Constance Bay
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Constance Bay clear sky chart from 2005-11-06 to 2025-09-19.

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% 10% 3 8% 5% 5% 5% 4 5% 7% 32%	126707
	Night	17% 12% 4 9% 5% 4 4 3 5% 6% 30%	122336
Transparency	Day	51% 6% 12% 21% 10% 1	130185
	Night	54% 5% 11% 19% 10% 1	126945
Seeing	Day	22% 15% 30% 20% 9% 3	15544
	Night	22% 12% 29% 26% 10% 1	24586

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	14% 17% 7% 16% 6% 5% 4 3 4 4 19%	3597
	11th to 20th	Day	13% 15% 6% 15% 7% 5% 5% 3 5% 5% 21%	3570
	21st to end	Day	11% 17% 7% 14% 7% 6% 5% 4 5% 5% 21%	4162
February	1st to 10th	Day	13% 15% 5% 13% 8% 5% 4 3 5% 5% 24%	1800
	11th to 20th	Day	12% 15% 6% 11% 6% 5% 5% 4 4 6% 27%	1753
	21st to end	Day	19% 12% 4 10% 7% 5% 4 4 5% 5% 26%	1640
March	1st to 10th	Day	20% 14% 5% 10% 6% 4 5% 3 4 5% 25%	4513
	11th to 20th	Day	13% 12% 4 9% 6% 5% 5% 4 6% 7% 29%	4980
	21st to end	Day	17% 12% 4 8% 5% 4 3 3 4 5% 36%	5740
April	1st to 10th	Day	22% 12% 3 6% 4 4 4 3 4 7% 32%	2400
	11th to 20th	Day	21% 10% 3 7% 4 3 4 3 4 5% 36%	2374
	21st to end	Day	21% 10% 3 7% 5% 4 4 3 4 5% 33%	2550
May	1st to 10th	Day	21% 9% 3 8% 5% 5% 5% 4 5% 6% 28%	6346
	11th to 20th	Day	16% 8% 3 8% 5% 6% 6% 4 6% 7% 32%	6318
	21st to end	Day	20% 9% 3 7% 5% 4 4 4 5% 6% 34%	6949
June	1st to 10th	Day	20% 9% 3 7% 5% 5% 5% 4 5% 7% 31%	2800
	11th to 20th	Day	14% 6% 2 7% 5% 5% 5% 4 6% 8% 39%	2800
	21st to end	Day	19% 6% 3 6% 5% 5% 5% 4 7% 8% 32%	2800
July	1st to 10th	Day	13% 6% 3 7% 5% 5% 6% 5% 7% 10% 35%	6360
	11th to 20th	Day	11% 5% 2 6% 4 5% 5% 4 6% 8% 45%	6360
	21st to end	Day	9% 5% 2 5% 4 4 5% 4 6% 10% 46%	6926
August	1st to 10th	Day	13% 6% 3 8% 5% 5% 5% 4 7% 8% 37%	4472
	11th to 20th	Day	16% 7% 3 5% 4 4 4 4 6% 10% 38%	4146
	21st to end	Day	13% 5% 2 6% 4 4 5% 4 6% 9% 41%	4312
September	1st to 10th	Day	13% 6% 2 5% 4 4 5% 4 6% 8% 43%	2400
	11th to 20th	Day	12% 5% 2 6% 4 4 4 3 6% 9% 46%	2376
	21st to end	Day	17% 6% 2 7% 3 4 4 4 5% 8% 39%	2020
October	1st to 10th	Day	21% 8% 3 7% 4 4 4 3 6% 6% 34%	1900
	11th to 20th	Day	23% 8% 3 7% 5% 5% 5% 3 5% 7% 29%	1900
	21st to end	Day	26% 10% 3 7% 5% 4 4 4 4 6% 28%	1947
November	1st to 10th	Day	16% 9% 3 6% 5% 5% 5% 4 6% 7% 33%	1750
	11th to 20th	Day	19% 13% 4 8% 6% 5% 4 4 5% 6% 27%	1600
	21st to end	Day	22% 15% 4 10% 7% 5% 5% 3 4 6% 20%	1600
December	1st to 10th	Day	22% 21% 5% 10% 5% 5% 5% 3 5% 5% 14%	2600
	11th to 20th	Day	17% 16% 6% 11% 6% 5% 3 3 4 4 24%	2986
	21st to end	Day	23% 16% 5% 12% 6% 4 4 3 3 4 20%	3960
January	1st to 10th	Night	14% 21% 8% 15% 6% 4 3 3 3 5% 19%	6349
	11th to 20th	Night	11% 15% 7% 15% 8% 6% 5% 3 4 5% 21%	6315
	21st to end	Night	12% 19% 6% 13% 7% 6% 4 3 4 4 21%	6790
February	1st to 10th	Night	13% 14% 5% 12% 7% 5% 5% 3 5% 6% 24%	2600
	11th to 20th	Night	13% 14% 5% 12% 7% 4 4 4 4 5% 28%	2533
	21st to end	Night	15% 13% 5% 10% 6% 4 5% 3 5% 6% 27%	1989
March	1st to 10th	Night	21% 15% 5% 9% 5% 4 3 3 3 4 28%	5323
	11th to 20th	Night	12% 13% 4 10% 5% 5% 5% 4 6% 8% 29%	4979
	21st to end	Night	18% 11% 4 9% 5% 4 4 3 5% 6% 34%	5192
April	1st to 10th	Night	20% 10% 3 9% 4 4 3 3 5% 5% 32%	2000
	11th to 20th	Night	19% 9% 4 8% 5% 4 4 3 4 5% 35%	1975
	21st to end	Night	20% 10% 3 8% 5% 4 4 3 5% 5% 34%	1865
May	1st to 10th	Night	22% 9% 3 8% 5% 5% 5% 4 7% 6% 26%	3594
	11th to 20th	Night	16% 8% 3 7% 4 4 4 4 5% 7% 38%	3575
	21st to end	Night	18% 9% 3 8% 5% 4 5% 4 6% 6% 32%	3926
June	1st to 10th	Night	16% 7% 3 6% 5% 5% 4 5% 7% 9% 34%	1600
	11th to 20th	Night	14% 5% 3 6% 4 5% 3 4 6% 7% 43%	1600
	21st to end	Night	18% 7% 2 6% 5% 4 5% 4 7% 7% 36%	1600
July	1st to 10th	Night	14% 7% 3 8% 6% 5% 5% 5% 6% 8% 34%	3600
	11th to 20th	Night	13% 5% 3 7% 5% 4 5% 4 6% 8% 43%	3600
	21st to end	Night	9% 4 2 6% 4 5% 4 4 6% 8% 47%	3926
August	1st to 10th	Night	12% 6% 3 7% 5% 4 4 4 6% 9% 39%	2674
	11th to 20th	Night	15% 7% 3 6% 3 4 4 4 6% 7% 41%	2875
	21st to end	Night	13% 6% 2 6% 4 5% 5% 4 6% 8% 41%	3580
September	1st to 10th	Night	11% 5% 3 6% 4 4 4 3 5% 7% 48%	2000
	11th to 20th	Night	14% 4 2 5% 3 4 3 3 5% 7% 50%	1981
	21st to end	Night	19% 6% 2 7% 4 3 4 3 4 6% 43%	2154
October	1st to 10th	Night	18% 8% 2 6% 4 5% 5% 3 5% 7% 38%	2280
	11th to 20th	Night	21% 9% 3 8% 5% 6% 4 4 5% 6% 31%	2280
	21st to end	Night	23% 11% 3 7% 6% 5% 4 3 5% 6% 27%	2650
November	1st to 10th	Night	19% 10% 3 7% 5% 4 4 3 5% 6% 33%	2547
	11th to 20th	Night	20% 11% 4 8% 5% 5% 5% 3 5% 6% 28%	2800
	21st to end	Night	22% 13% 4 10% 5% 4 4 3 6% 6% 22%	2800
December	1st to 10th	Night	20% 17% 5% 12% 7% 4 5% 4 5% 6% 15%	4580
	11th to 20th	Night	22% 15% 5% 10% 6% 4 4 2 3 4 25%	5208
	21st to end	Night	26% 18% 4 10% 4 4 3 3 3 4 21%	6996

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	67% 5% 10% 13% 5% 1	3598
	11th to 20th	Day	65% 4 10% 13% 7%	3600
	21st to end	Day	65% 5% 9% 12% 8% 1	4162
February	1st to 10th	Day	61% 4 10% 18% 5% 1	1800
	11th to 20th	Day	58% 4 12% 15% 9% 1	1762
	21st to end	Day	59% 5% 11% 13% 10% 2	1640
March	1st to 10th	Day	61% 4 8% 13% 11% 2	4516
	11th to 20th	Day	53% 6% 16% 16% 9% 1	4980
	21st to end	Day	50% 4 6% 21% 17% 3	5740
April	1st to 10th	Day	53% 4 8% 19% 13% 3	2400
	11th to 20th	Day	51% 4 7% 21% 15% 2	2374
	21st to end	Day	55% 5% 7% 16% 14% 4	2550
May	1st to 10th	Day	55% 6% 8% 20% 9% 3	6357
	11th to 20th	Day	50% 7% 12% 19% 12% 1	6322
	21st to end	Day	50% 5% 11% 20% 12% 1	6949
June	1st to 10th	Day	52% 6% 10% 20% 12% 1	2800
	11th to 20th	Day	42% 6% 12% 24% 13% 2	2800
	21st to end	Day	48% 7% 13% 21% 10% 1	2800
July	1st to 10th	Day	43% 6% 15% 27% 9%	6290
	11th to 20th	Day	36% 6% 13% 32% 13%	6285
	21st to end	Day	32% 6% 15% 36% 10%	6728
August	1st to 10th	Day	44% 6% 16% 24% 10%	4458
	11th to 20th	Day	42% 6% 15% 29% 9%	4185
	21st to end	Day	38% 6% 16% 27% 12% 1	4312
September	1st to 10th	Day	38% 7% 15% 29% 11%	2400
	11th to 20th	Day	35% 6% 15% 32% 11% 1	2376
	21st to end	Day	43% 7% 14% 26% 10%	2020
October	1st to 10th	Day	48% 5% 11% 28% 7%	3160
	11th to 20th	Day	47% 5% 14% 26% 8%	3140
	21st to end	Day	50% 6% 18% 19% 7%	3185
November	1st to 10th	Day	48% 6% 15% 24% 6% 1	1750
	11th to 20th	Day	57% 6% 12% 16% 8% 1	1600
	21st to end	Day	66% 7% 13% 9% 6% 1	1600
December	1st to 10th	Day	72% 5% 11% 9% 2	2600
	11th to 20th	Day	63% 5% 9% 15% 7%	2986
	21st to end	Day	69% 4 7% 16% 4	3960
January	1st to 10th	Night	70% 3 11% 12% 4	6349
	11th to 20th	Night	65% 6% 10% 14% 5%	6360
	21st to end	Night	67% 4 7% 10% 11% 1	6790
February	1st to 10th	Night	61% 5% 11% 16% 7% 1	2600
	11th to 20th	Night	58% 4 10% 19% 8% 1	2535
	21st to end	Night	56% 5% 10% 18% 10% 1	1989
March	1st to 10th	Night	61% 3 6% 15% 11% 3	5333
	11th to 20th	Night	51% 7% 15% 17% 8% 3	4980
	21st to end	Night	51% 5% 7% 22% 12% 3	5192
April	1st to 10th	Night	54% 5% 8% 18% 14% 2	2000
	11th to 20th	Night	50% 5% 7% 21% 15% 2	1975
	21st to end	Night	52% 4 8% 16% 16% 4	1865
May	1st to 10th	Night	55% 6% 9% 19% 8% 3	3600
	11th to 20th	Night	43% 7% 13% 20% 15% 1	3576
	21st to end	Night	50% 7% 12% 16% 13% 1	3927
June	1st to 10th	Night	45% 7% 13% 24% 9% 2	1600
	11th to 20th	Night	39% 6% 13% 24% 16% 2	1600
	21st to end	Night	45% 8% 12% 21% 14% 1	1600
July	1st to 10th	Night	46% 7% 15% 20% 11% 1	3560
	11th to 20th	Night	39% 6% 12% 32% 11%	3552
	21st to end	Night	34% 6% 13% 33% 15%	3809
August	1st to 10th	Night	40% 8% 15% 25% 11% 1	2664
	11th to 20th	Night	40% 6% 16% 29% 8%	2902
	21st to end	Night	39% 7% 14% 26% 11% 2	3580
September	1st to 10th	Night	36% 6% 13% 30% 14% 1	2000
	11th to 20th	Night	34% 6% 14% 34% 10% 1	1981
	21st to end	Night	42% 5% 12% 30% 11%	2154
October	1st to 10th	Night	40% 6% 13% 29% 12%	3800
	11th to 20th	Night	47% 5% 15% 24% 9%	3779
	21st to end	Night	50% 6% 17% 18% 8%	4362
November	1st to 10th	Night	51% 6% 14% 21% 8%	2547
	11th to 20th	Night	56% 5% 11% 18% 9% 1	2800
	21st to end	Night	61% 6% 13% 15% 5%	2800
December	1st to 10th	Night	69% 6% 12% 11% 2	4580
	11th to 20th	Night	64% 4 7% 14% 10% 1	5208
	21st to end	Night	69% 3 6% 12% 9%	6996

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	22% 22% 39% 14% 5%	200
	11th to 20th	Day	27% 19% 34% 16% 5% 1	200
	21st to end	Day	20% 17% 38% 21% 4	263
February	1st to 10th	Day	30% 23% 33% 12% 2	300
	11th to 20th	Day	30% 27% 35% 7% 1	300
	21st to end	Day	31% 22% 32% 13% 2	404
March	1st to 10th	Day	29% 18% 32% 17% 4 1	500
	11th to 20th	Day	25% 20% 36% 14% 3 1	500
	21st to end	Day	33% 18% 28% 16% 5% 1	550
April	1st to 10th	Day	34% 21% 26% 14% 5%	500
	11th to 20th	Day	32% 19% 23% 14% 10% 2	492
	21st to end	Day	28% 21% 31% 13% 6% 1	500
May	1st to 10th	Day	26% 15% 32% 17% 7% 2	503
	11th to 20th	Day	19% 16% 31% 21% 9% 3	527
	21st to end	Day	21% 15% 29% 23% 10% 2	573
June	1st to 10th	Day	25% 12% 31% 22% 8% 2	530
	11th to 20th	Day	15% 16% 31% 23% 12% 3	530
	21st to end	Day	22% 13% 32% 22% 9% 2	530
July	1st to 10th	Day	10% 9% 30% 34% 14% 3	525
	11th to 20th	Day	12% 8% 33% 28% 15% 4	524
	21st to end	Day	12% 9% 33% 30% 13% 4	576
August	1st to 10th	Day	12% 6% 28% 33% 17% 4	526
	11th to 20th	Day	14% 8% 23% 29% 19% 7%	519
	21st to end	Day	16% 10% 29% 24% 15% 7%	583
September	1st to 10th	Day	15% 12% 29% 21% 15% 8%	530
	11th to 20th	Day	11% 9% 23% 23% 20% 13%	522
	21st to end	Day	17% 13% 28% 22% 14% 6%	490
October	1st to 10th	Day	24% 14% 24% 21% 12% 4	490
	11th to 20th	Day	26% 18% 29% 18% 8% 1	490
	21st to end	Day	30% 18% 24% 22% 5%	463
November	1st to 10th	Day	26% 18% 29% 19% 8% 1	384
	11th to 20th	Day	27% 22% 35% 12% 6%	200
	21st to end	Day	31% 20% 35% 11% 3 1	200
December	1st to 10th	Day	34% 16% 35% 13% 4	200
	11th to 20th	Day	32% 25% 32% 9% 3 1	200
	21st to end	Day	27% 20% 35% 13% 4 1	220
January	1st to 10th	Night	28% 14% 35% 20% 3	800
	11th to 20th	Night	27% 17% 28% 21% 7% 1	800
	21st to end	Night	19% 13% 37% 25% 5%	880
February	1st to 10th	Night	21% 17% 36% 24% 3	800
	11th to 20th	Night	21% 19% 38% 19% 4	783
	21st to end	Night	23% 19% 28% 26% 4	660
March	1st to 10th	Night	24% 16% 33% 23% 4	800
	11th to 20th	Night	23% 19% 32% 22% 5%	800
	21st to end	Night	24% 17% 28% 24% 7%	880
April	1st to 10th	Night	27% 14% 29% 25% 4	800
	11th to 20th	Night	24% 18% 29% 21% 8%	791
	21st to end	Night	26% 13% 30% 23% 8%	665
May	1st to 10th	Night	24% 8% 27% 31% 9% 2	400
	11th to 20th	Night	23% 9% 28% 30% 9%	394
	21st to end	Night	17% 7% 36% 27% 13%	433
June	1st to 10th	Night	18% 6% 31% 35% 10%	400
	11th to 20th	Night	16% 7% 26% 36% 14% 1	400
	21st to end	Night	20% 7% 27% 33% 14%	400
July	1st to 10th	Night	10% 4 27% 41% 17%	390
	11th to 20th	Night	13% 5% 21% 45% 16% 1	388
	21st to end	Night	9% 5% 23% 46% 15% 1	438
August	1st to 10th	Night	9% 5% 19% 43% 23% 1	447
	11th to 20th	Night	12% 4 19% 39% 25% 3	594
	21st to end	Night	13% 7% 24% 36% 19% 2	874
September	1st to 10th	Night	14% 5% 28% 31% 20% 2	800
	11th to 20th	Night	15% 7% 20% 31% 22% 4	793
	21st to end	Night	20% 8% 24% 26% 20% 2	760
October	1st to 10th	Night	23% 10% 25% 25% 15% 1	760
	11th to 20th	Night	25% 13% 28% 24% 8% 1	760
	21st to end	Night	28% 14% 27% 25% 6%	836
November	1st to 10th	Night	26% 14% 26% 26% 7% 1	780
	11th to 20th	Night	25% 16% 33% 17% 9%	800
	21st to end	Night	31% 15% 26% 22% 6%	800
December	1st to 10th	Night	30% 15% 29% 21% 4	800
	11th to 20th	Night	28% 16% 30% 20% 5% 1	800
	21st to end	Night	32% 16% 29% 18% 4	880

Page generated at 2025-09-16 01:49:15. Version ac07

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