Northstar Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Northstar Observatory clear sky chart from 2005-11-06 to 2026-02-01.

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	11% 9% 3 7% 4 4 4 4 5% 6% 42%	131051
	Night	12% 10% 3 7% 5% 4 4 4 5% 6% 38%	129655
Transparency	Day	40% 7% 12% 22% 14% 4	130777
	Night	44% 8% 12% 20% 12% 4	129506
Seeing	Day	17% 12% 24% 25% 17% 5%	16352
	Night	20% 7% 17% 33% 20% 3	25655

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	20% 25% 8% 14% 5% 4 4 3 4 3 8%	3719
	11th to 20th	Day	15% 11% 3 9% 7% 5% 5% 4 5% 7% 27%	3696
	21st to end	Day	19% 15% 4 8% 4 5% 4 3 5% 6% 26%	4092
February	1st to 10th	Day	22% 16% 6% 9% 4 4 5% 3 5% 5% 20%	1600
	11th to 20th	Day	17% 18% 5% 10% 4 4 4 4 5% 5% 24%	1872
	21st to end	Day	17% 20% 6% 9% 6% 4 4 3 4 4 23%	1650
March	1st to 10th	Day	9% 14% 3 7% 5% 5% 6% 5% 7% 8% 30%	4518
	11th to 20th	Day	11% 11% 3 8% 5% 5% 5% 4 5% 6% 36%	4727
	21st to end	Day	16% 15% 4 10% 7% 6% 5% 4 6% 6% 20%	5871
April	1st to 10th	Day	16% 13% 3 9% 5% 6% 5% 5% 6% 7% 25%	2400
	11th to 20th	Day	12% 10% 3 8% 7% 5% 5% 4 5% 6% 34%	2460
	21st to end	Day	12% 9% 3 8% 6% 5% 6% 4 6% 7% 34%	2600
May	1st to 10th	Day	7% 5% 2 7% 4 5% 5% 4 7% 8% 45%	6143
	11th to 20th	Day	8% 8% 3 8% 5% 5% 5% 4 7% 11% 36%	6277
	21st to end	Day	8% 6% 2 7% 5% 5% 5% 4 6% 8% 43%	7036
June	1st to 10th	Day	16% 7% 2 7% 4 5% 4 4 5% 7% 39%	3000
	11th to 20th	Day	14% 6% 2 7% 6% 4 5% 4 6% 8% 38%	3000
	21st to end	Day	6% 4 1 5% 4 4 4 4 6% 7% 55%	3000
July	1st to 10th	Day	4 2 1 3 2 3 3 3 4 4 72%	6469
	11th to 20th	Day	2 1 1 2 2 2 2 2 3 4 78%	6320
	21st to end	Day	3 2 1 3 2 2 2 2 4 5% 73%	6874
August	1st to 10th	Day	3 3 1 3 3 3 3 2 4 7% 69%	4541
	11th to 20th	Day	3 3 1 4 3 3 3 3 5% 7% 65%	4253
	21st to end	Day	3 3 2 4 3 3 4 3 6% 6% 63%	4441
September	1st to 10th	Day	5% 3 1 4 2 3 4 3 4 6% 65%	2400
	11th to 20th	Day	8% 5% 2 6% 4 3 5% 4 5% 7% 51%	2270
	21st to end	Day	8% 6% 2 6% 4 4 4 4 6% 8% 48%	2200
October	1st to 10th	Day	9% 5% 2 5% 4 3 3 3 4 7% 56%	3509
	11th to 20th	Day	11% 7% 2 6% 5% 5% 5% 4 5% 8% 43%	3035
	21st to end	Day	17% 10% 4 8% 5% 5% 6% 4 6% 6% 30%	3200
November	1st to 10th	Day	25% 13% 5% 7% 5% 4 4 3 4 7% 24%	1854
	11th to 20th	Day	22% 15% 5% 11% 5% 5% 4 4 4 4 22%	1810
	21st to end	Day	20% 19% 5% 11% 6% 5% 4 3 4 4 20%	1491
December	1st to 10th	Day	15% 13% 7% 10% 8% 5% 5% 5% 6% 6% 20%	2360
	11th to 20th	Day	20% 24% 7% 13% 7% 5% 5% 3 4 3 10%	2712
	21st to end	Day	20% 23% 7% 12% 6% 5% 5% 3 4 4 11%	3651
January	1st to 10th	Night	23% 21% 5% 12% 6% 6% 5% 3 4 3 11%	6446
	11th to 20th	Night	13% 11% 4 9% 5% 4 5% 5% 7% 7% 30%	6422
	21st to end	Night	20% 15% 4 6% 4 4 4 3 5% 8% 27%	7106
February	1st to 10th	Night	19% 19% 5% 8% 5% 3 4 3 5% 5% 25%	2800
	11th to 20th	Night	16% 15% 4 9% 5% 5% 4 4 5% 5% 29%	2412
	21st to end	Night	14% 15% 3 8% 6% 5% 5% 4 6% 6% 28%	1980
March	1st to 10th	Night	7% 11% 3 7% 5% 5% 5% 4 7% 7% 38%	5376
	11th to 20th	Night	12% 12% 3 7% 4 4 4 4 4 5% 40%	5271
	21st to end	Night	12% 11% 4 10% 6% 6% 8% 6% 7% 7% 21%	5101
April	1st to 10th	Night	11% 9% 3 7% 6% 5% 6% 6% 8% 8% 33%	2000
	11th to 20th	Night	12% 9% 2 8% 5% 5% 4 3 4 5% 41%	1891
	21st to end	Night	11% 9% 2 8% 5% 5% 4 4 6% 7% 39%	1800
May	1st to 10th	Night	8% 5% 2 6% 5% 5% 5% 4 6% 8% 46%	3810
	11th to 20th	Night	11% 8% 3 8% 5% 5% 5% 4 6% 9% 37%	3616
	21st to end	Night	9% 5% 3 7% 5% 4 5% 4 6% 8% 45%	3834
June	1st to 10th	Night	12% 6% 2 7% 5% 4 5% 4 5% 8% 43%	1400
	11th to 20th	Night	10% 4 2 7% 5% 4 6% 5% 6% 9% 41%	1400
	21st to end	Night	5% 5% 2 5% 5% 4 5% 3 5% 7% 54%	1400
July	1st to 10th	Night	3 1 1 2 2 2 2 2 4 5% 76%	3472
	11th to 20th	Night	2 1 1 2 2 2 2 1 3 4 80%	3640
	21st to end	Night	3 3 1 3 2 2 3 2 4 5% 73%	3969
August	1st to 10th	Night	3 3 1 3 2 2 2 2 5% 6% 71%	2609
	11th to 20th	Night	2 3 2 5% 3 3 4 3 5% 7% 62%	2802
	21st to end	Night	3 3 1 4 4 3 4 3 5% 7% 63%	3471
September	1st to 10th	Night	4 2 2 4 3 3 2 3 5% 8% 65%	2000
	11th to 20th	Night	7% 5% 2 6% 4 5% 4 3 5% 7% 52%	2130
	21st to end	Night	11% 5% 2 4 3 4 4 4 5% 8% 52%	2200
October	1st to 10th	Night	7% 3 2 5% 3 4 3 3 5% 7% 59%	3688
	11th to 20th	Night	12% 5% 2 5% 3 5% 4 3 5% 7% 49%	4115
	21st to end	Night	13% 6% 2 7% 4 5% 5% 4 6% 7% 41%	4600
November	1st to 10th	Night	22% 13% 3 7% 4 4 4 4 6% 7% 26%	2665
	11th to 20th	Night	22% 14% 4 8% 5% 4 5% 4 4 5% 25%	2809
	21st to end	Night	22% 13% 3 9% 5% 5% 4 4 4 6% 25%	3104
December	1st to 10th	Night	12% 11% 5% 9% 6% 6% 7% 5% 8% 6% 28%	5040
	11th to 20th	Night	20% 21% 5% 12% 6% 5% 5% 4 5% 4 11%	5700
	21st to end	Night	19% 19% 7% 13% 6% 5% 6% 4 4 4 12%	7576

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	79% 8% 8% 4 1	3719
	11th to 20th	Day	53% 9% 16% 17% 4 1	3720
	21st to end	Day	57% 7% 14% 17% 4	4092
February	1st to 10th	Day	66% 8% 12% 9% 5% 1	1600
	11th to 20th	Day	62% 9% 12% 11% 6%	1882
	21st to end	Day	64% 8% 9% 15% 5%	1650
March	1st to 10th	Day	47% 9% 13% 19% 13%	4518
	11th to 20th	Day	45% 7% 16% 28% 4	4727
	21st to end	Day	61% 10% 13% 12% 4	5871
April	1st to 10th	Day	56% 9% 16% 15% 4	2400
	11th to 20th	Day	48% 8% 14% 19% 9% 1	2460
	21st to end	Day	48% 10% 14% 19% 8% 1	2600
May	1st to 10th	Day	33% 9% 12% 28% 18%	6143
	11th to 20th	Day	38% 8% 18% 28% 8%	6280
	21st to end	Day	35% 11% 18% 27% 8%	7037
June	1st to 10th	Day	43% 8% 13% 22% 13% 1	3000
	11th to 20th	Day	42% 8% 15% 22% 11% 3	3000
	21st to end	Day	27% 7% 15% 29% 18% 3	3000
July	1st to 10th	Day	16% 3 8% 23% 31% 19%	6400
	11th to 20th	Day	11% 3 6% 23% 30% 28%	6245
	21st to end	Day	14% 3 6% 31% 30% 16%	6681
August	1st to 10th	Day	17% 3 6% 24% 37% 13%	4524
	11th to 20th	Day	18% 4 10% 40% 24% 4	4294
	21st to end	Day	20% 5% 12% 35% 25% 4	4440
September	1st to 10th	Day	20% 5% 11% 30% 23% 10%	2400
	11th to 20th	Day	32% 7% 12% 24% 21% 4	2270
	21st to end	Day	33% 10% 18% 25% 12% 3	2200
October	1st to 10th	Day	28% 4 13% 38% 16% 1	3509
	11th to 20th	Day	41% 5% 19% 24% 12% 1	3035
	21st to end	Day	52% 8% 16% 18% 7%	3200
November	1st to 10th	Day	61% 8% 14% 14% 3	1854
	11th to 20th	Day	65% 9% 13% 8% 3 1	1810
	21st to end	Day	66% 8% 11% 11% 4	1491
December	1st to 10th	Day	62% 10% 14% 12% 2	2360
	11th to 20th	Day	78% 7% 8% 6% 1	2714
	21st to end	Day	73% 9% 9% 6% 3	3651
January	1st to 10th	Night	74% 9% 8% 7% 1	6446
	11th to 20th	Night	50% 10% 16% 16% 7% 1	6460
	21st to end	Night	54% 8% 13% 20% 5%	7106
February	1st to 10th	Night	61% 8% 13% 12% 5% 1	2800
	11th to 20th	Night	56% 8% 16% 12% 7% 1	2413
	21st to end	Night	55% 9% 12% 17% 6% 1	1980
March	1st to 10th	Night	40% 8% 13% 24% 14% 1	5376
	11th to 20th	Night	45% 6% 13% 28% 8%	5272
	21st to end	Night	53% 13% 13% 17% 3	5101
April	1st to 10th	Night	45% 10% 15% 22% 8%	2000
	11th to 20th	Night	44% 7% 12% 23% 14% 1	1891
	21st to end	Night	43% 8% 13% 24% 12% 1	1800
May	1st to 10th	Night	32% 8% 11% 26% 22% 1	3807
	11th to 20th	Night	40% 9% 13% 26% 13% 1	3616
	21st to end	Night	32% 11% 15% 30% 11% 1	3834
June	1st to 10th	Night	37% 9% 10% 23% 19% 2	1400
	11th to 20th	Night	37% 8% 12% 23% 16% 4	1400
	21st to end	Night	28% 6% 12% 26% 22% 6%	1400
July	1st to 10th	Night	12% 3 6% 19% 35% 26%	3432
	11th to 20th	Night	11% 2 4 18% 30% 37%	3592
	21st to end	Night	14% 3 4 24% 31% 24%	3857
August	1st to 10th	Night	15% 2 5% 19% 38% 19%	2601
	11th to 20th	Night	21% 3 8% 31% 29% 8%	2827
	21st to end	Night	21% 5% 10% 27% 29% 9%	3471
September	1st to 10th	Night	18% 5% 9% 27% 28% 12%	2000
	11th to 20th	Night	29% 7% 11% 23% 23% 7%	2130
	21st to end	Night	30% 6% 15% 27% 16% 6%	2200
October	1st to 10th	Night	25% 4 12% 40% 18% 1	3688
	11th to 20th	Night	36% 5% 14% 24% 20% 1	4115
	21st to end	Night	39% 9% 14% 28% 8% 2	4600
November	1st to 10th	Night	55% 9% 17% 14% 4 1	2665
	11th to 20th	Night	61% 8% 14% 10% 5% 1	2809
	21st to end	Night	59% 8% 15% 13% 5%	3104
December	1st to 10th	Night	53% 12% 17% 15% 2	5040
	11th to 20th	Night	73% 8% 11% 7% 1	5697
	21st to end	Night	69% 11% 11% 7% 2	7576

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	28% 20% 20% 20% 10% 3	220
	11th to 20th	Day	27% 14% 17% 26% 11% 5%	220
	21st to end	Day	28% 10% 24% 23% 13% 2	242
February	1st to 10th	Day	32% 17% 28% 14% 10% 1	200
	11th to 20th	Day	28% 18% 23% 21% 9% 1	445
	21st to end	Day	27% 19% 21% 22% 9% 1	414
March	1st to 10th	Day	23% 22% 24% 19% 10% 3	500
	11th to 20th	Day	23% 14% 21% 23% 16% 3	500
	21st to end	Day	27% 19% 24% 20% 10% 1	550
April	1st to 10th	Day	22% 16% 28% 22% 9% 3	500
	11th to 20th	Day	17% 21% 28% 18% 13% 4	492
	21st to end	Day	15% 16% 26% 26% 13% 4	500
May	1st to 10th	Day	12% 13% 27% 30% 14% 4	503
	11th to 20th	Day	17% 9% 28% 24% 17% 5%	528
	21st to end	Day	11% 12% 21% 28% 19% 9%	628
June	1st to 10th	Day	17% 10% 21% 25% 21% 5%	730
	11th to 20th	Day	15% 14% 30% 24% 15% 3	730
	21st to end	Day	7% 8% 24% 29% 26% 6%	730
July	1st to 10th	Day	6% 5% 24% 34% 27% 4	597
	11th to 20th	Day	4 8% 26% 33% 24% 5%	524
	21st to end	Day	3 6% 22% 34% 26% 8%	575
August	1st to 10th	Day	6% 6% 23% 33% 25% 8%	527
	11th to 20th	Day	4 3 22% 36% 26% 10%	517
	21st to end	Day	7% 8% 25% 27% 23% 10%	583
September	1st to 10th	Day	8% 7% 24% 26% 23% 12%	530
	11th to 20th	Day	10% 12% 22% 26% 22% 8%	530
	21st to end	Day	12% 13% 25% 25% 20% 5%	530
October	1st to 10th	Day	14% 8% 27% 27% 18% 6%	530
	11th to 20th	Day	19% 14% 22% 23% 14% 8%	394
	21st to end	Day	28% 12% 22% 24% 11% 3	363
November	1st to 10th	Day	33% 13% 18% 24% 12%	330
	11th to 20th	Day	33% 12% 20% 19% 14% 2	288
	21st to end	Day	27% 15% 19% 25% 14% 1	220
December	1st to 10th	Day	29% 15% 18% 16% 15% 6%	220
	11th to 20th	Day	37% 13% 24% 17% 8% 1	220
	21st to end	Day	35% 14% 19% 19% 11% 2	242
January	1st to 10th	Night	38% 9% 17% 25% 9% 2	840
	11th to 20th	Night	27% 9% 17% 32% 13% 3	840
	21st to end	Night	25% 7% 18% 34% 15% 1	924
February	1st to 10th	Night	30% 10% 22% 27% 10% 2	800
	11th to 20th	Night	26% 9% 17% 32% 13% 2	784
	21st to end	Night	23% 11% 21% 27% 17% 2	660
March	1st to 10th	Night	21% 10% 20% 31% 17% 1	800
	11th to 20th	Night	22% 10% 16% 32% 18% 1	800
	21st to end	Night	21% 10% 23% 33% 12% 2	880
April	1st to 10th	Night	17% 7% 20% 37% 18% 2	800
	11th to 20th	Night	15% 10% 24% 34% 15% 2	703
	21st to end	Night	14% 8% 21% 35% 20% 2	600
May	1st to 10th	Night	10% 6% 22% 34% 26% 3	479
	11th to 20th	Night	13% 5% 18% 28% 28% 8%	394
	21st to end	Night	13% 5% 16% 30% 30% 5%	433
June	1st to 10th	Night	14% 7% 16% 36% 25% 4	400
	11th to 20th	Night	12% 7% 23% 35% 21% 3	400
	21st to end	Night	6% 4 13% 40% 31% 5%	400
July	1st to 10th	Night	4 2 13% 39% 40% 2	390
	11th to 20th	Night	3 3 14% 46% 32% 2	388
	21st to end	Night	2 2 9% 42% 41% 4	438
August	1st to 10th	Night	2 3 10% 39% 39% 7%	398
	11th to 20th	Night	3 2 9% 38% 39% 9%	531
	21st to end	Night	5% 4 14% 35% 39% 4	795
September	1st to 10th	Night	6% 2 14% 38% 34% 6%	800
	11th to 20th	Night	9% 5% 14% 30% 35% 8%	800
	21st to end	Night	12% 5% 18% 34% 26% 5%	800
October	1st to 10th	Night	14% 6% 13% 40% 25% 2	800
	11th to 20th	Night	19% 6% 16% 32% 25% 3	800
	21st to end	Night	21% 8% 20% 32% 16% 3	880
November	1st to 10th	Night	30% 8% 16% 33% 13%	820
	11th to 20th	Night	31% 9% 18% 27% 13% 1	840
	21st to end	Night	29% 9% 16% 28% 16% 2	1004
December	1st to 10th	Night	27% 8% 17% 31% 16% 1	1050
	11th to 20th	Night	35% 9% 18% 29% 9%	1050
	21st to end	Night	35% 12% 18% 26% 9%	1134

Page generated at 2026-01-28 22:21:45. Version ac07

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