Sunset Farm Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sunset Farm Observatory clear sky chart from 2005-11-06 to 2025-09-19.

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	10% 9% 3 8% 6% 5% 5% 4 6% 8% 35%	135555
	Night	8% 9% 4 10% 6% 5% 5% 4 6% 7% 35%	130850
Transparency	Day	45% 7% 15% 23% 9%	130124
	Night	46% 7% 14% 23% 10% 1	127287
Seeing	Day	14% 11% 32% 27% 12% 3	17480
	Night	13% 10% 34% 30% 10% 1	27314

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	4 13% 5% 14% 9% 9% 7% 6% 8% 9% 17%	3178
	11th to 20th	Day	4 8% 3 12% 11% 8% 9% 9% 14% 9% 13%	3427
	21st to end	Day	5% 10% 5% 15% 7% 6% 8% 4 6% 8% 25%	4004
February	1st to 10th	Day	8% 13% 5% 12% 7% 6% 6% 5% 8% 7% 22%	1673
	11th to 20th	Day	5% 10% 4 10% 8% 8% 7% 6% 9% 8% 24%	1754
	21st to end	Day	6% 10% 5% 13% 7% 6% 7% 6% 7% 8% 24%	1640
March	1st to 10th	Day	8% 11% 4 10% 6% 6% 6% 4 8% 9% 28%	4500
	11th to 20th	Day	8% 11% 5% 12% 11% 6% 5% 4 6% 9% 23%	4980
	21st to end	Day	9% 14% 5% 15% 7% 5% 6% 3 6% 6% 24%	5464
April	1st to 10th	Day	9% 12% 5% 9% 5% 4 5% 4 5% 7% 35%	2440
	11th to 20th	Day	13% 13% 4 10% 5% 5% 5% 4 6% 5% 31%	2569
	21st to end	Day	17% 11% 4 8% 5% 4 4 3 6% 6% 32%	2600
May	1st to 10th	Day	14% 8% 3 5% 3 3 3 3 5% 8% 45%	5900
	11th to 20th	Day	20% 10% 3 7% 4 4 5% 4 5% 8% 30%	6678
	21st to end	Day	13% 7% 2 6% 4 4 4 3 5% 6% 43%	7427
June	1st to 10th	Day	13% 6% 2 6% 5% 5% 5% 4 6% 9% 38%	3000
	11th to 20th	Day	14% 6% 3 6% 6% 5% 5% 4 7% 8% 37%	3000
	21st to end	Day	11% 7% 2 6% 5% 6% 6% 5% 7% 9% 37%	3000
July	1st to 10th	Day	8% 4 2 6% 5% 5% 6% 5% 7% 11% 42%	6799
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 4 6% 8% 44%	6800
	21st to end	Day	8% 5% 2 6% 5% 4 5% 5% 7% 10% 43%	7048
August	1st to 10th	Day	9% 7% 3 7% 5% 4 4 5% 6% 9% 41%	6109
	11th to 20th	Day	11% 6% 2 5% 3 4 4 4 7% 9% 45%	5821
	21st to end	Day	7% 5% 2 5% 3 4 4 4 6% 9% 51%	6490
September	1st to 10th	Day	11% 6% 2 4 4 3 4 4 6% 7% 50%	2443
	11th to 20th	Day	15% 7% 2 6% 4 4 4 4 5% 8% 40%	2178
	21st to end	Day	13% 7% 2 5% 4 4 4 3 6% 9% 42%	2090
October	1st to 10th	Day	10% 8% 3 6% 3 3 4 4 4 6% 49%	3299
	11th to 20th	Day	10% 10% 3 7% 4 4 5% 3 6% 9% 40%	2997
	21st to end	Day	12% 11% 4 8% 5% 5% 4 3 6% 8% 33%	3091
November	1st to 10th	Day	14% 13% 5% 11% 5% 4 4 3 5% 6% 29%	1680
	11th to 20th	Day	9% 13% 6% 15% 8% 5% 4 4 5% 6% 26%	1600
	21st to end	Day	9% 15% 4 13% 9% 6% 5% 3 4 6% 26%	1445
December	1st to 10th	Day	9% 13% 6% 15% 9% 11% 7% 4 5% 6% 15%	2290
	11th to 20th	Day	9% 12% 5% 13% 10% 7% 6% 3 5% 8% 22%	2643
	21st to end	Day	7% 10% 5% 14% 9% 7% 6% 4 5% 6% 29%	3498
January	1st to 10th	Night	4 11% 5% 15% 10% 7% 7% 6% 8% 9% 19%	6787
	11th to 20th	Night	4 10% 5% 13% 8% 8% 9% 8% 8% 9% 18%	6552
	21st to end	Night	6% 14% 7% 15% 7% 6% 5% 4 6% 7% 23%	6974
February	1st to 10th	Night	7% 12% 4 11% 8% 7% 6% 6% 7% 6% 25%	2726
	11th to 20th	Night	5% 8% 4 8% 7% 7% 7% 5% 8% 9% 32%	2533
	21st to end	Night	6% 11% 4 9% 6% 6% 5% 4 7% 8% 32%	2006
March	1st to 10th	Night	6% 10% 4 9% 7% 6% 6% 5% 7% 8% 32%	5360
	11th to 20th	Night	8% 10% 4 11% 8% 5% 6% 4 6% 7% 32%	4980
	21st to end	Night	8% 12% 6% 16% 6% 5% 4 4 5% 5% 28%	5467
April	1st to 10th	Night	7% 8% 4 8% 5% 5% 5% 5% 6% 7% 40%	1958
	11th to 20th	Night	12% 12% 4 9% 5% 4 5% 4 5% 6% 33%	1779
	21st to end	Night	14% 8% 4 8% 5% 5% 4 3 5% 7% 37%	1800
May	1st to 10th	Night	15% 7% 3 6% 4 4 4 3 5% 6% 42%	4080
	11th to 20th	Night	16% 11% 3 7% 4 5% 4 4 5% 7% 35%	3219
	21st to end	Night	12% 6% 2 4 3 4 3 3 5% 6% 51%	3470
June	1st to 10th	Night	10% 4 2 7% 5% 4 4 4 7% 8% 45%	1400
	11th to 20th	Night	12% 6% 2 6% 4 4 4 3 8% 8% 43%	1400
	21st to end	Night	10% 5% 1 5% 3 4 5% 3 7% 9% 47%	1400
July	1st to 10th	Night	7% 4 1 4 3 3 3 4 7% 10% 54%	3180
	11th to 20th	Night	6% 4 2 5% 3 3 4 4 6% 8% 54%	3180
	21st to end	Night	4 5% 2 6% 4 4 5% 4 8% 9% 48%	3824
August	1st to 10th	Night	8% 7% 2 7% 4 4 4 3 6% 8% 48%	3703
	11th to 20th	Night	8% 4 2 5% 4 4 5% 4 6% 9% 50%	4028
	21st to end	Night	8% 4 1 4 3 3 4 4 6% 8% 54%	4487
September	1st to 10th	Night	8% 5% 2 6% 3 4 3 4 6% 7% 54%	1954
	11th to 20th	Night	11% 5% 2 6% 3 4 4 3 6% 7% 49%	2178
	21st to end	Night	13% 8% 2 6% 4 4 4 3 5% 6% 45%	2090
October	1st to 10th	Night	11% 8% 3 6% 4 3 4 3 5% 7% 47%	3628
	11th to 20th	Night	9% 9% 2 6% 4 4 4 3 7% 9% 43%	3926
	21st to end	Night	7% 9% 3 9% 5% 5% 4 4 5% 7% 41%	4467
November	1st to 10th	Night	11% 11% 4 10% 5% 5% 5% 4 6% 8% 31%	2616
	11th to 20th	Night	10% 15% 5% 11% 7% 6% 4 4 5% 6% 28%	2800
	21st to end	Night	9% 14% 4 11% 8% 5% 5% 3 6% 7% 28%	2954
December	1st to 10th	Night	7% 12% 5% 15% 10% 10% 8% 4 6% 6% 18%	4900
	11th to 20th	Night	8% 13% 5% 15% 8% 8% 6% 3 5% 6% 21%	5564
	21st to end	Night	7% 10% 4 12% 10% 8% 6% 3 5% 6% 28%	7480

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	60% 6% 11% 16% 6%	3179
	11th to 20th	Day	55% 8% 11% 17% 9%	3427
	21st to end	Day	56% 6% 15% 19% 4	4004
February	1st to 10th	Day	56% 6% 11% 18% 8% 1	1673
	11th to 20th	Day	52% 8% 11% 18% 10% 1	1763
	21st to end	Day	54% 7% 13% 17% 9% 1	1640
March	1st to 10th	Day	46% 8% 18% 17% 10%	4500
	11th to 20th	Day	55% 8% 14% 17% 6%	4980
	21st to end	Day	58% 8% 10% 19% 5%	5464
April	1st to 10th	Day	47% 7% 14% 23% 8% 1	2440
	11th to 20th	Day	52% 6% 12% 19% 9% 1	2572
	21st to end	Day	51% 7% 15% 17% 8% 1	2600
May	1st to 10th	Day	36% 6% 14% 26% 17% 1	5900
	11th to 20th	Day	49% 8% 11% 23% 9% 1	6682
	21st to end	Day	38% 7% 9% 25% 19% 1	7428
June	1st to 10th	Day	41% 9% 18% 22% 10% 1	3000
	11th to 20th	Day	43% 9% 16% 23% 9% 1	3000
	21st to end	Day	40% 11% 20% 24% 6% 1	3000
July	1st to 10th	Day	33% 10% 26% 26% 4	6726
	11th to 20th	Day	35% 7% 20% 32% 5%	6719
	21st to end	Day	33% 9% 23% 26% 8%	6846
August	1st to 10th	Day	35% 8% 17% 26% 13% 1	4446
	11th to 20th	Day	33% 7% 19% 34% 7%	4210
	21st to end	Day	32% 6% 20% 33% 9%	4674
September	1st to 10th	Day	33% 6% 19% 30% 12% 1	2443
	11th to 20th	Day	42% 7% 17% 24% 9% 1	2178
	21st to end	Day	39% 7% 15% 30% 9%	2090
October	1st to 10th	Day	36% 5% 11% 35% 11% 1	3299
	11th to 20th	Day	40% 5% 11% 30% 13%	2997
	21st to end	Day	48% 6% 12% 25% 9%	3091
November	1st to 10th	Day	55% 8% 11% 17% 8% 1	1680
	11th to 20th	Day	58% 7% 15% 14% 6%	1600
	21st to end	Day	60% 7% 12% 15% 7%	1445
December	1st to 10th	Day	69% 6% 11% 10% 3	2290
	11th to 20th	Day	60% 6% 11% 14% 9% 1	2640
	21st to end	Day	54% 6% 10% 19% 11%	3498
January	1st to 10th	Night	57% 8% 11% 17% 7%	6787
	11th to 20th	Night	56% 9% 11% 18% 6%	6552
	21st to end	Night	58% 6% 11% 19% 5%	6974
February	1st to 10th	Night	54% 7% 12% 16% 10% 1	2726
	11th to 20th	Night	45% 6% 14% 21% 13% 1	2534
	21st to end	Night	47% 6% 13% 22% 11% 1	2006
March	1st to 10th	Night	42% 10% 17% 19% 10% 2	5360
	11th to 20th	Night	48% 7% 13% 19% 13% 1	4980
	21st to end	Night	54% 8% 10% 20% 8%	5467
April	1st to 10th	Night	40% 7% 13% 24% 14% 3	1958
	11th to 20th	Night	50% 7% 11% 18% 13% 1	1779
	21st to end	Night	46% 6% 14% 21% 12% 1	1800
May	1st to 10th	Night	39% 7% 12% 25% 16% 1	4080
	11th to 20th	Night	46% 8% 12% 20% 12% 2	3220
	21st to end	Night	32% 6% 10% 26% 21% 4	3470
June	1st to 10th	Night	34% 9% 18% 28% 10% 2	1400
	11th to 20th	Night	37% 7% 18% 25% 12% 1	1400
	21st to end	Night	32% 10% 21% 31% 6% 1	1400
July	1st to 10th	Night	24% 9% 29% 30% 6% 1	3145
	11th to 20th	Night	26% 8% 19% 39% 8% 1	3138
	21st to end	Night	29% 9% 25% 30% 7% 1	3713
August	1st to 10th	Night	29% 7% 16% 31% 16% 1	2690
	11th to 20th	Night	29% 8% 21% 32% 9% 1	2915
	21st to end	Night	31% 6% 17% 34% 11% 1	3234
September	1st to 10th	Night	28% 7% 15% 34% 15% 2	1954
	11th to 20th	Night	34% 7% 19% 26% 13% 1	2178
	21st to end	Night	39% 6% 12% 30% 12% 1	2090
October	1st to 10th	Night	36% 6% 13% 29% 15% 1	3628
	11th to 20th	Night	38% 5% 13% 29% 15%	3926
	21st to end	Night	41% 6% 13% 31% 8%	4467
November	1st to 10th	Night	50% 8% 14% 18% 9% 2	2616
	11th to 20th	Night	57% 7% 14% 14% 8% 1	2800
	21st to end	Night	54% 8% 14% 17% 6%	2954
December	1st to 10th	Night	64% 7% 14% 11% 4	4900
	11th to 20th	Night	62% 6% 11% 16% 5% 1	5566
	21st to end	Night	55% 7% 10% 19% 10% 1	7480

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	10% 11% 38% 28% 11% 2	300
	11th to 20th	Day	10% 13% 40% 26% 10% 1	300
	21st to end	Day	15% 11% 40% 23% 9% 2	330
February	1st to 10th	Day	15% 17% 38% 24% 5% 2	300
	11th to 20th	Day	7% 16% 42% 26% 8% 1	300
	21st to end	Day	7% 11% 49% 25% 8%	249
March	1st to 10th	Day	13% 12% 36% 32% 6% 1	300
	11th to 20th	Day	17% 14% 32% 25% 10% 2	300
	21st to end	Day	13% 15% 34% 28% 9% 2	330
April	1st to 10th	Day	11% 20% 33% 25% 8% 2	540
	11th to 20th	Day	20% 13% 32% 25% 9% 1	690
	21st to end	Day	21% 13% 30% 25% 10% 2	700
May	1st to 10th	Day	15% 13% 29% 28% 12% 2	703
	11th to 20th	Day	20% 11% 32% 24% 11% 2	725
	21st to end	Day	18% 9% 23% 31% 16% 3	789
June	1st to 10th	Day	13% 12% 30% 25% 14% 4	730
	11th to 20th	Day	14% 12% 36% 25% 11% 2	730
	21st to end	Day	13% 10% 31% 30% 14% 3	730
July	1st to 10th	Day	8% 6% 29% 30% 20% 7%	721
	11th to 20th	Day	12% 8% 30% 26% 19% 5%	718
	21st to end	Day	5% 6% 28% 37% 18% 6%	794
August	1st to 10th	Day	7% 8% 28% 31% 20% 6%	726
	11th to 20th	Day	12% 5% 24% 34% 18% 7%	715
	21st to end	Day	11% 9% 27% 31% 17% 5%	803
September	1st to 10th	Day	12% 10% 35% 26% 14% 3	573
	11th to 20th	Day	21% 11% 29% 27% 11% 1	324
	21st to end	Day	22% 10% 31% 26% 10% 1	300
October	1st to 10th	Day	20% 16% 27% 25% 12%	300
	11th to 20th	Day	15% 14% 42% 22% 6% 2	300
	21st to end	Day	21% 15% 36% 19% 8% 2	330
November	1st to 10th	Day	21% 14% 31% 24% 9% 1	300
	11th to 20th	Day	16% 15% 41% 21% 6%	300
	21st to end	Day	14% 17% 43% 21% 5%	300
December	1st to 10th	Day	13% 14% 37% 27% 7% 2	300
	11th to 20th	Day	15% 14% 44% 23% 3 1	300
	21st to end	Day	10% 11% 39% 27% 9% 4	330
January	1st to 10th	Night	11% 13% 37% 29% 8% 1	1000
	11th to 20th	Night	14% 12% 41% 28% 6%	1000
	21st to end	Night	13% 14% 41% 25% 6% 1	1100
February	1st to 10th	Night	12% 13% 36% 31% 7% 2	1000
	11th to 20th	Night	9% 11% 40% 29% 9% 2	979
	21st to end	Night	11% 12% 41% 27% 9% 1	686
March	1st to 10th	Night	13% 12% 34% 28% 11% 2	799
	11th to 20th	Night	10% 11% 42% 27% 10% 2	600
	21st to end	Night	15% 10% 29% 32% 10% 3	660
April	1st to 10th	Night	10% 11% 39% 30% 9% 1	600
	11th to 20th	Night	17% 9% 35% 30% 9%	593
	21st to end	Night	16% 9% 29% 34% 11% 1	600
May	1st to 10th	Night	17% 8% 33% 31% 10% 1	600
	11th to 20th	Night	17% 9% 32% 28% 14%	591
	21st to end	Night	19% 8% 27% 34% 12%	650
June	1st to 10th	Night	12% 9% 30% 37% 12% 1	600
	11th to 20th	Night	13% 6% 34% 32% 15%	600
	21st to end	Night	10% 6% 28% 38% 16% 1	600
July	1st to 10th	Night	7% 4 25% 41% 22% 2	585
	11th to 20th	Night	7% 5% 28% 40% 19% 1	582
	21st to end	Night	5% 5% 24% 45% 19% 2	657
August	1st to 10th	Night	8% 4 24% 43% 20% 1	597
	11th to 20th	Night	9% 5% 27% 37% 19% 3	594
	21st to end	Night	11% 5% 29% 38% 16% 1	660
September	1st to 10th	Night	11% 4 32% 35% 16% 2	600
	11th to 20th	Night	12% 9% 31% 37% 11% 1	594
	21st to end	Night	16% 8% 34% 29% 12% 1	570
October	1st to 10th	Night	20% 11% 32% 28% 8%	653
	11th to 20th	Night	18% 12% 33% 29% 9%	844
	21st to end	Night	17% 13% 37% 26% 8% 1	1045
November	1st to 10th	Night	16% 15% 35% 26% 8% 1	975
	11th to 20th	Night	15% 14% 40% 22% 7% 2	1000
	21st to end	Night	18% 16% 40% 21% 5% 1	1000
December	1st to 10th	Night	11% 14% 39% 28% 7% 1	1000
	11th to 20th	Night	14% 15% 39% 25% 6% 1	1000
	21st to end	Night	18% 9% 35% 28% 8% 1	1100

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