Winkler
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Winkler clear sky chart from 2005-11-06 to 2026-06-16.

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% 6% 5% 5% 4 6% 8% 37%	137985
	Night	9% 10% 4 10% 6% 5% 5% 4 6% 7% 35%	134378
Transparency	Day	45% 5% 15% 25% 10% 1	141407
	Night	47% 5% 14% 24% 10% 1	139047
Seeing	Day	15% 9% 30% 29% 13% 4	18437
	Night	14% 8% 31% 34% 12% 1	28411

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	5% 14% 6% 14% 9% 7% 6% 5% 6% 6% 22%	4124
	11th to 20th	Day	5% 8% 3 13% 11% 10% 10% 10% 8% 7% 16%	4331
	21st to end	Day	4 11% 5% 10% 6% 8% 7% 5% 7% 7% 29%	5188
February	1st to 10th	Day	8% 14% 5% 11% 8% 7% 6% 5% 7% 5% 24%	1784
	11th to 20th	Day	7% 10% 3 10% 7% 7% 9% 6% 8% 6% 27%	1844
	21st to end	Day	6% 10% 3 11% 9% 8% 8% 5% 6% 7% 27%	1693
March	1st to 10th	Day	10% 9% 3 7% 7% 5% 5% 4 6% 9% 35%	4656
	11th to 20th	Day	9% 10% 4 12% 9% 6% 6% 4 6% 9% 25%	5090
	21st to end	Day	10% 14% 6% 13% 6% 5% 5% 4 5% 6% 27%	5585
April	1st to 10th	Day	9% 13% 5% 8% 6% 5% 5% 5% 5% 6% 34%	2553
	11th to 20th	Day	13% 12% 4 10% 5% 5% 5% 4 6% 6% 31%	2702
	21st to end	Day	18% 9% 3 8% 5% 4 4 3 5% 6% 33%	2730
May	1st to 10th	Day	11% 7% 2 7% 4 4 4 3 5% 7% 45%	7696
	11th to 20th	Day	18% 9% 3 6% 4 4 4 4 6% 7% 34%	8651
	21st to end	Day	12% 7% 2 6% 4 4 4 3 5% 7% 45%	9683
June	1st to 10th	Day	13% 7% 2 6% 5% 4 5% 5% 6% 9% 38%	3150
	11th to 20th	Day	13% 6% 3 6% 5% 4 5% 4 7% 8% 39%	3076
	21st to end	Day	12% 5% 2 6% 6% 5% 5% 5% 7% 9% 38%	3000
July	1st to 10th	Day	8% 4 2 5% 5% 5% 5% 5% 8% 10% 44%	6799
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 4 6% 8% 47%	6800
	21st to end	Day	7% 4 2 6% 4 5% 5% 4 7% 9% 45%	7136
August	1st to 10th	Day	8% 6% 2 6% 4 4 5% 4 6% 9% 46%	4425
	11th to 20th	Day	11% 5% 2 5% 4 3 4 3 5% 9% 48%	4171
	21st to end	Day	10% 5% 2 5% 4 4 4 3 5% 8% 49%	4675
September	1st to 10th	Day	11% 5% 2 5% 4 4 4 4 5% 8% 50%	2454
	11th to 20th	Day	14% 7% 2 7% 4 4 4 4 6% 6% 41%	2200
	21st to end	Day	14% 6% 2 5% 4 4 4 2 5% 8% 45%	2200
October	1st to 10th	Day	16% 9% 3 6% 4 4 3 3 5% 7% 40%	2070
	11th to 20th	Day	13% 11% 4 8% 4 4 5% 3 6% 8% 35%	1935
	21st to end	Day	15% 12% 4 9% 5% 4 5% 3 5% 7% 31%	1980
November	1st to 10th	Day	13% 14% 4 10% 5% 4 4 4 5% 5% 31%	1737
	11th to 20th	Day	9% 14% 6% 13% 7% 5% 4 4 4 6% 28%	1680
	21st to end	Day	11% 14% 4 11% 8% 6% 4 3 4 6% 28%	1543
December	1st to 10th	Day	9% 13% 6% 15% 10% 8% 5% 4 5% 6% 18%	2360
	11th to 20th	Day	9% 13% 6% 11% 7% 6% 6% 4 6% 8% 25%	2709
	21st to end	Day	7% 12% 5% 15% 8% 6% 4 3 5% 5% 28%	3575
January	1st to 10th	Night	5% 13% 6% 16% 8% 7% 6% 4 6% 6% 23%	8800
	11th to 20th	Night	4 12% 5% 13% 8% 9% 10% 6% 6% 6% 21%	8582
	21st to end	Night	5% 13% 5% 11% 5% 5% 6% 4 7% 8% 29%	9026
February	1st to 10th	Night	7% 13% 4 12% 7% 6% 5% 6% 6% 6% 28%	2835
	11th to 20th	Night	6% 10% 4 8% 5% 6% 6% 5% 8% 8% 35%	2663
	21st to end	Night	7% 10% 4 10% 7% 7% 6% 4 6% 6% 34%	2132
March	1st to 10th	Night	8% 10% 3 8% 6% 4 5% 5% 6% 7% 37%	5422
	11th to 20th	Night	8% 10% 4 9% 6% 5% 6% 4 6% 7% 36%	5089
	21st to end	Night	10% 14% 6% 13% 6% 4 4 3 4 5% 30%	5588
April	1st to 10th	Night	9% 9% 2 7% 4 4 5% 5% 7% 8% 40%	2063
	11th to 20th	Night	12% 12% 4 8% 5% 4 6% 3 5% 6% 35%	1869
	21st to end	Night	15% 8% 3 6% 5% 5% 6% 4 5% 7% 37%	1890
May	1st to 10th	Night	13% 5% 3 7% 4 4 5% 4 5% 8% 42%	5229
	11th to 20th	Night	17% 7% 2 6% 4 4 4 4 6% 7% 40%	4225
	21st to end	Night	13% 6% 1 5% 3 3 3 3 5% 6% 51%	4526
June	1st to 10th	Night	11% 5% 2 5% 4 5% 5% 4 7% 7% 46%	1470
	11th to 20th	Night	13% 5% 2 6% 5% 4 4 4 6% 8% 44%	1435
	21st to end	Night	10% 4 2 6% 4 4 4 5% 6% 10% 45%	1400
July	1st to 10th	Night	6% 4 1 4 4 4 4 4 6% 10% 53%	3180
	11th to 20th	Night	6% 4 2 6% 3 3 4 4 6% 9% 54%	3180
	21st to end	Night	5% 4 2 5% 4 4 6% 4 8% 10% 48%	3735
August	1st to 10th	Night	5% 4 2 5% 4 5% 5% 4 6% 7% 53%	2735
	11th to 20th	Night	9% 3 1 5% 3 4 4 4 6% 9% 51%	2888
	21st to end	Night	9% 5% 2 5% 3 3 4 3 5% 7% 52%	3234
September	1st to 10th	Night	9% 4 2 6% 4 3 3 3 6% 8% 54%	1944
	11th to 20th	Night	12% 5% 2 5% 4 3 4 3 5% 7% 49%	2200
	21st to end	Night	14% 5% 3 5% 4 4 3 4 5% 7% 47%	2200
October	1st to 10th	Night	15% 8% 3 6% 4 5% 4 3 4 7% 41%	2309
	11th to 20th	Night	14% 10% 3 7% 5% 4 4 3 5% 6% 39%	2462
	21st to end	Night	12% 12% 3 8% 5% 4 5% 3 5% 7% 35%	2860
November	1st to 10th	Night	12% 12% 3 8% 6% 5% 5% 4 6% 7% 32%	2781
	11th to 20th	Night	10% 16% 6% 11% 6% 4 4 3 4 6% 31%	2940
	21st to end	Night	11% 14% 5% 12% 6% 5% 4 3 4 6% 29%	3073
December	1st to 10th	Night	6% 14% 6% 17% 11% 7% 6% 3 5% 5% 20%	5050
	11th to 20th	Night	8% 14% 5% 15% 8% 7% 6% 3 5% 5% 24%	5718
	21st to end	Night	8% 11% 5% 16% 9% 6% 6% 4 5% 5% 26%	7645

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	61% 4 11% 19% 5%	4125
	11th to 20th	Day	62% 5% 11% 15% 7%	4360
	21st to end	Day	52% 4 14% 23% 7%	5188
February	1st to 10th	Day	58% 6% 12% 16% 8% 1	1784
	11th to 20th	Day	53% 5% 13% 20% 9%	1853
	21st to end	Day	54% 5% 13% 19% 8%	1693
March	1st to 10th	Day	46% 5% 20% 19% 10%	4658
	11th to 20th	Day	55% 4 17% 19% 6%	5090
	21st to end	Day	57% 4 11% 21% 6%	5585
April	1st to 10th	Day	49% 5% 11% 24% 9% 1	2553
	11th to 20th	Day	52% 5% 12% 19% 11% 1	2702
	21st to end	Day	52% 4 15% 20% 8% 2	2730
May	1st to 10th	Day	37% 4 11% 30% 16% 2	7707
	11th to 20th	Day	48% 4 11% 23% 13% 2	8655
	21st to end	Day	38% 4 10% 25% 20% 2	9683
June	1st to 10th	Day	40% 6% 19% 24% 10% 1	3150
	11th to 20th	Day	42% 5% 17% 25% 10% 1	3076
	21st to end	Day	41% 7% 19% 26% 7%	3000
July	1st to 10th	Day	32% 6% 29% 28% 5%	6726
	11th to 20th	Day	35% 5% 21% 33% 6%	6719
	21st to end	Day	33% 6% 24% 28% 8%	6930
August	1st to 10th	Day	34% 5% 19% 29% 13% 1	4409
	11th to 20th	Day	33% 4 20% 36% 6%	4210
	21st to end	Day	33% 5% 22% 31% 9%	4674
September	1st to 10th	Day	33% 5% 17% 33% 12% 1	2454
	11th to 20th	Day	43% 6% 18% 24% 8% 1	2200
	21st to end	Day	38% 4 16% 32% 10%	2200
October	1st to 10th	Day	37% 3 13% 33% 13% 1	3369
	11th to 20th	Day	40% 6% 11% 30% 13%	3129
	21st to end	Day	47% 4 13% 26% 9%	3190
November	1st to 10th	Day	55% 4 13% 20% 7% 1	1737
	11th to 20th	Day	58% 4 15% 15% 6% 1	1680
	21st to end	Day	59% 5% 14% 16% 5%	1543
December	1st to 10th	Day	66% 5% 13% 11% 4	2360
	11th to 20th	Day	58% 5% 14% 14% 9%	2710
	21st to end	Day	58% 5% 9% 22% 6%	3575
January	1st to 10th	Night	61% 4 13% 17% 5%	8800
	11th to 20th	Night	62% 5% 9% 20% 5%	8628
	21st to end	Night	51% 5% 13% 23% 8%	9026
February	1st to 10th	Night	55% 4 13% 17% 10%	2835
	11th to 20th	Night	44% 6% 15% 22% 13%	2664
	21st to end	Night	50% 4 12% 23% 11%	2132
March	1st to 10th	Night	45% 5% 17% 20% 10% 2	5433
	11th to 20th	Night	47% 5% 14% 22% 13% 1	5090
	21st to end	Night	56% 4 10% 21% 8% 1	5588
April	1st to 10th	Night	40% 5% 13% 27% 13% 3	2063
	11th to 20th	Night	50% 4 9% 23% 12% 2	1869
	21st to end	Night	46% 5% 13% 23% 12% 2	1890
May	1st to 10th	Night	40% 4 9% 31% 15%	5239
	11th to 20th	Night	43% 3 12% 24% 16% 2	4226
	21st to end	Night	33% 4 10% 26% 23% 3	4526
June	1st to 10th	Night	34% 7% 19% 27% 11% 2	1470
	11th to 20th	Night	37% 4 17% 27% 14% 2	1435
	21st to end	Night	34% 7% 18% 33% 7% 1	1400
July	1st to 10th	Night	27% 7% 26% 32% 8% 1	3145
	11th to 20th	Night	26% 5% 20% 41% 8% 1	3138
	21st to end	Night	29% 6% 27% 30% 8% 1	3627
August	1st to 10th	Night	28% 6% 17% 32% 16% 1	2727
	11th to 20th	Night	29% 5% 23% 34% 9%	2915
	21st to end	Night	31% 4 18% 34% 11% 1	3234
September	1st to 10th	Night	29% 5% 16% 34% 16% 1	1944
	11th to 20th	Night	34% 4 21% 28% 12% 1	2200
	21st to end	Night	38% 4 12% 32% 13% 1	2200
October	1st to 10th	Night	37% 3 12% 32% 14% 2	3777
	11th to 20th	Night	40% 3 12% 30% 15%	4011
	21st to end	Night	42% 5% 14% 31% 8%	4610
November	1st to 10th	Night	50% 5% 15% 19% 8% 2	2781
	11th to 20th	Night	55% 4 16% 16% 7% 1	2940
	21st to end	Night	56% 5% 15% 17% 6%	3073
December	1st to 10th	Night	66% 5% 13% 13% 3	5050
	11th to 20th	Night	62% 4 12% 14% 7%	5716
	21st to end	Night	58% 6% 10% 19% 6%	7645

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	12% 7% 32% 32% 12% 4	330
	11th to 20th	Day	9% 10% 35% 34% 10% 2	330
	21st to end	Day	13% 9% 35% 30% 11% 2	363
February	1st to 10th	Day	15% 12% 32% 33% 5% 2	330
	11th to 20th	Day	9% 12% 35% 30% 12% 2	330
	21st to end	Day	10% 7% 43% 29% 10% 1	273
March	1st to 10th	Day	14% 8% 35% 32% 10% 2	330
	11th to 20th	Day	15% 9% 35% 28% 10% 4	330
	21st to end	Day	15% 14% 31% 26% 10% 3	363
April	1st to 10th	Day	15% 14% 34% 25% 10% 2	573
	11th to 20th	Day	20% 10% 31% 26% 11% 1	740
	21st to end	Day	22% 11% 29% 26% 11% 2	750
May	1st to 10th	Day	16% 10% 31% 28% 12% 2	753
	11th to 20th	Day	23% 9% 30% 24% 12% 3	775
	21st to end	Day	18% 6% 25% 31% 16% 4	844
June	1st to 10th	Day	15% 10% 30% 27% 14% 6%	780
	11th to 20th	Day	15% 10% 35% 27% 12% 2	756
	21st to end	Day	13% 8% 28% 32% 15% 5%	730
July	1st to 10th	Day	8% 4 26% 32% 20% 10%	721
	11th to 20th	Day	11% 5% 30% 27% 20% 7%	718
	21st to end	Day	6% 4 25% 37% 21% 7%	794
August	1st to 10th	Day	9% 4 26% 33% 22% 6%	726
	11th to 20th	Day	10% 5% 21% 33% 20% 10%	715
	21st to end	Day	11% 7% 27% 31% 19% 5%	803
September	1st to 10th	Day	12% 8% 32% 29% 15% 4	584
	11th to 20th	Day	20% 8% 30% 28% 11% 3	330
	21st to end	Day	20% 8% 31% 27% 13% 2	330
October	1st to 10th	Day	19% 11% 27% 30% 12% 1	330
	11th to 20th	Day	16% 12% 38% 27% 6% 2	330
	21st to end	Day	21% 12% 33% 23% 9% 2	363
November	1st to 10th	Day	22% 12% 30% 25% 10% 2	330
	11th to 20th	Day	16% 15% 35% 24% 9% 1	330
	21st to end	Day	17% 12% 34% 28% 8% 1	330
December	1st to 10th	Day	14% 8% 35% 31% 9% 3	330
	11th to 20th	Day	15% 14% 35% 29% 7%	330
	21st to end	Day	15% 9% 31% 30% 10% 5%	363
January	1st to 10th	Night	11% 10% 34% 31% 13% 1	1050
	11th to 20th	Night	15% 10% 32% 37% 7%	1050
	21st to end	Night	13% 11% 34% 33% 8% 1	1155
February	1st to 10th	Night	13% 10% 34% 31% 10% 1	1050
	11th to 20th	Night	10% 8% 33% 34% 13% 2	1029
	21st to end	Night	13% 7% 34% 33% 11% 2	748
March	1st to 10th	Night	15% 9% 32% 31% 11% 1	818
	11th to 20th	Night	10% 8% 36% 34% 10% 1	630
	21st to end	Night	18% 8% 30% 31% 12% 3	693
April	1st to 10th	Night	14% 9% 34% 33% 10% 1	630
	11th to 20th	Night	18% 7% 32% 32% 11%	623
	21st to end	Night	20% 7% 24% 37% 10% 1	630
May	1st to 10th	Night	17% 6% 33% 32% 12% 1	630
	11th to 20th	Night	17% 7% 28% 34% 13%	621
	21st to end	Night	19% 6% 26% 36% 12% 1	683
June	1st to 10th	Night	13% 8% 28% 37% 14% 1	630
	11th to 20th	Night	15% 5% 29% 36% 14% 1	615
	21st to end	Night	11% 5% 26% 40% 17% 1	600
July	1st to 10th	Night	6% 2 26% 40% 24% 2	585
	11th to 20th	Night	7% 5% 27% 40% 20% 2	582
	21st to end	Night	6% 3 24% 43% 21% 2	657
August	1st to 10th	Night	7% 3 21% 47% 21% 1	597
	11th to 20th	Night	10% 3 23% 39% 22% 3	594
	21st to end	Night	10% 4 27% 39% 19% 1	660
September	1st to 10th	Night	11% 3 29% 37% 19% 2	600
	11th to 20th	Night	12% 6% 28% 41% 12% 1	600
	21st to end	Night	15% 6% 30% 34% 14% 1	600
October	1st to 10th	Night	20% 8% 34% 30% 8% 1	709
	11th to 20th	Night	19% 10% 31% 31% 10%	862
	21st to end	Night	18% 10% 35% 28% 10% 1	1100
November	1st to 10th	Night	17% 12% 33% 28% 9% 1	1025
	11th to 20th	Night	17% 11% 35% 27% 9% 1	1050
	21st to end	Night	17% 12% 37% 26% 6% 2	1050
December	1st to 10th	Night	14% 10% 31% 34% 10% 1	1050
	11th to 20th	Night	16% 11% 32% 31% 8% 1	1050
	21st to end	Night	19% 7% 31% 30% 11% 1	1155

Page generated at 2026-06-13 01:31:40. Version ac07

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