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 2024-12-24.

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%	101746
	Night	17% 8% 3 7% 5% 4 4 4 5% 6% 37%	98369
Transparency	Day	44% 7% 13% 21% 13% 2	105266
	Night	43% 7% 13% 21% 13% 2	102925
Seeing	Day	23% 18% 30% 19% 8% 2	15052
	Night	21% 17% 31% 23% 7% 1	24182

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	14% 9% 3 7% 4 4 4 3 5% 5% 41%	2517
	11th to 20th	Day	15% 10% 4 11% 6% 4 4 3 5% 6% 31%	2520
	21st to end	Day	21% 14% 6% 7% 4 4 4 3 4 4 29%	2897
February	1st to 10th	Day	17% 10% 3 9% 6% 4 4 5% 5% 6% 32%	1722
	11th to 20th	Day	14% 8% 3 9% 5% 5% 4 4 5% 6% 37%	1848
	21st to end	Day	18% 11% 3 8% 4 5% 5% 3 4 6% 34%	1570
March	1st to 10th	Day	24% 12% 4 7% 5% 3 3 3 4 4 31%	3116
	11th to 20th	Day	11% 9% 3 7% 6% 5% 6% 5% 6% 7% 36%	3292
	21st to end	Day	21% 8% 3 8% 4 4 3 3 5% 5% 36%	4166
April	1st to 10th	Day	20% 9% 3 7% 5% 4 4 4 5% 6% 34%	2280
	11th to 20th	Day	17% 7% 3 6% 6% 4 4 3 4 7% 39%	2254
	21st to end	Day	19% 8% 2 6% 4 4 4 4 5% 6% 37%	2358
May	1st to 10th	Day	26% 9% 3 6% 4 4 3 3 5% 6% 30%	4106
	11th to 20th	Day	20% 8% 3 7% 5% 4 4 4 5% 7% 33%	4385
	21st to end	Day	12% 6% 2 5% 4 4 5% 5% 7% 8% 43%	4837
June	1st to 10th	Day	17% 6% 2 6% 5% 5% 5% 5% 6% 10% 33%	2660
	11th to 20th	Day	16% 7% 2 6% 5% 5% 4 5% 7% 9% 34%	2660
	21st to end	Day	13% 8% 2 7% 6% 5% 5% 4 6% 9% 35%	2660
July	1st to 10th	Day	10% 7% 3 8% 5% 6% 6% 4 6% 8% 36%	4440
	11th to 20th	Day	11% 6% 2 6% 5% 5% 5% 4 7% 11% 38%	4440
	21st to end	Day	14% 8% 3 8% 6% 6% 5% 4 7% 7% 33%	4814
August	1st to 10th	Day	20% 7% 3 7% 6% 5% 5% 5% 6% 8% 29%	4298
	11th to 20th	Day	12% 7% 2 6% 4 4 5% 4 6% 8% 40%	3730
	21st to end	Day	10% 6% 2 6% 4 4 4 5% 6% 11% 43%	4180
September	1st to 10th	Day	13% 7% 2 6% 4 4 5% 4 6% 7% 42%	2280
	11th to 20th	Day	12% 6% 2 5% 4 3 4 3 5% 7% 49%	2179
	21st to end	Day	17% 6% 2 6% 4 4 4 4 6% 7% 39%	2090
October	1st to 10th	Day	21% 7% 2 6% 4 3 4 3 4 5% 39%	2009
	11th to 20th	Day	14% 8% 2 5% 4 4 4 3 5% 7% 44%	1900
	21st to end	Day	22% 8% 3 6% 4 4 5% 4 5% 6% 35%	1891
November	1st to 10th	Day	13% 8% 2 5% 3 3 3 3 5% 7% 47%	1761
	11th to 20th	Day	17% 8% 3 4 3 4 4 4 6% 6% 42%	1800
	21st to end	Day	18% 9% 2 5% 5% 4 4 2 5% 7% 39%	1704
December	1st to 10th	Day	18% 8% 4 9% 5% 4 5% 4 5% 6% 32%	1600
	11th to 20th	Day	20% 9% 2 6% 4 3 3 4 5% 5% 38%	1986
	21st to end	Day	19% 11% 4 9% 6% 5% 5% 3 5% 6% 27%	2796
January	1st to 10th	Night	16% 8% 2 7% 4 3 4 3 5% 6% 41%	4429
	11th to 20th	Night	15% 8% 4 9% 5% 5% 5% 3 4 7% 34%	4440
	21st to end	Night	23% 13% 4 7% 5% 4 4 3 4 5% 29%	4756
February	1st to 10th	Night	16% 10% 3 7% 4 5% 5% 4 6% 5% 35%	2456
	11th to 20th	Night	16% 8% 3 7% 6% 5% 5% 4 6% 5% 34%	2218
	21st to end	Night	18% 8% 3 7% 6% 4 4 3 5% 5% 36%	1884
March	1st to 10th	Night	23% 11% 3 7% 5% 4 4 4 5% 4 31%	3724
	11th to 20th	Night	11% 8% 3 9% 6% 6% 6% 4 6% 6% 37%	3663
	21st to end	Night	18% 9% 4 7% 5% 4 4 3 5% 5% 36%	3466
April	1st to 10th	Night	17% 9% 3 8% 6% 5% 5% 4 5% 7% 31%	1900
	11th to 20th	Night	18% 9% 3 7% 5% 3 3 3 5% 7% 38%	1875
	21st to end	Night	18% 9% 3 6% 4 3 3 4 5% 6% 39%	1820
May	1st to 10th	Night	25% 10% 3 7% 4 4 4 3 4 7% 30%	2834
	11th to 20th	Night	19% 8% 3 9% 5% 5% 4 3 6% 6% 32%	2537
	21st to end	Night	16% 7% 2 5% 4 4 4 3 7% 8% 41%	2738
June	1st to 10th	Night	16% 7% 2 7% 4 3 4 4 5% 7% 40%	1520
	11th to 20th	Night	14% 7% 2 7% 4 5% 4 5% 6% 8% 38%	1520
	21st to end	Night	14% 5% 2 5% 5% 4 5% 4 6% 9% 42%	1520
July	1st to 10th	Night	11% 6% 2 6% 5% 4 6% 5% 7% 8% 40%	2520
	11th to 20th	Night	13% 7% 3 6% 5% 4 5% 4 7% 10% 37%	2520
	21st to end	Night	15% 9% 3 8% 4 3 4 3 6% 7% 39%	2739
August	1st to 10th	Night	21% 9% 2 7% 4 4 5% 5% 6% 6% 31%	2584
	11th to 20th	Night	15% 6% 2 7% 4 4 4 4 6% 7% 40%	3097
	21st to end	Night	10% 5% 2 5% 3 3 4 3 5% 7% 52%	3476
September	1st to 10th	Night	15% 6% 2 6% 4 4 3 3 5% 6% 46%	1900
	11th to 20th	Night	13% 5% 2 3 3 3 4 3 5% 7% 53%	2000
	21st to end	Night	17% 7% 3 5% 4 3 5% 3 5% 6% 43%	2090
October	1st to 10th	Night	19% 8% 3 5% 5% 3 4 2 4 5% 41%	2166
	11th to 20th	Night	13% 6% 2 6% 4 4 4 4 6% 6% 45%	2280
	21st to end	Night	21% 8% 3 6% 4 4 4 3 5% 6% 37%	2705
November	1st to 10th	Night	14% 6% 3 6% 4 3 4 3 6% 6% 46%	2537
	11th to 20th	Night	17% 8% 3 6% 5% 4 3 4 4 6% 41%	2600
	21st to end	Night	13% 9% 3 7% 5% 4 4 4 5% 6% 40%	2694
December	1st to 10th	Night	20% 9% 3 8% 5% 4 4 4 5% 6% 33%	2800
	11th to 20th	Night	20% 9% 2 7% 5% 5% 5% 5% 5% 5% 33%	3428
	21st to end	Night	21% 12% 4 8% 4 4 4 4 5% 5% 30%	4933

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	41% 6% 13% 31% 7% 1	2518
	11th to 20th	Day	50% 8% 12% 22% 7% 1	2520
	21st to end	Day	57% 6% 9% 19% 8% 1	2897
February	1st to 10th	Day	51% 7% 12% 18% 11% 1	1722
	11th to 20th	Day	46% 6% 10% 22% 14% 2	1857
	21st to end	Day	51% 5% 11% 20% 11% 2	1570
March	1st to 10th	Day	55% 6% 12% 16% 10% 2	3119
	11th to 20th	Day	37% 12% 17% 25% 9% 1	3292
	21st to end	Day	47% 6% 7% 15% 20% 4	4166
April	1st to 10th	Day	49% 6% 10% 20% 12% 3	2280
	11th to 20th	Day	44% 7% 11% 20% 15% 2	2254
	21st to end	Day	45% 7% 11% 19% 15% 3	2358
May	1st to 10th	Day	52% 9% 13% 15% 11% 1	4120
	11th to 20th	Day	48% 10% 14% 19% 9% 1	4389
	21st to end	Day	33% 10% 14% 25% 16% 1	4837
June	1st to 10th	Day	43% 9% 15% 21% 11% 1	2660
	11th to 20th	Day	43% 9% 15% 22% 10% 1	2660
	21st to end	Day	43% 9% 14% 21% 12% 1	2660
July	1st to 10th	Day	44% 9% 12% 20% 15% 1	4370
	11th to 20th	Day	38% 7% 17% 24% 12% 1	4365
	21st to end	Day	48% 7% 13% 16% 14% 1	4661
August	1st to 10th	Day	50% 8% 14% 19% 8%	4284
	11th to 20th	Day	38% 8% 17% 29% 8%	3766
	21st to end	Day	35% 7% 13% 32% 12% 1	4180
September	1st to 10th	Day	39% 8% 11% 24% 17% 1	2280
	11th to 20th	Day	34% 6% 11% 24% 23% 2	2179
	21st to end	Day	41% 7% 13% 21% 16% 2	2090
October	1st to 10th	Day	41% 5% 9% 19% 23% 2	3334
	11th to 20th	Day	32% 5% 10% 25% 25% 4	3140
	21st to end	Day	33% 6% 15% 25% 18% 3	3091
November	1st to 10th	Day	36% 6% 15% 25% 16% 3	1761
	11th to 20th	Day	40% 6% 12% 23% 17% 3	1800
	21st to end	Day	44% 5% 14% 26% 9% 2	1704
December	1st to 10th	Day	52% 6% 15% 17% 10% 1	1600
	11th to 20th	Day	46% 6% 17% 20% 10% 2	1986
	21st to end	Day	55% 8% 11% 18% 8%	2796
January	1st to 10th	Night	41% 6% 14% 26% 10% 2	4429
	11th to 20th	Night	47% 7% 15% 19% 9% 2	4440
	21st to end	Night	56% 6% 11% 18% 9% 1	4756
February	1st to 10th	Night	47% 8% 14% 20% 10% 2	2456
	11th to 20th	Night	47% 6% 12% 17% 15% 3	2220
	21st to end	Night	47% 6% 11% 21% 11% 4	1884
March	1st to 10th	Night	51% 8% 11% 18% 10% 2	3734
	11th to 20th	Night	39% 11% 17% 21% 10% 2	3664
	21st to end	Night	45% 7% 9% 14% 21% 4	3466
April	1st to 10th	Night	49% 8% 11% 18% 11% 2	1900
	11th to 20th	Night	46% 8% 9% 19% 17% 1	1875
	21st to end	Night	44% 7% 10% 20% 15% 3	1820
May	1st to 10th	Night	53% 9% 12% 15% 9% 2	2840
	11th to 20th	Night	50% 11% 12% 17% 9% 1	2538
	21st to end	Night	38% 10% 16% 21% 13% 2	2739
June	1st to 10th	Night	42% 8% 13% 23% 13% 2	1520
	11th to 20th	Night	40% 8% 19% 23% 9% 1	1520
	21st to end	Night	36% 11% 14% 25% 14% 1	1520
July	1st to 10th	Night	39% 9% 16% 19% 15% 1	2480
	11th to 20th	Night	40% 8% 14% 25% 11%	2472
	21st to end	Night	44% 8% 14% 15% 17% 2	2651
August	1st to 10th	Night	51% 9% 14% 17% 9% 1	2574
	11th to 20th	Night	41% 8% 14% 28% 8%	3127
	21st to end	Night	31% 6% 13% 36% 13% 1	3476
September	1st to 10th	Night	39% 7% 11% 25% 16% 1	1900
	11th to 20th	Night	30% 7% 12% 25% 25% 1	2000
	21st to end	Night	40% 6% 11% 21% 20% 1	2090
October	1st to 10th	Night	42% 6% 11% 21% 18% 1	3608
	11th to 20th	Night	29% 5% 11% 27% 26% 2	3779
	21st to end	Night	35% 5% 14% 29% 16% 1	4455
November	1st to 10th	Night	38% 6% 12% 22% 19% 3	2537
	11th to 20th	Night	43% 5% 12% 20% 17% 3	2600
	21st to end	Night	42% 6% 18% 25% 9% 1	2694
December	1st to 10th	Night	49% 7% 14% 21% 10% 1	2800
	11th to 20th	Night	48% 8% 15% 15% 12% 2	3428
	21st to end	Night	52% 8% 13% 17% 9%	4933

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	28% 28% 25% 16% 2	180
	11th to 20th	Day	33% 27% 28% 9% 2	180
	21st to end	Day	23% 32% 32% 9% 2 1	233
February	1st to 10th	Day	34% 28% 30% 6% 2	282
	11th to 20th	Day	27% 30% 32% 10%	456
	21st to end	Day	33% 24% 31% 10% 3	382
March	1st to 10th	Day	28% 23% 34% 13% 2	460
	11th to 20th	Day	29% 27% 31% 12% 2	460
	21st to end	Day	32% 22% 31% 9% 5% 1	506
April	1st to 10th	Day	33% 24% 26% 10% 6% 1	460
	11th to 20th	Day	28% 28% 25% 12% 6% 1	452
	21st to end	Day	27% 20% 33% 14% 5% 1	460
May	1st to 10th	Day	33% 19% 27% 14% 5% 2	463
	11th to 20th	Day	22% 18% 34% 19% 6% 1	487
	21st to end	Day	19% 15% 29% 25% 11% 2	529
June	1st to 10th	Day	19% 14% 31% 23% 12% 1	490
	11th to 20th	Day	15% 16% 30% 26% 11% 2	490
	21st to end	Day	16% 17% 30% 25% 11% 2	490
July	1st to 10th	Day	13% 11% 31% 29% 13% 3	485
	11th to 20th	Day	13% 9% 28% 32% 15% 2	484
	21st to end	Day	12% 11% 35% 29% 11% 2	532
August	1st to 10th	Day	16% 8% 31% 30% 11% 6%	486
	11th to 20th	Day	14% 11% 30% 24% 15% 6%	479
	21st to end	Day	12% 11% 30% 26% 15% 6%	539
September	1st to 10th	Day	20% 11% 23% 23% 15% 6%	490
	11th to 20th	Day	13% 12% 29% 21% 16% 9%	490
	21st to end	Day	21% 12% 31% 23% 9% 5%	490
October	1st to 10th	Day	28% 12% 24% 18% 15% 3	490
	11th to 20th	Day	19% 19% 32% 20% 8% 1	490
	21st to end	Day	29% 15% 27% 21% 6% 2	434
November	1st to 10th	Day	21% 18% 29% 20% 9% 2	395
	11th to 20th	Day	29% 23% 28% 17% 3 1	400
	21st to end	Day	26% 26% 26% 18% 3	304
December	1st to 10th	Day	25% 29% 30% 11% 5% 1	200
	11th to 20th	Day	34% 29% 24% 13% 1	200
	21st to end	Day	25% 27% 28% 16% 3	204
January	1st to 10th	Night	20% 23% 40% 16% 2	760
	11th to 20th	Night	26% 23% 33% 17% 2	760
	21st to end	Night	19% 24% 37% 17% 3	836
February	1st to 10th	Night	22% 23% 35% 18% 1	760
	11th to 20th	Night	24% 25% 37% 13% 1	743
	21st to end	Night	27% 22% 33% 16% 2	628
March	1st to 10th	Night	23% 25% 33% 16% 3	760
	11th to 20th	Night	22% 27% 32% 16% 3	760
	21st to end	Night	24% 24% 32% 16% 5%	836
April	1st to 10th	Night	25% 24% 31% 18% 3	760
	11th to 20th	Night	24% 20% 31% 20% 5%	751
	21st to end	Night	23% 20% 34% 18% 5%	680
May	1st to 10th	Night	25% 18% 31% 20% 5%	570
	11th to 20th	Night	19% 16% 37% 24% 4	403
	21st to end	Night	19% 9% 28% 34% 10% 1	411
June	1st to 10th	Night	16% 9% 33% 31% 11% 1	380
	11th to 20th	Night	15% 9% 32% 30% 13%	380
	21st to end	Night	15% 6% 30% 38% 12%	380
July	1st to 10th	Night	14% 5% 22% 41% 17% 1	370
	11th to 20th	Night	14% 4 25% 41% 14% 1	368
	21st to end	Night	14% 6% 25% 40% 15%	416
August	1st to 10th	Night	10% 6% 26% 44% 13% 2	417
	11th to 20th	Night	14% 5% 22% 38% 19% 2	750
	21st to end	Night	15% 7% 23% 34% 20% 2	836
September	1st to 10th	Night	17% 6% 29% 31% 14% 2	760
	11th to 20th	Night	15% 8% 27% 28% 20% 4	760
	21st to end	Night	21% 7% 27% 31% 12% 2	760
October	1st to 10th	Night	24% 11% 27% 26% 11% 1	760
	11th to 20th	Night	17% 15% 32% 28% 8% 1	760
	21st to end	Night	26% 16% 31% 22% 6%	836
November	1st to 10th	Night	19% 16% 32% 23% 8% 1	780
	11th to 20th	Night	22% 20% 31% 20% 6% 1	800
	21st to end	Night	19% 22% 32% 21% 5% 1	800
December	1st to 10th	Night	26% 20% 31% 18% 5%	800
	11th to 20th	Night	25% 21% 31% 21% 3	800
	21st to end	Night	27% 19% 37% 15% 2	851

Page generated at 2024-12-21 04:13:53. Version ac07

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