Greeley Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Greeley Hill Observatory clear sky chart from 2005-11-06 to 2025-03-27.

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	8% 4 1 3 2 2 3 2 4 5% 67%	112876
	Night	10% 4 1 3 2 2 3 2 4 5% 63%	115241
Transparency	Day	21% 4 8% 17% 25% 26%	112627
	Night	24% 4 8% 19% 24% 21%	115080
Seeing	Day	10% 9% 28% 34% 15% 4	15668
	Night	11% 3 15% 39% 27% 6%	25216

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	14% 6% 2 4 3 3 3 3 4 6% 53%	3639
	11th to 20th	Day	13% 4 2 6% 4 4 3 2 3 4 55%	3944
	21st to end	Day	15% 6% 2 6% 4 3 4 3 5% 6% 45%	4510
February	1st to 10th	Day	20% 7% 2 4 3 2 3 3 4 5% 48%	1827
	11th to 20th	Day	15% 7% 2 4 3 3 3 3 5% 7% 48%	1950
	21st to end	Day	19% 7% 2 4 4 3 4 3 4 6% 45%	1650
March	1st to 10th	Day	16% 12% 3 6% 5% 5% 5% 5% 7% 8% 30%	3238
	11th to 20th	Day	13% 5% 2 4 2 3 3 3 4 6% 56%	3595
	21st to end	Day	17% 8% 2 6% 4 4 5% 3 6% 7% 38%	4110
April	1st to 10th	Day	14% 5% 1 4 3 3 3 3 4 6% 52%	2280
	11th to 20th	Day	12% 5% 2 4 3 3 3 2 4 7% 55%	2249
	21st to end	Day	9% 5% 1 4 3 2 3 3 4 6% 62%	2280
May	1st to 10th	Day	7% 3 1 2 2 2 2 2 4 5% 73%	3862
	11th to 20th	Day	3 1 2 1 2 2 2 3 4 78%	4222
	21st to end	Day	5% 2 1 2 2 2 2 2 3 3 76%	4813
June	1st to 10th	Day	4 3 1 3 2 2 3 2 4 6% 70%	2660
	11th to 20th	Day	1 1 1 2 2 1 2 2 2 4 83%	2660
	21st to end	Day	2 2 1 1 1 2 2 1 3 2 84%	2660
July	1st to 10th	Day	1 1 1 1 1 2 2 93%	4420
	11th to 20th	Day	1 1 1 1 1 1 2 2 3 3 83%	4420
	21st to end	Day	1 1 1 1 1 1 2 2 88%	4515
August	1st to 10th	Day	2 2 1 1 1 1 1 1 2 3 85%	3999
	11th to 20th	Day	1 1 1 1 1 1 2 3 90%	3724
	21st to end	Day	1 1 1 1 1 1 1 1 2 89%	4180
September	1st to 10th	Day	2 1 1 1 1 1 1 1 2 88%	2280
	11th to 20th	Day	3 1 1 2 1 2 2 2 3 4 79%	2258
	21st to end	Day	2 2 1 2 1 1 2 2 2 4 82%	2090
October	1st to 10th	Day	3 2 1 2 2 2 2 2 4 4 77%	3215
	11th to 20th	Day	4 2 1 2 2 1 2 2 3 4 78%	3161
	21st to end	Day	5% 3 1 3 3 3 3 2 4 6% 66%	3448
November	1st to 10th	Day	9% 5% 2 3 4 3 4 3 4 6% 57%	1770
	11th to 20th	Day	10% 3 1 3 4 2 3 3 5% 5% 60%	1800
	21st to end	Day	12% 5% 2 3 3 4 3 3 5% 7% 54%	1795
December	1st to 10th	Day	9% 3 1 4 3 3 4 3 4 6% 60%	2620
	11th to 20th	Day	24% 7% 2 6% 3 3 3 2 4 7% 38%	3028
	21st to end	Day	19% 7% 3 6% 4 5% 4 4 5% 8% 35%	4004
January	1st to 10th	Night	12% 6% 2 5% 3 2 3 3 6% 8% 51%	6306
	11th to 20th	Night	15% 6% 2 4 3 3 3 2 4 5% 54%	6012
	21st to end	Night	16% 6% 2 5% 3 3 3 3 4 5% 49%	6468
February	1st to 10th	Night	18% 6% 2 5% 3 3 3 3 3 4 49%	2570
	11th to 20th	Night	16% 6% 2 4 3 3 2 3 5% 6% 51%	2338
	21st to end	Night	17% 8% 2 4 2 3 3 3 3 5% 50%	1980
March	1st to 10th	Night	20% 10% 1 5% 4 4 4 3 6% 5% 38%	3856
	11th to 20th	Night	15% 5% 1 3 2 3 3 2 3 5% 58%	3600
	21st to end	Night	22% 7% 2 4 3 3 4 3 5% 6% 41%	3680
April	1st to 10th	Night	13% 5% 2 4 3 3 3 3 5% 6% 54%	1900
	11th to 20th	Night	10% 4 1 4 3 2 4 3 4 5% 61%	1879
	21st to end	Night	8% 4 1 3 2 2 3 3 4 6% 64%	1900
May	1st to 10th	Night	4 3 1 2 1 1 2 2 3 4 76%	3101
	11th to 20th	Night	3 1 1 1 1 2 2 3 6% 78%	2658
	21st to end	Night	3 2 1 2 1 2 2 2 3 4 80%	2762
June	1st to 10th	Night	5% 3 1 2 2 3 2 2 4 6% 72%	1520
	11th to 20th	Night	1 1 1 2 2 3 4 86%	1520
	21st to end	Night	1 2 2 1 2 2 1 2 3 85%	1520
July	1st to 10th	Night	1 1 1 1 1 94%	2540
	11th to 20th	Night	3 2 1 2 2 2 3 1 2 3 79%	2540
	21st to end	Night	1 1 1 2 1 2 2 2 2 3 83%	3041
August	1st to 10th	Night	2 1 1 2 1 2 2 2 3 3 80%	2940
	11th to 20th	Night	1 1 1 1 1 1 2 2 89%	3126
	21st to end	Night	3 1 1 1 1 1 1 1 2 88%	3498
September	1st to 10th	Night	1 1 1 1 1 1 1 1 90%	1900
	11th to 20th	Night	3 1 2 1 2 1 2 3 3 82%	1920
	21st to end	Night	2 2 2 1 2 2 1 2 4 82%	2090
October	1st to 10th	Night	3 1 1 2 2 2 2 1 3 4 80%	3761
	11th to 20th	Night	3 1 2 1 1 2 1 3 4 80%	3777
	21st to end	Night	6% 3 1 3 2 2 2 2 3 5% 71%	4121
November	1st to 10th	Night	11% 4 2 3 2 2 2 2 4 6% 62%	2526
	11th to 20th	Night	9% 4 1 3 2 2 4 3 5% 7% 59%	2600
	21st to end	Night	12% 4 1 3 3 3 4 3 4 6% 57%	2603
December	1st to 10th	Night	9% 3 1 3 2 3 4 3 4 5% 62%	4560
	11th to 20th	Night	21% 7% 2 5% 3 4 4 4 6% 9% 36%	5176
	21st to end	Night	17% 8% 3 8% 5% 6% 6% 4 5% 6% 32%	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	32% 5% 19% 22% 17% 5%	3639
	11th to 20th	Day	34% 5% 8% 13% 24% 17%	3944
	21st to end	Day	38% 6% 10% 14% 24% 8%	4510
February	1st to 10th	Day	39% 4 8% 25% 15% 9%	1827
	11th to 20th	Day	35% 4 12% 22% 23% 5%	1960
	21st to end	Day	39% 6% 9% 22% 20% 4	1650
March	1st to 10th	Day	44% 11% 14% 15% 14% 2	3238
	11th to 20th	Day	27% 6% 12% 32% 13% 9%	3595
	21st to end	Day	37% 11% 17% 21% 12% 2	4110
April	1st to 10th	Day	32% 5% 12% 25% 18% 7%	2280
	11th to 20th	Day	28% 6% 12% 29% 19% 6%	2252
	21st to end	Day	23% 5% 11% 30% 27% 4	2280
May	1st to 10th	Day	15% 3 8% 28% 31% 15%	3862
	11th to 20th	Day	11% 3 5% 24% 43% 14%	4225
	21st to end	Day	12% 4 7% 27% 44% 7%	4814
June	1st to 10th	Day	15% 5% 9% 21% 33% 17%	2660
	11th to 20th	Day	8% 2 6% 18% 37% 30%	2660
	21st to end	Day	8% 1 3 9% 30% 50%	2660
July	1st to 10th	Day	3 1 4 13% 79%	4351
	11th to 20th	Day	8% 1 2 5% 14% 69%	4345
	21st to end	Day	5% 1 2 5% 23% 63%	4373
August	1st to 10th	Day	8% 1 2 6% 22% 61%	3984
	11th to 20th	Day	3 1 1 5% 24% 65%	3760
	21st to end	Day	6% 1 3 5% 31% 55%	4179
September	1st to 10th	Day	6% 1 2 6% 27% 57%	2280
	11th to 20th	Day	10% 3 4 19% 38% 26%	2258
	21st to end	Day	10% 1 3 17% 37% 32%	2090
October	1st to 10th	Day	12% 2 4 10% 21% 51%	3215
	11th to 20th	Day	12% 2 4 19% 42% 21%	3161
	21st to end	Day	19% 3 5% 15% 41% 17%	3448
November	1st to 10th	Day	27% 5% 9% 23% 23% 13%	1770
	11th to 20th	Day	26% 4 11% 23% 26% 10%	1800
	21st to end	Day	29% 3 11% 19% 25% 13%	1795
December	1st to 10th	Day	26% 4 8% 23% 27% 12%	2620
	11th to 20th	Day	47% 4 9% 15% 19% 7%	3028
	21st to end	Day	44% 8% 16% 21% 8% 4	4004
January	1st to 10th	Night	28% 7% 18% 26% 16% 5%	6306
	11th to 20th	Night	32% 5% 9% 15% 28% 11%	6012
	21st to end	Night	36% 6% 8% 18% 22% 10%	6468
February	1st to 10th	Night	37% 4 9% 20% 19% 10%	2570
	11th to 20th	Night	33% 4 10% 25% 21% 7%	2339
	21st to end	Night	37% 4 12% 24% 18% 6%	1980
March	1st to 10th	Night	43% 9% 12% 19% 15% 2	3856
	11th to 20th	Night	29% 4 11% 32% 16% 9%	3600
	21st to end	Night	39% 8% 16% 23% 12% 3	3680
April	1st to 10th	Night	30% 5% 15% 23% 19% 7%	1900
	11th to 20th	Night	25% 6% 13% 28% 21% 7%	1879
	21st to end	Night	20% 6% 13% 31% 26% 4	1900
May	1st to 10th	Night	11% 5% 9% 33% 31% 11%	3101
	11th to 20th	Night	9% 3 7% 26% 42% 13%	2658
	21st to end	Night	10% 3 7% 33% 43% 5%	2762
June	1st to 10th	Night	14% 4 13% 20% 32% 17%	1520
	11th to 20th	Night	4 2 7% 21% 38% 28%	1520
	21st to end	Night	7% 2 3 8% 30% 50%	1520
July	1st to 10th	Night	2 1 1 3 15% 78%	2500
	11th to 20th	Night	13% 1 2 5% 13% 66%	2492
	21st to end	Night	7% 2 2 5% 22% 63%	2946
August	1st to 10th	Night	10% 1 2 7% 19% 61%	2931
	11th to 20th	Night	5% 1 2 6% 24% 63%	3156
	21st to end	Night	7% 1 1 7% 29% 55%	3498
September	1st to 10th	Night	5% 1 2 6% 27% 59%	1900
	11th to 20th	Night	9% 2 4 17% 40% 27%	1920
	21st to end	Night	9% 1 3 16% 39% 31%	2090
October	1st to 10th	Night	10% 2 4 10% 20% 54%	3761
	11th to 20th	Night	10% 3 4 24% 37% 23%	3777
	21st to end	Night	18% 2 3 19% 36% 22%	4121
November	1st to 10th	Night	23% 4 9% 24% 22% 18%	2526
	11th to 20th	Night	24% 4 11% 25% 25% 12%	2600
	21st to end	Night	28% 4 8% 21% 29% 10%	2603
December	1st to 10th	Night	24% 4 7% 26% 27% 12%	4560
	11th to 20th	Night	44% 4 11% 16% 20% 5%	5176
	21st to end	Night	49% 8% 14% 19% 7% 4	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	29% 8% 25% 26% 12% 2	200
	11th to 20th	Day	18% 8% 24% 33% 13% 5%	335
	21st to end	Day	19% 7% 23% 30% 18% 4	440
February	1st to 10th	Day	26% 11% 22% 23% 14% 3	415
	11th to 20th	Day	21% 11% 29% 28% 11% 1	495
	21st to end	Day	18% 13% 31% 27% 10%	414
March	1st to 10th	Day	23% 14% 30% 25% 6% 1	500
	11th to 20th	Day	23% 14% 28% 21% 12% 2	500
	21st to end	Day	20% 13% 34% 23% 9% 1	531
April	1st to 10th	Day	18% 18% 29% 22% 11% 1	460
	11th to 20th	Day	18% 17% 29% 27% 8% 2	452
	21st to end	Day	9% 16% 33% 27% 12% 3	460
May	1st to 10th	Day	10% 19% 34% 25% 10% 2	463
	11th to 20th	Day	9% 14% 31% 30% 12% 3	488
	21st to end	Day	10% 12% 25% 34% 14% 5%	529
June	1st to 10th	Day	4 12% 30% 36% 12% 6%	490
	11th to 20th	Day	3 12% 32% 36% 10% 6%	490
	21st to end	Day	3 7% 32% 35% 17% 6%	490
July	1st to 10th	Day	2 27% 48% 16% 7%	486
	11th to 20th	Day	1 1 20% 52% 17% 10%	484
	21st to end	Day	1 1 23% 48% 18% 9%	531
August	1st to 10th	Day	3 5% 25% 44% 15% 7%	487
	11th to 20th	Day	1 4 30% 42% 17% 7%	477
	21st to end	Day	1 3 24% 46% 18% 7%	539
September	1st to 10th	Day	2 4 24% 47% 18% 5%	490
	11th to 20th	Day	4 10% 31% 34% 20% 1	490
	21st to end	Day	3 7% 27% 43% 18% 2	490
October	1st to 10th	Day	4 10% 28% 33% 21% 4	490
	11th to 20th	Day	6% 7% 26% 34% 22% 4	490
	21st to end	Day	7% 7% 27% 35% 21% 3	539
November	1st to 10th	Day	9% 8% 26% 27% 26% 5%	306
	11th to 20th	Day	11% 7% 25% 30% 22% 5%	300
	21st to end	Day	14% 5% 26% 29% 23% 4	297
December	1st to 10th	Day	16% 6% 24% 31% 17% 7%	200
	11th to 20th	Day	22% 7% 20% 30% 20% 4	200
	21st to end	Day	22% 8% 24% 27% 15% 4	220
January	1st to 10th	Night	21% 2 19% 31% 19% 8%	800
	11th to 20th	Night	15% 4 18% 38% 20% 5%	800
	21st to end	Night	20% 2 16% 37% 20% 6%	880
February	1st to 10th	Night	21% 2 19% 35% 19% 4	800
	11th to 20th	Night	18% 5% 18% 33% 21% 7%	784
	21st to end	Night	19% 5% 20% 40% 14% 3	660
March	1st to 10th	Night	22% 5% 21% 34% 17% 3	800
	11th to 20th	Night	18% 6% 19% 30% 21% 6%	800
	21st to end	Night	17% 4 19% 39% 20% 1	862
April	1st to 10th	Night	15% 4 19% 39% 21% 2	760
	11th to 20th	Night	10% 6% 23% 36% 20% 5%	751
	21st to end	Night	7% 4 19% 41% 24% 4	760
May	1st to 10th	Night	8% 4 22% 40% 22% 4	720
	11th to 20th	Night	6% 3 18% 43% 28% 2	464
	21st to end	Night	5% 2 15% 43% 32% 2	411
June	1st to 10th	Night	6% 2 12% 44% 33% 3	380
	11th to 20th	Night	1 1 16% 42% 38% 2	380
	21st to end	Night	1 11% 44% 41% 3	380
July	1st to 10th	Night	1 5% 47% 44% 3	370
	11th to 20th	Night	2 4 46% 43% 5%	368
	21st to end	Night	2 5% 34% 50% 8%	582
August	1st to 10th	Night	2 1 7% 43% 39% 9%	620
	11th to 20th	Night	8% 46% 39% 6%	752
	21st to end	Night	2 5% 42% 45% 6%	836
September	1st to 10th	Night	1 1 8% 42% 41% 8%	760
	11th to 20th	Night	2 1 10% 43% 36% 8%	760
	21st to end	Night	2 1 11% 40% 38% 8%	760
October	1st to 10th	Night	4 1 11% 46% 31% 6%	760
	11th to 20th	Night	6% 2 13% 41% 31% 8%	760
	21st to end	Night	6% 1 16% 35% 28% 14%	836
November	1st to 10th	Night	12% 3 12% 36% 27% 10%	780
	11th to 20th	Night	11% 3 18% 37% 24% 6%	800
	21st to end	Night	14% 2 17% 39% 23% 7%	800
December	1st to 10th	Night	17% 3 14% 42% 18% 6%	800
	11th to 20th	Night	23% 2 15% 37% 20% 4	800
	21st to end	Night	21% 5% 20% 34% 17% 3	880

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