Seward
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Seward clear sky chart from 2005-11-06 to 2025-04-26.

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	7% 5% 2 5% 3 3 4 3 5% 7% 55%	118099
	Night	7% 5% 2 5% 3 3 3 3 5% 7% 57%	119306
Transparency	Day	28% 5% 11% 26% 23% 6%	117866
	Night	27% 5% 11% 26% 24% 7%	119129
Seeing	Day	11% 9% 31% 31% 15% 3	16131
	Night	10% 9% 36% 35% 10% 1	26185

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	10% 7% 3 7% 4 4 4 4 5% 7% 47%	4077
	11th to 20th	Day	5% 8% 3 9% 4 4 5% 3 5% 6% 49%	4080
	21st to end	Day	7% 7% 3 7% 3 3 4 3 3 5% 55%	4488
February	1st to 10th	Day	7% 10% 3 7% 5% 4 3 3 5% 7% 46%	1895
	11th to 20th	Day	5% 6% 2 7% 4 4 4 4 5% 7% 52%	1949
	21st to end	Day	6% 6% 2 5% 3 3 3 2 6% 7% 57%	1650
March	1st to 10th	Day	7% 6% 1 4 3 3 3 2 4 5% 61%	4627
	11th to 20th	Day	7% 5% 2 4 2 3 3 3 4 6% 60%	4980
	21st to end	Day	8% 5% 2 5% 4 3 4 4 6% 8% 51%	5464
April	1st to 10th	Day	10% 5% 2 4 4 3 3 3 5% 6% 55%	2270
	11th to 20th	Day	10% 7% 2 6% 4 4 4 3 6% 6% 48%	2444
	21st to end	Day	14% 6% 3 6% 4 4 4 3 5% 7% 45%	2536
May	1st to 10th	Day	13% 7% 2 6% 4 4 4 4 5% 7% 44%	4120
	11th to 20th	Day	11% 5% 2 6% 3 4 4 4 6% 7% 48%	4082
	21st to end	Day	10% 4 3 6% 5% 5% 4 4 6% 8% 46%	4489
June	1st to 10th	Day	5% 4 2 5% 3 4 4 3 5% 8% 58%	2470
	11th to 20th	Day	5% 3 1 4 4 4 4 4 6% 7% 57%	2470
	21st to end	Day	6% 2 1 3 3 4 5% 4 6% 8% 57%	2470
July	1st to 10th	Day	8% 5% 2 5% 4 4 4 4 5% 7% 53%	4120
	11th to 20th	Day	4 3 2 4 3 4 4 4 6% 11% 55%	4120
	21st to end	Day	4 3 1 5% 4 4 4 4 8% 9% 53%	4466
August	1st to 10th	Day	9% 5% 1 6% 3 4 4 4 6% 8% 51%	4103
	11th to 20th	Day	8% 4 2 4 3 4 5% 3 6% 8% 52%	4041
	21st to end	Day	4 3 1 4 3 4 4 4 6% 8% 58%	4249
September	1st to 10th	Day	8% 4 1 4 3 4 4 3 4 6% 59%	2138
	11th to 20th	Day	8% 4 2 5% 3 3 3 4 4 6% 59%	2090
	21st to end	Day	7% 5% 2 4 3 3 3 3 4 6% 60%	2090
October	1st to 10th	Day	7% 3 1 3 1 2 2 2 3 5% 74%	3480
	11th to 20th	Day	5% 3 1 3 2 2 2 3 4 7% 68%	3147
	21st to end	Day	7% 4 1 3 2 3 2 2 5% 5% 65%	3312
November	1st to 10th	Day	8% 4 1 3 3 2 3 3 5% 6% 64%	1763
	11th to 20th	Day	7% 5% 1 5% 3 3 2 3 5% 6% 61%	1800
	21st to end	Day	9% 7% 1 3 2 2 2 3 3 4 64%	1800
December	1st to 10th	Day	4 3 1 4 2 2 2 3 4 6% 69%	2940
	11th to 20th	Day	5% 5% 2 4 3 2 2 2 3 6% 65%	3391
	21st to end	Day	11% 11% 4 7% 4 3 4 3 5% 7% 42%	4488
January	1st to 10th	Night	10% 7% 2 9% 5% 4 4 3 4 6% 44%	5888
	11th to 20th	Night	4 4 2 5% 4 5% 4 3 5% 5% 59%	5900
	21st to end	Night	9% 7% 3 7% 3 3 2 2 3 4 56%	6490
February	1st to 10th	Night	8% 8% 4 7% 4 4 4 3 4 5% 49%	2521
	11th to 20th	Night	8% 7% 3 6% 5% 4 3 4 6% 6% 49%	2338
	21st to end	Night	7% 5% 3 6% 3 2 3 2 4 6% 59%	1980
March	1st to 10th	Night	6% 4 1 3 3 3 3 3 4 5% 66%	5240
	11th to 20th	Night	6% 4 2 4 3 3 4 4 5% 6% 59%	4980
	21st to end	Night	7% 7% 2 6% 4 3 4 3 4 6% 54%	5467
April	1st to 10th	Night	9% 5% 1 4 4 3 3 4 5% 8% 53%	2127
	11th to 20th	Night	11% 5% 2 5% 3 4 4 3 5% 7% 52%	1904
	21st to end	Night	13% 7% 2 5% 3 3 3 3 5% 6% 50%	1760
May	1st to 10th	Night	15% 7% 2 5% 3 3 4 3 5% 6% 47%	2850
	11th to 20th	Night	11% 4 1 4 3 3 4 4 6% 8% 50%	2823
	21st to end	Night	8% 4 1 4 3 3 4 4 7% 10% 52%	3098
June	1st to 10th	Night	6% 2 1 3 3 3 4 3 8% 10% 57%	1710
	11th to 20th	Night	5% 3 1 4 3 4 5% 3 7% 10% 56%	1710
	21st to end	Night	4 4 1 4 3 3 4 3 7% 9% 59%	1710
July	1st to 10th	Night	5% 4 2 5% 4 4 5% 4 6% 9% 52%	2850
	11th to 20th	Night	4 3 2 3 3 3 3 4 8% 12% 56%	2850
	21st to end	Night	5% 3 1 4 3 3 4 4 7% 11% 54%	3097
August	1st to 10th	Night	5% 3 2 5% 4 4 4 4 6% 10% 54%	2836
	11th to 20th	Night	7% 4 2 5% 4 4 4 4 6% 8% 52%	2798
	21st to end	Night	4 3 1 4 3 3 3 3 5% 9% 62%	3413
September	1st to 10th	Night	8% 3 1 4 3 3 3 3 4 7% 61%	2042
	11th to 20th	Night	7% 3 2 5% 2 3 3 3 4 7% 60%	2090
	21st to end	Night	6% 4 1 4 3 3 3 3 5% 6% 62%	2090
October	1st to 10th	Night	6% 2 1 3 2 2 2 2 3 4 73%	3480
	11th to 20th	Night	5% 2 1 2 2 2 3 2 4 6% 73%	3738
	21st to end	Night	5% 3 1 3 2 2 2 2 4 7% 69%	4235
November	1st to 10th	Night	8% 4 1 3 2 2 2 2 4 5% 66%	2535
	11th to 20th	Night	6% 4 1 4 3 3 2 3 5% 6% 63%	2600
	21st to end	Night	9% 6% 2 4 3 3 3 3 5% 6% 59%	2600
December	1st to 10th	Night	4 5% 2 4 3 2 3 2 3 5% 67%	4250
	11th to 20th	Night	6% 4 2 4 3 3 3 3 4 6% 63%	4816
	21st to end	Night	12% 10% 4 7% 3 3 3 2 4 5% 46%	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	37% 5% 11% 24% 18% 5%	4078
	11th to 20th	Day	35% 5% 11% 22% 22% 5%	4080
	21st to end	Day	34% 3 9% 25% 25% 4	4488
February	1st to 10th	Day	38% 4 13% 25% 14% 7%	1895
	11th to 20th	Day	31% 5% 15% 26% 17% 7%	1959
	21st to end	Day	26% 3 11% 26% 25% 9%	1650
March	1st to 10th	Day	26% 3 8% 17% 29% 16%	4627
	11th to 20th	Day	24% 4 8% 17% 32% 14%	4980
	21st to end	Day	29% 6% 13% 37% 13% 2	5464
April	1st to 10th	Day	30% 5% 10% 25% 22% 7%	2270
	11th to 20th	Day	35% 6% 9% 27% 18% 5%	2447
	21st to end	Day	38% 5% 10% 28% 17% 2	2536
May	1st to 10th	Day	37% 8% 12% 20% 14% 9%	4120
	11th to 20th	Day	32% 9% 13% 31% 15% 1	4086
	21st to end	Day	34% 8% 13% 22% 19% 4	4489
June	1st to 10th	Day	24% 6% 18% 32% 15% 4	2470
	11th to 20th	Day	25% 6% 13% 28% 22% 6%	2470
	21st to end	Day	22% 7% 12% 26% 26% 7%	2470
July	1st to 10th	Day	30% 6% 11% 23% 26% 3	4056
	11th to 20th	Day	22% 6% 14% 30% 24% 5%	4051
	21st to end	Day	23% 8% 13% 25% 28% 4	4325
August	1st to 10th	Day	30% 6% 15% 25% 21% 3	4090
	11th to 20th	Day	29% 6% 14% 27% 20% 4	4080
	21st to end	Day	22% 6% 12% 32% 23% 7%	4248
September	1st to 10th	Day	27% 5% 8% 26% 23% 11%	2138
	11th to 20th	Day	26% 5% 8% 27% 28% 5%	2090
	21st to end	Day	26% 5% 10% 25% 26% 8%	2090
October	1st to 10th	Day	16% 3 6% 21% 42% 13%	3480
	11th to 20th	Day	18% 3 10% 30% 28% 11%	3147
	21st to end	Day	22% 3 8% 24% 33% 10%	3312
November	1st to 10th	Day	23% 4 9% 28% 29% 8%	1763
	11th to 20th	Day	25% 3 9% 27% 28% 8%	1800
	21st to end	Day	27% 3 7% 29% 29% 7%	1800
December	1st to 10th	Day	18% 4 9% 39% 28% 4	2940
	11th to 20th	Day	22% 3 7% 38% 27% 3	3389
	21st to end	Day	42% 5% 8% 28% 14% 3	4488
January	1st to 10th	Night	40% 5% 10% 24% 17% 3	5888
	11th to 20th	Night	27% 4 11% 23% 28% 7%	5900
	21st to end	Night	34% 3 9% 23% 24% 7%	6490
February	1st to 10th	Night	38% 4 11% 23% 16% 7%	2521
	11th to 20th	Night	35% 5% 12% 24% 19% 5%	2338
	21st to end	Night	27% 3 8% 21% 28% 13%	1980
March	1st to 10th	Night	21% 3 6% 22% 30% 17%	5240
	11th to 20th	Night	23% 5% 8% 17% 29% 17%	4980
	21st to end	Night	31% 5% 16% 34% 12% 2	5467
April	1st to 10th	Night	28% 6% 12% 22% 26% 6%	2127
	11th to 20th	Night	32% 5% 13% 25% 20% 5%	1904
	21st to end	Night	34% 5% 13% 24% 21% 3	1760
May	1st to 10th	Night	36% 7% 12% 20% 14% 11%	2850
	11th to 20th	Night	29% 8% 16% 28% 18% 1	2824
	21st to end	Night	25% 8% 18% 26% 18% 5%	3099
June	1st to 10th	Night	20% 7% 20% 31% 18% 4	1710
	11th to 20th	Night	23% 6% 15% 30% 21% 5%	1710
	21st to end	Night	20% 7% 13% 28% 24% 8%	1710
July	1st to 10th	Night	27% 7% 14% 21% 26% 4	2805
	11th to 20th	Night	19% 5% 13% 29% 28% 6%	2796
	21st to end	Night	23% 7% 14% 28% 24% 3	2999
August	1st to 10th	Night	26% 6% 12% 28% 23% 5%	2826
	11th to 20th	Night	29% 8% 14% 25% 21% 3	2825
	21st to end	Night	20% 4 14% 37% 20% 6%	3413
September	1st to 10th	Night	25% 5% 10% 23% 26% 11%	2042
	11th to 20th	Night	25% 5% 10% 23% 30% 7%	2090
	21st to end	Night	23% 5% 10% 23% 29% 10%	2090
October	1st to 10th	Night	16% 3 6% 17% 47% 11%	3480
	11th to 20th	Night	15% 3 9% 27% 32% 14%	3738
	21st to end	Night	18% 2 7% 27% 36% 10%	4235
November	1st to 10th	Night	22% 3 9% 27% 27% 12%	2535
	11th to 20th	Night	23% 3 7% 28% 30% 9%	2600
	21st to end	Night	27% 4 8% 27% 24% 10%	2600
December	1st to 10th	Night	22% 3 9% 40% 23% 4	4250
	11th to 20th	Night	24% 3 6% 35% 25% 7%	4817
	21st to end	Night	40% 4 9% 30% 13% 4	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	10% 10% 41% 28% 10% 1	300
	11th to 20th	Day	13% 15% 41% 24% 6% 1	300
	21st to end	Day	8% 12% 51% 24% 5%	330
February	1st to 10th	Day	14% 18% 44% 20% 4 1	300
	11th to 20th	Day	13% 14% 44% 22% 6% 1	300
	21st to end	Day	12% 16% 49% 20% 3	249
March	1st to 10th	Day	11% 16% 43% 24% 6%	300
	11th to 20th	Day	16% 16% 39% 21% 8% 1	300
	21st to end	Day	17% 15% 43% 20% 4 1	330
April	1st to 10th	Day	14% 18% 36% 27% 5% 1	370
	11th to 20th	Day	14% 18% 40% 25% 3	565
	21st to end	Day	18% 16% 38% 19% 7% 2	676
May	1st to 10th	Day	14% 9% 38% 30% 8% 1	653
	11th to 20th	Day	17% 8% 31% 26% 16% 3	675
	21st to end	Day	13% 8% 34% 29% 12% 3	734
June	1st to 10th	Day	5% 4 28% 35% 19% 8%	680
	11th to 20th	Day	6% 7% 31% 38% 17% 2	680
	21st to end	Day	6% 6% 30% 43% 14% 2	680
July	1st to 10th	Day	8% 2 17% 39% 27% 7%	671
	11th to 20th	Day	4 2 15% 45% 28% 6%	668
	21st to end	Day	7% 2 13% 42% 28% 8%	739
August	1st to 10th	Day	8% 2 17% 41% 26% 6%	676
	11th to 20th	Day	7% 3 17% 43% 26% 3	665
	21st to end	Day	7% 3 18% 41% 25% 7%	582
September	1st to 10th	Day	11% 3 17% 37% 28% 4	348
	11th to 20th	Day	4 3 25% 42% 18% 9%	300
	21st to end	Day	12% 5% 29% 25% 24% 5%	300
October	1st to 10th	Day	15% 7% 30% 24% 19% 6%	300
	11th to 20th	Day	6% 9% 42% 27% 13% 3	300
	21st to end	Day	18% 14% 34% 22% 12% 2	330
November	1st to 10th	Day	13% 14% 33% 26% 11% 3	300
	11th to 20th	Day	13% 13% 38% 25% 9% 2	300
	21st to end	Day	15% 12% 34% 27% 9% 3	300
December	1st to 10th	Day	8% 10% 39% 30% 11% 3	300
	11th to 20th	Day	11% 10% 44% 27% 7%	300
	21st to end	Day	17% 14% 43% 22% 5%	330
January	1st to 10th	Night	9% 10% 40% 35% 6%	1000
	11th to 20th	Night	9% 12% 40% 32% 7% 1	1000
	21st to end	Night	9% 10% 45% 31% 5%	1100
February	1st to 10th	Night	13% 10% 42% 30% 4	921
	11th to 20th	Night	11% 13% 44% 30% 3	784
	21st to end	Night	9% 13% 44% 29% 4	660
March	1st to 10th	Night	9% 16% 42% 29% 4	744
	11th to 20th	Night	15% 11% 38% 29% 8%	600
	21st to end	Night	18% 14% 41% 24% 4	660
April	1st to 10th	Night	13% 13% 42% 26% 5%	600
	11th to 20th	Night	13% 15% 42% 25% 4	593
	21st to end	Night	18% 12% 37% 27% 6%	587
May	1st to 10th	Night	15% 10% 39% 30% 5%	570
	11th to 20th	Night	13% 8% 33% 34% 12% 1	561
	21st to end	Night	12% 9% 33% 33% 13% 1	617
June	1st to 10th	Night	6% 6% 30% 41% 15% 1	570
	11th to 20th	Night	4 7% 39% 39% 10%	570
	21st to end	Night	5% 3 34% 47% 10% 1	570
July	1st to 10th	Night	6% 3 21% 49% 20% 1	555
	11th to 20th	Night	3 1 27% 49% 20% 1	552
	21st to end	Night	8% 1 22% 46% 22% 1	624
August	1st to 10th	Night	5% 2 18% 54% 20% 1	567
	11th to 20th	Night	9% 2 20% 51% 18%	564
	21st to end	Night	4 2 22% 49% 23% 1	627
September	1st to 10th	Night	10% 3 24% 46% 16% 1	570
	11th to 20th	Night	9% 3 34% 39% 14% 1	570
	21st to end	Night	7% 6% 31% 40% 15% 1	570
October	1st to 10th	Night	11% 6% 32% 36% 15% 1	570
	11th to 20th	Night	10% 8% 40% 34% 8%	732
	21st to end	Night	12% 9% 35% 31% 11% 1	902
November	1st to 10th	Night	10% 9% 35% 35% 10% 1	975
	11th to 20th	Night	8% 11% 42% 31% 8%	1000
	21st to end	Night	12% 10% 34% 34% 11% 1	1000
December	1st to 10th	Night	8% 10% 39% 36% 7%	1000
	11th to 20th	Night	12% 12% 41% 30% 5%	1000
	21st to end	Night	13% 11% 38% 31% 5%	1100

Page generated at 2025-04-23 08:25:07. Version ac07

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