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 2024-04-25.

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	14% 9% 4 7% 6% 3 5% 5% 5% 7% 35%	83210
	Night	13% 10% 4 8% 6% 3 5% 5% 5% 6% 36%	85957
Transparency	Day	48% 7% 12% 22% 10% 1	82910
	Night	48% 6% 11% 23% 11% 1	85777
Seeing	Day	17% 10% 30% 26% 14% 4	15160
	Night	17% 9% 32% 31% 11% 1	25237

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	11% 16% 8% 15% 7% 3 5% 4 4 5% 23%	2527
	11th to 20th	Day	8% 12% 6% 14% 8% 5% 9% 10% 6% 5% 17%	2530
	21st to end	Day	13% 15% 10% 13% 7% 4 4 4 5% 7% 18%	2893
February	1st to 10th	Day	9% 11% 5% 10% 6% 3 6% 5% 6% 7% 32%	1710
	11th to 20th	Day	9% 11% 4 8% 6% 4 5% 6% 6% 6% 35%	1703
	21st to end	Day	13% 9% 5% 10% 9% 4 5% 4 6% 6% 29%	1570
March	1st to 10th	Day	13% 8% 4 5% 4 2 3 3 3 6% 48%	3120
	11th to 20th	Day	9% 7% 4 9% 7% 3 5% 5% 6% 8% 38%	3442
	21st to end	Day	20% 12% 5% 10% 7% 3 4 4 4 6% 27%	4059
April	1st to 10th	Day	17% 11% 5% 7% 6% 4 5% 4 4 6% 32%	2280
	11th to 20th	Day	20% 11% 4 7% 5% 3 4 4 4 6% 32%	2254
	21st to end	Day	19% 11% 3 7% 6% 4 4 4 5% 6% 30%	2276
May	1st to 10th	Day	16% 7% 3 7% 6% 4 5% 5% 5% 8% 33%	2520
	11th to 20th	Day	18% 7% 3 7% 5% 4 4 4 5% 6% 37%	2479
	21st to end	Day	15% 6% 3 6% 5% 4 5% 6% 6% 8% 37%	2725
June	1st to 10th	Day	14% 5% 3 4 6% 3 5% 4 5% 8% 44%	2520
	11th to 20th	Day	13% 6% 2 6% 5% 4 5% 5% 6% 7% 39%	2520
	21st to end	Day	11% 6% 2 4 7% 4 6% 6% 7% 8% 40%	2520
July	1st to 10th	Day	7% 3 2 5% 5% 3 6% 6% 7% 10% 45%	2520
	11th to 20th	Day	9% 3 2 5% 6% 4 6% 6% 7% 10% 43%	2520
	21st to end	Day	7% 5% 2 4 5% 3 5% 5% 7% 9% 47%	2743
August	1st to 10th	Day	9% 5% 3 6% 6% 4 6% 6% 6% 10% 39%	2477
	11th to 20th	Day	15% 5% 2 5% 6% 3 5% 6% 6% 8% 40%	2259
	21st to end	Day	13% 5% 2 6% 4 3 4 4 6% 7% 46%	2376
September	1st to 10th	Day	14% 6% 3 5% 4 3 4 5% 4 8% 44%	2160
	11th to 20th	Day	12% 6% 2 4 5% 2 4 6% 5% 8% 46%	2142
	21st to end	Day	15% 6% 3 6% 5% 3 5% 5% 6% 8% 38%	1949
October	1st to 10th	Day	15% 7% 3 6% 5% 3 4 4 5% 8% 40%	1800
	11th to 20th	Day	20% 10% 4 6% 6% 3 4 4 5% 7% 32%	1800
	21st to end	Day	19% 11% 4 7% 5% 3 4 3 4 6% 35%	1826
November	1st to 10th	Day	13% 8% 4 6% 5% 4 5% 4 5% 6% 40%	1672
	11th to 20th	Day	15% 12% 5% 10% 6% 3 5% 4 5% 7% 28%	1577
	21st to end	Day	14% 11% 4 7% 7% 3 5% 4 5% 6% 35%	1520
December	1st to 10th	Day	11% 12% 5% 11% 9% 6% 5% 4 4 6% 26%	1520
	11th to 20th	Day	14% 12% 7% 9% 9% 5% 5% 5% 4 6% 24%	1918
	21st to end	Day	20% 20% 7% 11% 8% 3 4 3 3 4 17%	2783
January	1st to 10th	Night	11% 14% 6% 13% 7% 3 4 4 4 5% 29%	4428
	11th to 20th	Night	9% 12% 5% 13% 10% 4 7% 9% 7% 5% 19%	4440
	21st to end	Night	13% 17% 9% 13% 6% 4 4 4 4 5% 22%	4757
February	1st to 10th	Night	8% 12% 6% 10% 6% 4 6% 5% 5% 5% 34%	2470
	11th to 20th	Night	9% 8% 4 8% 5% 3 5% 6% 6% 6% 39%	2363
	21st to end	Night	14% 10% 4 7% 8% 3 5% 4 5% 6% 33%	1884
March	1st to 10th	Night	10% 7% 3 5% 5% 4 4 4 6% 7% 45%	3733
	11th to 20th	Night	9% 8% 3 7% 7% 4 6% 5% 5% 7% 40%	3548
	21st to end	Night	23% 10% 4 11% 7% 3 3 3 3 4 28%	3572
April	1st to 10th	Night	19% 10% 3 6% 5% 3 4 4 5% 7% 34%	1900
	11th to 20th	Night	21% 7% 2 5% 4 3 5% 4 5% 6% 38%	1875
	21st to end	Night	19% 8% 3 5% 6% 3 4 4 5% 7% 37%	1779
May	1st to 10th	Night	19% 9% 3 7% 6% 4 5% 4 4 7% 32%	1440
	11th to 20th	Night	18% 6% 3 5% 5% 3 4 5% 6% 6% 39%	1416
	21st to end	Night	14% 5% 3 5% 5% 3 5% 5% 6% 8% 41%	1551
June	1st to 10th	Night	12% 6% 3 4 4 3 4 5% 6% 7% 46%	1440
	11th to 20th	Night	12% 4 3 6% 6% 3 5% 6% 6% 8% 41%	1440
	21st to end	Night	9% 5% 2 4 5% 3 5% 5% 6% 8% 48%	1440
July	1st to 10th	Night	7% 4 2 3 3 3 4 5% 6% 9% 55%	1440
	11th to 20th	Night	8% 4 3 4 4 3 4 5% 6% 9% 50%	1440
	21st to end	Night	4 4 2 3 4 3 4 3 6% 8% 58%	1575
August	1st to 10th	Night	8% 4 2 4 4 3 5% 6% 5% 7% 51%	1459
	11th to 20th	Night	11% 5% 2 5% 4 3 5% 4 4 6% 51%	1640
	21st to end	Night	12% 6% 2 3 3 3 4 4 6% 6% 51%	1980
September	1st to 10th	Night	10% 5% 3 5% 4 3 3 4 5% 8% 50%	1800
	11th to 20th	Night	11% 5% 2 4 5% 2 3 4 5% 7% 52%	1800
	21st to end	Night	11% 7% 3 5% 4 3 4 4 5% 7% 48%	2006
October	1st to 10th	Night	13% 6% 2 5% 4 3 4 5% 4 6% 48%	2160
	11th to 20th	Night	16% 10% 3 6% 4 3 4 5% 5% 7% 37%	2160
	21st to end	Night	17% 11% 3 6% 5% 3 5% 4 5% 6% 34%	2529
November	1st to 10th	Night	14% 8% 4 6% 5% 3 4 4 4 7% 41%	2406
	11th to 20th	Night	13% 11% 5% 9% 5% 4 4 5% 4 6% 33%	2603
	21st to end	Night	14% 10% 4 8% 6% 4 4 4 5% 7% 34%	2660
December	1st to 10th	Night	11% 12% 5% 8% 7% 4 6% 5% 5% 7% 30%	2660
	11th to 20th	Night	12% 14% 6% 10% 8% 4 5% 5% 5% 5% 26%	3279
	21st to end	Night	22% 16% 6% 10% 6% 3 4 3 4 4 21%	4884

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	63% 6% 10% 15% 5% 1	2528
	11th to 20th	Day	64% 7% 9% 13% 6% 1	2530
	21st to end	Day	66% 5% 12% 13% 4	2893
February	1st to 10th	Day	51% 6% 12% 20% 10% 1	1710
	11th to 20th	Day	47% 8% 12% 23% 9% 1	1712
	21st to end	Day	55% 6% 11% 17% 10% 2	1570
March	1st to 10th	Day	39% 4 12% 23% 19% 2	3120
	11th to 20th	Day	41% 6% 12% 26% 14% 1	3442
	21st to end	Day	59% 5% 8% 19% 7% 1	4059
April	1st to 10th	Day	53% 6% 10% 19% 11% 1	2280
	11th to 20th	Day	53% 5% 8% 20% 11% 2	2254
	21st to end	Day	54% 7% 10% 18% 11% 1	2276
May	1st to 10th	Day	49% 7% 14% 22% 9%	2520
	11th to 20th	Day	48% 7% 11% 23% 10% 1	2483
	21st to end	Day	44% 7% 12% 23% 12% 2	2725
June	1st to 10th	Day	37% 8% 13% 29% 12% 1	2520
	11th to 20th	Day	41% 8% 13% 24% 13% 1	2520
	21st to end	Day	40% 7% 16% 25% 10% 2	2520
July	1st to 10th	Day	31% 10% 15% 31% 12% 1	2451
	11th to 20th	Day	34% 9% 15% 25% 15% 2	2445
	21st to end	Day	30% 9% 17% 27% 16% 2	2591
August	1st to 10th	Day	37% 8% 15% 28% 9% 2	2462
	11th to 20th	Day	41% 7% 13% 27% 11%	2259
	21st to end	Day	36% 6% 13% 28% 16% 1	2376
September	1st to 10th	Day	39% 7% 12% 27% 14%	2160
	11th to 20th	Day	35% 8% 18% 30% 8% 1	2142
	21st to end	Day	43% 8% 12% 27% 9%	1949
October	1st to 10th	Day	43% 5% 11% 31% 10%	1800
	11th to 20th	Day	51% 7% 13% 20% 9% 1	1800
	21st to end	Day	51% 5% 14% 21% 9%	1826
November	1st to 10th	Day	44% 7% 11% 22% 14% 2	1672
	11th to 20th	Day	56% 6% 12% 19% 7% 1	1577
	21st to end	Day	50% 5% 15% 22% 7% 1	1520
December	1st to 10th	Day	59% 6% 12% 17% 5%	1520
	11th to 20th	Day	60% 6% 12% 13% 7% 2	1915
	21st to end	Day	72% 4 6% 13% 5%	2783
January	1st to 10th	Night	57% 6% 9% 20% 7% 1	4428
	11th to 20th	Night	62% 7% 10% 14% 6%	4440
	21st to end	Night	65% 6% 8% 16% 5% 1	4757
February	1st to 10th	Night	51% 4 11% 21% 11% 2	2470
	11th to 20th	Night	43% 7% 12% 26% 10% 2	2364
	21st to end	Night	51% 5% 12% 20% 10% 2	1884
March	1st to 10th	Night	37% 6% 13% 28% 12% 3	3733
	11th to 20th	Night	41% 7% 10% 25% 15% 2	3548
	21st to end	Night	61% 4 7% 19% 8% 1	3572
April	1st to 10th	Night	50% 6% 7% 18% 17% 2	1900
	11th to 20th	Night	47% 5% 8% 22% 15% 3	1875
	21st to end	Night	45% 7% 10% 22% 13% 2	1779
May	1st to 10th	Night	52% 7% 11% 16% 13% 1	1440
	11th to 20th	Night	44% 7% 10% 26% 12% 2	1417
	21st to end	Night	39% 8% 15% 21% 16% 1	1552
June	1st to 10th	Night	35% 8% 14% 28% 15% 1	1440
	11th to 20th	Night	38% 9% 12% 25% 14% 2	1440
	21st to end	Night	32% 8% 14% 30% 12% 3	1440
July	1st to 10th	Night	25% 7% 13% 34% 20% 2	1400
	11th to 20th	Night	28% 8% 16% 26% 19% 3	1392
	21st to end	Night	23% 6% 16% 32% 17% 4	1487
August	1st to 10th	Night	30% 7% 17% 28% 16% 3	1450
	11th to 20th	Night	35% 7% 13% 26% 18% 1	1640
	21st to end	Night	32% 7% 15% 26% 17% 3	1980
September	1st to 10th	Night	33% 7% 12% 29% 18% 1	1800
	11th to 20th	Night	32% 6% 13% 33% 15%	1800
	21st to end	Night	36% 6% 13% 32% 12% 1	2006
October	1st to 10th	Night	36% 8% 9% 32% 13% 2	2160
	11th to 20th	Night	44% 8% 13% 23% 11% 1	2160
	21st to end	Night	51% 6% 11% 21% 11%	2529
November	1st to 10th	Night	44% 5% 12% 24% 15% 1	2406
	11th to 20th	Night	53% 6% 13% 16% 11% 1	2603
	21st to end	Night	50% 6% 14% 23% 7% 1	2660
December	1st to 10th	Night	53% 8% 13% 20% 5%	2660
	11th to 20th	Night	60% 6% 11% 17% 6% 1	3281
	21st to end	Night	67% 5% 7% 16% 4	4884

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	19% 11% 31% 25% 12% 3	270
	11th to 20th	Day	17% 11% 40% 25% 6% 1	270
	21st to end	Day	18% 9% 35% 28% 9% 1	297
February	1st to 10th	Day	14% 14% 46% 16% 8% 3	270
	11th to 20th	Day	16% 17% 40% 18% 9%	270
	21st to end	Day	16% 12% 45% 25% 2	225
March	1st to 10th	Day	17% 13% 41% 18% 11% 1	270
	11th to 20th	Day	19% 13% 34% 21% 11% 2	270
	21st to end	Day	23% 13% 33% 20% 9% 3	439
April	1st to 10th	Day	25% 14% 32% 23% 6% 1	460
	11th to 20th	Day	28% 13% 28% 21% 8% 2	452
	21st to end	Day	24% 12% 29% 23% 11% 2	450
May	1st to 10th	Day	20% 14% 31% 25% 8% 1	603
	11th to 20th	Day	19% 12% 28% 25% 11% 4	625
	21st to end	Day	17% 9% 26% 26% 14% 7%	679
June	1st to 10th	Day	12% 7% 28% 29% 18% 6%	630
	11th to 20th	Day	15% 8% 32% 26% 15% 5%	630
	21st to end	Day	13% 6% 28% 30% 16% 7%	630
July	1st to 10th	Day	8% 3 19% 31% 27% 12%	621
	11th to 20th	Day	9% 6% 28% 32% 18% 8%	618
	21st to end	Day	6% 3 27% 33% 23% 8%	684
August	1st to 10th	Day	10% 4 24% 32% 23% 8%	626
	11th to 20th	Day	15% 5% 19% 24% 28% 9%	573
	21st to end	Day	15% 6% 22% 31% 20% 7%	495
September	1st to 10th	Day	18% 7% 22% 26% 21% 7%	450
	11th to 20th	Day	13% 6% 24% 33% 19% 4	450
	21st to end	Day	17% 10% 25% 32% 14% 2	419
October	1st to 10th	Day	17% 11% 27% 25% 14% 6%	270
	11th to 20th	Day	24% 16% 32% 19% 8% 1	270
	21st to end	Day	27% 13% 36% 19% 4 1	297
November	1st to 10th	Day	19% 11% 31% 27% 10% 2	270
	11th to 20th	Day	18% 16% 38% 21% 6% 1	270
	21st to end	Day	21% 12% 42% 16% 9%	270
December	1st to 10th	Day	15% 14% 42% 22% 5% 1	270
	11th to 20th	Day	20% 14% 37% 22% 6%	270
	21st to end	Day	29% 14% 35% 18% 4	297
January	1st to 10th	Night	15% 11% 36% 29% 8% 1	950
	11th to 20th	Night	16% 10% 37% 29% 6% 1	950
	21st to end	Night	18% 9% 36% 30% 7%	1045
February	1st to 10th	Night	13% 12% 37% 30% 7% 1	950
	11th to 20th	Night	15% 12% 40% 25% 8% 1	889
	21st to end	Night	20% 12% 37% 27% 4	628
March	1st to 10th	Night	16% 12% 35% 28% 7% 1	760
	11th to 20th	Night	16% 11% 36% 28% 8% 1	612
	21st to end	Night	22% 9% 30% 27% 9% 2	627
April	1st to 10th	Night	22% 11% 38% 23% 5% 1	570
	11th to 20th	Night	25% 9% 30% 29% 7%	563
	21st to end	Night	22% 7% 33% 30% 7% 1	555
May	1st to 10th	Night	22% 7% 34% 30% 6% 1	540
	11th to 20th	Night	21% 8% 30% 31% 9% 1	531
	21st to end	Night	15% 4 27% 38% 16% 1	584
June	1st to 10th	Night	14% 6% 26% 39% 13% 1	540
	11th to 20th	Night	14% 4 31% 39% 10%	540
	21st to end	Night	10% 5% 21% 44% 18% 2	540
July	1st to 10th	Night	7% 2 21% 37% 31% 2	525
	11th to 20th	Night	9% 4 21% 39% 24% 2	522
	21st to end	Night	5% 2 18% 50% 22% 3	591
August	1st to 10th	Night	8% 2 16% 47% 25% 2	537
	11th to 20th	Night	13% 3 19% 33% 29% 3	534
	21st to end	Night	14% 3 20% 38% 22% 3	594
September	1st to 10th	Night	13% 4 22% 39% 18% 4	540
	11th to 20th	Night	12% 4 27% 35% 19% 3	540
	21st to end	Night	13% 7% 31% 34% 14% 1	720
October	1st to 10th	Night	16% 7% 29% 29% 18% 1	720
	11th to 20th	Night	19% 9% 34% 29% 8%	720
	21st to end	Night	23% 8% 33% 27% 8% 1	792
November	1st to 10th	Night	19% 10% 32% 29% 9% 1	740
	11th to 20th	Night	18% 12% 41% 23% 7%	893
	21st to end	Night	19% 14% 36% 26% 6%	950
December	1st to 10th	Night	17% 11% 39% 25% 7%	950
	11th to 20th	Night	20% 12% 34% 26% 7% 1	950
	21st to end	Night	27% 11% 32% 25% 5%	1045

Page generated at 2024-04-22 08:25:43. Version ac07

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