North Wind Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the North Wind Observatory clear sky chart from 2005-11-06 to 2026-07-01.

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	14% 11% 5% 10% 5% 4 4 3 5% 6% 32%	137604
	Night	14% 13% 5% 11% 6% 5% 4 3 4 5% 29%	135029
Transparency	Day	52% 5% 10% 20% 12% 1	141104
	Night	55% 6% 10% 18% 9% 1	139593
Seeing	Day	23% 16% 31% 19% 8% 3	18169
	Night	22% 12% 29% 27% 9% 1	28052

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	13% 24% 12% 26% 7% 4 2 2 2 2 6%	4691
	11th to 20th	Day	10% 18% 12% 24% 13% 11% 5% 1 2 2 3	4710
	21st to end	Day	11% 16% 10% 16% 12% 8% 6% 3 3 3 13%	5156
February	1st to 10th	Day	11% 17% 8% 15% 14% 6% 5% 3 3 5% 14%	1842
	11th to 20th	Day	12% 16% 8% 17% 11% 6% 4 3 4 4 16%	2048
	21st to end	Day	12% 19% 8% 15% 7% 5% 4 3 4 5% 19%	1730
March	1st to 10th	Day	12% 12% 4 12% 7% 4 4 4 5% 5% 31%	4580
	11th to 20th	Day	13% 10% 7% 11% 5% 4 4 3 6% 7% 31%	4900
	21st to end	Day	22% 12% 8% 12% 5% 4 3 3 3 4 24%	5860
April	1st to 10th	Day	18% 17% 4 7% 4 4 3 3 5% 7% 28%	2520
	11th to 20th	Day	21% 14% 3 6% 4 4 4 3 4 5% 33%	2560
	21st to end	Day	16% 12% 3 8% 4 4 4 4 5% 7% 33%	2730
May	1st to 10th	Day	14% 8% 2 6% 4 4 4 3 5% 8% 42%	7616
	11th to 20th	Day	14% 10% 3 7% 5% 4 4 3 5% 6% 39%	7898
	21st to end	Day	14% 7% 2 6% 4 3 4 3 5% 7% 44%	9047
June	1st to 10th	Day	13% 6% 2 7% 5% 4 5% 3 5% 7% 44%	2940
	11th to 20th	Day	13% 7% 2 6% 5% 4 4 4 7% 7% 41%	2940
	21st to end	Day	14% 6% 2 6% 4 4 4 4 6% 8% 42%	2940
July	1st to 10th	Day	14% 6% 2 5% 4 4 4 4 6% 9% 42%	6361
	11th to 20th	Day	10% 5% 2 4 4 3 4 4 6% 8% 50%	6360
	21st to end	Day	9% 5% 2 5% 4 4 5% 4 6% 9% 46%	6924
August	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 5% 7% 8% 41%	4380
	11th to 20th	Day	14% 5% 2 5% 4 3 4 3 5% 6% 48%	4174
	21st to end	Day	12% 6% 2 6% 4 4 5% 4 6% 7% 43%	4383
September	1st to 10th	Day	14% 6% 2 7% 4 5% 4 3 6% 7% 42%	2390
	11th to 20th	Day	12% 6% 2 6% 3 4 5% 4 5% 7% 47%	2200
	21st to end	Day	15% 8% 2 7% 4 5% 5% 3 6% 8% 38%	2200
October	1st to 10th	Day	16% 9% 3 7% 4 4 3 3 5% 7% 41%	2200
	11th to 20th	Day	22% 13% 4 9% 5% 5% 5% 4 4 6% 23%	1906
	21st to end	Day	24% 14% 4 8% 6% 4 4 4 4 6% 23%	1980
November	1st to 10th	Day	17% 15% 4 5% 5% 4 4 4 6% 6% 30%	1852
	11th to 20th	Day	16% 21% 6% 12% 5% 5% 4 4 5% 6% 17%	1890
	21st to end	Day	19% 23% 8% 11% 5% 3 3 3 3 4 17%	1848
December	1st to 10th	Day	12% 28% 14% 17% 5% 5% 4 2 2 2 9%	2690
	11th to 20th	Day	13% 23% 13% 16% 10% 4 3 3 3 3 7%	3088
	21st to end	Day	22% 22% 8% 13% 7% 5% 3 2 3 4 11%	4070
January	1st to 10th	Night	13% 20% 10% 25% 7% 4 3 2 3 3 10%	8232
	11th to 20th	Night	10% 19% 10% 24% 12% 12% 3 2 2 2 5%	8280
	21st to end	Night	9% 17% 9% 20% 12% 8% 6% 3 3 3 9%	9058
February	1st to 10th	Night	11% 17% 6% 15% 13% 7% 4 3 4 4 16%	2773
	11th to 20th	Night	12% 16% 6% 14% 10% 6% 5% 3 5% 6% 18%	2458
	21st to end	Night	14% 17% 6% 13% 7% 5% 4 3 5% 4 22%	2076
March	1st to 10th	Night	16% 10% 4 11% 4 4 3 3 4 5% 35%	5493
	11th to 20th	Night	12% 11% 4 9% 5% 4 4 3 5% 8% 36%	5279
	21st to end	Night	18% 14% 4 11% 5% 4 4 3 4 5% 29%	5354
April	1st to 10th	Night	18% 12% 4 8% 4 3 4 4 6% 6% 31%	2100
	11th to 20th	Night	22% 9% 4 6% 4 4 4 3 3 5% 35%	2009
	21st to end	Night	18% 10% 3 7% 4 4 3 3 5% 5% 38%	1890
May	1st to 10th	Night	13% 7% 2 6% 5% 5% 5% 4 6% 8% 40%	5237
	11th to 20th	Night	12% 8% 2 6% 4 4 5% 4 6% 8% 41%	4961
	21st to end	Night	17% 6% 2 5% 4 3 4 3 4 6% 47%	5139
June	1st to 10th	Night	14% 6% 3 5% 4 4 3 3 7% 8% 43%	1680
	11th to 20th	Night	12% 6% 3 6% 4 3 5% 4 7% 8% 42%	1680
	21st to end	Night	12% 6% 2 6% 5% 4 4 4 5% 7% 45%	1680
July	1st to 10th	Night	13% 7% 2 5% 3 4 4 4 6% 7% 42%	3618
	11th to 20th	Night	8% 6% 2 4 3 3 4 3 5% 7% 55%	3617
	21st to end	Night	8% 6% 2 5% 4 4 5% 3 5% 7% 51%	3947
August	1st to 10th	Night	12% 6% 2 6% 4 4 4 3 7% 8% 43%	2771
	11th to 20th	Night	14% 5% 1 4 3 3 3 3 4 7% 51%	2875
	21st to end	Night	8% 6% 2 7% 4 4 5% 4 5% 7% 48%	3511
September	1st to 10th	Night	11% 5% 2 5% 3 3 4 4 4 6% 51%	2000
	11th to 20th	Night	12% 4 1 4 3 2 4 3 6% 8% 52%	2189
	21st to end	Night	14% 7% 3 6% 4 4 4 4 5% 7% 41%	2200
October	1st to 10th	Night	17% 8% 2 5% 4 3 3 3 4 6% 44%	2200
	11th to 20th	Night	23% 11% 3 7% 5% 4 4 3 5% 5% 30%	2490
	21st to end	Night	24% 13% 3 7% 4 3 4 3 4 6% 27%	2860
November	1st to 10th	Night	15% 14% 5% 8% 6% 4 4 3 5% 6% 32%	2666
	11th to 20th	Night	15% 19% 6% 11% 6% 5% 5% 3 5% 5% 20%	2730
	21st to end	Night	17% 22% 8% 11% 4 5% 5% 3 4 4 17%	2767
December	1st to 10th	Night	12% 22% 9% 20% 7% 4 4 3 3 4 13%	4720
	11th to 20th	Night	14% 26% 7% 16% 10% 4 3 2 3 4 10%	5339
	21st to end	Night	23% 21% 7% 12% 5% 4 3 3 4 4 14%	7150

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	86% 4 4 5% 1	4692
	11th to 20th	Day	90% 3 3 3 1	4710
	21st to end	Day	78% 5% 6% 9% 2	5156
February	1st to 10th	Day	73% 6% 6% 12% 2	1842
	11th to 20th	Day	71% 6% 8% 10% 5%	2058
	21st to end	Day	68% 4 10% 14% 4	1730
March	1st to 10th	Day	54% 5% 14% 17% 9% 1	4580
	11th to 20th	Day	52% 5% 15% 18% 9% 1	4900
	21st to end	Day	63% 5% 9% 15% 7% 1	5862
April	1st to 10th	Day	56% 5% 10% 18% 11%	2520
	11th to 20th	Day	53% 5% 8% 20% 13% 2	2560
	21st to end	Day	50% 6% 11% 18% 13% 2	2730
May	1st to 10th	Day	40% 5% 9% 26% 17% 3	7651
	11th to 20th	Day	44% 7% 9% 21% 18% 1	7902
	21st to end	Day	39% 6% 8% 21% 22% 4	9061
June	1st to 10th	Day	39% 6% 10% 26% 18% 2	2940
	11th to 20th	Day	41% 6% 12% 23% 17% 1	2940
	21st to end	Day	40% 6% 13% 25% 15% 1	2940
July	1st to 10th	Day	38% 5% 12% 28% 16% 1	6292
	11th to 20th	Day	31% 6% 12% 31% 19% 1	6285
	21st to end	Day	33% 7% 13% 29% 17% 1	6728
August	1st to 10th	Day	38% 7% 13% 27% 14% 1	4365
	11th to 20th	Day	36% 5% 13% 27% 17% 1	4213
	21st to end	Day	38% 6% 13% 28% 14% 1	4382
September	1st to 10th	Day	41% 5% 13% 25% 13% 2	2390
	11th to 20th	Day	35% 7% 13% 29% 15% 1	2200
	21st to end	Day	43% 6% 12% 26% 12%	2200
October	1st to 10th	Day	31% 4 9% 36% 19% 2	3590
	11th to 20th	Day	51% 5% 11% 23% 11%	3070
	21st to end	Day	57% 6% 14% 17% 7%	3180
November	1st to 10th	Day	53% 7% 11% 20% 9%	1852
	11th to 20th	Day	67% 6% 11% 11% 4	1890
	21st to end	Day	72% 4 11% 9% 3	1848
December	1st to 10th	Day	82% 5% 5% 6% 1	2690
	11th to 20th	Day	82% 5% 7% 5%	3085
	21st to end	Day	78% 5% 6% 9% 3	4070
January	1st to 10th	Night	80% 4 7% 6% 3	8233
	11th to 20th	Night	87% 4 5% 3 1	8280
	21st to end	Night	79% 6% 6% 7% 2	9058
February	1st to 10th	Night	71% 6% 7% 11% 4 1	2773
	11th to 20th	Night	66% 7% 8% 14% 5%	2459
	21st to end	Night	64% 6% 12% 14% 4	2076
March	1st to 10th	Night	51% 5% 15% 20% 8%	5493
	11th to 20th	Night	46% 5% 18% 20% 9% 1	5279
	21st to end	Night	57% 6% 10% 19% 8% 1	5354
April	1st to 10th	Night	52% 5% 10% 19% 13% 1	2100
	11th to 20th	Night	51% 6% 7% 20% 14% 2	2009
	21st to end	Night	47% 6% 11% 20% 13% 3	1890
May	1st to 10th	Night	39% 7% 12% 25% 15% 2	5264
	11th to 20th	Night	39% 7% 10% 28% 15% 1	4962
	21st to end	Night	39% 5% 8% 24% 18% 6%	5149
June	1st to 10th	Night	37% 6% 12% 25% 18% 2	1680
	11th to 20th	Night	37% 7% 13% 23% 18% 2	1680
	21st to end	Night	37% 7% 12% 28% 14% 2	1680
July	1st to 10th	Night	39% 6% 12% 24% 14% 4	3580
	11th to 20th	Night	29% 5% 13% 33% 18% 2	3572
	21st to end	Night	32% 6% 15% 26% 20% 1	3832
August	1st to 10th	Night	37% 7% 14% 27% 14% 1	2762
	11th to 20th	Night	33% 5% 12% 33% 16%	2902
	21st to end	Night	34% 6% 13% 24% 21% 2	3511
September	1st to 10th	Night	32% 7% 11% 30% 18% 2	2000
	11th to 20th	Night	28% 7% 14% 34% 15% 2	2189
	21st to end	Night	42% 7% 13% 26% 12% 1	2200
October	1st to 10th	Night	35% 5% 10% 36% 14% 1	3590
	11th to 20th	Night	43% 6% 11% 26% 13%	4052
	21st to end	Night	49% 5% 13% 25% 8%	4610
November	1st to 10th	Night	53% 7% 10% 21% 9% 1	2666
	11th to 20th	Night	64% 7% 12% 11% 5% 1	2730
	21st to end	Night	69% 6% 12% 10% 2 1	2767
December	1st to 10th	Night	75% 7% 9% 8% 1	4720
	11th to 20th	Night	80% 5% 6% 7% 1	5341
	21st to end	Night	74% 5% 9% 8% 4	7150

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	23% 20% 37% 13% 7%	330
	11th to 20th	Day	26% 25% 37% 11% 2	330
	21st to end	Day	28% 20% 37% 13% 3	363
February	1st to 10th	Day	23% 33% 27% 13% 3 1	330
	11th to 20th	Day	27% 32% 29% 11% 1	330
	21st to end	Day	33% 33% 29% 5% 1	273
March	1st to 10th	Day	29% 29% 32% 9% 1	330
	11th to 20th	Day	31% 17% 31% 13% 7% 1	454
	21st to end	Day	34% 18% 28% 14% 5% 1	594
April	1st to 10th	Day	33% 17% 33% 14% 3 1	540
	11th to 20th	Day	38% 16% 27% 12% 6% 1	532
	21st to end	Day	26% 17% 30% 18% 7% 1	540
May	1st to 10th	Day	26% 20% 33% 13% 6% 1	543
	11th to 20th	Day	21% 16% 33% 20% 6% 4	657
	21st to end	Day	18% 14% 34% 22% 10% 2	844
June	1st to 10th	Day	16% 14% 39% 22% 8% 2	780
	11th to 20th	Day	21% 15% 36% 20% 6% 2	780
	21st to end	Day	20% 12% 34% 24% 7% 2	780
July	1st to 10th	Day	11% 9% 34% 28% 13% 5%	722
	11th to 20th	Day	13% 11% 31% 29% 11% 5%	718
	21st to end	Day	11% 9% 36% 26% 13% 5%	794
August	1st to 10th	Day	12% 7% 29% 28% 19% 6%	621
	11th to 20th	Day	15% 6% 23% 29% 18% 10%	519
	21st to end	Day	19% 8% 25% 27% 16% 6%	583
September	1st to 10th	Day	18% 10% 25% 24% 17% 7%	530
	11th to 20th	Day	11% 5% 27% 29% 21% 7%	530
	21st to end	Day	19% 15% 22% 27% 13% 5%	530
October	1st to 10th	Day	21% 14% 28% 23% 12% 2	530
	11th to 20th	Day	29% 15% 38% 15% 3	386
	21st to end	Day	35% 20% 30% 12% 3	363
November	1st to 10th	Day	32% 19% 32% 12% 4	330
	11th to 20th	Day	31% 26% 31% 9% 3	330
	21st to end	Day	32% 22% 36% 8% 2	330
December	1st to 10th	Day	30% 25% 27% 13% 4	330
	11th to 20th	Day	32% 27% 29% 9% 2	330
	21st to end	Day	37% 24% 29% 6% 2 1	363
January	1st to 10th	Night	25% 18% 32% 21% 4	1050
	11th to 20th	Night	27% 18% 32% 18% 5%	1050
	21st to end	Night	25% 13% 35% 22% 5%	1155
February	1st to 10th	Night	22% 16% 32% 25% 4 1	940
	11th to 20th	Night	25% 16% 33% 22% 4	823
	21st to end	Night	25% 18% 35% 18% 3	692
March	1st to 10th	Night	21% 17% 32% 22% 8% 1	840
	11th to 20th	Night	21% 17% 36% 20% 5%	840
	21st to end	Night	23% 16% 30% 26% 5% 1	819
April	1st to 10th	Night	25% 15% 33% 21% 5%	630
	11th to 20th	Night	31% 11% 30% 22% 6% 1	623
	21st to end	Night	24% 11% 33% 26% 6%	630
May	1st to 10th	Night	16% 10% 39% 28% 6% 1	630
	11th to 20th	Night	18% 12% 33% 29% 8% 1	621
	21st to end	Night	16% 7% 29% 34% 13% 1	683
June	1st to 10th	Night	15% 6% 30% 38% 10% 1	630
	11th to 20th	Night	15% 7% 32% 32% 13% 1	630
	21st to end	Night	15% 5% 27% 39% 13% 1	630
July	1st to 10th	Night	10% 2 22% 41% 22% 2	585
	11th to 20th	Night	9% 3 23% 43% 20% 2	582
	21st to end	Night	11% 3 20% 44% 22% 1	657
August	1st to 10th	Night	11% 3 20% 44% 20% 2	597
	11th to 20th	Night	13% 3 18% 37% 23% 5%	594
	21st to end	Night	13% 4 21% 41% 19% 2	660
September	1st to 10th	Night	13% 5% 22% 36% 19% 5%	600
	11th to 20th	Night	13% 6% 21% 35% 22% 3	789
	21st to end	Night	18% 10% 25% 29% 17% 2	800
October	1st to 10th	Night	21% 7% 26% 34% 11% 1	800
	11th to 20th	Night	30% 9% 28% 26% 7% 1	800
	21st to end	Night	32% 11% 26% 23% 7% 1	880
November	1st to 10th	Night	25% 13% 31% 24% 7% 1	820
	11th to 20th	Night	29% 16% 32% 20% 3	840
	21st to end	Night	31% 13% 32% 21% 4	877
December	1st to 10th	Night	28% 18% 32% 18% 4	1050
	11th to 20th	Night	31% 16% 31% 17% 4 1	1050
	21st to end	Night	32% 16% 31% 19% 3	1155

Page generated at 2026-06-28 02:10:48. Version ac07

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