JGS Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the JGS Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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	16% 8% 3 7% 5% 4 4 4 5% 7% 36%	131382
	Night	16% 9% 3 7% 5% 4 4 4 5% 6% 37%	130770
Transparency	Day	44% 7% 12% 21% 14% 2	134884
	Night	44% 7% 12% 20% 14% 2	135342
Seeing	Day	23% 18% 29% 19% 8% 2	16710
	Night	21% 17% 31% 23% 7% 1	25901

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	11% 9% 3 12% 5% 5% 5% 3 5% 5% 36%	4661
	11th to 20th	Day	14% 10% 4 11% 5% 4 4 3 5% 5% 37%	4650
	21st to end	Day	15% 11% 4 6% 4 4 4 4 5% 7% 39%	5361
February	1st to 10th	Day	15% 9% 3 10% 7% 4 4 4 5% 6% 33%	1904
	11th to 20th	Day	14% 8% 3 10% 5% 5% 5% 4 5% 6% 35%	2048
	21st to end	Day	18% 11% 3 8% 4 4 4 3 4 6% 35%	1730
March	1st to 10th	Day	21% 10% 4 7% 5% 3 4 3 4 4 36%	4576
	11th to 20th	Day	13% 9% 3 7% 5% 5% 5% 4 5% 6% 39%	4797
	21st to end	Day	19% 8% 3 8% 5% 4 4 4 5% 6% 35%	6102
April	1st to 10th	Day	20% 9% 3 7% 5% 3 4 3 5% 6% 35%	2520
	11th to 20th	Day	17% 7% 3 6% 5% 4 4 3 4 7% 38%	2446
	21st to end	Day	19% 8% 2 6% 4 4 4 4 5% 6% 38%	2482
May	1st to 10th	Day	30% 10% 2 6% 4 4 3 4 5% 7% 26%	5886
	11th to 20th	Day	18% 9% 3 7% 5% 4 4 3 5% 7% 34%	6305
	21st to end	Day	19% 8% 3 5% 4 4 5% 5% 6% 8% 33%	6949
June	1st to 10th	Day	17% 6% 2 6% 5% 5% 5% 5% 6% 10% 33%	2800
	11th to 20th	Day	17% 7% 2 6% 5% 5% 4 5% 7% 8% 34%	2800
	21st to end	Day	14% 8% 2 7% 6% 5% 5% 4 6% 8% 36%	2800
July	1st to 10th	Day	11% 8% 3 8% 5% 5% 6% 4 6% 8% 35%	6360
	11th to 20th	Day	11% 6% 3 6% 5% 5% 6% 5% 8% 11% 34%	6360
	21st to end	Day	14% 7% 2 6% 4 4 4 4 6% 8% 41%	6926
August	1st to 10th	Day	19% 7% 3 7% 6% 5% 5% 4 6% 8% 30%	4435
	11th to 20th	Day	12% 7% 2 6% 4 4 5% 4 6% 8% 41%	3850
	21st to end	Day	10% 6% 2 5% 4 4 4 4 6% 11% 43%	4312
September	1st to 10th	Day	13% 7% 3 6% 4 4 5% 4 6% 7% 42%	2400
	11th to 20th	Day	12% 5% 2 5% 4 3 4 3 6% 7% 49%	2293
	21st to end	Day	17% 6% 2 6% 4 4 4 4 7% 7% 39%	2200
October	1st to 10th	Day	20% 7% 2 6% 3 3 4 3 4 5% 41%	2115
	11th to 20th	Day	14% 8% 2 6% 4 4 5% 3 5% 7% 43%	2000
	21st to end	Day	21% 8% 3 6% 4 4 5% 4 5% 5% 36%	1990
November	1st to 10th	Day	13% 8% 2 6% 3 4 4 3 5% 7% 46%	1851
	11th to 20th	Day	16% 8% 3 5% 4 4 4 4 6% 6% 41%	1890
	21st to end	Day	18% 9% 2 6% 5% 4 4 2 5% 7% 38%	1789
December	1st to 10th	Day	16% 8% 3 7% 4 4 4 3 5% 6% 39%	2680
	11th to 20th	Day	19% 9% 3 6% 4 3 3 4 5% 5% 39%	3066
	21st to end	Day	17% 11% 4 8% 5% 4 4 3 5% 6% 32%	4048
January	1st to 10th	Night	13% 9% 3 12% 5% 4 5% 4 5% 6% 34%	8234
	11th to 20th	Night	14% 9% 3 10% 6% 5% 5% 4 5% 6% 31%	8235
	21st to end	Night	15% 9% 3 7% 6% 5% 5% 3 5% 6% 36%	8817
February	1st to 10th	Night	15% 10% 3 7% 4 5% 5% 4 6% 6% 36%	2714
	11th to 20th	Night	16% 9% 3 7% 6% 5% 5% 4 7% 6% 33%	2458
	21st to end	Night	17% 9% 3 7% 6% 4 4 3 5% 6% 36%	2076
March	1st to 10th	Night	20% 9% 3 6% 4 4 4 4 5% 5% 36%	5484
	11th to 20th	Night	15% 8% 4 9% 5% 5% 5% 3 5% 6% 37%	5364
	21st to end	Night	19% 10% 4 7% 4 4 4 3 5% 5% 34%	5072
April	1st to 10th	Night	18% 9% 3 8% 6% 4 5% 4 5% 6% 31%	2100
	11th to 20th	Night	18% 9% 3 7% 5% 3 3 3 5% 7% 37%	2036
	21st to end	Night	18% 9% 3 6% 4 3 3 4 5% 6% 39%	1916
May	1st to 10th	Night	30% 11% 2 7% 4 4 4 3 4 6% 26%	4054
	11th to 20th	Night	18% 11% 4 8% 5% 4 4 3 6% 6% 31%	3631
	21st to end	Night	21% 8% 2 7% 4 4 5% 3 6% 7% 33%	3926
June	1st to 10th	Night	16% 7% 2 7% 4 3 4 4 5% 7% 40%	1600
	11th to 20th	Night	14% 7% 2 7% 4 5% 4 5% 7% 8% 37%	1600
	21st to end	Night	14% 5% 2 5% 5% 4 5% 4 6% 9% 42%	1600
July	1st to 10th	Night	12% 7% 3 6% 5% 4 5% 4 7% 8% 40%	3600
	11th to 20th	Night	14% 8% 3 7% 5% 4 5% 4 7% 10% 33%	3600
	21st to end	Night	11% 7% 3 6% 4 3 4 3 5% 6% 47%	3926
August	1st to 10th	Night	20% 8% 2 7% 4 4 5% 5% 6% 6% 32%	2666
	11th to 20th	Night	15% 6% 2 7% 4 5% 4 4 6% 7% 41%	3197
	21st to end	Night	10% 5% 2 5% 3 3 4 3 5% 7% 52%	3586
September	1st to 10th	Night	15% 6% 2 6% 4 4 3 3 5% 6% 47%	2000
	11th to 20th	Night	12% 4 2 3 3 3 4 3 5% 7% 53%	2106
	21st to end	Night	17% 8% 3 5% 4 3 5% 3 5% 6% 43%	2200
October	1st to 10th	Night	19% 7% 3 5% 5% 3 3 2 4 6% 43%	2280
	11th to 20th	Night	14% 6% 2 6% 4 4 4 4 6% 6% 45%	2400
	21st to end	Night	20% 8% 3 6% 4 4 4 4 5% 6% 38%	2848
November	1st to 10th	Night	14% 7% 3 6% 4 3 4 3 6% 7% 45%	2667
	11th to 20th	Night	16% 8% 3 6% 5% 4 3 4 4 6% 41%	2730
	21st to end	Night	13% 9% 3 7% 5% 4 4 4 5% 6% 39%	2829
December	1st to 10th	Night	17% 9% 3 8% 4 4 4 3 5% 6% 38%	4720
	11th to 20th	Night	22% 9% 3 7% 4 4 4 3 4 4 36%	5348
	21st to end	Night	17% 13% 4 8% 4 4 4 3 4 5% 33%	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	47% 6% 11% 24% 11% 1	4662
	11th to 20th	Day	47% 6% 10% 19% 13% 5%	4680
	21st to end	Day	44% 6% 9% 24% 17% 1	5361
February	1st to 10th	Day	50% 7% 12% 18% 13% 1	1904
	11th to 20th	Day	48% 6% 10% 21% 13% 2	2057
	21st to end	Day	50% 5% 12% 20% 12% 2	1730
March	1st to 10th	Day	50% 5% 11% 15% 11% 8%	4579
	11th to 20th	Day	39% 9% 16% 25% 8% 3	4797
	21st to end	Day	47% 6% 8% 14% 20% 5%	6102
April	1st to 10th	Day	49% 6% 10% 19% 13% 3	2520
	11th to 20th	Day	44% 7% 12% 20% 15% 2	2446
	21st to end	Day	45% 6% 11% 19% 15% 3	2482
May	1st to 10th	Day	56% 8% 13% 14% 8% 1	5897
	11th to 20th	Day	47% 8% 12% 20% 12% 1	6309
	21st to end	Day	45% 9% 13% 22% 11% 1	6949
June	1st to 10th	Day	43% 9% 15% 21% 11% 1	2800
	11th to 20th	Day	44% 9% 15% 21% 10% 1	2800
	21st to end	Day	43% 9% 14% 21% 12% 1	2800
July	1st to 10th	Day	46% 9% 12% 20% 13% 1	6290
	11th to 20th	Day	40% 8% 17% 23% 11% 1	6285
	21st to end	Day	41% 6% 12% 23% 17% 1	6728
August	1st to 10th	Day	49% 8% 14% 19% 8% 1	4421
	11th to 20th	Day	38% 8% 17% 29% 9%	3886
	21st to end	Day	34% 7% 13% 32% 13% 1	4312
September	1st to 10th	Day	39% 8% 11% 24% 17% 2	2400
	11th to 20th	Day	33% 6% 11% 24% 23% 2	2293
	21st to end	Day	41% 7% 13% 21% 16% 2	2200
October	1st to 10th	Day	40% 5% 9% 19% 24% 2	3440
	11th to 20th	Day	32% 5% 10% 25% 24% 4	3240
	21st to end	Day	33% 6% 15% 25% 18% 3	3190
November	1st to 10th	Day	37% 6% 14% 24% 16% 3	1851
	11th to 20th	Day	40% 7% 11% 23% 16% 2	1890
	21st to end	Day	46% 5% 14% 25% 9% 2	1789
December	1st to 10th	Day	45% 5% 16% 25% 8%	2680
	11th to 20th	Day	46% 5% 15% 20% 13% 2	3066
	21st to end	Day	51% 7% 8% 18% 13% 3	4048
January	1st to 10th	Night	47% 6% 12% 20% 12% 1	8234
	11th to 20th	Night	50% 7% 12% 15% 11% 4	8280
	21st to end	Night	46% 6% 10% 19% 17% 3	8817
February	1st to 10th	Night	46% 7% 13% 21% 11% 2	2714
	11th to 20th	Night	48% 6% 12% 17% 15% 3	2460
	21st to end	Night	48% 6% 11% 21% 11% 4	2076
March	1st to 10th	Night	45% 8% 12% 16% 14% 4	5494
	11th to 20th	Night	43% 9% 17% 20% 8% 3	5365
	21st to end	Night	49% 6% 7% 14% 21% 4	5072
April	1st to 10th	Night	50% 7% 11% 18% 12% 2	2100
	11th to 20th	Night	47% 8% 10% 19% 16% 1	2036
	21st to end	Night	44% 7% 10% 20% 16% 3	1916
May	1st to 10th	Night	58% 8% 13% 11% 9% 1	4060
	11th to 20th	Night	51% 10% 11% 15% 11% 2	3632
	21st to end	Night	48% 9% 13% 19% 10% 1	3927
June	1st to 10th	Night	43% 8% 12% 22% 14% 2	1600
	11th to 20th	Night	41% 8% 18% 23% 9% 1	1600
	21st to end	Night	36% 10% 14% 25% 14% 1	1600
July	1st to 10th	Night	41% 8% 15% 21% 15% 1	3560
	11th to 20th	Night	44% 9% 14% 21% 10% 1	3552
	21st to end	Night	36% 7% 12% 26% 18% 1	3809
August	1st to 10th	Night	50% 9% 14% 18% 9% 1	2656
	11th to 20th	Night	41% 8% 14% 28% 9%	3227
	21st to end	Night	31% 6% 12% 36% 14% 1	3586
September	1st to 10th	Night	39% 7% 11% 26% 17% 1	2000
	11th to 20th	Night	30% 7% 12% 25% 25% 1	2106
	21st to end	Night	40% 6% 11% 22% 19% 1	2200
October	1st to 10th	Night	42% 6% 11% 21% 19% 2	3722
	11th to 20th	Night	30% 5% 11% 27% 25% 2	3899
	21st to end	Night	35% 5% 14% 29% 16% 1	4598
November	1st to 10th	Night	38% 6% 12% 23% 18% 3	2667
	11th to 20th	Night	44% 5% 11% 20% 17% 3	2730
	21st to end	Night	42% 6% 17% 25% 9% 1	2829
December	1st to 10th	Night	45% 6% 13% 28% 8%	4720
	11th to 20th	Night	49% 6% 14% 15% 15% 2	5348
	21st to end	Night	50% 7% 10% 16% 14% 3	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	30% 28% 25% 15% 2	220
	11th to 20th	Day	36% 28% 26% 8% 2	220
	21st to end	Day	27% 31% 32% 8% 2 1	285
February	1st to 10th	Day	33% 27% 32% 6% 2	344
	11th to 20th	Day	28% 28% 33% 10%	536
	21st to end	Day	33% 26% 30% 9% 2	446
March	1st to 10th	Day	29% 24% 33% 13% 1	540
	11th to 20th	Day	30% 25% 30% 12% 2	540
	21st to end	Day	34% 24% 29% 8% 5% 1	594
April	1st to 10th	Day	36% 24% 25% 9% 5% 1	540
	11th to 20th	Day	29% 28% 26% 11% 5% 1	516
	21st to end	Day	27% 20% 33% 14% 5% 1	500
May	1st to 10th	Day	34% 19% 26% 14% 4 2	503
	11th to 20th	Day	23% 20% 34% 18% 6% 1	527
	21st to end	Day	21% 15% 28% 24% 10% 2	573
June	1st to 10th	Day	19% 14% 31% 23% 12% 1	530
	11th to 20th	Day	17% 16% 30% 25% 11% 2	530
	21st to end	Day	15% 16% 30% 25% 11% 2	530
July	1st to 10th	Day	14% 10% 32% 30% 12% 3	525
	11th to 20th	Day	13% 10% 28% 32% 15% 2	524
	21st to end	Day	13% 11% 35% 29% 11% 2	576
August	1st to 10th	Day	15% 8% 29% 30% 12% 6%	526
	11th to 20th	Day	14% 10% 29% 26% 16% 6%	519
	21st to end	Day	12% 12% 31% 25% 15% 5%	583
September	1st to 10th	Day	19% 11% 24% 24% 15% 6%	530
	11th to 20th	Day	12% 11% 29% 22% 17% 9%	530
	21st to end	Day	21% 12% 31% 23% 9% 5%	530
October	1st to 10th	Day	27% 12% 24% 18% 15% 4	530
	11th to 20th	Day	22% 18% 31% 19% 8% 1	530
	21st to end	Day	28% 16% 28% 20% 6% 2	467
November	1st to 10th	Day	22% 20% 29% 20% 9% 2	425
	11th to 20th	Day	28% 22% 28% 17% 4 1	430
	21st to end	Day	28% 26% 26% 17% 3	329
December	1st to 10th	Day	25% 29% 29% 10% 5%	220
	11th to 20th	Day	35% 29% 24% 12% 1	220
	21st to end	Day	27% 28% 29% 14% 3	242
January	1st to 10th	Night	20% 24% 39% 15% 2	840
	11th to 20th	Night	27% 23% 32% 16% 2	840
	21st to end	Night	20% 25% 36% 16% 3	924
February	1st to 10th	Night	23% 23% 34% 18% 1	840
	11th to 20th	Night	25% 24% 36% 13% 1	823
	21st to end	Night	26% 23% 34% 15% 2	692
March	1st to 10th	Night	25% 24% 33% 16% 3	840
	11th to 20th	Night	23% 26% 32% 16% 3	840
	21st to end	Night	25% 24% 32% 15% 4	924
April	1st to 10th	Night	26% 24% 30% 16% 3	840
	11th to 20th	Night	24% 20% 31% 19% 4	816
	21st to end	Night	23% 21% 33% 19% 4	716
May	1st to 10th	Night	27% 18% 31% 20% 5%	600
	11th to 20th	Night	19% 16% 37% 24% 4	424
	21st to end	Night	20% 9% 28% 33% 9% 1	433
June	1st to 10th	Night	16% 9% 32% 32% 10% 1	400
	11th to 20th	Night	16% 10% 32% 30% 13%	400
	21st to end	Night	15% 6% 29% 38% 13%	400
July	1st to 10th	Night	14% 5% 21% 42% 16% 1	390
	11th to 20th	Night	14% 4 25% 41% 14% 1	388
	21st to end	Night	13% 5% 25% 41% 15%	438
August	1st to 10th	Night	10% 5% 25% 44% 14% 2	439
	11th to 20th	Night	14% 5% 22% 38% 20% 2	790
	21st to end	Night	15% 6% 24% 34% 19% 1	880
September	1st to 10th	Night	17% 6% 29% 31% 15% 2	800
	11th to 20th	Night	14% 7% 26% 28% 20% 4	800
	21st to end	Night	21% 7% 26% 32% 12% 2	800
October	1st to 10th	Night	24% 10% 27% 26% 13% 1	800
	11th to 20th	Night	19% 15% 32% 27% 8% 1	800
	21st to end	Night	25% 15% 31% 22% 6%	880
November	1st to 10th	Night	20% 16% 32% 23% 8% 1	820
	11th to 20th	Night	22% 20% 32% 19% 6% 1	840
	21st to end	Night	19% 22% 32% 22% 5%	840
December	1st to 10th	Night	26% 20% 31% 19% 4	840
	11th to 20th	Night	26% 21% 31% 20% 3	840
	21st to end	Night	27% 20% 36% 15% 2	924

Page generated at 2026-04-14 02:44:25. Version ac07

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