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 2026-07-31.

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	7% 3 1 3 2 2 2 2 3 5% 68%	146621
	Night	9% 4 1 3 2 2 3 2 4 5% 64%	143847
Transparency	Day	20% 3 7% 17% 27% 26%	141615
	Night	24% 4 8% 19% 25% 21%	139836
Seeing	Day	10% 9% 28% 34% 15% 4	17488
	Night	11% 3 16% 39% 26% 5%	26868

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	18% 6% 2 4 2 2 3 3 4 6% 49%	4722
	11th to 20th	Day	10% 3 1 5% 3 4 3 1 3 5% 61%	5142
	21st to end	Day	12% 6% 2 6% 4 3 4 4 6% 7% 45%	5843
February	1st to 10th	Day	19% 7% 2 4 3 2 3 3 3 5% 48%	1919
	11th to 20th	Day	15% 8% 2 4 3 3 4 3 5% 7% 47%	2050
	21st to end	Day	18% 7% 2 4 4 3 4 3 4 6% 45%	1730
March	1st to 10th	Day	15% 11% 3 6% 4 4 5% 4 6% 7% 34%	4618
	11th to 20th	Day	16% 6% 2 4 3 3 3 3 4 6% 50%	5105
	21st to end	Day	17% 7% 2 5% 3 4 4 3 6% 8% 42%	5920
April	1st to 10th	Day	13% 6% 1 4 3 3 4 3 4 6% 52%	2520
	11th to 20th	Day	12% 5% 2 4 3 3 4 3 4 7% 54%	2489
	21st to end	Day	9% 5% 1 4 3 2 3 3 4 6% 60%	2520
May	1st to 10th	Day	6% 4 1 3 2 2 2 2 3 5% 71%	7170
	11th to 20th	Day	2 1 1 2 2 2 2 2 3 4 81%	7877
	21st to end	Day	3 2 1 2 2 2 2 2 3 3 78%	8992
June	1st to 10th	Day	3 2 1 3 2 2 3 2 4 6% 71%	2940
	11th to 20th	Day	1 1 1 2 1 1 2 1 2 4 84%	2940
	21st to end	Day	2 2 1 1 1 1 2 1 2 2 85%	2940
July	1st to 10th	Day	1 1 1 1 1 1 1 93%	6459
	11th to 20th	Day	1 1 1 1 1 1 1 2 2 88%	6460
	21st to end	Day	1 1 1 1 1 1 2 3 87%	6621
August	1st to 10th	Day	1 1 1 1 1 1 1 2 2 89%	5649
	11th to 20th	Day	1 1 1 2 2 93%	5364
	21st to end	Day	1 1 1 2 2 1 2 2 2 3 83%	5984
September	1st to 10th	Day	2 1 1 1 1 1 1 2 3 88%	2400
	11th to 20th	Day	3 2 1 2 1 2 2 2 3 4 79%	2377
	21st to end	Day	2 2 1 2 1 2 2 2 2 4 81%	2200
October	1st to 10th	Day	3 2 1 2 2 2 2 2 4 4 76%	3316
	11th to 20th	Day	4 2 1 2 2 2 2 2 3 4 78%	3261
	21st to end	Day	5% 3 1 3 3 3 3 2 4 6% 66%	3558
November	1st to 10th	Day	9% 5% 2 3 3 3 4 3 4 7% 58%	1860
	11th to 20th	Day	12% 3 1 4 4 2 3 3 5% 5% 57%	1890
	21st to end	Day	12% 5% 1 4 3 3 3 3 5% 7% 56%	1885
December	1st to 10th	Day	9% 3 1 3 3 3 4 3 4 6% 61%	2700
	11th to 20th	Day	23% 7% 2 6% 3 3 3 2 4 7% 39%	3108
	21st to end	Day	20% 7% 3 6% 4 5% 4 4 5% 8% 35%	4092
January	1st to 10th	Night	18% 7% 2 5% 3 2 3 3 5% 7% 46%	8172
	11th to 20th	Night	11% 5% 1 3 2 3 3 3 4 6% 60%	7821
	21st to end	Night	13% 6% 2 6% 4 3 3 3 4 6% 48%	8371
February	1st to 10th	Night	18% 6% 2 4 3 3 3 3 3 5% 50%	2698
	11th to 20th	Night	17% 6% 2 4 3 3 2 3 5% 6% 49%	2458
	21st to end	Night	17% 8% 2 5% 2 3 3 3 3 5% 50%	2076
March	1st to 10th	Night	18% 10% 2 5% 3 3 4 3 5% 5% 41%	5496
	11th to 20th	Night	17% 8% 2 5% 3 3 3 2 3 4 50%	5110
	21st to end	Night	21% 6% 2 4 2 3 3 3 5% 7% 44%	5296
April	1st to 10th	Night	13% 5% 1 4 3 3 3 3 5% 6% 54%	2100
	11th to 20th	Night	10% 4 1 4 3 2 4 3 4 5% 60%	2079
	21st to end	Night	8% 4 1 3 2 2 3 3 4 6% 63%	2100
May	1st to 10th	Night	4 4 1 3 2 2 2 2 3 5% 73%	5786
	11th to 20th	Night	2 1 1 2 1 2 2 2 3 4 80%	4980
	21st to end	Night	2 1 1 2 1 2 2 2 2 3 82%	5181
June	1st to 10th	Night	4 3 2 2 3 2 2 4 6% 73%	1680
	11th to 20th	Night	1 1 1 2 3 4 87%	1680
	21st to end	Night	1 2 2 1 1 2 1 2 2 86%	1680
July	1st to 10th	Night	1 1 1 1 95%	3720
	11th to 20th	Night	3 2 1 2 1 1 2 1 1 3 85%	3720
	21st to end	Night	1 1 1 1 2 2 2 2 3 84%	4464
August	1st to 10th	Night	1 1 1 1 1 1 1 2 2 2 86%	4166
	11th to 20th	Night	1 1 1 1 1 2 2 92%	4506
	21st to end	Night	3 1 1 2 1 1 2 2 2 3 83%	5015
September	1st to 10th	Night	1 1 1 1 1 1 1 2 90%	2000
	11th to 20th	Night	3 1 2 1 2 1 1 3 3 81%	2021
	21st to end	Night	2 2 2 1 2 2 1 3 4 81%	2200
October	1st to 10th	Night	3 2 1 2 2 2 2 1 3 4 80%	3880
	11th to 20th	Night	3 1 2 1 1 2 1 3 4 80%	3897
	21st to end	Night	6% 3 1 3 2 2 2 2 3 5% 71%	4253
November	1st to 10th	Night	11% 4 1 3 2 2 2 2 4 6% 63%	2656
	11th to 20th	Night	11% 4 2 3 3 3 4 3 5% 7% 57%	2730
	21st to end	Night	12% 4 1 3 3 3 3 2 4 7% 58%	2733
December	1st to 10th	Night	9% 3 1 3 2 3 4 3 4 6% 63%	4700
	11th to 20th	Night	21% 7% 2 5% 3 4 4 4 6% 9% 37%	5316
	21st to end	Night	17% 8% 3 8% 5% 6% 5% 4 5% 6% 32%	7106

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% 15% 4	4723
	11th to 20th	Day	27% 4 6% 11% 26% 25%	5142
	21st to end	Day	36% 6% 8% 17% 26% 8%	5843
February	1st to 10th	Day	39% 3 8% 24% 15% 10%	1919
	11th to 20th	Day	36% 4 11% 22% 22% 5%	2060
	21st to end	Day	40% 6% 10% 23% 19% 4	1730
March	1st to 10th	Day	42% 9% 11% 17% 18% 3	4618
	11th to 20th	Day	33% 5% 15% 29% 11% 7%	5105
	21st to end	Day	35% 9% 16% 24% 14% 2	5920
April	1st to 10th	Day	32% 5% 12% 25% 20% 7%	2520
	11th to 20th	Day	28% 6% 12% 29% 19% 6%	2492
	21st to end	Day	24% 4 11% 30% 27% 4	2520
May	1st to 10th	Day	18% 2 7% 27% 36% 11%	7170
	11th to 20th	Day	9% 2 4 24% 45% 16%	7880
	21st to end	Day	11% 3 4 20% 53% 8%	8994
June	1st to 10th	Day	14% 5% 8% 22% 34% 17%	2940
	11th to 20th	Day	7% 2 6% 17% 37% 32%	2940
	21st to end	Day	8% 1 3 8% 31% 50%	2940
July	1st to 10th	Day	3 1 1 3 16% 77%	6391
	11th to 20th	Day	6% 1 1 4 11% 77%	6385
	21st to end	Day	5% 2 2 7% 35% 50%	6434
August	1st to 10th	Day	8% 1 2 6% 22% 61%	4111
	11th to 20th	Day	3 1 1 5% 25% 64%	3880
	21st to end	Day	6% 1 3 5% 31% 54%	4311
September	1st to 10th	Day	6% 1 3 8% 28% 54%	2400
	11th to 20th	Day	10% 3 4 20% 38% 25%	2377
	21st to end	Day	11% 1 4 17% 36% 31%	2200
October	1st to 10th	Day	13% 2 4 10% 21% 50%	3316
	11th to 20th	Day	12% 2 4 19% 42% 21%	3261
	21st to end	Day	20% 3 5% 15% 41% 17%	3558
November	1st to 10th	Day	26% 5% 9% 23% 23% 13%	1860
	11th to 20th	Day	28% 5% 11% 22% 25% 9%	1890
	21st to end	Day	28% 3 10% 20% 26% 13%	1885
December	1st to 10th	Day	25% 4 8% 24% 27% 12%	2700
	11th to 20th	Day	47% 4 9% 15% 19% 8%	3108
	21st to end	Day	44% 8% 16% 21% 8% 4	4092
January	1st to 10th	Night	36% 6% 17% 23% 15% 4	8172
	11th to 20th	Night	26% 4 7% 14% 31% 18%	7821
	21st to end	Night	36% 5% 6% 21% 22% 9%	8371
February	1st to 10th	Night	37% 4 9% 20% 20% 10%	2698
	11th to 20th	Night	35% 4 9% 25% 21% 6%	2459
	21st to end	Night	37% 4 12% 24% 18% 5%	2076
March	1st to 10th	Night	41% 7% 9% 18% 20% 4	5496
	11th to 20th	Night	37% 4 11% 27% 14% 7%	5110
	21st to end	Night	37% 6% 17% 22% 15% 3	5296
April	1st to 10th	Night	30% 5% 14% 24% 20% 6%	2100
	11th to 20th	Night	25% 6% 13% 29% 20% 7%	2079
	21st to end	Night	22% 5% 13% 32% 25% 4	2100
May	1st to 10th	Night	15% 4 9% 30% 31% 11%	5784
	11th to 20th	Night	9% 2 5% 24% 45% 14%	4980
	21st to end	Night	9% 2 5% 25% 54% 5%	5182
June	1st to 10th	Night	14% 4 12% 21% 33% 18%	1680
	11th to 20th	Night	4 2 6% 19% 37% 32%	1680
	21st to end	Night	6% 2 3 9% 30% 50%	1680
July	1st to 10th	Night	1 1 1 2 16% 79%	3680
	11th to 20th	Night	9% 1 2 4 12% 72%	3672
	21st to end	Night	6% 2 2 7% 33% 50%	4340
August	1st to 10th	Night	10% 1 2 6% 20% 61%	3024
	11th to 20th	Night	5% 1 2 6% 24% 63%	3256
	21st to end	Night	7% 1 1 7% 30% 54%	3608
September	1st to 10th	Night	5% 1 2 8% 28% 56%	2000
	11th to 20th	Night	9% 2 4 18% 40% 27%	2021
	21st to end	Night	10% 2 3 17% 38% 31%	2200
October	1st to 10th	Night	10% 2 4 11% 21% 53%	3880
	11th to 20th	Night	10% 3 4 24% 37% 23%	3897
	21st to end	Night	17% 2 4 19% 36% 22%	4253
November	1st to 10th	Night	23% 4 9% 25% 22% 18%	2656
	11th to 20th	Night	26% 4 10% 24% 24% 12%	2730
	21st to end	Night	28% 4 7% 22% 29% 10%	2733
December	1st to 10th	Night	24% 4 7% 27% 27% 12%	4700
	11th to 20th	Night	43% 4 11% 16% 20% 6%	5316
	21st to end	Night	49% 8% 14% 18% 7% 4	7106

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% 8% 24% 24% 12% 1	220
	11th to 20th	Day	16% 7% 22% 34% 16% 5%	362
	21st to end	Day	18% 7% 23% 30% 18% 4	473
February	1st to 10th	Day	25% 10% 23% 24% 14% 4	447
	11th to 20th	Day	22% 12% 28% 27% 10% 1	535
	21st to end	Day	18% 13% 33% 27% 9%	446
March	1st to 10th	Day	22% 15% 31% 25% 6% 1	540
	11th to 20th	Day	21% 14% 28% 23% 12% 2	540
	21st to end	Day	20% 12% 34% 24% 9% 1	594
April	1st to 10th	Day	18% 16% 29% 24% 11% 1	540
	11th to 20th	Day	17% 17% 29% 28% 8% 2	532
	21st to end	Day	10% 16% 34% 26% 12% 3	540
May	1st to 10th	Day	10% 18% 32% 27% 10% 3	543
	11th to 20th	Day	10% 14% 31% 30% 11% 4	568
	21st to end	Day	9% 12% 26% 34% 14% 5%	617
June	1st to 10th	Day	3 11% 30% 36% 12% 7%	570
	11th to 20th	Day	3 11% 33% 36% 10% 6%	570
	21st to end	Day	3 8% 32% 34% 16% 7%	570
July	1st to 10th	Day	1 2 30% 46% 15% 7%	566
	11th to 20th	Day	2 1 20% 51% 16% 10%	564
	21st to end	Day	1 1 23% 48% 17% 9%	616
August	1st to 10th	Day	3 5% 26% 44% 16% 6%	527
	11th to 20th	Day	1 4 28% 42% 17% 8%	517
	21st to end	Day	1 3 24% 46% 19% 7%	583
September	1st to 10th	Day	2 4 26% 45% 18% 5%	530
	11th to 20th	Day	4 9% 31% 34% 20% 1	530
	21st to end	Day	3 7% 28% 43% 17% 2	530
October	1st to 10th	Day	4 11% 28% 33% 20% 5%	530
	11th to 20th	Day	7% 7% 26% 34% 22% 4	530
	21st to end	Day	8% 7% 27% 35% 21% 3	583
November	1st to 10th	Day	9% 7% 26% 28% 25% 6%	336
	11th to 20th	Day	15% 7% 25% 28% 20% 5%	330
	21st to end	Day	13% 5% 26% 31% 21% 5%	327
December	1st to 10th	Day	14% 5% 24% 32% 17% 7%	220
	11th to 20th	Day	20% 6% 20% 28% 22% 4	220
	21st to end	Day	25% 9% 23% 26% 14% 3	242
January	1st to 10th	Night	22% 3 20% 30% 18% 8%	840
	11th to 20th	Night	14% 4 18% 40% 20% 5%	840
	21st to end	Night	19% 2 18% 36% 19% 6%	924
February	1st to 10th	Night	20% 2 20% 35% 19% 4	840
	11th to 20th	Night	19% 5% 19% 32% 20% 6%	824
	21st to end	Night	18% 5% 22% 39% 13% 3	692
March	1st to 10th	Night	20% 5% 23% 34% 16% 3	840
	11th to 20th	Night	18% 6% 19% 31% 20% 6%	840
	21st to end	Night	17% 4 20% 39% 19% 1	924
April	1st to 10th	Night	14% 5% 20% 40% 20% 2	840
	11th to 20th	Night	10% 6% 24% 37% 19% 4	831
	21st to end	Night	8% 4 19% 42% 22% 4	840
May	1st to 10th	Night	7% 4 23% 40% 21% 4	796
	11th to 20th	Night	5% 4 21% 42% 26% 2	514
	21st to end	Night	5% 2 18% 44% 30% 2	455
June	1st to 10th	Night	5% 2 12% 45% 32% 3	420
	11th to 20th	Night	1 16% 44% 36% 2	420
	21st to end	Night	1 13% 44% 39% 2	420
July	1st to 10th	Night	1 9% 47% 40% 2	410
	11th to 20th	Night	2 4 47% 41% 5%	408
	21st to end	Night	2 6% 36% 49% 7%	641
August	1st to 10th	Night	2 1 7% 43% 38% 8%	653
	11th to 20th	Night	8% 45% 40% 6%	792
	21st to end	Night	2 5% 43% 44% 6%	880
September	1st to 10th	Night	1 1 8% 43% 40% 7%	800
	11th to 20th	Night	3 1 10% 43% 35% 7%	800
	21st to end	Night	3 1 12% 41% 37% 8%	800
October	1st to 10th	Night	5% 1 12% 45% 31% 6%	800
	11th to 20th	Night	6% 2 14% 40% 30% 8%	800
	21st to end	Night	6% 1 18% 35% 27% 13%	880
November	1st to 10th	Night	12% 3 12% 37% 28% 9%	820
	11th to 20th	Night	12% 3 19% 37% 23% 6%	840
	21st to end	Night	13% 1 18% 40% 22% 6%	840
December	1st to 10th	Night	16% 3 15% 41% 19% 6%	840
	11th to 20th	Night	22% 2 16% 37% 20% 4	840
	21st to end	Night	23% 5% 20% 33% 16% 3	924

Page generated at 2026-07-28 00:25:22. Version ac07

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