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 2024-10-20.

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%	100953
	Night	10% 4 1 3 2 2 3 2 4 5% 63%	97905
Transparency	Day	21% 4 8% 17% 24% 27%	100704
	Night	24% 4 9% 19% 22% 22%	97744
Seeing	Day	10% 9% 27% 34% 15% 4	15181
	Night	11% 2 14% 39% 28% 6%	24584

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% 8% 2 5% 3 3 3 3 3 5% 45%	2539
	11th to 20th	Day	19% 6% 3 8% 5% 6% 4 2 5% 6% 35%	2746
	21st to end	Day	20% 8% 2 6% 4 4 4 4 6% 7% 34%	3146
February	1st to 10th	Day	19% 7% 2 4 3 2 3 4 4 5% 49%	1735
	11th to 20th	Day	15% 7% 2 4 3 3 3 3 5% 7% 49%	1850
	21st to end	Day	19% 7% 2 4 4 3 4 3 3 6% 45%	1570
March	1st to 10th	Day	16% 12% 3 6% 5% 5% 4 5% 7% 8% 29%	3138
	11th to 20th	Day	12% 5% 2 4 2 3 3 3 4 6% 56%	3485
	21st to end	Day	17% 8% 2 6% 4 4 5% 3 6% 7% 38%	4040
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	5% 3 1 2 3 2 2 2 3 4 73%	1923
	11th to 20th	Day	5% 2 1 2 2 2 3 2 4 5% 73%	1891
	21st to end	Day	7% 2 1 3 2 2 3 2 4 5% 69%	1980
November	1st to 10th	Day	9% 5% 2 4 4 3 4 3 4 6% 56%	1680
	11th to 20th	Day	10% 3 1 3 4 2 3 3 5% 5% 61%	1710
	21st to end	Day	11% 5% 2 3 3 4 3 3 5% 7% 55%	1706
December	1st to 10th	Day	16% 5% 1 4 3 3 3 3 3 6% 53%	1520
	11th to 20th	Day	25% 9% 2 7% 4 3 3 2 4 5% 35%	1928
	21st to end	Day	14% 7% 3 7% 4 5% 5% 4 5% 7% 38%	2794
January	1st to 10th	Night	16% 8% 2 6% 3 2 3 3 5% 6% 46%	4406
	11th to 20th	Night	21% 8% 2 6% 3 4 4 3 5% 7% 36%	4203
	21st to end	Night	21% 7% 2 6% 4 4 4 4 5% 6% 36%	4521
February	1st to 10th	Night	17% 6% 2 4 3 3 3 3 4 5% 50%	2442
	11th to 20th	Night	16% 6% 2 4 3 3 2 3 5% 7% 51%	2218
	21st to end	Night	18% 9% 2 4 2 3 2 2 3 5% 50%	1884
March	1st to 10th	Night	20% 10% 1 5% 4 4 4 3 6% 5% 38%	3736
	11th to 20th	Night	15% 4 1 3 2 3 2 2 3 5% 59%	3490
	21st to end	Night	22% 7% 2 4 3 3 4 3 5% 6% 41%	3612
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	4 2 1 2 2 2 3 2 3 4 73%	2253
	11th to 20th	Night	5% 2 2 2 2 2 2 4 5% 75%	2273
	21st to end	Night	6% 2 1 2 1 1 2 2 3 5% 75%	2376
November	1st to 10th	Night	11% 4 2 3 2 2 2 2 4 6% 62%	2396
	11th to 20th	Night	9% 3 1 3 2 2 4 3 5% 6% 60%	2470
	21st to end	Night	12% 4 1 3 3 3 4 2 4 6% 58%	2472
December	1st to 10th	Night	16% 5% 2 4 3 3 4 3 4 5% 53%	2660
	11th to 20th	Night	24% 8% 2 5% 3 3 3 3 5% 6% 37%	3276
	21st to end	Night	16% 7% 3 7% 5% 5% 5% 4 5% 6% 37%	4862

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	39% 6% 17% 22% 12% 2	2539
	11th to 20th	Day	48% 8% 11% 13% 12% 7%	2746
	21st to end	Day	46% 8% 14% 18% 11% 3	3146
February	1st to 10th	Day	38% 4 9% 26% 16% 8%	1735
	11th to 20th	Day	34% 4 12% 21% 23% 6%	1860
	21st to end	Day	40% 6% 9% 22% 20% 2	1570
March	1st to 10th	Day	44% 11% 15% 15% 14% 2	3138
	11th to 20th	Day	26% 6% 12% 33% 13% 9%	3485
	21st to end	Day	37% 11% 17% 20% 12% 2	4040
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	15% 3 6% 16% 27% 33%	1923
	11th to 20th	Day	14% 3 6% 20% 27% 29%	1891
	21st to end	Day	17% 3 7% 20% 31% 23%	1980
November	1st to 10th	Day	27% 6% 9% 22% 22% 14%	1680
	11th to 20th	Day	25% 4 11% 22% 27% 10%	1710
	21st to end	Day	29% 4 11% 19% 25% 13%	1706
December	1st to 10th	Day	34% 5% 12% 21% 18% 11%	1520
	11th to 20th	Day	53% 4 8% 15% 14% 7%	1928
	21st to end	Day	40% 9% 15% 23% 9% 4	2794
January	1st to 10th	Night	36% 8% 16% 25% 12% 2	4406
	11th to 20th	Night	46% 7% 13% 16% 11% 7%	4203
	21st to end	Night	45% 8% 11% 20% 13% 4	4521
February	1st to 10th	Night	36% 4 9% 21% 19% 9%	2442
	11th to 20th	Night	32% 5% 10% 25% 22% 7%	2219
	21st to end	Night	37% 4 12% 24% 19% 4	1884
March	1st to 10th	Night	43% 9% 12% 19% 15% 2	3736
	11th to 20th	Night	28% 4 11% 32% 16% 9%	3490
	21st to end	Night	40% 8% 15% 23% 11% 3	3612
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	15% 3 6% 16% 27% 32%	2253
	11th to 20th	Night	13% 4 6% 18% 30% 30%	2273
	21st to end	Night	13% 2 5% 23% 32% 25%	2376
November	1st to 10th	Night	24% 4 9% 23% 22% 19%	2396
	11th to 20th	Night	23% 4 10% 25% 25% 13%	2470
	21st to end	Night	28% 4 8% 21% 29% 10%	2472
December	1st to 10th	Night	33% 5% 9% 22% 18% 12%	2660
	11th to 20th	Night	47% 5% 11% 15% 17% 6%	3276
	21st to end	Night	42% 10% 15% 19% 9% 5%	4862

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	32% 8% 23% 25% 12% 1	180
	11th to 20th	Day	19% 7% 23% 32% 13% 6%	308
	21st to end	Day	20% 7% 20% 31% 18% 3	407
February	1st to 10th	Day	25% 10% 21% 25% 15% 3	383
	11th to 20th	Day	20% 11% 28% 28% 11% 1	455
	21st to end	Day	19% 14% 30% 27% 10% 1	382
March	1st to 10th	Day	24% 14% 30% 25% 6% 1	460
	11th to 20th	Day	22% 15% 27% 22% 13% 2	460
	21st to end	Day	21% 14% 34% 23% 8%	506
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% 35% 22% 4	487
	21st to end	Day	7% 7% 25% 35% 22% 4	495
November	1st to 10th	Day	9% 8% 24% 27% 28% 5%	276
	11th to 20th	Day	10% 5% 24% 32% 24% 6%	270
	21st to end	Day	11% 4 25% 31% 24% 4	268
December	1st to 10th	Day	17% 6% 25% 29% 18% 6%	180
	11th to 20th	Day	21% 7% 21% 28% 19% 4	180
	21st to end	Day	22% 8% 24% 25% 17% 4	198
January	1st to 10th	Night	22% 2 17% 31% 20% 8%	760
	11th to 20th	Night	15% 4 16% 39% 21% 5%	760
	21st to end	Night	20% 2 15% 36% 21% 6%	836
February	1st to 10th	Night	20% 2 17% 37% 20% 4	760
	11th to 20th	Night	18% 4 16% 33% 22% 7%	744
	21st to end	Night	20% 5% 19% 39% 15% 3	628
March	1st to 10th	Night	22% 4 19% 34% 17% 3	760
	11th to 20th	Night	17% 6% 19% 30% 22% 7%	760
	21st to end	Night	18% 4 19% 38% 20% 1	836
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 12% 41% 31% 8%	758
	21st to end	Night	6% 1 14% 35% 29% 15%	792
November	1st to 10th	Night	13% 2 10% 36% 29% 10%	740
	11th to 20th	Night	11% 2 16% 39% 26% 7%	760
	21st to end	Night	13% 1 16% 39% 24% 7%	760
December	1st to 10th	Night	18% 3 13% 41% 18% 6%	760
	11th to 20th	Night	23% 2 14% 37% 20% 4	760
	21st to end	Night	21% 4 19% 35% 17% 3	836

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