Observatoire Isis
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire Isis 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	17% 10% 3 8% 5% 5% 4 4 5% 7% 32%	101483
	Night	18% 11% 4 8% 5% 4 4 3 5% 6% 32%	97323
Transparency	Day	50% 7% 12% 20% 10% 1	101241
	Night	53% 6% 11% 19% 9% 1	97185
Seeing	Day	24% 15% 31% 19% 8% 2	14431
	Night	24% 13% 28% 25% 9% 1	23471

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	16% 18% 7% 14% 6% 5% 4 3 4 4 20%	2517
	11th to 20th	Day	15% 14% 5% 16% 8% 6% 4 3 4 5% 21%	2520
	21st to end	Day	14% 18% 7% 13% 7% 7% 5% 4 4 5% 18%	2868
February	1st to 10th	Day	13% 16% 6% 13% 7% 5% 4 3 4 6% 23%	1710
	11th to 20th	Day	13% 16% 6% 12% 6% 4 4 3 4 5% 28%	1666
	21st to end	Day	19% 12% 4 11% 7% 5% 4 4 5% 5% 25%	1570
March	1st to 10th	Day	21% 16% 4 8% 5% 5% 3 3 4 4 26%	3116
	11th to 20th	Day	12% 17% 6% 10% 6% 5% 5% 3 4 5% 27%	3470
	21st to end	Day	14% 8% 3 7% 6% 4 4 4 5% 6% 39%	4032
April	1st to 10th	Day	23% 12% 3 8% 5% 4 3 2 4 6% 31%	2280
	11th to 20th	Day	23% 10% 3 6% 4 3 3 3 4 5% 35%	2254
	21st to end	Day	22% 10% 3 9% 5% 5% 4 3 4 5% 30%	2427
May	1st to 10th	Day	21% 9% 3 8% 5% 4 5% 4 6% 6% 30%	4426
	11th to 20th	Day	18% 8% 3 8% 5% 4 4 3 5% 7% 33%	4398
	21st to end	Day	17% 8% 2 6% 5% 5% 5% 3 5% 7% 36%	4837
June	1st to 10th	Day	23% 8% 3 8% 5% 4 5% 4 5% 7% 29%	2660
	11th to 20th	Day	15% 6% 3 6% 5% 5% 5% 4 6% 9% 37%	2660
	21st to end	Day	20% 7% 2 6% 4 5% 6% 5% 7% 7% 31%	2660
July	1st to 10th	Day	12% 7% 3 7% 5% 5% 6% 5% 7% 10% 34%	4440
	11th to 20th	Day	15% 5% 2 7% 4 5% 5% 5% 6% 8% 39%	4440
	21st to end	Day	12% 6% 2 5% 4 4 4 4 6% 8% 46%	4814
August	1st to 10th	Day	14% 6% 2 8% 5% 5% 6% 4 6% 8% 35%	4374
	11th to 20th	Day	17% 5% 2 7% 4 4 4 4 7% 9% 37%	3994
	21st to end	Day	14% 5% 2 6% 5% 4 5% 4 7% 9% 40%	4180
September	1st to 10th	Day	15% 6% 2 6% 4 4 4 4 5% 8% 43%	2280
	11th to 20th	Day	14% 5% 3 5% 4 4 5% 3 5% 7% 47%	2279
	21st to end	Day	20% 6% 2 5% 4 4 4 3 5% 8% 39%	2029
October	1st to 10th	Day	22% 8% 2 7% 4 4 4 3 5% 6% 34%	1900
	11th to 20th	Day	24% 10% 3 7% 5% 5% 5% 3 5% 6% 28%	1900
	21st to end	Day	27% 10% 3 7% 5% 5% 4 3 4 5% 26%	1873
November	1st to 10th	Day	15% 10% 3 8% 4 5% 5% 3 7% 7% 34%	1671
	11th to 20th	Day	20% 13% 4 9% 5% 5% 5% 3 5% 6% 25%	1520
	21st to end	Day	23% 14% 4 11% 6% 5% 4 3 5% 5% 20%	1520
December	1st to 10th	Day	21% 19% 6% 11% 5% 3 4 3 5% 5% 16%	1520
	11th to 20th	Day	21% 12% 5% 11% 6% 4 4 3 4 5% 24%	1906
	21st to end	Day	26% 20% 5% 9% 4 3 3 2 3 4 18%	2772
January	1st to 10th	Night	17% 19% 6% 11% 6% 4 4 3 5% 5% 21%	4429
	11th to 20th	Night	12% 16% 6% 12% 7% 5% 5% 3 5% 5% 24%	4440
	21st to end	Night	15% 18% 7% 12% 6% 7% 4 3 3 4 20%	4784
February	1st to 10th	Night	13% 15% 5% 12% 7% 5% 5% 4 4 5% 24%	2470
	11th to 20th	Night	13% 14% 6% 11% 6% 5% 5% 3 4 5% 29%	2399
	21st to end	Night	16% 14% 4 8% 6% 4 5% 4 4 6% 29%	1884
March	1st to 10th	Night	21% 12% 4 8% 5% 4 4 3 3 4 31%	3724
	11th to 20th	Night	15% 18% 5% 10% 5% 4 4 3 5% 5% 26%	3484
	21st to end	Night	15% 10% 4 9% 4 4 3 2 4 6% 40%	3599
April	1st to 10th	Night	21% 11% 3 8% 5% 4 3 3 4 6% 32%	1900
	11th to 20th	Night	19% 9% 3 7% 5% 4 4 3 5% 5% 35%	1875
	21st to end	Night	22% 9% 3 8% 4 4 3 3 5% 6% 34%	1768
May	1st to 10th	Night	20% 9% 4 8% 4 5% 4 4 5% 6% 30%	2514
	11th to 20th	Night	21% 7% 2 7% 4 4 4 3 5% 6% 36%	2495
	21st to end	Night	16% 6% 3 6% 4 5% 5% 4 6% 6% 38%	2738
June	1st to 10th	Night	18% 6% 2 7% 5% 4 5% 5% 7% 7% 35%	1520
	11th to 20th	Night	15% 5% 2 7% 4 4 5% 4 6% 6% 42%	1520
	21st to end	Night	19% 7% 2 5% 6% 5% 5% 4 5% 8% 35%	1520
July	1st to 10th	Night	13% 7% 3 7% 5% 6% 6% 4 6% 7% 36%	2520
	11th to 20th	Night	15% 5% 2 6% 5% 3 4 4 6% 8% 41%	2520
	21st to end	Night	11% 6% 2 5% 4 4 5% 4 7% 8% 45%	2739
August	1st to 10th	Night	13% 7% 3 7% 5% 5% 4 4 6% 8% 39%	2551
	11th to 20th	Night	16% 8% 2 5% 3 4 4 3 6% 8% 42%	2803
	21st to end	Night	15% 5% 2 6% 5% 4 4 4 5% 7% 41%	3476
September	1st to 10th	Night	12% 5% 2 6% 4 3 4 4 6% 6% 47%	1900
	11th to 20th	Night	15% 6% 1 6% 3 3 4 4 5% 5% 48%	1900
	21st to end	Night	18% 6% 2 6% 5% 3 3 3 4 6% 44%	2147
October	1st to 10th	Night	20% 7% 2 7% 3 4 3 4 5% 5% 40%	2280
	11th to 20th	Night	22% 8% 2 7% 6% 5% 4 4 5% 6% 32%	2280
	21st to end	Night	23% 10% 3 8% 5% 4 4 3 5% 6% 28%	2589
November	1st to 10th	Night	19% 11% 3 6% 5% 4 4 4 5% 6% 33%	2407
	11th to 20th	Night	19% 12% 4 9% 5% 5% 5% 3 4 5% 29%	2656
	21st to end	Night	21% 13% 4 10% 5% 4 5% 4 4 5% 24%	2660
December	1st to 10th	Night	21% 18% 5% 10% 5% 5% 4 4 5% 5% 18%	2660
	11th to 20th	Night	22% 15% 4 10% 6% 3 3 2 3 5% 27%	3288
	21st to end	Night	28% 19% 4 10% 4 3 3 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	66% 8% 10% 10% 5% 1	2518
	11th to 20th	Day	63% 6% 12% 13% 6%	2520
	21st to end	Day	67% 8% 10% 10% 5%	2868
February	1st to 10th	Day	61% 5% 10% 18% 5% 2	1710
	11th to 20th	Day	58% 4 12% 15% 9% 1	1675
	21st to end	Day	59% 6% 11% 13% 10% 2	1570
March	1st to 10th	Day	60% 6% 9% 16% 9% 1	3119
	11th to 20th	Day	57% 6% 12% 14% 10% 2	3470
	21st to end	Day	44% 5% 7% 23% 17% 3	4032
April	1st to 10th	Day	55% 4 8% 18% 12% 3	2280
	11th to 20th	Day	51% 4 7% 21% 15% 2	2254
	21st to end	Day	56% 4 8% 15% 13% 4	2427
May	1st to 10th	Day	51% 7% 8% 22% 10% 2	4440
	11th to 20th	Day	48% 8% 14% 19% 9% 2	4402
	21st to end	Day	45% 7% 11% 21% 14% 2	4837
June	1st to 10th	Day	53% 6% 11% 18% 11% 1	2660
	11th to 20th	Day	42% 8% 12% 23% 12% 2	2660
	21st to end	Day	49% 9% 13% 20% 10% 1	2660
July	1st to 10th	Day	43% 7% 14% 24% 11%	4370
	11th to 20th	Day	40% 7% 15% 30% 8%	4365
	21st to end	Day	35% 8% 15% 31% 12%	4661
August	1st to 10th	Day	45% 7% 16% 23% 9%	4360
	11th to 20th	Day	41% 8% 15% 28% 8%	4033
	21st to end	Day	39% 6% 17% 25% 12% 1	4180
September	1st to 10th	Day	39% 7% 14% 28% 12%	2280
	11th to 20th	Day	36% 6% 14% 31% 12% 1	2279
	21st to end	Day	42% 7% 14% 26% 10%	2029
October	1st to 10th	Day	48% 6% 11% 23% 10% 1	1900
	11th to 20th	Day	55% 7% 14% 17% 6%	1900
	21st to end	Day	59% 5% 13% 15% 7%	1873
November	1st to 10th	Day	48% 7% 14% 22% 7% 1	1671
	11th to 20th	Day	57% 6% 13% 15% 8% 1	1520
	21st to end	Day	63% 8% 12% 10% 6% 1	1520
December	1st to 10th	Day	67% 7% 12% 11% 3	1520
	11th to 20th	Day	64% 5% 12% 14% 5%	1906
	21st to end	Day	71% 5% 7% 14% 3	2772
January	1st to 10th	Night	65% 6% 14% 11% 3	4429
	11th to 20th	Night	61% 8% 14% 13% 5%	4440
	21st to end	Night	67% 6% 10% 10% 6% 1	4784
February	1st to 10th	Night	61% 5% 10% 16% 7% 1	2470
	11th to 20th	Night	58% 5% 10% 19% 8% 1	2401
	21st to end	Night	56% 6% 9% 18% 11% 1	1884
March	1st to 10th	Night	57% 5% 9% 18% 9% 2	3734
	11th to 20th	Night	60% 6% 11% 14% 8% 2	3485
	21st to end	Night	46% 5% 7% 22% 17% 3	3599
April	1st to 10th	Night	53% 5% 10% 17% 14% 2	1900
	11th to 20th	Night	49% 6% 7% 20% 16% 1	1875
	21st to end	Night	51% 5% 8% 16% 16% 3	1768
May	1st to 10th	Night	52% 9% 8% 19% 9% 3	2520
	11th to 20th	Night	47% 9% 10% 20% 12% 2	2496
	21st to end	Night	43% 7% 12% 19% 17% 2	2739
June	1st to 10th	Night	46% 7% 12% 24% 10% 1	1520
	11th to 20th	Night	40% 8% 11% 24% 16% 2	1520
	21st to end	Night	46% 9% 12% 19% 14% 1	1520
July	1st to 10th	Night	45% 9% 14% 19% 13% 1	2480
	11th to 20th	Night	40% 7% 13% 32% 9%	2472
	21st to end	Night	37% 8% 15% 26% 14%	2651
August	1st to 10th	Night	41% 9% 15% 25% 10%	2541
	11th to 20th	Night	40% 7% 14% 31% 8%	2830
	21st to end	Night	41% 8% 15% 23% 13% 2	3476
September	1st to 10th	Night	36% 8% 11% 30% 15% 1	1900
	11th to 20th	Night	35% 8% 12% 34% 10% 1	1900
	21st to end	Night	42% 5% 12% 30% 11%	2147
October	1st to 10th	Night	45% 7% 11% 24% 13%	2280
	11th to 20th	Night	52% 7% 11% 22% 8%	2280
	21st to end	Night	56% 6% 14% 17% 7%	2589
November	1st to 10th	Night	50% 6% 16% 20% 9%	2407
	11th to 20th	Night	56% 6% 12% 16% 9% 1	2656
	21st to end	Night	61% 6% 13% 15% 5%	2660
December	1st to 10th	Night	67% 6% 10% 12% 4	2660
	11th to 20th	Night	61% 5% 9% 17% 8% 1	3288
	21st to end	Night	71% 5% 8% 13% 4	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	29% 16% 33% 18% 4	180
	11th to 20th	Day	29% 17% 38% 12% 3 1	180
	21st to end	Day	23% 19% 36% 17% 5%	225
February	1st to 10th	Day	34% 23% 31% 9% 2	270
	11th to 20th	Day	29% 28% 36% 7% 1	273
	21st to end	Day	33% 22% 29% 14% 2	382
March	1st to 10th	Day	28% 19% 33% 14% 6%	460
	11th to 20th	Day	28% 20% 33% 14% 3 1	460
	21st to end	Day	32% 19% 30% 13% 5% 1	506
April	1st to 10th	Day	31% 22% 27% 14% 5% 1	460
	11th to 20th	Day	36% 19% 24% 14% 7% 1	452
	21st to end	Day	32% 21% 29% 12% 5% 1	460
May	1st to 10th	Day	26% 16% 32% 16% 6% 3	463
	11th to 20th	Day	19% 18% 34% 19% 8% 2	487
	21st to end	Day	23% 15% 33% 20% 8% 2	529
June	1st to 10th	Day	26% 11% 35% 20% 7% 1	490
	11th to 20th	Day	19% 15% 31% 22% 11% 3	490
	21st to end	Day	26% 11% 31% 20% 9% 3	490
July	1st to 10th	Day	14% 8% 32% 32% 11% 3	485
	11th to 20th	Day	14% 10% 35% 27% 11% 3	484
	21st to end	Day	14% 7% 35% 29% 12% 3	532
August	1st to 10th	Day	16% 9% 28% 30% 14% 3	486
	11th to 20th	Day	19% 4 27% 30% 13% 8%	479
	21st to end	Day	17% 11% 29% 25% 12% 6%	539
September	1st to 10th	Day	18% 11% 31% 22% 12% 7%	490
	11th to 20th	Day	14% 10% 26% 21% 19% 10%	490
	21st to end	Day	22% 12% 27% 21% 14% 4	490
October	1st to 10th	Day	25% 16% 23% 22% 10% 3	490
	11th to 20th	Day	27% 18% 32% 16% 7% 1	490
	21st to end	Day	33% 15% 28% 20% 4	436
November	1st to 10th	Day	27% 16% 31% 19% 7% 1	365
	11th to 20th	Day	29% 26% 28% 13% 3	180
	21st to end	Day	27% 24% 30% 13% 3 2	180
December	1st to 10th	Day	36% 13% 39% 11% 2	180
	11th to 20th	Day	28% 22% 37% 11% 2	180
	21st to end	Day	35% 20% 32% 11% 2 1	198
January	1st to 10th	Night	28% 17% 32% 19% 4	760
	11th to 20th	Night	30% 17% 26% 21% 5%	760
	21st to end	Night	20% 15% 35% 25% 5%	836
February	1st to 10th	Night	22% 20% 31% 24% 3	760
	11th to 20th	Night	22% 21% 36% 18% 4	743
	21st to end	Night	25% 19% 29% 23% 4	628
March	1st to 10th	Night	27% 16% 31% 22% 4	760
	11th to 20th	Night	25% 19% 31% 20% 5%	760
	21st to end	Night	25% 17% 29% 22% 7%	836
April	1st to 10th	Night	29% 15% 27% 24% 4	760
	11th to 20th	Night	25% 20% 28% 20% 7% 1	751
	21st to end	Night	27% 15% 26% 23% 8%	628
May	1st to 10th	Night	25% 10% 27% 29% 8% 2	380
	11th to 20th	Night	24% 9% 28% 30% 10%	374
	21st to end	Night	18% 8% 36% 27% 10% 1	411
June	1st to 10th	Night	19% 9% 27% 34% 10%	380
	11th to 20th	Night	16% 9% 26% 34% 14% 1	380
	21st to end	Night	21% 7% 23% 35% 14%	380
July	1st to 10th	Night	13% 3 28% 38% 19%	370
	11th to 20th	Night	13% 4 22% 43% 17% 1	368
	21st to end	Night	10% 5% 24% 42% 18% 2	416
August	1st to 10th	Night	13% 5% 19% 41% 22% 1	397
	11th to 20th	Night	14% 5% 18% 36% 24% 3	569
	21st to end	Night	15% 6% 25% 34% 18% 2	836
September	1st to 10th	Night	15% 7% 29% 29% 19% 1	760
	11th to 20th	Night	17% 7% 20% 30% 22% 4	760
	21st to end	Night	22% 8% 25% 26% 18% 2	760
October	1st to 10th	Night	23% 11% 26% 23% 15% 1	760
	11th to 20th	Night	27% 12% 27% 25% 8% 1	760
	21st to end	Night	29% 14% 27% 24% 5%	812
November	1st to 10th	Night	27% 14% 23% 26% 9% 1	740
	11th to 20th	Night	26% 17% 31% 16% 10%	760
	21st to end	Night	29% 19% 24% 22% 6%	760
December	1st to 10th	Night	30% 16% 29% 21% 3	760
	11th to 20th	Night	28% 19% 27% 20% 5% 1	760
	21st to end	Night	34% 16% 31% 16% 4	836

Page generated at 2024-10-22 20:21:05. Version ac07

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