Roberval
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Roberval clear sky chart from 2005-11-06 to 2026-03-18.

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	15% 11% 4 10% 6% 6% 5% 4 6% 7% 27%	131013
	Night	15% 13% 5% 11% 7% 6% 5% 4 5% 6% 25%	130517
Transparency	Day	56% 5% 13% 18% 8% 1	134430
	Night	60% 5% 11% 17% 6%	135188
Seeing	Day	21% 10% 28% 26% 13% 3	16829
	Night	24% 10% 28% 27% 10% 1	25412

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	12% 19% 7% 15% 6% 6% 6% 4 5% 5% 15%	4622
	11th to 20th	Day	8% 16% 7% 19% 10% 7% 7% 4 5% 4 13%	4650
	21st to end	Day	5% 7% 4 17% 8% 7% 8% 6% 7% 6% 24%	5124
February	1st to 10th	Day	6% 12% 5% 12% 10% 9% 8% 4 6% 5% 22%	1680
	11th to 20th	Day	11% 16% 6% 13% 8% 7% 6% 4 5% 5% 20%	1940
	21st to end	Day	9% 14% 5% 12% 7% 5% 6% 5% 6% 6% 25%	1730
March	1st to 10th	Day	13% 18% 5% 10% 5% 4 4 3 5% 6% 27%	4576
	11th to 20th	Day	14% 15% 5% 12% 6% 6% 6% 4 5% 6% 22%	4724
	21st to end	Day	12% 13% 4 9% 5% 5% 5% 4 6% 7% 31%	5942
April	1st to 10th	Day	17% 15% 5% 12% 6% 6% 4 3 3 4 25%	2400
	11th to 20th	Day	17% 12% 3 7% 4 4 4 3 5% 6% 34%	2461
	21st to end	Day	20% 12% 4 7% 5% 4 4 3 5% 6% 29%	2600
May	1st to 10th	Day	19% 11% 3 8% 5% 5% 5% 4 6% 6% 29%	6317
	11th to 20th	Day	23% 8% 3 8% 5% 5% 5% 3 5% 6% 30%	6318
	21st to end	Day	16% 9% 3 8% 5% 5% 5% 4 6% 6% 33%	7197
June	1st to 10th	Day	19% 8% 3 8% 5% 5% 5% 5% 5% 7% 31%	3000
	11th to 20th	Day	16% 6% 2 8% 6% 5% 5% 4 7% 8% 33%	3000
	21st to end	Day	21% 7% 3 7% 6% 5% 6% 4 7% 7% 28%	3000
July	1st to 10th	Day	17% 8% 3 7% 5% 5% 6% 5% 7% 8% 29%	6650
	11th to 20th	Day	14% 6% 3 8% 6% 6% 6% 6% 8% 9% 28%	6360
	21st to end	Day	11% 7% 3 7% 5% 5% 5% 4 7% 9% 37%	6926
August	1st to 10th	Day	14% 7% 3 9% 7% 7% 7% 5% 7% 9% 26%	4563
	11th to 20th	Day	15% 7% 3 7% 5% 6% 6% 5% 7% 9% 29%	4304
	21st to end	Day	13% 7% 4 8% 5% 5% 6% 4 6% 7% 33%	4452
September	1st to 10th	Day	16% 7% 3 7% 5% 5% 6% 4 6% 8% 34%	2400
	11th to 20th	Day	16% 7% 3 7% 5% 4 4 4 5% 7% 37%	2270
	21st to end	Day	22% 8% 3 7% 5% 5% 5% 3 5% 6% 32%	2200
October	1st to 10th	Day	17% 10% 3 8% 6% 6% 4 3 6% 7% 31%	2121
	11th to 20th	Day	23% 15% 4 10% 5% 5% 5% 4 5% 6% 19%	1848
	21st to end	Day	24% 15% 4 9% 4 5% 4 3 4 6% 22%	1980
November	1st to 10th	Day	18% 16% 4 10% 6% 6% 5% 3 4 4 23%	1851
	11th to 20th	Day	18% 17% 5% 11% 7% 5% 4 4 5% 4 21%	1746
	21st to end	Day	20% 19% 5% 12% 6% 6% 5% 4 4 6% 14%	1470
December	1st to 10th	Day	15% 17% 7% 13% 7% 5% 4 3 5% 5% 20%	2350
	11th to 20th	Day	18% 13% 5% 13% 9% 8% 4 2 4 5% 20%	2688
	21st to end	Day	15% 14% 6% 13% 9% 7% 5% 3 5% 5% 19%	3553
January	1st to 10th	Night	12% 22% 8% 14% 8% 6% 5% 3 4 4 13%	8259
	11th to 20th	Night	8% 20% 8% 20% 10% 6% 4 3 3 4 12%	8235
	21st to end	Night	5% 8% 5% 16% 10% 9% 9% 6% 7% 7% 21%	9057
February	1st to 10th	Night	9% 13% 5% 12% 8% 7% 6% 5% 5% 6% 26%	2940
	11th to 20th	Night	9% 17% 6% 14% 9% 6% 6% 3 4 4 21%	2560
	21st to end	Night	9% 15% 5% 12% 7% 5% 6% 3 6% 6% 27%	2076
March	1st to 10th	Night	17% 14% 5% 11% 7% 5% 7% 5% 5% 6% 20%	5484
	11th to 20th	Night	13% 15% 5% 12% 7% 6% 5% 4 5% 6% 21%	5389
	21st to end	Night	14% 13% 4 10% 6% 4 5% 4 5% 5% 29%	4973
April	1st to 10th	Night	19% 15% 4 9% 5% 5% 5% 3 4 5% 25%	2000
	11th to 20th	Night	18% 11% 3 7% 5% 6% 4 3 5% 6% 33%	1887
	21st to end	Night	20% 11% 3 8% 5% 5% 4 3 4 5% 32%	1800
May	1st to 10th	Night	18% 9% 3 7% 5% 5% 5% 3 5% 5% 35%	3653
	11th to 20th	Night	23% 9% 3 8% 5% 5% 4 4 5% 6% 28%	3575
	21st to end	Night	21% 8% 3 7% 4 4 4 3 5% 6% 33%	3689
June	1st to 10th	Night	19% 7% 3 8% 4 5% 5% 6% 5% 8% 31%	1400
	11th to 20th	Night	15% 8% 3 6% 6% 5% 5% 4 6% 7% 36%	1400
	21st to end	Night	18% 7% 3 8% 5% 4 5% 4 6% 8% 32%	1400
July	1st to 10th	Night	12% 6% 3 9% 6% 6% 7% 6% 8% 8% 29%	3306
	11th to 20th	Night	13% 7% 3 8% 5% 6% 6% 5% 8% 9% 32%	3600
	21st to end	Night	12% 5% 3 6% 5% 5% 5% 4 6% 8% 42%	3926
August	1st to 10th	Night	13% 7% 3 7% 5% 6% 6% 5% 7% 9% 34%	2586
	11th to 20th	Night	17% 7% 2 7% 5% 5% 4 4 6% 9% 34%	2710
	21st to end	Night	15% 7% 3 7% 4 5% 5% 4 7% 7% 38%	3444
September	1st to 10th	Night	16% 7% 2 6% 3 6% 4 3 5% 7% 40%	2000
	11th to 20th	Night	17% 8% 3 7% 5% 4 4 3 5% 5% 40%	2130
	21st to end	Night	19% 7% 2 7% 4 5% 5% 4 4 5% 38%	2200
October	1st to 10th	Night	19% 9% 3 6% 4 4 5% 3 6% 6% 35%	2275
	11th to 20th	Night	25% 12% 3 7% 5% 5% 5% 3 5% 6% 25%	2551
	21st to end	Night	26% 13% 3 7% 4 5% 5% 4 5% 6% 21%	2860
November	1st to 10th	Night	20% 14% 5% 10% 4 5% 4 4 5% 6% 24%	2667
	11th to 20th	Night	18% 17% 5% 11% 6% 5% 4 4 5% 5% 21%	2867
	21st to end	Night	19% 18% 5% 13% 7% 5% 4 4 5% 5% 15%	3150
December	1st to 10th	Night	13% 15% 8% 18% 9% 5% 4 3 4 4 15%	5060
	11th to 20th	Night	19% 15% 5% 12% 8% 5% 5% 4 5% 4 19%	5741
	21st to end	Night	14% 17% 7% 11% 7% 7% 4 3 4 4 21%	7667

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	71% 5% 10% 12% 2	4623
	11th to 20th	Day	72% 6% 9% 9% 5%	4680
	21st to end	Day	57% 5% 10% 19% 8%	5124
February	1st to 10th	Day	63% 4 9% 16% 8% 1	1680
	11th to 20th	Day	66% 3 8% 14% 8%	1949
	21st to end	Day	59% 4 9% 18% 8% 2	1730
March	1st to 10th	Day	58% 3 11% 14% 11% 2	4579
	11th to 20th	Day	62% 4 12% 14% 8%	4724
	21st to end	Day	51% 4 9% 17% 17% 2	5942
April	1st to 10th	Day	63% 3 9% 14% 9% 2	2400
	11th to 20th	Day	52% 3 9% 22% 13% 2	2461
	21st to end	Day	56% 2 10% 18% 13% 1	2600
May	1st to 10th	Day	53% 5% 12% 16% 13% 1	6328
	11th to 20th	Day	55% 5% 10% 19% 10% 1	6322
	21st to end	Day	50% 4 11% 20% 14% 1	7197
June	1st to 10th	Day	51% 4 10% 22% 11% 1	3000
	11th to 20th	Day	46% 4 14% 22% 12% 2	3000
	21st to end	Day	52% 5% 12% 20% 10% 1	3000
July	1st to 10th	Day	48% 6% 14% 28% 3	6578
	11th to 20th	Day	48% 8% 18% 22% 4	6285
	21st to end	Day	43% 6% 18% 26% 8%	6728
August	1st to 10th	Day	51% 7% 19% 21% 2	4549
	11th to 20th	Day	47% 7% 22% 21% 4	4346
	21st to end	Day	48% 6% 15% 21% 10%	4452
September	1st to 10th	Day	48% 5% 18% 22% 7%	2400
	11th to 20th	Day	45% 5% 19% 24% 7%	2270
	21st to end	Day	54% 5% 15% 19% 7%	2200
October	1st to 10th	Day	53% 6% 13% 23% 5%	3459
	11th to 20th	Day	65% 4 14% 14% 2	2986
	21st to end	Day	60% 7% 16% 14% 3	3180
November	1st to 10th	Day	67% 4 11% 13% 5% 1	1851
	11th to 20th	Day	69% 4 10% 13% 4	1746
	21st to end	Day	73% 6% 10% 8% 3	1470
December	1st to 10th	Day	71% 6% 9% 12% 2	2350
	11th to 20th	Day	69% 4 7% 14% 6%	2688
	21st to end	Day	69% 5% 7% 14% 5%	3553
January	1st to 10th	Night	76% 4 8% 9% 2	8259
	11th to 20th	Night	77% 4 6% 9% 4	8280
	21st to end	Night	61% 5% 11% 16% 6%	9057
February	1st to 10th	Night	60% 4 9% 18% 9%	2940
	11th to 20th	Night	67% 4 9% 13% 7%	2562
	21st to end	Night	59% 4 10% 19% 8%	2076
March	1st to 10th	Night	66% 3 9% 14% 8%	5494
	11th to 20th	Night	63% 5% 11% 13% 7% 1	5390
	21st to end	Night	56% 4 8% 22% 9% 1	4973
April	1st to 10th	Night	63% 3 8% 16% 8% 2	2000
	11th to 20th	Night	53% 4 8% 23% 12% 1	1887
	21st to end	Night	56% 2 9% 21% 11% 2	1800
May	1st to 10th	Night	51% 5% 9% 20% 14% 1	3659
	11th to 20th	Night	54% 6% 10% 19% 10% 1	3576
	21st to end	Night	52% 5% 11% 15% 14% 3	3690
June	1st to 10th	Night	50% 6% 11% 23% 10% 1	1400
	11th to 20th	Night	47% 5% 13% 21% 12% 3	1400
	21st to end	Night	51% 6% 12% 21% 11%	1400
July	1st to 10th	Night	48% 7% 14% 24% 7%	3268
	11th to 20th	Night	46% 7% 20% 23% 3	3552
	21st to end	Night	40% 6% 17% 28% 9%	3809
August	1st to 10th	Night	46% 8% 18% 22% 7%	2576
	11th to 20th	Night	46% 6% 23% 20% 4	2734
	21st to end	Night	44% 6% 17% 25% 8%	3444
September	1st to 10th	Night	44% 6% 16% 25% 9%	2000
	11th to 20th	Night	47% 6% 17% 25% 6%	2130
	21st to end	Night	49% 6% 13% 25% 8%	2200
October	1st to 10th	Night	50% 6% 15% 23% 5%	3717
	11th to 20th	Night	60% 4 14% 17% 5%	4152
	21st to end	Night	62% 7% 12% 13% 5%	4610
November	1st to 10th	Night	64% 4 13% 14% 5%	2667
	11th to 20th	Night	67% 4 11% 13% 4	2867
	21st to end	Night	74% 5% 9% 9% 3	3150
December	1st to 10th	Night	75% 5% 9% 9% 2	5060
	11th to 20th	Night	70% 5% 6% 15% 3	5742
	21st to end	Night	69% 4 8% 14% 4	7667

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	25% 10% 32% 20% 12% 2	220
	11th to 20th	Day	13% 11% 27% 30% 16% 1	220
	21st to end	Day	13% 6% 26% 32% 19% 2	242
February	1st to 10th	Day	9% 14% 37% 23% 14% 4	220
	11th to 20th	Day	20% 12% 34% 27% 7% 1	435
	21st to end	Day	16% 12% 38% 24% 8% 2	446
March	1st to 10th	Day	22% 11% 30% 25% 10% 1	540
	11th to 20th	Day	21% 13% 31% 24% 10% 2	528
	21st to end	Day	22% 11% 32% 25% 8% 2	550
April	1st to 10th	Day	24% 14% 30% 22% 9% 2	500
	11th to 20th	Day	22% 14% 25% 24% 11% 4	492
	21st to end	Day	25% 11% 31% 24% 10% 1	500
May	1st to 10th	Day	22% 11% 30% 23% 10% 4	503
	11th to 20th	Day	18% 8% 29% 27% 16% 3	527
	21st to end	Day	20% 12% 31% 26% 9% 3	678
June	1st to 10th	Day	22% 6% 29% 29% 12% 2	730
	11th to 20th	Day	19% 9% 25% 29% 16% 2	730
	21st to end	Day	23% 5% 24% 31% 14% 3	730
July	1st to 10th	Day	18% 5% 25% 35% 14% 3	657
	11th to 20th	Day	17% 7% 23% 30% 19% 4	524
	21st to end	Day	15% 5% 23% 29% 24% 4	576
August	1st to 10th	Day	14% 6% 21% 29% 23% 7%	526
	11th to 20th	Day	16% 3 20% 31% 22% 8%	519
	21st to end	Day	19% 9% 23% 24% 15% 9%	583
September	1st to 10th	Day	19% 8% 26% 27% 14% 6%	530
	11th to 20th	Day	19% 10% 21% 27% 17% 6%	530
	21st to end	Day	23% 9% 26% 21% 14% 6%	530
October	1st to 10th	Day	19% 13% 27% 22% 13% 5%	530
	11th to 20th	Day	29% 15% 26% 18% 9% 3	445
	21st to end	Day	33% 10% 28% 20% 8% 1	462
November	1st to 10th	Day	31% 16% 32% 15% 4 1	425
	11th to 20th	Day	28% 17% 27% 19% 8% 1	299
	21st to end	Day	29% 15% 31% 15% 8% 2	220
December	1st to 10th	Day	22% 6% 30% 26% 14% 2	220
	11th to 20th	Day	22% 15% 33% 20% 8% 2	220
	21st to end	Day	23% 13% 28% 26% 9% 1	242
January	1st to 10th	Night	25% 11% 29% 26% 8% 1	845
	11th to 20th	Night	23% 11% 26% 29% 10% 1	840
	21st to end	Night	13% 7% 29% 35% 13% 2	924
February	1st to 10th	Night	13% 9% 32% 31% 12% 2	840
	11th to 20th	Night	19% 10% 31% 30% 9% 1	823
	21st to end	Night	18% 12% 30% 28% 11% 1	692
March	1st to 10th	Night	20% 12% 32% 28% 8%	840
	11th to 20th	Night	23% 12% 29% 25% 9% 1	829
	21st to end	Night	21% 13% 30% 28% 7% 1	880
April	1st to 10th	Night	28% 9% 26% 29% 7% 1	800
	11th to 20th	Night	25% 9% 29% 28% 9% 1	703
	21st to end	Night	27% 11% 31% 25% 7%	600
May	1st to 10th	Night	29% 7% 27% 29% 7%	433
	11th to 20th	Night	23% 11% 26% 29% 11% 1	394
	21st to end	Night	23% 9% 33% 24% 9% 1	433
June	1st to 10th	Night	23% 5% 31% 30% 10% 1	400
	11th to 20th	Night	17% 7% 31% 31% 13% 1	400
	21st to end	Night	19% 7% 25% 34% 13% 2	400
July	1st to 10th	Night	20% 6% 24% 37% 13%	390
	11th to 20th	Night	15% 4 26% 39% 16%	388
	21st to end	Night	17% 6% 23% 39% 13% 2	438
August	1st to 10th	Night	16% 5% 24% 34% 19% 2	398
	11th to 20th	Night	19% 3 25% 33% 18% 3	491
	21st to end	Night	21% 9% 23% 28% 15% 4	790
September	1st to 10th	Night	20% 9% 26% 31% 14% 1	800
	11th to 20th	Night	23% 9% 26% 26% 14% 3	800
	21st to end	Night	25% 7% 30% 26% 11% 2	800
October	1st to 10th	Night	23% 10% 29% 26% 10% 2	800
	11th to 20th	Night	37% 10% 26% 19% 8% 1	800
	21st to end	Night	37% 11% 25% 19% 8%	880
November	1st to 10th	Night	32% 15% 30% 17% 5% 1	820
	11th to 20th	Night	28% 18% 30% 17% 7%	846
	21st to end	Night	31% 13% 30% 19% 6% 1	950
December	1st to 10th	Night	28% 11% 27% 25% 8% 1	950
	11th to 20th	Night	23% 13% 30% 24% 9% 1	950
	21st to end	Night	25% 12% 23% 30% 9% 1	1045

Page generated at 2026-03-15 02:23:51. Version ac07

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