OAGH
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the OAGH clear sky chart from 2005-11-06 to 2026-01-02.

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	3 3 1 4 3 4 4 4 5% 6% 62%	136043
	Night	2 3 1 4 3 3 4 3 5% 7% 64%	132811
Transparency	Day	20% 5% 7% 11% 24% 33%	130959
	Night	19% 4 7% 12% 26% 32%	128911
Seeing	Day	8% 12% 24% 29% 20% 7%	16380
	Night	5% 4 15% 39% 32% 6%	26528

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	2 3 2 4 3 3 4 4 5% 6% 64%	4107
	11th to 20th	Day	1 2 1 3 2 2 3 3 4 5% 74%	4100
	21st to end	Day	3 4 2 5% 3 3 4 4 6% 8% 58%	4510
February	1st to 10th	Day	4 4 2 5% 2 3 3 3 5% 5% 64%	1835
	11th to 20th	Day	4 5% 2 5% 3 3 3 3 4 5% 62%	1950
	21st to end	Day	4 4 1 5% 3 2 3 3 4 4 66%	1814
March	1st to 10th	Day	2 4 1 4 3 3 4 4 5% 5% 65%	4954
	11th to 20th	Day	2 3 1 4 3 3 4 3 4 6% 68%	5000
	21st to end	Day	1 3 1 4 3 5% 4 4 7% 8% 60%	5486
April	1st to 10th	Day	2 3 1 3 2 2 3 2 4 7% 71%	2200
	11th to 20th	Day	2 1 1 3 2 2 2 2 2 4 79%	2346
	21st to end	Day	1 2 1 2 2 2 2 2 3 4 80%	2400
May	1st to 10th	Day	1 2 1 1 2 2 2 4 85%	5660
	11th to 20th	Day	1 1 1 2 2 2 3 2 4 4 78%	5841
	21st to end	Day	1 1 1 2 1 1 2 2 2 3 84%	6423
June	1st to 10th	Day	3 2 1 3 2 2 3 2 5% 6% 71%	2600
	11th to 20th	Day	2 2 1 2 3 3 3 2 4 4 77%	2600
	21st to end	Day	3 3 2 5% 5% 4 6% 4 6% 7% 55%	2600
July	1st to 10th	Day	3 2 1 6% 6% 6% 9% 6% 8% 8% 44%	5879
	11th to 20th	Day	4 4 2 7% 7% 7% 8% 7% 9% 10% 35%	5880
	21st to end	Day	6% 4 2 8% 7% 7% 8% 6% 9% 9% 33%	6401
August	1st to 10th	Day	4 3 2 6% 6% 7% 7% 7% 10% 11% 37%	5779
	11th to 20th	Day	4 4 2 5% 6% 7% 9% 8% 11% 11% 34%	5632
	21st to end	Day	3 4 2 6% 6% 7% 7% 5% 7% 8% 45%	5951
September	1st to 10th	Day	9% 6% 2 6% 6% 5% 6% 4 6% 7% 42%	2200
	11th to 20th	Day	4 3 2 6% 4 4 4 5% 6% 6% 57%	2200
	21st to end	Day	4 3 1 3 2 4 4 3 5% 6% 66%	2200
October	1st to 10th	Day	1 1 1 2 2 2 3 2 3 5% 77%	3411
	11th to 20th	Day	1 2 1 2 1 2 2 2 3 3 80%	3250
	21st to end	Day	1 1 1 1 2 2 2 3 4 83%	3548
November	1st to 10th	Day	1 1 1 3 2 2 2 3 5% 7% 74%	2059
	11th to 20th	Day	2 3 1 4 2 2 3 3 4 5% 70%	2100
	21st to end	Day	5% 3 2 5% 3 3 3 4 6% 6% 61%	1979
December	1st to 10th	Day	2 3 1 2 2 3 2 2 4 5% 74%	3040
	11th to 20th	Day	2 4 2 6% 2 3 2 2 5% 6% 66%	3499
	21st to end	Day	2 3 1 3 2 3 3 3 5% 6% 69%	4609
January	1st to 10th	Night	2 3 1 4 3 4 4 4 5% 6% 64%	5880
	11th to 20th	Night	1 1 1 2 2 2 2 2 4 5% 79%	5880
	21st to end	Night	6% 6% 2 5% 3 4 4 3 5% 7% 57%	6468
February	1st to 10th	Night	3 4 1 4 4 4 3 2 4 6% 66%	2561
	11th to 20th	Night	3 4 2 5% 3 4 4 4 5% 6% 61%	2338
	21st to end	Night	2 4 2 5% 3 4 4 2 4 5% 65%	1815
March	1st to 10th	Night	1 3 1 4 3 3 4 3 4 5% 68%	4940
	11th to 20th	Night	1 3 2 3 2 2 2 3 5% 6% 71%	5000
	21st to end	Night	2 6% 2 5% 3 3 3 2 4 6% 64%	5489
April	1st to 10th	Night	1 3 1 3 2 2 3 3 5% 6% 72%	2200
	11th to 20th	Night	1 2 1 3 2 2 2 3 4 4 75%	2001
	21st to end	Night	1 2 1 1 2 1 3 1 3 5% 80%	2000
May	1st to 10th	Night	1 1 1 1 1 1 2 3 88%	4290
	11th to 20th	Night	1 1 2 2 3 5% 5% 79%	4074
	21st to end	Night	1 1 1 2 1 1 1 1 2 4 85%	4474
June	1st to 10th	Night	3 2 1 2 2 2 3 3 4 5% 74%	1800
	11th to 20th	Night	1 2 2 2 1 3 2 3 4 80%	1800
	21st to end	Night	3 4 2 4 4 3 5% 4 7% 8% 58%	1800
July	1st to 10th	Night	3 4 2 7% 5% 7% 7% 6% 8% 11% 41%	4100
	11th to 20th	Night	4 5% 2 8% 7% 7% 8% 6% 9% 10% 35%	4100
	21st to end	Night	4 5% 3 8% 5% 6% 6% 6% 11% 15% 31%	4472
August	1st to 10th	Night	3 3 2 6% 5% 6% 8% 7% 12% 14% 34%	4033
	11th to 20th	Night	4 4 2 8% 5% 8% 8% 7% 11% 15% 29%	4217
	21st to end	Night	4 3 2 6% 5% 5% 6% 5% 8% 11% 45%	5009
September	1st to 10th	Night	8% 6% 2 6% 5% 4 5% 4 7% 8% 45%	2200
	11th to 20th	Night	4 3 2 4 2 3 3 3 6% 7% 63%	2200
	21st to end	Night	3 3 1 3 2 3 3 2 4 5% 71%	2200
October	1st to 10th	Night	1 1 1 2 2 2 2 2 3 5% 79%	3782
	11th to 20th	Night	2 3 1 3 2 2 2 2 2 4 78%	3898
	21st to end	Night	1 1 1 1 1 1 3 4 85%	4252
November	1st to 10th	Night	1 2 1 2 1 2 2 2 4 5% 77%	2460
	11th to 20th	Night	3 2 1 3 2 3 4 3 4 6% 70%	2520
	21st to end	Night	4 4 2 5% 4 3 3 3 6% 5% 61%	2636
December	1st to 10th	Night	3 2 1 3 2 3 2 3 3 4 74%	4370
	11th to 20th	Night	2 3 2 5% 4 4 4 3 4 5% 65%	4941
	21st to end	Night	3 3 1 4 3 3 3 3 4 5% 67%	6611

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	19% 2 5% 7% 31% 36%	4108
	11th to 20th	Day	15% 2 4 7% 31% 42%	4100
	21st to end	Day	23% 3 4 10% 29% 32%	4510
February	1st to 10th	Day	21% 2 4 10% 25% 38%	1835
	11th to 20th	Day	24% 2 5% 10% 29% 29%	1960
	21st to end	Day	21% 3 4 7% 27% 39%	1814
March	1st to 10th	Day	18% 2 5% 5% 25% 45%	4954
	11th to 20th	Day	17% 2 5% 11% 32% 33%	5000
	21st to end	Day	20% 3 6% 7% 35% 29%	5486
April	1st to 10th	Day	14% 2 4 8% 33% 39%	2200
	11th to 20th	Day	12% 1 3 5% 27% 52%	2349
	21st to end	Day	10% 1 3 5% 31% 50%	2400
May	1st to 10th	Day	6% 1 2 5% 23% 62%	5660
	11th to 20th	Day	9% 2 4 8% 25% 53%	5844
	21st to end	Day	8% 1 2 2 15% 71%	6424
June	1st to 10th	Day	15% 2 4 6% 29% 44%	2600
	11th to 20th	Day	12% 2 5% 4 20% 56%	2600
	21st to end	Day	25% 7% 8% 10% 23% 28%	2600
July	1st to 10th	Day	28% 12% 13% 21% 14% 12%	5816
	11th to 20th	Day	34% 14% 16% 23% 11% 1	5811
	21st to end	Day	38% 13% 18% 15% 11% 5%	6217
August	1st to 10th	Day	37% 15% 19% 17% 10% 2	4208
	11th to 20th	Day	33% 14% 16% 23% 12% 3	4079
	21st to end	Day	31% 12% 16% 16% 16% 8%	4290
September	1st to 10th	Day	38% 8% 9% 13% 19% 12%	2200
	11th to 20th	Day	25% 6% 8% 13% 29% 20%	2200
	21st to end	Day	19% 4 8% 16% 32% 22%	2200
October	1st to 10th	Day	10% 3 6% 10% 20% 51%	3411
	11th to 20th	Day	10% 2 4 8% 34% 42%	3250
	21st to end	Day	7% 1 3 7% 30% 52%	3548
November	1st to 10th	Day	11% 2 5% 11% 35% 37%	2059
	11th to 20th	Day	17% 2 4 12% 32% 33%	2100
	21st to end	Day	22% 3 4 15% 29% 27%	1979
December	1st to 10th	Day	14% 2 4 11% 33% 38%	3040
	11th to 20th	Day	20% 1 4 8% 21% 44%	3498
	21st to end	Day	15% 2 5% 10% 38% 29%	4609
January	1st to 10th	Night	17% 3 6% 8% 35% 30%	5880
	11th to 20th	Night	10% 2 3 12% 31% 42%	5880
	21st to end	Night	26% 3 5% 10% 29% 27%	6468
February	1st to 10th	Night	21% 3 4 10% 25% 37%	2561
	11th to 20th	Night	23% 3 6% 12% 29% 27%	2339
	21st to end	Night	21% 2 4 8% 24% 40%	1815
March	1st to 10th	Night	18% 2 4 6% 27% 44%	4940
	11th to 20th	Night	13% 3 5% 13% 38% 29%	5000
	21st to end	Night	21% 2 5% 10% 35% 27%	5489
April	1st to 10th	Night	13% 2 3 11% 36% 34%	2200
	11th to 20th	Night	14% 1 4 8% 28% 46%	2001
	21st to end	Night	10% 1 4 8% 31% 47%	2000
May	1st to 10th	Night	5% 1 2 5% 36% 52%	4287
	11th to 20th	Night	6% 2 5% 10% 24% 53%	4075
	21st to end	Night	7% 1 2 3 20% 67%	4474
June	1st to 10th	Night	13% 2 4 6% 32% 42%	1800
	11th to 20th	Night	9% 2 4 4 27% 54%	1800
	21st to end	Night	22% 6% 8% 11% 22% 31%	1800
July	1st to 10th	Night	32% 10% 15% 20% 11% 12%	4055
	11th to 20th	Night	38% 11% 19% 20% 10% 3	4046
	21st to end	Night	35% 12% 23% 14% 9% 6%	4344
August	1st to 10th	Night	33% 14% 20% 20% 9% 4	2928
	11th to 20th	Night	32% 12% 21% 21% 10% 3	3046
	21st to end	Night	30% 10% 19% 20% 14% 8%	3613
September	1st to 10th	Night	34% 9% 14% 15% 18% 11%	2200
	11th to 20th	Night	19% 4 10% 18% 29% 19%	2200
	21st to end	Night	16% 4 7% 20% 34% 20%	2200
October	1st to 10th	Night	9% 3 5% 10% 21% 52%	3782
	11th to 20th	Night	13% 2 3 14% 32% 36%	3898
	21st to end	Night	6% 1 3 9% 30% 52%	4252
November	1st to 10th	Night	10% 2 4 13% 36% 35%	2460
	11th to 20th	Night	16% 3 4 15% 30% 32%	2520
	21st to end	Night	23% 3 5% 14% 28% 26%	2636
December	1st to 10th	Night	14% 3 4 10% 32% 37%	4370
	11th to 20th	Night	21% 2 4 9% 23% 41%	4941
	21st to end	Night	19% 2 5% 10% 33% 30%	6611

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	14% 14% 22% 36% 13% 1	303
	11th to 20th	Day	9% 11% 28% 33% 16% 3	300
	21st to end	Day	12% 14% 25% 34% 13% 2	330
February	1st to 10th	Day	10% 15% 24% 31% 16% 4	335
	11th to 20th	Day	17% 22% 30% 23% 8%	495
	21st to end	Day	15% 29% 26% 22% 7% 1	414
March	1st to 10th	Day	16% 22% 32% 21% 8% 1	500
	11th to 20th	Day	15% 22% 24% 23% 12% 4	500
	21st to end	Day	12% 25% 32% 20% 9% 2	550
April	1st to 10th	Day	7% 22% 31% 25% 12% 4	500
	11th to 20th	Day	7% 25% 36% 19% 12% 1	492
	21st to end	Day	7% 20% 36% 21% 12% 4	500
May	1st to 10th	Day	6% 18% 39% 22% 11% 3	503
	11th to 20th	Day	5% 15% 35% 29% 13% 4	528
	21st to end	Day	3 14% 42% 29% 9% 2	573
June	1st to 10th	Day	6% 8% 35% 34% 14% 4	530
	11th to 20th	Day	2 8% 32% 31% 22% 4	530
	21st to end	Day	4 4 23% 38% 25% 7%	530
July	1st to 10th	Day	6% 3 11% 36% 33% 11%	526
	11th to 20th	Day	8% 2 6% 35% 34% 15%	524
	21st to end	Day	10% 3 7% 28% 35% 17%	575
August	1st to 10th	Day	7% 2 4 29% 34% 23%	527
	11th to 20th	Day	6% 4 9% 30% 36% 16%	517
	21st to end	Day	6% 2 6% 29% 37% 20%	583
September	1st to 10th	Day	12% 2 15% 34% 29% 8%	530
	11th to 20th	Day	6% 4 16% 38% 26% 11%	530
	21st to end	Day	5% 7% 19% 35% 25% 10%	530
October	1st to 10th	Day	3 8% 23% 33% 23% 9%	419
	11th to 20th	Day	8% 8% 21% 28% 25% 10%	330
	21st to end	Day	5% 7% 24% 28% 24% 12%	363
November	1st to 10th	Day	5% 10% 25% 32% 21% 6%	330
	11th to 20th	Day	8% 8% 21% 27% 30% 6%	330
	21st to end	Day	9% 18% 26% 28% 16% 3	330
December	1st to 10th	Day	7% 14% 26% 32% 19% 2	330
	11th to 20th	Day	7% 9% 29% 35% 17% 3	330
	21st to end	Day	12% 17% 26% 25% 15% 3	363
January	1st to 10th	Night	7% 4 18% 43% 25% 4	804
	11th to 20th	Night	5% 5% 20% 43% 25% 2	800
	21st to end	Night	10% 7% 21% 40% 20% 2	880
February	1st to 10th	Night	5% 6% 19% 42% 24% 4	800
	11th to 20th	Night	8% 7% 22% 45% 17% 1	784
	21st to end	Night	5% 7% 27% 44% 13% 2	660
March	1st to 10th	Night	6% 10% 24% 42% 15% 2	800
	11th to 20th	Night	7% 8% 19% 38% 24% 4	800
	21st to end	Night	5% 7% 22% 42% 21% 3	880
April	1st to 10th	Night	3 5% 20% 47% 24% 1	800
	11th to 20th	Night	4 7% 27% 44% 17% 1	617
	21st to end	Night	3 4 26% 47% 19% 1	600
May	1st to 10th	Night	2 6% 22% 46% 22% 2	600
	11th to 20th	Night	1 4 16% 47% 28% 3	591
	21st to end	Night	1 3 20% 47% 28% 1	650
June	1st to 10th	Night	4 2 12% 45% 34% 3	600
	11th to 20th	Night	2 2 14% 45% 35% 2	600
	21st to end	Night	4 3 8% 38% 43% 5%	600
July	1st to 10th	Night	4 3 4 25% 56% 8%	585
	11th to 20th	Night	5% 2 4 22% 52% 14%	582
	21st to end	Night	7% 3 4 20% 52% 14%	657
August	1st to 10th	Night	6% 2 4 18% 51% 20%	597
	11th to 20th	Night	6% 3 3 17% 56% 14%	594
	21st to end	Night	8% 2 2 19% 52% 17%	663
September	1st to 10th	Night	11% 3 4 24% 45% 14%	800
	11th to 20th	Night	6% 1 5% 26% 54% 8%	800
	21st to end	Night	3 2 6% 39% 45% 6%	800
October	1st to 10th	Night	2 2 9% 43% 37% 7%	800
	11th to 20th	Night	4 3 11% 44% 31% 7%	800
	21st to end	Night	1 3 10% 40% 37% 8%	880
November	1st to 10th	Night	2 4 12% 41% 35% 6%	820
	11th to 20th	Night	4 3 13% 41% 34% 4	840
	21st to end	Night	8% 6% 20% 42% 21% 5%	840
December	1st to 10th	Night	7% 5% 13% 47% 24% 5%	840
	11th to 20th	Night	6% 5% 19% 45% 22% 3	840
	21st to end	Night	8% 5% 22% 39% 23% 4	924

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