Longway Planetarium
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Longway 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% 10% 3 8% 5% 5% 5% 4 5% 7% 33%	126112
	Night	14% 11% 4 9% 5% 4 4 3 5% 6% 33%	124370
Transparency	Day	49% 4 11% 22% 13% 1	129666
	Night	51% 3 11% 21% 12% 1	128886
Seeing	Day	20% 12% 29% 22% 13% 5%	16882
	Night	20% 10% 29% 29% 11% 1	26615

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	12% 17% 6% 18% 6% 5% 4 3 4 4 22%	3617
	11th to 20th	Day	13% 17% 7% 19% 10% 10% 4 3 4 4 9%	3620
	21st to end	Day	21% 18% 6% 14% 6% 5% 5% 3 4 4 14%	4263
February	1st to 10th	Day	13% 15% 6% 13% 8% 4 5% 4 5% 7% 20%	1800
	11th to 20th	Day	13% 15% 6% 13% 9% 5% 4 3 4 5% 23%	1894
	21st to end	Day	14% 17% 5% 11% 7% 4 5% 3 5% 5% 24%	1650
March	1st to 10th	Day	14% 12% 4 10% 5% 5% 4 3 5% 6% 33%	4560
	11th to 20th	Day	12% 13% 4 9% 6% 4 4 4 5% 8% 32%	4980
	21st to end	Day	18% 15% 4 10% 6% 4 4 3 4 5% 28%	5465
April	1st to 10th	Day	17% 13% 3 8% 5% 4 5% 3 5% 6% 30%	2351
	11th to 20th	Day	17% 10% 3 7% 4 4 4 4 5% 6% 36%	2531
	21st to end	Day	16% 9% 3 8% 5% 5% 5% 3 5% 7% 35%	2600
May	1st to 10th	Day	19% 9% 3 7% 3 4 4 3 5% 7% 35%	5862
	11th to 20th	Day	16% 9% 2 7% 4 5% 5% 4 6% 7% 35%	5861
	21st to end	Day	14% 7% 3 6% 5% 4 4 3 5% 7% 41%	6566
June	1st to 10th	Day	10% 7% 2 6% 5% 5% 5% 5% 6% 8% 41%	2890
	11th to 20th	Day	11% 7% 2 7% 5% 5% 4 4 6% 9% 40%	3000
	21st to end	Day	10% 6% 2 6% 4 6% 6% 5% 7% 9% 38%	2857
July	1st to 10th	Day	10% 6% 3 7% 4 5% 6% 5% 7% 10% 39%	6360
	11th to 20th	Day	8% 5% 2 5% 4 4 4 4 7% 9% 48%	6360
	21st to end	Day	8% 5% 2 6% 4 5% 4 4 6% 10% 45%	6657
August	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 4 7% 9% 40%	4234
	11th to 20th	Day	12% 6% 2 6% 5% 5% 5% 4 7% 8% 39%	4174
	21st to end	Day	9% 5% 2 7% 5% 6% 5% 5% 6% 9% 42%	4614
September	1st to 10th	Day	10% 6% 3 6% 6% 5% 6% 5% 6% 7% 41%	2280
	11th to 20th	Day	10% 5% 3 5% 3 4 4 4 6% 8% 48%	2200
	21st to end	Day	15% 8% 3 5% 5% 4 5% 4 6% 7% 37%	2200
October	1st to 10th	Day	13% 7% 2 7% 5% 5% 4 3 5% 7% 42%	2153
	11th to 20th	Day	18% 10% 3 7% 5% 4 4 4 6% 6% 32%	2000
	21st to end	Day	21% 10% 2 7% 5% 5% 5% 4 6% 7% 29%	2006
November	1st to 10th	Day	12% 11% 3 9% 6% 4 4 4 5% 7% 36%	1852
	11th to 20th	Day	14% 13% 5% 10% 6% 5% 5% 3 5% 7% 27%	1863
	21st to end	Day	18% 15% 5% 10% 6% 5% 4 3 5% 6% 24%	1684
December	1st to 10th	Day	13% 19% 8% 11% 5% 5% 5% 3 6% 6% 19%	2600
	11th to 20th	Day	17% 23% 6% 10% 6% 4 5% 3 5% 4 19%	2637
	21st to end	Day	24% 22% 6% 13% 5% 4 3 2 4 4 13%	3871
January	1st to 10th	Night	13% 14% 6% 15% 6% 5% 5% 3 4 5% 24%	6348
	11th to 20th	Night	13% 16% 7% 16% 9% 9% 5% 4 5% 4 13%	6360
	21st to end	Night	16% 18% 8% 10% 5% 5% 5% 3 4 6% 20%	6699
February	1st to 10th	Night	15% 16% 5% 12% 7% 6% 6% 4 4 4 22%	2600
	11th to 20th	Night	14% 14% 5% 11% 6% 5% 4 4 6% 5% 26%	2389
	21st to end	Night	17% 12% 4 10% 6% 4 5% 4 5% 5% 28%	1980
March	1st to 10th	Night	14% 12% 4 8% 4 4 4 3 4 6% 36%	5292
	11th to 20th	Night	10% 10% 3 7% 4 4 4 4 6% 9% 38%	4980
	21st to end	Night	17% 11% 4 8% 5% 4 4 3 4 5% 35%	5467
April	1st to 10th	Night	18% 12% 3 6% 4 4 3 4 5% 6% 34%	2065
	11th to 20th	Night	18% 8% 3 7% 5% 4 4 3 5% 6% 37%	1817
	21st to end	Night	17% 8% 2 7% 4 4 4 4 6% 7% 36%	1800
May	1st to 10th	Night	17% 10% 4 8% 4 4 4 3 5% 7% 34%	4033
	11th to 20th	Night	17% 9% 3 7% 5% 4 3 3 4 6% 40%	4032
	21st to end	Night	16% 6% 2 5% 4 4 4 3 4 7% 45%	4289
June	1st to 10th	Night	12% 5% 1 7% 4 4 4 3 5% 9% 46%	1500
	11th to 20th	Night	11% 6% 3 5% 4 4 5% 4 7% 8% 44%	1400
	21st to end	Night	9% 6% 2 6% 4 3 4 3 5% 9% 48%	1532
July	1st to 10th	Night	8% 5% 2 5% 4 5% 5% 4 7% 10% 45%	3618
	11th to 20th	Night	9% 5% 2 5% 4 3 5% 4 6% 9% 50%	3617
	21st to end	Night	9% 5% 2 7% 5% 4 4 4 5% 7% 48%	4201
August	1st to 10th	Night	10% 5% 2 5% 4 5% 4 4 6% 7% 48%	2915
	11th to 20th	Night	11% 5% 2 5% 4 5% 5% 4 5% 7% 47%	2875
	21st to end	Night	8% 6% 2 6% 4 4 4 4 6% 7% 49%	3281
September	1st to 10th	Night	10% 6% 2 5% 4 4 4 4 6% 6% 48%	2100
	11th to 20th	Night	10% 4 1 5% 3 3 4 3 5% 7% 54%	2200
	21st to end	Night	15% 6% 2 6% 4 4 4 4 6% 7% 42%	2200
October	1st to 10th	Night	15% 6% 2 6% 4 3 4 3 5% 6% 45%	2245
	11th to 20th	Night	20% 10% 3 7% 4 4 5% 3 5% 5% 34%	2400
	21st to end	Night	21% 9% 3 8% 5% 4 4 3 5% 6% 32%	2830
November	1st to 10th	Night	14% 9% 2 7% 4 4 4 3 6% 6% 40%	2666
	11th to 20th	Night	15% 14% 4 9% 5% 4 6% 4 5% 7% 28%	2692
	21st to end	Night	20% 15% 4 7% 5% 4 5% 3 5% 6% 26%	2712
December	1st to 10th	Night	12% 18% 7% 11% 5% 5% 5% 4 5% 6% 21%	4580
	11th to 20th	Night	15% 20% 7% 12% 7% 4 4 3 4 4 20%	5567
	21st to end	Night	22% 23% 6% 11% 4 4 3 2 3 5% 15%	7088

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	67% 2 11% 14% 5% 1	3618
	11th to 20th	Day	80% 3 8% 5% 4	3620
	21st to end	Day	75% 2 10% 8% 4	4263
February	1st to 10th	Day	63% 5% 10% 15% 7%	1800
	11th to 20th	Day	65% 3 8% 15% 9% 1	1904
	21st to end	Day	62% 3 11% 15% 7% 1	1650
March	1st to 10th	Day	54% 2 13% 20% 9% 2	4560
	11th to 20th	Day	52% 2 14% 22% 9% 1	4980
	21st to end	Day	60% 2 10% 17% 9% 2	5465
April	1st to 10th	Day	55% 3 9% 21% 10% 1	2351
	11th to 20th	Day	50% 3 11% 21% 14% 2	2531
	21st to end	Day	49% 4 11% 20% 14% 1	2600
May	1st to 10th	Day	49% 3 10% 20% 14% 3	5897
	11th to 20th	Day	48% 3 10% 23% 14% 1	5865
	21st to end	Day	44% 3 9% 21% 20% 3	6580
June	1st to 10th	Day	39% 5% 11% 27% 16% 2	2890
	11th to 20th	Day	41% 4 11% 26% 16% 2	3000
	21st to end	Day	40% 5% 12% 26% 16% 1	2857
July	1st to 10th	Day	39% 6% 13% 26% 15% 1	6291
	11th to 20th	Day	32% 5% 12% 32% 18% 1	6285
	21st to end	Day	33% 5% 14% 24% 22% 1	6467
August	1st to 10th	Day	39% 5% 13% 25% 17% 1	4221
	11th to 20th	Day	40% 6% 11% 30% 12% 1	4213
	21st to end	Day	37% 6% 13% 30% 13% 1	4613
September	1st to 10th	Day	40% 6% 11% 29% 11% 2	2280
	11th to 20th	Day	34% 4 12% 33% 16% 2	2200
	21st to end	Day	45% 4 12% 25% 14%	2200
October	1st to 10th	Day	33% 3 9% 35% 18% 2	3504
	11th to 20th	Day	41% 3 11% 28% 16%	3240
	21st to end	Day	48% 4 13% 24% 11% 1	3218
November	1st to 10th	Day	48% 4 11% 25% 11% 1	1852
	11th to 20th	Day	56% 3 12% 19% 9%	1863
	21st to end	Day	61% 3 14% 16% 5% 1	1684
December	1st to 10th	Day	64% 4 19% 10% 3	2600
	11th to 20th	Day	70% 2 9% 13% 4	2633
	21st to end	Day	77% 3 8% 9% 4	3871
January	1st to 10th	Night	63% 3 9% 16% 6% 2	6348
	11th to 20th	Night	74% 3 8% 10% 5% 1	6360
	21st to end	Night	67% 2 10% 14% 6%	6699
February	1st to 10th	Night	66% 4 8% 15% 7% 1	2600
	11th to 20th	Night	59% 4 8% 19% 9% 1	2390
	21st to end	Night	58% 3 11% 18% 9% 1	1980
March	1st to 10th	Night	51% 2 13% 23% 10% 1	5292
	11th to 20th	Night	43% 3 15% 24% 14% 1	4980
	21st to end	Night	52% 3 11% 20% 13% 1	5467
April	1st to 10th	Night	51% 3 9% 22% 13% 2	2065
	11th to 20th	Night	48% 3 10% 19% 18% 2	1817
	21st to end	Night	47% 4 10% 21% 16% 3	1800
May	1st to 10th	Night	51% 3 10% 17% 14% 5%	4060
	11th to 20th	Night	47% 3 9% 27% 11% 4	4033
	21st to end	Night	41% 3 9% 25% 17% 5%	4297
June	1st to 10th	Night	36% 3 10% 28% 21% 2	1500
	11th to 20th	Night	37% 4 12% 24% 20% 2	1400
	21st to end	Night	35% 4 13% 29% 18% 1	1532
July	1st to 10th	Night	34% 5% 13% 27% 18% 4	3580
	11th to 20th	Night	32% 4 13% 30% 20% 1	3572
	21st to end	Night	34% 5% 14% 23% 22% 1	4078
August	1st to 10th	Night	34% 5% 14% 30% 17% 1	2904
	11th to 20th	Night	36% 5% 13% 33% 12% 2	2902
	21st to end	Night	34% 5% 12% 33% 15% 1	3281
September	1st to 10th	Night	35% 6% 11% 30% 16% 2	2100
	11th to 20th	Night	30% 4 12% 33% 19% 2	2200
	21st to end	Night	42% 4 11% 28% 14% 1	2200
October	1st to 10th	Night	27% 3 10% 33% 25% 1	3674
	11th to 20th	Night	46% 3 10% 24% 17% 1	3899
	21st to end	Night	43% 3 10% 31% 12% 1	4568
November	1st to 10th	Night	44% 4 11% 26% 14% 1	2666
	11th to 20th	Night	55% 3 12% 20% 9% 1	2692
	21st to end	Night	59% 3 15% 17% 6%	2712
December	1st to 10th	Night	64% 4 17% 13% 3	4580
	11th to 20th	Night	68% 2 9% 13% 6% 1	5570
	21st to end	Night	74% 3 9% 8% 6%	7088

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	24% 17% 37% 15% 6% 1	300
	11th to 20th	Day	23% 20% 41% 12% 3 1	300
	21st to end	Day	31% 16% 31% 17% 5% 1	330
February	1st to 10th	Day	28% 22% 35% 13% 2 1	300
	11th to 20th	Day	25% 21% 37% 16% 1	300
	21st to end	Day	31% 25% 35% 7% 2	249
March	1st to 10th	Day	27% 21% 38% 11% 3 1	328
	11th to 20th	Day	25% 14% 33% 20% 8%	500
	21st to end	Day	29% 14% 31% 19% 6% 1	550
April	1st to 10th	Day	26% 15% 33% 18% 7% 1	500
	11th to 20th	Day	30% 17% 30% 13% 5% 5%	492
	21st to end	Day	26% 13% 27% 21% 11% 1	500
May	1st to 10th	Day	24% 17% 29% 20% 9% 2	503
	11th to 20th	Day	22% 10% 29% 22% 12% 6%	527
	21st to end	Day	17% 9% 25% 26% 17% 6%	628
June	1st to 10th	Day	12% 13% 29% 28% 14% 4	730
	11th to 20th	Day	16% 9% 31% 25% 14% 4	730
	21st to end	Day	13% 10% 29% 29% 15% 4	730
July	1st to 10th	Day	10% 7% 25% 28% 21% 9%	721
	11th to 20th	Day	12% 8% 29% 27% 17% 8%	718
	21st to end	Day	10% 5% 25% 32% 20% 8%	679
August	1st to 10th	Day	10% 5% 21% 26% 23% 14%	526
	11th to 20th	Day	15% 4 20% 28% 24% 10%	519
	21st to end	Day	13% 5% 22% 27% 22% 11%	583
September	1st to 10th	Day	13% 9% 22% 25% 20% 11%	530
	11th to 20th	Day	11% 4 19% 22% 29% 15%	530
	21st to end	Day	20% 10% 20% 24% 18% 8%	530
October	1st to 10th	Day	17% 11% 28% 20% 18% 5%	530
	11th to 20th	Day	22% 16% 28% 25% 7% 1	530
	21st to end	Day	30% 16% 35% 14% 4	389
November	1st to 10th	Day	22% 18% 38% 16% 5% 2	330
	11th to 20th	Day	26% 21% 32% 15% 6% 1	321
	21st to end	Day	30% 18% 34% 14% 3	242
December	1st to 10th	Day	26% 21% 31% 16% 6% 1	200
	11th to 20th	Day	30% 24% 35% 8% 3 1	200
	21st to end	Day	34% 22% 30% 13% 1	307
January	1st to 10th	Night	23% 15% 34% 22% 5% 1	1000
	11th to 20th	Night	26% 16% 31% 23% 4 1	1000
	21st to end	Night	22% 12% 34% 26% 6%	972
February	1st to 10th	Night	25% 12% 36% 23% 3	800
	11th to 20th	Night	23% 15% 33% 23% 5%	783
	21st to end	Night	26% 16% 34% 22% 2	660
March	1st to 10th	Night	22% 16% 32% 23% 7%	800
	11th to 20th	Night	19% 15% 34% 26% 6% 1	800
	21st to end	Night	25% 12% 30% 26% 7% 1	880
April	1st to 10th	Night	25% 13% 34% 23% 5%	665
	11th to 20th	Night	23% 11% 35% 21% 9% 1	593
	21st to end	Night	21% 11% 32% 30% 5%	600
May	1st to 10th	Night	20% 7% 32% 32% 8% 1	600
	11th to 20th	Night	22% 6% 30% 31% 10%	591
	21st to end	Night	15% 4 27% 37% 16% 2	650
June	1st to 10th	Night	15% 6% 25% 42% 12%	500
	11th to 20th	Night	13% 4 31% 42% 11%	400
	21st to end	Night	11% 5% 26% 42% 15% 1	532
July	1st to 10th	Night	9% 2 18% 42% 26% 2	585
	11th to 20th	Night	11% 3 22% 41% 20% 2	582
	21st to end	Night	9% 2 20% 47% 21% 1	657
August	1st to 10th	Night	9% 3 18% 42% 27% 2	597
	11th to 20th	Night	10% 4 17% 39% 27% 4	594
	21st to end	Night	12% 3 19% 36% 26% 3	660
September	1st to 10th	Night	12% 6% 20% 34% 24% 4	700
	11th to 20th	Night	13% 4 18% 33% 28% 4	800
	21st to end	Night	17% 7% 21% 32% 19% 5%	800
October	1st to 10th	Night	19% 7% 24% 32% 17% 1	800
	11th to 20th	Night	26% 11% 26% 28% 8% 1	800
	21st to end	Night	26% 10% 33% 26% 4	880
November	1st to 10th	Night	21% 15% 29% 26% 9%	820
	11th to 20th	Night	23% 18% 31% 22% 6% 1	829
	21st to end	Night	27% 13% 36% 19% 5%	800
December	1st to 10th	Night	26% 15% 34% 20% 5% 1	800
	11th to 20th	Night	24% 15% 36% 21% 4	985
	21st to end	Night	32% 13% 34% 18% 2	1100

Page generated at 2025-11-15 00:55:54. Version ac07

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