Saint-Zénon
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Saint-Zénon clear sky chart from 2005-11-06 to 2024-11-04.

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	16% 12% 4 10% 6% 5% 5% 4 5% 6% 28%	101589
	Night	15% 13% 5% 10% 6% 5% 4 3 5% 6% 28%	97434
Transparency	Day	54% 6% 11% 18% 9% 1	105039
	Night	55% 6% 12% 18% 9% 1	102073
Seeing	Day	25% 19% 31% 16% 7% 2	14483
	Night	23% 17% 29% 22% 8% 1	23403

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% 19% 7% 14% 8% 8% 5% 3 4 4 17%	2517
	11th to 20th	Day	9% 16% 7% 17% 11% 5% 4 3 4 3 20%	2520
	21st to end	Day	10% 12% 7% 16% 8% 6% 7% 5% 5% 7% 18%	2772
February	1st to 10th	Day	9% 18% 6% 13% 8% 8% 4 3 4 5% 21%	1638
	11th to 20th	Day	10% 18% 6% 13% 5% 6% 6% 4 5% 5% 23%	1736
	21st to end	Day	12% 18% 7% 12% 7% 5% 4 3 4 4 23%	1570
March	1st to 10th	Day	13% 18% 6% 12% 8% 5% 4 3 4 4 22%	3116
	11th to 20th	Day	10% 19% 8% 14% 7% 4 4 3 4 5% 23%	3283
	21st to end	Day	10% 13% 5% 11% 5% 5% 5% 3 5% 5% 33%	4161
April	1st to 10th	Day	18% 18% 4 10% 5% 4 3 3 4 5% 26%	2280
	11th to 20th	Day	21% 12% 4 8% 4 4 3 3 4 4 32%	2254
	21st to end	Day	20% 14% 3 9% 5% 4 4 3 4 5% 29%	2456
May	1st to 10th	Day	19% 12% 4 8% 5% 4 4 4 5% 6% 28%	4426
	11th to 20th	Day	17% 9% 3 8% 5% 5% 6% 5% 7% 7% 30%	4398
	21st to end	Day	17% 9% 3 8% 6% 5% 5% 4 5% 6% 32%	4837
June	1st to 10th	Day	22% 11% 3 8% 6% 4 5% 4 5% 6% 26%	2660
	11th to 20th	Day	17% 9% 3 8% 5% 5% 6% 4 6% 8% 31%	2660
	21st to end	Day	23% 10% 3 8% 5% 5% 5% 3 6% 7% 24%	2660
July	1st to 10th	Day	15% 9% 3 8% 6% 6% 6% 5% 6% 8% 28%	4440
	11th to 20th	Day	14% 7% 3 7% 6% 6% 6% 5% 6% 8% 31%	4440
	21st to end	Day	13% 7% 3 7% 5% 5% 5% 4 6% 7% 39%	4814
August	1st to 10th	Day	15% 8% 3 8% 5% 6% 6% 4 7% 8% 28%	4423
	11th to 20th	Day	17% 8% 3 7% 5% 6% 5% 4 7% 7% 32%	4090
	21st to end	Day	16% 7% 3 8% 5% 5% 5% 4 6% 8% 34%	4180
September	1st to 10th	Day	16% 7% 3 8% 4 3 5% 4 5% 9% 37%	2280
	11th to 20th	Day	14% 9% 3 6% 5% 4 4 3 5% 7% 40%	2207
	21st to end	Day	21% 11% 3 6% 4 4 4 3 4 6% 36%	2090
October	1st to 10th	Day	22% 11% 3 7% 5% 5% 4 4 6% 5% 29%	1948
	11th to 20th	Day	21% 15% 5% 9% 5% 5% 4 3 5% 4 24%	1847
	21st to end	Day	24% 14% 5% 10% 5% 4 2 3 3 5% 23%	1881
November	1st to 10th	Day	16% 16% 4 10% 5% 4 4 3 4 5% 29%	1698
	11th to 20th	Day	17% 18% 6% 11% 6% 5% 4 2 5% 6% 20%	1589
	21st to end	Day	15% 20% 7% 12% 7% 4 4 3 4 4 20%	1520
December	1st to 10th	Day	21% 18% 6% 13% 6% 4 4 4 3 4 18%	1520
	11th to 20th	Day	18% 16% 5% 14% 6% 3 4 3 4 6% 19%	1906
	21st to end	Day	17% 19% 8% 13% 6% 5% 4 3 4 4 18%	2772
January	1st to 10th	Night	12% 19% 7% 16% 8% 5% 4 3 4 4 18%	4429
	11th to 20th	Night	8% 13% 6% 15% 12% 9% 6% 5% 5% 5% 16%	4440
	21st to end	Night	8% 13% 8% 18% 9% 7% 6% 4 5% 5% 17%	4884
February	1st to 10th	Night	9% 15% 7% 12% 8% 6% 5% 4 5% 6% 22%	2537
	11th to 20th	Night	9% 15% 6% 13% 8% 6% 5% 5% 6% 5% 22%	2328
	21st to end	Night	11% 14% 5% 11% 6% 5% 5% 4 5% 6% 26%	1884
March	1st to 10th	Night	16% 18% 5% 10% 6% 4 5% 3 4 4 25%	3724
	11th to 20th	Night	12% 21% 6% 13% 6% 4 3 2 3 4 24%	3674
	21st to end	Night	12% 10% 5% 12% 6% 6% 5% 3 5% 4 33%	3471
April	1st to 10th	Night	21% 13% 4 10% 5% 4 4 3 5% 5% 27%	1900
	11th to 20th	Night	17% 12% 4 6% 5% 4 4 3 5% 6% 35%	1875
	21st to end	Night	20% 11% 4 10% 4 3 3 3 4 5% 33%	1729
May	1st to 10th	Night	18% 11% 3 8% 6% 5% 4 3 5% 6% 30%	2527
	11th to 20th	Night	18% 9% 3 7% 4 4 4 3 5% 7% 35%	2495
	21st to end	Night	18% 8% 3 8% 5% 6% 5% 4 6% 7% 30%	2738
June	1st to 10th	Night	19% 10% 3 6% 5% 3 4 4 6% 8% 31%	1520
	11th to 20th	Night	16% 9% 3 5% 5% 4 6% 4 6% 6% 36%	1520
	21st to end	Night	20% 8% 2 6% 4 5% 4 4 6% 7% 34%	1520
July	1st to 10th	Night	14% 8% 3 8% 6% 5% 5% 4 5% 8% 32%	2520
	11th to 20th	Night	14% 6% 3 7% 5% 5% 4 4 6% 7% 40%	2520
	21st to end	Night	10% 5% 2 6% 4 4 4 4 6% 8% 48%	2739
August	1st to 10th	Night	13% 9% 3 8% 5% 5% 5% 4 6% 7% 37%	2506
	11th to 20th	Night	17% 10% 3 9% 4 4 4 3 5% 6% 36%	2705
	21st to end	Night	17% 8% 3 7% 5% 4 5% 3 5% 7% 37%	3476
September	1st to 10th	Night	14% 9% 2 6% 5% 4 3 4 5% 7% 43%	1900
	11th to 20th	Night	15% 6% 2 6% 3 4 4 3 5% 6% 46%	1972
	21st to end	Night	16% 8% 3 6% 5% 3 4 3 4 7% 41%	2090
October	1st to 10th	Night	16% 10% 3 8% 5% 4 3 3 4 6% 38%	2229
	11th to 20th	Night	19% 16% 4 9% 4 3 3 2 5% 6% 29%	2332
	21st to end	Night	22% 14% 4 11% 5% 5% 3 3 4 5% 23%	2717
November	1st to 10th	Night	15% 14% 5% 9% 6% 4 5% 4 5% 6% 27%	2453
	11th to 20th	Night	16% 13% 5% 13% 6% 5% 5% 3 4 5% 24%	2588
	21st to end	Night	18% 17% 6% 13% 6% 3 4 3 4 5% 20%	2660
December	1st to 10th	Night	17% 18% 7% 15% 7% 5% 4 3 4 5% 15%	2660
	11th to 20th	Night	18% 17% 5% 13% 7% 5% 4 3 4 4 20%	3288
	21st to end	Night	18% 19% 7% 12% 8% 5% 4 3 3 4 16%	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	68% 4 10% 13% 5%	2518
	11th to 20th	Day	67% 4 6% 15% 7% 1	2520
	21st to end	Day	63% 8% 8% 14% 7% 1	2772
February	1st to 10th	Day	64% 3 8% 16% 8% 2	1638
	11th to 20th	Day	61% 4 10% 14% 11% 1	1745
	21st to end	Day	64% 3 7% 15% 9% 3	1570
March	1st to 10th	Day	64% 5% 6% 13% 11% 2	3119
	11th to 20th	Day	62% 5% 8% 10% 10% 3	3283
	21st to end	Day	50% 4 8% 20% 16% 3	4161
April	1st to 10th	Day	60% 3 8% 15% 10% 3	2280
	11th to 20th	Day	55% 4 7% 20% 12% 2	2254
	21st to end	Day	58% 4 7% 15% 12% 3	2456
May	1st to 10th	Day	54% 6% 9% 18% 11% 1	4440
	11th to 20th	Day	48% 9% 15% 19% 8% 1	4402
	21st to end	Day	52% 6% 9% 18% 14% 1	4837
June	1st to 10th	Day	56% 7% 9% 18% 10% 1	2660
	11th to 20th	Day	49% 7% 12% 20% 10% 2	2660
	21st to end	Day	58% 7% 11% 16% 8%	2660
July	1st to 10th	Day	50% 8% 13% 22% 7%	4370
	11th to 20th	Day	46% 8% 18% 23% 5%	4365
	21st to end	Day	42% 7% 15% 26% 10%	4661
August	1st to 10th	Day	50% 8% 16% 20% 6%	4409
	11th to 20th	Day	47% 8% 17% 23% 5%	4132
	21st to end	Day	46% 7% 13% 21% 13% 1	4180
September	1st to 10th	Day	44% 6% 14% 25% 11% 1	2280
	11th to 20th	Day	42% 5% 14% 27% 10% 1	2207
	21st to end	Day	49% 5% 11% 25% 10%	2090
October	1st to 10th	Day	51% 7% 13% 23% 6% 1	3234
	11th to 20th	Day	54% 6% 12% 22% 5%	3050
	21st to end	Day	54% 6% 18% 14% 6% 1	3081
November	1st to 10th	Day	57% 5% 11% 18% 9% 1	1698
	11th to 20th	Day	64% 5% 9% 14% 7% 1	1589
	21st to end	Day	66% 4 11% 13% 5% 1	1520
December	1st to 10th	Day	69% 4 9% 12% 6%	1520
	11th to 20th	Day	65% 4 9% 14% 8%	1906
	21st to end	Day	68% 4 6% 13% 8%	2772
January	1st to 10th	Night	67% 5% 10% 13% 5%	4429
	11th to 20th	Night	65% 6% 11% 12% 4 1	4440
	21st to end	Night	67% 5% 8% 11% 8% 1	4884
February	1st to 10th	Night	60% 4 9% 17% 7% 2	2537
	11th to 20th	Night	59% 5% 9% 17% 10% 1	2330
	21st to end	Night	55% 4 9% 18% 12% 2	1884
March	1st to 10th	Night	60% 5% 9% 12% 11% 3	3734
	11th to 20th	Night	63% 4 6% 13% 12% 2	3675
	21st to end	Night	53% 5% 7% 20% 12% 4	3471
April	1st to 10th	Night	57% 4 9% 16% 12% 2	1900
	11th to 20th	Night	49% 5% 9% 21% 13% 2	1875
	21st to end	Night	52% 4 9% 20% 13% 2	1729
May	1st to 10th	Night	53% 7% 13% 17% 8% 2	2533
	11th to 20th	Night	46% 8% 14% 18% 11% 2	2496
	21st to end	Night	51% 6% 11% 18% 12% 1	2739
June	1st to 10th	Night	49% 7% 13% 20% 11% 1	1520
	11th to 20th	Night	45% 8% 12% 21% 11% 3	1520
	21st to end	Night	48% 8% 14% 19% 11%	1520
July	1st to 10th	Night	49% 7% 18% 19% 7% 1	2480
	11th to 20th	Night	41% 8% 21% 25% 5% 1	2472
	21st to end	Night	34% 8% 20% 28% 11%	2651
August	1st to 10th	Night	44% 7% 19% 24% 6%	2496
	11th to 20th	Night	46% 8% 19% 21% 6%	2729
	21st to end	Night	44% 7% 15% 23% 9% 2	3476
September	1st to 10th	Night	40% 6% 16% 25% 13%	1900
	11th to 20th	Night	38% 5% 16% 31% 9% 1	1972
	21st to end	Night	42% 5% 14% 28% 11%	2090
October	1st to 10th	Night	46% 5% 14% 23% 11%	3723
	11th to 20th	Night	52% 5% 13% 22% 8%	3868
	21st to end	Night	53% 6% 16% 15% 8% 2	4467
November	1st to 10th	Night	55% 6% 12% 19% 7% 1	2453
	11th to 20th	Night	59% 5% 11% 15% 8% 2	2588
	21st to end	Night	63% 5% 11% 15% 5%	2660
December	1st to 10th	Night	71% 4 10% 11% 5%	2660
	11th to 20th	Night	65% 4 8% 13% 8% 1	3288
	21st to end	Night	70% 4 6% 13% 6% 1	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	21% 27% 37% 12% 3	180
	11th to 20th	Day	29% 15% 38% 12% 6% 1	180
	21st to end	Day	18% 25% 38% 16% 3 1	198
February	1st to 10th	Day	31% 31% 29% 8% 2	236
	11th to 20th	Day	26% 29% 36% 8% 1	343
	21st to end	Day	31% 27% 28% 12% 2	382
March	1st to 10th	Day	26% 23% 32% 14% 3	460
	11th to 20th	Day	27% 25% 32% 13% 3 1	460
	21st to end	Day	30% 24% 26% 12% 6% 2	506
April	1st to 10th	Day	29% 20% 32% 13% 5%	460
	11th to 20th	Day	34% 22% 21% 13% 8% 2	452
	21st to end	Day	31% 23% 32% 9% 5% 1	460
May	1st to 10th	Day	29% 24% 30% 13% 3 2	463
	11th to 20th	Day	25% 19% 31% 17% 6% 1	487
	21st to end	Day	25% 21% 31% 17% 5% 1	529
June	1st to 10th	Day	27% 19% 35% 14% 5% 1	490
	11th to 20th	Day	22% 19% 31% 17% 10% 1	490
	21st to end	Day	29% 13% 32% 18% 6% 2	490
July	1st to 10th	Day	18% 12% 40% 21% 7% 2	485
	11th to 20th	Day	16% 16% 35% 27% 5% 1	484
	21st to end	Day	14% 13% 36% 26% 9% 2	532
August	1st to 10th	Day	18% 13% 32% 25% 10% 2	486
	11th to 20th	Day	17% 11% 32% 25% 11% 5%	479
	21st to end	Day	18% 17% 29% 19% 12% 6%	539
September	1st to 10th	Day	21% 14% 32% 18% 11% 5%	490
	11th to 20th	Day	18% 12% 29% 22% 14% 6%	490
	21st to end	Day	23% 17% 25% 19% 12% 4	490
October	1st to 10th	Day	25% 18% 26% 17% 10% 4	490
	11th to 20th	Day	29% 23% 27% 13% 6% 2	462
	21st to end	Day	33% 20% 26% 15% 5%	429
November	1st to 10th	Day	29% 20% 29% 16% 5% 1	374
	11th to 20th	Day	31% 23% 30% 13% 3	249
	21st to end	Day	30% 25% 32% 9% 4 1	180
December	1st to 10th	Day	32% 19% 29% 13% 6% 1	180
	11th to 20th	Day	29% 32% 24% 12% 3	180
	21st to end	Day	34% 25% 26% 9% 5% 1	198
January	1st to 10th	Night	25% 21% 34% 16% 4	760
	11th to 20th	Night	21% 23% 32% 19% 5%	760
	21st to end	Night	14% 19% 39% 22% 5% 1	836
February	1st to 10th	Night	17% 24% 36% 19% 4 1	760
	11th to 20th	Night	20% 24% 34% 17% 3 1	743
	21st to end	Night	21% 21% 34% 20% 3	628
March	1st to 10th	Night	26% 19% 32% 19% 4	760
	11th to 20th	Night	26% 24% 28% 16% 6% 1	760
	21st to end	Night	23% 19% 30% 20% 7%	836
April	1st to 10th	Night	31% 16% 27% 21% 6% 1	760
	11th to 20th	Night	25% 18% 29% 20% 7% 1	751
	21st to end	Night	28% 16% 29% 19% 6% 1	589
May	1st to 10th	Night	25% 14% 30% 22% 9% 1	393
	11th to 20th	Night	22% 13% 30% 26% 7% 1	374
	21st to end	Night	20% 15% 35% 23% 8%	411
June	1st to 10th	Night	23% 11% 30% 29% 7% 1	380
	11th to 20th	Night	22% 10% 24% 31% 12% 1	380
	21st to end	Night	23% 8% 23% 33% 12% 1	380
July	1st to 10th	Night	14% 6% 31% 35% 13% 1	370
	11th to 20th	Night	13% 8% 26% 38% 14% 2	368
	21st to end	Night	14% 6% 26% 38% 15% 1	416
August	1st to 10th	Night	12% 7% 18% 44% 19%	378
	11th to 20th	Night	16% 6% 23% 34% 20% 1	507
	21st to end	Night	19% 11% 25% 26% 16% 3	836
September	1st to 10th	Night	17% 9% 29% 33% 12% 1	760
	11th to 20th	Night	17% 10% 25% 28% 18% 2	760
	21st to end	Night	21% 11% 28% 23% 16% 1	760
October	1st to 10th	Night	22% 17% 25% 23% 11% 1	760
	11th to 20th	Night	28% 17% 27% 19% 8% 1	760
	21st to end	Night	30% 18% 26% 22% 5%	836
November	1st to 10th	Night	24% 20% 29% 20% 7% 1	755
	11th to 20th	Night	25% 19% 30% 16% 9% 1	760
	21st to end	Night	29% 23% 24% 18% 5% 1	760
December	1st to 10th	Night	28% 21% 29% 18% 5% 1	760
	11th to 20th	Night	27% 21% 31% 16% 6% 1	760
	21st to end	Night	31% 22% 27% 16% 3 1	836

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