Golden Mean Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Golden Mean Observatory clear sky chart from 2005-11-06 to 2025-05-11.

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	13% 6% 2 6% 4 4 4 4 6% 8% 44%	113780
	Night	14% 7% 2 6% 4 4 4 3 5% 7% 44%	115167
Transparency	Day	38% 3 12% 25% 18% 4	117413
	Night	40% 3 11% 25% 18% 4	119673
Seeing	Day	18% 10% 28% 24% 14% 6%	15707
	Night	18% 9% 27% 31% 14% 2	25276

By Month

Percentage Cloud Cover SE (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% 4 2 5% 2 3 4 3 5% 6% 51%	3597
	11th to 20th	Day	14% 8% 3 7% 4 3 3 3 4 4 47%	4034
	21st to end	Day	14% 7% 3 6% 4 4 4 3 5% 6% 44%	4466
February	1st to 10th	Day	14% 8% 2 6% 5% 4 5% 4 6% 7% 39%	1800
	11th to 20th	Day	18% 7% 2 6% 3 3 3 3 5% 6% 43%	1884
	21st to end	Day	17% 8% 2 6% 5% 4 4 3 5% 6% 41%	1650
March	1st to 10th	Day	18% 6% 1 5% 4 3 3 4 6% 8% 43%	4681
	11th to 20th	Day	10% 6% 2 5% 3 3 3 3 5% 6% 53%	4980
	21st to end	Day	17% 6% 2 6% 4 4 4 3 5% 6% 43%	5684
April	1st to 10th	Day	13% 7% 2 5% 4 3 4 4 6% 8% 45%	2400
	11th to 20th	Day	11% 5% 2 4 4 4 4 3 5% 7% 50%	2374
	21st to end	Day	14% 6% 2 5% 4 3 4 3 5% 8% 46%	2400
May	1st to 10th	Day	20% 9% 2 6% 3 4 4 4 6% 9% 32%	3968
	11th to 20th	Day	18% 8% 3 7% 4 5% 4 4 6% 8% 34%	4393
	21st to end	Day	9% 5% 2 5% 4 4 4 4 6% 10% 48%	4823
June	1st to 10th	Day	9% 5% 2 5% 4 4 6% 5% 6% 10% 45%	2660
	11th to 20th	Day	9% 4 1 5% 3 5% 5% 4 6% 10% 47%	2660
	21st to end	Day	7% 4 1 4 4 4 4 4 7% 11% 48%	2660
July	1st to 10th	Day	6% 4 2 6% 4 4 5% 5% 9% 13% 42%	4440
	11th to 20th	Day	10% 4 2 6% 4 5% 5% 5% 8% 11% 40%	4422
	21st to end	Day	9% 5% 3 7% 6% 5% 6% 6% 8% 10% 36%	4466
August	1st to 10th	Day	15% 6% 2 5% 4 4 5% 4 8% 11% 34%	4104
	11th to 20th	Day	9% 5% 2 7% 5% 5% 5% 4 6% 9% 43%	3743
	21st to end	Day	8% 5% 2 5% 4 3 4 4 6% 9% 50%	4180
September	1st to 10th	Day	12% 5% 2 6% 4 4 5% 4 6% 7% 46%	2280
	11th to 20th	Day	11% 6% 2 6% 3 3 4 3 4 8% 49%	2280
	21st to end	Day	20% 8% 3 6% 3 3 4 3 5% 8% 37%	2038
October	1st to 10th	Day	16% 5% 2 6% 3 4 4 3 5% 6% 47%	1900
	11th to 20th	Day	17% 5% 2 3 3 3 2 3 3 6% 54%	1900
	21st to end	Day	14% 4 2 5% 4 3 3 3 4 6% 51%	2090
November	1st to 10th	Day	12% 5% 2 4 4 3 4 3 5% 7% 52%	1794
	11th to 20th	Day	13% 5% 2 4 3 4 4 4 5% 7% 49%	1800
	21st to end	Day	15% 5% 2 5% 4 3 3 3 5% 6% 49%	1679
December	1st to 10th	Day	11% 5% 2 6% 4 3 4 3 5% 8% 49%	2600
	11th to 20th	Day	19% 8% 2 6% 3 3 3 2 4 5% 46%	2990
	21st to end	Day	13% 6% 2 6% 4 4 5% 3 6% 7% 44%	3960
January	1st to 10th	Night	13% 8% 3 6% 3 3 4 3 4 6% 49%	6349
	11th to 20th	Night	13% 6% 2 6% 4 3 3 2 3 5% 52%	5923
	21st to end	Night	15% 8% 2 6% 4 3 4 3 4 5% 46%	6490
February	1st to 10th	Night	16% 7% 3 6% 4 3 4 3 4 7% 42%	2600
	11th to 20th	Night	20% 6% 2 5% 4 4 4 2 5% 6% 42%	2402
	21st to end	Night	18% 8% 2 5% 3 3 4 2 5% 6% 43%	1980
March	1st to 10th	Night	16% 7% 2 5% 4 3 3 3 4 5% 47%	5156
	11th to 20th	Night	11% 6% 2 5% 3 3 3 3 5% 8% 50%	4980
	21st to end	Night	14% 7% 3 6% 4 4 4 3 4 6% 45%	5248
April	1st to 10th	Night	16% 7% 3 6% 4 4 4 3 5% 6% 41%	2000
	11th to 20th	Night	12% 6% 2 7% 3 4 4 3 4 7% 47%	1975
	21st to end	Night	15% 6% 2 6% 4 4 4 3 6% 8% 43%	2000
May	1st to 10th	Night	18% 9% 3 7% 5% 5% 5% 3 5% 7% 32%	3176
	11th to 20th	Night	19% 9% 3 7% 5% 4 4 4 6% 6% 33%	2506
	21st to end	Night	10% 5% 2 6% 4 5% 4 4 7% 10% 43%	2732
June	1st to 10th	Night	11% 5% 2 6% 6% 5% 5% 4 7% 9% 41%	1520
	11th to 20th	Night	8% 6% 2 7% 5% 4 5% 4 5% 10% 42%	1520
	21st to end	Night	8% 5% 2 6% 5% 5% 5% 5% 7% 10% 41%	1520
July	1st to 10th	Night	8% 4 2 7% 6% 5% 6% 5% 7% 9% 41%	2520
	11th to 20th	Night	10% 5% 2 6% 5% 4 5% 5% 8% 10% 40%	2520
	21st to end	Night	11% 7% 3 9% 5% 5% 5% 5% 7% 8% 35%	3087
August	1st to 10th	Night	15% 7% 3 7% 4 5% 4 4 6% 8% 35%	2825
	11th to 20th	Night	11% 6% 3 7% 5% 5% 5% 4 7% 8% 38%	3052
	21st to end	Night	7% 6% 2 6% 4 4 4 3 5% 8% 51%	3476
September	1st to 10th	Night	13% 5% 2 6% 4 4 4 4 5% 7% 46%	1900
	11th to 20th	Night	10% 5% 1 5% 3 3 3 4 7% 7% 51%	1900
	21st to end	Night	18% 9% 2 7% 3 4 4 3 5% 6% 37%	2138
October	1st to 10th	Night	16% 6% 3 6% 5% 3 4 3 4 5% 45%	2280
	11th to 20th	Night	16% 5% 2 5% 3 3 3 3 4 6% 50%	2280
	21st to end	Night	16% 5% 2 4 3 3 3 3 4 7% 48%	2508
November	1st to 10th	Night	11% 6% 2 6% 4 3 3 3 4 6% 52%	2504
	11th to 20th	Night	16% 8% 3 5% 3 3 3 3 4 6% 47%	2600
	21st to end	Night	12% 5% 2 4 4 3 4 3 6% 7% 49%	2719
December	1st to 10th	Night	11% 6% 2 5% 3 3 3 3 5% 7% 51%	4580
	11th to 20th	Night	20% 7% 2 6% 4 3 4 3 5% 6% 39%	5205
	21st to end	Night	13% 8% 3 6% 4 4 4 4 5% 7% 41%	6996

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	35% 2 5% 28% 26% 5%	3598
	11th to 20th	Day	41% 2 7% 22% 22% 6%	4034
	21st to end	Day	43% 1 10% 29% 15% 2	4466
February	1st to 10th	Day	44% 4 11% 27% 13% 2	1800
	11th to 20th	Day	43% 3 8% 25% 18% 3	1893
	21st to end	Day	45% 2 12% 20% 18% 4	1650
March	1st to 10th	Day	42% 2 7% 22% 20% 8%	4681
	11th to 20th	Day	34% 1 9% 28% 22% 6%	4980
	21st to end	Day	43% 2 9% 19% 20% 7%	5684
April	1st to 10th	Day	38% 3 12% 22% 19% 6%	2400
	11th to 20th	Day	36% 2 11% 24% 21% 6%	2374
	21st to end	Day	38% 2 11% 25% 19% 5%	2400
May	1st to 10th	Day	50% 4 12% 22% 10% 2	4003
	11th to 20th	Day	51% 3 13% 23% 9% 1	4397
	21st to end	Day	33% 4 13% 29% 18% 2	4837
June	1st to 10th	Day	34% 5% 16% 28% 15% 2	2660
	11th to 20th	Day	32% 5% 16% 26% 18% 2	2660
	21st to end	Day	30% 6% 16% 28% 19% 2	2660
July	1st to 10th	Day	34% 5% 27% 17% 13% 4	4370
	11th to 20th	Day	38% 4 20% 28% 9% 2	4347
	21st to end	Day	42% 5% 15% 21% 15% 1	4324
August	1st to 10th	Day	43% 3 17% 28% 7% 1	4091
	11th to 20th	Day	39% 3 13% 30% 14% 1	3779
	21st to end	Day	32% 3 13% 30% 19% 3	4180
September	1st to 10th	Day	37% 5% 14% 21% 19% 4	2280
	11th to 20th	Day	35% 3 9% 27% 23% 3	2280
	21st to end	Day	47% 4 8% 21% 17% 4	2038
October	1st to 10th	Day	37% 2 7% 21% 27% 6%	3160
	11th to 20th	Day	27% 1 5% 28% 28% 11%	3140
	21st to end	Day	23% 2 7% 31% 27% 9%	3428
November	1st to 10th	Day	33% 2 9% 29% 22% 4	1794
	11th to 20th	Day	36% 3 11% 24% 22% 4	1800
	21st to end	Day	37% 2 9% 29% 19% 3	1679
December	1st to 10th	Day	36% 3 13% 30% 14% 4	2600
	11th to 20th	Day	43% 2 12% 28% 12% 3	2986
	21st to end	Day	40% 2 14% 26% 17% 1	3960
January	1st to 10th	Night	39% 1 5% 24% 25% 5%	6349
	11th to 20th	Night	38% 2 7% 26% 22% 6%	5923
	21st to end	Night	43% 1 7% 28% 19% 2	6490
February	1st to 10th	Night	43% 2 12% 25% 16% 2	2600
	11th to 20th	Night	45% 2 8% 21% 19% 5%	2404
	21st to end	Night	43% 2 9% 23% 18% 5%	1980
March	1st to 10th	Night	42% 2 6% 24% 17% 9%	5156
	11th to 20th	Night	35% 2 10% 25% 21% 6%	4980
	21st to end	Night	43% 2 8% 21% 20% 6%	5248
April	1st to 10th	Night	44% 2 10% 19% 20% 5%	2000
	11th to 20th	Night	39% 2 11% 22% 20% 6%	1975
	21st to end	Night	40% 3 10% 24% 18% 4	2000
May	1st to 10th	Night	52% 3 11% 22% 10% 2	3206
	11th to 20th	Night	52% 4 12% 21% 10% 1	2507
	21st to end	Night	37% 4 16% 25% 16% 3	2739
June	1st to 10th	Night	39% 5% 11% 27% 17% 1	1520
	11th to 20th	Night	37% 5% 14% 27% 14% 2	1520
	21st to end	Night	37% 5% 15% 23% 17% 3	1520
July	1st to 10th	Night	40% 7% 21% 19% 12% 2	2480
	11th to 20th	Night	38% 4 24% 26% 7% 1	2472
	21st to end	Night	46% 5% 14% 22% 13%	2988
August	1st to 10th	Night	47% 3 17% 23% 8%	2814
	11th to 20th	Night	43% 3 14% 28% 11% 1	3082
	21st to end	Night	34% 3 11% 29% 22% 2	3476
September	1st to 10th	Night	37% 5% 14% 21% 18% 5%	1900
	11th to 20th	Night	31% 5% 10% 28% 23% 3	1900
	21st to end	Night	48% 4 8% 20% 18% 3	2138
October	1st to 10th	Night	41% 2 9% 23% 21% 5%	3800
	11th to 20th	Night	27% 2 10% 22% 30% 10%	3779
	21st to end	Night	24% 1 9% 33% 26% 8%	4120
November	1st to 10th	Night	35% 2 9% 28% 21% 5%	2504
	11th to 20th	Night	40% 2 9% 22% 22% 6%	2600
	21st to end	Night	35% 2 11% 27% 21% 3	2719
December	1st to 10th	Night	33% 2 12% 29% 21% 3	4580
	11th to 20th	Night	47% 2 12% 21% 14% 4	5208
	21st to end	Night	43% 2 13% 24% 17% 1	6996

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	28% 20% 31% 17% 4 2	200
	11th to 20th	Day	27% 24% 34% 11% 4	295
	21st to end	Day	20% 20% 42% 13% 4	330
February	1st to 10th	Day	24% 20% 36% 14% 6%	300
	11th to 20th	Day	31% 20% 34% 13% 2	432
	21st to end	Day	25% 18% 36% 18% 3	414
March	1st to 10th	Day	23% 19% 32% 22% 3 1	500
	11th to 20th	Day	23% 16% 35% 19% 5% 2	500
	21st to end	Day	23% 16% 36% 19% 6% 1	550
April	1st to 10th	Day	23% 15% 37% 17% 6% 2	500
	11th to 20th	Day	17% 15% 33% 20% 12% 2	492
	21st to end	Day	22% 12% 32% 22% 11% 1	500
May	1st to 10th	Day	24% 14% 27% 21% 10% 4	503
	11th to 20th	Day	19% 7% 31% 29% 10% 3	487
	21st to end	Day	19% 4 23% 28% 18% 8%	529
June	1st to 10th	Day	13% 5% 28% 31% 17% 7%	490
	11th to 20th	Day	11% 6% 24% 30% 21% 9%	490
	21st to end	Day	11% 3 26% 35% 20% 6%	490
July	1st to 10th	Day	11% 2 19% 33% 24% 12%	485
	11th to 20th	Day	5% 16% 34% 32% 12%	484
	21st to end	Day	11% 1 20% 33% 25% 10%	532
August	1st to 10th	Day	12% 2 20% 31% 25% 10%	486
	11th to 20th	Day	11% 2 17% 30% 27% 12%	479
	21st to end	Day	9% 3 17% 32% 24% 15%	539
September	1st to 10th	Day	15% 4 19% 25% 20% 16%	490
	11th to 20th	Day	14% 2 22% 29% 20% 13%	490
	21st to end	Day	29% 6% 22% 23% 14% 6%	490
October	1st to 10th	Day	18% 6% 24% 24% 18% 9%	490
	11th to 20th	Day	19% 10% 25% 25% 17% 4	490
	21st to end	Day	17% 9% 33% 25% 13% 3	539
November	1st to 10th	Day	17% 10% 28% 25% 15% 4	412
	11th to 20th	Day	20% 13% 34% 21% 11% 2	400
	21st to end	Day	22% 16% 31% 19% 10% 3	279
December	1st to 10th	Day	18% 20% 39% 17% 6% 2	200
	11th to 20th	Day	26% 17% 35% 16% 8%	200
	21st to end	Day	23% 15% 27% 25% 9% 1	220
January	1st to 10th	Night	20% 19% 34% 22% 5%	800
	11th to 20th	Night	22% 13% 33% 26% 6%	800
	21st to end	Night	20% 16% 36% 25% 2	880
February	1st to 10th	Night	20% 12% 37% 27% 3	800
	11th to 20th	Night	25% 20% 33% 21% 1	783
	21st to end	Night	22% 17% 36% 23% 3	660
March	1st to 10th	Night	20% 16% 35% 24% 5%	800
	11th to 20th	Night	21% 14% 35% 22% 6% 1	800
	21st to end	Night	21% 12% 34% 27% 5%	880
April	1st to 10th	Night	20% 15% 40% 21% 5%	800
	11th to 20th	Night	15% 12% 34% 31% 8%	791
	21st to end	Night	19% 9% 32% 32% 9%	800
May	1st to 10th	Night	18% 7% 31% 33% 10% 1	772
	11th to 20th	Night	17% 6% 27% 40% 11%	381
	21st to end	Night	13% 2 19% 41% 21% 2	411
June	1st to 10th	Night	13% 3 21% 42% 20% 1	380
	11th to 20th	Night	9% 2 19% 45% 22% 2	380
	21st to end	Night	11% 1 18% 47% 20% 2	380
July	1st to 10th	Night	12% 1 12% 43% 30% 2	370
	11th to 20th	Night	6% 1 8% 44% 37% 3	368
	21st to end	Night	8% 2 16% 40% 31% 4	624
August	1st to 10th	Night	11% 1 11% 40% 33% 4	567
	11th to 20th	Night	13% 2 12% 39% 30% 4	717
	21st to end	Night	11% 2 10% 39% 32% 6%	836
September	1st to 10th	Night	16% 2 14% 35% 28% 5%	760
	11th to 20th	Night	13% 2 14% 36% 30% 6%	760
	21st to end	Night	23% 4 19% 31% 21% 3	760
October	1st to 10th	Night	19% 5% 17% 31% 22% 5%	760
	11th to 20th	Night	18% 9% 24% 30% 18% 1	760
	21st to end	Night	19% 9% 27% 29% 14% 2	836
November	1st to 10th	Night	14% 7% 31% 31% 14% 2	780
	11th to 20th	Night	21% 14% 27% 28% 10% 1	800
	21st to end	Night	15% 15% 29% 31% 9% 1	800
December	1st to 10th	Night	22% 14% 32% 23% 9% 1	800
	11th to 20th	Night	21% 12% 35% 26% 7%	800
	21st to end	Night	20% 11% 31% 34% 5%	880

Page generated at 2025-05-08 09:33:59. Version ac07

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