Terrarouge Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Terrarouge Observatory clear sky chart from 2005-11-06 to 2025-12-03.

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 9% 5% 4 4 3 5% 6% 28%	128027
	Night	19% 12% 4 10% 5% 5% 4 3 5% 6% 26%	122801
Transparency	Day	53% 7% 13% 18% 7% 1	131560
	Night	57% 7% 12% 16% 7% 1	127359
Seeing	Day	22% 12% 28% 27% 10% 2	19013
	Night	24% 12% 29% 26% 8% 1	26248

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	13% 21% 10% 23% 6% 4 3 3 3 3 11%	3616
	11th to 20th	Day	13% 18% 10% 16% 7% 5% 3 3 4 4 16%	3591
	21st to end	Day	9% 11% 6% 13% 6% 5% 5% 3 6% 6% 29%	4332
February	1st to 10th	Day	19% 14% 4 10% 4 5% 4 3 4 5% 26%	1800
	11th to 20th	Day	16% 16% 5% 12% 6% 4 4 3 4 5% 25%	1753
	21st to end	Day	11% 13% 6% 9% 7% 5% 4 3 5% 6% 30%	1552
March	1st to 10th	Day	19% 13% 5% 11% 6% 6% 4 2 4 5% 25%	4722
	11th to 20th	Day	13% 16% 7% 10% 6% 4 4 4 5% 6% 25%	4980
	21st to end	Day	23% 18% 6% 12% 5% 4 3 3 4 5% 18%	5593
April	1st to 10th	Day	21% 13% 4 9% 5% 4 4 3 4 5% 27%	2400
	11th to 20th	Day	22% 12% 3 6% 5% 4 4 3 5% 6% 31%	2399
	21st to end	Day	21% 14% 4 7% 4 4 3 3 5% 7% 29%	2600
May	1st to 10th	Day	27% 17% 4 6% 4 3 4 3 3 6% 24%	6289
	11th to 20th	Day	18% 11% 3 8% 4 4 4 3 5% 6% 33%	6318
	21st to end	Day	19% 11% 3 6% 4 4 4 3 5% 7% 34%	6948
June	1st to 10th	Day	25% 12% 3 6% 5% 4 5% 4 5% 7% 24%	2800
	11th to 20th	Day	18% 9% 3 7% 5% 4 4 3 5% 7% 35%	2878
	21st to end	Day	24% 11% 3 7% 4 4 4 3 5% 6% 30%	3000
July	1st to 10th	Day	18% 11% 3 7% 5% 4 5% 4 6% 8% 29%	6554
	11th to 20th	Day	14% 7% 3 7% 4 4 5% 4 5% 8% 41%	6360
	21st to end	Day	12% 7% 3 6% 5% 5% 5% 4 7% 9% 37%	6927
August	1st to 10th	Day	17% 9% 3 8% 6% 4 5% 5% 6% 8% 30%	4317
	11th to 20th	Day	14% 8% 2 7% 4 5% 5% 4 6% 8% 38%	4175
	21st to end	Day	11% 6% 2 6% 4 4 4 4 6% 8% 43%	4426
September	1st to 10th	Day	16% 8% 2 7% 4 4 4 4 6% 8% 39%	2400
	11th to 20th	Day	17% 8% 3 7% 5% 4 5% 3 6% 6% 37%	2400
	21st to end	Day	17% 8% 2 6% 5% 4 4 4 5% 7% 38%	2091
October	1st to 10th	Day	18% 8% 3 8% 6% 4 5% 4 5% 8% 33%	2000
	11th to 20th	Day	19% 12% 4 8% 5% 5% 6% 4 5% 6% 25%	2000
	21st to end	Day	20% 16% 4 9% 6% 4 5% 4 5% 6% 22%	2122
November	1st to 10th	Day	21% 14% 4 10% 4 6% 4 3 6% 5% 23%	1720
	11th to 20th	Day	22% 21% 5% 10% 5% 4 4 3 4 6% 17%	1680
	21st to end	Day	18% 19% 7% 12% 5% 5% 4 4 5% 6% 15%	1680
December	1st to 10th	Day	17% 16% 10% 19% 6% 5% 3 3 5% 4 12%	2626
	11th to 20th	Day	26% 15% 5% 13% 6% 4 3 2 3 4 17%	2996
	21st to end	Day	17% 15% 8% 17% 5% 5% 4 3 4 5% 16%	3982
January	1st to 10th	Night	14% 18% 8% 17% 6% 5% 4 3 4 4 17%	6349
	11th to 20th	Night	17% 15% 7% 19% 7% 4 4 3 4 4 17%	6314
	21st to end	Night	10% 10% 5% 14% 7% 7% 6% 4 6% 6% 25%	6628
February	1st to 10th	Night	20% 13% 4 9% 6% 5% 4 4 4 5% 26%	2600
	11th to 20th	Night	18% 15% 4 11% 6% 6% 5% 3 4 4 23%	2533
	21st to end	Night	13% 16% 5% 9% 7% 5% 4 4 4 4 30%	2076
March	1st to 10th	Night	22% 15% 5% 11% 6% 4 4 3 3 4 23%	5101
	11th to 20th	Night	19% 14% 7% 11% 6% 4 4 3 4 5% 26%	4979
	21st to end	Night	25% 17% 6% 10% 5% 4 3 2 3 3 20%	5339
April	1st to 10th	Night	22% 12% 4 8% 6% 5% 3 3 5% 6% 25%	2000
	11th to 20th	Night	21% 13% 4 7% 4 4 4 3 4 6% 31%	1946
	21st to end	Night	25% 12% 3 7% 4 3 4 3 5% 5% 29%	1800
May	1st to 10th	Night	29% 15% 4 7% 4 4 4 3 4 5% 20%	3669
	11th to 20th	Night	18% 11% 4 7% 5% 4 4 4 5% 8% 31%	3596
	21st to end	Night	17% 10% 3 7% 5% 5% 5% 4 5% 7% 32%	3948
June	1st to 10th	Night	27% 10% 3 6% 5% 3 3 3 5% 7% 27%	1600
	11th to 20th	Night	21% 7% 2 4 4 3 4 3 6% 7% 38%	1533
	21st to end	Night	24% 10% 3 7% 6% 4 4 4 6% 5% 28%	1400
July	1st to 10th	Night	15% 10% 4 9% 5% 6% 6% 5% 7% 8% 24%	3344
	11th to 20th	Night	13% 8% 3 6% 4 4 4 4 6% 8% 38%	3620
	21st to end	Night	15% 7% 3 6% 4 4 4 4 6% 8% 37%	3948
August	1st to 10th	Night	19% 9% 2 7% 5% 5% 5% 4 6% 8% 31%	2819
	11th to 20th	Night	15% 6% 3 6% 5% 4 5% 4 5% 9% 38%	2872
	21st to end	Night	12% 6% 2 7% 4 5% 4 4 6% 9% 41%	3472
September	1st to 10th	Night	14% 7% 2 6% 4 4 4 3 5% 8% 42%	2000
	11th to 20th	Night	17% 8% 2 5% 4 4 4 4 4 6% 43%	2000
	21st to end	Night	17% 7% 2 6% 4 4 4 4 5% 7% 40%	2306
October	1st to 10th	Night	19% 7% 3 7% 4 4 4 3 5% 7% 38%	2400
	11th to 20th	Night	20% 9% 3 8% 4 4 5% 4 5% 7% 32%	2400
	21st to end	Night	24% 13% 3 9% 5% 4 4 3 4 5% 25%	2717
November	1st to 10th	Night	20% 14% 4 8% 6% 5% 4 4 5% 6% 25%	2794
	11th to 20th	Night	24% 15% 4 8% 6% 5% 4 3 4 6% 20%	2940
	21st to end	Night	23% 19% 6% 10% 5% 4 4 3 5% 6% 17%	2940
December	1st to 10th	Night	17% 13% 7% 17% 7% 4 5% 4 5% 6% 17%	4609
	11th to 20th	Night	27% 13% 4 12% 6% 5% 4 3 4 4 19%	5213
	21st to end	Night	23% 14% 5% 15% 5% 5% 4 3 4 4 18%	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	78% 7% 9% 3 2	3617
	11th to 20th	Day	65% 8% 12% 10% 4	3620
	21st to end	Day	53% 6% 11% 18% 9% 2	4332
February	1st to 10th	Day	60% 5% 11% 15% 9% 1	1800
	11th to 20th	Day	60% 6% 9% 17% 6% 1	1761
	21st to end	Day	53% 5% 13% 16% 11% 2	1552
March	1st to 10th	Day	59% 5% 11% 16% 10% 1	4726
	11th to 20th	Day	56% 8% 16% 16% 5% 1	4980
	21st to end	Day	67% 5% 9% 13% 6% 1	5593
April	1st to 10th	Day	58% 5% 10% 15% 11% 2	2400
	11th to 20th	Day	52% 6% 10% 20% 12% 1	2399
	21st to end	Day	53% 6% 12% 16% 11% 2	2600
May	1st to 10th	Day	60% 7% 10% 16% 6% 1	6301
	11th to 20th	Day	50% 8% 14% 22% 7%	6322
	21st to end	Day	46% 10% 13% 23% 9%	6948
June	1st to 10th	Day	56% 8% 13% 18% 6% 1	2800
	11th to 20th	Day	47% 7% 13% 23% 9% 1	2878
	21st to end	Day	54% 7% 12% 16% 10% 1	3000
July	1st to 10th	Day	50% 7% 16% 17% 9%	6484
	11th to 20th	Day	40% 7% 16% 29% 8%	6284
	21st to end	Day	39% 10% 20% 25% 6%	6730
August	1st to 10th	Day	50% 9% 14% 22% 6%	4303
	11th to 20th	Day	41% 9% 15% 26% 9%	4214
	21st to end	Day	34% 7% 15% 32% 12%	4426
September	1st to 10th	Day	40% 8% 14% 26% 11%	2400
	11th to 20th	Day	45% 7% 14% 22% 12%	2400
	21st to end	Day	43% 6% 12% 26% 13%	2091
October	1st to 10th	Day	48% 10% 14% 21% 7%	3260
	11th to 20th	Day	51% 7% 15% 21% 7%	3240
	21st to end	Day	54% 7% 16% 19% 4	3416
November	1st to 10th	Day	61% 8% 11% 15% 5%	1720
	11th to 20th	Day	70% 6% 11% 11% 3	1680
	21st to end	Day	69% 8% 13% 8% 2	1680
December	1st to 10th	Day	73% 7% 8% 11% 1	2626
	11th to 20th	Day	71% 6% 9% 7% 7%	2995
	21st to end	Day	69% 6% 13% 9% 3	3982
January	1st to 10th	Night	68% 8% 13% 9% 2	6349
	11th to 20th	Night	65% 11% 11% 10% 4	6360
	21st to end	Night	56% 8% 12% 14% 9% 1	6628
February	1st to 10th	Night	60% 5% 9% 14% 12% 1	2600
	11th to 20th	Night	63% 5% 10% 14% 7% 1	2536
	21st to end	Night	55% 5% 11% 17% 10% 1	2076
March	1st to 10th	Night	63% 5% 6% 13% 11% 2	5110
	11th to 20th	Night	60% 5% 12% 14% 7% 1	4980
	21st to end	Night	68% 5% 6% 13% 8% 1	5339
April	1st to 10th	Night	59% 6% 11% 12% 10% 2	2000
	11th to 20th	Night	53% 5% 9% 19% 12% 1	1946
	21st to end	Night	57% 5% 9% 15% 12% 1	1800
May	1st to 10th	Night	66% 5% 7% 13% 8%	3674
	11th to 20th	Night	51% 8% 13% 21% 7% 1	3597
	21st to end	Night	48% 10% 17% 21% 4	3949
June	1st to 10th	Night	56% 7% 11% 18% 8%	1600
	11th to 20th	Night	45% 6% 13% 22% 14%	1533
	21st to end	Night	55% 8% 14% 14% 8% 1	1400
July	1st to 10th	Night	53% 9% 15% 17% 5% 2	3305
	11th to 20th	Night	41% 8% 17% 24% 9% 1	3572
	21st to end	Night	43% 8% 16% 25% 7%	3831
August	1st to 10th	Night	50% 10% 14% 21% 5%	2808
	11th to 20th	Night	42% 9% 17% 26% 6%	2899
	21st to end	Night	36% 10% 18% 25% 11%	3471
September	1st to 10th	Night	39% 8% 13% 28% 12%	2000
	11th to 20th	Night	43% 6% 12% 26% 12% 1	2000
	21st to end	Night	43% 7% 13% 26% 11%	2306
October	1st to 10th	Night	49% 7% 13% 23% 8%	3920
	11th to 20th	Night	43% 8% 16% 24% 9%	3899
	21st to end	Night	52% 6% 15% 20% 6%	4377
November	1st to 10th	Night	59% 8% 13% 14% 5%	2794
	11th to 20th	Night	65% 8% 11% 11% 4	2940
	21st to end	Night	69% 8% 13% 8% 2	2940
December	1st to 10th	Night	65% 8% 13% 13% 1	4609
	11th to 20th	Night	67% 6% 10% 12% 4 1	5215
	21st to end	Night	68% 7% 11% 10% 3	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	25% 19% 38% 16% 3	400
	11th to 20th	Day	25% 20% 28% 21% 7% 1	400
	21st to end	Day	18% 16% 38% 24% 4	440
February	1st to 10th	Day	26% 20% 30% 18% 6% 1	400
	11th to 20th	Day	24% 19% 34% 18% 4 1	396
	21st to end	Day	15% 13% 35% 28% 8% 1	331
March	1st to 10th	Day	23% 16% 31% 25% 5% 1	453
	11th to 20th	Day	16% 16% 33% 26% 7% 1	500
	21st to end	Day	27% 15% 31% 18% 8% 1	550
April	1st to 10th	Day	29% 16% 32% 18% 5%	500
	11th to 20th	Day	24% 14% 34% 20% 7% 1	492
	21st to end	Day	23% 12% 27% 25% 11% 1	500
May	1st to 10th	Day	31% 10% 27% 23% 6% 2	673
	11th to 20th	Day	22% 8% 24% 29% 13% 3	724
	21st to end	Day	21% 6% 29% 33% 9% 1	790
June	1st to 10th	Day	28% 7% 24% 29% 10% 2	730
	11th to 20th	Day	19% 7% 27% 35% 12% 1	730
	21st to end	Day	28% 8% 20% 29% 14% 3	730
July	1st to 10th	Day	18% 3 23% 39% 15% 2	720
	11th to 20th	Day	15% 4 22% 36% 20% 2	718
	21st to end	Day	19% 4 22% 38% 16% 1	795
August	1st to 10th	Day	14% 3 20% 38% 21% 3	582
	11th to 20th	Day	18% 5% 20% 33% 18% 6%	519
	21st to end	Day	15% 8% 22% 31% 20% 4	583
September	1st to 10th	Day	15% 8% 28% 32% 15% 2	530
	11th to 20th	Day	18% 9% 27% 32% 13% 1	530
	21st to end	Day	18% 12% 26% 28% 12% 3	464
October	1st to 10th	Day	18% 10% 27% 30% 12% 3	420
	11th to 20th	Day	24% 12% 27% 23% 12% 2	420
	21st to end	Day	25% 15% 32% 19% 6% 2	462
November	1st to 10th	Day	26% 19% 31% 18% 5% 1	425
	11th to 20th	Day	32% 20% 28% 16% 4 1	430
	21st to end	Day	28% 23% 28% 18% 3	430
December	1st to 10th	Day	30% 17% 31% 19% 3	406
	11th to 20th	Day	28% 27% 30% 13% 1	400
	21st to end	Day	23% 23% 34% 16% 3	440
January	1st to 10th	Night	26% 19% 33% 19% 3	900
	11th to 20th	Night	25% 15% 32% 22% 4 1	900
	21st to end	Night	20% 17% 34% 22% 6% 1	910
February	1st to 10th	Night	27% 14% 30% 24% 5%	800
	11th to 20th	Night	25% 15% 36% 21% 4	783
	21st to end	Night	19% 12% 33% 27% 9%	660
March	1st to 10th	Night	24% 11% 36% 25% 5%	800
	11th to 20th	Night	21% 11% 31% 28% 9% 1	800
	21st to end	Night	25% 13% 30% 23% 9% 1	830
April	1st to 10th	Night	30% 13% 28% 24% 6%	600
	11th to 20th	Night	25% 11% 31% 25% 8% 1	594
	21st to end	Night	28% 8% 28% 27% 8% 1	600
May	1st to 10th	Night	31% 8% 26% 24% 11% 2	600
	11th to 20th	Night	22% 6% 27% 31% 12% 2	592
	21st to end	Night	20% 6% 32% 35% 7%	649
June	1st to 10th	Night	29% 7% 23% 33% 9% 1	600
	11th to 20th	Night	23% 6% 25% 37% 10%	533
	21st to end	Night	25% 5% 23% 35% 11% 1	400
July	1st to 10th	Night	17% 3 22% 42% 16%	466
	11th to 20th	Night	16% 4 21% 44% 15% 1	582
	21st to end	Night	18% 4 21% 39% 16% 1	656
August	1st to 10th	Night	16% 3 17% 45% 18% 1	598
	11th to 20th	Night	17% 5% 21% 37% 18% 2	592
	21st to end	Night	14% 5% 22% 37% 21% 1	660
September	1st to 10th	Night	16% 7% 26% 35% 15% 1	600
	11th to 20th	Night	20% 6% 24% 35% 13% 1	600
	21st to end	Night	20% 9% 30% 31% 8% 3	789
October	1st to 10th	Night	22% 7% 30% 29% 12% 1	800
	11th to 20th	Night	23% 13% 30% 24% 10% 1	800
	21st to end	Night	30% 16% 30% 20% 5%	923
November	1st to 10th	Night	25% 19% 30% 19% 4 1	930
	11th to 20th	Night	30% 19% 27% 18% 5% 1	950
	21st to end	Night	32% 19% 28% 18% 3	950
December	1st to 10th	Night	31% 17% 32% 14% 5%	911
	11th to 20th	Night	28% 24% 30% 14% 3	900
	21st to end	Night	30% 19% 29% 18% 3 1	990

Page generated at 2025-11-30 02:00:51. Version ac07

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