Starbrite Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Starbrite Observatory clear sky chart from 2005-11-06 to 2026-08-15.

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	6% 5% 2 7% 6% 6% 7% 6% 9% 11% 35%	142605
	Night	6% 5% 2 6% 5% 5% 5% 5% 8% 10% 42%	139310
Transparency	Day	37% 11% 20% 21% 10% 2	146271
	Night	34% 10% 21% 22% 11% 3	143716
Seeing	Day	8% 5% 19% 29% 23% 16%	18473
	Night	8% 3 17% 32% 29% 12%	28563

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	5% 5% 2 5% 3 4 4 4 6% 10% 53%	5258
	11th to 20th	Day	14% 7% 3 9% 6% 5% 6% 5% 8% 9% 27%	5280
	21st to end	Day	11% 6% 3 7% 6% 5% 5% 5% 6% 8% 38%	5876
February	1st to 10th	Day	8% 6% 2 6% 5% 5% 5% 4 6% 9% 42%	2100
	11th to 20th	Day	6% 5% 2 5% 4 4 4 4 7% 10% 49%	2180
	21st to end	Day	6% 4 1 6% 5% 5% 5% 6% 8% 9% 46%	1903
March	1st to 10th	Day	4 4 2 7% 6% 6% 7% 6% 8% 9% 41%	5045
	11th to 20th	Day	4 3 1 3 3 3 4 5% 7% 8% 58%	5090
	21st to end	Day	7% 5% 2 6% 5% 6% 5% 5% 8% 9% 42%	5586
April	1st to 10th	Day	4 3 2 5% 5% 5% 6% 6% 10% 13% 43%	2310
	11th to 20th	Day	5% 5% 2 6% 6% 5% 6% 5% 8% 10% 41%	2389
	21st to end	Day	5% 4 2 6% 5% 6% 6% 6% 9% 10% 42%	2520
May	1st to 10th	Day	4 3 1 4 4 4 5% 4 8% 11% 53%	7021
	11th to 20th	Day	5% 4 2 6% 5% 6% 7% 6% 9% 11% 39%	7083
	21st to end	Day	4 4 2 6% 5% 6% 8% 7% 10% 14% 34%	8391
June	1st to 10th	Day	13% 9% 4 11% 8% 8% 8% 6% 7% 8% 17%	2730
	11th to 20th	Day	7% 8% 3 10% 8% 9% 7% 7% 9% 11% 20%	2730
	21st to end	Day	7% 8% 3 9% 8% 8% 8% 7% 10% 11% 21%	2730
July	1st to 10th	Day	5% 6% 3 8% 7% 8% 7% 7% 9% 13% 27%	7680
	11th to 20th	Day	4 4 2 8% 7% 7% 9% 7% 10% 13% 30%	7680
	21st to end	Day	3 3 2 6% 6% 6% 7% 6% 10% 16% 35%	8340
August	1st to 10th	Day	4 5% 3 10% 8% 8% 9% 8% 11% 13% 20%	4269
	11th to 20th	Day	5% 5% 2 7% 6% 8% 9% 9% 11% 14% 25%	3898
	21st to end	Day	5% 5% 2 8% 7% 8% 10% 8% 12% 17% 19%	4023
September	1st to 10th	Day	8% 7% 3 10% 8% 9% 10% 7% 10% 11% 17%	2200
	11th to 20th	Day	6% 7% 4 11% 10% 10% 9% 7% 8% 11% 19%	2200
	21st to end	Day	11% 10% 3 11% 7% 9% 7% 7% 8% 9% 19%	2200
October	1st to 10th	Day	8% 8% 3 9% 7% 8% 8% 7% 10% 12% 19%	2100
	11th to 20th	Day	10% 5% 3 8% 7% 7% 9% 6% 9% 10% 26%	2000
	21st to end	Day	11% 7% 2 7% 5% 5% 6% 5% 6% 11% 35%	2200
November	1st to 10th	Day	7% 6% 2 6% 6% 6% 7% 6% 8% 10% 36%	2057
	11th to 20th	Day	9% 7% 2 9% 7% 6% 7% 6% 7% 8% 33%	2100
	21st to end	Day	7% 5% 2 6% 6% 6% 5% 5% 9% 10% 39%	2100
December	1st to 10th	Day	5% 4 2 5% 5% 4 4 5% 7% 7% 52%	3308
	11th to 20th	Day	8% 6% 2 6% 6% 6% 6% 5% 7% 11% 37%	3463
	21st to end	Day	11% 6% 3 9% 6% 7% 7% 5% 8% 10% 29%	4565
January	1st to 10th	Night	5% 4 2 6% 4 4 4 4 6% 9% 53%	7635
	11th to 20th	Night	13% 9% 3 9% 5% 6% 5% 4 7% 8% 31%	7680
	21st to end	Night	10% 8% 3 7% 5% 4 4 3 5% 7% 43%	8301
February	1st to 10th	Night	7% 6% 2 4 4 4 4 4 6% 9% 48%	2520
	11th to 20th	Night	6% 4 2 4 3 3 5% 4 6% 8% 56%	2324
	21st to end	Night	5% 3 1 4 3 4 5% 4 6% 9% 56%	1903
March	1st to 10th	Night	4 4 2 6% 4 4 5% 4 6% 8% 51%	5028
	11th to 20th	Night	4 3 1 4 3 4 4 3 6% 9% 59%	5090
	21st to end	Night	6% 4 2 5% 5% 5% 5% 4 7% 8% 49%	5588
April	1st to 10th	Night	3 3 2 4 3 4 4 3 8% 9% 57%	2310
	11th to 20th	Night	4 4 2 4 4 4 5% 4 7% 9% 51%	2179
	21st to end	Night	5% 5% 2 5% 3 3 4 4 8% 9% 51%	2100
May	1st to 10th	Night	3 2 1 4 3 4 4 4 7% 11% 58%	5833
	11th to 20th	Night	5% 4 2 5% 4 5% 5% 5% 8% 11% 45%	5830
	21st to end	Night	4 4 2 6% 5% 5% 7% 6% 11% 13% 38%	5761
June	1st to 10th	Night	10% 8% 3 9% 7% 6% 7% 7% 8% 10% 26%	1890
	11th to 20th	Night	6% 6% 2 7% 5% 6% 6% 6% 8% 14% 33%	1890
	21st to end	Night	6% 5% 2 7% 5% 6% 7% 6% 10% 14% 31%	1890
July	1st to 10th	Night	4 5% 2 8% 5% 6% 6% 5% 8% 12% 38%	5278
	11th to 20th	Night	3 4 2 7% 4 5% 6% 6% 10% 13% 40%	5277
	21st to end	Night	2 3 1 5% 4 4 6% 5% 9% 14% 47%	5736
August	1st to 10th	Night	4 4 2 6% 7% 6% 8% 7% 10% 13% 34%	3063
	11th to 20th	Night	3 3 2 6% 5% 6% 7% 6% 10% 15% 38%	3238
	21st to end	Night	4 4 2 7% 6% 7% 8% 7% 13% 16% 26%	3868
September	1st to 10th	Night	6% 5% 2 8% 7% 7% 8% 6% 10% 13% 28%	2200
	11th to 20th	Night	7% 7% 3 8% 7% 7% 7% 5% 9% 12% 28%	2200
	21st to end	Night	8% 7% 3 9% 7% 8% 8% 6% 9% 10% 26%	2200
October	1st to 10th	Night	8% 6% 3 9% 7% 8% 6% 8% 9% 10% 27%	2298
	11th to 20th	Night	8% 6% 2 7% 5% 5% 6% 5% 7% 10% 37%	2400
	21st to end	Night	9% 6% 2 6% 4 5% 5% 5% 6% 9% 41%	2640
November	1st to 10th	Night	7% 5% 2 7% 5% 5% 6% 4 6% 9% 45%	2461
	11th to 20th	Night	7% 7% 3 6% 6% 6% 6% 5% 6% 9% 40%	2520
	21st to end	Night	7% 5% 2 6% 5% 5% 5% 4 7% 9% 45%	2520
December	1st to 10th	Night	6% 5% 2 5% 4 3 4 3 6% 9% 53%	4074
	11th to 20th	Night	12% 7% 3 7% 4 4 5% 4 7% 9% 37%	4952
	21st to end	Night	8% 8% 3 9% 6% 7% 6% 5% 7% 9% 30%	6633

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% 6% 9% 33% 20% 7%	5259
	11th to 20th	Day	50% 8% 14% 18% 7% 3	5280
	21st to end	Day	41% 6% 8% 27% 14% 4	5876
February	1st to 10th	Day	36% 7% 12% 24% 16% 6%	2100
	11th to 20th	Day	28% 7% 11% 27% 22% 6%	2190
	21st to end	Day	31% 7% 12% 20% 23% 7%	1903
March	1st to 10th	Day	32% 10% 13% 21% 21% 4	5045
	11th to 20th	Day	19% 7% 16% 28% 19% 11%	5090
	21st to end	Day	32% 8% 12% 28% 15% 5%	5586
April	1st to 10th	Day	27% 9% 18% 27% 15% 3	2310
	11th to 20th	Day	33% 8% 16% 20% 17% 5%	2389
	21st to end	Day	30% 8% 14% 24% 20% 3	2520
May	1st to 10th	Day	19% 7% 18% 39% 15% 2	7053
	11th to 20th	Day	31% 11% 23% 27% 7% 1	7087
	21st to end	Day	31% 13% 26% 24% 6%	8405
June	1st to 10th	Day	60% 11% 16% 10% 3	2730
	11th to 20th	Day	52% 14% 20% 13% 2	2730
	21st to end	Day	49% 14% 26% 10%	2730
July	1st to 10th	Day	39% 15% 35% 11%	7615
	11th to 20th	Day	39% 13% 34% 14%	7611
	21st to end	Day	29% 13% 34% 22% 1	8156
August	1st to 10th	Day	47% 17% 25% 10% 1	4256
	11th to 20th	Day	35% 17% 31% 16% 1	3934
	21st to end	Day	42% 18% 35% 5% 1	4023
September	1st to 10th	Day	50% 18% 21% 10% 1	2200
	11th to 20th	Day	51% 18% 21% 9% 2	2200
	21st to end	Day	55% 14% 16% 10% 5%	2200
October	1st to 10th	Day	48% 13% 20% 14% 4 1	3425
	11th to 20th	Day	48% 9% 16% 17% 9% 1	3240
	21st to end	Day	36% 7% 11% 27% 14% 4	3538
November	1st to 10th	Day	39% 9% 13% 17% 17% 6%	2057
	11th to 20th	Day	45% 8% 9% 17% 15% 5%	2100
	21st to end	Day	38% 8% 12% 18% 21% 4	2100
December	1st to 10th	Day	27% 7% 13% 29% 19% 4	3308
	11th to 20th	Day	37% 8% 12% 20% 18% 4	3460
	21st to end	Day	46% 8% 14% 15% 15% 2	4565
January	1st to 10th	Night	27% 7% 12% 30% 19% 5%	7636
	11th to 20th	Night	49% 7% 18% 16% 8% 3	7680
	21st to end	Night	39% 7% 10% 29% 12% 4	8301
February	1st to 10th	Night	30% 7% 13% 26% 17% 6%	2520
	11th to 20th	Night	24% 7% 12% 28% 22% 6%	2325
	21st to end	Night	24% 8% 11% 24% 26% 7%	1903
March	1st to 10th	Night	28% 8% 14% 23% 23% 4	5028
	11th to 20th	Night	20% 7% 19% 26% 17% 10%	5090
	21st to end	Night	29% 8% 14% 27% 20% 3	5588
April	1st to 10th	Night	20% 7% 19% 33% 17% 3	2310
	11th to 20th	Night	27% 8% 17% 22% 21% 5%	2179
	21st to end	Night	26% 8% 16% 27% 20% 3	2100
May	1st to 10th	Night	19% 7% 23% 37% 13% 2	5863
	11th to 20th	Night	28% 10% 23% 28% 10% 1	5831
	21st to end	Night	28% 14% 31% 22% 5%	5771
June	1st to 10th	Night	50% 13% 22% 13% 2	1890
	11th to 20th	Night	39% 14% 27% 17% 3	1890
	21st to end	Night	38% 17% 32% 12% 1	1890
July	1st to 10th	Night	36% 13% 39% 12%	5235
	11th to 20th	Night	32% 12% 41% 15%	5226
	21st to end	Night	25% 11% 37% 24% 2	5606
August	1st to 10th	Night	36% 16% 30% 15% 2	3052
	11th to 20th	Night	29% 17% 35% 19%	3268
	21st to end	Night	37% 20% 35% 6% 1	3868
September	1st to 10th	Night	41% 19% 27% 12% 1	2200
	11th to 20th	Night	44% 18% 24% 12% 2	2200
	21st to end	Night	49% 15% 16% 13% 6%	2200
October	1st to 10th	Night	48% 14% 18% 16% 3 1	3753
	11th to 20th	Night	46% 10% 18% 14% 11% 1	3899
	21st to end	Night	37% 8% 11% 27% 14% 3	4252
November	1st to 10th	Night	35% 9% 13% 19% 18% 6%	2461
	11th to 20th	Night	40% 9% 9% 22% 15% 5%	2520
	21st to end	Night	35% 8% 14% 20% 18% 5%	2520
December	1st to 10th	Night	28% 8% 14% 30% 17% 3	4074
	11th to 20th	Night	41% 8% 10% 20% 15% 5%	4954
	21st to end	Night	46% 9% 15% 16% 13% 2	6633

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% 10% 39% 29% 12% 2	330
	11th to 20th	Day	16% 13% 33% 28% 9% 1	330
	21st to end	Day	10% 13% 47% 25% 4 1	363
February	1st to 10th	Day	8% 10% 33% 35% 12% 2	330
	11th to 20th	Day	7% 6% 29% 39% 18% 2	463
	21st to end	Day	5% 7% 26% 38% 22% 2	446
March	1st to 10th	Day	5% 9% 32% 35% 16% 2	540
	11th to 20th	Day	7% 9% 32% 34% 16% 2	540
	21st to end	Day	5% 5% 30% 36% 19% 4	594
April	1st to 10th	Day	3 8% 36% 31% 18% 5%	540
	11th to 20th	Day	8% 7% 30% 31% 19% 5%	532
	21st to end	Day	5% 5% 27% 34% 22% 8%	540
May	1st to 10th	Day	4 2 23% 35% 24% 12%	543
	11th to 20th	Day	8% 4 21% 31% 25% 10%	567
	21st to end	Day	9% 3 11% 32% 27% 19%	617
June	1st to 10th	Day	14% 4 11% 26% 27% 18%	570
	11th to 20th	Day	8% 3 8% 25% 28% 28%	570
	21st to end	Day	4 2 6% 20% 28% 40%	570
July	1st to 10th	Day	5% 1 4 17% 32% 41%	565
	11th to 20th	Day	4 1 5% 20% 33% 36%	564
	21st to end	Day	4 1 5% 22% 28% 40%	620
August	1st to 10th	Day	6% 2 7% 18% 28% 39%	566
	11th to 20th	Day	5% 1 5% 19% 30% 40%	535
	21st to end	Day	9% 1 7% 21% 29% 32%	583
September	1st to 10th	Day	8% 3 5% 18% 27% 39%	530
	11th to 20th	Day	7% 1 8% 18% 30% 36%	530
	21st to end	Day	12% 2 11% 22% 27% 26%	530
October	1st to 10th	Day	10% 4 12% 36% 26% 12%	530
	11th to 20th	Day	11% 3 19% 31% 26% 11%	530
	21st to end	Day	9% 7% 21% 34% 22% 7%	583
November	1st to 10th	Day	8% 6% 25% 35% 21% 5%	535
	11th to 20th	Day	11% 6% 24% 33% 22% 5%	540
	21st to end	Day	6% 4 28% 35% 24% 3	540
December	1st to 10th	Day	11% 5% 23% 37% 21% 3	514
	11th to 20th	Day	6% 8% 32% 37% 14% 3	330
	21st to end	Day	10% 7% 33% 33% 12% 5%	363
January	1st to 10th	Night	8% 7% 22% 38% 23% 2	840
	11th to 20th	Night	9% 7% 30% 36% 17% 1	840
	21st to end	Night	9% 8% 29% 41% 13% 1	924
February	1st to 10th	Night	9% 7% 24% 40% 18% 2	840
	11th to 20th	Night	6% 6% 25% 42% 20% 1	823
	21st to end	Night	6% 5% 23% 47% 19% 1	692
March	1st to 10th	Night	5% 4 30% 42% 18% 1	840
	11th to 20th	Night	8% 6% 22% 43% 19% 1	840
	21st to end	Night	6% 5% 26% 42% 20% 2	924
April	1st to 10th	Night	4 3 22% 49% 20% 2	840
	11th to 20th	Night	5% 4 25% 45% 20% 1	831
	21st to end	Night	8% 2 20% 44% 24% 2	840
May	1st to 10th	Night	4 2 18% 44% 30% 2	840
	11th to 20th	Night	9% 4 15% 42% 28% 3	829
	21st to end	Night	7% 2 7% 30% 45% 8%	683
June	1st to 10th	Night	12% 2 7% 21% 44% 14%	630
	11th to 20th	Night	7% 1 4 14% 47% 27%	630
	21st to end	Night	5% 1 4 10% 41% 39%	630
July	1st to 10th	Night	6% 3 10% 40% 42%	615
	11th to 20th	Night	5% 3 11% 42% 37%	612
	21st to end	Night	3 1 2 9% 43% 42%	690
August	1st to 10th	Night	7% 3 12% 44% 34%	659
	11th to 20th	Night	3 1 4 14% 41% 37%	807
	21st to end	Night	9% 1 6% 18% 38% 28%	880
September	1st to 10th	Night	8% 1 6% 15% 38% 33%	800
	11th to 20th	Night	10% 1 8% 17% 37% 28%	800
	21st to end	Night	11% 2 9% 19% 37% 23%	800
October	1st to 10th	Night	10% 1 13% 34% 33% 9%	800
	11th to 20th	Night	10% 3 11% 32% 36% 9%	800
	21st to end	Night	12% 2 17% 36% 26% 6%	880
November	1st to 10th	Night	9% 3 24% 34% 26% 3	820
	11th to 20th	Night	10% 4 19% 35% 28% 4	840
	21st to end	Night	7% 4 22% 40% 24% 3	840
December	1st to 10th	Night	7% 4 19% 38% 30% 2	840
	11th to 20th	Night	9% 6% 21% 37% 23% 3	840
	21st to end	Night	8% 4 24% 37% 24% 3	924

Page generated at 2026-08-12 03:37:16. Version ac07

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