Viking Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Viking 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	12% 8% 3 8% 5% 4 4 4 5% 7% 38%	126599
	Night	13% 9% 3 8% 4 4 4 3 5% 6% 40%	122703
Transparency	Day	43% 6% 12% 23% 14% 2	130123
	Night	43% 5% 11% 23% 15% 2	127255
Seeing	Day	15% 11% 29% 26% 15% 5%	17025
	Night	16% 10% 31% 30% 12% 1	26778

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	10% 12% 5% 15% 7% 6% 5% 3 5% 5% 27%	3617
	11th to 20th	Day	8% 10% 5% 12% 7% 6% 7% 4 7% 6% 29%	3648
	21st to end	Day	13% 15% 7% 12% 6% 4 5% 3 4 5% 28%	4488
February	1st to 10th	Day	15% 11% 5% 11% 7% 5% 5% 3 4 6% 29%	1800
	11th to 20th	Day	15% 9% 3 9% 7% 5% 4 3 4 7% 34%	1868
	21st to end	Day	14% 12% 5% 9% 5% 4 2 3 4 6% 35%	1650
March	1st to 10th	Day	17% 11% 3 6% 3 3 3 2 3 5% 43%	4514
	11th to 20th	Day	8% 6% 2 6% 4 4 5% 4 6% 8% 46%	4980
	21st to end	Day	19% 11% 4 10% 5% 4 4 4 5% 6% 28%	5777
April	1st to 10th	Day	16% 12% 3 7% 4 4 4 3 5% 6% 35%	2400
	11th to 20th	Day	16% 10% 4 7% 5% 3 4 4 4 6% 37%	2374
	21st to end	Day	20% 8% 3 8% 5% 4 4 4 4 6% 35%	2400
May	1st to 10th	Day	15% 8% 3 7% 5% 5% 5% 4 6% 8% 36%	5926
	11th to 20th	Day	15% 7% 2 6% 4 4 4 3 5% 7% 43%	6319
	21st to end	Day	14% 7% 3 7% 5% 5% 4 4 6% 7% 38%	6935
June	1st to 10th	Day	9% 6% 2 7% 4 3 4 4 7% 8% 46%	2800
	11th to 20th	Day	9% 5% 2 6% 4 4 4 4 6% 8% 47%	2800
	21st to end	Day	10% 6% 2 5% 5% 5% 5% 4 6% 9% 42%	2800
July	1st to 10th	Day	8% 6% 2 7% 5% 6% 5% 5% 7% 9% 39%	6360
	11th to 20th	Day	10% 6% 3 6% 5% 5% 5% 5% 6% 9% 41%	6360
	21st to end	Day	11% 6% 2 7% 5% 5% 6% 5% 7% 10% 38%	6885
August	1st to 10th	Day	6% 4 2 6% 4 4 5% 4 8% 11% 44%	4234
	11th to 20th	Day	11% 6% 2 6% 4 4 5% 4 6% 9% 42%	3946
	21st to end	Day	6% 4 2 6% 4 4 4 4 6% 9% 49%	4312
September	1st to 10th	Day	11% 5% 2 5% 4 4 4 4 5% 7% 50%	2400
	11th to 20th	Day	10% 4 1 4 3 3 4 4 5% 8% 53%	2355
	21st to end	Day	10% 5% 2 5% 4 5% 4 4 6% 7% 48%	2070
October	1st to 10th	Day	13% 6% 2 6% 3 4 4 3 4 6% 50%	1900
	11th to 20th	Day	15% 8% 3 6% 4 3 4 3 4 6% 45%	1900
	21st to end	Day	19% 8% 3 6% 3 3 4 4 4 6% 40%	2009
November	1st to 10th	Day	12% 7% 2 6% 5% 3 4 3 5% 7% 46%	1762
	11th to 20th	Day	15% 11% 3 6% 5% 4 4 4 5% 7% 37%	1800
	21st to end	Day	16% 12% 3 8% 5% 4 4 2 5% 6% 37%	1610
December	1st to 10th	Day	8% 10% 3 8% 4 4 4 3 5% 6% 46%	2610
	11th to 20th	Day	14% 12% 6% 15% 5% 5% 4 4 5% 5% 24%	3008
	21st to end	Day	21% 20% 9% 13% 5% 4 3 2 3 4 15%	3982
January	1st to 10th	Night	12% 12% 6% 14% 4 4 4 3 4 5% 31%	6348
	11th to 20th	Night	10% 13% 6% 12% 8% 6% 5% 3 7% 4 26%	6315
	21st to end	Night	15% 12% 6% 11% 5% 4 4 3 4 5% 32%	6490
February	1st to 10th	Night	15% 11% 4 9% 6% 4 5% 3 5% 6% 33%	2600
	11th to 20th	Night	13% 10% 4 7% 5% 5% 4 3 6% 6% 37%	2417
	21st to end	Night	14% 11% 3 8% 4 4 3 3 4 5% 42%	1980
March	1st to 10th	Night	17% 10% 4 6% 3 3 3 3 3 4 42%	5373
	11th to 20th	Night	11% 7% 3 5% 3 4 3 3 5% 7% 50%	4990
	21st to end	Night	16% 12% 5% 10% 5% 5% 5% 3 5% 6% 28%	5151
April	1st to 10th	Night	14% 10% 3 6% 4 4 3 4 5% 7% 40%	2000
	11th to 20th	Night	16% 10% 3 6% 4 4 3 3 4 5% 42%	1975
	21st to end	Night	16% 6% 2 9% 5% 3 4 3 5% 7% 38%	2000
May	1st to 10th	Night	18% 9% 3 7% 5% 4 5% 3 5% 7% 36%	3961
	11th to 20th	Night	12% 6% 2 5% 4 4 5% 3 7% 9% 42%	3596
	21st to end	Night	13% 7% 2 6% 4 4 4 4 5% 8% 43%	3942
June	1st to 10th	Night	10% 4 2 5% 4 3 4 4 6% 9% 50%	1600
	11th to 20th	Night	7% 5% 2 6% 5% 4 4 4 6% 9% 47%	1600
	21st to end	Night	10% 5% 2 6% 4 4 4 4 6% 8% 48%	1600
July	1st to 10th	Night	11% 5% 2 6% 4 4 5% 4 6% 10% 42%	3618
	11th to 20th	Night	7% 5% 2 5% 3 4 5% 4 6% 8% 51%	3617
	21st to end	Night	11% 6% 2 6% 4 5% 5% 4 6% 8% 42%	3982
August	1st to 10th	Night	5% 3 2 4 3 4 4 4 7% 10% 56%	2926
	11th to 20th	Night	7% 5% 1 5% 4 4 4 4 5% 7% 54%	3105
	21st to end	Night	6% 3 1 5% 4 4 4 4 5% 8% 55%	3586
September	1st to 10th	Night	10% 4 2 4 3 3 3 2 4 5% 59%	2000
	11th to 20th	Night	9% 4 1 5% 3 3 3 3 6% 8% 53%	1980
	21st to end	Night	9% 4 2 6% 4 4 4 4 6% 7% 50%	2106
October	1st to 10th	Night	9% 6% 2 5% 3 4 3 3 4 6% 54%	2280
	11th to 20th	Night	14% 6% 2 5% 4 3 3 3 4 6% 50%	2280
	21st to end	Night	17% 8% 2 6% 4 4 4 3 5% 5% 40%	2587
November	1st to 10th	Night	12% 7% 2 5% 3 3 3 3 5% 7% 52%	2536
	11th to 20th	Night	14% 9% 3 7% 4 4 4 3 5% 7% 41%	2600
	21st to end	Night	16% 10% 4 8% 5% 3 4 3 4 5% 38%	2787
December	1st to 10th	Night	9% 9% 3 7% 4 3 4 3 5% 5% 47%	4580
	11th to 20th	Night	16% 12% 4 11% 5% 4 4 3 4 5% 33%	5199
	21st to end	Night	23% 18% 7% 11% 5% 5% 4 3 4 4 16%	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	59% 4 9% 20% 7% 1	3618
	11th to 20th	Day	55% 4 9% 14% 15% 4	3648
	21st to end	Day	60% 3 11% 18% 8% 1	4488
February	1st to 10th	Day	57% 4 15% 16% 7% 2	1800
	11th to 20th	Day	50% 5% 12% 21% 11% 1	1877
	21st to end	Day	51% 4 11% 22% 10% 2	1650
March	1st to 10th	Day	45% 4 8% 23% 17% 3	4514
	11th to 20th	Day	35% 5% 10% 32% 17%	4980
	21st to end	Day	55% 6% 13% 20% 5% 1	5779
April	1st to 10th	Day	49% 6% 11% 22% 12%	2400
	11th to 20th	Day	47% 5% 9% 23% 13% 2	2374
	21st to end	Day	50% 6% 10% 20% 13% 2	2400
May	1st to 10th	Day	44% 7% 11% 21% 16% 1	5963
	11th to 20th	Day	41% 5% 10% 27% 13% 4	6323
	21st to end	Day	43% 7% 11% 22% 16% 1	6949
June	1st to 10th	Day	34% 9% 13% 26% 16% 2	2800
	11th to 20th	Day	34% 8% 11% 25% 21% 2	2800
	21st to end	Day	37% 8% 16% 22% 16% 2	2800
July	1st to 10th	Day	39% 7% 13% 27% 14% 2	6291
	11th to 20th	Day	38% 7% 14% 26% 14% 1	6285
	21st to end	Day	40% 8% 18% 21% 12% 1	6689
August	1st to 10th	Day	31% 8% 18% 30% 12% 1	4220
	11th to 20th	Day	38% 7% 14% 27% 15% 1	3984
	21st to end	Day	31% 6% 14% 31% 16% 3	4311
September	1st to 10th	Day	32% 6% 11% 26% 20% 5%	2400
	11th to 20th	Day	28% 5% 11% 28% 24% 4	2355
	21st to end	Day	33% 6% 10% 27% 21% 2	2070
October	1st to 10th	Day	24% 3 6% 26% 40% 2	3160
	11th to 20th	Day	28% 3 10% 23% 32% 5%	3140
	21st to end	Day	41% 3 9% 26% 17% 3	3286
November	1st to 10th	Day	37% 5% 15% 24% 15% 3	1762
	11th to 20th	Day	47% 5% 12% 23% 12% 2	1800
	21st to end	Day	50% 5% 14% 22% 9%	1610
December	1st to 10th	Day	40% 4 13% 33% 9%	2610
	11th to 20th	Day	60% 4 11% 17% 6% 2	3005
	21st to end	Day	75% 4 8% 10% 4	3982
January	1st to 10th	Night	55% 4 12% 21% 8% 1	6348
	11th to 20th	Night	60% 3 11% 12% 13% 1	6315
	21st to end	Night	56% 4 8% 19% 12% 1	6490
February	1st to 10th	Night	52% 5% 12% 19% 12% 1	2600
	11th to 20th	Night	47% 5% 11% 24% 11% 2	2418
	21st to end	Night	45% 4 9% 24% 15% 3	1980
March	1st to 10th	Night	47% 4 8% 25% 15% 2	5373
	11th to 20th	Night	35% 4 9% 30% 18% 5%	4990
	21st to end	Night	56% 6% 13% 17% 7% 1	5151
April	1st to 10th	Night	43% 5% 10% 26% 14% 2	2000
	11th to 20th	Night	44% 5% 8% 23% 17% 4	1975
	21st to end	Night	45% 6% 12% 21% 14% 3	2000
May	1st to 10th	Night	47% 6% 11% 19% 15% 2	3989
	11th to 20th	Night	36% 7% 15% 24% 14% 4	3597
	21st to end	Night	39% 6% 11% 25% 17% 2	3950
June	1st to 10th	Night	30% 7% 15% 25% 19% 4	1600
	11th to 20th	Night	30% 7% 13% 25% 22% 4	1600
	21st to end	Night	33% 8% 15% 21% 20% 3	1600
July	1st to 10th	Night	37% 6% 14% 25% 16% 2	3580
	11th to 20th	Night	30% 6% 14% 29% 20% 1	3572
	21st to end	Night	38% 8% 15% 22% 15% 3	3867
August	1st to 10th	Night	23% 7% 19% 32% 17% 2	2916
	11th to 20th	Night	29% 7% 16% 30% 17% 1	3133
	21st to end	Night	27% 6% 15% 31% 17% 4	3586
September	1st to 10th	Night	28% 4 12% 27% 25% 5%	2000
	11th to 20th	Night	29% 5% 12% 28% 22% 5%	1980
	21st to end	Night	32% 6% 11% 26% 23% 3	2106
October	1st to 10th	Night	22% 4 7% 27% 36% 4	3800
	11th to 20th	Night	26% 3 9% 22% 34% 5%	3779
	21st to end	Night	41% 4 9% 21% 21% 4	4260
November	1st to 10th	Night	34% 5% 12% 26% 19% 4	2536
	11th to 20th	Night	42% 4 12% 23% 16% 2	2600
	21st to end	Night	48% 4 11% 23% 13% 1	2787
December	1st to 10th	Night	39% 3 12% 33% 12%	4580
	11th to 20th	Night	54% 4 11% 22% 8% 1	5201
	21st to end	Night	72% 5% 9% 11% 4	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	20% 18% 32% 23% 7% 1	300
	11th to 20th	Day	21% 22% 40% 14% 4	300
	21st to end	Day	19% 19% 37% 20% 5%	330
February	1st to 10th	Day	20% 17% 43% 15% 5%	300
	11th to 20th	Day	23% 18% 35% 19% 5% 1	300
	21st to end	Day	21% 15% 45% 16% 2	249
March	1st to 10th	Day	20% 20% 38% 18% 4	300
	11th to 20th	Day	22% 19% 33% 19% 6%	300
	21st to end	Day	26% 19% 34% 16% 4 1	464
April	1st to 10th	Day	23% 15% 35% 22% 5% 1	500
	11th to 20th	Day	21% 16% 32% 20% 11% 1	492
	21st to end	Day	23% 14% 33% 22% 7% 2	500
May	1st to 10th	Day	17% 10% 36% 25% 9% 3	582
	11th to 20th	Day	19% 13% 28% 23% 14% 3	725
	21st to end	Day	14% 9% 30% 26% 14% 7%	789
June	1st to 10th	Day	8% 8% 26% 31% 20% 7%	730
	11th to 20th	Day	8% 5% 30% 32% 20% 5%	730
	21st to end	Day	11% 7% 32% 30% 15% 5%	730
July	1st to 10th	Day	10% 4 18% 31% 25% 13%	721
	11th to 20th	Day	10% 3 23% 34% 21% 9%	718
	21st to end	Day	9% 2 19% 36% 28% 6%	794
August	1st to 10th	Day	6% 3 21% 37% 25% 8%	726
	11th to 20th	Day	9% 5% 18% 30% 25% 12%	580
	21st to end	Day	6% 3 18% 33% 29% 10%	583
September	1st to 10th	Day	11% 7% 17% 34% 25% 8%	530
	11th to 20th	Day	7% 3 18% 31% 30% 12%	522
	21st to end	Day	10% 10% 26% 24% 23% 7%	470
October	1st to 10th	Day	16% 6% 29% 29% 16% 4	300
	11th to 20th	Day	16% 20% 32% 20% 11%	300
	21st to end	Day	27% 16% 32% 19% 5% 1	330
November	1st to 10th	Day	12% 12% 42% 22% 9% 3	300
	11th to 20th	Day	22% 19% 30% 21% 7% 2	300
	21st to end	Day	24% 13% 34% 26% 3	300
December	1st to 10th	Day	13% 16% 44% 20% 3 3	300
	11th to 20th	Day	19% 15% 38% 21% 7%	300
	21st to end	Day	26% 23% 33% 15% 2	330
January	1st to 10th	Night	17% 16% 39% 22% 7%	1000
	11th to 20th	Night	17% 16% 40% 21% 6%	1000
	21st to end	Night	18% 14% 38% 27% 3	1100
February	1st to 10th	Night	23% 12% 39% 22% 4	1000
	11th to 20th	Night	18% 16% 38% 25% 3	862
	21st to end	Night	20% 14% 42% 21% 2 1	660
March	1st to 10th	Night	20% 15% 39% 22% 4	800
	11th to 20th	Night	17% 13% 35% 28% 6%	610
	21st to end	Night	24% 14% 33% 23% 5%	660
April	1st to 10th	Night	19% 15% 36% 26% 4	600
	11th to 20th	Night	21% 14% 31% 27% 7% 1	593
	21st to end	Night	18% 12% 34% 31% 5%	600
May	1st to 10th	Night	17% 9% 36% 31% 7% 1	600
	11th to 20th	Night	17% 9% 27% 36% 11%	591
	21st to end	Night	13% 6% 30% 34% 16% 1	650
June	1st to 10th	Night	9% 5% 24% 44% 16% 1	600
	11th to 20th	Night	9% 4 27% 41% 18%	600
	21st to end	Night	10% 4 30% 38% 17% 1	600
July	1st to 10th	Night	9% 3 12% 42% 29% 5%	585
	11th to 20th	Night	5% 3 21% 42% 26% 3	582
	21st to end	Night	8% 2 14% 45% 30% 2	657
August	1st to 10th	Night	6% 2 15% 44% 28% 4	597
	11th to 20th	Night	6% 2 15% 43% 29% 5%	594
	21st to end	Night	7% 1 15% 44% 31% 2	660
September	1st to 10th	Night	12% 4 18% 37% 26% 4	600
	11th to 20th	Night	9% 4 19% 35% 29% 4	594
	21st to end	Night	11% 7% 24% 34% 22% 2	760
October	1st to 10th	Night	12% 6% 30% 32% 19% 2	760
	11th to 20th	Night	17% 13% 29% 30% 11%	760
	21st to end	Night	22% 13% 33% 25% 8%	836
November	1st to 10th	Night	17% 10% 39% 26% 8% 1	780
	11th to 20th	Night	18% 15% 36% 24% 6%	800
	21st to end	Night	20% 13% 37% 24% 5%	987
December	1st to 10th	Night	19% 13% 40% 24% 5%	1000
	11th to 20th	Night	22% 12% 35% 26% 6%	1000
	21st to end	Night	26% 17% 33% 22% 3	1100

Page generated at 2025-09-16 00:49:13. Version ac07

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