Observatoire Iris
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire Iris clear sky chart from 2005-11-06 to 2024-02-25.

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	14% 8% 3 6% 6% 4 5% 5% 6% 7% 34%	77992
	Night	14% 10% 4 7% 6% 3 5% 5% 5% 6% 36%	81311
Transparency	Day	48% 9% 13% 19% 9% 1	77691
	Night	48% 8% 13% 20% 10% 1	81147
Seeing	Day	22% 13% 31% 23% 9% 2	14457
	Night	21% 9% 27% 32% 10% 1	22957

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	11% 14% 5% 7% 6% 3 4 5% 5% 7% 32%	2420
	11th to 20th	Day	8% 10% 4 9% 7% 4 7% 5% 5% 7% 33%	2520
	21st to end	Day	6% 7% 4 9% 7% 4 7% 6% 6% 7% 38%	2772
February	1st to 10th	Day	6% 9% 5% 8% 9% 5% 7% 5% 7% 7% 31%	1539
	11th to 20th	Day	8% 11% 5% 11% 9% 4 6% 5% 5% 6% 31%	1783
	21st to end	Day	10% 11% 3 8% 7% 4 5% 6% 5% 6% 35%	1520
March	1st to 10th	Day	12% 12% 4 8% 7% 4 5% 5% 5% 5% 33%	1797
	11th to 20th	Day	10% 11% 5% 7% 6% 4 5% 5% 6% 6% 34%	1914
	21st to end	Day	11% 12% 5% 8% 7% 4 5% 5% 4 6% 34%	2272
April	1st to 10th	Day	18% 13% 4 7% 6% 3 4 5% 5% 6% 29%	2160
	11th to 20th	Day	15% 8% 3 6% 6% 3 5% 5% 5% 5% 40%	2260
	21st to end	Day	20% 11% 2 5% 5% 3 5% 4 5% 5% 33%	2340
May	1st to 10th	Day	20% 7% 3 5% 5% 3 5% 5% 5% 7% 35%	2374
	11th to 20th	Day	17% 7% 3 6% 6% 4 5% 5% 5% 5% 36%	2478
	21st to end	Day	16% 6% 3 6% 5% 3 5% 7% 5% 8% 37%	2871
June	1st to 10th	Day	19% 7% 3 5% 7% 4 6% 6% 6% 7% 30%	2700
	11th to 20th	Day	18% 6% 3 5% 5% 4 6% 6% 6% 7% 36%	2700
	21st to end	Day	22% 7% 3 5% 5% 4 6% 6% 5% 8% 29%	2700
July	1st to 10th	Day	14% 5% 2 6% 6% 4 6% 6% 7% 10% 33%	2694
	11th to 20th	Day	11% 5% 3 5% 8% 5% 7% 7% 7% 9% 33%	2520
	21st to end	Day	12% 5% 3 6% 6% 4 6% 7% 7% 10% 34%	2744
August	1st to 10th	Day	13% 6% 2 6% 7% 5% 7% 7% 8% 10% 29%	2479
	11th to 20th	Day	11% 6% 3 5% 6% 4 7% 6% 6% 9% 37%	2304
	21st to end	Day	12% 5% 2 5% 6% 4 6% 5% 8% 8% 40%	2525
September	1st to 10th	Day	13% 5% 2 6% 6% 3 5% 5% 5% 8% 42%	2156
	11th to 20th	Day	14% 5% 2 5% 5% 3 4 4 5% 7% 46%	1980
	21st to end	Day	18% 4 2 4 5% 3 5% 4 5% 7% 42%	1980
October	1st to 10th	Day	17% 6% 2 6% 5% 3 5% 4 5% 7% 40%	1972
	11th to 20th	Day	20% 7% 3 6% 7% 4 5% 5% 6% 6% 30%	1649
	21st to end	Day	21% 10% 3 7% 6% 2 4 4 5% 7% 31%	1782
November	1st to 10th	Day	13% 9% 4 8% 6% 4 5% 4 6% 5% 36%	1671
	11th to 20th	Day	15% 10% 4 7% 6% 4 5% 6% 5% 7% 31%	1639
	21st to end	Day	15% 13% 5% 8% 6% 3 5% 4 5% 6% 29%	1348
December	1st to 10th	Day	15% 13% 6% 9% 6% 4 6% 4 4 5% 29%	1330
	11th to 20th	Day	17% 11% 4 8% 6% 4 6% 4 3 4 33%	1668
	21st to end	Day	17% 14% 4 7% 6% 3 4 3 3 4 36%	2431
January	1st to 10th	Night	10% 13% 6% 10% 7% 4 6% 5% 5% 6% 29%	4524
	11th to 20th	Night	8% 12% 5% 9% 8% 5% 5% 5% 5% 6% 32%	4440
	21st to end	Night	6% 10% 6% 12% 7% 5% 6% 5% 5% 7% 31%	4884
February	1st to 10th	Night	7% 9% 4 8% 7% 4 6% 5% 5% 7% 38%	2639
	11th to 20th	Night	7% 11% 6% 10% 8% 4 5% 4 6% 5% 34%	2282
	21st to end	Night	11% 11% 4 7% 6% 4 4 5% 5% 7% 35%	1831
March	1st to 10th	Night	11% 14% 5% 7% 5% 3 5% 4 4 7% 34%	2150
	11th to 20th	Night	12% 11% 5% 8% 6% 3 5% 4 4 6% 35%	2044
	21st to end	Night	11% 13% 4 6% 5% 3 4 5% 4 5% 38%	2081
April	1st to 10th	Night	18% 14% 4 6% 6% 3 3 4 5% 5% 32%	1800
	11th to 20th	Night	16% 8% 3 6% 6% 3 5% 4 4 8% 36%	1647
	21st to end	Night	22% 11% 3 5% 5% 2 4 4 4 5% 35%	1620
May	1st to 10th	Night	22% 8% 3 6% 5% 3 4 4 4 5% 36%	1602
	11th to 20th	Night	21% 7% 2 5% 4 3 5% 5% 4 6% 37%	1415
	21st to end	Night	20% 8% 3 4 4 2 4 4 5% 6% 40%	1401
June	1st to 10th	Night	18% 8% 4 6% 7% 2 5% 5% 4 8% 34%	1260
	11th to 20th	Night	15% 5% 2 5% 5% 4 4 5% 6% 8% 41%	1260
	21st to end	Night	19% 7% 4 7% 6% 3 5% 5% 6% 5% 33%	1260
July	1st to 10th	Night	18% 5% 3 4 4 3 5% 5% 6% 6% 42%	1263
	11th to 20th	Night	11% 5% 3 6% 6% 3 6% 6% 7% 8% 40%	1440
	21st to end	Night	13% 5% 3 4 5% 4 6% 6% 6% 9% 40%	1574
August	1st to 10th	Night	12% 4 2 5% 5% 3 5% 5% 7% 8% 44%	1440
	11th to 20th	Night	15% 5% 2 6% 6% 4 5% 6% 6% 7% 38%	1596
	21st to end	Night	13% 6% 3 5% 4 3 5% 3 5% 8% 46%	1828
September	1st to 10th	Night	14% 4 2 4 5% 2 5% 4 5% 7% 47%	1804
	11th to 20th	Night	16% 6% 2 5% 5% 3 4 4 5% 6% 44%	1980
	21st to end	Night	17% 6% 2 5% 4 3 4 4 4 6% 46%	1980
October	1st to 10th	Night	14% 7% 3 5% 5% 3 5% 3 5% 6% 45%	1988
	11th to 20th	Night	21% 8% 2 5% 4 3 5% 4 4 6% 36%	2308
	21st to end	Night	20% 8% 3 5% 5% 3 4 4 5% 7% 36%	2574
November	1st to 10th	Night	14% 8% 3 6% 5% 3 5% 6% 5% 7% 39%	2407
	11th to 20th	Night	16% 8% 4 6% 4 3 5% 5% 5% 7% 36%	2541
	21st to end	Night	14% 12% 5% 7% 6% 3 4 4 5% 7% 33%	2831
December	1st to 10th	Night	14% 14% 5% 9% 6% 4 5% 4 4 4 31%	2850
	11th to 20th	Night	17% 10% 4 7% 6% 3 5% 5% 6% 6% 32%	3531
	21st to end	Night	16% 16% 4 7% 6% 3 4 3 4 4 34%	5236

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	50% 8% 15% 21% 6%	2421
	11th to 20th	Day	50% 7% 10% 23% 9% 1	2520
	21st to end	Day	43% 10% 12% 19% 15% 1	2772
February	1st to 10th	Day	50% 8% 11% 16% 12% 2	1539
	11th to 20th	Day	54% 7% 11% 15% 11% 2	1791
	21st to end	Day	48% 9% 8% 18% 13% 3	1520
March	1st to 10th	Day	53% 6% 10% 14% 13% 3	1800
	11th to 20th	Day	49% 9% 11% 16% 13% 2	1914
	21st to end	Day	52% 6% 10% 18% 10% 4	2272
April	1st to 10th	Day	56% 8% 10% 13% 10% 3	2160
	11th to 20th	Day	46% 6% 12% 22% 11% 3	2260
	21st to end	Day	53% 8% 10% 16% 12% 2	2340
May	1st to 10th	Day	48% 8% 13% 17% 13% 2	2374
	11th to 20th	Day	48% 8% 12% 20% 10% 1	2482
	21st to end	Day	45% 11% 12% 21% 10% 1	2871
June	1st to 10th	Day	49% 12% 13% 18% 8% 1	2700
	11th to 20th	Day	46% 10% 13% 20% 10% 1	2700
	21st to end	Day	51% 12% 13% 16% 8%	2700
July	1st to 10th	Day	42% 12% 17% 21% 7%	2619
	11th to 20th	Day	44% 12% 15% 21% 7%	2445
	21st to end	Day	43% 14% 15% 19% 7%	2591
August	1st to 10th	Day	46% 16% 17% 18% 4	2465
	11th to 20th	Day	39% 14% 20% 20% 7%	2304
	21st to end	Day	38% 12% 18% 24% 9%	2525
September	1st to 10th	Day	39% 10% 17% 22% 12%	2156
	11th to 20th	Day	37% 8% 18% 26% 10% 1	1980
	21st to end	Day	41% 10% 14% 23% 11%	1980
October	1st to 10th	Day	44% 8% 13% 24% 11%	1972
	11th to 20th	Day	52% 11% 15% 17% 4	1649
	21st to end	Day	54% 8% 13% 16% 7% 1	1782
November	1st to 10th	Day	49% 9% 12% 19% 10% 1	1671
	11th to 20th	Day	52% 8% 10% 21% 8% 1	1639
	21st to end	Day	55% 7% 15% 15% 7% 1	1348
December	1st to 10th	Day	59% 7% 9% 17% 8%	1330
	11th to 20th	Day	56% 6% 9% 19% 10% 1	1668
	21st to end	Day	52% 5% 12% 19% 8% 3	2431
January	1st to 10th	Night	55% 8% 12% 14% 9% 1	4524
	11th to 20th	Night	50% 9% 12% 20% 8% 1	4440
	21st to end	Night	49% 8% 12% 18% 12% 1	4884
February	1st to 10th	Night	46% 8% 11% 20% 13% 1	2639
	11th to 20th	Night	53% 6% 11% 16% 14% 1	2285
	21st to end	Night	47% 7% 10% 19% 14% 2	1831
March	1st to 10th	Night	50% 7% 10% 19% 11% 2	2160
	11th to 20th	Night	51% 7% 11% 17% 12% 2	2045
	21st to end	Night	47% 7% 11% 19% 12% 4	2081
April	1st to 10th	Night	56% 5% 8% 16% 12% 3	1800
	11th to 20th	Night	45% 8% 10% 22% 13% 2	1647
	21st to end	Night	53% 5% 9% 19% 12% 2	1620
May	1st to 10th	Night	51% 6% 11% 16% 13% 2	1602
	11th to 20th	Night	47% 7% 12% 20% 13% 1	1416
	21st to end	Night	46% 8% 13% 19% 14% 1	1402
June	1st to 10th	Night	49% 8% 15% 20% 8% 1	1260
	11th to 20th	Night	41% 8% 12% 24% 12% 3	1260
	21st to end	Night	51% 10% 12% 17% 10%	1260
July	1st to 10th	Night	42% 9% 18% 24% 7%	1228
	11th to 20th	Night	40% 11% 16% 24% 8%	1392
	21st to end	Night	40% 11% 18% 23% 7%	1486
August	1st to 10th	Night	37% 12% 20% 27% 4	1430
	11th to 20th	Night	41% 13% 19% 18% 9%	1596
	21st to end	Night	37% 10% 15% 31% 8%	1828
September	1st to 10th	Night	37% 9% 16% 25% 12% 1	1804
	11th to 20th	Night	40% 9% 16% 26% 9% 1	1980
	21st to end	Night	40% 8% 13% 26% 12% 1	1980
October	1st to 10th	Night	41% 6% 13% 26% 13% 1	1988
	11th to 20th	Night	49% 8% 14% 23% 6%	2308
	21st to end	Night	49% 9% 14% 19% 9% 1	2574
November	1st to 10th	Night	45% 9% 14% 22% 9% 1	2407
	11th to 20th	Night	48% 8% 13% 20% 10% 1	2541
	21st to end	Night	52% 9% 11% 18% 9% 1	2831
December	1st to 10th	Night	58% 6% 10% 18% 7% 1	2850
	11th to 20th	Night	51% 8% 12% 18% 10% 1	3532
	21st to end	Night	55% 7% 12% 15% 9% 2	5236

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	20% 6% 37% 29% 7% 1	180
	11th to 20th	Day	26% 13% 33% 21% 8%	180
	21st to end	Day	16% 15% 38% 20% 8% 3	198
February	1st to 10th	Day	19% 17% 41% 18% 6% 1	199
	11th to 20th	Day	18% 17% 38% 21% 6%	424
	21st to end	Day	18% 18% 36% 22% 5% 1	362
March	1st to 10th	Day	24% 18% 30% 21% 7%	420
	11th to 20th	Day	21% 19% 31% 19% 9% 1	420
	21st to end	Day	22% 20% 28% 20% 9% 1	462
April	1st to 10th	Day	30% 17% 26% 19% 7% 1	420
	11th to 20th	Day	24% 15% 31% 20% 8% 2	412
	21st to end	Day	29% 13% 35% 15% 5% 2	420
May	1st to 10th	Day	29% 11% 29% 22% 8% 1	423
	11th to 20th	Day	23% 19% 29% 20% 8% 2	447
	21st to end	Day	22% 16% 32% 22% 7% 1	631
June	1st to 10th	Day	26% 12% 27% 25% 9% 1	630
	11th to 20th	Day	23% 12% 26% 24% 13% 1	630
	21st to end	Day	23% 9% 29% 26% 10% 2	630
July	1st to 10th	Day	18% 8% 30% 30% 12% 2	614
	11th to 20th	Day	16% 9% 35% 27% 11% 2	444
	21st to end	Day	16% 7% 34% 30% 11% 2	488
August	1st to 10th	Day	15% 11% 33% 25% 13% 4	446
	11th to 20th	Day	15% 6% 31% 31% 13% 4	439
	21st to end	Day	17% 14% 32% 18% 17% 3	495
September	1st to 10th	Day	18% 12% 30% 25% 10% 5%	450
	11th to 20th	Day	20% 10% 32% 24% 10% 5%	450
	21st to end	Day	22% 12% 28% 22% 12% 4	450
October	1st to 10th	Day	18% 15% 27% 27% 9% 3	446
	11th to 20th	Day	28% 11% 27% 19% 12% 2	360
	21st to end	Day	32% 11% 26% 23% 7% 1	396
November	1st to 10th	Day	25% 11% 32% 23% 6% 2	365
	11th to 20th	Day	25% 11% 27% 26% 10% 1	370
	21st to end	Day	27% 18% 31% 18% 5% 1	198
December	1st to 10th	Day	23% 11% 31% 21% 11% 3	180
	11th to 20th	Day	27% 16% 32% 18% 6% 2	180
	21st to end	Day	26% 16% 27% 23% 6% 2	198
January	1st to 10th	Night	20% 9% 31% 33% 6% 1	787
	11th to 20th	Night	18% 9% 34% 33% 7%	760
	21st to end	Night	13% 9% 27% 42% 9% 1	836
February	1st to 10th	Night	14% 8% 37% 31% 10% 1	760
	11th to 20th	Night	16% 12% 36% 27% 8% 1	743
	21st to end	Night	18% 9% 32% 33% 7%	611
March	1st to 10th	Night	19% 13% 30% 31% 7% 1	720
	11th to 20th	Night	20% 13% 30% 28% 8% 1	720
	21st to end	Night	21% 14% 27% 29% 8% 1	792
April	1st to 10th	Night	29% 9% 28% 27% 7% 1	720
	11th to 20th	Night	24% 10% 28% 29% 10%	583
	21st to end	Night	29% 9% 26% 27% 8% 1	540
May	1st to 10th	Night	27% 8% 28% 30% 7%	522
	11th to 20th	Night	26% 8% 22% 31% 13%	354
	21st to end	Night	28% 8% 28% 28% 8% 1	389
June	1st to 10th	Night	23% 6% 22% 36% 13% 1	360
	11th to 20th	Night	18% 3 23% 39% 16% 2	360
	21st to end	Night	22% 4 20% 39% 14% 1	360
July	1st to 10th	Night	21% 4 23% 41% 11% 1	350
	11th to 20th	Night	14% 3 20% 45% 18% 1	348
	21st to end	Night	15% 4 19% 46% 14% 2	394
August	1st to 10th	Night	14% 4 13% 46% 20% 2	366
	11th to 20th	Night	17% 3 18% 39% 21% 2	532
	21st to end	Night	16% 5% 23% 37% 17% 2	640
September	1st to 10th	Night	16% 6% 23% 39% 15% 1	720
	11th to 20th	Night	22% 4 23% 35% 13% 3	720
	21st to end	Night	20% 6% 21% 38% 15% 1	720
October	1st to 10th	Night	19% 10% 24% 34% 11% 2	720
	11th to 20th	Night	26% 7% 23% 31% 13% 1	720
	21st to end	Night	26% 9% 26% 30% 9%	792
November	1st to 10th	Night	22% 12% 26% 30% 9% 1	740
	11th to 20th	Night	24% 12% 27% 27% 9% 1	793
	21st to end	Night	24% 13% 24% 31% 7% 1	850
December	1st to 10th	Night	24% 10% 29% 28% 8% 1	850
	11th to 20th	Night	21% 13% 29% 28% 8%	850
	21st to end	Night	25% 11% 27% 26% 10% 1	935

Page generated at 2024-02-22 06:05:44. Version ac07

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