JMS Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the JMS Observatory clear sky chart from 2005-11-06 to 2026-09-14.

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	11% 9% 3 8% 5% 4 5% 4 6% 7% 39%	150301
	Night	11% 9% 4 8% 5% 4 5% 4 5% 7% 38%	141162
Transparency	Day	43% 5% 13% 26% 11% 1	153865
	Night	45% 5% 12% 24% 12% 1	145677
Seeing	Day	17% 11% 32% 26% 12% 3	18666
	Night	16% 10% 33% 31% 10% 1	28432

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% 14% 7% 20% 8% 5% 5% 3 5% 5% 20%	4691
	11th to 20th	Day	5% 11% 6% 14% 7% 7% 8% 8% 8% 4 21%	4747
	21st to end	Day	5% 12% 6% 11% 4 5% 7% 4 6% 7% 33%	5812
February	1st to 10th	Day	8% 13% 5% 10% 8% 5% 6% 4 6% 6% 31%	1890
	11th to 20th	Day	8% 12% 5% 10% 6% 5% 5% 4 5% 6% 35%	1844
	21st to end	Day	9% 13% 5% 10% 6% 5% 4 4 7% 7% 32%	1613
March	1st to 10th	Day	12% 9% 3 6% 4 4 3 3 5% 8% 43%	4947
	11th to 20th	Day	9% 10% 3 8% 4 5% 4 4 6% 7% 40%	5090
	21st to end	Day	21% 11% 4 10% 5% 5% 4 3 5% 6% 26%	5643
April	1st to 10th	Day	16% 9% 4 8% 5% 5% 4 3 5% 8% 34%	2520
	11th to 20th	Day	19% 12% 3 7% 5% 4 4 3 5% 6% 31%	2516
	21st to end	Day	21% 12% 2 6% 5% 4 4 4 4 6% 31%	2730
May	1st to 10th	Day	11% 5% 2 5% 4 4 5% 3 5% 6% 50%	7951
	11th to 20th	Day	16% 8% 2 5% 3 3 3 3 5% 6% 45%	8222
	21st to end	Day	15% 7% 2 7% 5% 4 5% 4 6% 8% 38%	9047
June	1st to 10th	Day	11% 5% 2 5% 4 4 4 5% 6% 9% 44%	2940
	11th to 20th	Day	14% 6% 3 7% 5% 5% 5% 4 6% 10% 35%	2940
	21st to end	Day	11% 5% 2 6% 5% 4 6% 4 8% 10% 39%	3143
July	1st to 10th	Day	10% 6% 2 6% 4 5% 6% 5% 8% 10% 39%	8434
	11th to 20th	Day	7% 3 1 4 3 3 3 3 6% 9% 58%	8280
	21st to end	Day	7% 5% 2 6% 4 4 5% 4 6% 9% 48%	8897
August	1st to 10th	Day	10% 5% 2 6% 4 4 5% 4 6% 8% 47%	6014
	11th to 20th	Day	11% 6% 2 6% 4 5% 5% 5% 7% 10% 40%	5948
	21st to end	Day	8% 6% 3 6% 4 4 4 4 6% 8% 47%	6316
September	1st to 10th	Day	10% 5% 2 5% 5% 4 3 4 6% 9% 46%	2520
	11th to 20th	Day	11% 6% 2 6% 4 4 5% 4 6% 8% 45%	2401
	21st to end	Day	14% 7% 2 5% 4 3 4 4 5% 6% 46%	2106
October	1st to 10th	Day	15% 8% 2 6% 4 5% 4 3 4 6% 43%	2000
	11th to 20th	Day	15% 12% 3 6% 4 3 4 3 5% 6% 38%	2000
	21st to end	Day	17% 13% 3 7% 4 3 3 3 4 6% 37%	2175
November	1st to 10th	Day	11% 13% 4 9% 5% 3 6% 3 5% 5% 36%	1716
	11th to 20th	Day	11% 14% 7% 12% 6% 5% 4 4 4 5% 29%	1680
	21st to end	Day	12% 13% 4 9% 5% 4 4 4 4 7% 33%	1680
December	1st to 10th	Day	7% 12% 5% 15% 11% 7% 6% 4 6% 6% 22%	2690
	11th to 20th	Day	10% 16% 7% 11% 6% 5% 4 3 5% 5% 28%	3088
	21st to end	Day	19% 21% 8% 19% 5% 4 3 2 3 3 14%	4070
January	1st to 10th	Night	9% 13% 6% 14% 8% 5% 4 3 5% 6% 29%	8232
	11th to 20th	Night	6% 12% 5% 13% 7% 7% 10% 8% 6% 5% 20%	8238
	21st to end	Night	6% 15% 5% 10% 5% 5% 5% 3 6% 6% 35%	8402
February	1st to 10th	Night	7% 12% 6% 10% 7% 5% 5% 4 5% 6% 34%	2730
	11th to 20th	Night	6% 11% 3 8% 7% 5% 5% 4 5% 7% 38%	2663
	21st to end	Night	11% 9% 3 9% 6% 5% 4 3 5% 6% 37%	2191
March	1st to 10th	Night	10% 7% 3 6% 4 3 4 4 5% 7% 47%	5170
	11th to 20th	Night	9% 8% 3 7% 4 5% 5% 4 7% 10% 38%	5090
	21st to end	Night	24% 12% 4 9% 4 4 3 3 4 5% 28%	5527
April	1st to 10th	Night	16% 11% 3 6% 4 4 5% 3 5% 6% 37%	2100
	11th to 20th	Night	19% 10% 3 7% 4 3 3 3 5% 5% 36%	2050
	21st to end	Night	20% 11% 2 6% 4 4 4 3 5% 6% 36%	1890
May	1st to 10th	Night	9% 7% 2 6% 4 3 4 3 5% 6% 51%	4932
	11th to 20th	Night	17% 6% 2 4 3 3 3 3 4 6% 49%	4670
	21st to end	Night	14% 9% 3 7% 5% 4 4 4 6% 7% 38%	5139
June	1st to 10th	Night	11% 5% 2 4 4 4 5% 3 7% 7% 47%	1680
	11th to 20th	Night	9% 5% 2 6% 4 5% 4 4 6% 8% 45%	1673
	21st to end	Night	8% 4 1 5% 4 4 5% 4 6% 8% 52%	1470
July	1st to 10th	Night	9% 4 1 5% 4 4 5% 4 7% 10% 45%	4532
	11th to 20th	Night	5% 3 1 4 3 3 4 4 6% 10% 58%	4707
	21st to end	Night	8% 5% 2 6% 4 4 5% 4 6% 8% 50%	5193
August	1st to 10th	Night	9% 5% 2 6% 4 4 3 3 4 7% 52%	4156
	11th to 20th	Night	13% 5% 2 5% 4 4 5% 4 6% 8% 45%	4118
	21st to end	Night	7% 5% 2 6% 4 4 4 3 6% 8% 50%	4820
September	1st to 10th	Night	8% 5% 2 5% 4 4 5% 3 5% 7% 53%	2100
	11th to 20th	Night	9% 5% 2 4 4 3 4 4 6% 7% 53%	2061
	21st to end	Night	13% 5% 2 4 4 3 4 3 5% 6% 52%	2273
October	1st to 10th	Night	13% 7% 2 5% 3 3 4 3 4 7% 49%	2400
	11th to 20th	Night	16% 10% 3 5% 4 3 4 3 5% 6% 41%	2400
	21st to end	Night	18% 11% 3 6% 4 4 4 3 4 6% 37%	2665
November	1st to 10th	Night	12% 10% 4 7% 4 4 4 4 5% 6% 40%	2801
	11th to 20th	Night	13% 11% 4 11% 6% 4 4 3 4 5% 34%	2940
	21st to end	Night	13% 11% 3 10% 6% 5% 4 4 4 7% 34%	2940
December	1st to 10th	Night	7% 10% 5% 14% 8% 8% 5% 3 5% 7% 29%	4720
	11th to 20th	Night	10% 17% 6% 14% 6% 5% 5% 5% 4 5% 23%	5339
	21st to end	Night	19% 17% 8% 16% 5% 4 3 3 3 4 18%	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	65% 5% 10% 15% 5%	4692
	11th to 20th	Day	61% 4 10% 16% 9%	4747
	21st to end	Day	51% 5% 13% 22% 9%	5812
February	1st to 10th	Day	53% 6% 14% 19% 8% 1	1890
	11th to 20th	Day	49% 6% 12% 25% 8%	1853
	21st to end	Day	51% 5% 13% 20% 10%	1613
March	1st to 10th	Day	39% 5% 18% 26% 10% 2	4947
	11th to 20th	Day	42% 5% 15% 29% 9% 1	5090
	21st to end	Day	58% 5% 10% 19% 7%	5645
April	1st to 10th	Day	49% 5% 12% 21% 12% 1	2520
	11th to 20th	Day	53% 6% 10% 18% 11% 1	2516
	21st to end	Day	53% 6% 11% 20% 9%	2730
May	1st to 10th	Day	34% 5% 11% 30% 19% 2	7989
	11th to 20th	Day	40% 5% 7% 25% 18% 4	8226
	21st to end	Day	43% 5% 10% 30% 11% 1	9061
June	1st to 10th	Day	34% 8% 14% 31% 12% 1	2940
	11th to 20th	Day	44% 7% 13% 22% 12% 1	2940
	21st to end	Day	36% 9% 17% 27% 10% 2	3143
July	1st to 10th	Day	38% 5% 18% 31% 8%	8365
	11th to 20th	Day	24% 3 14% 40% 18% 1	8205
	21st to end	Day	33% 4 14% 33% 17% 1	8702
August	1st to 10th	Day	35% 5% 13% 34% 12% 1	6000
	11th to 20th	Day	38% 5% 19% 30% 8%	5987
	21st to end	Day	35% 4 18% 30% 12% 1	6315
September	1st to 10th	Day	34% 7% 16% 32% 12%	2520
	11th to 20th	Day	36% 8% 17% 31% 8%	2401
	21st to end	Day	38% 7% 11% 33% 11%	2106
October	1st to 10th	Day	32% 3 8% 35% 21%	3260
	11th to 20th	Day	40% 4 11% 28% 16% 1	3240
	21st to end	Day	45% 3 14% 28% 9%	3489
November	1st to 10th	Day	48% 6% 12% 19% 12% 2	1716
	11th to 20th	Day	56% 5% 14% 16% 9%	1680
	21st to end	Day	51% 6% 15% 20% 6% 1	1680
December	1st to 10th	Day	62% 6% 15% 12% 4 1	2690
	11th to 20th	Day	58% 4 14% 18% 6%	3085
	21st to end	Day	77% 4 7% 8% 4	4070
January	1st to 10th	Night	57% 4 15% 17% 7%	8233
	11th to 20th	Night	63% 4 10% 15% 9%	8238
	21st to end	Night	50% 4 10% 25% 10%	8402
February	1st to 10th	Night	51% 5% 13% 21% 10% 1	2730
	11th to 20th	Night	44% 6% 14% 25% 9% 1	2664
	21st to end	Night	47% 5% 14% 21% 12% 1	2191
March	1st to 10th	Night	37% 5% 14% 26% 16% 3	5170
	11th to 20th	Night	38% 5% 16% 27% 13% 1	5090
	21st to end	Night	60% 3 10% 18% 9% 1	5527
April	1st to 10th	Night	48% 6% 8% 24% 14% 1	2100
	11th to 20th	Night	48% 6% 9% 21% 14% 2	2050
	21st to end	Night	49% 6% 8% 21% 13% 2	1890
May	1st to 10th	Night	33% 5% 8% 31% 20% 2	4959
	11th to 20th	Night	36% 4 8% 21% 22% 8%	4671
	21st to end	Night	44% 5% 9% 27% 13% 1	5149
June	1st to 10th	Night	33% 7% 14% 28% 16% 2	1680
	11th to 20th	Night	35% 8% 13% 24% 17% 2	1673
	21st to end	Night	29% 6% 16% 30% 15% 3	1470
July	1st to 10th	Night	34% 4 19% 30% 12% 1	4494
	11th to 20th	Night	23% 3 13% 39% 20% 1	4662
	21st to end	Night	33% 4 14% 32% 16% 2	5077
August	1st to 10th	Night	33% 4 12% 32% 16% 3	4146
	11th to 20th	Night	37% 6% 17% 29% 12% 1	4145
	21st to end	Night	32% 5% 18% 30% 13% 1	4820
September	1st to 10th	Night	31% 6% 14% 33% 16% 1	2100
	11th to 20th	Night	29% 7% 18% 31% 15%	2061
	21st to end	Night	33% 5% 12% 32% 16% 1	2273
October	1st to 10th	Night	24% 3 7% 35% 26% 4	3920
	11th to 20th	Night	35% 4 12% 24% 21% 3	3899
	21st to end	Night	44% 4 11% 28% 12%	4301
November	1st to 10th	Night	44% 5% 13% 23% 13% 1	2801
	11th to 20th	Night	53% 5% 12% 19% 10% 1	2940
	21st to end	Night	50% 5% 15% 22% 6% 1	2940
December	1st to 10th	Night	55% 5% 17% 18% 5%	4720
	11th to 20th	Night	62% 6% 10% 17% 4 1	5341
	21st to end	Night	71% 4 9% 12% 4	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	14% 12% 34% 26% 12% 2	330
	11th to 20th	Day	16% 13% 38% 28% 3 1	330
	21st to end	Day	17% 12% 39% 23% 9% 1	363
February	1st to 10th	Day	19% 16% 41% 19% 5% 1	330
	11th to 20th	Day	15% 18% 36% 24% 6% 1	330
	21st to end	Day	17% 13% 45% 22% 3	273
March	1st to 10th	Day	18% 15% 42% 19% 6%	330
	11th to 20th	Day	20% 12% 39% 21% 7% 1	330
	21st to end	Day	26% 19% 27% 19% 6% 2	385
April	1st to 10th	Day	25% 15% 34% 21% 4	540
	11th to 20th	Day	30% 13% 28% 19% 9%	554
	21st to end	Day	28% 15% 29% 19% 7% 2	750
May	1st to 10th	Day	19% 16% 30% 24% 9% 2	753
	11th to 20th	Day	20% 13% 29% 22% 13% 2	775
	21st to end	Day	19% 9% 29% 27% 13% 4	844
June	1st to 10th	Day	11% 8% 31% 30% 17% 3	780
	11th to 20th	Day	17% 10% 37% 26% 9% 2	780
	21st to end	Day	14% 9% 32% 27% 14% 4	780
July	1st to 10th	Day	9% 5% 22% 35% 20% 8%	771
	11th to 20th	Day	9% 7% 30% 30% 17% 7%	768
	21st to end	Day	7% 5% 28% 35% 19% 5%	849
August	1st to 10th	Day	11% 6% 24% 36% 19% 4	776
	11th to 20th	Day	14% 5% 25% 30% 21% 5%	765
	21st to end	Day	11% 7% 27% 33% 17% 5%	737
September	1st to 10th	Day	14% 9% 31% 27% 15% 3	570
	11th to 20th	Day	12% 8% 30% 29% 18% 3	507
	21st to end	Day	16% 11% 30% 23% 15% 5%	330
October	1st to 10th	Day	20% 12% 27% 26% 13% 3	330
	11th to 20th	Day	19% 19% 38% 18% 6%	330
	21st to end	Day	23% 17% 38% 16% 5% 1	363
November	1st to 10th	Day	21% 18% 35% 20% 6% 1	330
	11th to 20th	Day	22% 18% 38% 18% 4	330
	21st to end	Day	23% 15% 41% 17% 4	330
December	1st to 10th	Day	14% 15% 42% 21% 8%	330
	11th to 20th	Day	21% 17% 36% 24% 3	330
	21st to end	Day	28% 15% 36% 19% 1 1	363
January	1st to 10th	Night	11% 13% 36% 31% 8% 1	1050
	11th to 20th	Night	17% 13% 37% 28% 5% 1	1050
	21st to end	Night	17% 13% 37% 27% 5%	1155
February	1st to 10th	Night	13% 14% 40% 27% 6% 1	1050
	11th to 20th	Night	12% 14% 41% 26% 7%	1029
	21st to end	Night	16% 12% 37% 27% 7%	807
March	1st to 10th	Night	17% 10% 36% 29% 7%	698
	11th to 20th	Night	18% 11% 40% 26% 6%	630
	21st to end	Night	24% 11% 29% 24% 10% 1	693
April	1st to 10th	Night	22% 13% 32% 28% 5%	630
	11th to 20th	Night	23% 11% 31% 28% 7%	623
	21st to end	Night	25% 9% 32% 25% 8% 1	630
May	1st to 10th	Night	18% 13% 33% 30% 6% 1	630
	11th to 20th	Night	17% 10% 33% 30% 10%	621
	21st to end	Night	15% 8% 27% 36% 14%	683
June	1st to 10th	Night	12% 6% 27% 42% 13%	630
	11th to 20th	Night	12% 6% 31% 44% 7%	630
	21st to end	Night	11% 6% 25% 42% 15% 1	630
July	1st to 10th	Night	6% 2 22% 40% 28% 1	615
	11th to 20th	Night	8% 3 26% 45% 17% 2	612
	21st to end	Night	6% 3 27% 44% 18% 2	690
August	1st to 10th	Night	8% 4 22% 46% 19% 1	627
	11th to 20th	Night	13% 4 20% 36% 25% 2	624
	21st to end	Night	8% 4 27% 36% 23% 2	693
September	1st to 10th	Night	10% 6% 27% 38% 17% 2	630
	11th to 20th	Night	10% 6% 32% 33% 17% 2	609
	21st to end	Night	13% 10% 30% 30% 15% 1	673
October	1st to 10th	Night	18% 10% 30% 30% 12% 1	800
	11th to 20th	Night	22% 11% 33% 26% 7% 1	800
	21st to end	Night	26% 10% 30% 28% 6%	880
November	1st to 10th	Night	19% 12% 32% 30% 6% 1	955
	11th to 20th	Night	19% 14% 34% 25% 7%	1050
	21st to end	Night	19% 16% 35% 23% 6%	1050
December	1st to 10th	Night	16% 12% 37% 28% 7% 1	1050
	11th to 20th	Night	19% 13% 36% 25% 6%	1050
	21st to end	Night	25% 12% 36% 22% 5%	1155

Page generated at 2026-09-11 03:11:54. Version ac07

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