Sisk Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sisk Observatory clear sky chart from 2005-11-06 to 2025-07-21.

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	12% 7% 2 6% 4 4 4 3 5% 7% 46%	119719
	Night	11% 7% 2 5% 4 3 3 3 5% 6% 50%	119628
Transparency	Day	36% 6% 11% 25% 18% 4	123372
	Night	34% 5% 11% 26% 20% 5%	124118
Seeing	Day	14% 9% 27% 28% 17% 5%	16533
	Night	14% 9% 31% 32% 13% 1	26361

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	18% 9% 4 6% 4 4 4 2 4 6% 39%	4077
	11th to 20th	Day	8% 10% 4 8% 5% 4 4 3 4 6% 45%	4080
	21st to end	Day	15% 11% 3 6% 3 3 3 2 4 5% 44%	4488
February	1st to 10th	Day	17% 9% 3 7% 5% 4 4 3 4 5% 39%	1800
	11th to 20th	Day	13% 10% 3 7% 4 4 3 2 4 5% 45%	1765
	21st to end	Day	13% 9% 3 6% 5% 4 4 5% 5% 6% 41%	1747
March	1st to 10th	Day	12% 7% 3 5% 3 4 3 3 4 6% 50%	4936
	11th to 20th	Day	10% 6% 2 4 3 3 3 3 5% 7% 54%	4980
	21st to end	Day	13% 10% 3 8% 5% 5% 5% 4 6% 7% 34%	5462
April	1st to 10th	Day	13% 8% 3 5% 4 4 4 4 5% 7% 44%	2295
	11th to 20th	Day	15% 6% 2 8% 4 4 4 3 5% 6% 42%	2374
	21st to end	Day	20% 8% 3 6% 4 5% 4 3 5% 6% 36%	2491
May	1st to 10th	Day	19% 8% 3 7% 4 4 4 3 5% 7% 37%	5862
	11th to 20th	Day	12% 6% 2 6% 4 4 4 3 6% 8% 44%	5862
	21st to end	Day	20% 7% 2 6% 4 3 4 3 6% 8% 37%	6434
June	1st to 10th	Day	7% 4 2 5% 5% 4 4 5% 6% 10% 48%	2600
	11th to 20th	Day	7% 4 2 6% 5% 4 5% 4 7% 8% 48%	2600
	21st to end	Day	5% 2 1 4 4 4 3 4 7% 10% 56%	2600
July	1st to 10th	Day	9% 6% 2 7% 4 4 5% 5% 6% 9% 41%	4250
	11th to 20th	Day	6% 4 2 5% 4 4 4 4 6% 11% 50%	4250
	21st to end	Day	6% 4 2 5% 5% 5% 5% 5% 8% 11% 43%	4466
August	1st to 10th	Day	5% 4 1 4 3 5% 4 4 6% 9% 55%	4104
	11th to 20th	Day	8% 5% 2 7% 5% 5% 5% 4 6% 8% 45%	4041
	21st to end	Day	5% 4 2 4 3 4 4 4 5% 8% 56%	4222
September	1st to 10th	Day	11% 5% 2 5% 3 3 3 3 5% 6% 54%	2280
	11th to 20th	Day	11% 4 2 6% 4 4 5% 3 5% 8% 48%	2091
	21st to end	Day	9% 4 2 5% 4 4 4 3 5% 7% 54%	2082
October	1st to 10th	Day	11% 4 2 5% 3 4 4 3 4 6% 55%	1900
	11th to 20th	Day	12% 6% 2 3 2 3 3 2 5% 5% 57%	1900
	21st to end	Day	18% 8% 2 5% 3 2 3 2 3 5% 50%	2090
November	1st to 10th	Day	10% 6% 2 5% 3 2 3 3 4 5% 57%	1958
	11th to 20th	Day	13% 7% 2 4 3 3 3 2 4 5% 54%	1857
	21st to end	Day	14% 7% 2 6% 3 2 2 2 4 6% 51%	1641
December	1st to 10th	Day	10% 8% 2 4 3 3 3 3 4 5% 54%	2610
	11th to 20th	Day	11% 7% 3 6% 3 3 3 2 3 5% 53%	3036
	21st to end	Day	20% 18% 7% 8% 4 4 3 2 4 5% 27%	4488
January	1st to 10th	Night	15% 9% 3 8% 5% 4 3 3 3 5% 42%	5888
	11th to 20th	Night	8% 7% 3 7% 5% 4 4 4 5% 5% 47%	5900
	21st to end	Night	16% 9% 4 7% 3 2 2 2 3 5% 47%	6490
February	1st to 10th	Night	15% 10% 2 6% 4 4 4 3 4 5% 42%	2600
	11th to 20th	Night	14% 9% 3 6% 3 4 3 3 4 6% 45%	2533
	21st to end	Night	14% 7% 3 5% 3 4 3 3 4 7% 47%	1886
March	1st to 10th	Night	13% 8% 3 7% 4 3 3 3 4 5% 49%	4919
	11th to 20th	Night	8% 6% 2 4 3 3 3 3 4 6% 57%	4980
	21st to end	Night	11% 6% 3 6% 4 5% 5% 4 5% 7% 45%	5467
April	1st to 10th	Night	11% 7% 2 5% 4 3 3 3 5% 6% 51%	2103
	11th to 20th	Night	14% 6% 3 5% 3 4 3 3 4 5% 50%	1975
	21st to end	Night	15% 7% 3 6% 4 4 4 3 5% 6% 44%	1905
May	1st to 10th	Night	16% 9% 3 5% 3 3 3 3 4 6% 45%	4053
	11th to 20th	Night	13% 7% 2 6% 3 3 3 3 6% 8% 45%	4053
	21st to end	Night	17% 6% 2 5% 3 3 3 3 5% 7% 47%	4443
June	1st to 10th	Night	6% 4 2 4 4 3 5% 4 6% 10% 53%	1800
	11th to 20th	Night	5% 3 2 4 3 4 5% 4 7% 11% 53%	1800
	21st to end	Night	4 3 1 4 4 3 5% 3 6% 9% 59%	1800
July	1st to 10th	Night	10% 6% 2 5% 3 3 3 4 7% 7% 49%	2940
	11th to 20th	Night	3 2 1 4 4 4 3 4 6% 10% 59%	2940
	21st to end	Night	6% 4 2 4 3 4 4 4 8% 11% 50%	3098
August	1st to 10th	Night	3 2 1 3 3 3 3 4 5% 8% 64%	2836
	11th to 20th	Night	7% 5% 2 6% 3 4 4 4 6% 8% 51%	2798
	21st to end	Night	3 3 1 4 3 3 3 3 5% 8% 64%	3435
September	1st to 10th	Night	9% 5% 2 4 3 3 4 3 4 6% 56%	1900
	11th to 20th	Night	10% 4 2 5% 5% 3 3 3 4 8% 53%	2085
	21st to end	Night	7% 5% 2 4 4 4 3 3 4 6% 59%	2090
October	1st to 10th	Night	8% 4 1 4 3 3 3 3 5% 6% 62%	2267
	11th to 20th	Night	13% 5% 1 4 3 2 2 3 4 5% 58%	2280
	21st to end	Night	15% 6% 1 5% 3 3 3 2 4 5% 53%	2508
November	1st to 10th	Night	11% 5% 1 4 2 3 2 2 3 5% 61%	2340
	11th to 20th	Night	10% 7% 3 5% 3 3 2 3 4 6% 53%	2540
	21st to end	Night	15% 6% 2 4 4 3 2 3 4 5% 54%	2753
December	1st to 10th	Night	8% 8% 2 4 3 3 3 3 5% 6% 55%	4580
	11th to 20th	Night	14% 7% 3 6% 3 3 3 2 4 5% 50%	5153
	21st to end	Night	17% 15% 5% 8% 4 4 4 3 4 4 32%	6490

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	47% 3 10% 24% 15% 1	4078
	11th to 20th	Day	41% 3 10% 25% 16% 4	4080
	21st to end	Day	44% 3 8% 20% 19% 7%	4488
February	1st to 10th	Day	48% 5% 9% 24% 11% 2	1800
	11th to 20th	Day	44% 4 10% 22% 16% 4	1774
	21st to end	Day	43% 4 7% 22% 16% 7%	1747
March	1st to 10th	Day	36% 4 7% 22% 20% 11%	4936
	11th to 20th	Day	30% 5% 9% 22% 23% 11%	4980
	21st to end	Day	47% 7% 13% 25% 7% 1	5464
April	1st to 10th	Day	39% 6% 12% 23% 17% 3	2295
	11th to 20th	Day	42% 6% 14% 21% 16% 2	2374
	21st to end	Day	50% 5% 9% 23% 11% 1	2491
May	1st to 10th	Day	46% 7% 9% 22% 15% 1	5900
	11th to 20th	Day	37% 8% 12% 25% 16% 3	5866
	21st to end	Day	44% 7% 14% 20% 14% 1	6447
June	1st to 10th	Day	30% 8% 16% 27% 18% 2	2600
	11th to 20th	Day	30% 8% 14% 29% 17% 2	2600
	21st to end	Day	22% 7% 17% 30% 19% 5%	2600
July	1st to 10th	Day	36% 8% 14% 21% 17% 3	4186
	11th to 20th	Day	27% 7% 12% 36% 17% 2	4181
	21st to end	Day	29% 9% 22% 30% 10% 1	4326
August	1st to 10th	Day	24% 6% 17% 25% 25% 3	4090
	11th to 20th	Day	36% 7% 15% 25% 15% 1	4080
	21st to end	Day	25% 7% 12% 40% 13% 4	4221
September	1st to 10th	Day	31% 5% 11% 26% 17% 9%	2280
	11th to 20th	Day	33% 6% 10% 25% 23% 2	2091
	21st to end	Day	29% 6% 10% 25% 25% 4	2082
October	1st to 10th	Day	19% 2 7% 18% 37% 17%	3160
	11th to 20th	Day	18% 2 4 27% 36% 12%	3140
	21st to end	Day	30% 3 7% 29% 26% 5%	3428
November	1st to 10th	Day	30% 5% 7% 29% 23% 5%	1958
	11th to 20th	Day	34% 4 8% 24% 24% 7%	1857
	21st to end	Day	37% 3 9% 27% 22% 1	1641
December	1st to 10th	Day	33% 4 8% 35% 19%	2610
	11th to 20th	Day	35% 3 7% 30% 22% 2	3033
	21st to end	Day	61% 5% 7% 19% 7% 1	4488
January	1st to 10th	Night	45% 4 12% 25% 13% 2	5888
	11th to 20th	Night	38% 5% 10% 25% 20% 3	5900
	21st to end	Night	43% 3 9% 19% 21% 5%	6490
February	1st to 10th	Night	44% 5% 11% 24% 13% 3	2600
	11th to 20th	Night	41% 4 10% 24% 17% 3	2534
	21st to end	Night	38% 4 8% 23% 20% 7%	1886
March	1st to 10th	Night	38% 5% 8% 20% 24% 5%	4919
	11th to 20th	Night	28% 5% 7% 23% 28% 9%	4980
	21st to end	Night	37% 6% 13% 29% 13% 2	5467
April	1st to 10th	Night	34% 5% 12% 23% 21% 6%	2103
	11th to 20th	Night	37% 5% 9% 24% 20% 5%	1975
	21st to end	Night	41% 6% 11% 24% 16% 2	1905
May	1st to 10th	Night	40% 6% 12% 24% 14% 3	4080
	11th to 20th	Night	36% 6% 13% 24% 15% 6%	4054
	21st to end	Night	36% 7% 12% 27% 15% 2	4452
June	1st to 10th	Night	26% 9% 17% 28% 17% 4	1800
	11th to 20th	Night	23% 8% 18% 30% 20% 2	1800
	21st to end	Night	21% 6% 15% 33% 22% 4	1800
July	1st to 10th	Night	32% 7% 14% 24% 18% 5%	2895
	11th to 20th	Night	20% 6% 11% 38% 22% 2	2886
	21st to end	Night	25% 7% 21% 33% 14%	2999
August	1st to 10th	Night	18% 6% 15% 34% 22% 5%	2826
	11th to 20th	Night	29% 7% 14% 29% 19% 1	2825
	21st to end	Night	18% 6% 13% 40% 19% 4	3435
September	1st to 10th	Night	28% 6% 10% 25% 22% 9%	1900
	11th to 20th	Night	30% 5% 10% 25% 26% 5%	2085
	21st to end	Night	27% 5% 9% 25% 29% 5%	2090
October	1st to 10th	Night	14% 3 7% 17% 42% 17%	3787
	11th to 20th	Night	21% 2 5% 20% 34% 17%	3779
	21st to end	Night	30% 4 7% 29% 26% 5%	4120
November	1st to 10th	Night	28% 4 9% 27% 25% 7%	2340
	11th to 20th	Night	33% 4 7% 24% 26% 6%	2540
	21st to end	Night	35% 4 9% 24% 24% 4	2753
December	1st to 10th	Night	31% 4 11% 35% 18% 1	4580
	11th to 20th	Night	38% 3 8% 25% 22% 3	5155
	21st to end	Night	56% 5% 8% 21% 8% 2	6490

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	21% 12% 37% 25% 4	300
	11th to 20th	Day	21% 21% 36% 22% 1	300
	21st to end	Day	15% 14% 47% 19% 5%	330
February	1st to 10th	Day	26% 18% 35% 18% 3	300
	11th to 20th	Day	21% 20% 39% 16% 4	300
	21st to end	Day	21% 20% 41% 14% 3	249
March	1st to 10th	Day	21% 18% 35% 22% 4	300
	11th to 20th	Day	23% 17% 35% 22% 3	300
	21st to end	Day	30% 15% 38% 12% 3 1	330
April	1st to 10th	Day	20% 16% 39% 20% 5% 1	395
	11th to 20th	Day	19% 17% 38% 20% 6%	492
	21st to end	Day	23% 14% 29% 24% 9% 1	591
May	1st to 10th	Day	17% 13% 35% 25% 9% 1	703
	11th to 20th	Day	16% 10% 31% 25% 15% 3	725
	21st to end	Day	16% 7% 30% 25% 16% 6%	789
June	1st to 10th	Day	8% 3 20% 33% 28% 8%	730
	11th to 20th	Day	8% 3 20% 40% 22% 6%	730
	21st to end	Day	4 4 28% 39% 20% 5%	730
July	1st to 10th	Day	9% 3 15% 32% 29% 12%	721
	11th to 20th	Day	4 3 15% 39% 30% 9%	718
	21st to end	Day	7% 3 11% 34% 33% 12%	740
August	1st to 10th	Day	9% 2 13% 40% 31% 5%	676
	11th to 20th	Day	9% 3 16% 35% 31% 6%	665
	21st to end	Day	9% 1 9% 30% 40% 11%	568
September	1st to 10th	Day	10% 2 12% 32% 34% 10%	490
	11th to 20th	Day	5% 2 15% 39% 29% 11%	301
	21st to end	Day	12% 4 18% 37% 21% 8%	300
October	1st to 10th	Day	10% 9% 24% 27% 21% 9%	300
	11th to 20th	Day	12% 12% 38% 27% 9% 2	300
	21st to end	Day	27% 13% 31% 23% 5% 1	330
November	1st to 10th	Day	17% 12% 36% 23% 10% 2	300
	11th to 20th	Day	16% 16% 35% 21% 9% 3	300
	21st to end	Day	21% 17% 30% 23% 8% 1	300
December	1st to 10th	Day	16% 13% 43% 18% 7% 2	300
	11th to 20th	Day	18% 16% 39% 21% 6% 1	300
	21st to end	Day	24% 18% 40% 16% 2	330
January	1st to 10th	Night	16% 13% 40% 25% 6%	1000
	11th to 20th	Night	17% 15% 41% 23% 5%	1000
	21st to end	Night	15% 13% 40% 28% 4	1100
February	1st to 10th	Night	21% 13% 40% 23% 3	1000
	11th to 20th	Night	18% 15% 40% 24% 3	979
	21st to end	Night	19% 15% 36% 27% 2	566
March	1st to 10th	Night	17% 13% 40% 26% 5%	600
	11th to 20th	Night	21% 11% 39% 24% 5%	600
	21st to end	Night	21% 12% 40% 22% 5%	660
April	1st to 10th	Night	14% 13% 37% 30% 5%	600
	11th to 20th	Night	18% 13% 36% 28% 6%	593
	21st to end	Night	18% 10% 39% 26% 8%	600
May	1st to 10th	Night	16% 5% 33% 37% 8% 1	600
	11th to 20th	Night	16% 5% 28% 34% 17% 1	591
	21st to end	Night	15% 4 25% 37% 17% 2	650
June	1st to 10th	Night	7% 3 17% 44% 29% 1	600
	11th to 20th	Night	6% 1 23% 46% 23% 1	600
	21st to end	Night	3 3 23% 51% 20% 1	600
July	1st to 10th	Night	8% 1 12% 41% 35% 3	585
	11th to 20th	Night	4 1 12% 46% 34% 3	582
	21st to end	Night	7% 1 12% 42% 36% 3	624
August	1st to 10th	Night	5% 2 15% 44% 34% 1	567
	11th to 20th	Night	6% 2 10% 53% 26% 3	564
	21st to end	Night	5% 1 10% 39% 42% 4	627
September	1st to 10th	Night	10% 2 13% 43% 31% 1	570
	11th to 20th	Night	11% 2 15% 43% 25% 4	570
	21st to end	Night	9% 2 20% 41% 23% 3	570
October	1st to 10th	Night	8% 3 27% 37% 23% 3	747
	11th to 20th	Night	15% 6% 37% 32% 10% 1	760
	21st to end	Night	18% 9% 33% 29% 10%	836
November	1st to 10th	Night	14% 7% 38% 31% 10%	780
	11th to 20th	Night	14% 13% 35% 29% 8%	940
	21st to end	Night	18% 11% 37% 27% 7%	1000
December	1st to 10th	Night	16% 11% 39% 28% 5%	1000
	11th to 20th	Night	19% 11% 36% 29% 4	1000
	21st to end	Night	18% 14% 37% 27% 3	1100

Page generated at 2025-07-18 00:10:01. Version ac07

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