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 2024-01-13.

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% 11% 4 8% 6% 3 4 4 5% 7% 28%	75042
	Night	21% 11% 4 8% 6% 3 4 4 5% 6% 27%	74682
Transparency	Day	55% 7% 12% 18% 8% 1	74745
	Night	58% 7% 12% 16% 7% 1	74514
Seeing	Day	22% 11% 27% 27% 11% 2	16349
	Night	24% 11% 29% 27% 9% 1	23745

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	18% 17% 9% 11% 9% 3 5% 3 4 5% 17%	1516
	11th to 20th	Day	21% 16% 6% 12% 7% 4 4 4 5% 6% 16%	1456
	21st to end	Day	12% 11% 6% 12% 9% 3 5% 4 5% 6% 26%	1726
February	1st to 10th	Day	19% 13% 5% 10% 5% 4 5% 4 4 5% 26%	1620
	11th to 20th	Day	17% 15% 5% 10% 7% 3 5% 4 4 5% 24%	1573
	21st to end	Day	11% 12% 6% 8% 8% 4 5% 4 6% 7% 29%	1391
March	1st to 10th	Day	15% 13% 5% 9% 6% 4 6% 4 5% 6% 27%	1890
	11th to 20th	Day	14% 11% 6% 9% 8% 4 5% 4 4 5% 29%	1980
	21st to end	Day	18% 15% 9% 9% 6% 3 2 3 3 4 28%	2222
April	1st to 10th	Day	20% 13% 5% 8% 6% 3 4 4 4 5% 28%	2160
	11th to 20th	Day	22% 11% 3 6% 5% 2 4 4 4 7% 32%	2156
	21st to end	Day	22% 13% 4 6% 5% 3 4 4 5% 7% 27%	2340
May	1st to 10th	Day	25% 12% 4 6% 5% 3 4 4 4 6% 26%	2492
	11th to 20th	Day	20% 10% 3 6% 5% 3 4 3 5% 7% 33%	2478
	21st to end	Day	20% 11% 3 6% 4 3 4 4 6% 8% 32%	2724
June	1st to 10th	Day	27% 11% 3 5% 5% 3 5% 4 5% 7% 25%	2520
	11th to 20th	Day	20% 10% 2 6% 5% 3 4 4 5% 7% 35%	2590
	21st to end	Day	24% 10% 3 6% 5% 2 4 4 5% 6% 30%	2700
July	1st to 10th	Day	15% 8% 2 5% 5% 4 4 5% 5% 9% 38%	2602
	11th to 20th	Day	16% 6% 4 6% 4 3 4 6% 6% 9% 36%	2520
	21st to end	Day	19% 9% 4 6% 6% 4 5% 4 5% 7% 31%	2745
August	1st to 10th	Day	16% 6% 2 5% 6% 2 4 6% 6% 9% 37%	2378
	11th to 20th	Day	18% 8% 2 6% 5% 3 4 6% 6% 8% 35%	2304
	21st to end	Day	14% 7% 2 5% 5% 3 4 5% 6% 9% 40%	2444
September	1st to 10th	Day	16% 7% 2 6% 5% 3 4 5% 6% 9% 38%	2160
	11th to 20th	Day	18% 8% 3 6% 5% 4 3 5% 5% 6% 36%	2160
	21st to end	Day	18% 8% 2 5% 6% 3 4 4 5% 7% 38%	1883
October	1st to 10th	Day	19% 7% 3 7% 6% 3 5% 5% 5% 8% 32%	1800
	11th to 20th	Day	20% 12% 4 7% 7% 4 5% 5% 5% 6% 24%	1800
	21st to end	Day	21% 15% 5% 8% 7% 3 4 5% 5% 6% 21%	1910
November	1st to 10th	Day	21% 13% 4 9% 5% 4 4 4 6% 5% 24%	1553
	11th to 20th	Day	21% 18% 6% 9% 6% 4 4 4 5% 6% 19%	1520
	21st to end	Day	19% 18% 8% 11% 6% 4 4 5% 5% 7% 15%	1520
December	1st to 10th	Day	23% 18% 9% 11% 8% 4 4 3 4 5% 12%	1520
	11th to 20th	Day	28% 16% 7% 12% 7% 4 4 4 3 4 9%	1906
	21st to end	Day	21% 15% 9% 13% 7% 4 4 4 3 5% 16%	2783
January	1st to 10th	Night	19% 15% 7% 11% 7% 3 4 4 4 5% 20%	2649
	11th to 20th	Night	20% 14% 6% 10% 7% 3 5% 4 5% 6% 21%	2549
	21st to end	Night	15% 12% 6% 9% 6% 4 5% 5% 5% 6% 27%	2628
February	1st to 10th	Night	20% 12% 4 8% 6% 3 5% 4 4 6% 27%	2340
	11th to 20th	Night	19% 15% 5% 10% 7% 4 6% 4 4 5% 22%	2273
	21st to end	Night	12% 15% 6% 9% 7% 4 4 5% 4 5% 29%	1863
March	1st to 10th	Night	14% 14% 5% 11% 8% 3 5% 5% 4 5% 26%	2054
	11th to 20th	Night	17% 10% 7% 8% 8% 3 4 4 5% 6% 28%	1979
	21st to end	Night	19% 13% 6% 8% 6% 3 4 3 4 5% 29%	2134
April	1st to 10th	Night	23% 12% 4 7% 7% 3 4 4 5% 6% 25%	1800
	11th to 20th	Night	21% 12% 4 6% 5% 3 4 4 4 6% 32%	1750
	21st to end	Night	25% 11% 4 6% 5% 2 3 4 5% 5% 29%	1620
May	1st to 10th	Night	27% 9% 3 5% 5% 3 4 4 4 5% 30%	1468
	11th to 20th	Night	21% 9% 3 7% 5% 3 3 4 5% 7% 33%	1416
	21st to end	Night	17% 10% 3 6% 5% 3 5% 6% 5% 8% 31%	1550
June	1st to 10th	Night	28% 9% 4 6% 6% 2 4 4 5% 7% 26%	1440
	11th to 20th	Night	23% 7% 2 4 5% 2 4 3 5% 6% 38%	1381
	21st to end	Night	23% 9% 4 6% 7% 3 3 5% 5% 5% 29%	1260
July	1st to 10th	Night	14% 7% 4 6% 4 3 6% 6% 6% 9% 35%	1332
	11th to 20th	Night	15% 6% 3 5% 5% 4 5% 7% 7% 10% 33%	1440
	21st to end	Night	16% 9% 4 6% 6% 3 6% 5% 7% 7% 30%	1573
August	1st to 10th	Night	15% 6% 2 5% 6% 3 5% 6% 5% 9% 38%	1558
	11th to 20th	Night	18% 6% 3 7% 6% 3 5% 5% 5% 8% 34%	1596
	21st to end	Night	14% 5% 2 5% 5% 4 5% 5% 7% 10% 39%	1912
September	1st to 10th	Night	14% 6% 3 6% 5% 3 5% 5% 5% 8% 41%	1800
	11th to 20th	Night	19% 8% 2 5% 5% 2 4 4 4 6% 41%	1800
	21st to end	Night	18% 7% 2 4 5% 3 4 5% 4 7% 40%	2074
October	1st to 10th	Night	20% 7% 2 6% 5% 3 4 4 4 6% 38%	2160
	11th to 20th	Night	21% 8% 3 6% 4 3 5% 5% 6% 7% 32%	2160
	21st to end	Night	26% 12% 4 8% 6% 3 4 4 4 5% 24%	2445
November	1st to 10th	Night	21% 12% 4 7% 6% 4 4 5% 5% 6% 26%	2521
	11th to 20th	Night	23% 12% 5% 7% 7% 4 5% 5% 5% 7% 21%	2660
	21st to end	Night	23% 17% 7% 9% 6% 2 4 4 4 6% 17%	2660
December	1st to 10th	Night	25% 15% 8% 10% 7% 3 5% 4 4 5% 13%	2660
	11th to 20th	Night	30% 13% 5% 10% 7% 3 4 4 3 4 16%	3293
	21st to end	Night	27% 15% 5% 13% 5% 3 4 3 3 4 18%	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	72% 6% 10% 7% 5%	1517
	11th to 20th	Day	71% 6% 9% 9% 5%	1456
	21st to end	Day	59% 6% 10% 13% 12%	1726
February	1st to 10th	Day	60% 5% 11% 15% 8%	1620
	11th to 20th	Day	61% 6% 10% 17% 6% 1	1581
	21st to end	Day	53% 5% 14% 16% 11% 2	1391
March	1st to 10th	Day	56% 5% 11% 18% 9% 1	1894
	11th to 20th	Day	56% 6% 11% 17% 8% 2	1980
	21st to end	Day	61% 4 9% 15% 8% 2	2222
April	1st to 10th	Day	58% 5% 10% 15% 11% 2	2160
	11th to 20th	Day	51% 5% 10% 20% 12% 1	2156
	21st to end	Day	54% 6% 13% 16% 10% 1	2340
May	1st to 10th	Day	59% 6% 9% 17% 9% 1	2492
	11th to 20th	Day	51% 6% 11% 22% 11%	2482
	21st to end	Day	50% 7% 13% 20% 9%	2724
June	1st to 10th	Day	57% 8% 12% 18% 5%	2520
	11th to 20th	Day	49% 6% 12% 23% 9% 1	2590
	21st to end	Day	54% 7% 13% 16% 10% 1	2700
July	1st to 10th	Day	40% 10% 17% 23% 9%	2532
	11th to 20th	Day	42% 9% 16% 23% 10%	2444
	21st to end	Day	51% 8% 15% 19% 6%	2592
August	1st to 10th	Day	40% 10% 18% 25% 7%	2364
	11th to 20th	Day	44% 10% 18% 23% 6%	2304
	21st to end	Day	38% 7% 18% 27% 9%	2444
September	1st to 10th	Day	41% 8% 15% 26% 10%	2160
	11th to 20th	Day	47% 7% 13% 21% 12%	2160
	21st to end	Day	44% 6% 12% 26% 12%	1883
October	1st to 10th	Day	49% 6% 12% 24% 9%	1800
	11th to 20th	Day	59% 6% 15% 15% 5%	1800
	21st to end	Day	63% 6% 11% 15% 5%	1910
November	1st to 10th	Day	61% 7% 11% 16% 5%	1553
	11th to 20th	Day	68% 7% 12% 11% 3	1520
	21st to end	Day	68% 8% 13% 8% 3	1520
December	1st to 10th	Day	75% 7% 8% 7% 2	1520
	11th to 20th	Day	79% 7% 6% 6% 2 1	1905
	21st to end	Day	72% 5% 14% 6% 2	2783
January	1st to 10th	Night	68% 7% 10% 11% 4	2649
	11th to 20th	Night	65% 7% 12% 12% 5% 1	2549
	21st to end	Night	57% 6% 11% 18% 8% 1	2628
February	1st to 10th	Night	59% 5% 9% 14% 11% 1	2340
	11th to 20th	Night	64% 5% 9% 13% 7% 1	2276
	21st to end	Night	56% 5% 11% 16% 10% 2	1863
March	1st to 10th	Night	60% 6% 7% 15% 10% 1	2063
	11th to 20th	Night	57% 5% 10% 18% 8% 2	1980
	21st to end	Night	59% 5% 9% 16% 10% 2	2134
April	1st to 10th	Night	59% 6% 12% 12% 9% 2	1800
	11th to 20th	Night	53% 6% 9% 18% 12% 1	1750
	21st to end	Night	57% 6% 9% 15% 12% 1	1620
May	1st to 10th	Night	57% 5% 8% 19% 10% 1	1468
	11th to 20th	Night	51% 7% 8% 24% 9% 1	1417
	21st to end	Night	49% 9% 13% 20% 8% 1	1551
June	1st to 10th	Night	58% 6% 11% 17% 7%	1440
	11th to 20th	Night	48% 6% 11% 22% 13%	1381
	21st to end	Night	55% 7% 14% 15% 9% 1	1260
July	1st to 10th	Night	42% 9% 19% 24% 6%	1293
	11th to 20th	Night	43% 10% 16% 22% 8% 1	1392
	21st to end	Night	49% 10% 16% 18% 6%	1486
August	1st to 10th	Night	42% 10% 16% 25% 7%	1547
	11th to 20th	Night	48% 8% 18% 22% 5%	1596
	21st to end	Night	39% 10% 18% 25% 8%	1912
September	1st to 10th	Night	40% 9% 13% 27% 11%	1800
	11th to 20th	Night	44% 6% 11% 25% 12% 1	1800
	21st to end	Night	43% 7% 13% 26% 11%	2074
October	1st to 10th	Night	48% 5% 12% 24% 11%	2160
	11th to 20th	Night	51% 7% 15% 23% 4	2160
	21st to end	Night	63% 6% 9% 16% 6%	2445
November	1st to 10th	Night	59% 7% 13% 15% 5% 1	2521
	11th to 20th	Night	63% 9% 12% 11% 5%	2660
	21st to end	Night	69% 8% 13% 8% 2	2660
December	1st to 10th	Night	73% 7% 10% 8% 2	2660
	11th to 20th	Night	74% 6% 7% 10% 3	3295
	21st to end	Night	72% 5% 11% 9% 2	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	24% 18% 39% 16% 3	370
	11th to 20th	Day	25% 21% 29% 18% 7% 1	346
	21st to end	Day	17% 14% 38% 27% 3	374
February	1st to 10th	Day	25% 19% 32% 20% 4	340
	11th to 20th	Day	25% 16% 35% 19% 4 1	336
	21st to end	Day	15% 12% 37% 28% 7% 1	280
March	1st to 10th	Day	20% 15% 32% 27% 5% 1	382
	11th to 20th	Day	17% 15% 31% 27% 8% 1	420
	21st to end	Day	27% 13% 30% 20% 9% 1	462
April	1st to 10th	Day	28% 13% 34% 18% 6%	420
	11th to 20th	Day	23% 13% 34% 20% 8% 1	412
	21st to end	Day	24% 9% 26% 28% 12% 2	420
May	1st to 10th	Day	31% 9% 28% 24% 6% 3	575
	11th to 20th	Day	23% 6% 26% 30% 13% 2	624
	21st to end	Day	22% 6% 30% 32% 9% 1	680
June	1st to 10th	Day	31% 5% 23% 29% 10% 2	630
	11th to 20th	Day	21% 5% 25% 35% 13% 1	630
	21st to end	Day	27% 6% 18% 30% 16% 3	630
July	1st to 10th	Day	17% 3 21% 40% 17% 2	620
	11th to 20th	Day	16% 3 21% 36% 22% 2	618
	21st to end	Day	21% 3 19% 38% 18% 2	685
August	1st to 10th	Day	14% 2 18% 39% 23% 4	497
	11th to 20th	Day	21% 5% 18% 33% 17% 7%	439
	21st to end	Day	16% 6% 19% 33% 22% 4	495
September	1st to 10th	Day	16% 6% 28% 33% 16% 2	450
	11th to 20th	Day	20% 9% 27% 30% 13% 1	450
	21st to end	Day	20% 10% 26% 29% 13% 4	396
October	1st to 10th	Day	19% 9% 26% 29% 14% 3	360
	11th to 20th	Day	23% 11% 27% 24% 13% 2	360
	21st to end	Day	26% 15% 31% 20% 6% 2	396
November	1st to 10th	Day	23% 17% 33% 21% 6% 1	365
	11th to 20th	Day	26% 20% 31% 17% 5% 1	370
	21st to end	Day	27% 22% 28% 20% 3	370
December	1st to 10th	Day	29% 16% 31% 20% 3 1	370
	11th to 20th	Day	28% 28% 30% 13% 1	370
	21st to end	Day	24% 22% 34% 16% 3	407
January	1st to 10th	Night	26% 18% 33% 20% 3	850
	11th to 20th	Night	25% 16% 32% 22% 5%	811
	21st to end	Night	20% 14% 35% 24% 6% 1	816
February	1st to 10th	Night	27% 13% 32% 25% 4	720
	11th to 20th	Night	24% 14% 37% 20% 4	703
	21st to end	Night	19% 12% 33% 28% 9%	592
March	1st to 10th	Night	22% 10% 38% 26% 5%	720
	11th to 20th	Night	21% 10% 30% 29% 9% 1	720
	21st to end	Night	24% 11% 29% 25% 10% 1	748
April	1st to 10th	Night	30% 12% 27% 25% 6%	540
	11th to 20th	Night	25% 9% 32% 24% 8% 1	534
	21st to end	Night	28% 7% 28% 27% 8% 1	540
May	1st to 10th	Night	31% 6% 26% 24% 11% 2	540
	11th to 20th	Night	23% 6% 27% 31% 12% 2	532
	21st to end	Night	21% 5% 33% 33% 7% 1	583
June	1st to 10th	Night	29% 6% 23% 32% 9% 1	540
	11th to 20th	Night	24% 5% 22% 37% 11%	481
	21st to end	Night	24% 6% 21% 37% 12% 1	360
July	1st to 10th	Night	16% 2 22% 42% 17%	418
	11th to 20th	Night	16% 4 20% 44% 16% 1	522
	21st to end	Night	19% 3 19% 41% 17% 1	590
August	1st to 10th	Night	16% 2 17% 46% 19% 1	538
	11th to 20th	Night	18% 5% 21% 36% 19% 2	532
	21st to end	Night	15% 4 19% 39% 22% 1	594
September	1st to 10th	Night	16% 6% 26% 36% 16% 1	540
	11th to 20th	Night	21% 6% 24% 34% 14% 1	540
	21st to end	Night	21% 9% 29% 30% 8% 3	709
October	1st to 10th	Night	22% 6% 29% 28% 13% 1	720
	11th to 20th	Night	23% 11% 30% 25% 10% 1	720
	21st to end	Night	30% 14% 31% 20% 5%	827
November	1st to 10th	Night	24% 17% 31% 21% 5% 1	830
	11th to 20th	Night	27% 18% 29% 19% 5% 1	850
	21st to end	Night	32% 18% 29% 19% 3	850
December	1st to 10th	Night	31% 18% 32% 14% 5%	850
	11th to 20th	Night	28% 24% 31% 14% 3	850
	21st to end	Night	30% 19% 29% 18% 3 1	935

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