Starlight Hills Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Starlight Hills Observatory clear sky chart from 2005-11-06 to 2025-04-26.

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 5% 2 5% 4 4 4 4 6% 8% 55%	117570
	Night	3 5% 2 5% 3 3 4 3 5% 7% 60%	119947
Transparency	Day	23% 7% 11% 23% 25% 11%	117315
	Night	21% 7% 11% 25% 25% 11%	119792
Seeing	Day	7% 11% 30% 29% 17% 6%	16097
	Night	6% 6% 32% 39% 15% 2	26259

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	1 5% 2 4 2 3 3 3 5% 10% 63%	3638
	11th to 20th	Day	1 2 1 4 4 5% 5% 4 6% 8% 59%	3796
	21st to end	Day	1 4 2 6% 4 3 3 3 5% 6% 62%	4510
February	1st to 10th	Day	1 6% 3 5% 5% 3 4 3 6% 7% 59%	1820
	11th to 20th	Day	1 3 2 5% 4 4 4 4 6% 9% 57%	1949
	21st to end	Day	1 5% 2 8% 4 3 4 3 5% 6% 58%	1650
March	1st to 10th	Day	1 4 2 5% 3 2 3 3 5% 7% 64%	4500
	11th to 20th	Day	3 9% 3 7% 4 4 4 3 6% 8% 50%	4899
	21st to end	Day	2 5% 3 5% 4 4 5% 4 6% 8% 54%	5852
April	1st to 10th	Day	3 6% 2 5% 3 4 4 5% 6% 9% 55%	2400
	11th to 20th	Day	6% 7% 3 6% 4 4 4 4 7% 8% 49%	2371
	21st to end	Day	6% 8% 2 5% 3 4 4 4 6% 9% 49%	2341
May	1st to 10th	Day	7% 9% 2 5% 4 4 5% 4 6% 8% 46%	3990
	11th to 20th	Day	7% 7% 2 6% 4 5% 5% 5% 7% 9% 44%	4281
	21st to end	Day	4 4 1 5% 4 4 5% 5% 9% 9% 49%	4825
June	1st to 10th	Day	5% 4 2 4 4 4 5% 6% 8% 9% 47%	2660
	11th to 20th	Day	5% 3 1 4 4 4 5% 5% 8% 9% 53%	2660
	21st to end	Day	3 3 2 5% 5% 5% 6% 6% 9% 9% 48%	2660
July	1st to 10th	Day	4 4 2 5% 3 4 5% 5% 7% 9% 52%	4430
	11th to 20th	Day	3 4 1 6% 5% 6% 6% 6% 8% 10% 45%	4430
	21st to end	Day	5% 4 2 5% 5% 5% 5% 5% 7% 8% 50%	4690
August	1st to 10th	Day	9% 5% 2 7% 6% 6% 6% 6% 9% 9% 37%	4014
	11th to 20th	Day	4 4 2 5% 5% 5% 7% 6% 7% 9% 48%	3726
	21st to end	Day	5% 4 2 5% 4 5% 6% 5% 8% 9% 48%	4180
September	1st to 10th	Day	5% 3 2 4 3 4 5% 4 7% 7% 56%	2280
	11th to 20th	Day	6% 3 1 4 3 3 4 4 6% 7% 59%	2242
	21st to end	Day	5% 4 1 4 2 3 3 3 4 6% 65%	2090
October	1st to 10th	Day	4 3 1 3 1 2 2 2 4 5% 74%	3242
	11th to 20th	Day	4 5% 2 6% 3 3 3 3 4 6% 60%	3150
	21st to end	Day	3 3 1 3 2 2 2 3 4 6% 72%	3319
November	1st to 10th	Day	2 4 1 3 3 2 3 3 6% 7% 67%	1763
	11th to 20th	Day	1 3 1 3 3 2 3 3 5% 7% 69%	1800
	21st to end	Day	1 4 2 5% 3 3 3 3 5% 7% 64%	1762
December	1st to 10th	Day	1 4 2 3 3 2 3 3 4 6% 71%	2620
	11th to 20th	Day	1 3 1 3 3 2 3 3 4 8% 69%	3026
	21st to end	Day	1 4 2 5% 3 3 4 4 5% 7% 61%	4004
January	1st to 10th	Night	1 5% 3 7% 4 3 4 3 5% 8% 57%	6327
	11th to 20th	Night	1 3 2 4 4 4 4 3 5% 7% 64%	6227
	21st to end	Night	1 3 1 6% 4 3 3 3 4 6% 65%	6468
February	1st to 10th	Night	1 5% 2 6% 4 3 3 3 5% 8% 58%	2578
	11th to 20th	Night	1 6% 3 7% 4 3 4 4 6% 7% 54%	2338
	21st to end	Night	1 5% 2 6% 3 2 3 2 5% 4 66%	1980
March	1st to 10th	Night	1 5% 1 5% 3 4 4 3 5% 7% 63%	5374
	11th to 20th	Night	4 8% 2 5% 4 4 3 3 5% 6% 56%	5093
	21st to end	Night	3 7% 2 5% 3 3 4 3 5% 7% 57%	5105
April	1st to 10th	Night	3 5% 1 4 3 3 4 3 5% 8% 61%	2000
	11th to 20th	Night	6% 9% 2 5% 2 4 3 4 5% 7% 53%	1977
	21st to end	Night	7% 7% 3 5% 3 4 4 3 5% 8% 52%	1955
May	1st to 10th	Night	8% 6% 2 4 3 3 4 3 5% 7% 55%	2978
	11th to 20th	Night	7% 8% 2 4 3 4 4 4 6% 9% 49%	2620
	21st to end	Night	7% 5% 1 4 3 4 4 4 5% 7% 56%	2762
June	1st to 10th	Night	5% 4 2 4 2 3 3 4 7% 10% 57%	1520
	11th to 20th	Night	4 3 1 3 3 3 3 3 6% 9% 63%	1520
	21st to end	Night	4 4 1 4 4 3 5% 5% 8% 8% 52%	1520
July	1st to 10th	Night	5% 4 1 5% 4 5% 5% 4 5% 7% 55%	2540
	11th to 20th	Night	3 4 1 4 4 4 5% 5% 9% 11% 50%	2540
	21st to end	Night	4 4 1 4 3 3 5% 4 7% 9% 56%	2870
August	1st to 10th	Night	9% 7% 2 5% 5% 5% 7% 5% 9% 10% 36%	2925
	11th to 20th	Night	4 4 1 6% 4 5% 5% 5% 8% 10% 48%	3113
	21st to end	Night	3 3 1 5% 4 5% 6% 6% 9% 11% 47%	3487
September	1st to 10th	Night	4 4 1 5% 4 4 6% 4 7% 8% 53%	1900
	11th to 20th	Night	6% 4 1 3 2 3 3 3 4 7% 63%	1938
	21st to end	Night	5% 4 1 4 3 3 3 3 4 6% 65%	2090
October	1st to 10th	Night	5% 3 1 2 2 2 2 2 3 5% 72%	3723
	11th to 20th	Night	7% 5% 1 4 3 3 3 3 5% 6% 60%	3778
	21st to end	Night	3 3 2 3 2 2 2 2 3 6% 71%	4206
November	1st to 10th	Night	2 4 1 4 3 3 3 2 5% 7% 66%	2535
	11th to 20th	Night	1 2 2 3 3 2 3 3 5% 5% 71%	2600
	21st to end	Night	2 4 2 5% 3 3 2 3 5% 7% 64%	2635
December	1st to 10th	Night	2 1 4 3 2 2 2 3 5% 75%	4570
	11th to 20th	Night	1 3 2 4 3 3 3 3 5% 10% 65%	5181
	21st to end	Night	2 5% 2 5% 3 3 4 3 5% 7% 60%	6974

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	18% 4 9% 33% 26% 10%	3639
	11th to 20th	Day	19% 7% 11% 22% 30% 11%	3796
	21st to end	Day	22% 5% 8% 26% 28% 11%	4510
February	1st to 10th	Day	24% 5% 12% 26% 23% 11%	1820
	11th to 20th	Day	21% 7% 9% 26% 28% 8%	1959
	21st to end	Day	24% 5% 7% 21% 24% 19%	1650
March	1st to 10th	Day	17% 5% 8% 16% 33% 21%	4500
	11th to 20th	Day	27% 8% 10% 22% 27% 7%	4899
	21st to end	Day	21% 9% 12% 37% 18% 3	5852
April	1st to 10th	Day	23% 7% 13% 24% 22% 10%	2400
	11th to 20th	Day	30% 6% 12% 22% 18% 11%	2374
	21st to end	Day	30% 7% 13% 25% 20% 4	2341
May	1st to 10th	Day	29% 8% 13% 25% 14% 12%	3990
	11th to 20th	Day	31% 10% 15% 21% 20% 3	4285
	21st to end	Day	21% 12% 16% 22% 20% 10%	4825
June	1st to 10th	Day	26% 10% 16% 22% 17% 10%	2660
	11th to 20th	Day	21% 8% 12% 17% 24% 17%	2660
	21st to end	Day	26% 9% 12% 19% 22% 11%	2660
July	1st to 10th	Day	24% 9% 11% 16% 27% 14%	4361
	11th to 20th	Day	27% 9% 13% 21% 20% 10%	4355
	21st to end	Day	26% 8% 13% 17% 19% 16%	4542
August	1st to 10th	Day	37% 9% 17% 22% 10% 4	4000
	11th to 20th	Day	26% 11% 13% 18% 19% 14%	3762
	21st to end	Day	25% 8% 14% 20% 26% 6%	4179
September	1st to 10th	Day	23% 7% 10% 22% 24% 14%	2280
	11th to 20th	Day	21% 7% 9% 17% 29% 17%	2242
	21st to end	Day	20% 5% 8% 20% 33% 14%	2090
October	1st to 10th	Day	14% 4 7% 16% 38% 22%	3242
	11th to 20th	Day	22% 6% 8% 21% 32% 11%	3150
	21st to end	Day	13% 3 6% 22% 38% 18%	3319
November	1st to 10th	Day	16% 4 9% 24% 29% 18%	1763
	11th to 20th	Day	15% 5% 6% 30% 33% 12%	1800
	21st to end	Day	19% 5% 7% 23% 30% 16%	1762
December	1st to 10th	Day	15% 5% 8% 30% 33% 10%	2620
	11th to 20th	Day	15% 4 6% 31% 30% 13%	3024
	21st to end	Day	19% 6% 10% 27% 26% 12%	4004
January	1st to 10th	Night	24% 5% 8% 31% 25% 7%	6327
	11th to 20th	Night	17% 6% 10% 26% 25% 15%	6227
	21st to end	Night	20% 6% 11% 26% 27% 11%	6468
February	1st to 10th	Night	24% 5% 9% 28% 24% 10%	2578
	11th to 20th	Night	27% 6% 9% 28% 21% 8%	2338
	21st to end	Night	20% 4 7% 25% 27% 17%	1980
March	1st to 10th	Night	19% 5% 7% 16% 38% 15%	5374
	11th to 20th	Night	26% 7% 10% 18% 28% 12%	5093
	21st to end	Night	23% 8% 12% 32% 21% 3	5105
April	1st to 10th	Night	21% 6% 13% 24% 25% 11%	2000
	11th to 20th	Night	28% 7% 12% 25% 20% 8%	1977
	21st to end	Night	29% 7% 14% 27% 18% 5%	1955
May	1st to 10th	Night	27% 8% 14% 26% 16% 10%	2978
	11th to 20th	Night	29% 11% 17% 25% 10% 8%	2621
	21st to end	Night	23% 10% 17% 23% 20% 7%	2762
June	1st to 10th	Night	22% 7% 22% 24% 15% 10%	1520
	11th to 20th	Night	17% 8% 15% 23% 20% 18%	1520
	21st to end	Night	24% 9% 12% 22% 23% 11%	1520
July	1st to 10th	Night	26% 9% 11% 19% 28% 7%	2500
	11th to 20th	Night	22% 9% 17% 30% 16% 6%	2492
	21st to end	Night	21% 10% 16% 24% 14% 15%	2780
August	1st to 10th	Night	38% 10% 19% 19% 9% 5%	2916
	11th to 20th	Night	26% 11% 19% 20% 19% 5%	3143
	21st to end	Night	23% 11% 19% 26% 17% 4	3487
September	1st to 10th	Night	26% 7% 11% 23% 22% 11%	1900
	11th to 20th	Night	19% 5% 10% 23% 26% 16%	1938
	21st to end	Night	20% 6% 8% 23% 32% 11%	2090
October	1st to 10th	Night	14% 5% 7% 21% 35% 17%	3723
	11th to 20th	Night	22% 5% 8% 27% 27% 10%	3778
	21st to end	Night	16% 4 7% 26% 32% 15%	4206
November	1st to 10th	Night	16% 5% 9% 25% 28% 16%	2535
	11th to 20th	Night	15% 4 8% 28% 33% 12%	2600
	21st to end	Night	20% 4 9% 24% 31% 13%	2635
December	1st to 10th	Night	13% 4 8% 33% 33% 10%	4570
	11th to 20th	Night	16% 5% 7% 28% 34% 10%	5182
	21st to end	Night	22% 6% 11% 22% 26% 12%	6974

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	6% 7% 34% 37% 13% 4	300
	11th to 20th	Day	6% 11% 35% 37% 9% 2	300
	21st to end	Day	3 12% 35% 35% 12% 3	330
February	1st to 10th	Day	4 17% 47% 25% 6% 2	300
	11th to 20th	Day	3 15% 46% 26% 10% 1	300
	21st to end	Day	2 18% 46% 28% 6% 1	249
March	1st to 10th	Day	3 15% 39% 31% 10% 3	300
	11th to 20th	Day	10% 18% 38% 21% 10% 3	451
	21st to end	Day	11% 21% 39% 20% 6% 2	550
April	1st to 10th	Day	11% 18% 39% 23% 7% 2	500
	11th to 20th	Day	12% 24% 36% 20% 6% 2	492
	21st to end	Day	15% 16% 33% 26% 8% 2	481
May	1st to 10th	Day	14% 17% 34% 25% 8% 2	575
	11th to 20th	Day	13% 11% 28% 30% 15% 4	675
	21st to end	Day	9% 12% 30% 26% 20% 4	734
June	1st to 10th	Day	7% 10% 22% 32% 22% 7%	680
	11th to 20th	Day	6% 10% 27% 33% 18% 5%	680
	21st to end	Day	4 8% 24% 34% 23% 7%	680
July	1st to 10th	Day	6% 5% 18% 31% 30% 10%	671
	11th to 20th	Day	5% 3 14% 34% 33% 11%	668
	21st to end	Day	7% 4 14% 28% 34% 14%	676
August	1st to 10th	Day	10% 2 18% 35% 27% 8%	487
	11th to 20th	Day	5% 6% 22% 33% 22% 12%	477
	21st to end	Day	7% 3 23% 37% 21% 9%	539
September	1st to 10th	Day	8% 4 25% 32% 20% 11%	490
	11th to 20th	Day	6% 8% 29% 27% 22% 8%	452
	21st to end	Day	10% 8% 18% 26% 26% 11%	300
October	1st to 10th	Day	9% 12% 19% 25% 24% 11%	300
	11th to 20th	Day	3 9% 33% 32% 17% 6%	300
	21st to end	Day	6% 12% 34% 28% 17% 3	330
November	1st to 10th	Day	7% 12% 34% 27% 13% 6%	300
	11th to 20th	Day	3 7% 40% 32% 15% 3	300
	21st to end	Day	6% 12% 34% 31% 12% 4	300
December	1st to 10th	Day	3 13% 39% 26% 15% 4	300
	11th to 20th	Day	5% 8% 41% 30% 15% 1	300
	21st to end	Day	3 14% 44% 29% 8% 2	330
January	1st to 10th	Night	3 7% 39% 36% 13% 4	1000
	11th to 20th	Night	3 7% 42% 37% 9% 2	952
	21st to end	Night	2 6% 38% 43% 10% 1	880
February	1st to 10th	Night	3 6% 52% 33% 6% 1	800
	11th to 20th	Night	3 7% 48% 33% 8% 1	784
	21st to end	Night	3 8% 49% 34% 4	660
March	1st to 10th	Night	3 7% 46% 35% 8% 1	800
	11th to 20th	Night	8% 7% 35% 39% 11% 1	800
	21st to end	Night	5% 10% 43% 35% 7%	716
April	1st to 10th	Night	7% 11% 43% 34% 5%	600
	11th to 20th	Night	14% 10% 34% 34% 7% 1	593
	21st to end	Night	13% 9% 33% 35% 9% 1	587
May	1st to 10th	Night	10% 9% 31% 36% 12% 1	570
	11th to 20th	Night	12% 7% 26% 39% 14% 2	561
	21st to end	Night	11% 6% 30% 35% 16% 2	617
June	1st to 10th	Night	5% 6% 21% 45% 21% 3	570
	11th to 20th	Night	4 4 25% 51% 15% 1	570
	21st to end	Night	5% 3 18% 49% 23% 2	570
July	1st to 10th	Night	6% 3 13% 42% 33% 3	555
	11th to 20th	Night	4 2 6% 46% 37% 5%	552
	21st to end	Night	8% 2 8% 35% 42% 4	624
August	1st to 10th	Night	8% 3 8% 40% 37% 4	567
	11th to 20th	Night	7% 1 9% 49% 30% 4	564
	21st to end	Night	4 2 10% 45% 34% 4	627
September	1st to 10th	Night	6% 3 12% 46% 28% 6%	570
	11th to 20th	Night	8% 3 15% 47% 23% 4	570
	21st to end	Night	7% 3 21% 44% 21% 4	570
October	1st to 10th	Night	10% 4 24% 40% 18% 3	709
	11th to 20th	Night	5% 4 32% 39% 17% 4	760
	21st to end	Night	6% 5% 32% 37% 17% 3	886
November	1st to 10th	Night	4 4 34% 42% 15% 2	975
	11th to 20th	Night	3 6% 37% 42% 10% 2	1000
	21st to end	Night	5% 7% 35% 39% 12% 2	1000
December	1st to 10th	Night	2 6% 41% 38% 11% 2	1000
	11th to 20th	Night	3 7% 40% 38% 11% 1	1000
	21st to end	Night	5% 8% 44% 34% 8% 1	1100

Page generated at 2025-04-23 07:25:01. Version ac07

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