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 2025-01-22.

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	19% 13% 4 9% 5% 5% 5% 3 5% 6% 26%	104677
	Night	18% 15% 5% 11% 6% 4 4 3 4 5% 25%	104690
Transparency	Day	58% 6% 11% 17% 7% 1	108163
	Night	61% 5% 10% 15% 7% 1	109294
Seeing	Day	28% 19% 29% 17% 6% 1	15280
	Night	26% 17% 29% 21% 7% 1	24082

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	14% 19% 9% 17% 10% 6% 6% 3 3 3 10%	2597
	11th to 20th	Day	11% 16% 6% 24% 13% 5% 3 2 2 2 17%	2600
	21st to end	Day	11% 17% 9% 16% 7% 7% 5% 3 4 4 17%	2780
February	1st to 10th	Day	13% 20% 6% 15% 9% 6% 5% 2 4 3 18%	1561
	11th to 20th	Day	13% 21% 8% 15% 7% 6% 4 2 4 4 16%	1848
	21st to end	Day	12% 19% 6% 12% 7% 7% 5% 4 4 5% 19%	1570
March	1st to 10th	Day	20% 16% 4 10% 6% 5% 3 3 4 5% 23%	3116
	11th to 20th	Day	18% 22% 7% 13% 5% 4 3 2 3 3 20%	3339
	21st to end	Day	18% 17% 5% 11% 4 4 3 2 3 4 30%	4030
April	1st to 10th	Day	22% 18% 4 10% 6% 3 3 2 3 4 24%	2280
	11th to 20th	Day	18% 16% 3 6% 3 4 4 3 4 5% 33%	2337
	21st to end	Day	24% 15% 4 8% 4 3 3 3 4 6% 27%	2470
May	1st to 10th	Day	24% 10% 3 8% 5% 5% 5% 3 4 6% 27%	4106
	11th to 20th	Day	21% 9% 3 7% 5% 5% 5% 4 6% 7% 28%	4398
	21st to end	Day	17% 8% 2 7% 5% 5% 5% 5% 7% 8% 29%	4837
June	1st to 10th	Day	28% 9% 2 6% 4 4 5% 4 5% 7% 25%	2764
	11th to 20th	Day	20% 7% 2 6% 5% 5% 5% 4 7% 8% 31%	2850
	21st to end	Day	24% 9% 4 8% 6% 5% 5% 4 5% 6% 25%	2762
July	1st to 10th	Day	16% 8% 3 8% 5% 6% 6% 5% 7% 8% 29%	4440
	11th to 20th	Day	17% 8% 3 8% 5% 5% 6% 4 7% 8% 29%	4440
	21st to end	Day	17% 7% 2 6% 4 4 5% 4 6% 6% 39%	4814
August	1st to 10th	Day	20% 7% 3 9% 5% 5% 7% 5% 7% 9% 25%	4334
	11th to 20th	Day	20% 9% 3 7% 5% 6% 5% 5% 7% 8% 24%	4045
	21st to end	Day	23% 7% 2 6% 4 4 5% 4 5% 7% 35%	4313
September	1st to 10th	Day	19% 7% 2 6% 4 4 4 3 5% 6% 39%	2280
	11th to 20th	Day	20% 7% 2 5% 3 3 4 3 5% 7% 41%	2099
	21st to end	Day	20% 8% 3 6% 4 4 4 4 6% 8% 34%	2090
October	1st to 10th	Day	22% 11% 3 6% 4 4 4 4 6% 7% 30%	2090
	11th to 20th	Day	20% 16% 4 8% 5% 5% 3 4 5% 5% 25%	1765
	21st to end	Day	26% 20% 3 7% 3 4 4 2 4 5% 22%	1881
November	1st to 10th	Day	20% 19% 5% 7% 4 4 4 3 4 4 27%	1761
	11th to 20th	Day	17% 19% 7% 10% 5% 3 4 3 3 5% 23%	1800
	21st to end	Day	19% 20% 7% 13% 5% 4 3 3 4 3 18%	1575
December	1st to 10th	Day	16% 22% 9% 17% 8% 4 5% 2 3 3 11%	2280
	11th to 20th	Day	18% 18% 8% 13% 6% 5% 4 2 3 4 21%	2618
	21st to end	Day	18% 19% 5% 15% 5% 4 5% 3 4 5% 17%	3807
January	1st to 10th	Night	14% 18% 10% 20% 9% 5% 4 3 3 3 11%	4569
	11th to 20th	Night	11% 18% 8% 20% 12% 6% 5% 3 3 2 12%	4580
	21st to end	Night	11% 19% 8% 16% 8% 7% 5% 3 3 4 15%	4899
February	1st to 10th	Night	12% 17% 6% 14% 9% 6% 5% 3 4 4 21%	2614
	11th to 20th	Night	12% 19% 7% 16% 8% 6% 5% 3 3 3 18%	2218
	21st to end	Night	13% 18% 7% 11% 6% 5% 5% 4 4 4 24%	1884
March	1st to 10th	Night	22% 17% 4 8% 5% 3 5% 2 3 4 26%	3724
	11th to 20th	Night	16% 22% 7% 12% 4 4 4 2 3 4 22%	3618
	21st to end	Night	20% 17% 5% 11% 5% 3 3 2 3 4 27%	3599
April	1st to 10th	Night	21% 17% 5% 7% 4 3 4 2 4 4 28%	1900
	11th to 20th	Night	20% 13% 3 7% 4 4 4 4 5% 5% 32%	1792
	21st to end	Night	23% 14% 3 7% 4 3 4 3 5% 5% 29%	1710
May	1st to 10th	Night	26% 9% 3 7% 4 4 4 3 5% 7% 27%	2834
	11th to 20th	Night	21% 9% 3 8% 5% 4 3 3 4 6% 33%	2509
	21st to end	Night	20% 6% 2 6% 4 4 4 4 6% 8% 36%	2738
June	1st to 10th	Night	27% 9% 3 6% 5% 4 4 3 5% 7% 27%	1411
	11th to 20th	Night	21% 6% 3 6% 4 4 4 4 6% 8% 35%	1330
	21st to end	Night	22% 8% 3 7% 5% 5% 4 5% 5% 6% 29%	1412
July	1st to 10th	Night	15% 10% 3 8% 6% 5% 5% 4 6% 7% 31%	2520
	11th to 20th	Night	21% 8% 2 7% 4 5% 4 4 6% 6% 32%	2520
	21st to end	Night	17% 7% 3 6% 4 4 5% 4 5% 6% 40%	2739
August	1st to 10th	Night	24% 9% 3 6% 4 4 4 3 5% 7% 30%	2565
	11th to 20th	Night	19% 9% 3 8% 4 6% 6% 4 5% 7% 29%	2782
	21st to end	Night	20% 7% 2 7% 5% 4 4 3 4 5% 39%	3339
September	1st to 10th	Night	19% 7% 2 7% 4 4 3 4 6% 6% 38%	1900
	11th to 20th	Night	19% 5% 2 5% 4 4 3 3 5% 6% 45%	2080
	21st to end	Night	20% 7% 2 5% 3 4 3 3 5% 6% 41%	2090
October	1st to 10th	Night	20% 10% 2 6% 4 4 4 4 5% 6% 35%	2090
	11th to 20th	Night	22% 14% 3 7% 5% 4 4 3 4 5% 28%	2413
	21st to end	Night	26% 16% 3 7% 4 3 3 3 4 5% 25%	2717
November	1st to 10th	Night	20% 18% 4 8% 5% 4 4 2 5% 4 26%	2537
	11th to 20th	Night	18% 17% 6% 10% 5% 3 3 3 4 5% 26%	2600
	21st to end	Night	17% 22% 6% 12% 6% 4 3 2 4 4 20%	2820
December	1st to 10th	Night	15% 22% 12% 20% 8% 4 3 2 3 2 10%	4910
	11th to 20th	Night	18% 20% 7% 16% 8% 5% 3 2 3 3 16%	5591
	21st to end	Night	19% 21% 5% 10% 5% 4 4 2 3 4 24%	7136

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	79% 5% 8% 7% 2	2598
	11th to 20th	Day	77% 3 4 12% 3 1	2600
	21st to end	Day	70% 6% 7% 8% 7% 2	2780
February	1st to 10th	Day	73% 4 5% 10% 7% 1	1561
	11th to 20th	Day	74% 3 7% 9% 5% 1	1857
	21st to end	Day	67% 5% 7% 13% 5% 2	1570
March	1st to 10th	Day	64% 6% 7% 11% 9% 2	3119
	11th to 20th	Day	69% 5% 8% 11% 6% 1	3339
	21st to end	Day	60% 5% 6% 14% 14% 2	4030
April	1st to 10th	Day	65% 4 8% 14% 8% 2	2280
	11th to 20th	Day	54% 5% 8% 20% 12% 1	2337
	21st to end	Day	60% 5% 7% 16% 10% 2	2470
May	1st to 10th	Day	57% 9% 10% 17% 7% 1	4120
	11th to 20th	Day	52% 9% 18% 14% 6% 1	4402
	21st to end	Day	48% 9% 13% 20% 9% 1	4837
June	1st to 10th	Day	57% 7% 10% 20% 6% 1	2764
	11th to 20th	Day	49% 7% 13% 21% 9% 1	2850
	21st to end	Day	59% 7% 10% 17% 7%	2762
July	1st to 10th	Day	50% 7% 15% 21% 7%	4370
	11th to 20th	Day	51% 8% 15% 24% 3	4365
	21st to end	Day	46% 7% 13% 28% 6%	4661
August	1st to 10th	Day	54% 7% 16% 19% 3	4320
	11th to 20th	Day	55% 9% 16% 15% 5%	4084
	21st to end	Day	48% 6% 14% 18% 12% 2	4313
September	1st to 10th	Day	46% 7% 14% 22% 10%	2280
	11th to 20th	Day	44% 5% 12% 26% 12%	2099
	21st to end	Day	48% 7% 11% 24% 10%	2090
October	1st to 10th	Day	53% 6% 14% 22% 6%	3480
	11th to 20th	Day	57% 5% 11% 23% 4	2903
	21st to end	Day	57% 3 15% 17% 7%	3081
November	1st to 10th	Day	61% 5% 10% 17% 7%	1761
	11th to 20th	Day	66% 5% 8% 13% 7% 1	1800
	21st to end	Day	71% 5% 11% 11% 2	1575
December	1st to 10th	Day	80% 4 7% 5% 4	2280
	11th to 20th	Day	70% 4 6% 13% 8%	2618
	21st to end	Day	71% 3 6% 9% 9% 1	3807
January	1st to 10th	Night	79% 6% 7% 6% 2	4569
	11th to 20th	Night	78% 6% 5% 7% 2	4580
	21st to end	Night	72% 6% 7% 8% 5% 1	4899
February	1st to 10th	Night	69% 5% 7% 12% 7% 1	2614
	11th to 20th	Night	72% 4 7% 10% 6% 1	2220
	21st to end	Night	64% 5% 7% 14% 10% 1	1884
March	1st to 10th	Night	62% 5% 9% 15% 6% 2	3734
	11th to 20th	Night	67% 6% 7% 12% 7% 1	3619
	21st to end	Night	63% 3 6% 15% 10% 2	3599
April	1st to 10th	Night	61% 3 8% 15% 12% 1	1900
	11th to 20th	Night	53% 6% 9% 20% 10% 2	1792
	21st to end	Night	56% 6% 11% 16% 9% 2	1710
May	1st to 10th	Night	55% 7% 14% 15% 7% 1	2840
	11th to 20th	Night	51% 6% 15% 20% 7% 1	2510
	21st to end	Night	44% 8% 16% 17% 14%	2739
June	1st to 10th	Night	56% 6% 11% 19% 6%	1411
	11th to 20th	Night	47% 7% 12% 23% 11% 2	1330
	21st to end	Night	55% 8% 11% 18% 8%	1412
July	1st to 10th	Night	51% 7% 17% 18% 7%	2480
	11th to 20th	Night	50% 8% 17% 20% 6%	2472
	21st to end	Night	44% 8% 16% 26% 6%	2651
August	1st to 10th	Night	54% 7% 15% 21% 3	2555
	11th to 20th	Night	54% 9% 16% 16% 5%	2809
	21st to end	Night	47% 6% 13% 21% 14%	3339
September	1st to 10th	Night	45% 5% 15% 24% 10%	1900
	11th to 20th	Night	40% 7% 15% 29% 10% 1	2080
	21st to end	Night	44% 7% 12% 25% 11% 1	2090
October	1st to 10th	Night	51% 5% 11% 23% 10%	3480
	11th to 20th	Night	56% 5% 11% 23% 5%	4014
	21st to end	Night	56% 4 14% 17% 8% 1	4467
November	1st to 10th	Night	62% 5% 11% 15% 6% 1	2537
	11th to 20th	Night	62% 6% 10% 13% 8% 1	2600
	21st to end	Night	69% 4 11% 11% 4	2820
December	1st to 10th	Night	83% 3 6% 6% 2	4910
	11th to 20th	Night	74% 4 6% 11% 5%	5592
	21st to end	Night	66% 4 7% 13% 9%	7136

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% 34% 11%	200
	11th to 20th	Day	31% 24% 32% 12% 2 1	200
	21st to end	Day	22% 28% 35% 13% 2 1	200
February	1st to 10th	Day	34% 29% 26% 7% 3	219
	11th to 20th	Day	29% 34% 29% 7% 2	456
	21st to end	Day	30% 27% 29% 14% 1	382
March	1st to 10th	Day	28% 26% 31% 12% 3	460
	11th to 20th	Day	31% 27% 26% 12% 4 1	460
	21st to end	Day	32% 25% 24% 13% 6% 1	506
April	1st to 10th	Day	39% 18% 26% 11% 4 1	460
	11th to 20th	Day	35% 23% 23% 13% 5% 1	452
	21st to end	Day	36% 21% 28% 10% 4 1	460
May	1st to 10th	Day	33% 19% 28% 15% 4 1	463
	11th to 20th	Day	27% 16% 31% 19% 5% 1	487
	21st to end	Day	21% 23% 33% 17% 5%	529
June	1st to 10th	Day	33% 16% 29% 16% 6%	594
	11th to 20th	Day	24% 15% 28% 23% 10%	680
	21st to end	Day	27% 16% 30% 17% 9% 1	592
July	1st to 10th	Day	19% 14% 33% 25% 7%	485
	11th to 20th	Day	20% 14% 37% 19% 8% 1	484
	21st to end	Day	22% 14% 32% 27% 5% 1	532
August	1st to 10th	Day	23% 13% 30% 24% 9% 1	486
	11th to 20th	Day	21% 11% 28% 28% 11% 2	479
	21st to end	Day	25% 16% 24% 20% 11% 4	539
September	1st to 10th	Day	20% 15% 31% 21% 9% 4	490
	11th to 20th	Day	24% 9% 26% 21% 14% 5%	490
	21st to end	Day	21% 16% 28% 22% 9% 4	490
October	1st to 10th	Day	27% 18% 24% 18% 10% 3	490
	11th to 20th	Day	30% 17% 29% 14% 9% 1	407
	21st to end	Day	41% 19% 24% 14% 2	429
November	1st to 10th	Day	37% 17% 29% 14% 4	395
	11th to 20th	Day	28% 23% 30% 13% 5% 1	400
	21st to end	Day	32% 24% 27% 14% 3	264
December	1st to 10th	Day	36% 22% 27% 13% 4	200
	11th to 20th	Day	36% 29% 24% 10% 3	200
	21st to end	Day	35% 25% 25% 11% 3	220
January	1st to 10th	Night	26% 27% 34% 12% 2	800
	11th to 20th	Night	26% 25% 32% 15% 3	800
	21st to end	Night	19% 25% 35% 18% 3	841
February	1st to 10th	Night	21% 27% 35% 14% 3	760
	11th to 20th	Night	24% 26% 35% 14% 2	743
	21st to end	Night	26% 20% 36% 16% 2	628
March	1st to 10th	Night	27% 19% 35% 17% 2	760
	11th to 20th	Night	27% 22% 30% 17% 4	760
	21st to end	Night	28% 17% 27% 22% 4 1	836
April	1st to 10th	Night	32% 18% 28% 18% 4	760
	11th to 20th	Night	28% 14% 31% 21% 5%	668
	21st to end	Night	31% 19% 28% 18% 4	570
May	1st to 10th	Night	31% 14% 31% 19% 5% 1	570
	11th to 20th	Night	23% 12% 32% 27% 5% 1	388
	21st to end	Night	23% 10% 34% 27% 6%	411
June	1st to 10th	Night	30% 8% 27% 27% 8%	380
	11th to 20th	Night	22% 7% 25% 34% 12%	380
	21st to end	Night	23% 11% 26% 29% 11%	380
July	1st to 10th	Night	18% 5% 35% 30% 11% 1	370
	11th to 20th	Night	18% 6% 28% 36% 11% 1	368
	21st to end	Night	19% 7% 24% 39% 11%	416
August	1st to 10th	Night	20% 4 21% 38% 16%	411
	11th to 20th	Night	17% 6% 27% 31% 17% 1	562
	21st to end	Night	21% 9% 25% 29% 14% 2	751
September	1st to 10th	Night	21% 8% 28% 31% 11% 1	760
	11th to 20th	Night	22% 8% 23% 27% 18% 2	760
	21st to end	Night	23% 10% 28% 26% 12% 1	760
October	1st to 10th	Night	25% 12% 26% 26% 11% 1	760
	11th to 20th	Night	29% 14% 27% 21% 8% 2	760
	21st to end	Night	34% 17% 28% 17% 3	836
November	1st to 10th	Night	33% 17% 27% 18% 5%	780
	11th to 20th	Night	28% 19% 29% 16% 8% 1	800
	21st to end	Night	31% 22% 27% 16% 4 1	843
December	1st to 10th	Night	32% 22% 28% 14% 4	900
	11th to 20th	Night	31% 24% 25% 15% 4	900
	21st to end	Night	33% 24% 26% 14% 4	910

Page generated at 2025-01-19 03:06:48. Version ac07

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