J. Weiskopf Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the J. Weiskopf Observatory clear sky chart from 2005-11-06 to 2026-01-17.

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	13% 9% 3 8% 5% 4 4 4 6% 7% 37%	127582
	Night	13% 10% 3 8% 5% 4 4 4 5% 6% 39%	124369
Transparency	Day	45% 4 12% 25% 14% 1	131089
	Night	46% 3 11% 24% 15% 2	128936
Seeing	Day	17% 11% 29% 24% 14% 5%	17452
	Night	17% 10% 30% 30% 12% 1	27381

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	9% 12% 8% 15% 6% 6% 5% 4 5% 5% 25%	3697
	11th to 20th	Day	9% 11% 6% 12% 4 4 4 6% 8% 6% 30%	3668
	21st to end	Day	13% 15% 8% 9% 4 3 3 3 5% 6% 31%	4240
February	1st to 10th	Day	13% 14% 5% 9% 7% 5% 5% 3 5% 6% 28%	1800
	11th to 20th	Day	13% 11% 3 8% 6% 5% 5% 4 4 5% 35%	1910
	21st to end	Day	12% 13% 5% 11% 5% 4 4 3 5% 6% 32%	1650
March	1st to 10th	Day	16% 9% 3 6% 4 3 3 3 4 6% 44%	4480
	11th to 20th	Day	11% 6% 3 7% 5% 4 4 4 5% 8% 43%	4792
	21st to end	Day	19% 12% 5% 12% 6% 5% 5% 4 5% 5% 23%	5911
April	1st to 10th	Day	17% 12% 4 8% 6% 4 5% 3 5% 6% 31%	2400
	11th to 20th	Day	16% 9% 3 8% 5% 5% 4 5% 5% 6% 34%	2374
	21st to end	Day	18% 10% 3 8% 5% 4 4 3 5% 6% 33%	2449
May	1st to 10th	Day	15% 9% 2 7% 4 5% 4 4 6% 7% 36%	5949
	11th to 20th	Day	14% 5% 2 6% 4 4 5% 4 6% 8% 42%	6319
	21st to end	Day	13% 6% 3 6% 5% 5% 5% 4 7% 9% 38%	6935
June	1st to 10th	Day	10% 6% 2 6% 5% 4 5% 5% 7% 8% 43%	2800
	11th to 20th	Day	9% 5% 2 6% 5% 4 4 4 7% 9% 45%	2800
	21st to end	Day	11% 7% 3 6% 5% 5% 5% 5% 7% 9% 39%	2800
July	1st to 10th	Day	8% 7% 3 8% 5% 5% 5% 5% 7% 10% 37%	6360
	11th to 20th	Day	8% 6% 2 6% 5% 4 5% 4 6% 10% 43%	6360
	21st to end	Day	10% 7% 3 7% 6% 5% 5% 4 6% 10% 38%	6924
August	1st to 10th	Day	9% 6% 2 6% 4 4 5% 4 7% 10% 42%	4444
	11th to 20th	Day	11% 5% 2 5% 4 4 4 4 7% 10% 45%	3885
	21st to end	Day	8% 6% 2 6% 4 4 4 4 5% 9% 48%	4312
September	1st to 10th	Day	12% 5% 2 5% 4 4 4 4 6% 7% 48%	2400
	11th to 20th	Day	13% 4 1 3 3 3 3 3 5% 8% 54%	2326
	21st to end	Day	11% 6% 3 6% 4 4 5% 3 5% 7% 46%	2200
October	1st to 10th	Day	12% 6% 2 6% 4 3 4 3 5% 6% 49%	2060
	11th to 20th	Day	17% 10% 3 7% 5% 4 4 3 4 6% 38%	2000
	21st to end	Day	20% 9% 3 6% 5% 3 3 2 4 6% 39%	2025
November	1st to 10th	Day	11% 9% 3 6% 4 4 4 3 4 7% 45%	1852
	11th to 20th	Day	15% 12% 3 9% 5% 4 4 3 5% 6% 32%	1890
	21st to end	Day	16% 14% 4 9% 4 3 3 3 4 5% 35%	1722
December	1st to 10th	Day	9% 10% 4 8% 5% 4 4 3 5% 6% 43%	2690
	11th to 20th	Day	12% 14% 5% 13% 6% 6% 6% 4 5% 5% 23%	3088
	21st to end	Day	22% 23% 8% 12% 5% 4 4 3 3 4 13%	4070
January	1st to 10th	Night	10% 13% 7% 14% 6% 4 4 3 3 4 32%	6488
	11th to 20th	Night	11% 14% 6% 13% 6% 5% 5% 5% 7% 5% 22%	6445
	21st to end	Night	15% 15% 6% 10% 6% 4 4 3 4 5% 31%	6724
February	1st to 10th	Night	13% 14% 4 9% 6% 5% 5% 3 4 5% 33%	2600
	11th to 20th	Night	13% 11% 3 9% 5% 4 4 3 4 5% 39%	2374
	21st to end	Night	15% 12% 3 8% 4 3 4 2 5% 6% 37%	1980
March	1st to 10th	Night	17% 9% 4 6% 4 3 4 3 4 5% 42%	5373
	11th to 20th	Night	9% 7% 2 6% 4 4 4 3 5% 7% 49%	5210
	21st to end	Night	20% 12% 5% 10% 5% 5% 4 3 4 5% 27%	5016
April	1st to 10th	Night	17% 10% 2 7% 5% 4 4 3 5% 5% 39%	2000
	11th to 20th	Night	18% 8% 3 7% 4 3 3 4 4 6% 39%	1975
	21st to end	Night	16% 9% 3 7% 4 4 4 4 5% 7% 36%	1950
May	1st to 10th	Night	17% 8% 3 7% 5% 4 5% 4 5% 6% 37%	3946
	11th to 20th	Night	11% 6% 2 6% 4 5% 4 4 6% 8% 44%	3596
	21st to end	Night	14% 7% 2 6% 4 4 4 4 6% 7% 40%	3942
June	1st to 10th	Night	8% 5% 2 6% 4 4 5% 5% 6% 9% 48%	1600
	11th to 20th	Night	9% 5% 2 4 4 4 5% 5% 7% 9% 46%	1600
	21st to end	Night	8% 5% 2 8% 5% 5% 5% 5% 6% 7% 44%	1600
July	1st to 10th	Night	12% 6% 3 7% 4 5% 4 4 6% 9% 41%	3618
	11th to 20th	Night	7% 5% 2 5% 4 4 4 4 5% 7% 52%	3617
	21st to end	Night	10% 6% 2 7% 4 4 4 4 6% 8% 44%	3947
August	1st to 10th	Night	4 4 2 5% 4 5% 4 4 6% 9% 52%	2713
	11th to 20th	Night	6% 5% 2 6% 4 4 5% 5% 6% 8% 49%	3165
	21st to end	Night	8% 4 2 5% 4 4 5% 4 6% 8% 51%	3586
September	1st to 10th	Night	9% 5% 2 4 4 3 3 3 4 7% 55%	2000
	11th to 20th	Night	9% 3 2 5% 3 4 4 3 5% 9% 53%	2053
	21st to end	Night	10% 6% 2 5% 4 4 4 3 6% 7% 49%	2200
October	1st to 10th	Night	11% 6% 2 5% 4 3 3 3 4 5% 52%	2337
	11th to 20th	Night	15% 8% 2 5% 4 3 3 3 4 6% 46%	2400
	21st to end	Night	19% 9% 3 6% 4 4 4 3 5% 6% 37%	2815
November	1st to 10th	Night	12% 7% 3 5% 4 3 4 4 4 6% 48%	2666
	11th to 20th	Night	15% 10% 3 8% 5% 4 3 3 5% 6% 38%	2730
	21st to end	Night	17% 12% 4 8% 4 3 4 3 4 5% 36%	2894
December	1st to 10th	Night	10% 9% 3 8% 5% 5% 5% 4 5% 6% 40%	4720
	11th to 20th	Night	16% 16% 4 9% 5% 4 4 3 4 5% 30%	5339
	21st to end	Night	22% 19% 6% 11% 5% 5% 4 3 4 4 17%	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	61% 3 12% 17% 7% 1	3698
	11th to 20th	Day	53% 2 12% 12% 18% 3	3668
	21st to end	Day	54% 2 13% 20% 9%	4240
February	1st to 10th	Day	57% 2 12% 17% 10% 1	1800
	11th to 20th	Day	51% 3 10% 25% 9% 2	1920
	21st to end	Day	53% 4 10% 21% 10% 2	1650
March	1st to 10th	Day	45% 2 10% 29% 10% 4	4480
	11th to 20th	Day	40% 3 11% 33% 12% 1	4792
	21st to end	Day	64% 3 11% 14% 7% 1	5913
April	1st to 10th	Day	55% 3 10% 23% 9%	2400
	11th to 20th	Day	50% 4 9% 24% 11% 2	2374
	21st to end	Day	52% 3 11% 19% 13% 1	2449
May	1st to 10th	Day	47% 3 9% 23% 17%	5984
	11th to 20th	Day	41% 5% 10% 27% 16% 1	6323
	21st to end	Day	43% 4 11% 27% 12% 3	6949
June	1st to 10th	Day	37% 4 13% 28% 15% 1	2800
	11th to 20th	Day	36% 4 12% 26% 20% 2	2800
	21st to end	Day	40% 5% 13% 25% 14% 2	2800
July	1st to 10th	Day	40% 6% 14% 27% 12% 1	6291
	11th to 20th	Day	35% 6% 14% 29% 15% 1	6285
	21st to end	Day	40% 7% 16% 20% 15% 2	6728
August	1st to 10th	Day	36% 6% 16% 29% 12% 1	4429
	11th to 20th	Day	33% 6% 12% 34% 15% 1	3921
	21st to end	Day	33% 5% 12% 31% 17% 1	4311
September	1st to 10th	Day	36% 4 10% 28% 20% 2	2400
	11th to 20th	Day	30% 4 11% 32% 21% 3	2326
	21st to end	Day	37% 4 9% 29% 20% 1	2200
October	1st to 10th	Day	24% 3 6% 35% 30% 1	3359
	11th to 20th	Day	34% 3 9% 27% 25% 2	3240
	21st to end	Day	42% 2 9% 29% 14% 3	3250
November	1st to 10th	Day	40% 3 13% 27% 14% 2	1852
	11th to 20th	Day	53% 3 11% 22% 9% 1	1890
	21st to end	Day	53% 3 12% 23% 8% 1	1722
December	1st to 10th	Day	43% 3 18% 28% 7% 1	2690
	11th to 20th	Day	62% 4 11% 16% 6% 1	3085
	21st to end	Day	77% 3 7% 8% 5%	4070
January	1st to 10th	Night	57% 2 10% 21% 10%	6488
	11th to 20th	Night	62% 3 10% 10% 14% 1	6445
	21st to end	Night	58% 2 11% 18% 10% 1	6724
February	1st to 10th	Night	55% 2 9% 20% 13% 1	2600
	11th to 20th	Night	48% 3 10% 24% 14% 1	2375
	21st to end	Night	49% 2 11% 22% 12% 4	1980
March	1st to 10th	Night	46% 2 11% 24% 12% 6%	5373
	11th to 20th	Night	36% 2 10% 35% 14% 3	5210
	21st to end	Night	61% 3 9% 19% 8% 1	5016
April	1st to 10th	Night	49% 2 10% 24% 14% 2	2000
	11th to 20th	Night	47% 3 7% 22% 17% 4	1975
	21st to end	Night	47% 3 11% 25% 12% 2	1950
May	1st to 10th	Night	48% 3 11% 18% 18% 2	3973
	11th to 20th	Night	38% 4 13% 25% 15% 3	3597
	21st to end	Night	42% 4 10% 26% 16% 2	3950
June	1st to 10th	Night	33% 4 14% 29% 19% 2	1600
	11th to 20th	Night	34% 5% 11% 24% 24% 3	1600
	21st to end	Night	38% 5% 14% 22% 18% 3	1600
July	1st to 10th	Night	40% 6% 13% 25% 14% 3	3580
	11th to 20th	Night	30% 6% 12% 30% 20% 1	3572
	21st to end	Night	35% 7% 15% 24% 16% 3	3832
August	1st to 10th	Night	28% 5% 15% 32% 18% 2	2704
	11th to 20th	Night	30% 7% 13% 32% 17% 1	3195
	21st to end	Night	30% 5% 13% 32% 18% 3	3586
September	1st to 10th	Night	30% 3 11% 30% 23% 3	2000
	11th to 20th	Night	30% 4 8% 35% 20% 3	2053
	21st to end	Night	34% 4 10% 30% 21% 2	2200
October	1st to 10th	Night	24% 2 6% 35% 30% 2	3818
	11th to 20th	Night	34% 3 8% 26% 27% 3	3899
	21st to end	Night	42% 3 9% 25% 17% 3	4540
November	1st to 10th	Night	38% 4 9% 29% 18% 3	2666
	11th to 20th	Night	48% 3 11% 24% 13% 1	2730
	21st to end	Night	51% 3 11% 24% 11% 1	2894
December	1st to 10th	Night	44% 4 13% 28% 10% 1	4720
	11th to 20th	Night	57% 3 12% 18% 9% 1	5341
	21st to end	Night	72% 2 8% 13% 5%	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% 15% 38% 21% 7% 1	330
	11th to 20th	Day	20% 19% 38% 18% 3 1	318
	21st to end	Day	21% 14% 39% 22% 3 1	330
February	1st to 10th	Day	20% 19% 40% 15% 5%	300
	11th to 20th	Day	21% 23% 34% 18% 3	300
	21st to end	Day	28% 20% 39% 11% 2	249
March	1st to 10th	Day	21% 23% 37% 16% 4	300
	11th to 20th	Day	24% 18% 37% 13% 7% 1	300
	21st to end	Day	28% 15% 31% 18% 7% 1	520
April	1st to 10th	Day	26% 13% 36% 20% 5%	500
	11th to 20th	Day	24% 18% 26% 22% 8% 2	492
	21st to end	Day	24% 14% 29% 22% 9% 1	500
May	1st to 10th	Day	20% 13% 32% 25% 8% 2	538
	11th to 20th	Day	20% 12% 29% 23% 12% 4	725
	21st to end	Day	16% 11% 31% 21% 15% 7%	789
June	1st to 10th	Day	10% 9% 29% 30% 17% 5%	730
	11th to 20th	Day	11% 6% 34% 28% 16% 5%	730
	21st to end	Day	12% 9% 30% 30% 15% 4	730
July	1st to 10th	Day	10% 3 19% 30% 24% 14%	721
	11th to 20th	Day	10% 5% 23% 31% 22% 9%	718
	21st to end	Day	8% 5% 21% 36% 24% 6%	794
August	1st to 10th	Day	8% 4 19% 33% 25% 11%	726
	11th to 20th	Day	13% 5% 17% 26% 24% 14%	543
	21st to end	Day	8% 4 18% 33% 27% 9%	583
September	1st to 10th	Day	15% 6% 22% 27% 23% 7%	530
	11th to 20th	Day	12% 2 19% 30% 25% 11%	530
	21st to end	Day	13% 10% 25% 27% 17% 7%	530
October	1st to 10th	Day	16% 8% 27% 27% 18% 4	390
	11th to 20th	Day	20% 19% 33% 19% 8%	330
	21st to end	Day	29% 15% 30% 22% 4 1	363
November	1st to 10th	Day	19% 18% 35% 17% 9% 2	330
	11th to 20th	Day	25% 20% 32% 16% 6% 1	330
	21st to end	Day	28% 12% 39% 17% 5%	330
December	1st to 10th	Day	18% 16% 40% 18% 6% 1	330
	11th to 20th	Day	19% 16% 38% 20% 6%	330
	21st to end	Day	28% 20% 36% 14% 2	363
January	1st to 10th	Night	18% 14% 37% 26% 5%	1050
	11th to 20th	Night	20% 15% 36% 24% 5%	1031
	21st to end	Night	21% 12% 33% 27% 6%	1100
February	1st to 10th	Night	22% 12% 36% 24% 5%	1000
	11th to 20th	Night	19% 13% 37% 26% 5%	819
	21st to end	Night	21% 14% 40% 22% 4	660
March	1st to 10th	Night	21% 12% 37% 23% 6% 1	800
	11th to 20th	Night	17% 11% 36% 28% 7%	700
	21st to end	Night	24% 11% 35% 25% 5% 1	660
April	1st to 10th	Night	22% 12% 38% 22% 6%	600
	11th to 20th	Night	22% 14% 30% 28% 6%	593
	21st to end	Night	22% 10% 29% 29% 9% 1	600
May	1st to 10th	Night	18% 9% 31% 33% 9%	600
	11th to 20th	Night	17% 8% 31% 32% 11% 1	591
	21st to end	Night	14% 4 29% 35% 16% 1	650
June	1st to 10th	Night	9% 5% 25% 46% 15% 1	600
	11th to 20th	Night	11% 4 29% 42% 13% 1	600
	21st to end	Night	10% 4 29% 40% 17% 1	600
July	1st to 10th	Night	10% 3 14% 40% 30% 4	585
	11th to 20th	Night	9% 3 18% 41% 26% 3	582
	21st to end	Night	8% 2 16% 47% 26% 2	657
August	1st to 10th	Night	4 3 13% 47% 30% 3	597
	11th to 20th	Night	7% 3 13% 42% 31% 5%	594
	21st to end	Night	10% 2 15% 43% 28% 3	660
September	1st to 10th	Night	11% 4 19% 34% 28% 5%	600
	11th to 20th	Night	9% 4 20% 36% 28% 4	653
	21st to end	Night	11% 9% 23% 34% 21% 2	800
October	1st to 10th	Night	13% 7% 28% 32% 18% 2	800
	11th to 20th	Night	19% 11% 32% 28% 10% 1	800
	21st to end	Night	24% 11% 32% 25% 8%	880
November	1st to 10th	Night	16% 10% 39% 24% 10% 1	820
	11th to 20th	Night	20% 15% 35% 24% 6%	840
	21st to end	Night	23% 13% 34% 25% 6%	1004
December	1st to 10th	Night	23% 12% 37% 23% 5%	1050
	11th to 20th	Night	23% 13% 33% 25% 5% 1	1050
	21st to end	Night	29% 13% 33% 22% 3	1155

Page generated at 2026-01-14 00:14:27. Version ac07

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