Four Winds Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Four Winds Observatory clear sky chart from 2005-11-06 to 2026-04-02.

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% 11% 5% 4 4 3 5% 6% 33%	130486
	Night	14% 14% 6% 13% 7% 5% 4 3 4 5% 27%	131498
Transparency	Day	51% 6% 11% 20% 10% 1	133966
	Night	58% 7% 11% 16% 8% 1	136105
Seeing	Day	17% 10% 26% 27% 16% 4	17590
	Night	20% 10% 26% 29% 13% 2	27742

By Month

Percentage Cloud Cover NE (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% 22% 13% 31% 6% 4 2 1 2 2 4	4691
	11th to 20th	Day	11% 18% 10% 29% 13% 9% 3 1 2 1 3	4679
	21st to end	Day	11% 16% 8% 18% 11% 10% 7% 2 3 3 10%	5156
February	1st to 10th	Day	9% 19% 6% 19% 15% 7% 3 2 3 4 12%	1785
	11th to 20th	Day	12% 14% 7% 19% 12% 7% 4 3 3 4 15%	2048
	21st to end	Day	13% 16% 7% 13% 9% 6% 5% 3 5% 4 20%	1730
March	1st to 10th	Day	13% 11% 4 13% 7% 4 4 3 5% 6% 33%	4578
	11th to 20th	Day	11% 10% 5% 14% 6% 5% 5% 4 5% 7% 29%	4919
	21st to end	Day	22% 11% 7% 15% 5% 4 3 2 4 4 23%	5855
April	1st to 10th	Day	17% 14% 4 8% 5% 4 4 3 6% 6% 30%	2412
	11th to 20th	Day	19% 12% 4 6% 3 4 3 3 5% 6% 36%	2462
	21st to end	Day	15% 9% 3 9% 4 4 4 4 6% 7% 35%	2600
May	1st to 10th	Day	13% 9% 3 6% 4 3 3 3 5% 6% 45%	5888
	11th to 20th	Day	11% 8% 2 7% 5% 4 4 3 5% 6% 45%	6135
	21st to end	Day	13% 7% 2 6% 4 4 4 3 4 7% 45%	6949
June	1st to 10th	Day	13% 4 2 7% 4 4 4 3 6% 8% 45%	2800
	11th to 20th	Day	11% 5% 2 6% 4 4 5% 4 6% 9% 44%	2806
	21st to end	Day	13% 6% 2 6% 3 3 5% 4 6% 7% 45%	2800
July	1st to 10th	Day	16% 6% 2 4 3 3 4 3 6% 8% 46%	6359
	11th to 20th	Day	10% 4 1 4 3 3 4 3 6% 8% 55%	6360
	21st to end	Day	9% 5% 2 5% 3 3 3 4 6% 9% 51%	6926
August	1st to 10th	Day	7% 5% 1 7% 5% 5% 5% 4 7% 9% 45%	4415
	11th to 20th	Day	15% 5% 2 5% 3 3 3 3 4 6% 50%	4174
	21st to end	Day	13% 5% 3 7% 4 4 4 3 4 6% 47%	4446
September	1st to 10th	Day	14% 6% 2 5% 4 4 4 4 6% 6% 44%	2390
	11th to 20th	Day	13% 4 2 6% 4 3 4 4 5% 9% 46%	2200
	21st to end	Day	17% 7% 2 6% 4 4 5% 4 6% 8% 38%	2200
October	1st to 10th	Day	19% 8% 3 7% 5% 4 4 3 5% 7% 37%	2200
	11th to 20th	Day	23% 16% 3 9% 5% 5% 5% 3 4 5% 21%	1884
	21st to end	Day	24% 15% 4 10% 6% 5% 6% 3 4 5% 19%	1980
November	1st to 10th	Day	18% 20% 4 6% 4 4 4 3 5% 6% 26%	1852
	11th to 20th	Day	15% 23% 7% 12% 5% 5% 4 3 5% 4 15%	1890
	21st to end	Day	18% 26% 9% 13% 5% 3 4 3 3 4 13%	1782
December	1st to 10th	Day	13% 32% 14% 17% 5% 4 3 2 2 1 5%	2350
	11th to 20th	Day	13% 26% 11% 19% 9% 5% 3 3 2 3 5%	2715
	21st to end	Day	21% 22% 10% 15% 6% 4 4 2 3 3 8%	4070
January	1st to 10th	Night	13% 22% 10% 31% 8% 3 2 1 2 2 6%	8233
	11th to 20th	Night	10% 19% 10% 23% 17% 8% 4 2 2 2 4	8236
	21st to end	Night	9% 16% 9% 19% 14% 9% 7% 2 3 3 9%	9058
February	1st to 10th	Night	11% 18% 6% 19% 14% 7% 4 3 3 3 12%	2832
	11th to 20th	Night	12% 14% 7% 17% 11% 6% 4 3 3 4 19%	2458
	21st to end	Night	15% 17% 5% 14% 7% 4 4 3 4 7% 21%	2076
March	1st to 10th	Night	16% 10% 4 10% 5% 3 3 3 4 6% 35%	5482
	11th to 20th	Night	11% 12% 5% 9% 5% 4 5% 3 5% 8% 32%	5259
	21st to end	Night	19% 12% 5% 12% 5% 4 3 3 4 5% 27%	5361
April	1st to 10th	Night	18% 13% 3 9% 5% 4 4 4 5% 5% 30%	2010
	11th to 20th	Night	20% 10% 3 8% 4 4 4 3 5% 5% 34%	1887
	21st to end	Night	17% 9% 3 8% 5% 4 5% 4 4 6% 36%	1800
May	1st to 10th	Night	14% 8% 3 6% 4 4 4 3 5% 6% 43%	4053
	11th to 20th	Night	13% 9% 3 8% 5% 4 4 4 5% 5% 41%	3756
	21st to end	Night	18% 7% 2 6% 3 3 4 3 5% 6% 44%	3949
June	1st to 10th	Night	13% 7% 2 6% 4 5% 4 3 4 7% 44%	1600
	11th to 20th	Night	13% 6% 2 6% 5% 4 4 4 6% 8% 41%	1495
	21st to end	Night	13% 6% 2 7% 4 4 4 4 5% 7% 44%	1600
July	1st to 10th	Night	12% 6% 3 6% 4 4 4 4 6% 7% 44%	3620
	11th to 20th	Night	9% 6% 2 6% 3 3 3 3 5% 6% 54%	3620
	21st to end	Night	8% 5% 2 6% 4 3 4 4 5% 8% 52%	3948
August	1st to 10th	Night	12% 6% 2 7% 5% 5% 4 3 5% 7% 44%	2737
	11th to 20th	Night	15% 4 2 5% 4 4 3 3 5% 7% 49%	2875
	21st to end	Night	13% 5% 2 6% 4 4 4 3 5% 6% 48%	3449
September	1st to 10th	Night	12% 5% 3 6% 4 4 4 3 5% 7% 48%	2000
	11th to 20th	Night	13% 5% 2 5% 3 4 4 4 5% 7% 47%	2190
	21st to end	Night	17% 6% 3 7% 5% 4 4 3 6% 7% 38%	2200
October	1st to 10th	Night	19% 8% 2 7% 5% 4 4 2 4 6% 38%	2200
	11th to 20th	Night	25% 13% 3 8% 4 4 4 4 4 6% 25%	2514
	21st to end	Night	24% 15% 3 8% 6% 5% 4 3 4 5% 23%	2860
November	1st to 10th	Night	17% 17% 4 8% 5% 5% 4 3 4 6% 27%	2666
	11th to 20th	Night	16% 24% 7% 11% 6% 5% 4 3 3 4 18%	2730
	21st to end	Night	17% 25% 8% 12% 6% 5% 3 3 4 4 13%	2827
December	1st to 10th	Night	12% 26% 15% 16% 6% 5% 4 2 3 3 8%	5060
	11th to 20th	Night	13% 24% 9% 20% 9% 4 4 2 3 3 8%	5707
	21st to end	Night	23% 21% 9% 15% 5% 3 3 2 3 3 11%	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	88% 4 5% 2	4692
	11th to 20th	Day	90% 4 3 3	4710
	21st to end	Day	79% 6% 8% 7% 1	5156
February	1st to 10th	Day	78% 5% 7% 9% 1	1785
	11th to 20th	Day	73% 7% 8% 9% 3	2058
	21st to end	Day	68% 7% 8% 13% 3 1	1730
March	1st to 10th	Day	52% 6% 16% 16% 9% 1	4580
	11th to 20th	Day	53% 6% 15% 17% 8% 1	4919
	21st to end	Day	64% 6% 10% 14% 6%	5855
April	1st to 10th	Day	55% 6% 10% 19% 10%	2412
	11th to 20th	Day	49% 6% 10% 21% 13% 2	2462
	21st to end	Day	47% 7% 13% 16% 15% 2	2600
May	1st to 10th	Day	39% 7% 11% 23% 19% 2	5897
	11th to 20th	Day	39% 7% 10% 25% 17% 1	6139
	21st to end	Day	38% 7% 10% 23% 19% 2	6949
June	1st to 10th	Day	36% 8% 13% 26% 16% 2	2800
	11th to 20th	Day	36% 8% 14% 24% 17% 1	2806
	21st to end	Day	37% 7% 15% 25% 14% 1	2800
July	1st to 10th	Day	36% 5% 15% 30% 13% 1	6291
	11th to 20th	Day	28% 6% 15% 37% 13% 1	6285
	21st to end	Day	29% 6% 15% 32% 17% 1	6728
August	1st to 10th	Day	34% 7% 18% 29% 12% 1	4400
	11th to 20th	Day	35% 6% 14% 31% 13% 1	4213
	21st to end	Day	39% 6% 14% 29% 11% 1	4445
September	1st to 10th	Day	38% 7% 15% 25% 13% 1	2390
	11th to 20th	Day	33% 10% 16% 29% 11% 1	2200
	21st to end	Day	43% 8% 15% 24% 10%	2200
October	1st to 10th	Day	33% 5% 9% 32% 19% 1	3590
	11th to 20th	Day	55% 6% 10% 19% 10%	3035
	21st to end	Day	62% 6% 12% 16% 3	3180
November	1st to 10th	Day	58% 7% 11% 17% 7%	1852
	11th to 20th	Day	70% 7% 10% 9% 4	1890
	21st to end	Day	75% 6% 11% 7% 1	1782
December	1st to 10th	Day	86% 4 5% 4	2350
	11th to 20th	Day	84% 7% 5% 3 1	2715
	21st to end	Day	80% 8% 6% 5% 1	4070
January	1st to 10th	Night	85% 5% 6% 3	8233
	11th to 20th	Night	87% 5% 5% 3 1	8280
	21st to end	Night	78% 7% 7% 7% 1	9058
February	1st to 10th	Night	77% 6% 7% 8% 2	2832
	11th to 20th	Night	68% 7% 9% 13% 3	2459
	21st to end	Night	64% 6% 12% 14% 3	2076
March	1st to 10th	Night	50% 6% 17% 19% 8%	5493
	11th to 20th	Night	49% 7% 18% 19% 7% 1	5260
	21st to end	Night	57% 8% 12% 16% 7%	5361
April	1st to 10th	Night	53% 8% 9% 20% 10% 1	2010
	11th to 20th	Night	51% 6% 9% 19% 13% 3	1887
	21st to end	Night	48% 7% 9% 20% 14% 3	1800
May	1st to 10th	Night	40% 7% 11% 20% 18% 3	4060
	11th to 20th	Night	42% 8% 8% 24% 16% 1	3757
	21st to end	Night	42% 6% 12% 22% 14% 5%	3950
June	1st to 10th	Night	38% 8% 11% 28% 14% 1	1600
	11th to 20th	Night	39% 8% 14% 22% 16% 2	1495
	21st to end	Night	38% 8% 11% 26% 15% 2	1600
July	1st to 10th	Night	37% 5% 15% 26% 14% 3	3580
	11th to 20th	Night	30% 6% 13% 30% 19% 2	3572
	21st to end	Night	30% 6% 16% 29% 18% 1	3832
August	1st to 10th	Night	39% 7% 15% 26% 12% 1	2728
	11th to 20th	Night	35% 6% 15% 29% 14% 1	2902
	21st to end	Night	36% 6% 15% 24% 18% 1	3449
September	1st to 10th	Night	35% 7% 15% 23% 19% 1	2000
	11th to 20th	Night	35% 7% 17% 28% 13%	2190
	21st to end	Night	44% 8% 15% 23% 10% 1	2200
October	1st to 10th	Night	40% 6% 9% 32% 12% 1	3590
	11th to 20th	Night	51% 6% 10% 22% 11%	4102
	21st to end	Night	55% 6% 12% 21% 6%	4610
November	1st to 10th	Night	57% 8% 11% 17% 6% 1	2666
	11th to 20th	Night	69% 7% 10% 9% 5% 1	2730
	21st to end	Night	74% 8% 10% 7% 1	2827
December	1st to 10th	Night	80% 7% 7% 6% 1	5060
	11th to 20th	Night	80% 7% 7% 5% 1	5706
	21st to end	Night	76% 8% 8% 5% 2	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	18% 16% 35% 23% 6% 2	330
	11th to 20th	Day	22% 13% 38% 21% 5% 1	330
	21st to end	Day	19% 15% 35% 25% 5% 1	363
February	1st to 10th	Day	19% 19% 37% 18% 6% 1	330
	11th to 20th	Day	17% 18% 38% 19% 6% 1	330
	21st to end	Day	18% 26% 38% 16% 2	273
March	1st to 10th	Day	18% 18% 37% 22% 4	330
	11th to 20th	Day	21% 15% 30% 24% 9% 2	460
	21st to end	Day	26% 12% 30% 22% 9% 1	594
April	1st to 10th	Day	23% 13% 30% 25% 8% 1	504
	11th to 20th	Day	25% 13% 28% 20% 11% 3	492
	21st to end	Day	18% 10% 30% 27% 13% 3	500
May	1st to 10th	Day	17% 9% 31% 27% 13% 3	503
	11th to 20th	Day	15% 5% 28% 33% 15% 4	645
	21st to end	Day	14% 6% 23% 29% 22% 5%	789
June	1st to 10th	Day	13% 6% 25% 33% 19% 4	730
	11th to 20th	Day	15% 4 25% 33% 19% 4	730
	21st to end	Day	16% 5% 21% 34% 20% 4	730
July	1st to 10th	Day	10% 2 17% 36% 27% 7%	721
	11th to 20th	Day	9% 4 21% 34% 24% 8%	718
	21st to end	Day	8% 3 20% 36% 28% 5%	794
August	1st to 10th	Day	8% 3 17% 33% 28% 10%	643
	11th to 20th	Day	13% 3 16% 30% 27% 10%	519
	21st to end	Day	15% 4 19% 29% 25% 8%	583
September	1st to 10th	Day	16% 8% 21% 24% 22% 9%	530
	11th to 20th	Day	10% 5% 21% 25% 27% 12%	530
	21st to end	Day	18% 8% 16% 29% 22% 7%	530
October	1st to 10th	Day	20% 11% 24% 25% 16% 4	530
	11th to 20th	Day	27% 13% 29% 25% 6% 1	384
	21st to end	Day	28% 14% 26% 22% 8% 1	363
November	1st to 10th	Day	28% 15% 29% 17% 10% 1	330
	11th to 20th	Day	19% 20% 38% 17% 5%	330
	21st to end	Day	29% 21% 32% 16% 3	313
December	1st to 10th	Day	32% 17% 30% 14% 6%	220
	11th to 20th	Day	23% 25% 32% 16% 3	226
	21st to end	Day	29% 18% 35% 13% 3 1	363
January	1st to 10th	Night	20% 15% 36% 22% 6% 1	1050
	11th to 20th	Night	24% 16% 31% 22% 7% 1	1050
	21st to end	Night	19% 13% 35% 25% 8% 1	1155
February	1st to 10th	Night	17% 15% 34% 25% 8% 1	960
	11th to 20th	Night	20% 14% 34% 25% 8%	823
	21st to end	Night	23% 15% 30% 27% 5%	692
March	1st to 10th	Night	17% 11% 30% 31% 10% 1	840
	11th to 20th	Night	17% 11% 34% 29% 8% 1	840
	21st to end	Night	21% 9% 27% 33% 9% 1	826
April	1st to 10th	Night	23% 10% 29% 31% 7% 1	603
	11th to 20th	Night	23% 10% 24% 31% 11% 1	593
	21st to end	Night	19% 7% 27% 35% 12% 1	600
May	1st to 10th	Night	17% 5% 26% 36% 16% 1	600
	11th to 20th	Night	18% 3 21% 38% 18% 2	591
	21st to end	Night	14% 4 22% 34% 24% 2	650
June	1st to 10th	Night	17% 3 21% 38% 20% 1	600
	11th to 20th	Night	14% 4 18% 40% 22% 2	495
	21st to end	Night	14% 3 19% 38% 25% 2	600
July	1st to 10th	Night	10% 2 12% 43% 28% 4	585
	11th to 20th	Night	11% 1 17% 38% 28% 5%	582
	21st to end	Night	9% 2 17% 38% 30% 3	657
August	1st to 10th	Night	11% 1 15% 41% 28% 4	597
	11th to 20th	Night	11% 4 15% 32% 32% 6%	594
	21st to end	Night	15% 3 15% 37% 28% 2	660
September	1st to 10th	Night	14% 4 20% 31% 27% 4	600
	11th to 20th	Night	16% 4 17% 31% 27% 6%	790
	21st to end	Night	19% 7% 19% 30% 20% 5%	800
October	1st to 10th	Night	22% 9% 21% 30% 17% 2	800
	11th to 20th	Night	29% 11% 25% 26% 8% 1	800
	21st to end	Night	31% 11% 27% 23% 8% 1	880
November	1st to 10th	Night	26% 14% 25% 28% 6% 1	820
	11th to 20th	Night	28% 17% 30% 20% 5%	840
	21st to end	Night	28% 16% 30% 21% 4	914
December	1st to 10th	Night	26% 20% 31% 19% 4	1050
	11th to 20th	Night	25% 17% 35% 18% 5% 1	1050
	21st to end	Night	30% 13% 34% 20% 3	1155

Page generated at 2026-03-30 02:18:40. Version ac07

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