Vanier
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Vanier clear sky chart from 2005-11-06 to 2026-03-18.

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	18% 10% 3 8% 5% 5% 5% 4 5% 7% 31%	130666
	Night	18% 12% 4 9% 5% 5% 4 3 5% 6% 29%	130890
Transparency	Day	52% 6% 13% 19% 9% 1	134108
	Night	56% 6% 12% 18% 8% 1	135523
Seeing	Day	23% 14% 30% 22% 9% 2	16883
	Night	24% 11% 28% 28% 8% 1	25544

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	15% 17% 6% 12% 5% 4 3 3 4 5% 28%	4661
	11th to 20th	Day	10% 17% 8% 15% 7% 6% 4 3 4 4 21%	4650
	21st to end	Day	6% 13% 5% 10% 5% 5% 4 5% 7% 8% 33%	5124
February	1st to 10th	Day	10% 14% 6% 12% 6% 6% 5% 4 5% 6% 27%	1680
	11th to 20th	Day	14% 14% 6% 10% 5% 5% 4 4 5% 6% 28%	2032
	21st to end	Day	12% 14% 5% 10% 7% 5% 5% 4 4 6% 28%	1730
March	1st to 10th	Day	21% 14% 4 9% 6% 5% 4 3 3 5% 26%	4576
	11th to 20th	Day	15% 14% 4 7% 5% 5% 4 4 6% 7% 29%	4820
	21st to end	Day	17% 14% 4 7% 4 4 4 3 4 5% 34%	5818
April	1st to 10th	Day	21% 12% 3 7% 5% 5% 4 3 5% 5% 30%	2400
	11th to 20th	Day	20% 7% 2 6% 5% 5% 4 3 5% 6% 37%	2472
	21st to end	Day	24% 8% 3 7% 4 4 4 4 4 6% 31%	2600
May	1st to 10th	Day	26% 9% 2 7% 4 4 4 3 5% 6% 29%	6022
	11th to 20th	Day	22% 6% 2 7% 5% 5% 5% 3 6% 8% 31%	6318
	21st to end	Day	20% 8% 2 8% 5% 5% 6% 4 6% 6% 29%	7031
June	1st to 10th	Day	19% 6% 3 7% 5% 5% 6% 4 8% 7% 28%	3000
	11th to 20th	Day	19% 7% 2 5% 5% 5% 5% 4 6% 8% 34%	3000
	21st to end	Day	23% 8% 3 7% 6% 5% 5% 4 5% 7% 27%	3000
July	1st to 10th	Day	17% 7% 3 8% 5% 5% 6% 5% 7% 9% 28%	6454
	11th to 20th	Day	16% 6% 3 7% 4 5% 5% 5% 7% 9% 34%	6360
	21st to end	Day	12% 5% 2 5% 4 5% 5% 4 7% 9% 42%	6926
August	1st to 10th	Day	15% 5% 2 7% 5% 5% 6% 5% 7% 10% 32%	4538
	11th to 20th	Day	19% 6% 2 7% 5% 6% 4 4 6% 9% 30%	4175
	21st to end	Day	21% 5% 2 7% 4 5% 3 3 5% 7% 38%	4479
September	1st to 10th	Day	16% 7% 2 6% 4 4 4 4 6% 9% 38%	2400
	11th to 20th	Day	17% 5% 2 4 3 3 4 2 5% 8% 47%	2221
	21st to end	Day	20% 7% 2 7% 3 4 4 3 6% 6% 38%	2200
October	1st to 10th	Day	20% 7% 3 6% 4 4 5% 4 5% 7% 35%	2197
	11th to 20th	Day	23% 9% 3 7% 6% 5% 5% 5% 5% 7% 25%	1829
	21st to end	Day	27% 10% 4 7% 4 4 4 4 5% 6% 24%	1980
November	1st to 10th	Day	20% 11% 3 8% 5% 5% 4 3 4 6% 30%	1851
	11th to 20th	Day	22% 13% 5% 8% 5% 4 4 3 5% 4 27%	1890
	21st to end	Day	23% 16% 5% 8% 5% 5% 4 4 4 5% 22%	1510
December	1st to 10th	Day	18% 16% 5% 9% 5% 5% 5% 4 6% 6% 21%	2350
	11th to 20th	Day	20% 14% 5% 9% 6% 5% 4 3 4 5% 27%	2688
	21st to end	Day	20% 17% 4 8% 5% 5% 4 4 5% 4 25%	3684
January	1st to 10th	Night	14% 19% 6% 14% 6% 5% 4 3 3 4 23%	8234
	11th to 20th	Night	12% 23% 8% 15% 6% 5% 4 3 3 4 16%	8235
	21st to end	Night	6% 12% 5% 13% 7% 6% 5% 5% 7% 8% 26%	9057
February	1st to 10th	Night	11% 13% 6% 11% 8% 5% 4 4 4 5% 29%	2940
	11th to 20th	Night	13% 17% 5% 10% 6% 4 4 3 5% 6% 26%	2469
	21st to end	Night	14% 15% 4 10% 5% 5% 4 3 4 6% 30%	2076
March	1st to 10th	Night	22% 14% 4 9% 4 4 5% 3 4 5% 25%	5484
	11th to 20th	Night	16% 14% 3 9% 6% 6% 5% 4 6% 6% 25%	5293
	21st to end	Night	19% 15% 4 8% 5% 4 3 3 3 5% 31%	5110
April	1st to 10th	Night	22% 14% 4 7% 5% 4 3 3 4 5% 29%	2000
	11th to 20th	Night	21% 8% 3 7% 4 4 4 4 5% 6% 33%	1877
	21st to end	Night	24% 10% 2 7% 5% 3 3 3 4 5% 35%	1800
May	1st to 10th	Night	25% 8% 3 6% 5% 5% 5% 3 4 6% 30%	3962
	11th to 20th	Night	27% 7% 3 7% 4 4 4 3 4 6% 31%	3575
	21st to end	Night	19% 7% 2 6% 5% 5% 5% 4 5% 7% 35%	3843
June	1st to 10th	Night	22% 8% 3 6% 5% 4 5% 5% 5% 8% 29%	1400
	11th to 20th	Night	20% 6% 3 6% 4 5% 3 4 6% 7% 36%	1400
	21st to end	Night	22% 9% 4 7% 5% 4 5% 3 4 7% 30%	1400
July	1st to 10th	Night	15% 8% 2 7% 5% 6% 6% 5% 7% 8% 31%	3504
	11th to 20th	Night	21% 7% 2 6% 5% 4 4 3 5% 7% 36%	3600
	21st to end	Night	13% 5% 2 7% 5% 5% 5% 3 5% 7% 42%	3926
August	1st to 10th	Night	18% 7% 3 7% 5% 5% 5% 4 5% 7% 35%	2586
	11th to 20th	Night	19% 7% 3 8% 5% 5% 5% 4 6% 7% 32%	2864
	21st to end	Night	21% 6% 3 7% 5% 5% 5% 3 5% 6% 34%	3416
September	1st to 10th	Night	18% 6% 2 6% 5% 3 4 3 5% 7% 42%	2000
	11th to 20th	Night	17% 6% 2 5% 4 4 3 3 5% 6% 45%	2178
	21st to end	Night	20% 7% 3 5% 4 4 4 3 5% 5% 41%	2200
October	1st to 10th	Night	19% 8% 2 7% 4 4 4 4 5% 6% 38%	2200
	11th to 20th	Night	24% 8% 3 7% 5% 5% 4 3 5% 4 31%	2570
	21st to end	Night	24% 11% 3 7% 4 4 3 3 5% 7% 29%	2860
November	1st to 10th	Night	20% 12% 3 7% 5% 5% 4 3 5% 6% 30%	2667
	11th to 20th	Night	20% 11% 3 9% 5% 4 4 4 4 5% 31%	2730
	21st to end	Night	22% 16% 5% 9% 4 5% 4 3 4 4 23%	3107
December	1st to 10th	Night	17% 16% 6% 11% 7% 6% 5% 4 5% 4 20%	5060
	11th to 20th	Night	22% 13% 4 10% 6% 5% 4 3 4 6% 22%	5741
	21st to end	Night	21% 18% 4 9% 4 4 4 3 4 5% 24%	7526

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	61% 4 9% 18% 9%	4662
	11th to 20th	Day	66% 5% 8% 14% 6% 1	4680
	21st to end	Day	47% 5% 12% 22% 14%	5124
February	1st to 10th	Day	57% 5% 11% 16% 8% 2	1680
	11th to 20th	Day	58% 5% 12% 15% 9% 1	2041
	21st to end	Day	56% 5% 10% 19% 8% 2	1730
March	1st to 10th	Day	60% 5% 9% 12% 10% 3	4579
	11th to 20th	Day	53% 6% 16% 16% 9% 1	4820
	21st to end	Day	53% 4 8% 17% 14% 3	5818
April	1st to 10th	Day	57% 4 11% 14% 11% 3	2400
	11th to 20th	Day	48% 5% 11% 23% 11% 2	2472
	21st to end	Day	54% 6% 11% 17% 12% 2	2600
May	1st to 10th	Day	54% 5% 12% 15% 13% 1	6033
	11th to 20th	Day	50% 8% 16% 17% 6% 2	6322
	21st to end	Day	52% 8% 11% 17% 10% 3	7031
June	1st to 10th	Day	50% 10% 12% 19% 8% 1	3000
	11th to 20th	Day	46% 8% 14% 20% 11% 1	3000
	21st to end	Day	56% 7% 11% 16% 9%	3000
July	1st to 10th	Day	50% 8% 14% 22% 6%	6384
	11th to 20th	Day	44% 8% 20% 24% 5%	6285
	21st to end	Day	37% 7% 19% 29% 9%	6728
August	1st to 10th	Day	44% 9% 20% 20% 6%	4524
	11th to 20th	Day	49% 8% 20% 18% 5%	4214
	21st to end	Day	45% 8% 15% 19% 12% 1	4479
September	1st to 10th	Day	41% 7% 17% 24% 10%	2400
	11th to 20th	Day	36% 6% 17% 30% 10%	2221
	21st to end	Day	46% 7% 13% 26% 9%	2200
October	1st to 10th	Day	52% 7% 12% 23% 6%	3574
	11th to 20th	Day	52% 8% 18% 21% 2	2954
	21st to end	Day	53% 6% 17% 16% 7%	3180
November	1st to 10th	Day	56% 6% 12% 19% 7%	1851
	11th to 20th	Day	61% 5% 9% 18% 6% 1	1890
	21st to end	Day	66% 5% 12% 12% 5%	1510
December	1st to 10th	Day	63% 5% 12% 12% 7%	2350
	11th to 20th	Day	60% 4 11% 14% 11%	2688
	21st to end	Day	61% 6% 11% 13% 8% 1	3684
January	1st to 10th	Night	66% 5% 9% 13% 7%	8234
	11th to 20th	Night	72% 4 7% 13% 3	8280
	21st to end	Night	55% 5% 12% 17% 12%	9057
February	1st to 10th	Night	57% 5% 9% 19% 9% 1	2940
	11th to 20th	Night	59% 5% 10% 15% 9% 1	2471
	21st to end	Night	56% 5% 8% 18% 13% 1	2076
March	1st to 10th	Night	62% 4 9% 12% 11% 1	5494
	11th to 20th	Night	59% 7% 12% 14% 7% 1	5294
	21st to end	Night	58% 3 6% 21% 10% 2	5110
April	1st to 10th	Night	57% 4 9% 16% 12% 2	2000
	11th to 20th	Night	50% 5% 10% 20% 12% 2	1877
	21st to end	Night	53% 5% 8% 18% 15% 2	1800
May	1st to 10th	Night	55% 6% 10% 16% 12% 1	3968
	11th to 20th	Night	53% 7% 11% 19% 9% 1	3576
	21st to end	Night	48% 7% 12% 17% 15% 2	3844
June	1st to 10th	Night	51% 7% 12% 19% 10% 1	1400
	11th to 20th	Night	46% 7% 13% 20% 12% 3	1400
	21st to end	Night	54% 6% 12% 17% 11%	1400
July	1st to 10th	Night	48% 7% 20% 19% 6%	3464
	11th to 20th	Night	47% 7% 19% 23% 4	3552
	21st to end	Night	42% 7% 18% 26% 8%	3809
August	1st to 10th	Night	48% 8% 16% 23% 6%	2576
	11th to 20th	Night	50% 9% 18% 17% 6% 1	2891
	21st to end	Night	50% 6% 13% 20% 11%	3416
September	1st to 10th	Night	43% 6% 14% 25% 11% 1	2000
	11th to 20th	Night	40% 6% 17% 29% 8% 1	2178
	21st to end	Night	45% 6% 11% 26% 13%	2200
October	1st to 10th	Night	48% 6% 11% 26% 7% 1	3590
	11th to 20th	Night	53% 6% 13% 24% 3	4184
	21st to end	Night	51% 6% 18% 16% 9%	4610
November	1st to 10th	Night	55% 6% 12% 20% 6% 1	2667
	11th to 20th	Night	55% 6% 12% 18% 8%	2730
	21st to end	Night	64% 6% 11% 13% 6% 1	3107
December	1st to 10th	Night	67% 6% 11% 12% 3	5060
	11th to 20th	Night	63% 4 8% 17% 7%	5742
	21st to end	Night	63% 4 9% 16% 7% 1	7526

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	27% 20% 32% 16% 5% 1	220
	11th to 20th	Day	28% 23% 30% 13% 5% 1	220
	21st to end	Day	16% 21% 36% 22% 4 1	242
February	1st to 10th	Day	22% 22% 34% 16% 4 1	220
	11th to 20th	Day	23% 18% 37% 18% 3	528
	21st to end	Day	21% 22% 30% 22% 5%	446
March	1st to 10th	Day	24% 21% 30% 19% 6% 1	540
	11th to 20th	Day	24% 18% 34% 17% 6% 1	528
	21st to end	Day	23% 17% 27% 21% 10% 1	550
April	1st to 10th	Day	28% 16% 29% 19% 8%	500
	11th to 20th	Day	27% 13% 30% 21% 8% 1	492
	21st to end	Day	29% 17% 31% 17% 5% 1	500
May	1st to 10th	Day	28% 16% 33% 17% 5% 1	503
	11th to 20th	Day	20% 14% 31% 24% 9% 2	527
	21st to end	Day	24% 18% 31% 20% 6% 1	603
June	1st to 10th	Day	22% 14% 29% 26% 9% 1	730
	11th to 20th	Day	21% 11% 28% 26% 12% 1	730
	21st to end	Day	26% 10% 27% 24% 11% 2	730
July	1st to 10th	Day	18% 12% 30% 26% 12% 2	567
	11th to 20th	Day	15% 11% 35% 26% 11% 2	524
	21st to end	Day	14% 10% 34% 27% 12% 2	576
August	1st to 10th	Day	16% 8% 28% 28% 15% 4	526
	11th to 20th	Day	18% 5% 28% 27% 18% 4	519
	21st to end	Day	21% 11% 25% 21% 15% 7%	583
September	1st to 10th	Day	17% 12% 27% 26% 14% 4	530
	11th to 20th	Day	17% 11% 27% 22% 18% 6%	530
	21st to end	Day	20% 13% 25% 23% 14% 5%	530
October	1st to 10th	Day	20% 14% 27% 26% 11% 3	530
	11th to 20th	Day	28% 13% 29% 19% 10% 1	435
	21st to end	Day	33% 15% 28% 18% 6%	462
November	1st to 10th	Day	31% 15% 24% 22% 7%	425
	11th to 20th	Day	28% 17% 30% 17% 8%	430
	21st to end	Day	31% 20% 30% 16% 3	225
December	1st to 10th	Day	29% 13% 30% 20% 6% 2	220
	11th to 20th	Day	30% 20% 28% 16% 4 2	220
	21st to end	Day	31% 18% 31% 15% 4 1	242
January	1st to 10th	Night	25% 16% 32% 22% 4	840
	11th to 20th	Night	26% 14% 30% 25% 5%	840
	21st to end	Night	16% 12% 34% 30% 6% 1	924
February	1st to 10th	Night	18% 12% 34% 28% 7%	840
	11th to 20th	Night	21% 15% 38% 21% 4	823
	21st to end	Night	23% 13% 32% 28% 5%	692
March	1st to 10th	Night	23% 12% 35% 25% 5%	840
	11th to 20th	Night	25% 14% 32% 22% 6% 1	829
	21st to end	Night	24% 10% 30% 28% 6% 1	880
April	1st to 10th	Night	28% 14% 27% 25% 6%	800
	11th to 20th	Night	26% 10% 33% 25% 6%	693
	21st to end	Night	29% 11% 30% 24% 6%	600
May	1st to 10th	Night	24% 10% 33% 27% 6%	560
	11th to 20th	Night	25% 9% 27% 31% 7%	394
	21st to end	Night	27% 6% 29% 30% 8%	433
June	1st to 10th	Night	25% 7% 27% 36% 5% 1	400
	11th to 20th	Night	19% 7% 25% 37% 12%	400
	21st to end	Night	24% 6% 23% 38% 9%	400
July	1st to 10th	Night	17% 5% 29% 37% 11% 1	390
	11th to 20th	Night	19% 4 22% 41% 13% 1	388
	21st to end	Night	15% 5% 18% 47% 14%	438
August	1st to 10th	Night	18% 4 14% 47% 17% 1	398
	11th to 20th	Night	16% 5% 21% 40% 17% 1	584
	21st to end	Night	19% 5% 24% 33% 18% 2	775
September	1st to 10th	Night	20% 5% 25% 34% 16% 1	800
	11th to 20th	Night	19% 6% 23% 32% 19% 2	800
	21st to end	Night	22% 6% 26% 31% 14% 1	800
October	1st to 10th	Night	22% 9% 26% 34% 9% 1	800
	11th to 20th	Night	28% 10% 25% 28% 9% 1	800
	21st to end	Night	29% 12% 29% 25% 5%	880
November	1st to 10th	Night	27% 14% 25% 25% 8%	820
	11th to 20th	Night	25% 13% 27% 26% 8% 1	840
	21st to end	Night	31% 14% 30% 19% 7%	931
December	1st to 10th	Night	29% 13% 27% 24% 7% 1	950
	11th to 20th	Night	26% 18% 29% 21% 6% 1	950
	21st to end	Night	31% 15% 28% 19% 5%	1012

Page generated at 2026-03-15 02:23:54. Version ac07

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