Ashern
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ashern clear sky chart from 2005-11-06 to 2025-12-18.

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% 8% 3 8% 5% 5% 5% 4 6% 8% 37%	131449
	Night	8% 9% 4 10% 6% 5% 5% 5% 6% 7% 34%	128355
Transparency	Day	43% 8% 16% 24% 8% 1	131147
	Night	46% 8% 14% 22% 9% 1	128238
Seeing	Day	14% 10% 31% 28% 14% 3	17846
	Night	14% 7% 31% 34% 13% 1	27825

By Month

Percentage Cloud Cover NW (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 14% 6% 14% 7% 7% 9% 6% 7% 8% 17%	3179
	11th to 20th	Day	4 10% 5% 15% 9% 9% 7% 8% 13% 7% 12%	3159
	21st to end	Day	4 13% 7% 14% 7% 6% 7% 5% 6% 8% 24%	3985
February	1st to 10th	Day	8% 14% 5% 12% 9% 6% 7% 6% 7% 7% 21%	1613
	11th to 20th	Day	5% 11% 4 8% 6% 7% 8% 6% 7% 8% 31%	1754
	21st to end	Day	7% 10% 4 10% 8% 6% 7% 6% 6% 7% 30%	1595
March	1st to 10th	Day	8% 9% 5% 10% 8% 6% 6% 5% 7% 8% 30%	4500
	11th to 20th	Day	7% 11% 3 12% 8% 6% 6% 5% 6% 7% 28%	4973
	21st to end	Day	8% 8% 3 9% 6% 5% 6% 4 6% 7% 39%	5481
April	1st to 10th	Day	9% 10% 3 8% 5% 6% 5% 3 5% 7% 38%	2479
	11th to 20th	Day	12% 11% 4 7% 5% 6% 4 4 5% 6% 37%	2572
	21st to end	Day	12% 8% 3 8% 4 5% 5% 4 6% 8% 38%	2600
May	1st to 10th	Day	12% 7% 2 5% 4 4 4 4 5% 6% 48%	6228
	11th to 20th	Day	17% 9% 3 8% 5% 5% 5% 4 5% 6% 33%	6755
	21st to end	Day	13% 5% 2 5% 4 4 3 3 5% 7% 50%	7428
June	1st to 10th	Day	15% 8% 3 7% 5% 4 4 4 6% 8% 37%	3000
	11th to 20th	Day	13% 6% 2 7% 6% 5% 5% 4 6% 7% 39%	3000
	21st to end	Day	12% 6% 2 6% 5% 5% 5% 5% 7% 9% 38%	3000
July	1st to 10th	Day	9% 5% 2 6% 4 4 5% 5% 7% 9% 46%	6800
	11th to 20th	Day	9% 5% 2 6% 4 5% 4 4 7% 9% 46%	6800
	21st to end	Day	8% 4 2 5% 4 4 5% 4 6% 9% 50%	7320
August	1st to 10th	Day	7% 4 2 6% 5% 5% 6% 4 8% 10% 43%	4552
	11th to 20th	Day	11% 5% 2 6% 4 4 5% 3 6% 8% 45%	4180
	21st to end	Day	13% 5% 2 5% 4 4 4 4 6% 8% 44%	4675
September	1st to 10th	Day	11% 6% 2 6% 4 3 4 4 7% 8% 46%	2495
	11th to 20th	Day	15% 9% 3 8% 5% 4 4 4 6% 7% 35%	2200
	21st to end	Day	12% 7% 2 7% 4 5% 4 4 7% 8% 40%	2200
October	1st to 10th	Day	13% 8% 2 7% 5% 5% 4 4 6% 8% 39%	3371
	11th to 20th	Day	10% 10% 3 7% 5% 5% 5% 4 6% 10% 35%	3035
	21st to end	Day	12% 13% 5% 9% 5% 5% 5% 5% 6% 7% 28%	3190
November	1st to 10th	Day	16% 16% 5% 11% 6% 5% 3 3 4 6% 25%	1687
	11th to 20th	Day	9% 13% 6% 14% 8% 6% 5% 3 5% 8% 23%	1627
	21st to end	Day	11% 16% 5% 12% 6% 6% 6% 3 5% 7% 24%	1470
December	1st to 10th	Day	9% 14% 6% 13% 10% 11% 7% 6% 5% 5% 13%	2360
	11th to 20th	Day	9% 13% 4 11% 7% 5% 6% 5% 9% 9% 23%	2688
	21st to end	Day	5% 9% 6% 17% 8% 6% 6% 4 5% 5% 30%	3498
January	1st to 10th	Night	4 12% 6% 15% 8% 7% 7% 5% 6% 8% 22%	6787
	11th to 20th	Night	5% 12% 6% 15% 10% 9% 9% 8% 8% 5% 13%	6759
	21st to end	Night	6% 14% 7% 16% 7% 7% 6% 5% 7% 8% 18%	6992
February	1st to 10th	Night	8% 12% 5% 12% 6% 5% 5% 6% 6% 8% 27%	2786
	11th to 20th	Night	4 9% 4 7% 5% 6% 7% 6% 8% 8% 35%	2533
	21st to end	Night	5% 9% 4 11% 7% 6% 6% 5% 5% 7% 35%	2054
March	1st to 10th	Night	9% 11% 4 10% 7% 5% 6% 4 6% 8% 31%	5374
	11th to 20th	Night	5% 9% 4 9% 7% 5% 5% 5% 7% 8% 36%	4985
	21st to end	Night	9% 10% 3 8% 5% 5% 5% 4 6% 7% 38%	5438
April	1st to 10th	Night	8% 8% 3 6% 6% 5% 5% 6% 7% 7% 39%	1921
	11th to 20th	Night	13% 8% 3 8% 6% 6% 5% 5% 6% 6% 35%	1779
	21st to end	Night	11% 8% 4 9% 5% 4 5% 3 5% 8% 38%	1800
May	1st to 10th	Night	14% 9% 2 7% 4 4 5% 4 5% 7% 39%	3734
	11th to 20th	Night	14% 8% 3 9% 6% 4 5% 4 6% 7% 33%	3161
	21st to end	Night	10% 5% 2 6% 4 4 4 3 6% 6% 50%	3470
June	1st to 10th	Night	13% 7% 3 7% 4 5% 5% 5% 7% 8% 37%	1400
	11th to 20th	Night	15% 5% 3 5% 6% 4 5% 3 7% 10% 39%	1400
	21st to end	Night	11% 5% 2 6% 4 5% 7% 5% 8% 7% 41%	1400
July	1st to 10th	Night	6% 3 2 4 4 4 5% 4 7% 9% 54%	3180
	11th to 20th	Night	7% 5% 2 6% 4 5% 4 4 7% 10% 47%	3180
	21st to end	Night	6% 4 2 5% 4 4 4 4 7% 8% 52%	3544
August	1st to 10th	Night	9% 4 2 5% 4 5% 5% 4 7% 8% 47%	2608
	11th to 20th	Night	8% 4 2 4 4 4 5% 4 7% 10% 50%	2879
	21st to end	Night	11% 5% 2 6% 3 4 3 3 6% 7% 49%	3234
September	1st to 10th	Night	8% 4 2 5% 4 3 4 4 6% 11% 49%	1902
	11th to 20th	Night	12% 7% 3 6% 5% 4 5% 3 6% 7% 42%	2200
	21st to end	Night	11% 6% 2 5% 5% 5% 4 4 5% 8% 45%	2200
October	1st to 10th	Night	11% 7% 3 7% 4 4 4 3 5% 8% 44%	3775
	11th to 20th	Night	8% 8% 3 8% 5% 5% 5% 4 7% 9% 39%	4095
	21st to end	Night	10% 10% 3 9% 5% 5% 5% 4 6% 7% 35%	4610
November	1st to 10th	Night	12% 14% 4 11% 7% 6% 5% 4 5% 6% 26%	2830
	11th to 20th	Night	9% 15% 5% 13% 6% 6% 5% 4 6% 6% 26%	2991
	21st to end	Night	10% 14% 6% 12% 8% 6% 6% 4 6% 6% 22%	3150
December	1st to 10th	Night	9% 16% 5% 13% 10% 7% 7% 5% 5% 6% 17%	5050
	11th to 20th	Night	7% 12% 6% 14% 6% 5% 5% 6% 6% 6% 26%	5674
	21st to end	Night	6% 10% 5% 13% 8% 6% 5% 4 7% 7% 29%	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% 8% 13% 15% 4	3179
	11th to 20th	Day	60% 8% 7% 18% 7%	3180
	21st to end	Day	56% 9% 15% 17% 4	3985
February	1st to 10th	Day	58% 8% 12% 14% 7% 1	1613
	11th to 20th	Day	47% 9% 16% 20% 8%	1763
	21st to end	Day	50% 7% 13% 21% 9%	1595
March	1st to 10th	Day	48% 9% 15% 18% 9% 1	4500
	11th to 20th	Day	48% 11% 13% 20% 7%	4975
	21st to end	Day	41% 8% 12% 28% 11%	5481
April	1st to 10th	Day	44% 7% 14% 22% 12% 1	2479
	11th to 20th	Day	47% 7% 12% 20% 14% 1	2572
	21st to end	Day	41% 7% 15% 26% 8% 2	2600
May	1st to 10th	Day	35% 6% 11% 31% 17% 1	6228
	11th to 20th	Day	48% 8% 11% 22% 10% 1	6759
	21st to end	Day	33% 7% 9% 22% 24% 4	7428
June	1st to 10th	Day	44% 7% 15% 24% 9% 1	3000
	11th to 20th	Day	43% 8% 16% 22% 10% 1	3000
	21st to end	Day	40% 10% 21% 23% 6%	3000
July	1st to 10th	Day	33% 8% 26% 27% 6%	6726
	11th to 20th	Day	31% 7% 22% 35% 4	6719
	21st to end	Day	29% 8% 25% 31% 7%	7114
August	1st to 10th	Day	32% 9% 21% 30% 8%	4536
	11th to 20th	Day	36% 8% 24% 29% 4	4219
	21st to end	Day	36% 8% 25% 27% 4	4674
September	1st to 10th	Day	34% 9% 20% 29% 8% 1	2495
	11th to 20th	Day	46% 9% 18% 22% 4	2200
	21st to end	Day	40% 9% 18% 26% 8%	2200
October	1st to 10th	Day	42% 7% 17% 30% 4	3371
	11th to 20th	Day	43% 7% 15% 28% 7%	3035
	21st to end	Day	54% 7% 18% 17% 4	3190
November	1st to 10th	Day	61% 7% 12% 15% 4 1	1687
	11th to 20th	Day	59% 10% 14% 12% 4	1627
	21st to end	Day	59% 8% 15% 14% 4	1470
December	1st to 10th	Day	69% 9% 11% 8% 3	2360
	11th to 20th	Day	54% 8% 15% 17% 6%	2689
	21st to end	Day	53% 7% 11% 17% 10% 1	3498
January	1st to 10th	Night	56% 8% 11% 17% 7%	6787
	11th to 20th	Night	66% 7% 8% 15% 4	6800
	21st to end	Night	60% 9% 12% 14% 5%	6992
February	1st to 10th	Night	53% 7% 14% 17% 9%	2786
	11th to 20th	Night	41% 9% 18% 23% 9%	2534
	21st to end	Night	46% 6% 13% 26% 9%	2054
March	1st to 10th	Night	49% 7% 14% 19% 9% 3	5374
	11th to 20th	Night	39% 11% 14% 23% 12% 1	4985
	21st to end	Night	42% 7% 10% 28% 12% 1	5438
April	1st to 10th	Night	39% 6% 13% 23% 16% 3	1921
	11th to 20th	Night	47% 6% 9% 19% 16% 3	1779
	21st to end	Night	43% 7% 13% 23% 12% 2	1800
May	1st to 10th	Night	42% 7% 12% 22% 17% 1	3734
	11th to 20th	Night	48% 7% 12% 22% 9% 1	3161
	21st to end	Night	32% 8% 10% 21% 22% 8%	3470
June	1st to 10th	Night	42% 8% 14% 24% 10% 1	1400
	11th to 20th	Night	40% 8% 17% 22% 11% 3	1400
	21st to end	Night	37% 12% 19% 25% 7% 1	1400
July	1st to 10th	Night	25% 8% 26% 34% 7%	3145
	11th to 20th	Night	31% 7% 21% 35% 5%	3138
	21st to end	Night	27% 8% 24% 29% 11%	3445
August	1st to 10th	Night	32% 9% 16% 31% 12%	2600
	11th to 20th	Night	28% 7% 25% 32% 7%	2906
	21st to end	Night	34% 7% 21% 31% 7%	3234
September	1st to 10th	Night	29% 7% 20% 31% 12% 1	1902
	11th to 20th	Night	39% 10% 18% 25% 8% 1	2200
	21st to end	Night	37% 8% 15% 30% 10% 1	2200
October	1st to 10th	Night	37% 6% 15% 30% 12%	3775
	11th to 20th	Night	40% 6% 15% 28% 10% 1	4095
	21st to end	Night	46% 7% 17% 26% 4	4610
November	1st to 10th	Night	57% 9% 15% 13% 5% 1	2830
	11th to 20th	Night	58% 9% 16% 12% 5%	2991
	21st to end	Night	60% 9% 14% 13% 4	3150
December	1st to 10th	Night	66% 8% 13% 10% 3	5050
	11th to 20th	Night	55% 6% 12% 14% 12%	5672
	21st to end	Night	52% 7% 10% 18% 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	8% 9% 36% 35% 10% 3	300
	11th to 20th	Day	9% 9% 37% 32% 11% 1	300
	21st to end	Day	13% 11% 32% 29% 14% 2	330
February	1st to 10th	Day	13% 10% 36% 31% 9% 2	300
	11th to 20th	Day	12% 8% 33% 32% 13% 3	300
	21st to end	Day	6% 12% 36% 28% 17% 1	249
March	1st to 10th	Day	14% 9% 35% 26% 15% 1	300
	11th to 20th	Day	12% 14% 38% 24% 10% 3	300
	21st to end	Day	13% 9% 32% 31% 14% 1	335
April	1st to 10th	Day	14% 11% 32% 31% 10% 2	579
	11th to 20th	Day	17% 10% 31% 30% 11% 1	690
	21st to end	Day	13% 11% 32% 30% 12% 1	700
May	1st to 10th	Day	13% 11% 32% 29% 13% 2	703
	11th to 20th	Day	18% 10% 28% 28% 14% 3	725
	21st to end	Day	18% 10% 25% 26% 18% 2	789
June	1st to 10th	Day	16% 11% 31% 24% 13% 5%	730
	11th to 20th	Day	15% 10% 31% 27% 13% 3	730
	21st to end	Day	13% 7% 25% 33% 18% 4	730
July	1st to 10th	Day	9% 6% 28% 30% 21% 6%	721
	11th to 20th	Day	11% 7% 29% 30% 17% 6%	718
	21st to end	Day	9% 8% 25% 30% 21% 8%	794
August	1st to 10th	Day	7% 8% 28% 30% 21% 6%	726
	11th to 20th	Day	12% 8% 23% 27% 22% 8%	715
	21st to end	Day	14% 9% 26% 27% 21% 3	803
September	1st to 10th	Day	12% 9% 31% 32% 13% 3	625
	11th to 20th	Day	20% 9% 35% 22% 13% 2	330
	21st to end	Day	18% 8% 35% 28% 8% 2	330
October	1st to 10th	Day	21% 12% 27% 26% 11% 2	330
	11th to 20th	Day	22% 12% 36% 22% 8% 1	330
	21st to end	Day	23% 13% 33% 22% 8% 2	363
November	1st to 10th	Day	24% 12% 32% 22% 9% 1	330
	11th to 20th	Day	14% 14% 38% 23% 9% 2	330
	21st to end	Day	18% 12% 37% 26% 7% 1	330
December	1st to 10th	Day	15% 9% 38% 22% 14% 2	330
	11th to 20th	Day	16% 12% 36% 26% 8% 2	321
	21st to end	Day	9% 7% 35% 29% 17% 3	330
January	1st to 10th	Night	11% 7% 34% 35% 12% 2	1000
	11th to 20th	Night	14% 7% 33% 33% 11% 1	1000
	21st to end	Night	14% 8% 33% 34% 10% 1	1100
February	1st to 10th	Night	13% 8% 30% 36% 12% 1	1000
	11th to 20th	Night	9% 6% 31% 35% 18% 1	979
	21st to end	Night	9% 7% 32% 36% 16% 1	734
March	1st to 10th	Night	15% 7% 27% 34% 15% 2	800
	11th to 20th	Night	11% 8% 37% 34% 10% 1	605
	21st to end	Night	17% 7% 30% 31% 13% 2	660
April	1st to 10th	Night	11% 6% 35% 36% 12% 1	600
	11th to 20th	Night	16% 6% 30% 36% 12% 1	593
	21st to end	Night	13% 8% 34% 36% 10% 1	600
May	1st to 10th	Night	18% 7% 29% 34% 11% 1	600
	11th to 20th	Night	16% 8% 28% 34% 13% 2	591
	21st to end	Night	17% 8% 24% 36% 14% 1	650
June	1st to 10th	Night	17% 7% 27% 32% 17% 1	600
	11th to 20th	Night	17% 5% 27% 37% 14% 1	600
	21st to end	Night	13% 6% 19% 42% 20% 1	600
July	1st to 10th	Night	6% 2 24% 43% 22% 2	585
	11th to 20th	Night	9% 4 26% 40% 19% 3	582
	21st to end	Night	7% 5% 27% 34% 24% 4	657
August	1st to 10th	Night	8% 5% 22% 42% 22% 2	597
	11th to 20th	Night	9% 4 24% 37% 20% 5%	594
	21st to end	Night	15% 4 25% 35% 20% 2	660
September	1st to 10th	Night	10% 5% 30% 36% 17% 2	600
	11th to 20th	Night	14% 8% 27% 39% 12% 1	600
	21st to end	Night	14% 6% 32% 35% 13% 1	600
October	1st to 10th	Night	19% 9% 32% 32% 8%	707
	11th to 20th	Night	18% 8% 32% 31% 11% 1	920
	21st to end	Night	20% 11% 31% 29% 7% 1	1100
November	1st to 10th	Night	16% 12% 34% 26% 11% 2	1025
	11th to 20th	Night	16% 12% 35% 26% 8% 2	1050
	21st to end	Night	15% 11% 40% 28% 6% 1	1050
December	1st to 10th	Night	11% 8% 32% 35% 12% 2	1050
	11th to 20th	Night	14% 9% 38% 29% 8% 1	1036
	21st to end	Night	15% 6% 32% 31% 14% 2	1100

Page generated at 2025-12-15 00:13:53. Version ac07

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