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 2024-11-25.

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 5% 4 4 4 3 5% 7% 60%	105300
	Night	3 5% 2 5% 4 4 4 3 5% 7% 59%	102929
Transparency	Day	22% 4 7% 17% 27% 22%	105066
	Night	24% 4 8% 18% 26% 20%	102752
Seeing	Day	6% 8% 22% 28% 24% 12%	14657
	Night	6% 4 21% 40% 25% 3	24750

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 7% 3 6% 3 3 3 3 5% 6% 57%	2858
	11th to 20th	Day	3 5% 2 5% 3 4 4 4 5% 6% 60%	2860
	21st to end	Day	5% 10% 4 7% 4 4 3 3 5% 8% 47%	3146
February	1st to 10th	Day	2 7% 2 6% 4 4 2 3 5% 7% 59%	1710
	11th to 20th	Day	3 8% 2 5% 4 3 4 3 5% 7% 56%	1689
	21st to end	Day	4 7% 2 6% 5% 4 3 3 4 6% 56%	1727
March	1st to 10th	Day	2 4 2 4 3 3 3 3 5% 8% 62%	3444
	11th to 20th	Day	4 8% 2 7% 6% 5% 5% 4 6% 7% 45%	3490
	21st to end	Day	3 5% 2 5% 3 3 3 4 6% 6% 60%	3825
April	1st to 10th	Day	3 3 1 4 3 4 4 3 5% 7% 64%	2175
	11th to 20th	Day	2 4 1 5% 3 3 4 2 4 5% 66%	2251
	21st to end	Day	2 3 2 4 3 3 3 3 4 6% 68%	2280
May	1st to 10th	Day	2 2 1 2 2 2 2 3 4 5% 76%	4048
	11th to 20th	Day	2 3 2 4 4 4 4 4 6% 6% 62%	4071
	21st to end	Day	2 2 1 2 1 2 2 1 3 4 80%	4477
June	1st to 10th	Day	1 2 1 2 2 3 3 3 5% 7% 70%	2470
	11th to 20th	Day	2 1 1 2 2 2 2 2 3 4 79%	2470
	21st to end	Day	2 2 2 4 4 4 4 4 6% 6% 62%	2470
July	1st to 10th	Day	2 3 2 4 4 3 4 4 5% 6% 63%	4110
	11th to 20th	Day	3 3 1 5% 5% 5% 7% 5% 8% 10% 47%	4110
	21st to end	Day	3 3 2 6% 5% 6% 7% 6% 9% 10% 43%	4454
August	1st to 10th	Day	5% 4 2 8% 6% 6% 7% 5% 8% 9% 41%	4092
	11th to 20th	Day	3 3 1 5% 4 5% 6% 5% 8% 10% 49%	3945
	21st to end	Day	5% 3 2 6% 5% 5% 6% 6% 7% 9% 46%	4180
September	1st to 10th	Day	4 4 2 5% 5% 4 5% 4 6% 8% 54%	2280
	11th to 20th	Day	3 3 1 4 4 3 3 3 5% 6% 65%	2092
	21st to end	Day	3 2 1 4 3 3 3 3 5% 5% 68%	2090
October	1st to 10th	Day	3 2 1 3 2 2 2 2 3 4 75%	3173
	11th to 20th	Day	5% 3 1 4 2 2 3 3 5% 5% 67%	3150
	21st to end	Day	2 2 2 2 2 2 2 4 6% 78%	3438
November	1st to 10th	Day	2 3 1 2 2 2 3 3 4 8% 70%	1959
	11th to 20th	Day	2 2 1 4 2 3 3 2 3 5% 72%	1995
	21st to end	Day	4 5% 2 4 3 3 3 3 4 5% 64%	1746
December	1st to 10th	Day	4 5% 1 5% 3 3 3 3 4 6% 62%	1710
	11th to 20th	Day	4 8% 3 8% 5% 4 4 4 5% 5% 50%	2169
	21st to end	Day	4 7% 3 6% 4 4 4 3 5% 5% 55%	3146
January	1st to 10th	Night	3 7% 4 8% 4 4 4 4 5% 6% 52%	4097
	11th to 20th	Night	3 5% 1 4 3 3 3 3 5% 7% 65%	4110
	21st to end	Night	6% 11% 3 7% 4 4 5% 4 5% 5% 46%	4521
February	1st to 10th	Night	2 5% 2 5% 3 3 3 2 4 7% 62%	2470
	11th to 20th	Night	4 8% 3 5% 4 3 4 3 5% 5% 57%	2373
	21st to end	Night	4 6% 2 5% 3 3 3 3 4 7% 60%	1727
March	1st to 10th	Night	2 5% 1 4 2 2 2 2 4 7% 67%	3430
	11th to 20th	Night	6% 9% 2 7% 4 5% 5% 3 5% 7% 47%	3490
	21st to end	Night	3 5% 2 4 3 4 4 4 5% 8% 59%	3828
April	1st to 10th	Night	3 4 1 4 3 3 3 4 5% 6% 64%	2004
	11th to 20th	Night	3 4 1 4 3 3 3 3 4 6% 66%	1877
	21st to end	Night	3 4 1 4 3 2 3 2 4 6% 68%	1900
May	1st to 10th	Night	2 3 1 3 3 2 2 2 4 5% 73%	2905
	11th to 20th	Night	4 4 2 4 3 3 3 2 4 5% 64%	2834
	21st to end	Night	2 2 1 2 1 1 2 2 2 3 82%	3110
June	1st to 10th	Night	4 3 1 4 3 3 3 3 5% 6% 66%	1710
	11th to 20th	Night	3 2 1 2 2 2 2 1 3 5% 78%	1710
	21st to end	Night	2 3 1 5% 4 4 5% 4 6% 7% 58%	1710
July	1st to 10th	Night	3 3 2 6% 4 3 5% 4 6% 5% 59%	2860
	11th to 20th	Night	4 4 3 10% 7% 7% 7% 7% 8% 11% 32%	2860
	21st to end	Night	4 5% 2 7% 6% 8% 8% 7% 10% 11% 33%	3109
August	1st to 10th	Night	3 5% 2 11% 7% 8% 8% 7% 9% 9% 30%	2847
	11th to 20th	Night	5% 4 2 8% 7% 7% 7% 5% 9% 9% 37%	2894
	21st to end	Night	5% 5% 2 7% 6% 6% 7% 6% 9% 10% 38%	3487
September	1st to 10th	Night	4 5% 2 8% 6% 6% 6% 5% 7% 8% 43%	1900
	11th to 20th	Night	3 3 1 4 4 3 4 3 5% 5% 64%	2082
	21st to end	Night	4 3 1 4 3 3 3 3 5% 5% 67%	2090
October	1st to 10th	Night	3 2 1 3 3 2 3 3 4 5% 72%	3795
	11th to 20th	Night	3 2 1 4 3 2 2 2 4 6% 70%	3778
	21st to end	Night	2 2 1 2 2 3 2 2 4 6% 73%	4120
November	1st to 10th	Night	3 2 1 3 3 3 3 2 4 5% 70%	2340
	11th to 20th	Night	2 3 1 2 2 2 2 2 4 4 75%	2401
	21st to end	Night	3 6% 1 4 2 3 3 3 5% 5% 65%	2528
December	1st to 10th	Night	6% 5% 2 4 3 3 3 3 4 7% 62%	2470
	11th to 20th	Night	5% 7% 2 5% 5% 4 4 4 5% 8% 51%	3041
	21st to end	Night	4 6% 2 5% 4 4 4 4 5% 7% 56%	4521

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	28% 3 8% 27% 26% 7%	2859
	11th to 20th	Day	24% 4 9% 31% 27% 5%	2860
	21st to end	Day	34% 5% 7% 22% 26% 6%	3146
February	1st to 10th	Day	25% 3 9% 19% 29% 15%	1710
	11th to 20th	Day	27% 4 5% 23% 24% 17%	1698
	21st to end	Day	28% 3 5% 17% 28% 18%	1727
March	1st to 10th	Day	20% 3 5% 18% 39% 15%	3444
	11th to 20th	Day	33% 5% 7% 22% 22% 11%	3490
	21st to end	Day	23% 3 6% 21% 36% 10%	3825
April	1st to 10th	Day	20% 3 5% 15% 35% 22%	2175
	11th to 20th	Day	21% 2 4 13% 32% 28%	2254
	21st to end	Day	18% 3 5% 14% 36% 24%	2280
May	1st to 10th	Day	11% 2 4 11% 35% 38%	4048
	11th to 20th	Day	19% 4 7% 10% 32% 28%	4074
	21st to end	Day	11% 1 2 6% 24% 55%	4477
June	1st to 10th	Day	13% 2 4 9% 31% 40%	2470
	11th to 20th	Day	11% 2 3 6% 24% 55%	2470
	21st to end	Day	20% 4 7% 11% 21% 36%	2470
July	1st to 10th	Day	21% 4 6% 10% 10% 48%	4046
	11th to 20th	Day	27% 9% 11% 26% 17% 11%	4041
	21st to end	Day	29% 8% 16% 28% 14% 4	4314
August	1st to 10th	Day	35% 9% 18% 18% 11% 9%	4078
	11th to 20th	Day	27% 7% 11% 22% 19% 14%	3984
	21st to end	Day	30% 6% 14% 18% 22% 9%	4179
September	1st to 10th	Day	28% 5% 9% 16% 22% 21%	2280
	11th to 20th	Day	19% 4 7% 15% 27% 27%	2092
	21st to end	Day	17% 3 6% 12% 29% 32%	2090
October	1st to 10th	Day	14% 2 4 12% 26% 41%	3173
	11th to 20th	Day	18% 3 3 11% 41% 24%	3150
	21st to end	Day	10% 2 3 15% 36% 33%	3438
November	1st to 10th	Day	13% 3 5% 22% 38% 20%	1959
	11th to 20th	Day	16% 3 4 18% 38% 21%	1995
	21st to end	Day	22% 3 4 19% 32% 20%	1746
December	1st to 10th	Day	23% 4 7% 24% 28% 14%	1710
	11th to 20th	Day	35% 4 8% 21% 23% 9%	2168
	21st to end	Day	30% 4 6% 23% 27% 10%	3146
January	1st to 10th	Night	32% 4 7% 25% 26% 7%	4097
	11th to 20th	Night	20% 3 8% 33% 30% 6%	4110
	21st to end	Night	38% 5% 7% 20% 24% 6%	4521
February	1st to 10th	Night	23% 3 8% 21% 30% 15%	2470
	11th to 20th	Night	29% 3 5% 22% 24% 17%	2374
	21st to end	Night	24% 3 5% 17% 33% 18%	1727
March	1st to 10th	Night	17% 3 6% 26% 33% 16%	3430
	11th to 20th	Night	32% 5% 8% 20% 24% 10%	3490
	21st to end	Night	21% 4 8% 30% 28% 8%	3828
April	1st to 10th	Night	20% 2 6% 18% 33% 20%	2004
	11th to 20th	Night	19% 3 5% 13% 36% 25%	1877
	21st to end	Night	18% 3 6% 14% 37% 23%	1900
May	1st to 10th	Night	14% 2 5% 12% 42% 24%	2905
	11th to 20th	Night	21% 3 7% 11% 30% 28%	2835
	21st to end	Night	9% 2 3 7% 26% 53%	3110
June	1st to 10th	Night	19% 3 4 10% 25% 40%	1710
	11th to 20th	Night	12% 2 3 7% 22% 55%	1710
	21st to end	Night	22% 5% 7% 13% 19% 34%	1710
July	1st to 10th	Night	24% 6% 9% 8% 9% 44%	2815
	11th to 20th	Night	40% 8% 13% 22% 7% 9%	2806
	21st to end	Night	39% 11% 19% 21% 9% 1	3011
August	1st to 10th	Night	43% 11% 19% 11% 8% 8%	2838
	11th to 20th	Night	38% 9% 17% 14% 10% 12%	2921
	21st to end	Night	35% 8% 16% 18% 16% 7%	3487
September	1st to 10th	Night	33% 7% 12% 15% 16% 17%	1900
	11th to 20th	Night	20% 5% 8% 17% 26% 24%	2082
	21st to end	Night	19% 4 6% 12% 28% 32%	2090
October	1st to 10th	Night	14% 4 5% 12% 22% 43%	3795
	11th to 20th	Night	15% 3 3 16% 35% 27%	3778
	21st to end	Night	13% 3 3 12% 39% 30%	4120
November	1st to 10th	Night	17% 3 3 19% 35% 22%	2340
	11th to 20th	Night	14% 2 5% 19% 36% 24%	2401
	21st to end	Night	21% 3 5% 18% 32% 21%	2528
December	1st to 10th	Night	24% 2 6% 27% 27% 14%	2470
	11th to 20th	Night	31% 4 8% 20% 27% 10%	3041
	21st to end	Night	27% 4 7% 21% 25% 16%	4521

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% 26% 35% 21% 3	270
	11th to 20th	Day	9% 7% 22% 36% 19% 8%	270
	21st to end	Day	4 9% 22% 34% 21% 11%	297
February	1st to 10th	Day	4 10% 23% 35% 21% 7%	270
	11th to 20th	Day	10% 12% 27% 34% 17% 1	294
	21st to end	Day	12% 18% 32% 23% 12% 2	382
March	1st to 10th	Day	7% 20% 30% 27% 13% 4	460
	11th to 20th	Day	9% 17% 24% 29% 16% 5%	460
	21st to end	Day	11% 21% 35% 20% 10% 3	506
April	1st to 10th	Day	7% 15% 29% 27% 16% 6%	460
	11th to 20th	Day	10% 14% 36% 22% 13% 4	452
	21st to end	Day	5% 15% 34% 26% 14% 6%	460
May	1st to 10th	Day	7% 11% 33% 30% 16% 3	463
	11th to 20th	Day	6% 12% 29% 24% 22% 8%	488
	21st to end	Day	7% 12% 28% 28% 19% 6%	529
June	1st to 10th	Day	3 6% 22% 31% 29% 10%	490
	11th to 20th	Day	2 7% 27% 31% 22% 12%	490
	21st to end	Day	4 3 22% 34% 26% 11%	490
July	1st to 10th	Day	6% 2 12% 28% 33% 20%	486
	11th to 20th	Day	7% 1 8% 23% 29% 34%	484
	21st to end	Day	7% 1 7% 21% 31% 33%	531
August	1st to 10th	Day	6% 2 7% 22% 34% 28%	487
	11th to 20th	Day	8% 2 11% 23% 33% 23%	477
	21st to end	Day	6% 1 9% 25% 33% 26%	539
September	1st to 10th	Day	5% 2 13% 22% 33% 24%	490
	11th to 20th	Day	4 2 18% 30% 32% 13%	490
	21st to end	Day	3 6% 18% 30% 28% 16%	490
October	1st to 10th	Day	6% 4 18% 27% 26% 18%	303
	11th to 20th	Day	7% 4 19% 28% 26% 16%	300
	21st to end	Day	4 5% 17% 29% 31% 14%	330
November	1st to 10th	Day	5% 5% 22% 36% 25% 7%	300
	11th to 20th	Day	3 5% 17% 30% 32% 12%	300
	21st to end	Day	5% 9% 24% 31% 23% 9%	282
December	1st to 10th	Day	7% 5% 22% 30% 28% 7%	270
	11th to 20th	Day	4 7% 26% 36% 19% 7%	270
	21st to end	Day	12% 9% 24% 34% 18% 2	297
January	1st to 10th	Night	8% 3 21% 43% 22% 4	760
	11th to 20th	Night	7% 6% 21% 43% 20% 4	760
	21st to end	Night	10% 4 23% 41% 19% 3	836
February	1st to 10th	Night	4 6% 27% 41% 19% 3	760
	11th to 20th	Night	8% 6% 29% 42% 14% 1	744
	21st to end	Night	7% 7% 34% 40% 11%	628
March	1st to 10th	Night	7% 9% 30% 39% 16%	760
	11th to 20th	Night	8% 7% 26% 39% 18% 2	760
	21st to end	Night	6% 8% 30% 41% 13% 1	836
April	1st to 10th	Night	5% 6% 32% 42% 14% 1	674
	11th to 20th	Night	4 10% 34% 40% 11% 1	563
	21st to end	Night	5% 9% 33% 39% 13% 2	570
May	1st to 10th	Night	5% 5% 32% 43% 14% 1	570
	11th to 20th	Night	5% 4 28% 42% 19% 2	561
	21st to end	Night	4 5% 29% 41% 19% 1	617
June	1st to 10th	Night	4 3 21% 50% 21%	570
	11th to 20th	Night	3 3 26% 49% 19% 1	570
	21st to end	Night	4 1 16% 46% 31% 1	570
July	1st to 10th	Night	4 1 12% 41% 40% 2	555
	11th to 20th	Night	5% 1 8% 31% 49% 5%	552
	21st to end	Night	8% 2 6% 23% 53% 9%	624
August	1st to 10th	Night	7% 1 5% 25% 56% 7%	567
	11th to 20th	Night	7% 2 8% 31% 47% 5%	564
	21st to end	Night	6% 7% 31% 53% 4	627
September	1st to 10th	Night	6% 2 11% 39% 41% 1	570
	11th to 20th	Night	4 1 10% 43% 38% 4	752
	21st to end	Night	4 3 16% 39% 34% 4	760
October	1st to 10th	Night	4 3 20% 45% 25% 3	760
	11th to 20th	Night	3 2 19% 46% 28% 3	760
	21st to end	Night	2 4 20% 41% 28% 5%	836
November	1st to 10th	Night	5% 3 18% 44% 26% 3	780
	11th to 20th	Night	3 4 18% 46% 25% 4	800
	21st to end	Night	7% 5% 23% 35% 26% 3	778
December	1st to 10th	Night	8% 4 20% 43% 23% 3	760
	11th to 20th	Night	10% 5% 21% 43% 20% 1	760
	21st to end	Night	8% 6% 23% 45% 15% 3	836

Page generated at 2024-11-22 06:02:28. Version ac07

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