GuideStar Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the GuideStar Observatory clear sky chart from 2005-11-06 to 2026-01-17.

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 4 2 4 3 4 4 3 5% 7% 62%	136269
	Night	3 4 2 5% 4 4 4 3 5% 6% 59%	132933
Transparency	Day	22% 4 7% 17% 28% 24%	131160
	Night	23% 4 7% 17% 27% 21%	129045
Seeing	Day	6% 8% 22% 28% 24% 12%	16135
	Night	6% 4 21% 41% 25% 3	26262

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 6% 3 5% 3 3 4 3 5% 6% 60%	4188
	11th to 20th	Day	2 3 2 4 2 3 3 3 4 4 70%	4154
	21st to end	Day	3 8% 3 6% 3 3 3 3 4 6% 56%	4510
February	1st to 10th	Day	2 7% 2 6% 4 4 2 3 4 7% 60%	1800
	11th to 20th	Day	3 8% 2 5% 4 3 4 3 5% 7% 56%	1781
	21st to end	Day	4 7% 2 5% 5% 4 3 3 4 6% 57%	1815
March	1st to 10th	Day	2 5% 2 4 3 3 3 3 5% 7% 63%	4954
	11th to 20th	Day	3 6% 2 6% 6% 6% 6% 4 7% 7% 48%	5000
	21st to end	Day	3 4 2 5% 4 3 3 4 5% 6% 60%	5486
April	1st to 10th	Day	3 3 1 4 3 4 4 3 5% 7% 64%	2290
	11th to 20th	Day	2 4 1 5% 3 3 4 3 5% 5% 65%	2371
	21st to end	Day	2 3 2 4 3 3 3 3 4 6% 69%	2400
May	1st to 10th	Day	2 3 1 5% 4 3 3 3 4 6% 65%	5792
	11th to 20th	Day	1 3 1 3 3 3 3 3 5% 6% 68%	5841
	21st to end	Day	2 2 1 3 2 2 3 2 3 5% 76%	6423
June	1st to 10th	Day	2 2 1 2 2 3 3 3 6% 7% 69%	2600
	11th to 20th	Day	2 1 1 2 2 2 2 2 3 4 79%	2600
	21st to end	Day	2 2 1 4 4 4 4 3 5% 6% 63%	2600
July	1st to 10th	Day	3 2 2 4 4 3 3 3 5% 5% 66%	5879
	11th to 20th	Day	4 3 2 6% 5% 5% 6% 5% 8% 10% 47%	5880
	21st to end	Day	3 3 2 6% 5% 6% 6% 5% 8% 9% 49%	6401
August	1st to 10th	Day	4 3 1 6% 4 5% 6% 5% 7% 9% 51%	5779
	11th to 20th	Day	3 2 1 5% 4 5% 6% 5% 7% 9% 52%	5676
	21st to end	Day	4 3 1 6% 6% 5% 6% 6% 7% 9% 47%	5984
September	1st to 10th	Day	4 4 2 5% 5% 4 5% 4 6% 8% 53%	2400
	11th to 20th	Day	3 3 1 4 4 3 3 3 5% 6% 65%	2202
	21st to end	Day	3 3 1 4 3 3 3 3 5% 5% 68%	2200
October	1st to 10th	Day	3 2 1 3 2 2 2 2 3 4 75%	3273
	11th to 20th	Day	5% 3 1 4 2 2 3 3 5% 5% 67%	3250
	21st to end	Day	2 2 1 2 1 2 2 2 4 6% 78%	3548
November	1st to 10th	Day	2 3 1 2 2 2 3 3 4 8% 72%	2059
	11th to 20th	Day	2 2 1 4 3 3 3 2 3 5% 72%	2095
	21st to end	Day	3 5% 2 4 3 3 3 3 4 6% 63%	1890
December	1st to 10th	Day	2 3 1 3 1 2 2 2 3 4 76%	3040
	11th to 20th	Day	3 5% 2 6% 3 3 2 3 5% 7% 60%	3499
	21st to end	Day	3 5% 2 5% 3 3 4 3 5% 7% 61%	4609
January	1st to 10th	Night	2 6% 3 7% 4 4 4 4 5% 6% 54%	5997
	11th to 20th	Night	2 3 1 3 2 2 2 2 4 5% 72%	5958
	21st to end	Night	4 10% 3 7% 4 4 4 3 4 4 53%	6468
February	1st to 10th	Night	2 5% 2 5% 3 3 3 2 4 7% 63%	2600
	11th to 20th	Night	4 7% 3 5% 4 4 4 3 5% 5% 56%	2500
	21st to end	Night	3 6% 2 5% 3 3 3 3 4 7% 61%	1815
March	1st to 10th	Night	2 7% 2 5% 3 3 3 3 4 6% 63%	4940
	11th to 20th	Night	5% 7% 2 6% 4 5% 5% 3 5% 7% 51%	5000
	21st to end	Night	2 4 2 4 3 4 4 4 5% 7% 62%	5489
April	1st to 10th	Night	3 4 2 4 3 3 3 4 5% 6% 63%	2109
	11th to 20th	Night	3 4 2 4 3 3 3 3 4 6% 66%	1977
	21st to end	Night	3 4 1 3 3 2 3 2 4 6% 69%	2000
May	1st to 10th	Night	4 4 2 5% 4 3 4 3 5% 6% 62%	4171
	11th to 20th	Night	3 3 2 4 3 3 3 2 4 5% 68%	4074
	21st to end	Night	1 2 1 3 2 2 3 3 4 5% 76%	4474
June	1st to 10th	Night	4 3 1 4 3 3 3 3 5% 6% 65%	1800
	11th to 20th	Night	3 2 1 2 2 2 2 1 3 5% 78%	1800
	21st to end	Night	2 3 1 5% 4 4 5% 3 6% 7% 59%	1800
July	1st to 10th	Night	3 3 2 6% 4 3 5% 4 6% 5% 59%	4100
	11th to 20th	Night	4 4 3 10% 7% 6% 7% 6% 8% 11% 33%	4100
	21st to end	Night	4 4 2 8% 6% 7% 8% 6% 9% 9% 38%	4472
August	1st to 10th	Night	3 4 2 9% 6% 7% 7% 6% 8% 9% 38%	4033
	11th to 20th	Night	5% 4 2 8% 6% 6% 7% 6% 9% 9% 38%	4173
	21st to end	Night	6% 5% 3 8% 7% 6% 7% 6% 9% 9% 34%	4993
September	1st to 10th	Night	4 5% 2 8% 6% 6% 6% 5% 7% 8% 43%	2000
	11th to 20th	Night	3 3 1 5% 4 4 4 3 5% 5% 63%	2192
	21st to end	Night	3 3 1 5% 3 3 4 3 5% 5% 66%	2200
October	1st to 10th	Night	3 2 1 3 2 2 3 3 4 5% 72%	3915
	11th to 20th	Night	3 2 1 4 3 2 2 2 4 6% 70%	3898
	21st to end	Night	2 2 1 2 2 2 2 2 4 6% 74%	4252
November	1st to 10th	Night	3 2 1 3 2 2 3 2 4 5% 71%	2460
	11th to 20th	Night	2 3 1 3 2 2 2 3 4 5% 74%	2521
	21st to end	Night	4 6% 1 4 2 3 3 3 5% 5% 64%	2730
December	1st to 10th	Night	4 3 1 3 2 2 2 2 4 7% 68%	4370
	11th to 20th	Night	3 4 1 4 4 3 3 3 5% 7% 63%	4941
	21st to end	Night	3 5% 1 4 3 4 4 3 4 5% 63%	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	25% 3 6% 26% 32% 7%	4189
	11th to 20th	Day	18% 3 6% 22% 24% 27%	4154
	21st to end	Day	29% 4 5% 16% 23% 23%	4510
February	1st to 10th	Day	25% 3 8% 19% 30% 16%	1800
	11th to 20th	Day	27% 4 5% 23% 24% 17%	1790
	21st to end	Day	27% 3 5% 17% 27% 21%	1815
March	1st to 10th	Day	20% 3 4 13% 37% 23%	4954
	11th to 20th	Day	30% 5% 6% 19% 25% 14%	5000
	21st to end	Day	23% 3 5% 24% 33% 12%	5486
April	1st to 10th	Day	19% 3 5% 15% 35% 22%	2290
	11th to 20th	Day	21% 2 4 13% 32% 28%	2374
	21st to end	Day	17% 3 5% 14% 37% 24%	2400
May	1st to 10th	Day	19% 3 4 16% 30% 27%	5792
	11th to 20th	Day	15% 3 5% 9% 34% 35%	5844
	21st to end	Day	13% 2 2 5% 26% 52%	6424
June	1st to 10th	Day	13% 3 4 10% 30% 40%	2600
	11th to 20th	Day	11% 2 3 6% 23% 56%	2600
	21st to end	Day	20% 4 7% 11% 21% 38%	2600
July	1st to 10th	Day	20% 3 5% 8% 15% 48%	5816
	11th to 20th	Day	27% 8% 12% 24% 15% 14%	5811
	21st to end	Day	28% 7% 15% 21% 18% 11%	6217
August	1st to 10th	Day	34% 9% 18% 18% 12% 9%	4208
	11th to 20th	Day	27% 7% 11% 22% 19% 14%	4111
	21st to end	Day	30% 6% 14% 18% 22% 9%	4311
September	1st to 10th	Day	28% 5% 9% 16% 23% 20%	2400
	11th to 20th	Day	19% 4 7% 16% 28% 27%	2202
	21st to end	Day	18% 3 6% 12% 29% 32%	2200
October	1st to 10th	Day	15% 2 4 12% 26% 41%	3273
	11th to 20th	Day	19% 2 3 11% 41% 24%	3250
	21st to end	Day	10% 2 3 15% 36% 33%	3548
November	1st to 10th	Day	12% 3 4 22% 39% 20%	2059
	11th to 20th	Day	16% 3 4 19% 38% 20%	2095
	21st to end	Day	23% 3 5% 20% 32% 18%	1890
December	1st to 10th	Day	14% 3 4 24% 39% 17%	3040
	11th to 20th	Day	25% 3 5% 19% 39% 8%	3498
	21st to end	Day	24% 3 7% 26% 32% 8%	4609
January	1st to 10th	Night	30% 4 6% 24% 31% 6%	5997
	11th to 20th	Night	15% 3 6% 23% 30% 23%	5958
	21st to end	Night	33% 4 5% 14% 22% 21%	6468
February	1st to 10th	Night	22% 3 8% 21% 30% 16%	2600
	11th to 20th	Night	28% 3 5% 23% 24% 17%	2501
	21st to end	Night	23% 3 5% 17% 32% 21%	1815
March	1st to 10th	Night	21% 3 4 19% 33% 19%	4940
	11th to 20th	Night	30% 4 6% 20% 27% 13%	5000
	21st to end	Night	19% 4 6% 27% 34% 10%	5489
April	1st to 10th	Night	21% 2 6% 18% 33% 20%	2109
	11th to 20th	Night	19% 3 4 13% 35% 25%	1977
	21st to end	Night	17% 3 6% 14% 38% 23%	2000
May	1st to 10th	Night	22% 3 7% 17% 34% 17%	4168
	11th to 20th	Night	18% 3 6% 10% 35% 29%	4075
	21st to end	Night	12% 3 3 7% 26% 50%	4474
June	1st to 10th	Night	20% 3 4 10% 24% 40%	1800
	11th to 20th	Night	12% 2 3 7% 22% 55%	1800
	21st to end	Night	22% 5% 7% 12% 18% 36%	1800
July	1st to 10th	Night	24% 6% 8% 8% 13% 40%	4055
	11th to 20th	Night	40% 7% 14% 19% 7% 12%	4046
	21st to end	Night	37% 9% 16% 17% 16% 6%	4344
August	1st to 10th	Night	43% 11% 18% 12% 8% 8%	2928
	11th to 20th	Night	37% 9% 17% 14% 10% 12%	3014
	21st to end	Night	35% 8% 15% 18% 16% 7%	3597
September	1st to 10th	Night	33% 7% 12% 15% 17% 16%	2000
	11th to 20th	Night	20% 5% 7% 18% 26% 24%	2192
	21st to end	Night	20% 4 6% 12% 27% 31%	2200
October	1st to 10th	Night	14% 4 5% 11% 22% 43%	3915
	11th to 20th	Night	15% 3 3 16% 35% 28%	3898
	21st to end	Night	13% 3 3 12% 39% 30%	4252
November	1st to 10th	Night	16% 3 3 19% 37% 22%	2460
	11th to 20th	Night	15% 2 6% 19% 35% 23%	2521
	21st to end	Night	23% 2 5% 19% 32% 19%	2730
December	1st to 10th	Night	16% 3 4 25% 31% 20%	4370
	11th to 20th	Night	21% 3 5% 18% 39% 14%	4941
	21st to end	Night	23% 4 6% 26% 30% 12%	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	8% 7% 27% 36% 19% 3	330
	11th to 20th	Day	8% 7% 22% 35% 20% 8%	318
	21st to end	Day	4 10% 22% 33% 20% 11%	330
February	1st to 10th	Day	3 11% 24% 35% 20% 7%	300
	11th to 20th	Day	10% 12% 28% 33% 17% 1	326
	21st to end	Day	12% 17% 33% 24% 12% 2	414
March	1st to 10th	Day	8% 20% 30% 25% 12% 4	500
	11th to 20th	Day	10% 17% 25% 28% 15% 4	500
	21st to end	Day	11% 21% 34% 20% 11% 4	550
April	1st to 10th	Day	8% 14% 28% 26% 17% 7%	500
	11th to 20th	Day	10% 15% 35% 22% 14% 4	492
	21st to end	Day	5% 14% 33% 26% 15% 6%	500
May	1st to 10th	Day	6% 10% 33% 30% 17% 4	503
	11th to 20th	Day	6% 13% 29% 23% 21% 7%	528
	21st to end	Day	6% 12% 27% 27% 21% 6%	573
June	1st to 10th	Day	3 6% 23% 31% 28% 9%	530
	11th to 20th	Day	2 6% 27% 31% 22% 12%	530
	21st to end	Day	4 4 22% 33% 27% 11%	530
July	1st to 10th	Day	5% 2 12% 27% 34% 20%	526
	11th to 20th	Day	7% 1 8% 23% 29% 32%	524
	21st to end	Day	7% 1 7% 23% 31% 31%	575
August	1st to 10th	Day	6% 2 7% 24% 35% 26%	527
	11th to 20th	Day	7% 2 11% 24% 33% 24%	517
	21st to end	Day	5% 1 9% 25% 33% 27%	583
September	1st to 10th	Day	5% 2 13% 23% 33% 23%	530
	11th to 20th	Day	4 2 19% 31% 32% 13%	530
	21st to end	Day	3 5% 18% 30% 28% 15%	530
October	1st to 10th	Day	6% 4 18% 29% 26% 17%	333
	11th to 20th	Day	7% 4 20% 29% 25% 15%	330
	21st to end	Day	4 5% 18% 28% 31% 14%	363
November	1st to 10th	Day	5% 5% 22% 34% 26% 8%	330
	11th to 20th	Day	4 5% 17% 30% 32% 12%	330
	21st to end	Day	6% 8% 22% 31% 23% 9%	330
December	1st to 10th	Day	6% 5% 20% 30% 29% 9%	330
	11th to 20th	Day	4 7% 24% 33% 23% 9%	330
	21st to end	Day	11% 10% 24% 34% 18% 3	363
January	1st to 10th	Night	8% 3 22% 43% 20% 4	840
	11th to 20th	Night	6% 5% 22% 43% 19% 4	824
	21st to end	Night	10% 3 24% 41% 18% 3	880
February	1st to 10th	Night	4 6% 28% 41% 19% 3	800
	11th to 20th	Night	7% 7% 29% 42% 13% 1	784
	21st to end	Night	7% 7% 34% 40% 11%	660
March	1st to 10th	Night	7% 9% 30% 39% 15%	800
	11th to 20th	Night	7% 8% 27% 39% 18% 2	800
	21st to end	Night	6% 8% 31% 41% 14% 1	880
April	1st to 10th	Night	5% 6% 32% 41% 15% 1	709
	11th to 20th	Night	5% 10% 34% 39% 11% 1	593
	21st to end	Night	5% 9% 33% 39% 13% 2	600
May	1st to 10th	Night	5% 6% 32% 44% 14% 1	600
	11th to 20th	Night	4 5% 29% 41% 18% 2	591
	21st to end	Night	4 4 29% 42% 19% 1	650
June	1st to 10th	Night	5% 4 22% 50% 20%	600
	11th to 20th	Night	3 3 25% 50% 19% 1	600
	21st to end	Night	4 1 17% 47% 31% 1	600
July	1st to 10th	Night	4 1 12% 42% 39% 2	585
	11th to 20th	Night	5% 1 8% 31% 49% 5%	582
	21st to end	Night	8% 2 6% 25% 50% 9%	657
August	1st to 10th	Night	7% 1 5% 26% 54% 7%	597
	11th to 20th	Night	7% 2 8% 32% 47% 5%	594
	21st to end	Night	5% 1 7% 31% 53% 4	660
September	1st to 10th	Night	6% 2 11% 40% 41% 2	600
	11th to 20th	Night	4 1 10% 44% 38% 4	792
	21st to end	Night	4 3 16% 40% 34% 4	800
October	1st to 10th	Night	4 3 20% 46% 25% 3	800
	11th to 20th	Night	3 3 19% 46% 27% 2	800
	21st to end	Night	2 4 20% 41% 28% 5%	880
November	1st to 10th	Night	5% 3 18% 44% 27% 3	820
	11th to 20th	Night	3 4 18% 46% 25% 4	840
	21st to end	Night	8% 5% 23% 36% 26% 3	840
December	1st to 10th	Night	7% 4 20% 43% 23% 2	840
	11th to 20th	Night	9% 4 21% 43% 22% 2	840
	21st to end	Night	8% 6% 23% 46% 15% 3	924

Page generated at 2026-01-13 23:14:28. Version ac07

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