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 2025-04-11.

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% 11% 4 9% 5% 5% 4 4 5% 6% 31%	113566
	Night	15% 12% 4 10% 6% 5% 4 4 5% 6% 30%	114695
Transparency	Day	52% 5% 11% 20% 10% 1	117118
	Night	54% 5% 11% 19% 10% 1	119307
Seeing	Day	24% 16% 31% 19% 8% 3	14893
	Night	23% 14% 31% 23% 8% 1	24196

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	13% 15% 7% 14% 9% 7% 5% 3 4 5% 20%	3597
	11th to 20th	Day	11% 16% 6% 15% 8% 6% 4 4 5% 4 20%	3570
	21st to end	Day	11% 20% 8% 16% 6% 5% 4 3 4 5% 19%	4254
February	1st to 10th	Day	12% 16% 5% 14% 8% 6% 5% 4 4 5% 22%	1800
	11th to 20th	Day	10% 15% 6% 12% 7% 6% 4 4 4 5% 27%	1753
	21st to end	Day	17% 14% 3 11% 6% 5% 5% 4 5% 5% 25%	1620
March	1st to 10th	Day	17% 14% 5% 12% 6% 4 4 3 4 6% 25%	4586
	11th to 20th	Day	10% 14% 5% 10% 7% 5% 5% 4 6% 6% 29%	4980
	21st to end	Day	15% 13% 4 8% 5% 4 3 2 3 5% 37%	5689
April	1st to 10th	Day	20% 14% 3 7% 5% 4 4 3 5% 6% 30%	2400
	11th to 20th	Day	21% 10% 3 7% 4 3 4 3 4 4 37%	2254
	21st to end	Day	21% 10% 4 8% 4 4 4 3 4 5% 32%	2413
May	1st to 10th	Day	19% 9% 3 8% 5% 5% 4 4 6% 6% 32%	4426
	11th to 20th	Day	15% 9% 4 8% 6% 4 5% 4 6% 8% 32%	4398
	21st to end	Day	14% 8% 2 7% 5% 5% 5% 4 5% 8% 38%	4837
June	1st to 10th	Day	20% 10% 3 7% 5% 5% 5% 4 6% 6% 30%	2660
	11th to 20th	Day	14% 7% 3 6% 4 5% 5% 5% 7% 7% 38%	2660
	21st to end	Day	17% 7% 2 7% 6% 5% 4 4 6% 7% 33%	2660
July	1st to 10th	Day	11% 7% 3 8% 5% 6% 6% 4 6% 9% 34%	4440
	11th to 20th	Day	11% 6% 2 7% 5% 5% 5% 5% 7% 9% 39%	4440
	21st to end	Day	10% 6% 2 7% 4 4 4 5% 6% 8% 44%	4814
August	1st to 10th	Day	14% 8% 3 9% 5% 5% 5% 4 6% 7% 35%	4265
	11th to 20th	Day	16% 7% 2 7% 5% 4 4 4 6% 8% 37%	4044
	21st to end	Day	12% 5% 2 6% 4 5% 5% 5% 7% 10% 40%	4185
September	1st to 10th	Day	14% 6% 2 6% 5% 5% 4 4 5% 8% 40%	2280
	11th to 20th	Day	12% 5% 1 6% 4 5% 4 4 6% 8% 45%	2280
	21st to end	Day	17% 7% 3 7% 4 5% 4 4 5% 7% 39%	2012
October	1st to 10th	Day	19% 10% 3 7% 4 4 4 3 5% 6% 34%	1900
	11th to 20th	Day	20% 12% 3 8% 5% 5% 5% 3 4 7% 28%	1900
	21st to end	Day	24% 11% 4 9% 4 4 5% 3 4 5% 26%	1976
November	1st to 10th	Day	14% 11% 4 8% 5% 4 4 4 5% 7% 34%	1727
	11th to 20th	Day	16% 12% 5% 9% 7% 6% 5% 3 6% 5% 28%	1600
	21st to end	Day	19% 16% 5% 11% 6% 6% 5% 3 5% 6% 20%	1600
December	1st to 10th	Day	20% 21% 5% 13% 7% 5% 4 3 5% 5% 12%	2600
	11th to 20th	Day	15% 17% 6% 13% 7% 5% 4 3 3 3 24%	2986
	21st to end	Day	21% 18% 4 16% 5% 4 3 3 3 4 19%	3960
January	1st to 10th	Night	13% 17% 9% 15% 7% 5% 4 3 4 4 19%	6349
	11th to 20th	Night	10% 14% 6% 15% 9% 6% 6% 4 5% 5% 20%	6315
	21st to end	Night	12% 18% 7% 13% 7% 7% 4 4 4 5% 20%	6700
February	1st to 10th	Night	11% 16% 5% 12% 7% 5% 5% 4 5% 6% 23%	2600
	11th to 20th	Night	12% 14% 5% 11% 7% 5% 5% 3 4 5% 28%	2533
	21st to end	Night	13% 13% 4 9% 8% 5% 5% 4 5% 5% 28%	2010
March	1st to 10th	Night	20% 14% 5% 11% 6% 4 3 3 3 3 28%	5237
	11th to 20th	Night	10% 12% 4 9% 6% 5% 6% 4 7% 7% 29%	4979
	21st to end	Night	16% 11% 4 9% 6% 5% 4 3 5% 6% 31%	5241
April	1st to 10th	Night	19% 12% 4 8% 5% 3 4 3 4 6% 32%	2000
	11th to 20th	Night	19% 10% 3 7% 4 4 4 3 4 6% 36%	1882
	21st to end	Night	20% 10% 3 7% 5% 5% 4 3 4 6% 33%	1782
May	1st to 10th	Night	19% 10% 4 7% 5% 4 5% 4 6% 5% 33%	2514
	11th to 20th	Night	17% 9% 3 7% 4 4 4 4 6% 7% 36%	2495
	21st to end	Night	12% 6% 3 7% 5% 5% 5% 4 6% 8% 40%	2738
June	1st to 10th	Night	15% 7% 3 8% 5% 5% 4 5% 7% 8% 33%	1520
	11th to 20th	Night	13% 6% 3 7% 3 4 4 4 5% 7% 43%	1520
	21st to end	Night	17% 7% 2 7% 4 4 5% 3 5% 9% 36%	1520
July	1st to 10th	Night	9% 8% 3 9% 6% 6% 6% 4 6% 7% 36%	2520
	11th to 20th	Night	12% 5% 2 6% 5% 4 4 5% 6% 8% 43%	2520
	21st to end	Night	9% 6% 2 6% 5% 5% 5% 3 7% 7% 45%	2739
August	1st to 10th	Night	11% 6% 2 7% 5% 5% 4 4 7% 8% 41%	2660
	11th to 20th	Night	13% 6% 3 7% 4 4 4 3 6% 7% 43%	2785
	21st to end	Night	12% 6% 2 6% 4 5% 5% 4 6% 8% 44%	3438
September	1st to 10th	Night	11% 6% 2 6% 4 4 4 5% 5% 6% 47%	1900
	11th to 20th	Night	14% 5% 1 4 4 4 4 3 5% 6% 50%	1900
	21st to end	Night	16% 6% 2 6% 4 4 4 3 5% 6% 43%	2165
October	1st to 10th	Night	17% 9% 2 6% 4 3 4 4 5% 6% 39%	2280
	11th to 20th	Night	19% 11% 3 8% 5% 4 4 3 5% 6% 31%	2280
	21st to end	Night	21% 11% 3 9% 5% 4 5% 4 5% 7% 27%	2622
November	1st to 10th	Night	16% 13% 3 7% 4 4 5% 3 5% 6% 34%	2567
	11th to 20th	Night	16% 12% 4 9% 6% 5% 5% 4 5% 6% 29%	2800
	21st to end	Night	20% 15% 5% 10% 6% 4 4 3 5% 6% 23%	2800
December	1st to 10th	Night	18% 18% 5% 13% 6% 5% 5% 4 5% 5% 15%	4580
	11th to 20th	Night	20% 15% 5% 13% 6% 4 4 3 3 4 24%	5208
	21st to end	Night	24% 18% 5% 12% 5% 4 4 3 3 4 20%	6996

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	67% 6% 9% 12% 7%	3598
	11th to 20th	Day	66% 5% 10% 10% 8% 1	3600
	21st to end	Day	69% 4 7% 12% 8% 1	4254
February	1st to 10th	Day	63% 5% 9% 17% 5% 1	1800
	11th to 20th	Day	59% 5% 11% 16% 9% 1	1762
	21st to end	Day	62% 5% 10% 12% 10% 1	1620
March	1st to 10th	Day	60% 4 8% 13% 12% 2	4589
	11th to 20th	Day	53% 6% 14% 15% 10% 2	4980
	21st to end	Day	50% 3 6% 19% 18% 3	5689
April	1st to 10th	Day	54% 4 7% 19% 13% 3	2400
	11th to 20th	Day	51% 4 7% 21% 15% 2	2254
	21st to end	Day	54% 5% 8% 15% 15% 3	2413
May	1st to 10th	Day	50% 7% 8% 21% 12% 2	4440
	11th to 20th	Day	48% 7% 13% 20% 10% 1	4402
	21st to end	Day	42% 7% 12% 21% 15% 2	4837
June	1st to 10th	Day	52% 6% 10% 19% 11% 1	2660
	11th to 20th	Day	42% 7% 12% 24% 13% 2	2660
	21st to end	Day	48% 6% 13% 21% 11% 1	2660
July	1st to 10th	Day	44% 6% 13% 25% 11% 1	4370
	11th to 20th	Day	38% 8% 14% 30% 10%	4365
	21st to end	Day	37% 7% 12% 33% 12%	4661
August	1st to 10th	Day	47% 6% 14% 23% 10%	4251
	11th to 20th	Day	44% 6% 13% 28% 8% 1	4083
	21st to end	Day	37% 7% 15% 30% 11% 1	4185
September	1st to 10th	Day	40% 7% 14% 28% 11%	2280
	11th to 20th	Day	36% 6% 14% 29% 15%	2280
	21st to end	Day	46% 5% 14% 26% 9%	2012
October	1st to 10th	Day	48% 5% 9% 31% 7%	3160
	11th to 20th	Day	47% 4 14% 22% 12%	3140
	21st to end	Day	52% 4 17% 20% 6%	3240
November	1st to 10th	Day	49% 5% 16% 23% 7% 1	1727
	11th to 20th	Day	56% 6% 12% 15% 10% 1	1600
	21st to end	Day	66% 5% 12% 10% 7% 1	1600
December	1st to 10th	Day	74% 5% 11% 8% 2	2600
	11th to 20th	Day	65% 4 9% 14% 7%	2986
	21st to end	Day	70% 3 7% 15% 5%	3960
January	1st to 10th	Night	68% 6% 10% 12% 5%	6349
	11th to 20th	Night	65% 5% 12% 12% 6%	6360
	21st to end	Night	68% 4 7% 9% 11% 1	6700
February	1st to 10th	Night	60% 5% 10% 17% 6% 1	2600
	11th to 20th	Night	58% 4 10% 19% 8% 1	2535
	21st to end	Night	56% 5% 9% 19% 10% 1	2010
March	1st to 10th	Night	62% 3 7% 15% 10% 4	5247
	11th to 20th	Night	50% 7% 18% 16% 8% 2	4980
	21st to end	Night	52% 5% 8% 20% 12% 3	5241
April	1st to 10th	Night	54% 4 7% 19% 14% 2	2000
	11th to 20th	Night	50% 4 8% 22% 15% 1	1882
	21st to end	Night	53% 5% 7% 18% 16% 2	1782
May	1st to 10th	Night	50% 7% 10% 21% 10% 3	2520
	11th to 20th	Night	44% 8% 13% 23% 10% 2	2496
	21st to end	Night	39% 8% 14% 20% 17% 2	2739
June	1st to 10th	Night	45% 7% 12% 24% 11% 1	1520
	11th to 20th	Night	39% 7% 12% 24% 18% 2	1520
	21st to end	Night	45% 8% 14% 19% 13% 1	1520
July	1st to 10th	Night	44% 7% 14% 20% 14% 2	2480
	11th to 20th	Night	38% 5% 13% 34% 10%	2472
	21st to end	Night	36% 7% 14% 28% 14%	2651
August	1st to 10th	Night	39% 6% 15% 26% 13% 1	2650
	11th to 20th	Night	39% 5% 16% 31% 8%	2812
	21st to end	Night	37% 7% 15% 28% 12% 2	3438
September	1st to 10th	Night	35% 8% 13% 29% 15% 1	1900
	11th to 20th	Night	35% 5% 13% 35% 11% 1	1900
	21st to end	Night	41% 5% 13% 30% 10%	2165
October	1st to 10th	Night	40% 4 11% 31% 14%	3800
	11th to 20th	Night	47% 5% 16% 23% 10%	3779
	21st to end	Night	50% 5% 16% 21% 8%	4308
November	1st to 10th	Night	51% 5% 13% 21% 10% 1	2567
	11th to 20th	Night	54% 6% 11% 18% 9% 1	2800
	21st to end	Night	62% 6% 12% 16% 5%	2800
December	1st to 10th	Night	70% 5% 12% 11% 2	4580
	11th to 20th	Night	65% 4 7% 14% 10% 1	5208
	21st to end	Night	69% 4 6% 12% 9%	6996

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	23% 21% 38% 15% 5%	200
	11th to 20th	Day	31% 18% 37% 11% 4 1	200
	21st to end	Day	23% 24% 35% 12% 5% 1	283
February	1st to 10th	Day	30% 27% 30% 10% 3	300
	11th to 20th	Day	31% 26% 38% 4 1	300
	21st to end	Day	31% 24% 34% 10% 1	384
March	1st to 10th	Day	28% 20% 36% 13% 3	500
	11th to 20th	Day	29% 19% 32% 17% 2 1	500
	21st to end	Day	33% 22% 27% 13% 4	550
April	1st to 10th	Day	34% 21% 28% 12% 5%	500
	11th to 20th	Day	36% 18% 21% 16% 8% 2	452
	21st to end	Day	31% 17% 32% 12% 7% 1	460
May	1st to 10th	Day	26% 16% 32% 17% 7% 2	463
	11th to 20th	Day	21% 16% 31% 22% 8% 2	487
	21st to end	Day	20% 16% 33% 19% 9% 3	529
June	1st to 10th	Day	25% 13% 31% 21% 10%	490
	11th to 20th	Day	17% 15% 31% 22% 12% 2	490
	21st to end	Day	22% 12% 32% 21% 11% 1	490
July	1st to 10th	Day	12% 8% 31% 33% 13% 3	485
	11th to 20th	Day	14% 10% 33% 27% 13% 4	484
	21st to end	Day	15% 8% 35% 29% 11% 3	532
August	1st to 10th	Day	17% 6% 30% 30% 13% 5%	486
	11th to 20th	Day	14% 7% 29% 29% 16% 5%	479
	21st to end	Day	15% 11% 30% 24% 14% 6%	539
September	1st to 10th	Day	16% 12% 30% 22% 13% 7%	490
	11th to 20th	Day	13% 9% 26% 24% 17% 11%	490
	21st to end	Day	20% 13% 29% 19% 13% 6%	490
October	1st to 10th	Day	25% 17% 25% 20% 8% 4	490
	11th to 20th	Day	27% 21% 30% 15% 6% 1	490
	21st to end	Day	32% 19% 28% 16% 4	479
November	1st to 10th	Day	27% 19% 30% 17% 6% 1	361
	11th to 20th	Day	28% 22% 33% 12% 3 4	200
	21st to end	Day	31% 24% 33% 8% 4 1	200
December	1st to 10th	Day	35% 15% 38% 9% 4	200
	11th to 20th	Day	32% 29% 29% 7% 4 1	200
	21st to end	Day	30% 19% 32% 14% 4	220
January	1st to 10th	Night	26% 20% 35% 16% 3	800
	11th to 20th	Night	27% 19% 30% 18% 5% 1	800
	21st to end	Night	20% 15% 37% 23% 5%	880
February	1st to 10th	Night	23% 19% 36% 19% 3	800
	11th to 20th	Night	22% 19% 41% 15% 3	783
	21st to end	Night	23% 21% 31% 21% 3	660
March	1st to 10th	Night	27% 19% 32% 20% 4	800
	11th to 20th	Night	25% 20% 32% 18% 4 1	800
	21st to end	Night	24% 16% 31% 21% 7%	880
April	1st to 10th	Night	27% 17% 31% 21% 4	800
	11th to 20th	Night	25% 20% 29% 20% 7%	754
	21st to end	Night	25% 15% 29% 24% 7% 1	642
May	1st to 10th	Night	21% 10% 30% 27% 10% 1	380
	11th to 20th	Night	21% 10% 34% 27% 8% 1	374
	21st to end	Night	13% 9% 37% 30% 12%	411
June	1st to 10th	Night	16% 10% 30% 35% 8%	380
	11th to 20th	Night	14% 12% 31% 31% 10% 1	380
	21st to end	Night	23% 8% 26% 31% 12%	380
July	1st to 10th	Night	9% 7% 31% 38% 14% 1	370
	11th to 20th	Night	12% 6% 29% 40% 13% 1	368
	21st to end	Night	10% 4 30% 43% 13% 1	416
August	1st to 10th	Night	12% 5% 26% 37% 20%	467
	11th to 20th	Night	12% 4 24% 37% 20% 3	564
	21st to end	Night	13% 7% 29% 32% 17% 2	811
September	1st to 10th	Night	15% 8% 30% 30% 16% 1	760
	11th to 20th	Night	17% 6% 23% 30% 21% 4	760
	21st to end	Night	20% 10% 24% 24% 20% 2	760
October	1st to 10th	Night	24% 11% 26% 25% 13% 2	760
	11th to 20th	Night	27% 13% 31% 20% 8% 1	760
	21st to end	Night	28% 17% 28% 21% 6%	836
November	1st to 10th	Night	28% 14% 30% 21% 6% 1	780
	11th to 20th	Night	25% 18% 33% 15% 8% 1	800
	21st to end	Night	32% 17% 27% 19% 5%	800
December	1st to 10th	Night	31% 16% 34% 16% 3	800
	11th to 20th	Night	28% 20% 30% 20% 3 1	800
	21st to end	Night	34% 15% 30% 17% 4	880

Page generated at 2025-04-08 09:17:28. Version ac07

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