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 2025-02-21.

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 8% 6% 5% 5% 4 6% 7% 36%	108339
	Night	9% 10% 4 10% 6% 5% 5% 4 6% 7% 35%	110459
Transparency	Day	46% 5% 16% 25% 8% 1	111808
	Night	48% 5% 14% 24% 9% 1	115153
Seeing	Day	14% 8% 30% 29% 14% 4	16543
	Night	14% 8% 31% 34% 12% 1	26669

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	4 15% 6% 14% 7% 7% 6% 5% 7% 7% 21%	3178
	11th to 20th	Day	5% 8% 3 12% 10% 10% 8% 10% 8% 7% 17%	3315
	21st to end	Day	5% 13% 6% 12% 7% 7% 7% 4 6% 6% 27%	4004
February	1st to 10th	Day	9% 14% 5% 11% 8% 7% 6% 5% 7% 5% 25%	1699
	11th to 20th	Day	6% 10% 3 10% 7% 8% 9% 6% 8% 6% 27%	1754
	21st to end	Day	6% 10% 3 11% 8% 8% 7% 5% 6% 7% 29%	1536
March	1st to 10th	Day	13% 9% 3 8% 7% 5% 5% 4 5% 8% 32%	3172
	11th to 20th	Day	9% 11% 4 11% 9% 5% 6% 4 6% 8% 27%	3480
	21st to end	Day	10% 13% 5% 13% 6% 5% 4 4 5% 6% 29%	3814
April	1st to 10th	Day	9% 12% 5% 8% 6% 5% 5% 5% 5% 6% 34%	2310
	11th to 20th	Day	13% 12% 3 10% 5% 5% 5% 4 6% 6% 32%	2442
	21st to end	Day	19% 10% 3 8% 5% 4 4 3 5% 6% 33%	2470
May	1st to 10th	Day	19% 10% 3 7% 5% 4 4 3 6% 7% 33%	4134
	11th to 20th	Day	18% 8% 2 6% 4 5% 5% 4 6% 7% 34%	4633
	21st to end	Day	19% 9% 3 7% 5% 5% 4 4 5% 6% 34%	5173
June	1st to 10th	Day	13% 7% 2 6% 5% 5% 5% 5% 6% 9% 38%	2850
	11th to 20th	Day	14% 6% 2 5% 5% 4 5% 4 7% 7% 39%	2850
	21st to end	Day	12% 5% 2 6% 6% 5% 5% 5% 7% 9% 39%	2850
July	1st to 10th	Day	8% 3 1 5% 5% 5% 6% 5% 9% 10% 44%	4750
	11th to 20th	Day	7% 3 2 5% 3 4 5% 4 6% 9% 53%	4750
	21st to end	Day	5% 3 1 5% 4 5% 5% 5% 7% 9% 52%	4965
August	1st to 10th	Day	8% 6% 2 6% 4 4 5% 4 6% 8% 47%	4289
	11th to 20th	Day	11% 5% 2 5% 4 3 4 4 5% 9% 48%	4041
	21st to end	Day	10% 5% 2 5% 4 4 4 3 5% 8% 49%	4532
September	1st to 10th	Day	11% 5% 2 5% 3 3 4 3 4 8% 51%	2332
	11th to 20th	Day	13% 7% 2 7% 4 5% 4 3 6% 6% 43%	2090
	21st to end	Day	15% 6% 2 5% 4 4 4 2 5% 9% 43%	2090
October	1st to 10th	Day	17% 10% 3 6% 4 4 3 3 5% 7% 39%	1967
	11th to 20th	Day	12% 11% 4 8% 4 4 5% 3 6% 8% 35%	1838
	21st to end	Day	15% 12% 4 9% 5% 4 5% 3 5% 7% 31%	1881
November	1st to 10th	Day	14% 14% 4 10% 6% 4 4 4 5% 5% 31%	1653
	11th to 20th	Day	10% 14% 6% 13% 8% 5% 4 4 5% 6% 26%	1600
	21st to end	Day	11% 15% 4 10% 8% 6% 4 3 4 6% 28%	1470
December	1st to 10th	Day	9% 13% 7% 15% 10% 8% 5% 4 5% 6% 18%	2290
	11th to 20th	Day	9% 13% 6% 11% 7% 6% 6% 4 6% 8% 25%	2639
	21st to end	Day	7% 12% 5% 16% 8% 6% 4 3 4 5% 29%	3498
January	1st to 10th	Night	4 12% 6% 16% 9% 8% 6% 5% 7% 6% 22%	6787
	11th to 20th	Night	4 12% 5% 13% 9% 9% 10% 7% 7% 6% 20%	6588
	21st to end	Night	7% 16% 7% 13% 6% 5% 5% 3 5% 6% 26%	6974
February	1st to 10th	Night	8% 13% 4 12% 7% 6% 5% 6% 6% 6% 28%	2700
	11th to 20th	Night	5% 10% 4 8% 6% 6% 6% 5% 8% 8% 34%	2533
	21st to end	Night	7% 11% 4 9% 7% 6% 5% 4 6% 6% 35%	1937
March	1st to 10th	Night	11% 11% 3 9% 6% 5% 5% 4 5% 5% 36%	3677
	11th to 20th	Night	7% 8% 3 8% 6% 4 5% 4 5% 7% 41%	3479
	21st to end	Night	9% 13% 5% 14% 6% 4 4 3 5% 6% 31%	3817
April	1st to 10th	Night	8% 9% 2 7% 4 4 5% 5% 7% 8% 42%	1867
	11th to 20th	Night	12% 12% 4 8% 4 3 6% 4 5% 6% 36%	1689
	21st to end	Night	15% 8% 3 5% 5% 5% 6% 4 5% 7% 37%	1710
May	1st to 10th	Night	22% 6% 3 6% 4 4 4 3 5% 7% 36%	2814
	11th to 20th	Night	15% 7% 2 6% 4 4 4 4 6% 6% 42%	2269
	21st to end	Night	19% 7% 1 5% 3 3 3 3 5% 6% 44%	2414
June	1st to 10th	Night	11% 5% 2 5% 4 5% 5% 4 7% 7% 46%	1330
	11th to 20th	Night	13% 5% 2 5% 5% 4 4 4 6% 8% 44%	1330
	21st to end	Night	10% 4 2 6% 4 4 4 5% 6% 10% 45%	1330
July	1st to 10th	Night	7% 3 1 4 3 4 3 4 6% 10% 54%	2220
	11th to 20th	Night	5% 3 1 4 2 3 4 4 6% 10% 59%	2220
	21st to end	Night	5% 3 2 4 4 4 6% 5% 9% 10% 50%	2596
August	1st to 10th	Night	5% 4 2 5% 4 5% 5% 3 6% 7% 53%	2651
	11th to 20th	Night	9% 3 2 5% 3 4 4 4 6% 10% 51%	2798
	21st to end	Night	9% 5% 2 5% 3 3 4 3 5% 7% 52%	3135
September	1st to 10th	Night	9% 3 2 6% 4 3 3 2 6% 8% 55%	1846
	11th to 20th	Night	11% 5% 2 5% 4 3 4 3 5% 7% 51%	2090
	21st to end	Night	15% 6% 3 5% 4 4 3 4 5% 6% 45%	2090
October	1st to 10th	Night	15% 8% 3 7% 4 5% 4 3 4 7% 40%	2193
	11th to 20th	Night	14% 10% 3 7% 5% 4 4 3 5% 6% 40%	2339
	21st to end	Night	11% 12% 3 8% 6% 5% 5% 3 5% 7% 35%	2717
November	1st to 10th	Night	13% 12% 3 8% 6% 4 5% 4 6% 6% 32%	2645
	11th to 20th	Night	10% 16% 6% 11% 6% 4 5% 2 4 6% 30%	2800
	21st to end	Night	11% 14% 5% 11% 6% 5% 4 3 4 6% 29%	2926
December	1st to 10th	Night	6% 14% 6% 18% 11% 7% 6% 3 5% 5% 20%	4900
	11th to 20th	Night	8% 14% 5% 15% 8% 7% 6% 3 5% 5% 24%	5568
	21st to end	Night	8% 11% 5% 16% 9% 6% 6% 4 5% 5% 26%	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	59% 5% 12% 20% 4	3179
	11th to 20th	Day	60% 6% 10% 16% 8%	3344
	21st to end	Day	57% 4 16% 19% 4	4004
February	1st to 10th	Day	57% 5% 12% 17% 8% 1	1699
	11th to 20th	Day	53% 5% 13% 19% 10%	1763
	21st to end	Day	54% 5% 13% 19% 9%	1536
March	1st to 10th	Day	50% 5% 17% 21% 7%	3174
	11th to 20th	Day	55% 4 15% 19% 6%	3480
	21st to end	Day	56% 4 11% 21% 7%	3814
April	1st to 10th	Day	49% 6% 11% 25% 9% 1	2310
	11th to 20th	Day	52% 5% 12% 20% 11% 1	2442
	21st to end	Day	52% 4 15% 19% 8% 2	2470
May	1st to 10th	Day	50% 4 11% 23% 11% 1	4145
	11th to 20th	Day	48% 4 13% 23% 11% 1	4637
	21st to end	Day	50% 4 12% 24% 9% 1	5173
June	1st to 10th	Day	41% 6% 19% 23% 11% 1	2850
	11th to 20th	Day	43% 5% 17% 25% 10% 1	2850
	21st to end	Day	41% 6% 19% 26% 7%	2850
July	1st to 10th	Day	32% 7% 27% 28% 6% 1	4676
	11th to 20th	Day	27% 5% 23% 37% 8%	4669
	21st to end	Day	27% 6% 24% 34% 9% 1	4807
August	1st to 10th	Day	34% 5% 19% 29% 13% 1	4273
	11th to 20th	Day	33% 4 20% 36% 7%	4080
	21st to end	Day	33% 5% 22% 31% 9%	4531
September	1st to 10th	Day	32% 5% 17% 33% 12% 1	2332
	11th to 20th	Day	41% 5% 19% 26% 9% 1	2090
	21st to end	Day	39% 4 16% 31% 9%	2090
October	1st to 10th	Day	37% 3 13% 33% 12% 1	3266
	11th to 20th	Day	39% 6% 11% 30% 13%	3032
	21st to end	Day	47% 4 13% 26% 9%	3091
November	1st to 10th	Day	55% 5% 14% 20% 6% 1	1653
	11th to 20th	Day	60% 4 15% 15% 6% 1	1600
	21st to end	Day	59% 5% 15% 16% 5%	1470
December	1st to 10th	Day	66% 5% 14% 11% 4	2290
	11th to 20th	Day	57% 5% 14% 14% 9%	2640
	21st to end	Day	58% 5% 9% 22% 7%	3498
January	1st to 10th	Night	61% 4 12% 19% 4	6787
	11th to 20th	Night	62% 5% 8% 20% 5%	6634
	21st to end	Night	59% 4 13% 18% 5%	6974
February	1st to 10th	Night	55% 4 13% 17% 11%	2700
	11th to 20th	Night	44% 6% 15% 22% 13%	2534
	21st to end	Night	49% 4 12% 23% 11%	1937
March	1st to 10th	Night	48% 5% 13% 24% 9%	3688
	11th to 20th	Night	42% 5% 15% 25% 12% 1	3480
	21st to end	Night	55% 4 11% 21% 9% 1	3817
April	1st to 10th	Night	38% 5% 13% 27% 14% 3	1867
	11th to 20th	Night	49% 4 9% 22% 13% 2	1689
	21st to end	Night	45% 5% 14% 23% 11% 2	1710
May	1st to 10th	Night	48% 3 10% 28% 10% 1	2823
	11th to 20th	Night	43% 3 13% 27% 11% 3	2270
	21st to end	Night	40% 5% 13% 29% 13%	2414
June	1st to 10th	Night	35% 7% 19% 27% 11% 2	1330
	11th to 20th	Night	37% 4 16% 26% 14% 2	1330
	21st to end	Night	33% 7% 18% 33% 7% 1	1330
July	1st to 10th	Night	25% 7% 27% 32% 8% 1	2185
	11th to 20th	Night	21% 5% 18% 44% 11% 1	2178
	21st to end	Night	27% 5% 25% 33% 10% 1	2514
August	1st to 10th	Night	28% 6% 16% 33% 16% 1	2643
	11th to 20th	Night	29% 5% 23% 33% 10%	2825
	21st to end	Night	31% 4 18% 34% 11% 1	3135
September	1st to 10th	Night	29% 4 15% 34% 17% 1	1846
	11th to 20th	Night	33% 4 20% 29% 13% 1	2090
	21st to end	Night	39% 4 12% 32% 13% 1	2090
October	1st to 10th	Night	37% 3 12% 31% 14% 2	3661
	11th to 20th	Night	39% 3 12% 31% 15%	3888
	21st to end	Night	42% 5% 14% 31% 9%	4467
November	1st to 10th	Night	51% 5% 15% 19% 8% 2	2645
	11th to 20th	Night	57% 4 15% 16% 7% 1	2800
	21st to end	Night	56% 5% 15% 17% 6%	2926
December	1st to 10th	Night	67% 5% 13% 12% 3	4900
	11th to 20th	Night	63% 4 12% 14% 7%	5566
	21st to end	Night	58% 6% 10% 19% 6%	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	11% 6% 32% 33% 13% 5%	300
	11th to 20th	Day	10% 9% 32% 36% 11% 2	300
	21st to end	Day	15% 8% 34% 31% 11% 2	330
February	1st to 10th	Day	16% 11% 32% 34% 5% 2	300
	11th to 20th	Day	8% 12% 37% 30% 11% 2	300
	21st to end	Day	10% 5% 44% 29% 10% 2	225
March	1st to 10th	Day	14% 7% 33% 33% 11% 2	270
	11th to 20th	Day	13% 9% 33% 29% 11% 4	270
	21st to end	Day	15% 10% 30% 29% 12% 3	297
April	1st to 10th	Day	14% 14% 32% 27% 10% 3	490
	11th to 20th	Day	21% 10% 30% 26% 12% 1	640
	21st to end	Day	22% 11% 29% 26% 11% 2	650
May	1st to 10th	Day	17% 10% 30% 28% 12% 2	653
	11th to 20th	Day	21% 8% 30% 25% 13% 3	675
	21st to end	Day	19% 7% 26% 30% 15% 3	734
June	1st to 10th	Day	15% 9% 29% 27% 15% 6%	680
	11th to 20th	Day	16% 9% 35% 26% 12% 3	680
	21st to end	Day	13% 7% 26% 33% 16% 5%	680
July	1st to 10th	Day	9% 4 26% 31% 20% 10%	671
	11th to 20th	Day	11% 5% 29% 28% 20% 7%	668
	21st to end	Day	6% 4 25% 37% 21% 7%	739
August	1st to 10th	Day	9% 4 26% 32% 22% 7%	676
	11th to 20th	Day	10% 5% 20% 33% 21% 11%	665
	21st to end	Day	11% 7% 27% 32% 18% 5%	748
September	1st to 10th	Day	12% 8% 32% 29% 15% 4	542
	11th to 20th	Day	18% 8% 31% 29% 11% 4	300
	21st to end	Day	22% 9% 32% 26% 12%	300
October	1st to 10th	Day	20% 11% 27% 29% 12% 1	300
	11th to 20th	Day	14% 10% 39% 28% 7% 2	300
	21st to end	Day	22% 10% 35% 23% 8% 2	330
November	1st to 10th	Day	22% 10% 29% 26% 11% 2	300
	11th to 20th	Day	17% 14% 36% 24% 9% 1	300
	21st to end	Day	18% 11% 34% 29% 8% 1	300
December	1st to 10th	Day	13% 7% 34% 32% 10% 3	300
	11th to 20th	Day	14% 12% 35% 32% 8%	300
	21st to end	Day	15% 7% 32% 30% 10% 6%	330
January	1st to 10th	Night	11% 10% 33% 31% 14% 1	1000
	11th to 20th	Night	15% 9% 30% 38% 7%	1000
	21st to end	Night	14% 11% 33% 33% 8% 1	1100
February	1st to 10th	Night	13% 9% 34% 32% 11% 1	1000
	11th to 20th	Night	9% 8% 33% 35% 13% 2	979
	21st to end	Night	13% 6% 33% 33% 12% 2	681
March	1st to 10th	Night	16% 8% 31% 32% 12% 1	740
	11th to 20th	Night	10% 8% 36% 34% 11% 1	570
	21st to end	Night	18% 7% 27% 33% 12% 3	627
April	1st to 10th	Night	12% 8% 36% 32% 11% 1	570
	11th to 20th	Night	18% 6% 32% 32% 12% 1	563
	21st to end	Night	21% 5% 23% 39% 11% 1	570
May	1st to 10th	Night	18% 5% 33% 32% 11% 1	570
	11th to 20th	Night	17% 7% 26% 35% 14% 1	561
	21st to end	Night	20% 7% 26% 35% 11% 1	617
June	1st to 10th	Night	13% 7% 26% 38% 15% 1	570
	11th to 20th	Night	16% 5% 29% 34% 15% 1	570
	21st to end	Night	11% 5% 25% 41% 18% 1	570
July	1st to 10th	Night	6% 2 26% 40% 24% 2	555
	11th to 20th	Night	6% 5% 26% 40% 21% 2	552
	21st to end	Night	6% 3 24% 43% 21% 2	624
August	1st to 10th	Night	7% 3 21% 48% 21% 1	567
	11th to 20th	Night	10% 3 23% 39% 22% 4	564
	21st to end	Night	10% 4 26% 40% 19% 1	627
September	1st to 10th	Night	11% 3 30% 36% 19% 2	570
	11th to 20th	Night	11% 6% 27% 42% 13% 1	570
	21st to end	Night	16% 6% 31% 34% 12% 1	570
October	1st to 10th	Night	20% 8% 33% 30% 8% 1	673
	11th to 20th	Night	18% 9% 32% 31% 10%	819
	21st to end	Night	17% 10% 35% 27% 10% 1	1045
November	1st to 10th	Night	18% 11% 33% 28% 9% 1	975
	11th to 20th	Night	17% 11% 35% 27% 9% 1	1000
	21st to end	Night	18% 12% 36% 26% 6% 2	1000
December	1st to 10th	Night	14% 10% 31% 34% 10% 1	1000
	11th to 20th	Night	16% 11% 32% 31% 9% 1	1000
	21st to end	Night	19% 7% 31% 30% 12% 1	1100

Page generated at 2025-02-18 02:26:33. Version ac07

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