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 2025-04-26.

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	12% 9% 3 8% 5% 4 5% 4 6% 7% 37%	114196
	Night	11% 10% 4 9% 5% 4 4 4 5% 6% 37%	114631
Transparency	Day	45% 6% 13% 24% 11% 1	117804
	Night	45% 5% 12% 23% 12% 1	119165
Seeing	Day	16% 11% 32% 26% 12% 3	16586
	Night	16% 10% 33% 31% 10% 1	26644

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	8% 13% 6% 18% 8% 5% 5% 3 5% 6% 24%	3617
	11th to 20th	Day	6% 10% 6% 14% 7% 6% 9% 8% 9% 5% 20%	3643
	21st to end	Day	6% 16% 8% 13% 5% 5% 4 3 5% 6% 31%	4488
February	1st to 10th	Day	8% 13% 5% 10% 7% 5% 6% 3 6% 6% 31%	1800
	11th to 20th	Day	8% 12% 5% 9% 6% 5% 5% 4 5% 6% 34%	1754
	21st to end	Day	9% 13% 5% 10% 6% 4 4 4 7% 7% 31%	1538
March	1st to 10th	Day	12% 9% 3 6% 4 4 3 3 5% 8% 44%	4839
	11th to 20th	Day	9% 10% 3 8% 4 5% 4 4 6% 7% 40%	4980
	21st to end	Day	21% 11% 4 10% 5% 4 5% 3 5% 6% 26%	5521
April	1st to 10th	Day	17% 9% 4 7% 4 5% 4 3 5% 8% 34%	2400
	11th to 20th	Day	19% 12% 3 8% 5% 4 4 3 4 6% 31%	2395
	21st to end	Day	21% 12% 3 7% 5% 4 4 4 4 6% 31%	2536
May	1st to 10th	Day	17% 7% 2 6% 4 4 5% 3 6% 7% 40%	4264
	11th to 20th	Day	14% 6% 2 6% 4 4 4 4 5% 7% 45%	4399
	21st to end	Day	17% 7% 2 7% 5% 4 5% 4 6% 8% 34%	4824
June	1st to 10th	Day	11% 5% 2 5% 3 4 4 5% 6% 9% 46%	2660
	11th to 20th	Day	13% 6% 3 7% 6% 5% 4 4 6% 10% 36%	2660
	21st to end	Day	10% 5% 2 6% 5% 4 6% 5% 9% 9% 39%	2843
July	1st to 10th	Day	10% 6% 2 6% 5% 5% 6% 5% 8% 10% 37%	4516
	11th to 20th	Day	6% 3 1 4 3 3 3 4 6% 9% 59%	4440
	21st to end	Day	6% 5% 2 5% 4 5% 5% 4 7% 9% 48%	4762
August	1st to 10th	Day	12% 6% 2 6% 4 4 4 4 6% 8% 43%	4104
	11th to 20th	Day	12% 5% 2 5% 4 4 5% 4 6% 8% 43%	4041
	21st to end	Day	9% 6% 3 6% 5% 5% 4 4 5% 8% 46%	4345
September	1st to 10th	Day	10% 5% 2 5% 5% 4 3 4 5% 9% 47%	2280
	11th to 20th	Day	12% 6% 2 5% 4 4 5% 3 6% 7% 46%	2247
	21st to end	Day	15% 7% 2 5% 4 3 4 4 5% 6% 45%	2001
October	1st to 10th	Day	16% 8% 2 6% 4 5% 4 3 4 6% 42%	1900
	11th to 20th	Day	14% 12% 3 6% 4 3 4 3 5% 6% 38%	1900
	21st to end	Day	17% 12% 3 7% 4 3 3 3 4 6% 38%	2066
November	1st to 10th	Day	11% 13% 4 8% 5% 3 6% 3 5% 5% 36%	1633
	11th to 20th	Day	11% 14% 7% 12% 7% 5% 4 4 4 5% 28%	1600
	21st to end	Day	12% 13% 4 9% 5% 3 4 4 4 7% 33%	1600
December	1st to 10th	Day	7% 13% 4 15% 11% 7% 6% 4 6% 6% 22%	2610
	11th to 20th	Day	10% 16% 7% 11% 6% 5% 4 3 5% 5% 28%	3008
	21st to end	Day	19% 21% 8% 19% 5% 4 3 2 2 3 14%	3982
January	1st to 10th	Night	8% 12% 6% 14% 8% 6% 4 3 4 6% 30%	6348
	11th to 20th	Night	6% 11% 5% 12% 7% 6% 10% 8% 7% 5% 20%	6332
	21st to end	Night	7% 18% 6% 10% 5% 5% 4 3 5% 5% 32%	6490
February	1st to 10th	Night	7% 12% 6% 9% 7% 5% 5% 4 5% 6% 35%	2600
	11th to 20th	Night	6% 11% 3 8% 7% 6% 5% 4 5% 7% 38%	2533
	21st to end	Night	11% 10% 3 9% 6% 5% 4 3 5% 6% 37%	2090
March	1st to 10th	Night	10% 7% 3 6% 4 3 4 4 5% 7% 47%	5057
	11th to 20th	Night	9% 8% 3 7% 4 5% 5% 4 7% 10% 38%	4980
	21st to end	Night	24% 12% 4 9% 4 4 3 3 4 5% 28%	5407
April	1st to 10th	Night	15% 11% 3 6% 4 4 5% 3 5% 7% 37%	2000
	11th to 20th	Night	20% 11% 3 7% 4 3 3 3 5% 5% 35%	1951
	21st to end	Night	21% 10% 2 6% 3 4 4 3 5% 6% 35%	1760
May	1st to 10th	Night	14% 8% 3 7% 5% 4 4 4 5% 6% 42%	2641
	11th to 20th	Night	14% 5% 1 4 3 3 4 4 5% 8% 49%	2506
	21st to end	Night	14% 9% 2 6% 4 4 3 4 6% 7% 41%	2743
June	1st to 10th	Night	11% 5% 2 4 4 4 5% 3 7% 7% 48%	1520
	11th to 20th	Night	8% 5% 2 6% 4 5% 5% 4 7% 8% 46%	1514
	21st to end	Night	7% 4 1 5% 3 4 4 4 6% 7% 54%	1330
July	1st to 10th	Night	11% 4 1 5% 4 4 5% 5% 7% 10% 46%	2432
	11th to 20th	Night	4 3 1 3 3 4 3 4 6% 9% 60%	2530
	21st to end	Night	5% 4 1 5% 3 4 4 4 6% 7% 57%	2798
August	1st to 10th	Night	11% 6% 2 7% 4 4 3 4 5% 7% 48%	2836
	11th to 20th	Night	13% 4 1 4 3 3 4 4 5% 8% 50%	2798
	21st to end	Night	8% 7% 2 7% 4 5% 4 3 5% 7% 47%	3316
September	1st to 10th	Night	8% 5% 2 5% 4 4 5% 3 5% 7% 54%	1900
	11th to 20th	Night	9% 5% 2 4 4 3 4 3 6% 6% 54%	1929
	21st to end	Night	13% 5% 2 5% 4 3 4 3 5% 6% 51%	2159
October	1st to 10th	Night	14% 7% 2 5% 3 3 4 3 4 6% 49%	2280
	11th to 20th	Night	15% 11% 3 5% 4 3 4 3 5% 6% 41%	2280
	21st to end	Night	18% 11% 3 6% 5% 4 4 3 4 6% 37%	2532
November	1st to 10th	Night	13% 10% 3 7% 4 4 4 4 5% 6% 40%	2664
	11th to 20th	Night	14% 11% 4 11% 6% 4 4 3 4 5% 33%	2800
	21st to end	Night	13% 11% 3 9% 6% 5% 4 4 4 7% 35%	2800
December	1st to 10th	Night	7% 10% 5% 13% 8% 8% 5% 4 5% 7% 29%	4580
	11th to 20th	Night	10% 17% 6% 14% 6% 5% 5% 5% 4 5% 24%	5199
	21st to end	Night	19% 17% 8% 16% 5% 4 3 3 3 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	61% 6% 9% 18% 6%	3618
	11th to 20th	Day	60% 6% 9% 17% 8%	3643
	21st to end	Day	55% 5% 15% 17% 7%	4488
February	1st to 10th	Day	53% 6% 14% 19% 8% 1	1800
	11th to 20th	Day	49% 6% 12% 25% 8%	1763
	21st to end	Day	52% 5% 13% 20% 10% 1	1538
March	1st to 10th	Day	39% 5% 18% 25% 11% 2	4839
	11th to 20th	Day	41% 5% 15% 29% 9% 1	4980
	21st to end	Day	59% 5% 10% 19% 7% 1	5523
April	1st to 10th	Day	48% 6% 13% 21% 12% 1	2400
	11th to 20th	Day	53% 7% 9% 18% 11% 1	2395
	21st to end	Day	53% 7% 12% 19% 8%	2536
May	1st to 10th	Day	40% 7% 13% 24% 13% 2	4302
	11th to 20th	Day	37% 8% 11% 29% 14% 2	4403
	21st to end	Day	45% 8% 13% 24% 10% 1	4837
June	1st to 10th	Day	33% 8% 15% 31% 12% 2	2660
	11th to 20th	Day	43% 8% 13% 21% 13% 1	2660
	21st to end	Day	35% 9% 17% 27% 9% 2	2843
July	1st to 10th	Day	39% 6% 21% 22% 11% 1	4447
	11th to 20th	Day	22% 5% 17% 37% 17% 2	4365
	21st to end	Day	31% 6% 16% 29% 17% 1	4612
August	1st to 10th	Day	38% 6% 12% 30% 12% 2	4090
	11th to 20th	Day	37% 6% 13% 34% 10%	4080
	21st to end	Day	37% 5% 16% 28% 13% 1	4344
September	1st to 10th	Day	34% 7% 15% 33% 12%	2280
	11th to 20th	Day	35% 8% 16% 32% 8%	2247
	21st to end	Day	39% 7% 11% 33% 10%	2001
October	1st to 10th	Day	32% 3 8% 35% 21%	3160
	11th to 20th	Day	40% 4 11% 28% 15% 1	3140
	21st to end	Day	45% 3 15% 28% 9%	3380
November	1st to 10th	Day	48% 7% 12% 19% 12% 2	1633
	11th to 20th	Day	58% 5% 13% 15% 9% 1	1600
	21st to end	Day	50% 7% 16% 20% 6% 2	1600
December	1st to 10th	Day	61% 7% 15% 12% 4 1	2610
	11th to 20th	Day	58% 4 14% 18% 6%	3005
	21st to end	Day	77% 4 7% 8% 4	3982
January	1st to 10th	Night	56% 4 11% 21% 8%	6348
	11th to 20th	Night	60% 5% 11% 14% 9%	6332
	21st to end	Night	55% 5% 11% 23% 6%	6490
February	1st to 10th	Night	50% 5% 13% 21% 10% 1	2600
	11th to 20th	Night	44% 7% 14% 25% 9% 1	2534
	21st to end	Night	47% 6% 13% 20% 12% 1	2090
March	1st to 10th	Night	36% 5% 14% 26% 16% 3	5057
	11th to 20th	Night	38% 6% 16% 27% 13% 1	4980
	21st to end	Night	60% 3 10% 18% 9% 1	5407
April	1st to 10th	Night	47% 6% 8% 24% 14% 1	2000
	11th to 20th	Night	48% 6% 9% 20% 14% 2	1951
	21st to end	Night	49% 7% 9% 20% 13% 2	1760
May	1st to 10th	Night	41% 8% 11% 21% 19% 1	2668
	11th to 20th	Night	32% 7% 11% 27% 19% 4	2507
	21st to end	Night	40% 8% 12% 24% 15% 1	2751
June	1st to 10th	Night	31% 7% 15% 28% 17% 2	1520
	11th to 20th	Night	33% 9% 14% 24% 18% 2	1514
	21st to end	Night	27% 7% 16% 31% 16% 3	1330
July	1st to 10th	Night	33% 6% 21% 25% 14% 1	2392
	11th to 20th	Night	21% 5% 14% 39% 19% 2	2482
	21st to end	Night	25% 6% 16% 32% 18% 3	2709
August	1st to 10th	Night	36% 6% 11% 29% 14% 4	2826
	11th to 20th	Night	32% 7% 13% 33% 15% 1	2825
	21st to end	Night	36% 5% 17% 28% 11% 2	3316
September	1st to 10th	Night	30% 6% 14% 33% 17% 1	1900
	11th to 20th	Night	29% 7% 17% 31% 16%	1929
	21st to end	Night	33% 6% 13% 32% 15% 1	2159
October	1st to 10th	Night	24% 4 7% 35% 27% 4	3800
	11th to 20th	Night	35% 4 12% 24% 21% 3	3779
	21st to end	Night	44% 4 11% 28% 12%	4168
November	1st to 10th	Night	44% 5% 13% 23% 13% 1	2664
	11th to 20th	Night	53% 6% 12% 18% 11% 1	2800
	21st to end	Night	50% 6% 15% 22% 6% 1	2800
December	1st to 10th	Night	55% 5% 17% 18% 4	4580
	11th to 20th	Night	62% 6% 10% 17% 5% 1	5201
	21st to end	Night	72% 4 9% 12% 4	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	12% 13% 34% 26% 13% 2	300
	11th to 20th	Day	15% 14% 38% 29% 4 1	300
	21st to end	Day	17% 11% 39% 22% 9% 2	330
February	1st to 10th	Day	18% 16% 41% 20% 5%	300
	11th to 20th	Day	15% 19% 38% 22% 5% 1	300
	21st to end	Day	18% 12% 45% 22% 2	249
March	1st to 10th	Day	18% 14% 41% 20% 6%	300
	11th to 20th	Day	18% 11% 40% 22% 7% 1	300
	21st to end	Day	26% 18% 27% 21% 7% 2	351
April	1st to 10th	Day	24% 16% 33% 23% 4	500
	11th to 20th	Day	30% 12% 28% 20% 9%	513
	21st to end	Day	27% 14% 29% 20% 8% 2	676
May	1st to 10th	Day	20% 16% 31% 23% 8% 1	653
	11th to 20th	Day	18% 13% 30% 22% 14% 3	675
	21st to end	Day	18% 9% 28% 27% 14% 4	734
June	1st to 10th	Day	10% 8% 30% 31% 18% 4	680
	11th to 20th	Day	14% 9% 38% 28% 10% 2	680
	21st to end	Day	13% 8% 31% 29% 15% 4	680
July	1st to 10th	Day	10% 5% 22% 35% 20% 9%	671
	11th to 20th	Day	9% 7% 29% 30% 18% 7%	668
	21st to end	Day	6% 5% 26% 36% 21% 6%	739
August	1st to 10th	Day	11% 5% 24% 35% 21% 4	676
	11th to 20th	Day	14% 5% 23% 30% 23% 5%	665
	21st to end	Day	11% 8% 26% 32% 18% 5%	639
September	1st to 10th	Day	13% 8% 32% 28% 16% 3	490
	11th to 20th	Day	12% 7% 29% 29% 19% 3	457
	21st to end	Day	17% 11% 32% 22% 14% 5%	300
October	1st to 10th	Day	21% 11% 26% 25% 13% 4	300
	11th to 20th	Day	16% 19% 38% 19% 7%	300
	21st to end	Day	23% 17% 39% 15% 5% 1	330
November	1st to 10th	Day	19% 17% 36% 21% 6% 1	300
	11th to 20th	Day	20% 19% 39% 18% 4	300
	21st to end	Day	22% 15% 42% 16% 4	300
December	1st to 10th	Day	15% 15% 41% 21% 9%	300
	11th to 20th	Day	20% 17% 36% 24% 3	300
	21st to end	Day	28% 15% 34% 20% 2 1	330
January	1st to 10th	Night	11% 13% 37% 31% 8% 1	1000
	11th to 20th	Night	17% 12% 37% 29% 5% 1	1000
	21st to end	Night	17% 12% 37% 27% 5%	1100
February	1st to 10th	Night	13% 13% 39% 28% 6% 1	1000
	11th to 20th	Night	11% 14% 41% 26% 7%	979
	21st to end	Night	16% 12% 36% 28% 8%	770
March	1st to 10th	Night	16% 10% 37% 29% 8%	665
	11th to 20th	Night	17% 11% 40% 26% 6%	600
	21st to end	Night	24% 11% 28% 25% 11% 2	660
April	1st to 10th	Night	22% 12% 32% 29% 6%	600
	11th to 20th	Night	23% 11% 31% 28% 6%	593
	21st to end	Night	24% 9% 33% 25% 8% 1	587
May	1st to 10th	Night	18% 13% 33% 30% 5% 1	570
	11th to 20th	Night	16% 8% 34% 30% 10% 1	561
	21st to end	Night	14% 9% 27% 35% 15%	617
June	1st to 10th	Night	12% 6% 27% 41% 14%	570
	11th to 20th	Night	11% 5% 31% 45% 7%	570
	21st to end	Night	10% 5% 24% 43% 16% 1	570
July	1st to 10th	Night	6% 2 22% 39% 28% 2	555
	11th to 20th	Night	7% 4 25% 45% 17% 2	552
	21st to end	Night	6% 2 27% 44% 20% 2	624
August	1st to 10th	Night	8% 4 20% 47% 19% 1	567
	11th to 20th	Night	12% 4 20% 36% 26% 2	564
	21st to end	Night	8% 4 27% 36% 23% 2	627
September	1st to 10th	Night	9% 6% 27% 38% 18% 2	570
	11th to 20th	Night	10% 6% 31% 33% 17% 2	570
	21st to end	Night	13% 10% 31% 29% 15% 1	639
October	1st to 10th	Night	18% 9% 30% 30% 13% 1	760
	11th to 20th	Night	22% 10% 33% 27% 8% 1	760
	21st to end	Night	25% 10% 29% 29% 6%	836
November	1st to 10th	Night	18% 12% 31% 31% 7% 1	908
	11th to 20th	Night	19% 14% 35% 25% 7%	1000
	21st to end	Night	19% 16% 35% 24% 6%	1000
December	1st to 10th	Night	16% 11% 37% 29% 7% 1	1000
	11th to 20th	Night	19% 12% 37% 26% 6%	1000
	21st to end	Night	25% 12% 35% 23% 5%	1100

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