Sunset Bay
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sunset Bay 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	15% 10% 4 9% 5% 5% 5% 4 5% 7% 32%	144453
	Night	15% 12% 4 10% 6% 5% 5% 4 5% 6% 30%	137401
Transparency	Day	51% 5% 11% 21% 11% 1	147959
	Night	53% 5% 11% 20% 10% 1	141984
Seeing	Day	24% 16% 31% 18% 8% 2	16660
	Night	23% 14% 31% 24% 8% 1	25731

By Month

Percentage Cloud Cover NE (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	11% 15% 6% 14% 8% 7% 5% 3 5% 6% 19%	4661
	11th to 20th	Day	10% 18% 7% 16% 8% 6% 4 3 4 4 18%	4650
	21st to end	Day	9% 17% 7% 16% 9% 8% 5% 3 3 4 18%	5500
February	1st to 10th	Day	11% 16% 5% 14% 8% 6% 5% 4 4 5% 23%	1890
	11th to 20th	Day	10% 15% 6% 11% 7% 5% 5% 4 5% 5% 26%	1843
	21st to end	Day	17% 14% 3 12% 6% 5% 5% 4 5% 5% 24%	1699
March	1st to 10th	Day	17% 14% 5% 12% 6% 4 4 3 4 6% 25%	4688
	11th to 20th	Day	10% 14% 5% 10% 7% 5% 5% 4 6% 6% 28%	5090
	21st to end	Day	15% 13% 4 8% 5% 4 4 2 3 5% 37%	5815
April	1st to 10th	Day	20% 14% 4 7% 5% 4 4 3 4 6% 30%	2520
	11th to 20th	Day	21% 11% 3 7% 4 3 4 3 4 4 36%	2494
	21st to end	Day	21% 10% 4 8% 4 4 4 3 4 5% 33%	2668
May	1st to 10th	Day	19% 10% 3 8% 5% 5% 5% 4 6% 7% 29%	8237
	11th to 20th	Day	15% 9% 3 8% 5% 5% 5% 4 6% 8% 33%	8221
	21st to end	Day	19% 9% 3 6% 4 4 4 4 5% 6% 36%	9060
June	1st to 10th	Day	19% 10% 3 7% 5% 4 5% 4 6% 6% 31%	2940
	11th to 20th	Day	14% 7% 3 6% 4 5% 5% 5% 7% 7% 38%	2940
	21st to end	Day	16% 7% 2 7% 6% 5% 4 5% 6% 8% 34%	2940
July	1st to 10th	Day	12% 6% 2 7% 4 6% 6% 4 6% 10% 37%	8280
	11th to 20th	Day	9% 5% 2 6% 4 5% 5% 4 7% 9% 46%	8280
	21st to end	Day	11% 5% 2 6% 4 4 5% 5% 7% 10% 41%	8993
August	1st to 10th	Day	14% 8% 3 8% 5% 5% 5% 4 6% 7% 36%	4535
	11th to 20th	Day	16% 7% 2 7% 5% 4 4 4 6% 8% 37%	4226
	21st to end	Day	12% 5% 2 6% 4 5% 5% 4 7% 10% 39%	4317
September	1st to 10th	Day	14% 6% 2 6% 5% 5% 5% 4 6% 8% 41%	2400
	11th to 20th	Day	12% 5% 1 6% 4 5% 4 4 5% 8% 46%	2400
	21st to end	Day	17% 7% 3 7% 4 5% 4 4 4 7% 39%	2117
October	1st to 10th	Day	19% 9% 3 7% 4 4 4 3 5% 6% 35%	2000
	11th to 20th	Day	19% 11% 3 7% 5% 5% 5% 4 4 7% 29%	2000
	21st to end	Day	24% 11% 4 9% 4 4 5% 3 4 5% 26%	2080
November	1st to 10th	Day	14% 11% 4 8% 5% 4 4 4 5% 7% 33%	1815
	11th to 20th	Day	16% 13% 5% 9% 7% 5% 4 3 5% 5% 27%	1680
	21st to end	Day	19% 16% 5% 11% 6% 5% 5% 3 5% 6% 20%	1680
December	1st to 10th	Day	20% 21% 6% 14% 7% 5% 4 3 5% 5% 12%	2680
	11th to 20th	Day	14% 17% 6% 13% 7% 5% 4 3 4 3 24%	3066
	21st to end	Day	21% 17% 4 16% 5% 5% 3 3 3 4 19%	4048
January	1st to 10th	Night	12% 16% 8% 16% 7% 6% 5% 3 4 4 18%	8234
	11th to 20th	Night	11% 17% 6% 15% 8% 6% 6% 4 5% 5% 18%	8235
	21st to end	Night	10% 16% 6% 12% 8% 8% 5% 5% 5% 5% 21%	8666
February	1st to 10th	Night	11% 16% 5% 12% 7% 5% 5% 4 5% 6% 23%	2730
	11th to 20th	Night	11% 14% 6% 12% 7% 5% 5% 3 4 5% 28%	2663
	21st to end	Night	13% 13% 5% 9% 8% 5% 5% 4 5% 6% 27%	2107
March	1st to 10th	Night	19% 14% 5% 11% 6% 4 3 3 3 3 28%	5355
	11th to 20th	Night	10% 12% 4 9% 6% 5% 6% 4 7% 7% 29%	5089
	21st to end	Night	16% 11% 4 9% 6% 5% 4 3 5% 6% 31%	5357
April	1st to 10th	Night	20% 12% 4 8% 5% 4 3 3 4 6% 32%	2100
	11th to 20th	Night	19% 10% 3 8% 4 4 4 3 4 6% 36%	2075
	21st to end	Night	20% 10% 3 7% 5% 5% 4 3 4 6% 34%	1969
May	1st to 10th	Night	18% 10% 4 8% 5% 5% 5% 4 5% 7% 28%	4660
	11th to 20th	Night	13% 8% 3 7% 4 4 4 4 6% 7% 40%	4639
	21st to end	Night	18% 7% 3 7% 5% 4 5% 4 6% 8% 34%	5114
June	1st to 10th	Night	15% 7% 3 7% 5% 5% 4 5% 7% 8% 35%	1680
	11th to 20th	Night	13% 6% 3 7% 3 4 4 4 5% 7% 43%	1680
	21st to end	Night	16% 7% 2 7% 5% 4 5% 4 5% 8% 36%	1680
July	1st to 10th	Night	10% 7% 2 7% 5% 5% 5% 4 7% 10% 38%	4680
	11th to 20th	Night	10% 5% 2 6% 4 3 4 4 6% 8% 48%	4680
	21st to end	Night	10% 5% 2 7% 5% 6% 5% 4 6% 7% 44%	5084
August	1st to 10th	Night	11% 6% 2 6% 5% 5% 5% 4 7% 8% 42%	2830
	11th to 20th	Night	13% 6% 3 7% 4 3 4 3 5% 7% 44%	2911
	21st to end	Night	12% 6% 2 7% 4 4 5% 4 6% 8% 43%	3547
September	1st to 10th	Night	11% 6% 2 6% 4 4 4 5% 6% 6% 47%	2000
	11th to 20th	Night	13% 5% 2 4 4 3 4 3 5% 6% 51%	2000
	21st to end	Night	16% 6% 2 6% 4 4 4 3 5% 6% 43%	2279
October	1st to 10th	Night	17% 9% 2 6% 4 3 4 4 5% 6% 41%	2400
	11th to 20th	Night	18% 11% 3 8% 5% 4 5% 3 5% 6% 32%	2400
	21st to end	Night	20% 10% 3 9% 5% 4 5% 4 5% 7% 28%	2760
November	1st to 10th	Night	16% 13% 3 8% 5% 4 5% 3 5% 6% 33%	2699
	11th to 20th	Night	16% 12% 4 9% 6% 5% 5% 4 5% 6% 29%	2940
	21st to end	Night	20% 15% 5% 10% 6% 4 4 4 5% 6% 22%	2940
December	1st to 10th	Night	17% 18% 5% 13% 6% 5% 5% 4 5% 5% 15%	4720
	11th to 20th	Night	20% 15% 5% 13% 6% 4 4 3 3 4 24%	5348
	21st to end	Night	23% 18% 5% 12% 5% 4 4 3 3 4 20%	7150

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	65% 5% 11% 13% 6%	4662
	11th to 20th	Day	69% 4 8% 9% 9% 1	4680
	21st to end	Day	70% 4 6% 11% 9% 1	5500
February	1st to 10th	Day	63% 5% 8% 17% 5% 1	1890
	11th to 20th	Day	58% 4 11% 17% 8% 1	1852
	21st to end	Day	62% 4 10% 12% 10% 1	1699
March	1st to 10th	Day	60% 4 9% 13% 12% 2	4691
	11th to 20th	Day	54% 6% 14% 15% 10% 2	5090
	21st to end	Day	51% 3 6% 19% 18% 3	5815
April	1st to 10th	Day	55% 4 7% 18% 13% 3	2520
	11th to 20th	Day	52% 4 7% 20% 15% 2	2494
	21st to end	Day	54% 5% 8% 16% 15% 3	2668
May	1st to 10th	Day	53% 5% 7% 21% 11% 2	8249
	11th to 20th	Day	48% 6% 12% 21% 12% 1	8225
	21st to end	Day	48% 5% 10% 23% 13% 1	9061
June	1st to 10th	Day	52% 6% 9% 19% 13% 1	2940
	11th to 20th	Day	43% 7% 11% 24% 13% 2	2940
	21st to end	Day	47% 6% 13% 22% 11% 1	2940
July	1st to 10th	Day	42% 6% 15% 26% 11%	8210
	11th to 20th	Day	33% 6% 12% 34% 14% 1	8205
	21st to end	Day	37% 6% 14% 33% 9%	8795
August	1st to 10th	Day	46% 6% 14% 23% 10% 1	4521
	11th to 20th	Day	44% 6% 14% 28% 8% 1	4265
	21st to end	Day	37% 7% 15% 31% 10% 1	4317
September	1st to 10th	Day	40% 7% 14% 29% 10%	2400
	11th to 20th	Day	35% 6% 13% 31% 15%	2400
	21st to end	Day	46% 5% 13% 25% 10%	2117
October	1st to 10th	Day	47% 5% 9% 31% 8%	3260
	11th to 20th	Day	47% 4 14% 22% 12% 1	3240
	21st to end	Day	53% 4 17% 20% 6% 1	3344
November	1st to 10th	Day	50% 5% 15% 22% 7% 1	1815
	11th to 20th	Day	57% 6% 12% 15% 9% 1	1680
	21st to end	Day	66% 5% 12% 10% 7% 1	1680
December	1st to 10th	Day	74% 5% 10% 8% 2	2680
	11th to 20th	Day	65% 4 9% 14% 7%	3066
	21st to end	Day	70% 3 7% 15% 5%	4048
January	1st to 10th	Night	68% 5% 9% 12% 5%	8234
	11th to 20th	Night	67% 4 9% 12% 7%	8280
	21st to end	Night	65% 4 7% 12% 12% 1	8666
February	1st to 10th	Night	60% 5% 10% 18% 6% 1	2730
	11th to 20th	Night	58% 4 10% 20% 8% 1	2665
	21st to end	Night	57% 5% 9% 18% 10% 1	2107
March	1st to 10th	Night	61% 3 7% 16% 10% 3	5365
	11th to 20th	Night	50% 7% 18% 16% 8% 2	5090
	21st to end	Night	52% 5% 8% 20% 12% 3	5357
April	1st to 10th	Night	54% 4 7% 19% 13% 2	2100
	11th to 20th	Night	51% 4 7% 22% 15% 1	2075
	21st to end	Night	52% 5% 6% 19% 16% 2	1969
May	1st to 10th	Night	54% 5% 9% 21% 8% 3	4666
	11th to 20th	Night	41% 7% 13% 23% 14% 2	4640
	21st to end	Night	46% 6% 12% 20% 15% 1	5115
June	1st to 10th	Night	44% 7% 11% 24% 13% 1	1680
	11th to 20th	Night	39% 6% 12% 24% 17% 2	1680
	21st to end	Night	45% 8% 13% 21% 13% 1	1680
July	1st to 10th	Night	40% 7% 17% 22% 14% 1	4640
	11th to 20th	Night	34% 5% 13% 33% 15% 1	4632
	21st to end	Night	38% 7% 11% 30% 13%	4967
August	1st to 10th	Night	38% 6% 15% 26% 13% 1	2820
	11th to 20th	Night	39% 5% 16% 31% 8% 1	2938
	21st to end	Night	37% 7% 15% 29% 11% 1	3547
September	1st to 10th	Night	35% 8% 13% 30% 14% 1	2000
	11th to 20th	Night	34% 5% 13% 36% 11% 1	2000
	21st to end	Night	42% 5% 12% 30% 11%	2279
October	1st to 10th	Night	39% 4 11% 31% 14%	3920
	11th to 20th	Night	46% 5% 16% 23% 10%	3899
	21st to end	Night	49% 5% 16% 21% 8%	4446
November	1st to 10th	Night	51% 5% 13% 21% 9% 1	2699
	11th to 20th	Night	55% 6% 11% 19% 8% 1	2940
	21st to end	Night	62% 6% 11% 16% 5%	2940
December	1st to 10th	Night	69% 5% 12% 11% 2	4720
	11th to 20th	Night	65% 4 7% 14% 10% 1	5348
	21st to end	Night	70% 4 6% 12% 9%	7150

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	22% 20% 39% 15% 4	220
	11th to 20th	Day	32% 19% 35% 10% 4	220
	21st to end	Day	22% 23% 36% 14% 5% 1	311
February	1st to 10th	Day	29% 25% 32% 11% 3	330
	11th to 20th	Day	29% 26% 40% 5% 1	330
	21st to end	Day	31% 24% 34% 10% 1	415
March	1st to 10th	Day	28% 21% 36% 12% 3	540
	11th to 20th	Day	31% 19% 30% 16% 2 1	540
	21st to end	Day	33% 22% 28% 12% 4	594
April	1st to 10th	Day	34% 21% 29% 12% 5%	540
	11th to 20th	Day	36% 19% 21% 15% 7% 2	532
	21st to end	Day	30% 18% 33% 11% 6% 1	540
May	1st to 10th	Day	27% 17% 32% 16% 7% 2	543
	11th to 20th	Day	21% 16% 32% 21% 7% 2	567
	21st to end	Day	23% 17% 31% 18% 8% 3	617
June	1st to 10th	Day	24% 14% 32% 20% 9% 1	570
	11th to 20th	Day	18% 16% 33% 21% 11% 2	570
	21st to end	Day	20% 14% 33% 21% 11% 1	570
July	1st to 10th	Day	12% 9% 34% 31% 12% 3	565
	11th to 20th	Day	14% 11% 33% 27% 12% 3	564
	21st to end	Day	15% 9% 34% 28% 11% 3	620
August	1st to 10th	Day	16% 6% 29% 31% 13% 4	566
	11th to 20th	Day	13% 8% 29% 29% 16% 5%	535
	21st to end	Day	16% 11% 31% 23% 14% 6%	583
September	1st to 10th	Day	16% 12% 29% 22% 13% 7%	530
	11th to 20th	Day	12% 9% 25% 26% 17% 11%	530
	21st to end	Day	20% 12% 30% 19% 12% 5%	530
October	1st to 10th	Day	23% 17% 25% 21% 9% 4	530
	11th to 20th	Day	27% 20% 32% 15% 5% 1	530
	21st to end	Day	32% 18% 28% 17% 4	517
November	1st to 10th	Day	29% 19% 30% 16% 6% 1	389
	11th to 20th	Day	28% 21% 33% 12% 3 3	220
	21st to end	Day	32% 25% 32% 7% 4	220
December	1st to 10th	Day	34% 15% 40% 9% 3	220
	11th to 20th	Day	31% 28% 31% 6% 3	220
	21st to end	Day	33% 19% 31% 14% 3	242
January	1st to 10th	Night	25% 20% 35% 16% 3	840
	11th to 20th	Night	28% 19% 30% 18% 5% 1	840
	21st to end	Night	20% 16% 35% 23% 5%	924
February	1st to 10th	Night	22% 19% 36% 20% 3	840
	11th to 20th	Night	21% 19% 41% 15% 3	823
	21st to end	Night	24% 21% 31% 21% 3	692
March	1st to 10th	Night	27% 19% 32% 19% 3	840
	11th to 20th	Night	25% 21% 31% 18% 4 1	840
	21st to end	Night	25% 17% 31% 21% 6%	924
April	1st to 10th	Night	28% 16% 31% 20% 4	840
	11th to 20th	Night	25% 19% 29% 20% 7%	831
	21st to end	Night	25% 16% 29% 23% 7% 1	709
May	1st to 10th	Night	21% 12% 30% 27% 10% 1	420
	11th to 20th	Night	20% 11% 35% 26% 7%	414
	21st to end	Night	14% 9% 38% 29% 11%	455
June	1st to 10th	Night	17% 10% 30% 36% 8%	420
	11th to 20th	Night	15% 14% 32% 30% 9% 1	420
	21st to end	Night	22% 8% 26% 33% 12%	420
July	1st to 10th	Night	9% 7% 32% 38% 13% 1	410
	11th to 20th	Night	12% 7% 29% 39% 12% 1	408
	21st to end	Night	10% 4 30% 42% 12% 1	460
August	1st to 10th	Night	11% 4 26% 38% 20%	517
	11th to 20th	Night	11% 4 24% 38% 19% 3	606
	21st to end	Night	13% 7% 29% 32% 17% 2	854
September	1st to 10th	Night	15% 8% 30% 30% 16% 1	800
	11th to 20th	Night	17% 5% 22% 30% 22% 4	800
	21st to end	Night	20% 9% 25% 24% 20% 2	800
October	1st to 10th	Night	23% 11% 27% 26% 13% 2	800
	11th to 20th	Night	26% 14% 32% 20% 8% 1	800
	21st to end	Night	28% 16% 28% 22% 6%	880
November	1st to 10th	Night	28% 15% 30% 21% 6%	820
	11th to 20th	Night	25% 18% 33% 15% 8% 1	840
	21st to end	Night	32% 18% 27% 19% 5%	840
December	1st to 10th	Night	30% 17% 34% 16% 3	840
	11th to 20th	Night	28% 20% 30% 19% 3	840
	21st to end	Night	34% 16% 30% 16% 4	924

Page generated at 2026-08-12 03:37:19. Version ac07

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