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 2024-05-10.

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	13% 8% 4 7% 6% 3 4 5% 5% 7% 38%	83522
	Night	14% 9% 4 7% 5% 3 4 4 5% 6% 40%	85965
Transparency	Day	45% 6% 12% 22% 13% 2	83223
	Night	45% 5% 12% 22% 14% 2	85784
Seeing	Day	15% 10% 29% 26% 15% 5%	15020
	Night	16% 10% 31% 30% 12% 1	25032

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% 7% 14% 8% 4 5% 4 4 6% 26%	2527
	11th to 20th	Day	12% 12% 5% 11% 10% 4 8% 5% 7% 6% 21%	2544
	21st to end	Day	16% 19% 9% 13% 7% 3 4 3 3 4 18%	3135
February	1st to 10th	Day	16% 10% 6% 9% 7% 4 6% 4 3 6% 29%	1710
	11th to 20th	Day	15% 8% 4 7% 8% 3 4 4 4 7% 35%	1772
	21st to end	Day	14% 12% 6% 8% 6% 3 3 3 4 7% 33%	1570
March	1st to 10th	Day	15% 12% 4 6% 5% 3 3 3 4 6% 38%	3140
	11th to 20th	Day	10% 6% 3 5% 4 3 5% 5% 6% 7% 46%	3480
	21st to end	Day	20% 11% 4 10% 6% 3 4 4 5% 6% 27%	4031
April	1st to 10th	Day	16% 10% 4 7% 5% 3 5% 3 5% 6% 36%	2280
	11th to 20th	Day	16% 9% 4 6% 6% 3 4 4 4 6% 38%	2254
	21st to end	Day	20% 8% 3 7% 6% 3 5% 4 4 6% 35%	2280
May	1st to 10th	Day	17% 8% 3 5% 6% 4 5% 4 6% 8% 33%	2480
	11th to 20th	Day	19% 9% 3 5% 5% 3 4 5% 5% 7% 35%	2479
	21st to end	Day	14% 7% 3 6% 7% 4 5% 5% 7% 8% 34%	2725
June	1st to 10th	Day	9% 5% 3 6% 5% 3 4 5% 7% 9% 46%	2520
	11th to 20th	Day	9% 5% 3 6% 5% 3 5% 5% 5% 8% 48%	2520
	21st to end	Day	11% 6% 3 5% 5% 4 5% 5% 6% 9% 41%	2520
July	1st to 10th	Day	8% 4 2 4 5% 3 4 5% 7% 10% 47%	2520
	11th to 20th	Day	8% 4 2 5% 5% 3 5% 5% 6% 10% 47%	2520
	21st to end	Day	7% 4 2 4 6% 4 5% 6% 7% 10% 45%	2731
August	1st to 10th	Day	8% 5% 2 5% 6% 3 6% 7% 8% 12% 39%	2328
	11th to 20th	Day	8% 6% 3 5% 4 3 4 5% 5% 8% 47%	2180
	21st to end	Day	7% 4 2 5% 5% 4 4 5% 6% 9% 50%	2376
September	1st to 10th	Day	12% 5% 2 4 5% 3 4 5% 5% 7% 47%	2160
	11th to 20th	Day	11% 3 1 4 3 2 4 5% 5% 8% 53%	2141
	21st to end	Day	9% 5% 2 5% 4 4 4 5% 6% 8% 48%	1962
October	1st to 10th	Day	13% 6% 2 5% 4 3 4 4 4 6% 48%	1800
	11th to 20th	Day	16% 8% 3 6% 4 3 4 3 4 6% 43%	1800
	21st to end	Day	20% 8% 4 5% 4 2 4 4 4 6% 39%	1904
November	1st to 10th	Day	11% 7% 2 5% 6% 2 4 4 5% 7% 47%	1672
	11th to 20th	Day	16% 10% 4 6% 5% 3 4 4 5% 7% 36%	1710
	21st to end	Day	16% 10% 4 8% 5% 2 4 3 4 5% 38%	1530
December	1st to 10th	Day	13% 12% 5% 9% 5% 3 5% 4 5% 5% 33%	1520
	11th to 20th	Day	14% 11% 4 8% 6% 3 5% 5% 5% 6% 32%	1918
	21st to end	Day	21% 18% 10% 12% 6% 2 4 3 4 4 16%	2783
January	1st to 10th	Night	14% 11% 6% 12% 6% 3 4 4 4 5% 30%	4428
	11th to 20th	Night	12% 16% 8% 10% 7% 3 5% 4 8% 4 23%	4409
	21st to end	Night	19% 15% 7% 12% 6% 3 4 3 3 5% 23%	4532
February	1st to 10th	Night	16% 10% 4 8% 6% 3 5% 4 4 6% 33%	2470
	11th to 20th	Night	13% 9% 4 6% 6% 4 4 5% 5% 6% 39%	2293
	21st to end	Night	15% 10% 4 7% 5% 3 4 3 4 6% 40%	1884
March	1st to 10th	Night	18% 10% 3 5% 5% 3 3 4 4 5% 41%	3733
	11th to 20th	Night	12% 6% 3 4 4 2 3 4 4 7% 50%	3490
	21st to end	Night	21% 11% 6% 10% 6% 4 4 4 4 5% 25%	3599
April	1st to 10th	Night	14% 9% 3 6% 5% 3 4 4 5% 7% 40%	1900
	11th to 20th	Night	16% 9% 3 5% 4 2 4 4 4 5% 43%	1875
	21st to end	Night	17% 6% 2 8% 6% 2 4 4 5% 7% 39%	1900
May	1st to 10th	Night	15% 7% 4 5% 7% 3 5% 5% 5% 7% 37%	1680
	11th to 20th	Night	17% 7% 3 6% 5% 3 4 4 6% 7% 40%	1416
	21st to end	Night	11% 5% 3 6% 6% 3 5% 5% 6% 9% 41%	1551
June	1st to 10th	Night	9% 4 2 4 5% 3 4 5% 5% 9% 50%	1440
	11th to 20th	Night	8% 5% 3 5% 6% 3 4 5% 6% 9% 47%	1440
	21st to end	Night	10% 4 2 5% 5% 3 5% 5% 6% 8% 47%	1440
July	1st to 10th	Night	7% 4 1 3 5% 3 5% 5% 6% 10% 51%	1440
	11th to 20th	Night	5% 4 1 3 2 2 4 5% 5% 9% 59%	1440
	21st to end	Night	5% 3 2 4 4 4 5% 6% 6% 8% 52%	1583
August	1st to 10th	Night	5% 3 2 4 4 4 3 5% 6% 10% 53%	1611
	11th to 20th	Night	7% 4 2 4 4 3 5% 5% 5% 6% 55%	1720
	21st to end	Night	7% 3 1 3 4 2 3 5% 5% 8% 58%	1980
September	1st to 10th	Night	11% 5% 2 4 3 2 4 3 4 6% 57%	1800
	11th to 20th	Night	10% 4 2 4 4 2 4 4 6% 7% 53%	1800
	21st to end	Night	8% 4 3 5% 5% 3 4 5% 6% 7% 51%	1995
October	1st to 10th	Night	10% 6% 2 5% 4 3 4 4 4 6% 53%	2160
	11th to 20th	Night	15% 6% 2 5% 4 2 3 4 4 7% 48%	2160
	21st to end	Night	18% 8% 3 5% 5% 3 4 4 5% 5% 40%	2450
November	1st to 10th	Night	12% 6% 2 5% 4 2 3 3 4 7% 53%	2406
	11th to 20th	Night	14% 8% 3 6% 4 3 3 4 5% 7% 42%	2470
	21st to end	Night	17% 9% 4 6% 6% 2 4 4 4 6% 39%	2647
December	1st to 10th	Night	14% 10% 5% 7% 6% 3 5% 4 5% 6% 36%	2660
	11th to 20th	Night	14% 11% 4 8% 6% 3 4 4 4 5% 36%	3279
	21st to end	Night	22% 16% 7% 10% 7% 4 4 4 3 4 18%	4884

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% 5% 10% 18% 6%	2528
	11th to 20th	Day	63% 5% 10% 15% 8%	2544
	21st to end	Day	70% 4 9% 11% 6%	3135
February	1st to 10th	Day	57% 4 14% 16% 7% 2	1710
	11th to 20th	Day	49% 6% 12% 22% 11% 1	1781
	21st to end	Day	53% 4 11% 22% 9% 2	1570
March	1st to 10th	Day	48% 5% 11% 20% 14% 2	3140
	11th to 20th	Day	36% 6% 13% 27% 17% 1	3480
	21st to end	Day	57% 6% 14% 19% 3 1	4031
April	1st to 10th	Day	49% 6% 11% 22% 12%	2280
	11th to 20th	Day	47% 5% 9% 23% 14% 2	2254
	21st to end	Day	50% 6% 10% 19% 13% 2	2280
May	1st to 10th	Day	48% 8% 13% 21% 10% 1	2480
	11th to 20th	Day	48% 7% 10% 22% 13%	2483
	21st to end	Day	46% 9% 13% 16% 14% 2	2725
June	1st to 10th	Day	33% 9% 14% 26% 16% 2	2520
	11th to 20th	Day	35% 8% 10% 23% 22% 3	2520
	21st to end	Day	38% 8% 16% 21% 15% 2	2520
July	1st to 10th	Day	31% 8% 15% 27% 15% 4	2451
	11th to 20th	Day	32% 8% 16% 27% 14% 4	2445
	21st to end	Day	32% 8% 16% 27% 15% 1	2579
August	1st to 10th	Day	34% 10% 17% 27% 11% 1	2314
	11th to 20th	Day	34% 8% 13% 28% 17% 1	2180
	21st to end	Day	30% 7% 12% 27% 19% 4	2376
September	1st to 10th	Day	34% 7% 12% 25% 18% 5%	2160
	11th to 20th	Day	29% 5% 12% 27% 23% 4	2141
	21st to end	Day	32% 7% 11% 27% 21% 2	1962
October	1st to 10th	Day	38% 4 10% 27% 20% 2	1800
	11th to 20th	Day	44% 4 9% 25% 17% 2	1800
	21st to end	Day	46% 5% 13% 22% 12% 3	1904
November	1st to 10th	Day	37% 5% 15% 24% 15% 4	1672
	11th to 20th	Day	47% 5% 12% 23% 12% 1	1710
	21st to end	Day	50% 5% 14% 22% 10% 1	1530
December	1st to 10th	Day	53% 5% 12% 20% 9% 1	1520
	11th to 20th	Day	50% 5% 12% 24% 7% 3	1915
	21st to end	Day	72% 5% 8% 10% 5%	2783
January	1st to 10th	Night	55% 5% 13% 18% 8%	4428
	11th to 20th	Night	62% 3 11% 14% 9% 1	4409
	21st to end	Night	65% 5% 9% 16% 6% 1	4532
February	1st to 10th	Night	52% 5% 12% 19% 12% 1	2470
	11th to 20th	Night	46% 5% 12% 24% 12% 2	2294
	21st to end	Night	46% 4 8% 24% 14% 3	1884
March	1st to 10th	Night	47% 6% 10% 24% 13% 1	3733
	11th to 20th	Night	35% 4 11% 30% 16% 4	3490
	21st to end	Night	59% 6% 13% 16% 5% 1	3599
April	1st to 10th	Night	44% 5% 10% 25% 14% 2	1900
	11th to 20th	Night	43% 5% 8% 23% 17% 4	1875
	21st to end	Night	45% 6% 12% 20% 14% 3	1900
May	1st to 10th	Night	46% 6% 13% 21% 13% 1	1680
	11th to 20th	Night	43% 5% 13% 22% 16% 1	1417
	21st to end	Night	39% 7% 14% 20% 17% 3	1552
June	1st to 10th	Night	30% 7% 15% 24% 20% 4	1440
	11th to 20th	Night	31% 7% 13% 23% 22% 5%	1440
	21st to end	Night	33% 8% 15% 20% 21% 3	1440
July	1st to 10th	Night	29% 8% 15% 28% 16% 4	1400
	11th to 20th	Night	23% 6% 16% 32% 21% 2	1392
	21st to end	Night	27% 7% 14% 28% 20% 4	1495
August	1st to 10th	Night	26% 9% 18% 31% 14% 3	1601
	11th to 20th	Night	29% 7% 12% 29% 20% 3	1720
	21st to end	Night	24% 7% 14% 28% 20% 7%	1980
September	1st to 10th	Night	30% 4 12% 26% 23% 5%	1800
	11th to 20th	Night	30% 5% 12% 28% 20% 5%	1800
	21st to end	Night	31% 6% 11% 26% 24% 3	1995
October	1st to 10th	Night	33% 5% 11% 23% 24% 4	2160
	11th to 20th	Night	37% 4 11% 25% 20% 3	2160
	21st to end	Night	45% 5% 11% 20% 17% 2	2450
November	1st to 10th	Night	32% 5% 13% 26% 19% 4	2406
	11th to 20th	Night	42% 4 13% 24% 16% 2	2470
	21st to end	Night	47% 4 11% 24% 13% 1	2647
December	1st to 10th	Night	49% 5% 13% 22% 10% 1	2660
	11th to 20th	Night	50% 5% 12% 25% 8% 1	3281
	21st to end	Night	70% 5% 8% 12% 5%	4884

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% 22% 7% 1	270
	11th to 20th	Day	23% 21% 39% 14% 4	270
	21st to end	Day	20% 18% 36% 21% 5%	297
February	1st to 10th	Day	21% 16% 44% 14% 5%	270
	11th to 20th	Day	23% 16% 33% 21% 5% 1	270
	21st to end	Day	21% 16% 44% 16% 2	225
March	1st to 10th	Day	19% 22% 37% 18% 4	270
	11th to 20th	Day	22% 18% 34% 19% 7%	270
	21st to end	Day	27% 18% 33% 16% 4 1	424
April	1st to 10th	Day	22% 14% 35% 23% 5% 1	460
	11th to 20th	Day	20% 15% 33% 19% 11% 2	452
	21st to end	Day	23% 13% 33% 22% 7% 2	460
May	1st to 10th	Day	18% 11% 35% 26% 8% 2	534
	11th to 20th	Day	20% 12% 28% 23% 13% 2	625
	21st to end	Day	14% 8% 29% 27% 15% 8%	679
June	1st to 10th	Day	8% 6% 26% 31% 22% 7%	630
	11th to 20th	Day	9% 4 29% 33% 20% 5%	630
	21st to end	Day	11% 8% 30% 31% 15% 5%	630
July	1st to 10th	Day	10% 3 16% 29% 27% 14%	621
	11th to 20th	Day	10% 3 22% 35% 22% 8%	618
	21st to end	Day	7% 2 21% 36% 28% 6%	684
August	1st to 10th	Day	6% 3 21% 35% 27% 9%	626
	11th to 20th	Day	10% 5% 17% 30% 25% 12%	494
	21st to end	Day	6% 3 19% 33% 28% 11%	495
September	1st to 10th	Day	13% 6% 17% 33% 25% 7%	450
	11th to 20th	Day	8% 4 20% 30% 27% 11%	450
	21st to end	Day	8% 9% 27% 24% 25% 7%	432
October	1st to 10th	Day	18% 5% 30% 28% 14% 4	270
	11th to 20th	Day	17% 22% 34% 17% 9%	270
	21st to end	Day	29% 16% 31% 19% 4 1	297
November	1st to 10th	Day	10% 10% 44% 23% 9% 3	270
	11th to 20th	Day	22% 18% 30% 21% 6% 2	270
	21st to end	Day	26% 11% 34% 27% 3	270
December	1st to 10th	Day	14% 16% 41% 22% 4 4	270
	11th to 20th	Day	19% 13% 38% 23% 8%	270
	21st to end	Day	24% 25% 35% 15% 2	297
January	1st to 10th	Night	17% 15% 40% 21% 6%	950
	11th to 20th	Night	18% 15% 40% 21% 6%	950
	21st to end	Night	18% 13% 37% 28% 3	1045
February	1st to 10th	Night	23% 11% 39% 23% 4	950
	11th to 20th	Night	17% 15% 38% 26% 3	818
	21st to end	Night	21% 14% 42% 20% 3 1	628
March	1st to 10th	Night	20% 14% 39% 22% 5%	760
	11th to 20th	Night	18% 13% 34% 28% 7%	580
	21st to end	Night	25% 13% 33% 23% 5%	627
April	1st to 10th	Night	18% 15% 36% 26% 4	570
	11th to 20th	Night	21% 13% 31% 27% 7% 1	563
	21st to end	Night	18% 11% 35% 30% 5%	570
May	1st to 10th	Night	17% 10% 36% 31% 6% 1	569
	11th to 20th	Night	17% 9% 27% 35% 11%	531
	21st to end	Night	12% 6% 30% 34% 18% 1	584
June	1st to 10th	Night	9% 5% 23% 44% 17% 1	540
	11th to 20th	Night	9% 5% 27% 41% 19%	540
	21st to end	Night	10% 4 28% 39% 17% 1	540
July	1st to 10th	Night	9% 2 11% 42% 31% 6%	525
	11th to 20th	Night	5% 3 21% 43% 26% 2	522
	21st to end	Night	7% 1 15% 46% 30% 2	591
August	1st to 10th	Night	6% 2 13% 45% 29% 4	537
	11th to 20th	Night	6% 1 14% 42% 30% 5%	534
	21st to end	Night	7% 1 15% 43% 31% 2	594
September	1st to 10th	Night	13% 4 17% 38% 24% 4	540
	11th to 20th	Night	10% 4 19% 34% 28% 5%	540
	21st to end	Night	10% 8% 24% 34% 23% 2	720
October	1st to 10th	Night	12% 6% 31% 31% 19% 2	720
	11th to 20th	Night	18% 13% 29% 29% 10%	720
	21st to end	Night	22% 12% 34% 24% 7%	792
November	1st to 10th	Night	15% 10% 40% 26% 9% 1	740
	11th to 20th	Night	18% 15% 36% 25% 6%	760
	21st to end	Night	20% 12% 37% 24% 6%	937
December	1st to 10th	Night	19% 13% 39% 24% 5%	950
	11th to 20th	Night	22% 11% 35% 26% 6%	950
	21st to end	Night	25% 17% 33% 22% 3	1045

Page generated at 2024-05-07 06:24:59. Version ac07

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