Grand Coteau Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grand Coteau Observatory clear sky chart from 2005-11-06 to 2026-07-31.

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	12% 6% 2 7% 5% 5% 5% 4 6% 8% 40%	137357
	Night	13% 6% 2 6% 4 4 4 4 6% 7% 43%	135634
Transparency	Day	41% 5% 12% 18% 17% 7%	141082
	Night	39% 5% 12% 18% 18% 8%	139987
Seeing	Day	14% 9% 21% 25% 21% 10%	17008
	Night	15% 7% 21% 30% 22% 5%	26506

By Month

Percentage Cloud Cover SE (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	18% 6% 4 5% 3 4 4 3 5% 6% 43%	5362
	11th to 20th	Day	10% 6% 3 6% 3 3 3 3 4 5% 54%	5553
	21st to end	Day	22% 9% 3 7% 4 3 4 3 5% 5% 36%	6436
February	1st to 10th	Day	17% 8% 3 6% 3 5% 4 3 5% 5% 40%	2100
	11th to 20th	Day	18% 6% 2 6% 5% 4 4 3 5% 8% 40%	2048
	21st to end	Day	16% 8% 3 7% 4 5% 4 3 5% 6% 39%	1730
March	1st to 10th	Day	18% 9% 3 7% 4 4 4 4 5% 6% 35%	4663
	11th to 20th	Day	14% 6% 2 6% 4 4 4 3 5% 7% 46%	5090
	21st to end	Day	13% 7% 3 8% 5% 5% 4 3 5% 5% 40%	5947
April	1st to 10th	Day	13% 7% 2 7% 5% 5% 5% 4 5% 7% 41%	2520
	11th to 20th	Day	14% 6% 3 6% 4 4 5% 4 6% 6% 42%	2494
	21st to end	Day	9% 5% 2 6% 5% 5% 5% 4 6% 8% 47%	2520
May	1st to 10th	Day	21% 7% 3 6% 4 5% 5% 4 6% 7% 32%	7021
	11th to 20th	Day	9% 5% 2 6% 5% 5% 6% 5% 8% 9% 41%	7031
	21st to end	Day	11% 7% 3 7% 6% 6% 6% 4 8% 11% 33%	7970
June	1st to 10th	Day	10% 6% 3 7% 6% 6% 8% 6% 8% 9% 31%	2730
	11th to 20th	Day	6% 5% 2 7% 7% 7% 7% 6% 7% 10% 35%	2850
	21st to end	Day	7% 6% 2 8% 7% 8% 9% 6% 9% 10% 28%	2837
July	1st to 10th	Day	5% 6% 3 8% 7% 7% 8% 7% 9% 12% 28%	6030
	11th to 20th	Day	5% 4 2 7% 6% 7% 8% 6% 9% 11% 34%	5842
	21st to end	Day	6% 6% 2 8% 6% 7% 8% 6% 9% 13% 29%	6042
August	1st to 10th	Day	3 3 1 5% 4 5% 5% 5% 6% 8% 53%	3905
	11th to 20th	Day	7% 4 2 7% 5% 6% 6% 5% 7% 10% 41%	3849
	21st to end	Day	8% 6% 3 9% 6% 6% 7% 5% 8% 7% 35%	4312
September	1st to 10th	Day	10% 7% 2 8% 6% 7% 6% 5% 6% 8% 35%	2400
	11th to 20th	Day	11% 7% 2 7% 5% 5% 6% 4 6% 7% 42%	2310
	21st to end	Day	9% 4 2 5% 4 4 5% 4 5% 8% 50%	2200
October	1st to 10th	Day	6% 5% 2 6% 4 4 4 3 5% 6% 54%	2168
	11th to 20th	Day	8% 4 2 5% 4 4 3 4 5% 7% 56%	2000
	21st to end	Day	10% 5% 1 5% 4 3 4 3 3 5% 56%	2200
November	1st to 10th	Day	10% 5% 2 5% 4 3 3 3 5% 7% 53%	2058
	11th to 20th	Day	11% 5% 2 4 3 3 4 3 5% 6% 55%	1895
	21st to end	Day	14% 6% 3 6% 4 4 4 3 5% 7% 46%	1890
December	1st to 10th	Day	18% 7% 2 5% 3 3 4 4 4 5% 43%	3020
	11th to 20th	Day	11% 8% 2 7% 4 4 4 3 5% 6% 46%	3463
	21st to end	Day	17% 7% 2 6% 3 3 3 3 4 6% 44%	4871
January	1st to 10th	Night	21% 7% 4 5% 3 3 3 2 4 5% 43%	7501
	11th to 20th	Night	16% 8% 3 5% 3 2 3 2 4 5% 49%	7383
	21st to end	Night	24% 9% 3 8% 3 4 4 3 4 5% 36%	7745
February	1st to 10th	Night	17% 8% 3 6% 4 3 4 3 5% 6% 42%	2520
	11th to 20th	Night	21% 9% 3 5% 4 4 3 2 4 5% 39%	2458
	21st to end	Night	20% 7% 3 7% 4 3 3 3 4 6% 39%	2076
March	1st to 10th	Night	20% 10% 3 6% 4 3 4 3 5% 6% 36%	5451
	11th to 20th	Night	14% 7% 2 5% 4 4 4 3 4 5% 48%	5090
	21st to end	Night	17% 7% 2 5% 4 4 3 4 5% 5% 45%	5223
April	1st to 10th	Night	15% 7% 2 7% 4 4 4 3 5% 6% 44%	2100
	11th to 20th	Night	13% 5% 3 5% 5% 3 4 4 5% 6% 47%	2075
	21st to end	Night	8% 4 2 5% 4 3 4 4 5% 6% 55%	2100
May	1st to 10th	Night	22% 8% 3 7% 4 3 3 3 4 7% 37%	5832
	11th to 20th	Night	12% 6% 2 7% 4 4 4 3 6% 9% 43%	5829
	21st to end	Night	11% 7% 3 8% 5% 5% 5% 5% 7% 11% 33%	6160
June	1st to 10th	Night	8% 6% 3 6% 5% 5% 6% 5% 7% 11% 40%	1890
	11th to 20th	Night	4 4 2 5% 4 5% 7% 5% 8% 10% 45%	1760
	21st to end	Night	7% 6% 2 6% 5% 5% 6% 5% 8% 11% 41%	1772
July	1st to 10th	Night	4 4 2 7% 6% 6% 7% 6% 9% 13% 36%	4168
	11th to 20th	Night	3 5% 2 7% 5% 6% 6% 5% 9% 13% 39%	4341
	21st to end	Night	5% 5% 3 8% 6% 6% 7% 6% 9% 13% 33%	5029
August	1st to 10th	Night	2 2 1 3 3 4 4 4 7% 12% 56%	3244
	11th to 20th	Night	4 4 1 5% 4 5% 6% 5% 8% 14% 43%	3200
	21st to end	Night	6% 5% 2 8% 6% 6% 6% 6% 8% 10% 38%	3586
September	1st to 10th	Night	8% 6% 3 7% 5% 5% 6% 4 7% 9% 39%	2000
	11th to 20th	Night	9% 6% 2 6% 4 4 6% 4 6% 9% 45%	2070
	21st to end	Night	8% 4 2 6% 4 4 4 3 5% 7% 52%	2200
October	1st to 10th	Night	7% 4 2 4 4 3 4 3 6% 7% 56%	2227
	11th to 20th	Night	7% 4 2 4 4 3 3 3 4 6% 61%	2400
	21st to end	Night	12% 5% 2 5% 3 3 3 2 4 5% 57%	2640
November	1st to 10th	Night	10% 4 2 5% 3 3 3 3 4 5% 58%	2460
	11th to 20th	Night	12% 5% 2 4 2 3 3 3 5% 6% 55%	2720
	21st to end	Night	13% 7% 2 5% 3 3 3 3 6% 7% 47%	2730
December	1st to 10th	Night	17% 7% 2 5% 3 4 4 3 5% 5% 44%	4380
	11th to 20th	Night	14% 7% 3 5% 4 3 4 3 5% 5% 46%	4952
	21st to end	Night	21% 8% 3 5% 4 3 3 2 3 5% 44%	6322

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	43% 3 7% 18% 24% 6%	5363
	11th to 20th	Day	34% 3 3 18% 31% 11%	5553
	21st to end	Day	52% 2 5% 11% 16% 14%	6436
February	1st to 10th	Day	46% 4 6% 14% 21% 10%	2100
	11th to 20th	Day	43% 4 6% 16% 20% 9%	2058
	21st to end	Day	47% 3 6% 15% 18% 10%	1730
March	1st to 10th	Day	49% 3 6% 15% 18% 9%	4663
	11th to 20th	Day	38% 3 7% 16% 19% 17%	5090
	21st to end	Day	46% 3 10% 16% 17% 8%	5949
April	1st to 10th	Day	43% 4 9% 18% 18% 9%	2520
	11th to 20th	Day	41% 4 7% 18% 22% 9%	2494
	21st to end	Day	36% 5% 11% 22% 21% 5%	2520
May	1st to 10th	Day	49% 4 12% 18% 13% 3	7056
	11th to 20th	Day	38% 4 15% 23% 18% 2	7035
	21st to end	Day	44% 6% 16% 23% 10% 2	7984
June	1st to 10th	Day	45% 8% 14% 21% 10% 2	2730
	11th to 20th	Day	42% 7% 17% 20% 12% 2	2850
	21st to end	Day	47% 9% 17% 20% 6% 1	2837
July	1st to 10th	Day	45% 7% 26% 18% 4 1	5966
	11th to 20th	Day	41% 9% 35% 14% 2	5773
	21st to end	Day	44% 9% 21% 22% 3	5872
August	1st to 10th	Day	27% 6% 14% 27% 25% 1	3893
	11th to 20th	Day	37% 5% 18% 26% 12% 1	3885
	21st to end	Day	46% 8% 15% 15% 15% 2	4311
September	1st to 10th	Day	45% 7% 14% 17% 13% 5%	2400
	11th to 20th	Day	41% 6% 11% 18% 17% 7%	2310
	21st to end	Day	33% 5% 10% 23% 19% 9%	2200
October	1st to 10th	Day	28% 3 5% 11% 23% 29%	3532
	11th to 20th	Day	18% 3 4 16% 29% 30%	3240
	21st to end	Day	22% 3 3 14% 37% 21%	3538
November	1st to 10th	Day	33% 3 6% 19% 26% 13%	2058
	11th to 20th	Day	31% 3 6% 17% 30% 13%	1895
	21st to end	Day	39% 3 7% 18% 22% 11%	1890
December	1st to 10th	Day	44% 2 7% 16% 23% 7%	3020
	11th to 20th	Day	40% 2 9% 16% 23% 10%	3460
	21st to end	Day	42% 4 6% 18% 26% 4	4871
January	1st to 10th	Night	46% 2 8% 16% 22% 6%	7502
	11th to 20th	Night	40% 2 3 16% 26% 12%	7383
	21st to end	Night	53% 2 4 13% 19% 9%	7745
February	1st to 10th	Night	43% 4 6% 16% 22% 9%	2520
	11th to 20th	Night	49% 3 5% 12% 20% 11%	2459
	21st to end	Night	47% 4 8% 15% 14% 12%	2076
March	1st to 10th	Night	49% 3 8% 12% 20% 8%	5451
	11th to 20th	Night	38% 4 7% 16% 20% 15%	5090
	21st to end	Night	41% 3 10% 21% 17% 8%	5223
April	1st to 10th	Night	41% 4 10% 16% 17% 11%	2100
	11th to 20th	Night	38% 4 8% 22% 19% 9%	2075
	21st to end	Night	29% 4 12% 23% 24% 7%	2100
May	1st to 10th	Night	50% 3 13% 16% 15% 3	5862
	11th to 20th	Night	39% 4 19% 22% 15% 2	5830
	21st to end	Night	44% 6% 19% 21% 9% 1	6171
June	1st to 10th	Night	37% 8% 18% 25% 11% 2	1890
	11th to 20th	Night	30% 11% 20% 26% 11% 2	1760
	21st to end	Night	36% 8% 21% 25% 8% 2	1772
July	1st to 10th	Night	38% 10% 29% 18% 5%	4125
	11th to 20th	Night	35% 13% 38% 12% 2	4290
	21st to end	Night	41% 13% 19% 24% 3	4885
August	1st to 10th	Night	19% 8% 19% 30% 22% 1	3232
	11th to 20th	Night	29% 10% 23% 28% 9% 1	3230
	21st to end	Night	40% 11% 17% 18% 11% 4	3586
September	1st to 10th	Night	39% 8% 18% 16% 14% 5%	2000
	11th to 20th	Night	35% 7% 17% 18% 15% 8%	2070
	21st to end	Night	31% 6% 11% 25% 16% 11%	2200
October	1st to 10th	Night	28% 4 7% 13% 22% 26%	3643
	11th to 20th	Night	16% 3 4 20% 24% 32%	3899
	21st to end	Night	21% 2 4 13% 36% 24%	4252
November	1st to 10th	Night	31% 3 7% 19% 25% 16%	2460
	11th to 20th	Night	30% 4 6% 16% 28% 16%	2720
	21st to end	Night	37% 3 8% 20% 20% 13%	2730
December	1st to 10th	Night	43% 2 6% 17% 25% 7%	4380
	11th to 20th	Night	41% 3 8% 18% 21% 9%	4954
	21st to end	Night	45% 3 5% 18% 23% 7%	6322

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	26% 16% 36% 16% 5% 1	330
	11th to 20th	Day	23% 18% 35% 17% 6% 1	330
	21st to end	Day	28% 23% 33% 14% 2	363
February	1st to 10th	Day	28% 16% 32% 18% 5% 1	330
	11th to 20th	Day	29% 19% 33% 15% 4	330
	21st to end	Day	23% 25% 38% 11% 2	273
March	1st to 10th	Day	21% 18% 42% 15% 4	330
	11th to 20th	Day	27% 18% 32% 18% 4 1	330
	21st to end	Day	18% 15% 32% 26% 8% 1	520
April	1st to 10th	Day	18% 15% 35% 22% 9% 1	540
	11th to 20th	Day	21% 14% 30% 23% 11% 2	532
	21st to end	Day	9% 12% 31% 30% 14% 4	540
May	1st to 10th	Day	16% 13% 27% 24% 17% 4	543
	11th to 20th	Day	13% 4 25% 32% 18% 8%	567
	21st to end	Day	8% 3 21% 37% 22% 9%	617
June	1st to 10th	Day	12% 4 15% 32% 24% 14%	570
	11th to 20th	Day	8% 3 12% 34% 29% 14%	690
	21st to end	Day	9% 3 11% 29% 31% 17%	780
July	1st to 10th	Day	5% 4 7% 28% 37% 19%	771
	11th to 20th	Day	8% 2 3 22% 40% 26%	684
	21st to end	Day	9% 2 6% 20% 40% 21%	616
August	1st to 10th	Day	4 1 6% 24% 35% 30%	526
	11th to 20th	Day	8% 2 4 16% 37% 33%	519
	21st to end	Day	11% 3 9% 22% 33% 22%	583
September	1st to 10th	Day	11% 2 8% 28% 34% 17%	530
	11th to 20th	Day	10% 4 11% 25% 35% 15%	530
	21st to end	Day	12% 3 11% 30% 33% 11%	530
October	1st to 10th	Day	7% 6% 22% 34% 24% 7%	498
	11th to 20th	Day	7% 7% 26% 32% 23% 5%	330
	21st to end	Day	18% 10% 21% 29% 19% 3	363
November	1st to 10th	Day	17% 8% 26% 33% 14% 2	330
	11th to 20th	Day	16% 11% 30% 27% 14% 1	330
	21st to end	Day	16% 17% 36% 20% 8% 3	330
December	1st to 10th	Day	23% 12% 35% 22% 6% 1	330
	11th to 20th	Day	23% 15% 36% 19% 6% 1	330
	21st to end	Day	26% 14% 28% 23% 8% 1	363
January	1st to 10th	Night	23% 12% 31% 28% 5%	1006
	11th to 20th	Night	21% 13% 30% 30% 7%	948
	21st to end	Night	27% 11% 31% 27% 4	924
February	1st to 10th	Night	22% 13% 34% 27% 5%	840
	11th to 20th	Night	28% 15% 27% 25% 5%	823
	21st to end	Night	24% 18% 34% 21% 4	692
March	1st to 10th	Night	21% 13% 32% 28% 6%	824
	11th to 20th	Night	19% 10% 32% 31% 8%	630
	21st to end	Night	17% 11% 32% 31% 9%	693
April	1st to 10th	Night	19% 12% 28% 31% 10%	630
	11th to 20th	Night	16% 8% 34% 29% 13% 1	623
	21st to end	Night	10% 5% 29% 40% 14% 1	630
May	1st to 10th	Night	14% 6% 24% 39% 16% 1	630
	11th to 20th	Night	11% 4 22% 38% 21% 4	621
	21st to end	Night	8% 2 15% 41% 29% 5%	683
June	1st to 10th	Night	9% 2 11% 38% 33% 7%	630
	11th to 20th	Night	6% 2 8% 30% 44% 10%	630
	21st to end	Night	9% 2 7% 27% 41% 15%	630
July	1st to 10th	Night	6% 1 2 21% 51% 19%	615
	11th to 20th	Night	5% 1 2 17% 51% 23%	612
	21st to end	Night	6% 1 4 21% 49% 19%	687
August	1st to 10th	Night	4 2 2 17% 50% 25%	597
	11th to 20th	Night	8% 1 3 17% 47% 24%	594
	21st to end	Night	8% 2 5% 23% 40% 21%	660
September	1st to 10th	Night	9% 3 7% 27% 41% 13%	600
	11th to 20th	Night	10% 3 7% 27% 42% 10%	670
	21st to end	Night	10% 3 10% 32% 38% 7%	800
October	1st to 10th	Night	9% 2 19% 36% 30% 4	800
	11th to 20th	Night	9% 5% 21% 37% 25% 3	800
	21st to end	Night	14% 8% 20% 36% 22% 2	880
November	1st to 10th	Night	13% 5% 20% 38% 20% 2	820
	11th to 20th	Night	14% 8% 27% 33% 16% 1	840
	21st to end	Night	17% 9% 25% 34% 14% 1	840
December	1st to 10th	Night	20% 13% 27% 28% 12%	840
	11th to 20th	Night	22% 8% 30% 31% 9%	840
	21st to end	Night	26% 10% 30% 27% 7%	924

Page generated at 2026-07-28 02:25:25. Version ac07

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