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 2024-10-25.

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% 9% 3 7% 5% 5% 5% 4 6% 7% 34%	101484
	Night	14% 10% 3 8% 5% 5% 5% 4 5% 6% 36%	97369
Transparency	Day	47% 9% 14% 19% 9% 1	101236
	Night	48% 8% 14% 20% 10% 1	97238
Seeing	Day	22% 13% 30% 23% 9% 2	15465
	Night	21% 9% 26% 33% 10% 1	23700

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% 8% 5% 4 4 4 5% 7% 32%	2420
	11th to 20th	Day	8% 11% 4 10% 6% 6% 6% 4 5% 6% 33%	2520
	21st to end	Day	6% 7% 4 11% 6% 5% 6% 4 6% 7% 38%	2772
February	1st to 10th	Day	6% 11% 4 9% 9% 6% 7% 4 7% 6% 31%	1539
	11th to 20th	Day	8% 12% 4 13% 8% 5% 5% 3 5% 6% 31%	1783
	21st to end	Day	10% 11% 3 8% 6% 5% 6% 5% 6% 6% 34%	1570
March	1st to 10th	Day	14% 11% 4 9% 6% 4 5% 4 5% 6% 32%	3116
	11th to 20th	Day	11% 17% 5% 9% 6% 5% 4 4 6% 6% 29%	3372
	21st to end	Day	11% 15% 4 9% 5% 4 4 3 4 5% 36%	3992
April	1st to 10th	Day	18% 14% 3 8% 5% 4 4 4 5% 5% 29%	2280
	11th to 20th	Day	15% 9% 2 7% 5% 4 5% 3 5% 5% 39%	2388
	21st to end	Day	20% 11% 2 6% 5% 4 4 4 5% 5% 34%	2470
May	1st to 10th	Day	16% 8% 3 7% 5% 5% 5% 4 5% 6% 35%	4168
	11th to 20th	Day	17% 8% 2 7% 5% 6% 5% 4 5% 7% 34%	4398
	21st to end	Day	15% 7% 3 7% 5% 5% 5% 5% 6% 8% 33%	5109
June	1st to 10th	Day	19% 7% 3 6% 6% 6% 6% 5% 6% 7% 30%	2850
	11th to 20th	Day	17% 6% 2 6% 5% 6% 5% 4 6% 7% 36%	2850
	21st to end	Day	22% 8% 3 6% 5% 5% 5% 4 5% 8% 28%	2850
July	1st to 10th	Day	14% 6% 2 7% 6% 6% 7% 5% 8% 9% 32%	4740
	11th to 20th	Day	11% 5% 2 7% 6% 6% 6% 6% 8% 9% 33%	4440
	21st to end	Day	11% 5% 2 6% 5% 6% 6% 5% 7% 9% 39%	4814
August	1st to 10th	Day	11% 5% 2 7% 5% 7% 7% 6% 8% 10% 31%	4369
	11th to 20th	Day	13% 6% 3 7% 6% 6% 7% 6% 7% 8% 30%	4045
	21st to end	Day	16% 5% 2 5% 4 5% 5% 4 6% 7% 41%	4441
September	1st to 10th	Day	12% 5% 2 6% 5% 5% 5% 4 5% 8% 42%	2275
	11th to 20th	Day	13% 5% 2 5% 4 4 4 3 6% 6% 48%	2090
	21st to end	Day	19% 4 2 6% 4 4 4 4 5% 8% 41%	2090
October	1st to 10th	Day	17% 7% 2 7% 5% 5% 5% 3 5% 6% 39%	2081
	11th to 20th	Day	20% 8% 2 7% 5% 5% 6% 4 7% 6% 31%	1740
	21st to end	Day	21% 10% 3 7% 4 4 4 3 5% 7% 31%	1825
November	1st to 10th	Day	13% 10% 3 9% 5% 5% 4 3 6% 5% 36%	1671
	11th to 20th	Day	15% 11% 4 8% 5% 6% 4 5% 5% 7% 31%	1639
	21st to end	Day	15% 14% 4 9% 6% 5% 4 4 5% 6% 29%	1348
December	1st to 10th	Day	15% 14% 5% 9% 6% 6% 4 4 4 4 28%	1330
	11th to 20th	Day	17% 12% 4 9% 5% 5% 5% 3 3 4 33%	1668
	21st to end	Day	16% 14% 3 8% 4 4 3 3 3 4 36%	2431
January	1st to 10th	Night	9% 14% 6% 10% 6% 6% 5% 4 5% 5% 29%	4524
	11th to 20th	Night	8% 12% 5% 10% 7% 6% 5% 4 5% 6% 32%	4440
	21st to end	Night	6% 11% 6% 13% 6% 6% 5% 4 5% 7% 31%	4884
February	1st to 10th	Night	6% 10% 3 9% 7% 5% 5% 4 5% 7% 38%	2639
	11th to 20th	Night	7% 12% 5% 11% 7% 5% 5% 3 6% 5% 33%	2282
	21st to end	Night	11% 12% 4 7% 5% 5% 4 4 6% 7% 35%	1884
March	1st to 10th	Night	15% 15% 4 9% 5% 4 5% 4 4 5% 30%	3724
	11th to 20th	Night	10% 15% 4 12% 7% 6% 6% 4 5% 6% 26%	3586
	21st to end	Night	13% 15% 4 8% 5% 4 4 3 4 5% 35%	3637
April	1st to 10th	Night	17% 15% 4 7% 4 4 4 2 5% 5% 33%	1900
	11th to 20th	Night	17% 8% 3 7% 5% 4 6% 3 4 8% 35%	1739
	21st to end	Night	21% 11% 3 6% 4 3 4 4 5% 5% 36%	1710
May	1st to 10th	Night	17% 8% 3 6% 5% 4 4 3 5% 6% 38%	2802
	11th to 20th	Night	18% 9% 2 7% 5% 5% 5% 4 5% 7% 34%	2495
	21st to end	Night	17% 6% 2 5% 4 4 5% 5% 6% 9% 39%	2472
June	1st to 10th	Night	18% 9% 3 7% 5% 4 5% 4 5% 7% 34%	1330
	11th to 20th	Night	15% 6% 2 5% 5% 5% 4 3 6% 8% 42%	1330
	21st to end	Night	19% 8% 3 8% 4 5% 5% 4 6% 5% 33%	1330
July	1st to 10th	Night	14% 6% 2 6% 4 5% 5% 4 7% 8% 40%	2226
	11th to 20th	Night	14% 5% 2 5% 4 4 5% 4 6% 8% 41%	2520
	21st to end	Night	11% 4 2 3 4 4 4 4 6% 9% 49%	2739
August	1st to 10th	Night	15% 6% 2 6% 5% 5% 6% 4 6% 7% 40%	2528
	11th to 20th	Night	13% 5% 2 8% 6% 7% 6% 5% 7% 7% 34%	2782
	21st to end	Night	15% 6% 3 7% 3 4 4 3 4 7% 44%	3212
September	1st to 10th	Night	14% 5% 1 5% 5% 4 4 3 5% 6% 48%	1905
	11th to 20th	Night	15% 6% 2 6% 3 5% 3 4 5% 6% 45%	2090
	21st to end	Night	16% 6% 2 7% 4 4 4 3 4 6% 44%	2090
October	1st to 10th	Night	15% 7% 3 6% 4 4 4 3 5% 6% 44%	2099
	11th to 20th	Night	21% 9% 2 6% 3 4 5% 3 4 6% 36%	2437
	21st to end	Night	19% 8% 3 6% 4 4 4 3 5% 7% 36%	2637
November	1st to 10th	Night	14% 9% 2 7% 5% 4 4 5% 5% 7% 39%	2407
	11th to 20th	Night	16% 9% 3 7% 4 4 4 4 5% 6% 36%	2541
	21st to end	Night	14% 12% 4 7% 6% 4 4 3 5% 6% 33%	2831
December	1st to 10th	Night	14% 15% 4 10% 6% 5% 5% 3 4 4 31%	2850
	11th to 20th	Night	16% 11% 3 8% 6% 4 4 4 6% 5% 32%	3531
	21st to end	Night	16% 16% 4 8% 5% 4 3 2 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% 9% 18% 13% 3	1570
March	1st to 10th	Day	52% 10% 12% 13% 10% 3	3119
	11th to 20th	Day	54% 9% 14% 13% 8% 1	3372
	21st to end	Day	51% 6% 9% 17% 13% 5%	3992
April	1st to 10th	Day	55% 8% 10% 13% 11% 3	2280
	11th to 20th	Day	46% 6% 12% 21% 11% 3	2388
	21st to end	Day	52% 7% 10% 16% 12% 2	2470
May	1st to 10th	Day	46% 9% 15% 16% 13% 1	4182
	11th to 20th	Day	46% 9% 17% 20% 6% 1	4402
	21st to end	Day	45% 10% 13% 19% 12%	5109
June	1st to 10th	Day	49% 12% 13% 19% 7% 1	2850
	11th to 20th	Day	44% 10% 14% 20% 10% 1	2850
	21st to end	Day	51% 11% 13% 16% 8%	2850
July	1st to 10th	Day	42% 11% 18% 21% 8%	4665
	11th to 20th	Day	41% 12% 19% 24% 5%	4365
	21st to end	Day	38% 12% 18% 22% 10%	4661
August	1st to 10th	Day	42% 14% 19% 18% 8%	4355
	11th to 20th	Day	45% 13% 20% 17% 4	4084
	21st to end	Day	40% 9% 19% 19% 12% 1	4441
September	1st to 10th	Day	39% 10% 16% 23% 12%	2275
	11th to 20th	Day	36% 8% 18% 27% 10%	2090
	21st to end	Day	41% 10% 14% 23% 11%	2090
October	1st to 10th	Day	44% 8% 13% 24% 11%	2081
	11th to 20th	Day	51% 11% 15% 18% 4	1740
	21st to end	Day	53% 8% 14% 16% 7% 1	1825
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	1884
March	1st to 10th	Night	54% 8% 12% 16% 8% 2	3734
	11th to 20th	Night	57% 9% 11% 14% 7% 1	3587
	21st to end	Night	52% 6% 10% 20% 10% 2	3637
April	1st to 10th	Night	54% 4 8% 17% 13% 3	1900
	11th to 20th	Night	47% 8% 9% 22% 13% 1	1739
	21st to end	Night	52% 5% 9% 19% 12% 2	1710
May	1st to 10th	Night	45% 8% 14% 20% 12% 1	2808
	11th to 20th	Night	45% 10% 15% 19% 10% 1	2496
	21st to end	Night	40% 9% 16% 21% 13% 1	2473
June	1st to 10th	Night	48% 8% 14% 21% 7% 1	1330
	11th to 20th	Night	39% 8% 14% 23% 13% 3	1330
	21st to end	Night	51% 10% 12% 17% 10%	1330
July	1st to 10th	Night	39% 8% 23% 26% 4	2191
	11th to 20th	Night	39% 9% 21% 26% 5%	2472
	21st to end	Night	31% 9% 25% 23% 11%	2651
August	1st to 10th	Night	42% 9% 17% 24% 7%	2518
	11th to 20th	Night	45% 13% 20% 13% 9%	2809
	21st to end	Night	40% 8% 13% 27% 12%	3212
September	1st to 10th	Night	36% 9% 16% 25% 13% 1	1905
	11th to 20th	Night	39% 8% 16% 27% 9% 1	2090
	21st to end	Night	41% 8% 13% 25% 12% 1	2090
October	1st to 10th	Night	42% 6% 13% 26% 12% 1	2099
	11th to 20th	Night	48% 8% 14% 24% 6%	2437
	21st to end	Night	48% 8% 16% 19% 9% 1	2637
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	19% 17% 36% 22% 5% 1	382
March	1st to 10th	Day	24% 17% 30% 21% 7%	460
	11th to 20th	Day	21% 19% 31% 19% 9% 1	460
	21st to end	Day	22% 21% 28% 19% 8% 1	506
April	1st to 10th	Day	30% 17% 27% 19% 7% 1	460
	11th to 20th	Day	25% 16% 31% 19% 8% 2	452
	21st to end	Day	29% 15% 36% 15% 4 2	460
May	1st to 10th	Day	28% 11% 29% 22% 9% 1	463
	11th to 20th	Day	23% 18% 28% 21% 9% 2	487
	21st to end	Day	22% 17% 32% 21% 6% 1	684
June	1st to 10th	Day	26% 11% 26% 25% 10% 1	680
	11th to 20th	Day	23% 12% 27% 24% 13% 1	680
	21st to end	Day	24% 10% 28% 25% 10% 2	680
July	1st to 10th	Day	18% 9% 30% 30% 11% 2	663
	11th to 20th	Day	16% 9% 35% 27% 10% 2	484
	21st to end	Day	16% 8% 34% 30% 11% 2	532
August	1st to 10th	Day	15% 10% 33% 25% 13% 3	486
	11th to 20th	Day	15% 8% 29% 31% 13% 4	479
	21st to end	Day	17% 13% 31% 19% 16% 3	539
September	1st to 10th	Day	18% 13% 29% 26% 10% 4	490
	11th to 20th	Day	18% 9% 31% 25% 11% 5%	490
	21st to end	Day	22% 11% 27% 22% 13% 5%	490
October	1st to 10th	Day	19% 15% 27% 26% 9% 3	485
	11th to 20th	Day	27% 13% 27% 19% 12% 2	390
	21st to end	Day	31% 11% 27% 23% 8%	411
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%	628
March	1st to 10th	Night	20% 13% 29% 31% 6% 1	760
	11th to 20th	Night	20% 13% 30% 29% 8% 1	760
	21st to end	Night	22% 13% 28% 28% 8% 1	836
April	1st to 10th	Night	29% 9% 29% 26% 6% 1	760
	11th to 20th	Night	25% 9% 28% 28% 9%	615
	21st to end	Night	30% 9% 26% 27% 8% 1	570
May	1st to 10th	Night	26% 8% 28% 30% 7%	551
	11th to 20th	Night	26% 7% 21% 32% 13%	374
	21st to end	Night	27% 8% 27% 29% 8% 1	411
June	1st to 10th	Night	23% 5% 22% 36% 13% 1	380
	11th to 20th	Night	17% 3 23% 38% 17% 2	380
	21st to end	Night	22% 5% 21% 38% 13% 1	380
July	1st to 10th	Night	20% 5% 22% 42% 11% 1	370
	11th to 20th	Night	14% 3 19% 45% 17% 1	368
	21st to end	Night	15% 4 19% 47% 15% 1	416
August	1st to 10th	Night	14% 4 14% 47% 20% 2	387
	11th to 20th	Night	17% 4 18% 40% 21% 1	562
	21st to end	Night	16% 5% 23% 36% 18% 1	676
September	1st to 10th	Night	16% 6% 24% 39% 14% 1	760
	11th to 20th	Night	21% 4 22% 36% 15% 3	760
	21st to end	Night	19% 6% 20% 38% 16% 1	760
October	1st to 10th	Night	20% 10% 24% 34% 11% 2	760
	11th to 20th	Night	26% 8% 23% 31% 12% 1	760
	21st to end	Night	26% 9% 26% 30% 10%	812
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 2024-10-22 20:21:00. Version ac07

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