RJM Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the RJM Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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	13% 8% 3 8% 5% 4 4 4 5% 7% 37%	126946
	Night	12% 9% 4 9% 5% 4 4 3 5% 6% 37%	122719
Transparency	Day	45% 6% 12% 25% 12% 1	130419
	Night	46% 6% 11% 23% 13% 1	127319
Seeing	Day	17% 10% 30% 26% 14% 4	17306
	Night	17% 9% 32% 31% 11% 1	27098

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% 12% 6% 18% 8% 6% 4 3 4 5% 27%	3617
	11th to 20th	Day	8% 12% 6% 13% 5% 5% 7% 8% 8% 6% 22%	3620
	21st to end	Day	9% 13% 8% 14% 5% 4 3 3 5% 7% 29%	4148
February	1st to 10th	Day	9% 12% 5% 11% 5% 5% 6% 4 6% 7% 31%	1800
	11th to 20th	Day	9% 12% 4 9% 5% 5% 5% 5% 6% 6% 35%	1796
	21st to end	Day	12% 11% 5% 11% 8% 5% 5% 3 6% 5% 30%	1650
March	1st to 10th	Day	14% 8% 3 5% 3 3 3 3 4 7% 47%	4480
	11th to 20th	Day	11% 7% 3 8% 5% 4 4 4 6% 9% 39%	4928
	21st to end	Day	23% 11% 4 10% 4 3 3 3 4 6% 28%	5819
April	1st to 10th	Day	17% 13% 3 8% 5% 4 5% 3 4 5% 32%	2400
	11th to 20th	Day	19% 12% 4 7% 5% 4 3 3 5% 6% 31%	2374
	21st to end	Day	20% 11% 3 8% 6% 5% 4 3 5% 6% 30%	2475
May	1st to 10th	Day	15% 6% 2 6% 4 4 3 3 5% 6% 46%	6319
	11th to 20th	Day	15% 6% 2 6% 4 5% 4 4 6% 7% 41%	6319
	21st to end	Day	18% 7% 2 6% 5% 4 4 4 5% 6% 37%	6935
June	1st to 10th	Day	14% 5% 2 6% 5% 4 5% 3 5% 8% 43%	2800
	11th to 20th	Day	15% 7% 2 7% 5% 5% 5% 4 6% 8% 37%	2800
	21st to end	Day	12% 6% 2 6% 6% 5% 5% 4 7% 8% 38%	2800
July	1st to 10th	Day	11% 4 2 5% 5% 5% 5% 4 6% 10% 44%	6360
	11th to 20th	Day	8% 4 2 5% 4 4 5% 4 6% 9% 50%	6360
	21st to end	Day	9% 6% 2 7% 4 5% 5% 4 6% 8% 44%	6924
August	1st to 10th	Day	13% 6% 3 7% 5% 4 5% 4 5% 7% 41%	4503
	11th to 20th	Day	14% 5% 1 5% 4 4 5% 4 6% 8% 43%	4084
	21st to end	Day	12% 6% 2 7% 4 5% 5% 3 6% 8% 41%	4312
September	1st to 10th	Day	14% 6% 2 5% 4 4 4 4 4 8% 44%	2400
	11th to 20th	Day	12% 6% 2 5% 4 3 4 5% 6% 8% 45%	2380
	21st to end	Day	14% 6% 3 7% 4 4 5% 3 6% 8% 41%	2165
October	1st to 10th	Day	14% 7% 3 7% 4 4 4 3 6% 7% 42%	1930
	11th to 20th	Day	19% 11% 3 7% 5% 4 4 3 5% 7% 33%	1900
	21st to end	Day	18% 11% 3 7% 4 4 4 3 4 6% 35%	1927
November	1st to 10th	Day	14% 8% 3 7% 5% 5% 4 4 5% 6% 39%	1762
	11th to 20th	Day	14% 13% 5% 10% 6% 4 5% 3 5% 7% 28%	1659
	21st to end	Day	13% 12% 4 9% 6% 4 5% 3 5% 5% 34%	1600
December	1st to 10th	Day	7% 9% 5% 17% 10% 8% 6% 4 5% 6% 24%	2610
	11th to 20th	Day	13% 16% 7% 10% 7% 5% 4 3 3 5% 27%	3008
	21st to end	Day	20% 21% 6% 14% 6% 4 4 3 4 4 16%	3982
January	1st to 10th	Night	8% 12% 5% 13% 9% 5% 3 3 4 5% 33%	6348
	11th to 20th	Night	7% 12% 5% 14% 8% 5% 8% 6% 7% 5% 23%	6360
	21st to end	Night	9% 15% 7% 13% 6% 4 4 3 4 6% 29%	6813
February	1st to 10th	Night	8% 12% 6% 10% 6% 5% 5% 3 6% 5% 33%	2600
	11th to 20th	Night	9% 9% 3 9% 4 4 6% 5% 6% 6% 39%	2490
	21st to end	Night	14% 11% 3 9% 6% 5% 5% 3 6% 6% 33%	1980
March	1st to 10th	Night	11% 7% 3 6% 4 4 4 3 6% 7% 46%	5373
	11th to 20th	Night	9% 9% 3 9% 5% 5% 6% 4 5% 8% 37%	5076
	21st to end	Night	23% 11% 4 11% 5% 4 3 3 3 5% 29%	5110
April	1st to 10th	Night	18% 12% 3 7% 4 3 4 3 5% 7% 34%	2000
	11th to 20th	Night	21% 8% 2 6% 4 3 4 3 5% 5% 38%	1975
	21st to end	Night	20% 8% 3 6% 5% 4 3 3 5% 7% 37%	1920
May	1st to 10th	Night	14% 7% 3 7% 4 4 4 3 4 6% 43%	3596
	11th to 20th	Night	12% 5% 3 6% 4 3 4 3 5% 7% 49%	3596
	21st to end	Night	18% 7% 3 7% 4 4 4 3 5% 7% 38%	3942
June	1st to 10th	Night	12% 7% 2 4 4 4 5% 4 6% 7% 46%	1600
	11th to 20th	Night	14% 5% 3 7% 5% 4 5% 5% 6% 7% 39%	1600
	21st to end	Night	11% 6% 2 5% 4 4 5% 4 6% 8% 46%	1600
July	1st to 10th	Night	11% 4 2 5% 4 5% 4 4 6% 9% 45%	3618
	11th to 20th	Night	9% 4 2 4 3 4 3 3 6% 7% 55%	3617
	21st to end	Night	7% 5% 2 6% 4 4 4 3 6% 8% 50%	3947
August	1st to 10th	Night	11% 5% 3 6% 4 4 5% 4 6% 6% 46%	2654
	11th to 20th	Night	11% 6% 2 6% 4 4 4 3 5% 7% 49%	2970
	21st to end	Night	12% 6% 2 6% 4 4 4 3 6% 6% 47%	3586
September	1st to 10th	Night	10% 5% 2 5% 4 4 3 3 5% 7% 51%	2000
	11th to 20th	Night	11% 6% 2 5% 4 4 3 3 5% 8% 50%	2000
	21st to end	Night	11% 7% 2 6% 3 4 3 3 5% 7% 50%	2230
October	1st to 10th	Night	12% 6% 2 6% 4 4 5% 3 4 5% 49%	2317
	11th to 20th	Night	15% 10% 3 7% 4 4 4 4 6% 7% 38%	2280
	21st to end	Night	17% 12% 3 7% 4 4 5% 3 5% 6% 34%	2670
November	1st to 10th	Night	15% 9% 3 7% 5% 3 4 3 4 7% 40%	2536
	11th to 20th	Night	13% 13% 5% 10% 5% 5% 4 4 5% 5% 33%	2740
	21st to end	Night	13% 12% 4 9% 5% 4 4 3 5% 6% 33%	2800
December	1st to 10th	Night	8% 10% 4 10% 5% 5% 5% 4 6% 8% 32%	4580
	11th to 20th	Night	12% 17% 6% 14% 6% 4 4 3 4 4 25%	5199
	21st to end	Night	21% 16% 6% 12% 5% 4 3 3 4 4 21%	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	60% 4 9% 17% 9%	3618
	11th to 20th	Day	58% 6% 7% 15% 12% 1	3620
	21st to end	Day	56% 4 13% 19% 9%	4148
February	1st to 10th	Day	52% 5% 12% 19% 10% 1	1800
	11th to 20th	Day	46% 7% 12% 24% 9% 1	1805
	21st to end	Day	54% 6% 11% 18% 10% 2	1650
March	1st to 10th	Day	38% 4 13% 24% 17% 5%	4480
	11th to 20th	Day	41% 5% 15% 24% 15% 2	4928
	21st to end	Day	58% 4 8% 16% 13% 1	5821
April	1st to 10th	Day	53% 6% 10% 19% 11% 1	2400
	11th to 20th	Day	53% 5% 9% 20% 11% 2	2374
	21st to end	Day	54% 7% 10% 18% 11% 1	2475
May	1st to 10th	Day	38% 5% 10% 29% 16% 1	6357
	11th to 20th	Day	40% 7% 12% 26% 14% 1	6323
	21st to end	Day	45% 6% 9% 25% 14% 1	6949
June	1st to 10th	Day	37% 8% 13% 28% 13% 1	2800
	11th to 20th	Day	44% 8% 12% 23% 13% 1	2800
	21st to end	Day	42% 7% 15% 24% 10% 1	2800
July	1st to 10th	Day	34% 8% 16% 35% 7% 1	6291
	11th to 20th	Day	29% 7% 14% 35% 14% 1	6285
	21st to end	Day	37% 7% 12% 26% 17% 1	6728
August	1st to 10th	Day	40% 6% 13% 27% 13% 1	4488
	11th to 20th	Day	38% 6% 12% 32% 12%	4123
	21st to end	Day	40% 6% 13% 29% 11% 1	4311
September	1st to 10th	Day	38% 7% 13% 28% 14%	2400
	11th to 20th	Day	35% 8% 18% 30% 8% 1	2380
	21st to end	Day	40% 7% 12% 29% 11%	2165
October	1st to 10th	Day	28% 3 9% 38% 22%	3190
	11th to 20th	Day	43% 5% 11% 24% 16% 1	3140
	21st to end	Day	44% 4 15% 28% 9%	3152
November	1st to 10th	Day	45% 6% 11% 22% 13% 2	1762
	11th to 20th	Day	55% 6% 12% 19% 8% 1	1659
	21st to end	Day	52% 5% 15% 21% 7% 1	1600
December	1st to 10th	Day	60% 6% 11% 17% 5%	2610
	11th to 20th	Day	61% 4 8% 16% 10% 2	3005
	21st to end	Day	73% 4 6% 11% 6%	3982
January	1st to 10th	Night	54% 4 9% 20% 13%	6348
	11th to 20th	Night	59% 5% 8% 13% 14%	6360
	21st to end	Night	57% 5% 9% 17% 11%	6813
February	1st to 10th	Night	52% 4 11% 21% 11% 2	2600
	11th to 20th	Night	43% 7% 12% 26% 10% 2	2491
	21st to end	Night	51% 5% 12% 19% 10% 2	1980
March	1st to 10th	Night	37% 5% 14% 25% 13% 6%	5373
	11th to 20th	Night	44% 6% 15% 22% 12% 2	5076
	21st to end	Night	59% 4 9% 17% 12%	5110
April	1st to 10th	Night	50% 6% 7% 18% 17% 2	2000
	11th to 20th	Night	47% 5% 9% 21% 15% 3	1975
	21st to end	Night	45% 7% 10% 22% 14% 2	1920
May	1st to 10th	Night	41% 7% 11% 19% 20% 2	3620
	11th to 20th	Night	34% 6% 12% 27% 18% 3	3597
	21st to end	Night	45% 7% 11% 23% 15% 1	3950
June	1st to 10th	Night	34% 8% 13% 28% 16% 1	1600
	11th to 20th	Night	41% 8% 12% 24% 14% 2	1600
	21st to end	Night	35% 8% 13% 29% 12% 3	1600
July	1st to 10th	Night	35% 7% 16% 30% 10% 1	3580
	11th to 20th	Night	28% 6% 14% 36% 15% 1	3572
	21st to end	Night	32% 6% 14% 29% 17% 2	3832
August	1st to 10th	Night	37% 6% 14% 26% 16% 2	2645
	11th to 20th	Night	34% 7% 14% 30% 14% 1	2997
	21st to end	Night	36% 6% 15% 26% 15% 2	3586
September	1st to 10th	Night	32% 6% 13% 31% 17% 1	2000
	11th to 20th	Night	33% 7% 13% 33% 14%	2000
	21st to end	Night	34% 6% 12% 32% 15% 1	2230
October	1st to 10th	Night	27% 5% 6% 36% 23% 3	3837
	11th to 20th	Night	34% 5% 10% 31% 19%	3779
	21st to end	Night	50% 4 9% 23% 14%	4395
November	1st to 10th	Night	45% 4 12% 23% 14% 1	2536
	11th to 20th	Night	53% 6% 13% 17% 11% 1	2740
	21st to end	Night	52% 5% 13% 22% 6% 1	2800
December	1st to 10th	Night	48% 7% 17% 21% 5% 2	4580
	11th to 20th	Night	63% 4 8% 14% 10% 1	5201
	21st to end	Night	67% 4 6% 14% 8%	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	17% 12% 32% 25% 12% 2	300
	11th to 20th	Day	18% 11% 40% 25% 5% 1	300
	21st to end	Day	17% 11% 37% 26% 8% 1	330
February	1st to 10th	Day	15% 14% 47% 14% 7% 2	300
	11th to 20th	Day	15% 17% 40% 19% 8%	300
	21st to end	Day	17% 12% 45% 23% 2	249
March	1st to 10th	Day	17% 14% 41% 18% 10% 1	300
	11th to 20th	Day	19% 14% 34% 21% 10% 2	300
	21st to end	Day	24% 13% 33% 20% 8% 3	480
April	1st to 10th	Day	25% 15% 32% 22% 6% 1	500
	11th to 20th	Day	27% 15% 28% 21% 7% 2	492
	21st to end	Day	25% 13% 30% 21% 9% 2	515
May	1st to 10th	Day	20% 13% 31% 26% 9% 2	703
	11th to 20th	Day	19% 13% 26% 25% 13% 4	725
	21st to end	Day	19% 10% 26% 26% 13% 6%	789
June	1st to 10th	Day	12% 8% 29% 28% 17% 5%	730
	11th to 20th	Day	18% 8% 32% 24% 14% 4	730
	21st to end	Day	16% 7% 29% 28% 14% 6%	730
July	1st to 10th	Day	9% 3 19% 31% 27% 11%	721
	11th to 20th	Day	9% 6% 28% 31% 18% 8%	718
	21st to end	Day	7% 3 27% 35% 22% 8%	794
August	1st to 10th	Day	10% 5% 23% 31% 22% 8%	726
	11th to 20th	Day	15% 6% 19% 26% 26% 8%	667
	21st to end	Day	14% 6% 22% 32% 19% 7%	583
September	1st to 10th	Day	17% 8% 23% 26% 19% 7%	530
	11th to 20th	Day	12% 6% 23% 35% 20% 4	530
	21st to end	Day	15% 9% 24% 31% 16% 4	495
October	1st to 10th	Day	15% 12% 27% 27% 14% 5%	309
	11th to 20th	Day	22% 15% 35% 19% 7% 1	300
	21st to end	Day	26% 13% 38% 19% 4 1	330
November	1st to 10th	Day	20% 11% 33% 25% 9% 2	300
	11th to 20th	Day	17% 18% 38% 21% 6% 1	300
	21st to end	Day	21% 13% 43% 15% 8%	300
December	1st to 10th	Day	14% 14% 44% 22% 5% 1	300
	11th to 20th	Day	20% 15% 38% 21% 6%	300
	21st to end	Day	29% 14% 33% 19% 4 1	330
January	1st to 10th	Night	15% 12% 36% 29% 8% 1	1000
	11th to 20th	Night	16% 11% 38% 29% 6% 1	1000
	21st to end	Night	18% 9% 36% 29% 7%	1100
February	1st to 10th	Night	14% 13% 37% 29% 6% 1	1000
	11th to 20th	Night	15% 13% 40% 24% 7% 1	936
	21st to end	Night	19% 12% 38% 27% 4	660
March	1st to 10th	Night	17% 12% 36% 28% 7% 1	800
	11th to 20th	Night	16% 12% 36% 27% 7% 1	644
	21st to end	Night	23% 9% 30% 27% 9% 2	660
April	1st to 10th	Night	23% 11% 37% 24% 5% 1	600
	11th to 20th	Night	24% 9% 30% 29% 7%	593
	21st to end	Night	23% 8% 33% 30% 6% 1	600
May	1st to 10th	Night	21% 7% 35% 31% 6% 1	600
	11th to 20th	Night	19% 8% 30% 32% 9% 1	591
	21st to end	Night	16% 5% 26% 37% 15%	650
June	1st to 10th	Night	14% 7% 27% 39% 12% 1	600
	11th to 20th	Night	16% 5% 31% 39% 9%	600
	21st to end	Night	13% 6% 22% 42% 17% 2	600
July	1st to 10th	Night	8% 2 21% 39% 29% 2	585
	11th to 20th	Night	9% 4 21% 39% 24% 2	582
	21st to end	Night	5% 2 19% 50% 21% 3	657
August	1st to 10th	Night	8% 3 16% 47% 25% 2	597
	11th to 20th	Night	12% 3 19% 35% 28% 3	594
	21st to end	Night	14% 4 20% 38% 22% 3	660
September	1st to 10th	Night	13% 4 23% 38% 19% 4	600
	11th to 20th	Night	12% 3 26% 37% 20% 3	600
	21st to end	Night	12% 7% 28% 36% 16% 1	800
October	1st to 10th	Night	15% 8% 29% 31% 16% 1	773
	11th to 20th	Night	18% 9% 35% 30% 8%	760
	21st to end	Night	23% 8% 33% 27% 8% 1	836
November	1st to 10th	Night	19% 11% 33% 28% 9% 1	780
	11th to 20th	Night	19% 12% 40% 23% 6%	940
	21st to end	Night	19% 14% 36% 26% 5%	1000
December	1st to 10th	Night	17% 11% 39% 25% 7%	1000
	11th to 20th	Night	21% 12% 34% 26% 7% 1	1000
	21st to end	Night	27% 11% 31% 25% 5%	1100

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