ROC St-Romain
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the ROC St-Romain clear sky chart from 2005-11-06 to 2026-05-17.

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	18% 12% 4 10% 6% 5% 5% 3 5% 6% 25%	131240
	Night	17% 15% 5% 12% 6% 5% 4 3 4 5% 24%	131585
Transparency	Day	60% 6% 10% 16% 7% 1	134708
	Night	63% 5% 10% 15% 7% 1	136205
Seeing	Day	28% 19% 29% 16% 6% 1	16985
	Night	26% 17% 29% 21% 7%	25773

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% 16% 10% 20% 8% 7% 7% 3 3 3 10%	4661
	11th to 20th	Day	8% 17% 9% 26% 14% 5% 3 2 2 2 11%	4650
	21st to end	Day	7% 13% 8% 16% 9% 13% 8% 4 5% 3 14%	5124
February	1st to 10th	Day	12% 19% 6% 15% 10% 7% 5% 2 4 3 19%	1726
	11th to 20th	Day	12% 20% 8% 16% 7% 6% 4 2 3 4 17%	2048
	21st to end	Day	12% 18% 6% 13% 8% 7% 5% 3 4 5% 20%	1730
March	1st to 10th	Day	17% 17% 6% 13% 7% 5% 4 2 3 4 20%	4576
	11th to 20th	Day	15% 17% 5% 11% 5% 4 4 3 4 5% 26%	4888
	21st to end	Day	21% 16% 5% 11% 5% 4 3 3 3 4 25%	5906
April	1st to 10th	Day	22% 19% 4 10% 6% 4 3 2 3 4 24%	2520
	11th to 20th	Day	19% 16% 3 6% 3 4 4 3 4 5% 32%	2586
	21st to end	Day	23% 14% 4 7% 4 3 3 3 4 6% 27%	2730
May	1st to 10th	Day	28% 10% 4 9% 5% 5% 6% 3 4 5% 21%	6016
	11th to 20th	Day	22% 9% 3 8% 5% 5% 5% 4 6% 6% 28%	6402
	21st to end	Day	20% 10% 3 7% 5% 5% 5% 4 6% 7% 28%	6949
June	1st to 10th	Day	28% 9% 2 6% 4 4 5% 4 5% 7% 26%	2910
	11th to 20th	Day	20% 7% 2 6% 5% 5% 5% 4 7% 8% 31%	3000
	21st to end	Day	24% 9% 4 7% 6% 5% 5% 4 5% 6% 26%	2907
July	1st to 10th	Day	18% 9% 3 9% 6% 6% 7% 5% 7% 7% 24%	6360
	11th to 20th	Day	16% 7% 3 7% 5% 5% 5% 4 7% 9% 32%	6360
	21st to end	Day	16% 7% 2 5% 4 4 5% 4 6% 8% 40%	6926
August	1st to 10th	Day	20% 6% 3 8% 5% 5% 6% 5% 7% 9% 26%	4471
	11th to 20th	Day	20% 9% 3 7% 5% 5% 5% 5% 7% 8% 26%	4175
	21st to end	Day	22% 7% 2 6% 4 4 5% 4 5% 7% 35%	4449
September	1st to 10th	Day	19% 7% 2 6% 4 4 4 3 5% 6% 39%	2400
	11th to 20th	Day	20% 7% 2 5% 3 3 4 3 5% 7% 42%	2209
	21st to end	Day	20% 8% 3 6% 4 4 4 4 6% 7% 35%	2200
October	1st to 10th	Day	22% 10% 3 6% 4 4 4 4 5% 7% 32%	2200
	11th to 20th	Day	20% 16% 4 8% 5% 5% 3 4 5% 5% 25%	1857
	21st to end	Day	25% 20% 4 7% 3 4 4 3 4 4 21%	1980
November	1st to 10th	Day	20% 21% 5% 7% 4 3 4 3 4 4 26%	1851
	11th to 20th	Day	17% 21% 7% 11% 5% 3 4 2 3 5% 22%	1890
	21st to end	Day	18% 21% 7% 13% 5% 4 3 3 4 3 17%	1653
December	1st to 10th	Day	16% 21% 9% 17% 9% 4 5% 2 3 3 11%	2350
	11th to 20th	Day	17% 18% 8% 14% 6% 5% 3 2 3 4 20%	2688
	21st to end	Day	18% 19% 5% 15% 5% 4 5% 3 4 5% 17%	3892
January	1st to 10th	Night	11% 18% 9% 21% 9% 7% 5% 3 3 3 11%	8234
	11th to 20th	Night	9% 21% 10% 23% 12% 6% 4 2 3 2 8%	8235
	21st to end	Night	6% 13% 7% 17% 11% 12% 7% 5% 4 4 14%	9057
February	1st to 10th	Night	11% 17% 6% 15% 9% 6% 5% 3 3 4 21%	2888
	11th to 20th	Night	12% 18% 7% 16% 9% 6% 5% 2 3 3 19%	2458
	21st to end	Night	12% 19% 7% 11% 6% 5% 5% 4 4 4 24%	2076
March	1st to 10th	Night	19% 17% 5% 13% 6% 4 6% 2 3 4 21%	5484
	11th to 20th	Night	15% 17% 6% 11% 5% 5% 4 3 4 5% 24%	5289
	21st to end	Night	18% 17% 6% 11% 6% 3 3 2 4 5% 26%	5265
April	1st to 10th	Night	22% 17% 5% 7% 4 3 4 2 4 4 27%	2100
	11th to 20th	Night	20% 14% 3 7% 4 4 4 3 4 5% 31%	1983
	21st to end	Night	22% 13% 3 7% 4 4 4 3 5% 5% 30%	1890
May	1st to 10th	Night	28% 10% 3 9% 5% 5% 4 3 4 6% 23%	4144
	11th to 20th	Night	25% 8% 3 8% 5% 4 3 3 4 5% 31%	3652
	21st to end	Night	21% 8% 3 6% 4 4 4 4 5% 7% 33%	3926
June	1st to 10th	Night	27% 9% 3 6% 5% 4 4 3 5% 6% 27%	1485
	11th to 20th	Night	21% 6% 3 6% 4 4 4 4 6% 8% 34%	1400
	21st to end	Night	22% 8% 3 7% 5% 5% 4 5% 5% 7% 30%	1487
July	1st to 10th	Night	18% 10% 3 8% 5% 5% 5% 3 6% 8% 28%	3600
	11th to 20th	Night	21% 7% 2 7% 4 4 4 4 5% 7% 35%	3600
	21st to end	Night	15% 6% 3 7% 4 4 5% 4 6% 7% 38%	3926
August	1st to 10th	Night	24% 9% 3 5% 4 4 4 3 5% 7% 32%	2647
	11th to 20th	Night	18% 9% 3 8% 4 6% 6% 4 5% 7% 30%	2872
	21st to end	Night	19% 7% 2 7% 5% 4 4 3 4 5% 39%	3445
September	1st to 10th	Night	18% 7% 2 6% 4 4 3 4 6% 6% 39%	2000
	11th to 20th	Night	18% 5% 2 5% 3 4 3 3 5% 6% 46%	2190
	21st to end	Night	20% 7% 2 5% 4 4 3 3 5% 6% 41%	2200
October	1st to 10th	Night	20% 9% 2 5% 4 4 4 4 5% 6% 38%	2200
	11th to 20th	Night	21% 14% 3 7% 5% 4 4 3 4 5% 29%	2540
	21st to end	Night	25% 17% 3 7% 4 3 3 3 4 5% 25%	2860
November	1st to 10th	Night	21% 18% 5% 8% 5% 4 4 2 5% 4 25%	2667
	11th to 20th	Night	18% 17% 7% 10% 5% 3 3 3 4 5% 24%	2730
	21st to end	Night	17% 22% 7% 13% 6% 4 3 2 4 4 19%	2961
December	1st to 10th	Night	15% 21% 12% 20% 8% 4 3 2 3 3 10%	5060
	11th to 20th	Night	17% 20% 6% 16% 8% 4 3 2 3 3 16%	5741
	21st to end	Night	18% 21% 5% 10% 5% 4 4 2 3 4 24%	7293

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	80% 3 6% 7% 2	4662
	11th to 20th	Day	83% 2 3 7% 3 1	4680
	21st to end	Day	74% 3 6% 7% 9% 1	5124
February	1st to 10th	Day	72% 4 4 11% 7% 1	1726
	11th to 20th	Day	73% 3 7% 10% 5% 1	2057
	21st to end	Day	67% 5% 7% 13% 6% 2	1730
March	1st to 10th	Day	69% 5% 6% 9% 8% 3	4579
	11th to 20th	Day	61% 5% 13% 12% 9% 1	4888
	21st to end	Day	64% 4 5% 13% 12% 1	5906
April	1st to 10th	Day	65% 4 7% 14% 8% 2	2520
	11th to 20th	Day	56% 4 8% 18% 12% 1	2586
	21st to end	Day	59% 5% 7% 17% 10% 2	2730
May	1st to 10th	Day	64% 7% 8% 15% 5% 1	6027
	11th to 20th	Day	54% 8% 15% 15% 6% 2	6406
	21st to end	Day	53% 8% 12% 18% 9% 1	6949
June	1st to 10th	Day	56% 7% 10% 20% 7% 1	2910
	11th to 20th	Day	49% 7% 13% 21% 9% 1	3000
	21st to end	Day	58% 7% 10% 17% 7%	2907
July	1st to 10th	Day	55% 7% 15% 17% 5%	6290
	11th to 20th	Day	47% 7% 15% 26% 5%	6285
	21st to end	Day	43% 6% 14% 29% 7%	6728
August	1st to 10th	Day	53% 7% 17% 19% 4	4457
	11th to 20th	Day	54% 8% 16% 17% 5%	4214
	21st to end	Day	48% 6% 14% 18% 11% 2	4449
September	1st to 10th	Day	45% 7% 14% 24% 9%	2400
	11th to 20th	Day	43% 5% 11% 27% 12%	2209
	21st to end	Day	47% 7% 11% 24% 11%	2200
October	1st to 10th	Day	51% 6% 14% 23% 7%	3590
	11th to 20th	Day	57% 5% 11% 23% 4	2995
	21st to end	Day	58% 3 14% 17% 7%	3180
November	1st to 10th	Day	63% 5% 9% 16% 6%	1851
	11th to 20th	Day	68% 5% 8% 12% 7% 1	1890
	21st to end	Day	72% 5% 10% 11% 2	1653
December	1st to 10th	Day	79% 4 7% 6% 4	2350
	11th to 20th	Day	70% 4 6% 13% 7%	2688
	21st to end	Day	71% 3 6% 9% 9% 1	3892
January	1st to 10th	Night	80% 4 7% 6% 3	8234
	11th to 20th	Night	84% 4 4 5% 3	8280
	21st to end	Night	72% 4 6% 8% 9% 1	9057
February	1st to 10th	Night	68% 5% 7% 13% 7% 1	2888
	11th to 20th	Night	71% 4 7% 10% 7% 1	2460
	21st to end	Night	64% 5% 6% 15% 9% 1	2076
March	1st to 10th	Night	69% 4 7% 11% 7% 2	5494
	11th to 20th	Night	63% 6% 12% 12% 7% 1	5290
	21st to end	Night	64% 3 6% 15% 12% 1	5265
April	1st to 10th	Night	62% 3 8% 15% 11% 1	2100
	11th to 20th	Night	55% 6% 9% 20% 10% 1	1983
	21st to end	Night	55% 6% 10% 17% 10% 2	1890
May	1st to 10th	Night	62% 6% 12% 13% 7% 1	4150
	11th to 20th	Night	55% 6% 13% 21% 5% 1	3653
	21st to end	Night	50% 7% 15% 15% 13%	3927
June	1st to 10th	Night	57% 6% 11% 19% 7%	1485
	11th to 20th	Night	48% 7% 12% 22% 11% 2	1400
	21st to end	Night	54% 8% 11% 19% 9%	1487
July	1st to 10th	Night	53% 7% 17% 18% 5%	3560
	11th to 20th	Night	48% 7% 16% 23% 6%	3552
	21st to end	Night	44% 7% 16% 26% 7%	3809
August	1st to 10th	Night	52% 7% 15% 22% 4	2637
	11th to 20th	Night	53% 9% 16% 16% 6%	2899
	21st to end	Night	47% 5% 13% 21% 13%	3445
September	1st to 10th	Night	45% 5% 15% 25% 10%	2000
	11th to 20th	Night	39% 7% 14% 30% 10% 1	2190
	21st to end	Night	44% 7% 12% 25% 11% 1	2200
October	1st to 10th	Night	49% 5% 11% 24% 10%	3590
	11th to 20th	Night	56% 5% 11% 24% 5%	4141
	21st to end	Night	56% 4 13% 17% 8% 1	4610
November	1st to 10th	Night	63% 5% 10% 15% 6%	2667
	11th to 20th	Night	63% 5% 9% 13% 8% 1	2730
	21st to end	Night	70% 4 11% 11% 4	2961
December	1st to 10th	Night	83% 3 6% 6% 2	5060
	11th to 20th	Night	75% 4 6% 11% 5%	5742
	21st to end	Night	66% 4 7% 13% 9%	7293

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	30% 26% 32% 11%	220
	11th to 20th	Day	32% 25% 30% 10% 1 1	220
	21st to end	Day	25% 29% 32% 13% 1 1	242
February	1st to 10th	Day	30% 30% 26% 10% 4	264
	11th to 20th	Day	28% 32% 30% 8% 1	536
	21st to end	Day	30% 25% 30% 13% 1	446
March	1st to 10th	Day	28% 29% 30% 11% 3	540
	11th to 20th	Day	32% 28% 25% 11% 4 1	540
	21st to end	Day	36% 23% 24% 11% 5% 1	594
April	1st to 10th	Day	40% 19% 26% 11% 4 1	540
	11th to 20th	Day	38% 21% 24% 11% 4 1	532
	21st to end	Day	35% 21% 28% 10% 5% 1	540
May	1st to 10th	Day	35% 20% 29% 13% 3 1	543
	11th to 20th	Day	28% 18% 29% 19% 5% 1	551
	21st to end	Day	23% 22% 32% 17% 5%	573
June	1st to 10th	Day	32% 17% 30% 15% 6%	640
	11th to 20th	Day	24% 16% 28% 23% 9%	730
	21st to end	Day	27% 16% 31% 17% 8% 1	637
July	1st to 10th	Day	20% 15% 33% 25% 7%	525
	11th to 20th	Day	20% 15% 38% 19% 7% 1	524
	21st to end	Day	21% 14% 33% 26% 5% 1	576
August	1st to 10th	Day	21% 13% 31% 25% 9% 1	526
	11th to 20th	Day	20% 10% 28% 28% 12% 2	519
	21st to end	Day	25% 16% 26% 20% 10% 4	583
September	1st to 10th	Day	20% 15% 30% 22% 9% 4	530
	11th to 20th	Day	23% 8% 27% 22% 15% 5%	530
	21st to end	Day	22% 16% 27% 22% 8% 4	530
October	1st to 10th	Day	25% 18% 25% 19% 10% 2	530
	11th to 20th	Day	30% 16% 30% 14% 9% 1	438
	21st to end	Day	40% 19% 25% 14% 2	462
November	1st to 10th	Day	37% 17% 29% 13% 3	425
	11th to 20th	Day	29% 23% 30% 13% 5% 1	430
	21st to end	Day	32% 25% 28% 13% 2	287
December	1st to 10th	Day	35% 21% 29% 12% 3	220
	11th to 20th	Day	35% 29% 25% 9% 2	220
	21st to end	Day	36% 25% 26% 11% 3	242
January	1st to 10th	Night	26% 27% 34% 12% 2	840
	11th to 20th	Night	26% 25% 31% 15% 3	840
	21st to end	Night	19% 25% 35% 17% 3	924
February	1st to 10th	Night	20% 26% 35% 15% 4	840
	11th to 20th	Night	25% 24% 36% 14% 2	823
	21st to end	Night	26% 20% 37% 16% 2	692
March	1st to 10th	Night	27% 21% 35% 15% 2	840
	11th to 20th	Night	28% 22% 29% 16% 4	840
	21st to end	Night	29% 18% 28% 21% 4 1	924
April	1st to 10th	Night	33% 18% 28% 16% 4	840
	11th to 20th	Night	29% 15% 31% 20% 5%	739
	21st to end	Night	30% 19% 29% 17% 5%	630
May	1st to 10th	Night	31% 14% 31% 19% 5% 1	630
	11th to 20th	Night	23% 12% 33% 26% 5%	422
	21st to end	Night	23% 10% 33% 27% 7%	433
June	1st to 10th	Night	30% 8% 28% 27% 7%	400
	11th to 20th	Night	22% 8% 25% 35% 11%	400
	21st to end	Night	23% 11% 27% 30% 10%	400
July	1st to 10th	Night	18% 6% 34% 30% 11% 1	390
	11th to 20th	Night	18% 5% 28% 36% 11% 1	388
	21st to end	Night	19% 7% 25% 39% 11%	438
August	1st to 10th	Night	19% 4 21% 38% 17%	433
	11th to 20th	Night	17% 6% 26% 32% 19% 1	592
	21st to end	Night	21% 9% 25% 29% 14% 2	791
September	1st to 10th	Night	21% 9% 27% 31% 12% 1	800
	11th to 20th	Night	21% 8% 23% 27% 19% 2	800
	21st to end	Night	23% 10% 28% 26% 12% 1	800
October	1st to 10th	Night	24% 12% 27% 26% 11% 1	800
	11th to 20th	Night	29% 14% 27% 21% 8% 2	800
	21st to end	Night	34% 18% 28% 17% 3	880
November	1st to 10th	Night	34% 17% 27% 18% 4	820
	11th to 20th	Night	28% 20% 29% 15% 8% 1	840
	21st to end	Night	30% 23% 26% 16% 4 1	887
December	1st to 10th	Night	32% 23% 28% 14% 4	950
	11th to 20th	Night	31% 25% 25% 15% 4	950
	21st to end	Night	33% 24% 26% 13% 3	957

Page generated at 2026-05-14 01:58:56. Version ac07

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