	4044
	21st to end	Day	4 2 1 8% 38% 46%	4315
August	1st to 10th	Day	11% 3 2 13% 35% 36%	4076
	11th to 20th	Day	5% 1 1 9% 48% 35%	3833
	21st to end	Day	7% 3 3 12% 42% 32%	4179
September	1st to 10th	Day	8% 1 2 11% 33% 45%	2280
	11th to 20th	Day	16% 3 3 25% 34% 18%	2280
	21st to end	Day	11% 2 4 23% 37% 23%	2020
October	1st to 10th	Day	13% 2 4 12% 29% 39%	3180
	11th to 20th	Day	16% 2 7% 27% 31% 17%	3161
	21st to end	Day	17% 2 6% 26% 32% 17%	3448
November	1st to 10th	Day	28% 4 8% 23% 24% 12%	1924
	11th to 20th	Day	30% 5% 10% 24% 23% 9%	1800
	21st to end	Day	32% 3 8% 18% 30% 9%	1665
December	1st to 10th	Day	27% 3 7% 24% 33% 7%	2620
	11th to 20th	Day	44% 5% 11% 19% 16% 5%	3028
	21st to end	Day	48% 8% 14% 17% 11% 2	4004
January	1st to 10th	Night	34% 4 19% 24% 16% 3	6042
	11th to 20th	Night	39% 4 10% 13% 21% 12%	5880
	21st to end	Night	43% 4 8% 12% 18% 16%	6468
February	1st to 10th	Night	38% 3 7% 20% 20% 12%	2600
	11th to 20th	Night	37% 3 10% 20% 23% 7%	2427
	21st to end	Night	34% 3 10% 30% 18% 5%	1980
March	1st to 10th	Night	45% 4 12% 20% 16% 3	5029
	11th to 20th	Night	34% 5% 11% 26% 17% 7%	5000
	21st to end	Night	42% 7% 13% 24% 12% 2	5402
April	1st to 10th	Night	34% 4 14% 25% 18% 5%	2000
	11th to 20th	Night	30% 3 11% 29% 19% 8%	1979
	21st to end	Night	21% 3 12% 33% 24% 7%	1954
May	1st to 10th	Night	18% 3 10% 29% 27% 14%	3090
	11th to 20th	Night	12% 2 6% 43% 25% 11%	2747
	21st to end	Night	14% 1 10% 38% 32% 5%	2762
June	1st to 10th	Night	18% 3 11% 28% 29% 11%	1520
	11th to 20th	Night	9% 2 7% 21% 37% 25%	1520
	21st to end	Night	12% 1 2 16% 33% 36%	1520
July	1st to 10th	Night	4 6% 18% 71%	2500
	11th to 20th	Night	8% 3 2 9% 21% 57%	2798
	21st to end	Night	8% 3 1 10% 34% 44%	3011
August	1st to 10th	Night	16% 5% 2 11% 34% 32%	2839
	11th to 20th	Night	8% 2 1 8% 43% 38%	3077
	21st to end	Night	9% 4 4 10% 45% 29%	3498
September	1st to 10th	Night	8% 2 2 11% 30% 47%	1900
	11th to 20th	Night	17% 4 3 23% 33% 19%	1900
	21st to end	Night	12% 2 4 21% 34% 27%	2155
October	1st to 10th	Night	12% 1 5% 12% 26% 44%	3800
	11th to 20th	Night	19% 3 6% 25% 29% 18%	3777
	21st to end	Night	19% 2 6% 24% 28% 20%	4121
November	1st to 10th	Night	26% 3 8% 26% 22% 15%	2371
	11th to 20th	Night	29% 4 10% 25% 23% 9%	2600
	21st to end	Night	29% 3 9% 22% 24% 13%	2730
December	1st to 10th	Night	26% 3 7% 25% 28% 11%	4560
	11th to 20th	Night	44% 6% 10% 17% 18% 5%	5176
	21st to end	Night	50% 5% 16% 16% 9% 3	6952

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% 12% 26% 23% 16% 3	315
	11th to 20th	Day	13% 10% 28% 30% 15% 5%	400
	21st to end	Day	13% 11% 27% 27% 19% 5%	440
February	1st to 10th	Day	14% 17% 26% 25% 14% 5%	400
	11th to 20th	Day	14% 18% 30% 25% 11% 2	449
	21st to end	Day	10% 15% 29% 32% 13% 2	414
March	1st to 10th	Day	16% 16% 33% 23% 10% 1	500
	11th to 20th	Day	15% 19% 24% 23% 15% 4	500
	21st to end	Day	12% 19% 33% 24% 10% 3	550
April	1st to 10th	Day	11% 15% 31% 26% 13% 5%	500
	11th to 20th	Day	11% 16% 28% 25% 15% 5%	492
	21st to end	Day	6% 17% 28% 29% 13% 7%	481
May	1st to 10th	Day	7% 12% 34% 29% 14% 4	463
	11th to 20th	Day	9% 12% 28% 30% 15% 6%	488
	21st to end	Day	6% 10% 26% 31% 19% 7%	529
June	1st to 10th	Day	4 10% 28% 29% 22% 7%	490
	11th to 20th	Day	3 8% 28% 35% 20% 7%	490
	21st to end	Day	2 3 21% 36% 28% 10%	490
July	1st to 10th	Day	1 2 16% 38% 32% 10%	486
	11th to 20th	Day	1 13% 41% 31% 13%	484
	21st to end	Day	1 13% 40% 31% 15%	531
August	1st to 10th	Day	2 1 18% 39% 28% 11%	487
	11th to 20th	Day	1 1 19% 39% 26% 15%	477
	21st to end	Day	1 2 14% 42% 29% 13%	539
September	1st to 10th	Day	2 3 13% 38% 33% 11%	490
	11th to 20th	Day	2 5% 23% 38% 22% 11%	490
	21st to end	Day	3 6% 18% 33% 29% 11%	490
October	1st to 10th	Day	3 8% 19% 32% 25% 13%	490
	11th to 20th	Day	5% 9% 18% 29% 28% 11%	490
	21st to end	Day	6% 6% 24% 33% 24% 7%	539
November	1st to 10th	Day	7% 11% 23% 24% 27% 9%	460
	11th to 20th	Day	5% 8% 20% 31% 26% 10%	300
	21st to end	Day	10% 5% 23% 32% 24% 6%	233
December	1st to 10th	Day	7% 11% 21% 28% 23% 11%	200
	11th to 20th	Day	10% 14% 23% 23% 22% 10%	200
	21st to end	Day	12% 18% 23% 29% 15% 4	220
January	1st to 10th	Night	15% 7% 16% 34% 22% 6%	800
	11th to 20th	Night	11% 8% 14% 37% 23% 7%	800
	21st to end	Night	15% 6% 13% 38% 23% 6%	880
February	1st to 10th	Night	13% 7% 17% 36% 22% 5%	800
	11th to 20th	Night	13% 9% 20% 35% 20% 5%	784
	21st to end	Night	11% 8% 20% 39% 19% 3	660
March	1st to 10th	Night	14% 10% 20% 34% 20% 3	800
	11th to 20th	Night	14% 8% 19% 33% 23% 4	800
	21st to end	Night	11% 8% 23% 37% 20% 2	880
April	1st to 10th	Night	10% 6% 22% 36% 23% 3	800
	11th to 20th	Night	10% 10% 20% 35% 20% 4	791
	21st to end	Night	5% 5% 24% 36% 24% 6%	782
May	1st to 10th	Night	7% 2 24% 38% 25% 5%	709
	11th to 20th	Night	6% 3 13% 39% 34% 6%	513
	21st to end	Night	4 4 16% 40% 32% 3	411
June	1st to 10th	Night	4 3 17% 44% 28% 3	380
	11th to 20th	Night	2 2 13% 48% 33% 1	380
	21st to end	Night	1 11% 40% 45% 3	380
July	1st to 10th	Night	1 7% 34% 55% 3	370
	11th to 20th	Night	2 2 30% 54% 12%	544
	21st to end	Night	2 2 29% 58% 10%	624
August	1st to 10th	Night	4 1 4 32% 51% 8%	567
	11th to 20th	Night	2 1 8% 40% 44% 6%	698
	21st to end	Night	3 6% 35% 45% 11%	836
September	1st to 10th	Night	2 1 8% 36% 44% 10%	760
	11th to 20th	Night	5% 1 11% 38% 38% 7%	760
	21st to end	Night	3 2 10% 34% 40% 11%	760
October	1st to 10th	Night	4 2 11% 38% 36% 9%	760
	11th to 20th	Night	6% 3 11% 31% 38% 11%	760
	21st to end	Night	5% 5% 11% 35% 31% 13%	836
November	1st to 10th	Night	6% 3 16% 33% 32% 9%	780
	11th to 20th	Night	9% 3 16% 36% 30% 8%	800
	21st to end	Night	9% 4 16% 40% 25% 6%	800
December	1st to 10th	Night	10% 6% 16% 37% 25% 7%	800
	11th to 20th	Night	16% 5% 18% 36% 21% 5%	800
	21st to end	Night	14% 9% 22% 34% 18% 3	880

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