Sunset Field
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sunset Field clear sky chart from 2005-11-06 to 2025-08-05.

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	14% 6% 2 6% 4 5% 5% 4 6% 8% 40%	124983
	Night	14% 6% 2 6% 4 4 4 3 6% 7% 44%	123032
Transparency	Day	38% 6% 12% 22% 17% 4	128588
	Night	37% 5% 11% 23% 19% 5%	127496
Seeing	Day	20% 11% 24% 24% 15% 6%	16051
	Night	18% 9% 23% 32% 17% 2	26235

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	14% 5% 2 6% 5% 4 4 3 4 6% 48%	4056
	11th to 20th	Day	12% 7% 2 6% 4 3 4 3 4 5% 50%	4060
	21st to end	Day	17% 7% 3 6% 4 3 3 3 4 5% 46%	4466
February	1st to 10th	Day	12% 7% 3 7% 5% 5% 6% 4 5% 7% 38%	1800
	11th to 20th	Day	19% 7% 3 5% 5% 5% 4 3 5% 6% 39%	1753
	21st to end	Day	19% 7% 3 6% 4 4 4 3 5% 5% 39%	1755
March	1st to 10th	Day	16% 7% 2 6% 4 4 4 4 7% 8% 38%	4935
	11th to 20th	Day	11% 5% 2 5% 3 4 4 4 6% 7% 50%	4980
	21st to end	Day	15% 7% 2 8% 5% 4 5% 3 5% 6% 40%	5465
April	1st to 10th	Day	12% 7% 2 6% 4 4 4 3 6% 7% 46%	2304
	11th to 20th	Day	14% 6% 2 5% 4 4 5% 3 5% 7% 44%	2374
	21st to end	Day	15% 6% 2 7% 5% 5% 4 4 5% 6% 41%	2497
May	1st to 10th	Day	13% 5% 2 6% 5% 6% 7% 5% 8% 10% 31%	5862
	11th to 20th	Day	22% 7% 3 7% 5% 5% 5% 4 6% 8% 29%	5861
	21st to end	Day	15% 6% 2 5% 4 4 5% 4 7% 9% 39%	6433
June	1st to 10th	Day	10% 4 2 5% 5% 5% 5% 5% 7% 10% 42%	2623
	11th to 20th	Day	11% 5% 2 7% 5% 5% 5% 5% 7% 11% 39%	2726
	21st to end	Day	7% 5% 2 6% 6% 5% 5% 6% 7% 11% 41%	2600
July	1st to 10th	Day	7% 4 2 7% 5% 5% 7% 5% 8% 11% 39%	5900
	11th to 20th	Day	8% 6% 3 7% 6% 6% 7% 6% 9% 13% 29%	5900
	21st to end	Day	10% 6% 3 8% 6% 6% 6% 6% 8% 11% 30%	6424
August	1st to 10th	Day	15% 7% 2 7% 5% 5% 5% 5% 8% 11% 30%	4156
	11th to 20th	Day	9% 5% 2 6% 5% 6% 6% 5% 8% 11% 36%	4044
	21st to end	Day	8% 4 2 5% 4 4 5% 4 7% 9% 47%	4180
September	1st to 10th	Day	13% 5% 2 5% 3 5% 5% 4 5% 7% 46%	2280
	11th to 20th	Day	16% 7% 2 5% 4 4 4 4 6% 7% 41%	2105
	21st to end	Day	24% 7% 2 6% 4 3 4 3 5% 5% 36%	2076
October	1st to 10th	Day	16% 6% 2 4 3 4 3 3 4 6% 49%	1900
	11th to 20th	Day	15% 5% 2 4 2 3 3 3 5% 6% 51%	1900
	21st to end	Day	14% 6% 2 5% 3 3 3 3 5% 5% 50%	2090
November	1st to 10th	Day	12% 5% 2 5% 4 3 4 3 4 6% 54%	1957
	11th to 20th	Day	15% 5% 1 4 3 3 4 3 5% 7% 50%	1870
	21st to end	Day	13% 5% 2 5% 4 4 3 4 4 8% 48%	1618
December	1st to 10th	Day	12% 7% 3 6% 4 4 4 4 5% 6% 46%	2600
	11th to 20th	Day	20% 7% 2 4 3 3 4 3 5% 7% 43%	2990
	21st to end	Day	22% 7% 3 6% 4 4 4 4 7% 7% 34%	4443
January	1st to 10th	Night	12% 5% 2 4 3 4 3 3 5% 5% 53%	5889
	11th to 20th	Night	12% 6% 2 5% 3 3 3 3 4 5% 53%	5900
	21st to end	Night	17% 6% 2 5% 3 3 3 2 4 5% 50%	6490
February	1st to 10th	Night	14% 7% 2 7% 4 3 4 3 7% 6% 42%	2600
	11th to 20th	Night	20% 7% 2 6% 4 4 4 3 4 6% 40%	2528
	21st to end	Night	15% 7% 3 5% 5% 4 4 4 5% 7% 41%	1873
March	1st to 10th	Night	18% 6% 2 6% 4 4 4 4 5% 6% 43%	4918
	11th to 20th	Night	11% 4 1 4 3 4 3 4 5% 7% 54%	4980
	21st to end	Night	13% 6% 2 6% 5% 4 5% 4 6% 6% 43%	5467
April	1st to 10th	Night	15% 7% 2 6% 4 3 3 3 6% 7% 43%	2095
	11th to 20th	Night	13% 5% 2 5% 4 3 4 4 5% 8% 48%	1975
	21st to end	Night	15% 6% 2 6% 4 4 5% 4 6% 8% 43%	1919
May	1st to 10th	Night	16% 7% 3 6% 4 4 4 3 5% 9% 36%	4033
	11th to 20th	Night	23% 8% 3 8% 4 4 5% 3 5% 6% 31%	4032
	21st to end	Night	15% 6% 2 6% 4 4 5% 4 7% 9% 39%	4421
June	1st to 10th	Night	9% 5% 2 6% 4 5% 4 4 7% 10% 44%	1754
	11th to 20th	Night	10% 5% 2 7% 4 4 5% 4 7% 10% 43%	1649
	21st to end	Night	8% 4 2 5% 4 6% 5% 4 7% 10% 45%	1800
July	1st to 10th	Night	10% 6% 2 5% 5% 5% 6% 5% 7% 10% 40%	4058
	11th to 20th	Night	10% 7% 3 8% 5% 6% 7% 5% 8% 11% 29%	4057
	21st to end	Night	9% 7% 3 8% 6% 6% 6% 5% 7% 9% 32%	4427
August	1st to 10th	Night	17% 7% 2 7% 5% 4 4 4 7% 10% 33%	2861
	11th to 20th	Night	10% 6% 2 6% 5% 5% 6% 5% 7% 9% 38%	2785
	21st to end	Night	9% 5% 3 6% 4 4 4 4 7% 8% 47%	3442
September	1st to 10th	Night	11% 6% 3 6% 4 4 5% 4 5% 7% 47%	1900
	11th to 20th	Night	16% 6% 2 6% 5% 4 4 3 5% 6% 42%	2071
	21st to end	Night	24% 7% 3 7% 4 3 3 3 5% 5% 37%	2104
October	1st to 10th	Night	16% 7% 2 6% 3 3 3 2 5% 6% 48%	2280
	11th to 20th	Night	14% 6% 2 4 4 3 3 2 4 6% 51%	2280
	21st to end	Night	14% 3 1 4 3 3 3 2 6% 6% 55%	2508
November	1st to 10th	Night	14% 6% 2 5% 3 3 3 3 4 4 53%	2341
	11th to 20th	Night	14% 4 2 5% 4 3 4 4 4 5% 51%	2525
	21st to end	Night	13% 5% 2 5% 4 3 4 3 5% 6% 49%	2777
December	1st to 10th	Night	11% 6% 2 5% 3 3 4 3 6% 6% 51%	4580
	11th to 20th	Night	18% 8% 2 6% 4 4 4 3 5% 6% 40%	5205
	21st to end	Night	18% 5% 2 5% 4 4 5% 4 6% 8% 40%	6508

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	34% 4 6% 24% 25% 7%	4057
	11th to 20th	Day	35% 4 6% 18% 26% 10%	4060
	21st to end	Day	42% 3 6% 25% 21% 4	4466
February	1st to 10th	Day	42% 5% 12% 23% 16% 3	1800
	11th to 20th	Day	44% 3 8% 22% 17% 4	1762
	21st to end	Day	46% 4 9% 18% 17% 6%	1755
March	1st to 10th	Day	42% 4 8% 20% 17% 9%	4935
	11th to 20th	Day	32% 5% 8% 23% 22% 10%	4980
	21st to end	Day	44% 4 10% 19% 18% 5%	5465
April	1st to 10th	Day	35% 4 11% 23% 18% 8%	2304
	11th to 20th	Day	37% 4 11% 24% 19% 5%	2374
	21st to end	Day	42% 5% 12% 23% 14% 4	2497
May	1st to 10th	Day	43% 8% 14% 21% 12% 2	5897
	11th to 20th	Day	51% 6% 11% 19% 12% 1	5865
	21st to end	Day	39% 6% 15% 20% 17% 3	6447
June	1st to 10th	Day	34% 8% 17% 26% 15% 1	2623
	11th to 20th	Day	36% 8% 17% 21% 16% 1	2726
	21st to end	Day	32% 10% 15% 26% 14% 2	2600
July	1st to 10th	Day	35% 10% 20% 21% 10% 4	5835
	11th to 20th	Day	39% 12% 22% 20% 7%	5831
	21st to end	Day	42% 10% 16% 23% 10%	6240
August	1st to 10th	Day	44% 8% 18% 23% 6%	4143
	11th to 20th	Day	36% 9% 15% 28% 11% 1	4083
	21st to end	Day	30% 6% 21% 28% 13% 1	4180
September	1st to 10th	Day	35% 7% 13% 20% 21% 4	2280
	11th to 20th	Day	41% 5% 9% 26% 17% 3	2105
	21st to end	Day	49% 4 8% 18% 18% 2	2076
October	1st to 10th	Day	33% 3 8% 20% 28% 8%	3160
	11th to 20th	Day	24% 3 9% 23% 28% 13%	3140
	21st to end	Day	22% 3 12% 29% 23% 12%	3428
November	1st to 10th	Day	32% 4 9% 23% 27% 5%	1957
	11th to 20th	Day	34% 4 9% 24% 23% 8%	1870
	21st to end	Day	34% 5% 9% 25% 24% 4	1618
December	1st to 10th	Day	36% 5% 9% 23% 21% 7%	2600
	11th to 20th	Day	39% 5% 13% 20% 18% 5%	2986
	21st to end	Day	47% 4 10% 24% 14% 1	4443
January	1st to 10th	Night	30% 4 5% 24% 27% 10%	5889
	11th to 20th	Night	33% 3 5% 20% 29% 9%	5900
	21st to end	Night	38% 3 6% 27% 20% 6%	6490
February	1st to 10th	Night	39% 5% 12% 24% 18% 2	2600
	11th to 20th	Night	45% 3 9% 19% 18% 6%	2530
	21st to end	Night	40% 5% 10% 23% 16% 7%	1873
March	1st to 10th	Night	40% 4 6% 21% 20% 8%	4918
	11th to 20th	Night	28% 4 8% 20% 30% 10%	4980
	21st to end	Night	39% 5% 9% 22% 20% 5%	5467
April	1st to 10th	Night	38% 5% 11% 20% 21% 5%	2095
	11th to 20th	Night	34% 5% 13% 22% 21% 5%	1975
	21st to end	Night	37% 6% 12% 24% 18% 3	1919
May	1st to 10th	Night	44% 7% 17% 19% 11% 2	4060
	11th to 20th	Night	52% 6% 11% 20% 11% 1	4033
	21st to end	Night	39% 6% 17% 25% 11% 3	4429
June	1st to 10th	Night	33% 9% 15% 27% 16% 1	1754
	11th to 20th	Night	35% 8% 18% 21% 16% 2	1649
	21st to end	Night	31% 10% 16% 23% 17% 3	1800
July	1st to 10th	Night	38% 10% 19% 18% 13% 2	4015
	11th to 20th	Night	46% 11% 20% 17% 6% 1	4006
	21st to end	Night	45% 8% 16% 22% 9%	4297
August	1st to 10th	Night	45% 7% 19% 22% 7%	2850
	11th to 20th	Night	37% 8% 17% 26% 11% 1	2812
	21st to end	Night	33% 7% 16% 33% 9% 2	3442
September	1st to 10th	Night	36% 8% 13% 19% 18% 6%	1900
	11th to 20th	Night	41% 7% 8% 23% 18% 3	2071
	21st to end	Night	50% 5% 7% 20% 15% 3	2104
October	1st to 10th	Night	37% 4 9% 19% 25% 6%	3800
	11th to 20th	Night	25% 3 9% 22% 28% 14%	3779
	21st to end	Night	19% 2 9% 30% 28% 11%	4120
November	1st to 10th	Night	35% 3 8% 22% 25% 6%	2341
	11th to 20th	Night	35% 4 8% 22% 24% 7%	2525
	21st to end	Night	34% 4 9% 26% 22% 5%	2777
December	1st to 10th	Night	32% 3 10% 28% 24% 3	4580
	11th to 20th	Night	44% 4 11% 20% 15% 6%	5208
	21st to end	Night	39% 5% 11% 25% 18% 2	6508

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	32% 26% 22% 16% 2 1	300
	11th to 20th	Day	41% 15% 29% 15%	300
	21st to end	Day	32% 24% 28% 14% 3	330
February	1st to 10th	Day	25% 23% 35% 12% 5%	300
	11th to 20th	Day	44% 18% 29% 6% 3	300
	21st to end	Day	30% 23% 28% 17% 3	354
March	1st to 10th	Day	28% 17% 28% 23% 4	500
	11th to 20th	Day	24% 21% 29% 21% 5% 1	500
	21st to end	Day	25% 18% 30% 20% 6%	550
April	1st to 10th	Day	25% 22% 25% 19% 8% 1	500
	11th to 20th	Day	21% 15% 30% 21% 10% 3	492
	21st to end	Day	23% 13% 29% 21% 11% 3	500
May	1st to 10th	Day	19% 16% 32% 20% 9% 3	503
	11th to 20th	Day	23% 11% 24% 23% 15% 5%	527
	21st to end	Day	21% 8% 18% 27% 18% 8%	573
June	1st to 10th	Day	10% 7% 28% 30% 17% 8%	530
	11th to 20th	Day	12% 7% 21% 32% 19% 10%	530
	21st to end	Day	10% 6% 23% 32% 22% 7%	530
July	1st to 10th	Day	11% 3 17% 29% 26% 13%	525
	11th to 20th	Day	6% 2 16% 32% 31% 13%	524
	21st to end	Day	13% 2 19% 29% 24% 13%	576
August	1st to 10th	Day	17% 4 18% 24% 24% 14%	502
	11th to 20th	Day	12% 5% 18% 27% 24% 14%	479
	21st to end	Day	10% 3 12% 29% 26% 20%	539
September	1st to 10th	Day	13% 3 15% 25% 26% 17%	490
	11th to 20th	Day	19% 3 14% 24% 28% 11%	490
	21st to end	Day	27% 9% 19% 24% 17% 5%	490
October	1st to 10th	Day	22% 5% 20% 28% 20% 5%	490
	11th to 20th	Day	16% 13% 21% 28% 18% 3	490
	21st to end	Day	17% 12% 28% 29% 12% 2	539
November	1st to 10th	Day	17% 11% 26% 31% 12% 3	495
	11th to 20th	Day	22% 18% 25% 22% 11% 1	370
	21st to end	Day	27% 16% 25% 22% 9% 2	208
December	1st to 10th	Day	27% 19% 31% 18% 6%	200
	11th to 20th	Day	27% 16% 29% 19% 9% 2	200
	21st to end	Day	29% 15% 29% 19% 8% 1	325
January	1st to 10th	Night	24% 16% 31% 26% 3	800
	11th to 20th	Night	21% 16% 31% 31% 3	800
	21st to end	Night	20% 14% 37% 25% 3	880
February	1st to 10th	Night	18% 14% 35% 30% 3	800
	11th to 20th	Night	28% 17% 33% 22% 1	783
	21st to end	Night	21% 16% 34% 27% 2	660
March	1st to 10th	Night	18% 16% 31% 32% 4	800
	11th to 20th	Night	24% 13% 30% 26% 7% 1	800
	21st to end	Night	19% 13% 31% 31% 6%	880
April	1st to 10th	Night	18% 18% 30% 28% 6%	800
	11th to 20th	Night	16% 14% 28% 30% 11% 1	791
	21st to end	Night	18% 5% 28% 38% 11%	719
May	1st to 10th	Night	16% 7% 26% 36% 14% 1	600
	11th to 20th	Night	19% 6% 19% 37% 17% 2	591
	21st to end	Night	17% 4 10% 33% 31% 4	650
June	1st to 10th	Night	11% 2 13% 42% 31% 2	554
	11th to 20th	Night	10% 2 14% 39% 33% 2	449
	21st to end	Night	9% 2 10% 44% 31% 4	600
July	1st to 10th	Night	11% 1 8% 34% 40% 5%	585
	11th to 20th	Night	7% 1 8% 30% 47% 7%	582
	21st to end	Night	9% 2 8% 39% 37% 5%	657
August	1st to 10th	Night	15% 2 10% 34% 36% 3	579
	11th to 20th	Night	13% 2 10% 34% 37% 4	564
	21st to end	Night	12% 1 9% 34% 37% 7%	815
September	1st to 10th	Night	14% 2 11% 32% 35% 7%	760
	11th to 20th	Night	19% 3 10% 30% 33% 5%	760
	21st to end	Night	25% 4 17% 31% 21% 2	760
October	1st to 10th	Night	18% 7% 18% 28% 26% 4	760
	11th to 20th	Night	16% 9% 23% 34% 17% 1	760
	21st to end	Night	16% 7% 21% 40% 14% 2	836
November	1st to 10th	Night	18% 8% 22% 37% 14% 2	780
	11th to 20th	Night	17% 12% 25% 34% 11% 1	800
	21st to end	Night	17% 12% 25% 35% 10% 1	800
December	1st to 10th	Night	23% 14% 29% 28% 6%	800
	11th to 20th	Night	19% 10% 32% 35% 5%	800
	21st to end	Night	20% 13% 28% 35% 4	880

Page generated at 2025-08-02 01:32:28. Version ac07

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