U-46 Planetarium
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the U-46 Planetarium clear sky chart from 2005-11-06 to 2025-11-18.

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% 9% 3 7% 5% 4 4 4 5% 7% 37%	126917
	Night	14% 9% 4 7% 4 4 4 3 5% 6% 39%	123698
Transparency	Day	44% 6% 12% 23% 14% 2	130426
	Night	45% 5% 11% 22% 15% 2	128263
Seeing	Day	17% 15% 30% 23% 11% 4	17250
	Night	18% 12% 31% 27% 11% 1	27069

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	11% 11% 7% 13% 6% 5% 5% 4 5% 6% 26%	3617
	11th to 20th	Day	10% 13% 6% 12% 4 4 4 6% 6% 5% 31%	3620
	21st to end	Day	15% 16% 7% 8% 4 3 3 3 4 6% 32%	4102
February	1st to 10th	Day	14% 14% 5% 9% 6% 5% 5% 4 5% 6% 27%	1825
	11th to 20th	Day	14% 11% 4 9% 6% 4 3 3 5% 5% 36%	1948
	21st to end	Day	13% 13% 5% 10% 5% 4 4 3 5% 7% 32%	1650
March	1st to 10th	Day	16% 11% 3 6% 4 3 3 3 3 6% 41%	4480
	11th to 20th	Day	10% 6% 3 7% 4 5% 5% 4 5% 9% 42%	4689
	21st to end	Day	20% 14% 6% 11% 6% 5% 4 3 4 6% 20%	5967
April	1st to 10th	Day	18% 13% 4 8% 5% 4 5% 4 5% 5% 30%	2400
	11th to 20th	Day	16% 11% 2 6% 5% 5% 4 4 5% 7% 34%	2374
	21st to end	Day	20% 8% 3 7% 6% 4 4 4 5% 6% 33%	2503
May	1st to 10th	Day	17% 7% 3 6% 4 5% 4 4 6% 8% 36%	5862
	11th to 20th	Day	14% 6% 2 6% 4 5% 5% 4 6% 7% 41%	6222
	21st to end	Day	15% 7% 2 6% 5% 4 5% 4 6% 8% 38%	6935
June	1st to 10th	Day	10% 5% 2 6% 5% 4 5% 4 6% 7% 46%	2800
	11th to 20th	Day	10% 6% 2 6% 5% 4 4 4 7% 8% 45%	2800
	21st to end	Day	11% 6% 3 7% 5% 5% 4 4 7% 9% 40%	2800
July	1st to 10th	Day	11% 6% 3 7% 6% 5% 5% 5% 7% 9% 37%	6360
	11th to 20th	Day	10% 4 2 6% 4 5% 6% 4 7% 9% 42%	6360
	21st to end	Day	11% 6% 2 7% 5% 5% 5% 4 6% 9% 39%	6924
August	1st to 10th	Day	11% 5% 2 6% 4 4 5% 4 6% 9% 44%	4500
	11th to 20th	Day	12% 5% 2 6% 5% 5% 5% 5% 6% 9% 42%	3855
	21st to end	Day	8% 5% 2 6% 4 4 5% 4 6% 8% 48%	4312
September	1st to 10th	Day	12% 5% 2 5% 4 4 5% 3 5% 7% 48%	2400
	11th to 20th	Day	11% 4 1 4 3 3 4 4 5% 6% 54%	2280
	21st to end	Day	11% 7% 3 6% 4 5% 4 4 6% 7% 45%	2200
October	1st to 10th	Day	11% 6% 3 6% 4 4 4 3 6% 6% 48%	2132
	11th to 20th	Day	19% 9% 3 6% 4 4 4 3 4 7% 37%	1993
	21st to end	Day	19% 10% 3 6% 3 4 4 3 5% 5% 38%	1980
November	1st to 10th	Day	12% 9% 3 5% 4 3 4 3 5% 6% 44%	1852
	11th to 20th	Day	16% 13% 3 8% 5% 4 3 4 5% 6% 33%	1863
	21st to end	Day	18% 12% 3 9% 4 4 3 3 3 4 35%	1712
December	1st to 10th	Day	9% 10% 4 7% 5% 4 4 3 4 6% 43%	2610
	11th to 20th	Day	12% 15% 5% 11% 6% 6% 5% 4 5% 7% 24%	3008
	21st to end	Day	23% 24% 8% 10% 4 4 3 2 3 4 12%	3982
January	1st to 10th	Night	12% 13% 6% 13% 5% 4 4 2 4 4 32%	6348
	11th to 20th	Night	13% 14% 7% 12% 6% 5% 4 5% 6% 4 23%	6360
	21st to end	Night	17% 15% 7% 9% 4 3 3 3 4 5% 32%	6859
February	1st to 10th	Night	14% 15% 4 9% 5% 5% 4 3 4 6% 31%	2574
	11th to 20th	Night	13% 12% 3 8% 5% 4 4 4 5% 6% 36%	2338
	21st to end	Night	16% 12% 3 8% 5% 3 3 2 4 6% 37%	1980
March	1st to 10th	Night	16% 10% 4 7% 3 3 3 3 4 4 42%	5373
	11th to 20th	Night	10% 6% 2 6% 4 4 4 3 4 7% 48%	5314
	21st to end	Night	20% 11% 5% 9% 6% 5% 4 3 4 5% 29%	4962
April	1st to 10th	Night	16% 8% 3 7% 4 4 4 4 6% 6% 38%	2000
	11th to 20th	Night	18% 10% 2 7% 4 4 4 3 4 5% 39%	1975
	21st to end	Night	17% 8% 3 6% 5% 3 4 4 5% 7% 38%	1895
May	1st to 10th	Night	18% 7% 3 7% 5% 5% 4 4 5% 7% 37%	4033
	11th to 20th	Night	13% 5% 2 6% 3 4 4 4 6% 8% 44%	3684
	21st to end	Night	17% 8% 2 6% 4 4 4 3 5% 6% 41%	3942
June	1st to 10th	Night	10% 5% 1 5% 4 4 5% 4 7% 8% 48%	1600
	11th to 20th	Night	9% 5% 2 6% 5% 4 4 4 5% 9% 46%	1600
	21st to end	Night	10% 5% 2 6% 5% 5% 4 5% 7% 8% 43%	1600
July	1st to 10th	Night	11% 5% 2 6% 5% 4 6% 4 6% 9% 41%	3618
	11th to 20th	Night	8% 5% 2 6% 4 4 4 4 7% 7% 51%	3617
	21st to end	Night	10% 5% 2 7% 4 5% 5% 4 6% 8% 45%	3947
August	1st to 10th	Night	7% 4 1 5% 5% 4 5% 5% 7% 8% 49%	2655
	11th to 20th	Night	10% 5% 2 5% 4 3 4 3 5% 8% 51%	3191
	21st to end	Night	9% 3 2 5% 3 3 3 3 5% 8% 54%	3586
September	1st to 10th	Night	11% 5% 2 6% 4 3 3 3 4 7% 55%	2000
	11th to 20th	Night	9% 4 2 5% 4 3 3 3 5% 7% 56%	2099
	21st to end	Night	11% 5% 2 7% 4 4 5% 3 6% 7% 48%	2200
October	1st to 10th	Night	11% 6% 2 6% 4 3 3 3 4 6% 53%	2265
	11th to 20th	Night	17% 7% 2 5% 4 3 3 3 4 5% 46%	2405
	21st to end	Night	20% 9% 3 5% 4 4 4 3 5% 7% 38%	2860
November	1st to 10th	Night	13% 8% 3 6% 4 3 3 3 4 5% 48%	2666
	11th to 20th	Night	14% 11% 3 8% 4 3 3 3 6% 6% 38%	2692
	21st to end	Night	15% 12% 5% 8% 5% 3 4 3 4 5% 36%	2685
December	1st to 10th	Night	11% 9% 3 7% 4 4 4 3 4 7% 43%	4580
	11th to 20th	Night	17% 16% 5% 8% 5% 4 3 3 4 5% 29%	5199
	21st to end	Night	23% 19% 6% 9% 5% 5% 4 3 5% 4 17%	6996

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	58% 5% 11% 17% 8%	3618
	11th to 20th	Day	53% 5% 11% 12% 15% 5%	3620
	21st to end	Day	54% 4 14% 18% 10%	4102
February	1st to 10th	Day	58% 5% 12% 14% 9% 2	1825
	11th to 20th	Day	51% 5% 10% 23% 9% 2	1958
	21st to end	Day	53% 7% 10% 18% 10% 2	1650
March	1st to 10th	Day	45% 5% 9% 25% 11% 4	4480
	11th to 20th	Day	38% 6% 12% 32% 11% 1	4689
	21st to end	Day	64% 7% 11% 11% 6% 1	5969
April	1st to 10th	Day	55% 6% 10% 19% 10%	2400
	11th to 20th	Day	47% 7% 10% 21% 11% 3	2374
	21st to end	Day	50% 7% 12% 17% 12% 2	2503
May	1st to 10th	Day	44% 8% 11% 22% 14% 1	5897
	11th to 20th	Day	41% 7% 10% 25% 16% 1	6226
	21st to end	Day	41% 7% 11% 24% 13% 3	6949
June	1st to 10th	Day	35% 7% 11% 27% 17% 2	2800
	11th to 20th	Day	35% 8% 12% 23% 19% 3	2800
	21st to end	Day	40% 8% 13% 22% 15% 2	2800
July	1st to 10th	Day	42% 6% 13% 24% 13% 1	6291
	11th to 20th	Day	36% 7% 14% 26% 16% 1	6285
	21st to end	Day	40% 7% 15% 20% 15% 2	6728
August	1st to 10th	Day	35% 8% 16% 27% 13% 1	4485
	11th to 20th	Day	36% 7% 13% 28% 15% 1	3891
	21st to end	Day	33% 6% 13% 26% 20% 2	4311
September	1st to 10th	Day	35% 7% 12% 26% 17% 4	2400
	11th to 20th	Day	29% 6% 12% 30% 21% 3	2280
	21st to end	Day	37% 6% 10% 26% 20% 1	2200
October	1st to 10th	Day	25% 4 7% 33% 29% 1	3470
	11th to 20th	Day	33% 5% 12% 27% 23% 1	3221
	21st to end	Day	43% 4 9% 27% 13% 3	3180
November	1st to 10th	Day	40% 5% 13% 24% 16% 2	1852
	11th to 20th	Day	51% 5% 11% 22% 10% 1	1863
	21st to end	Day	53% 5% 11% 22% 8%	1712
December	1st to 10th	Day	43% 4 18% 27% 8% 1	2610
	11th to 20th	Day	58% 6% 13% 16% 6% 1	3005
	21st to end	Day	77% 3 6% 9% 5%	3982
January	1st to 10th	Night	56% 4 9% 19% 11% 1	6348
	11th to 20th	Night	62% 5% 9% 9% 14% 1	6360
	21st to end	Night	56% 4 13% 15% 11% 1	6859
February	1st to 10th	Night	54% 4 11% 16% 12% 1	2574
	11th to 20th	Night	48% 6% 10% 21% 14% 1	2339
	21st to end	Night	50% 4 12% 20% 10% 4	1980
March	1st to 10th	Night	45% 5% 11% 24% 11% 4	5373
	11th to 20th	Night	36% 5% 12% 31% 14% 3	5314
	21st to end	Night	58% 6% 9% 17% 10% 1	4962
April	1st to 10th	Night	45% 6% 11% 23% 13% 2	2000
	11th to 20th	Night	47% 6% 8% 19% 16% 4	1975
	21st to end	Night	45% 7% 10% 23% 12% 3	1895
May	1st to 10th	Night	46% 6% 12% 17% 16% 3	4060
	11th to 20th	Night	34% 9% 16% 23% 14% 4	3685
	21st to end	Night	42% 7% 10% 21% 16% 4	3950
June	1st to 10th	Night	32% 8% 13% 25% 20% 3	1600
	11th to 20th	Night	35% 6% 11% 25% 22% 2	1600
	21st to end	Night	37% 9% 16% 20% 17% 2	1600
July	1st to 10th	Night	39% 6% 14% 25% 13% 3	3580
	11th to 20th	Night	30% 7% 14% 29% 19% 1	3572
	21st to end	Night	35% 8% 14% 23% 18% 2	3832
August	1st to 10th	Night	29% 9% 16% 28% 18% 1	2646
	11th to 20th	Night	30% 7% 15% 30% 18% 1	3221
	21st to end	Night	28% 5% 13% 33% 17% 3	3586
September	1st to 10th	Night	30% 6% 11% 27% 23% 3	2000
	11th to 20th	Night	28% 5% 8% 33% 24% 2	2099
	21st to end	Night	35% 6% 11% 29% 18% 2	2200
October	1st to 10th	Night	22% 4 7% 37% 28% 2	3707
	11th to 20th	Night	35% 4 8% 23% 27% 2	3916
	21st to end	Night	41% 4 9% 26% 18% 3	4610
November	1st to 10th	Night	39% 5% 9% 26% 18% 3	2666
	11th to 20th	Night	46% 5% 12% 23% 12% 2	2692
	21st to end	Night	50% 5% 11% 22% 11%	2685
December	1st to 10th	Night	42% 4 15% 27% 10% 2	4580
	11th to 20th	Night	57% 5% 11% 16% 11% 1	5201
	21st to end	Night	69% 5% 8% 11% 6%	6996

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	22% 17% 40% 18% 3	300
	11th to 20th	Day	23% 22% 39% 12% 3	300
	21st to end	Day	24% 23% 34% 14% 4 1	330
February	1st to 10th	Day	23% 26% 35% 13% 3	300
	11th to 20th	Day	27% 24% 32% 14% 2	300
	21st to end	Day	27% 26% 36% 7% 4	249
March	1st to 10th	Day	21% 30% 36% 11% 1	300
	11th to 20th	Day	28% 25% 29% 15% 3	317
	21st to end	Day	28% 18% 31% 16% 5% 1	550
April	1st to 10th	Day	28% 19% 30% 19% 4	500
	11th to 20th	Day	24% 25% 22% 19% 8% 1	492
	21st to end	Day	21% 20% 32% 19% 7% 1	500
May	1st to 10th	Day	22% 16% 32% 20% 8% 2	503
	11th to 20th	Day	21% 14% 31% 24% 9% 2	680
	21st to end	Day	17% 13% 32% 20% 14% 4	789
June	1st to 10th	Day	12% 12% 33% 28% 12% 3	730
	11th to 20th	Day	11% 10% 36% 27% 12% 3	730
	21st to end	Day	12% 12% 34% 28% 10% 4	730
July	1st to 10th	Day	8% 6% 25% 32% 18% 10%	721
	11th to 20th	Day	10% 8% 30% 31% 14% 8%	718
	21st to end	Day	8% 6% 31% 34% 15% 6%	794
August	1st to 10th	Day	9% 4 26% 34% 19% 8%	689
	11th to 20th	Day	12% 5% 22% 29% 23% 9%	519
	21st to end	Day	7% 7% 22% 33% 23% 8%	583
September	1st to 10th	Day	13% 9% 24% 27% 21% 6%	530
	11th to 20th	Day	8% 5% 20% 33% 23% 10%	530
	21st to end	Day	12% 15% 25% 26% 17% 5%	530
October	1st to 10th	Day	15% 13% 28% 28% 14% 3	462
	11th to 20th	Day	23% 23% 33% 17% 5%	330
	21st to end	Day	30% 18% 33% 14% 5%	363
November	1st to 10th	Day	21% 19% 35% 18% 6% 2	330
	11th to 20th	Day	27% 23% 30% 13% 6% 1	321
	21st to end	Day	27% 16% 38% 18% 1 1	300
December	1st to 10th	Day	17% 27% 38% 13% 5%	300
	11th to 20th	Day	19% 28% 33% 15% 4 1	300
	21st to end	Day	28% 23% 33% 12% 2	330
January	1st to 10th	Night	19% 17% 40% 21% 4	1000
	11th to 20th	Night	21% 20% 35% 19% 4 1	1000
	21st to end	Night	21% 15% 37% 23% 4	1100
February	1st to 10th	Night	23% 16% 36% 21% 4	974
	11th to 20th	Night	20% 19% 35% 23% 4	783
	21st to end	Night	22% 18% 38% 19% 3	660
March	1st to 10th	Night	20% 17% 36% 21% 6%	800
	11th to 20th	Night	17% 13% 39% 24% 7%	770
	21st to end	Night	24% 13% 31% 24% 7% 1	660
April	1st to 10th	Night	20% 15% 35% 23% 6%	600
	11th to 20th	Night	24% 14% 30% 24% 7%	593
	21st to end	Night	20% 13% 32% 28% 7%	600
May	1st to 10th	Night	18% 11% 35% 28% 7% 1	600
	11th to 20th	Night	18% 8% 31% 31% 13%	591
	21st to end	Night	15% 7% 29% 33% 16% 1	650
June	1st to 10th	Night	11% 6% 29% 39% 15%	600
	11th to 20th	Night	10% 6% 34% 37% 12% 1	600
	21st to end	Night	10% 7% 31% 35% 16% 1	600
July	1st to 10th	Night	8% 3 20% 41% 25% 3	585
	11th to 20th	Night	8% 4 26% 37% 23% 2	582
	21st to end	Night	6% 2 23% 44% 23% 2	657
August	1st to 10th	Night	5% 2 18% 45% 26% 3	597
	11th to 20th	Night	8% 3 18% 39% 30% 3	594
	21st to end	Night	9% 3 20% 39% 28% 2	660
September	1st to 10th	Night	11% 6% 22% 33% 24% 4	600
	11th to 20th	Night	9% 4 21% 33% 28% 4	699
	21st to end	Night	12% 10% 24% 32% 21% 2	800
October	1st to 10th	Night	13% 7% 27% 34% 17% 2	800
	11th to 20th	Night	21% 14% 32% 24% 9%	800
	21st to end	Night	25% 14% 31% 22% 8%	880
November	1st to 10th	Night	17% 15% 36% 23% 9% 1	820
	11th to 20th	Night	21% 19% 33% 22% 4	829
	21st to end	Night	20% 16% 37% 21% 5%	885
December	1st to 10th	Night	22% 16% 37% 21% 5%	1000
	11th to 20th	Night	24% 15% 34% 24% 4	1000
	21st to end	Night	30% 16% 34% 18% 2	1100

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