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 2025-03-27.

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% 3 7% 5% 5% 5% 4 6% 8% 40%	109352
	Night	13% 6% 2 6% 4 4 4 4 5% 7% 44%	109954
Transparency	Day	41% 5% 11% 18% 18% 7%	113113
	Night	38% 5% 11% 18% 19% 9%	114332
Seeing	Day	14% 9% 20% 25% 21% 10%	15147
	Night	15% 7% 21% 30% 22% 5%	24848

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	17% 5% 5% 5% 3 3 3 3 5% 5% 46%	4138
	11th to 20th	Day	12% 6% 3 6% 4 3 3 3 4 4 52%	4271
	21st to end	Day	22% 9% 3 7% 4 3 3 3 5% 5% 37%	4972
February	1st to 10th	Day	18% 9% 3 6% 3 5% 4 3 5% 6% 38%	2000
	11th to 20th	Day	19% 6% 2 5% 4 3 3 3 5% 7% 41%	1948
	21st to end	Day	17% 8% 3 7% 4 5% 4 3 5% 6% 38%	1650
March	1st to 10th	Day	22% 10% 3 8% 5% 4 4 4 5% 6% 30%	3276
	11th to 20th	Day	16% 7% 3 7% 5% 5% 4 3 5% 8% 36%	3590
	21st to end	Day	11% 7% 3 7% 5% 5% 5% 4 5% 6% 40%	4128
April	1st to 10th	Day	12% 7% 2 6% 5% 5% 5% 4 5% 7% 42%	2280
	11th to 20th	Day	15% 6% 3 7% 4 4 5% 4 5% 6% 40%	2254
	21st to end	Day	9% 5% 2 6% 5% 5% 5% 4 5% 7% 48%	2280
May	1st to 10th	Day	11% 5% 2 7% 5% 6% 6% 5% 8% 9% 36%	3765
	11th to 20th	Day	15% 6% 2 6% 5% 4 5% 3 6% 7% 40%	3764
	21st to end	Day	7% 4 2 6% 5% 6% 6% 4 7% 12% 41%	4251
June	1st to 10th	Day	10% 6% 3 7% 6% 6% 8% 6% 8% 9% 30%	2470
	11th to 20th	Day	5% 5% 2 7% 7% 7% 7% 6% 7% 10% 36%	2578
	21st to end	Day	7% 7% 2 8% 8% 8% 9% 7% 9% 9% 27%	2567
July	1st to 10th	Day	7% 7% 3 9% 8% 8% 8% 7% 9% 11% 23%	4120
	11th to 20th	Day	7% 6% 2 7% 7% 8% 8% 6% 9% 10% 30%	3992
	21st to end	Day	9% 7% 3 10% 7% 7% 8% 6% 8% 12% 24%	4118
August	1st to 10th	Day	3 3 1 5% 4 5% 5% 4 6% 8% 55%	3785
	11th to 20th	Day	7% 4 2 7% 5% 6% 6% 5% 7% 10% 41%	3729
	21st to end	Day	8% 6% 3 9% 7% 6% 7% 5% 7% 7% 34%	4180
September	1st to 10th	Day	10% 8% 2 8% 6% 7% 6% 5% 6% 8% 34%	2280
	11th to 20th	Day	11% 7% 2 7% 5% 5% 5% 4 6% 7% 40%	2195
	21st to end	Day	9% 5% 2 5% 4 4 5% 4 5% 8% 50%	2090
October	1st to 10th	Day	6% 5% 2 6% 4 5% 4 3 5% 6% 55%	2060
	11th to 20th	Day	8% 4 2 5% 4 4 3 4 5% 7% 54%	1900
	21st to end	Day	10% 5% 1 5% 4 3 4 3 3 5% 56%	2090
November	1st to 10th	Day	11% 5% 2 6% 4 4 3 3 5% 7% 51%	1958
	11th to 20th	Day	11% 5% 2 4 3 3 4 3 5% 6% 54%	1805
	21st to end	Day	14% 6% 3 6% 4 4 4 3 4 7% 46%	1800
December	1st to 10th	Day	18% 7% 2 5% 3 3 4 4 4 5% 43%	2930
	11th to 20th	Day	11% 8% 2 7% 4 4 4 3 5% 6% 46%	3373
	21st to end	Day	18% 7% 2 6% 3 3 3 3 4 6% 44%	4765
January	1st to 10th	Night	18% 7% 4 5% 2 3 3 2 4 4 48%	5781
	11th to 20th	Night	18% 8% 3 4 2 2 2 2 4 4 49%	5673
	21st to end	Night	24% 9% 2 5% 3 3 3 3 4 5% 38%	5984
February	1st to 10th	Night	18% 8% 3 6% 4 3 4 3 5% 7% 40%	2400
	11th to 20th	Night	21% 9% 3 5% 4 4 3 2 4 5% 41%	2338
	21st to end	Night	20% 7% 3 7% 5% 3 3 3 5% 6% 38%	1980
March	1st to 10th	Night	23% 11% 3 7% 4 3 4 3 5% 6% 31%	3825
	11th to 20th	Night	14% 8% 2 6% 5% 5% 5% 4 5% 6% 39%	3590
	21st to end	Night	15% 7% 2 5% 4 4 4 4 5% 6% 45%	3641
April	1st to 10th	Night	14% 7% 2 7% 4 4 4 3 5% 7% 45%	1900
	11th to 20th	Night	14% 5% 3 5% 6% 4 4 4 5% 5% 45%	1875
	21st to end	Night	8% 3 2 5% 4 3 4 4 5% 6% 57%	1900
May	1st to 10th	Night	13% 7% 3 6% 3 3 3 3 4 8% 45%	3130
	11th to 20th	Night	14% 6% 2 7% 4 4 4 3 5% 7% 43%	3129
	21st to end	Night	5% 4 2 5% 5% 5% 5% 4 8% 12% 45%	3283
June	1st to 10th	Night	7% 6% 3 5% 5% 4 6% 5% 7% 11% 40%	1710
	11th to 20th	Night	3 4 2 5% 4 4 7% 6% 8% 11% 46%	1592
	21st to end	Night	7% 6% 2 6% 5% 5% 6% 4 8% 11% 41%	1603
July	1st to 10th	Night	5% 5% 3 8% 6% 6% 6% 6% 9% 14% 32%	2850
	11th to 20th	Night	4 5% 2 7% 6% 6% 7% 6% 10% 13% 36%	2968
	21st to end	Night	6% 6% 3 8% 7% 6% 7% 5% 9% 13% 29%	3435
August	1st to 10th	Night	2 2 1 3 3 3 4 4 7% 12% 57%	3144
	11th to 20th	Night	4 4 1 5% 4 5% 6% 5% 8% 14% 43%	3100
	21st to end	Night	6% 5% 2 8% 6% 6% 6% 5% 8% 10% 38%	3476
September	1st to 10th	Night	9% 6% 3 7% 5% 5% 6% 4 7% 9% 39%	1900
	11th to 20th	Night	9% 6% 2 6% 5% 4 6% 4 6% 9% 44%	1966
	21st to end	Night	8% 4 2 6% 4 4 4 4 5% 7% 52%	2090
October	1st to 10th	Night	7% 4 2 4 4 3 4 3 5% 7% 57%	2115
	11th to 20th	Night	7% 4 2 4 4 3 3 3 4 7% 60%	2280
	21st to end	Night	12% 5% 2 5% 3 3 3 2 3 5% 58%	2508
November	1st to 10th	Night	11% 4 2 5% 3 3 3 3 4 5% 57%	2340
	11th to 20th	Night	13% 5% 2 4 2 3 3 3 5% 6% 54%	2590
	21st to end	Night	13% 7% 2 5% 3 3 3 3 6% 6% 48%	2600
December	1st to 10th	Night	17% 6% 2 5% 4 4 4 3 5% 5% 45%	4250
	11th to 20th	Night	14% 7% 3 5% 4 3 4 3 5% 5% 46%	4822
	21st to end	Night	21% 8% 3 5% 3 3 3 2 3 5% 44%	6186

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	40% 3 4 19% 27% 7%	4139
	11th to 20th	Day	36% 3 3 16% 31% 10%	4271
	21st to end	Day	51% 2 6% 12% 16% 12%	4972
February	1st to 10th	Day	47% 4 7% 13% 19% 10%	2000
	11th to 20th	Day	43% 4 6% 16% 21% 10%	1958
	21st to end	Day	48% 3 6% 16% 17% 10%	1650
March	1st to 10th	Day	54% 4 7% 16% 14% 5%	3276
	11th to 20th	Day	46% 4 8% 18% 18% 7%	3590
	21st to end	Day	44% 5% 9% 14% 19% 9%	4128
April	1st to 10th	Day	42% 4 9% 17% 18% 9%	2280
	11th to 20th	Day	43% 4 8% 17% 20% 8%	2254
	21st to end	Day	35% 5% 9% 22% 23% 6%	2280
May	1st to 10th	Day	40% 7% 14% 21% 15% 4	3800
	11th to 20th	Day	43% 5% 10% 19% 20% 3	3768
	21st to end	Day	34% 7% 16% 31% 11% 2	4264
June	1st to 10th	Day	46% 8% 14% 21% 9% 2	2470
	11th to 20th	Day	40% 7% 16% 21% 13% 2	2578
	21st to end	Day	48% 9% 16% 19% 7% 1	2567
July	1st to 10th	Day	51% 8% 22% 15% 3	4056
	11th to 20th	Day	45% 9% 32% 12% 2	3923
	21st to end	Day	51% 10% 18% 15% 5% 1	3987
August	1st to 10th	Day	27% 6% 13% 27% 25% 1	3773
	11th to 20th	Day	37% 6% 17% 26% 13% 1	3765
	21st to end	Day	46% 8% 14% 15% 15% 2	4179
September	1st to 10th	Day	46% 7% 13% 16% 12% 5%	2280
	11th to 20th	Day	42% 6% 11% 17% 16% 7%	2195
	21st to end	Day	33% 5% 10% 23% 19% 10%	2090
October	1st to 10th	Day	27% 3 5% 11% 24% 30%	3424
	11th to 20th	Day	18% 3 4 16% 29% 29%	3140
	21st to end	Day	22% 3 3 14% 37% 21%	3428
November	1st to 10th	Day	34% 3 7% 18% 25% 12%	1958
	11th to 20th	Day	32% 3 6% 16% 30% 13%	1805
	21st to end	Day	39% 3 6% 18% 22% 11%	1800
December	1st to 10th	Day	44% 2 7% 16% 23% 8%	2930
	11th to 20th	Day	40% 3 9% 16% 23% 10%	3370
	21st to end	Day	42% 4 6% 18% 26% 4	4765
January	1st to 10th	Night	41% 2 5% 18% 27% 7%	5781
	11th to 20th	Night	41% 3 3 16% 28% 10%	5673
	21st to end	Night	50% 2 5% 12% 21% 10%	5984
February	1st to 10th	Night	44% 4 6% 16% 21% 9%	2400
	11th to 20th	Night	48% 3 5% 13% 20% 11%	2339
	21st to end	Night	48% 4 8% 15% 13% 12%	1980
March	1st to 10th	Night	55% 4 9% 13% 13% 6%	3825
	11th to 20th	Night	44% 5% 9% 15% 19% 7%	3590
	21st to end	Night	39% 4 10% 21% 19% 6%	3641
April	1st to 10th	Night	39% 5% 10% 16% 18% 11%	1900
	11th to 20th	Night	40% 4 9% 21% 17% 9%	1875
	21st to end	Night	27% 5% 11% 23% 26% 8%	1900
May	1st to 10th	Night	39% 5% 14% 21% 17% 4	3160
	11th to 20th	Night	40% 6% 12% 23% 17% 2	3130
	21st to end	Night	29% 7% 22% 26% 14% 2	3291
June	1st to 10th	Night	35% 8% 18% 26% 11% 2	1710
	11th to 20th	Night	27% 11% 20% 28% 12% 2	1592
	21st to end	Night	36% 8% 20% 26% 9% 2	1603
July	1st to 10th	Night	40% 11% 29% 15% 4	2805
	11th to 20th	Night	36% 12% 38% 11% 2 1	2914
	21st to end	Night	44% 16% 20% 17% 3	3325
August	1st to 10th	Night	18% 8% 19% 30% 23% 1	3132
	11th to 20th	Night	29% 10% 22% 28% 9% 1	3130
	21st to end	Night	40% 11% 16% 18% 11% 4	3476
September	1st to 10th	Night	39% 8% 19% 14% 14% 6%	1900
	11th to 20th	Night	36% 7% 17% 17% 14% 8%	1966
	21st to end	Night	31% 6% 11% 25% 16% 11%	2090
October	1st to 10th	Night	28% 4 6% 12% 22% 27%	3531
	11th to 20th	Night	17% 3 4 20% 24% 32%	3779
	21st to end	Night	21% 2 4 13% 36% 25%	4120
November	1st to 10th	Night	31% 3 7% 19% 24% 15%	2340
	11th to 20th	Night	31% 4 6% 15% 28% 16%	2590
	21st to end	Night	37% 3 7% 20% 20% 13%	2600
December	1st to 10th	Night	42% 2 6% 17% 25% 7%	4250
	11th to 20th	Night	41% 3 8% 18% 21% 9%	4824
	21st to end	Night	45% 3 5% 18% 23% 7%	6186

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% 37% 16% 4 1	300
	11th to 20th	Day	24% 17% 35% 17% 7% 1	300
	21st to end	Day	25% 24% 34% 15% 2	330
February	1st to 10th	Day	30% 15% 29% 19% 6% 1	300
	11th to 20th	Day	30% 19% 33% 15% 3	300
	21st to end	Day	22% 26% 38% 12% 2	249
March	1st to 10th	Day	23% 19% 41% 14% 3	300
	11th to 20th	Day	28% 19% 31% 18% 3 1	300
	21st to end	Day	19% 17% 32% 22% 9% 1	460
April	1st to 10th	Day	17% 16% 34% 23% 10% 1	460
	11th to 20th	Day	23% 15% 28% 21% 11% 2	452
	21st to end	Day	9% 12% 30% 32% 15% 3	460
May	1st to 10th	Day	12% 12% 27% 25% 19% 4	463
	11th to 20th	Day	14% 4 22% 32% 19% 9%	487
	21st to end	Day	7% 3 20% 36% 24% 11%	529
June	1st to 10th	Day	13% 4 13% 32% 23% 15%	490
	11th to 20th	Day	7% 2 11% 35% 30% 14%	598
	21st to end	Day	9% 3 11% 30% 30% 16%	680
July	1st to 10th	Day	6% 4 7% 27% 37% 19%	671
	11th to 20th	Day	9% 2 2 21% 40% 26%	592
	21st to end	Day	11% 3 5% 20% 39% 23%	532
August	1st to 10th	Day	5% 1 6% 24% 34% 30%	486
	11th to 20th	Day	9% 2 4 17% 38% 31%	479
	21st to end	Day	11% 3 9% 20% 34% 22%	539
September	1st to 10th	Day	11% 3 8% 26% 34% 18%	490
	11th to 20th	Day	11% 4 12% 25% 35% 13%	490
	21st to end	Day	13% 3 10% 29% 33% 11%	490
October	1st to 10th	Day	6% 7% 22% 34% 25% 7%	460
	11th to 20th	Day	7% 6% 26% 33% 22% 5%	300
	21st to end	Day	17% 10% 18% 30% 20% 4	330
November	1st to 10th	Day	17% 7% 27% 34% 13% 2	300
	11th to 20th	Day	18% 12% 30% 26% 14% 1	300
	21st to end	Day	16% 15% 35% 21% 9% 3	300
December	1st to 10th	Day	22% 11% 36% 23% 7% 1	300
	11th to 20th	Day	24% 16% 36% 18% 6% 1	300
	21st to end	Day	27% 15% 29% 23% 7% 1	330
January	1st to 10th	Night	22% 12% 32% 28% 6%	958
	11th to 20th	Night	22% 12% 30% 29% 7%	903
	21st to end	Night	27% 11% 31% 27% 4	880
February	1st to 10th	Night	22% 13% 34% 27% 5%	800
	11th to 20th	Night	27% 16% 27% 26% 5%	783
	21st to end	Night	24% 17% 35% 21% 4	660
March	1st to 10th	Night	20% 14% 32% 28% 6%	785
	11th to 20th	Night	18% 10% 33% 31% 8%	600
	21st to end	Night	17% 11% 32% 30% 10%	647
April	1st to 10th	Night	17% 13% 28% 31% 10% 1	570
	11th to 20th	Night	16% 8% 33% 29% 12% 1	563
	21st to end	Night	9% 5% 29% 41% 14% 1	570
May	1st to 10th	Night	11% 6% 24% 40% 17% 1	570
	11th to 20th	Night	10% 4 20% 40% 22% 4	561
	21st to end	Night	6% 2 15% 41% 31% 6%	617
June	1st to 10th	Night	9% 2 10% 37% 34% 8%	570
	11th to 20th	Night	4 2 8% 30% 45% 11%	570
	21st to end	Night	9% 2 7% 27% 40% 15%	570
July	1st to 10th	Night	6% 1 2 22% 50% 19%	555
	11th to 20th	Night	5% 1 3 18% 51% 24%	552
	21st to end	Night	5% 1 4 20% 50% 19%	624
August	1st to 10th	Night	4 1 2 18% 48% 26%	567
	11th to 20th	Night	8% 1 3 17% 48% 23%	564
	21st to end	Night	8% 2 5% 23% 41% 21%	627
September	1st to 10th	Night	9% 3 6% 26% 42% 14%	570
	11th to 20th	Night	11% 3 8% 27% 41% 10%	636
	21st to end	Night	10% 3 10% 31% 39% 7%	760
October	1st to 10th	Night	8% 2 19% 36% 31% 4	760
	11th to 20th	Night	10% 5% 21% 37% 25% 3	760
	21st to end	Night	14% 8% 19% 35% 22% 2	836
November	1st to 10th	Night	13% 5% 21% 38% 20% 2	780
	11th to 20th	Night	15% 8% 27% 33% 15% 1	800
	21st to end	Night	17% 9% 25% 34% 14% 1	800
December	1st to 10th	Night	18% 13% 28% 29% 12% 1	800
	11th to 20th	Night	22% 8% 31% 31% 9%	800
	21st to end	Night	26% 10% 30% 27% 7%	880

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