Varennes
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Varennes clear sky chart from 2005-11-06 to 2024-12-06.

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% 9% 3 8% 5% 5% 5% 4 6% 7% 33%	101806
	Night	18% 11% 4 8% 5% 4 4 3 5% 6% 32%	97929
Transparency	Day	48% 7% 13% 21% 9% 1	105282
	Night	52% 6% 12% 19% 10% 1	102542
Seeing	Day	21% 11% 27% 25% 12% 3	14652
	Night	23% 9% 29% 29% 10% 1	23626

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% 16% 5% 12% 6% 6% 4 3 4 4 23%	2517
	11th to 20th	Day	15% 12% 6% 10% 6% 5% 3 4 5% 7% 27%	2520
	21st to end	Day	15% 15% 5% 10% 6% 5% 5% 4 6% 6% 24%	2772
February	1st to 10th	Day	15% 16% 5% 9% 5% 5% 5% 3 4 6% 27%	1648
	11th to 20th	Day	13% 12% 5% 11% 7% 4 5% 5% 6% 5% 28%	1770
	21st to end	Day	18% 16% 4 8% 6% 4 4 2 5% 6% 28%	1570
March	1st to 10th	Day	19% 12% 3 10% 6% 6% 5% 3 5% 5% 27%	3116
	11th to 20th	Day	13% 16% 5% 11% 6% 5% 4 3 5% 6% 27%	3272
	21st to end	Day	14% 9% 3 10% 5% 5% 4 3 5% 5% 37%	4166
April	1st to 10th	Day	22% 11% 3 9% 5% 4 5% 3 3 5% 31%	2280
	11th to 20th	Day	20% 9% 2 7% 4 5% 3 3 4 5% 37%	2254
	21st to end	Day	21% 9% 3 8% 5% 4 4 4 4 5% 33%	2450
May	1st to 10th	Day	19% 9% 3 8% 5% 4 5% 4 6% 7% 30%	4349
	11th to 20th	Day	17% 8% 3 8% 5% 5% 5% 4 6% 8% 31%	4398
	21st to end	Day	14% 7% 2 6% 5% 4 5% 4 7% 9% 36%	4837
June	1st to 10th	Day	20% 8% 3 7% 5% 5% 5% 3 6% 8% 30%	2660
	11th to 20th	Day	14% 6% 3 6% 5% 4 4 4 6% 9% 38%	2660
	21st to end	Day	19% 8% 3 8% 6% 5% 5% 4 5% 7% 31%	2660
July	1st to 10th	Day	11% 6% 2 7% 6% 5% 6% 4 7% 9% 37%	4440
	11th to 20th	Day	12% 5% 2 6% 5% 5% 6% 5% 7% 9% 37%	4440
	21st to end	Day	11% 6% 2 5% 4 4 4 4 7% 8% 45%	4814
August	1st to 10th	Day	13% 6% 2 8% 6% 5% 6% 5% 7% 10% 32%	4423
	11th to 20th	Day	13% 6% 3 8% 5% 4 4 4 6% 9% 38%	4021
	21st to end	Day	15% 6% 2 6% 4 4 4 4 6% 8% 40%	4180
September	1st to 10th	Day	11% 5% 2 7% 5% 4 5% 5% 6% 8% 42%	2280
	11th to 20th	Day	14% 5% 2 5% 3 3 4 3 6% 7% 49%	2190
	21st to end	Day	15% 6% 2 7% 4 4 5% 4 5% 7% 41%	2090
October	1st to 10th	Day	18% 6% 3 7% 6% 5% 4 3 5% 8% 35%	1976
	11th to 20th	Day	17% 9% 3 8% 5% 7% 5% 5% 6% 7% 29%	1843
	21st to end	Day	24% 9% 3 7% 4 4 4 3 5% 5% 31%	1881
November	1st to 10th	Day	17% 10% 3 7% 5% 4 5% 4 7% 6% 32%	1761
	11th to 20th	Day	17% 10% 3 9% 5% 5% 5% 5% 6% 5% 31%	1730
	21st to end	Day	22% 12% 3 9% 5% 4 6% 4 4 6% 25%	1600
December	1st to 10th	Day	23% 16% 4 11% 6% 4 3 4 4 6% 19%	1560
	11th to 20th	Day	22% 14% 4 9% 3 4 4 3 5% 6% 26%	1906
	21st to end	Day	27% 16% 4 10% 5% 4 4 3 4 4 20%	2772
January	1st to 10th	Night	17% 16% 6% 10% 6% 5% 5% 3 4 5% 23%	4429
	11th to 20th	Night	12% 13% 5% 13% 7% 6% 6% 3 5% 6% 23%	4440
	21st to end	Night	16% 17% 5% 10% 7% 4 4 3 4 5% 24%	4884
February	1st to 10th	Night	15% 15% 6% 12% 7% 5% 4 3 5% 5% 22%	2528
	11th to 20th	Night	15% 12% 5% 9% 5% 6% 5% 3 5% 5% 30%	2294
	21st to end	Night	16% 14% 4 10% 5% 5% 4 3 4 6% 31%	1884
March	1st to 10th	Night	21% 11% 3 8% 4 4 4 2 4 5% 33%	3724
	11th to 20th	Night	18% 19% 4 9% 5% 4 4 3 4 4 28%	3685
	21st to end	Night	13% 11% 4 9% 6% 5% 4 3 4 5% 38%	3466
April	1st to 10th	Night	23% 10% 3 7% 5% 4 4 3 5% 6% 30%	1900
	11th to 20th	Night	22% 9% 3 6% 4 4 4 4 4 6% 34%	1875
	21st to end	Night	22% 10% 3 6% 4 4 4 3 5% 5% 35%	1729
May	1st to 10th	Night	21% 9% 3 7% 5% 5% 5% 4 5% 6% 31%	2620
	11th to 20th	Night	22% 8% 3 7% 4 4 4 3 6% 7% 33%	2495
	21st to end	Night	14% 8% 3 7% 5% 5% 4 4 6% 8% 36%	2738
June	1st to 10th	Night	22% 7% 2 8% 4 4 5% 4 6% 7% 32%	1520
	11th to 20th	Night	15% 6% 3 7% 4 4 5% 3 8% 7% 38%	1520
	21st to end	Night	18% 7% 2 7% 5% 6% 5% 4 5% 8% 32%	1520
July	1st to 10th	Night	13% 7% 3 8% 5% 5% 6% 4 7% 9% 34%	2520
	11th to 20th	Night	14% 6% 2 6% 4 4 5% 4 7% 8% 40%	2520
	21st to end	Night	10% 6% 2 6% 5% 5% 4 4 7% 8% 43%	2739
August	1st to 10th	Night	14% 9% 4 8% 5% 5% 5% 3 6% 6% 36%	2506
	11th to 20th	Night	15% 7% 3 8% 5% 5% 5% 3 6% 7% 37%	2772
	21st to end	Night	16% 6% 3 7% 5% 5% 4 4 6% 7% 38%	3476
September	1st to 10th	Night	12% 5% 2 7% 5% 4 4 4 6% 7% 44%	1900
	11th to 20th	Night	15% 6% 2 5% 3 2 3 3 5% 7% 49%	1990
	21st to end	Night	16% 7% 2 5% 5% 4 3 3 5% 7% 43%	2090
October	1st to 10th	Night	17% 7% 2 7% 4 4 5% 3 5% 7% 39%	2199
	11th to 20th	Night	21% 9% 3 7% 5% 4 4 3 4 6% 34%	2337
	21st to end	Night	23% 9% 3 9% 5% 4 4 3 4 5% 31%	2717
November	1st to 10th	Night	19% 9% 3 8% 5% 5% 4 3 5% 5% 33%	2537
	11th to 20th	Night	19% 11% 3 8% 5% 5% 4 3 5% 6% 33%	2666
	21st to end	Night	22% 13% 3 8% 6% 4 4 3 4 6% 26%	2800
December	1st to 10th	Night	21% 16% 6% 11% 5% 4 5% 3 4 5% 21%	2737
	11th to 20th	Night	23% 15% 4 9% 5% 4 3 3 3 5% 26%	3288
	21st to end	Night	27% 16% 5% 10% 4 3 3 3 4 4 22%	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	60% 9% 12% 15% 4	2518
	11th to 20th	Day	55% 7% 13% 17% 7%	2520
	21st to end	Day	59% 7% 14% 14% 6% 1	2772
February	1st to 10th	Day	60% 5% 9% 18% 6% 2	1648
	11th to 20th	Day	57% 7% 11% 14% 10% 1	1779
	21st to end	Day	59% 4 10% 16% 10% 2	1570
March	1st to 10th	Day	58% 7% 8% 18% 8% 1	3119
	11th to 20th	Day	57% 7% 11% 12% 10% 3	3272
	21st to end	Day	46% 7% 9% 20% 16% 2	4166
April	1st to 10th	Day	56% 5% 7% 19% 11% 2	2280
	11th to 20th	Day	49% 5% 9% 23% 13% 2	2254
	21st to end	Day	53% 6% 9% 18% 12% 3	2450
May	1st to 10th	Day	49% 8% 11% 22% 9% 1	4363
	11th to 20th	Day	47% 10% 16% 17% 9% 1	4402
	21st to end	Day	42% 8% 13% 23% 13% 1	4837
June	1st to 10th	Day	50% 7% 11% 19% 11% 2	2660
	11th to 20th	Day	41% 7% 13% 24% 12% 3	2660
	21st to end	Day	52% 7% 13% 19% 10% 1	2660
July	1st to 10th	Day	42% 7% 15% 27% 9% 1	4370
	11th to 20th	Day	40% 8% 17% 28% 7%	4365
	21st to end	Day	36% 7% 15% 33% 9%	4661
August	1st to 10th	Day	46% 7% 18% 22% 7%	4409
	11th to 20th	Day	42% 7% 20% 25% 6%	4063
	21st to end	Day	42% 7% 15% 21% 14%	4180
September	1st to 10th	Day	37% 8% 15% 29% 11% 1	2280
	11th to 20th	Day	35% 6% 14% 32% 13% 1	2190
	21st to end	Day	42% 8% 14% 25% 11%	2090
October	1st to 10th	Day	44% 6% 15% 27% 7%	3288
	11th to 20th	Day	44% 6% 18% 25% 7%	3046
	21st to end	Day	43% 6% 19% 23% 9% 1	3081
November	1st to 10th	Day	49% 7% 13% 22% 9% 1	1761
	11th to 20th	Day	51% 7% 11% 19% 12%	1730
	21st to end	Day	58% 8% 14% 14% 6%	1600
December	1st to 10th	Day	67% 5% 10% 13% 5%	1560
	11th to 20th	Day	60% 5% 10% 19% 6%	1906
	21st to end	Day	68% 5% 12% 9% 5%	2772
January	1st to 10th	Night	60% 8% 15% 12% 5%	4429
	11th to 20th	Night	57% 10% 12% 15% 5% 1	4440
	21st to end	Night	61% 7% 12% 13% 6% 1	4884
February	1st to 10th	Night	63% 5% 9% 15% 7% 1	2528
	11th to 20th	Night	58% 5% 8% 18% 10% 1	2296
	21st to end	Night	55% 6% 9% 16% 13% 2	1884
March	1st to 10th	Night	54% 6% 10% 18% 9% 4	3734
	11th to 20th	Night	59% 6% 8% 12% 13% 2	3686
	21st to end	Night	49% 6% 7% 22% 14% 3	3466
April	1st to 10th	Night	55% 5% 7% 18% 14% 2	1900
	11th to 20th	Night	52% 4 7% 20% 14% 2	1875
	21st to end	Night	51% 4 8% 19% 14% 3	1729
May	1st to 10th	Night	51% 9% 10% 19% 8% 3	2626
	11th to 20th	Night	48% 9% 12% 19% 10% 2	2496
	21st to end	Night	45% 6% 11% 20% 17% 2	2739
June	1st to 10th	Night	49% 7% 10% 21% 12% 1	1520
	11th to 20th	Night	44% 7% 11% 21% 14% 3	1520
	21st to end	Night	49% 7% 10% 18% 14% 1	1520
July	1st to 10th	Night	45% 7% 15% 20% 12% 1	2480
	11th to 20th	Night	40% 7% 18% 26% 9%	2472
	21st to end	Night	38% 7% 19% 24% 12%	2651
August	1st to 10th	Night	48% 7% 12% 25% 9%	2496
	11th to 20th	Night	46% 7% 17% 22% 7%	2796
	21st to end	Night	45% 6% 13% 21% 14% 1	3476
September	1st to 10th	Night	38% 8% 12% 26% 15% 1	1900
	11th to 20th	Night	36% 5% 14% 31% 13% 1	1990
	21st to end	Night	40% 8% 10% 27% 15%	2090
October	1st to 10th	Night	44% 6% 13% 26% 10%	3667
	11th to 20th	Night	45% 5% 11% 30% 8%	3873
	21st to end	Night	48% 4 15% 18% 12% 2	4467
November	1st to 10th	Night	53% 5% 12% 21% 8% 1	2537
	11th to 20th	Night	52% 6% 11% 19% 11% 2	2666
	21st to end	Night	60% 5% 11% 17% 7%	2800
December	1st to 10th	Night	66% 6% 11% 13% 4	2737
	11th to 20th	Night	61% 5% 8% 17% 8%	3288
	21st to end	Night	66% 5% 9% 15% 5%	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	28% 13% 29% 18% 12%	180
	11th to 20th	Day	25% 15% 34% 19% 6% 1	180
	21st to end	Day	19% 11% 35% 23% 11% 1	198
February	1st to 10th	Day	30% 16% 34% 18% 3	240
	11th to 20th	Day	21% 15% 34% 25% 4 1	378
	21st to end	Day	28% 12% 31% 24% 4	382
March	1st to 10th	Day	23% 12% 29% 26% 10%	460
	11th to 20th	Day	21% 14% 35% 22% 7% 1	460
	21st to end	Day	27% 11% 32% 21% 8% 1	506
April	1st to 10th	Day	28% 14% 27% 21% 8% 2	460
	11th to 20th	Day	27% 18% 25% 19% 8% 3	452
	21st to end	Day	27% 13% 30% 18% 10% 1	460
May	1st to 10th	Day	23% 11% 33% 20% 11% 2	463
	11th to 20th	Day	17% 11% 28% 26% 14% 4	487
	21st to end	Day	19% 13% 30% 26% 10% 2	529
June	1st to 10th	Day	22% 11% 25% 29% 10% 2	490
	11th to 20th	Day	16% 10% 26% 32% 14% 2	490
	21st to end	Day	23% 11% 23% 26% 14% 3	490
July	1st to 10th	Day	13% 8% 24% 31% 19% 5%	485
	11th to 20th	Day	14% 8% 26% 33% 16% 4	484
	21st to end	Day	14% 7% 24% 33% 18% 3	532
August	1st to 10th	Day	15% 5% 23% 30% 21% 6%	486
	11th to 20th	Day	12% 6% 20% 30% 22% 9%	479
	21st to end	Day	16% 8% 22% 26% 18% 10%	539
September	1st to 10th	Day	12% 9% 27% 29% 16% 7%	490
	11th to 20th	Day	14% 8% 20% 27% 19% 13%	490
	21st to end	Day	18% 8% 26% 24% 18% 6%	490
October	1st to 10th	Day	20% 13% 26% 23% 14% 4	490
	11th to 20th	Day	23% 12% 29% 23% 11% 2	460
	21st to end	Day	30% 11% 27% 24% 8%	429
November	1st to 10th	Day	26% 13% 30% 21% 9%	395
	11th to 20th	Day	23% 15% 31% 21% 8% 2	330
	21st to end	Day	33% 13% 30% 16% 9% 1	200
December	1st to 10th	Day	30% 11% 24% 21% 12% 3	190
	11th to 20th	Day	24% 22% 33% 16% 4 1	180
	21st to end	Day	34% 17% 26% 18% 4 1	198
January	1st to 10th	Night	28% 10% 30% 27% 5%	760
	11th to 20th	Night	25% 10% 34% 25% 6%	760
	21st to end	Night	20% 9% 33% 31% 6% 1	836
February	1st to 10th	Night	22% 10% 36% 27% 5% 1	760
	11th to 20th	Night	19% 11% 38% 26% 5% 1	743
	21st to end	Night	22% 10% 33% 30% 5%	628
March	1st to 10th	Night	26% 8% 31% 30% 6%	760
	11th to 20th	Night	26% 10% 30% 26% 7%	760
	21st to end	Night	22% 9% 31% 29% 8%	836
April	1st to 10th	Night	28% 11% 26% 28% 6%	760
	11th to 20th	Night	25% 13% 29% 26% 6% 1	751
	21st to end	Night	28% 11% 30% 24% 7% 1	589
May	1st to 10th	Night	26% 7% 32% 27% 8% 1	447
	11th to 20th	Night	23% 9% 28% 30% 9% 1	374
	21st to end	Night	18% 9% 36% 29% 9%	411
June	1st to 10th	Night	23% 6% 26% 35% 9% 1	380
	11th to 20th	Night	19% 3 29% 36% 13%	380
	21st to end	Night	18% 7% 27% 33% 14% 1	380
July	1st to 10th	Night	12% 5% 29% 38% 15% 1	370
	11th to 20th	Night	12% 2 24% 46% 14% 1	368
	21st to end	Night	13% 4 21% 45% 16%	416
August	1st to 10th	Night	14% 5% 17% 44% 19% 2	378
	11th to 20th	Night	14% 3 20% 38% 23% 2	548
	21st to end	Night	16% 4 21% 36% 20% 2	836
September	1st to 10th	Night	15% 5% 23% 41% 15% 1	760
	11th to 20th	Night	17% 5% 21% 34% 21% 2	760
	21st to end	Night	18% 6% 26% 31% 15% 3	760
October	1st to 10th	Night	21% 7% 27% 31% 13% 1	760
	11th to 20th	Night	26% 8% 27% 29% 9% 1	760
	21st to end	Night	27% 10% 31% 25% 7%	836
November	1st to 10th	Night	24% 12% 31% 25% 8%	780
	11th to 20th	Night	25% 10% 33% 23% 9% 1	800
	21st to end	Night	30% 12% 30% 21% 7%	800
December	1st to 10th	Night	29% 10% 30% 24% 7%	783
	11th to 20th	Night	28% 12% 31% 22% 6% 1	760
	21st to end	Night	32% 11% 30% 21% 6%	836

Page generated at 2024-12-03 06:45:15. Version ac07

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