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 2024-10-20.

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%	100477
	Night	12% 6% 2 6% 4 4 4 4 6% 8% 44%	98117
Transparency	Day	42% 6% 12% 18% 17% 6%	100296
	Night	38% 6% 12% 19% 18% 7%	97968
Seeing	Day	14% 9% 20% 25% 22% 11%	14670
	Night	15% 7% 21% 30% 22% 6%	24221

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% 6% 2 5% 3 4 4 3 6% 6% 43%	2895
	11th to 20th	Day	12% 6% 2 5% 3 3 3 3 4 5% 55%	2976
	21st to end	Day	24% 9% 3 6% 3 2 3 3 4 5% 39%	3476
February	1st to 10th	Day	18% 8% 3 6% 4 5% 4 3 5% 6% 38%	1900
	11th to 20th	Day	19% 6% 2 5% 4 3 3 3 5% 7% 42%	1848
	21st to end	Day	17% 8% 3 7% 5% 5% 4 3 5% 6% 38%	1570
March	1st to 10th	Day	22% 10% 3 8% 5% 4 4 4 5% 6% 29%	3174
	11th to 20th	Day	16% 8% 3 7% 5% 5% 4 3 6% 8% 35%	3480
	21st to end	Day	11% 7% 3 7% 5% 6% 5% 4 5% 6% 40%	4059
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%	1890
	21st to end	Day	10% 5% 1 5% 4 3 4 4 3 5% 55%	1980
November	1st to 10th	Day	10% 5% 2 5% 4 3 3 3 5% 7% 53%	1858
	11th to 20th	Day	11% 5% 2 4 3 3 4 3 5% 6% 54%	1715
	21st to end	Day	15% 6% 3 6% 4 4 4 3 4 6% 45%	1710
December	1st to 10th	Day	14% 7% 3 6% 4 3 5% 4 5% 6% 44%	1710
	11th to 20th	Day	16% 10% 3 7% 4 3 3 3 5% 5% 41%	2153
	21st to end	Day	17% 8% 2 6% 4 4 4 4 6% 7% 39%	3325
January	1st to 10th	Night	18% 7% 2 4 3 3 3 3 4 6% 47%	4029
	11th to 20th	Night	16% 8% 2 4 2 2 3 2 4 5% 52%	3963
	21st to end	Night	27% 9% 2 4 2 3 3 2 3 5% 41%	4180
February	1st to 10th	Night	18% 8% 3 6% 4 3 4 3 6% 7% 40%	2280
	11th to 20th	Night	20% 9% 3 4 4 4 3 2 4 5% 42%	2218
	21st to end	Night	20% 7% 3 7% 5% 3 4 3 4 6% 38%	1884
March	1st to 10th	Night	23% 12% 3 7% 4 3 4 3 5% 6% 30%	3706
	11th to 20th	Night	14% 8% 2 6% 6% 5% 5% 4 6% 6% 38%	3480
	21st to end	Night	15% 7% 2 5% 4 4 4 4 5% 6% 44%	3572
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%	2275
	21st to end	Night	13% 5% 2 5% 3 3 3 2 4 5% 56%	2376
November	1st to 10th	Night	10% 4 2 5% 3 3 3 3 4 5% 59%	2220
	11th to 20th	Night	12% 5% 2 4 2 3 3 3 5% 6% 54%	2460
	21st to end	Night	14% 7% 2 5% 3 3 3 3 6% 6% 47%	2470
December	1st to 10th	Night	15% 7% 3 5% 3 3 3 3 6% 6% 45%	2470
	11th to 20th	Night	17% 8% 3 6% 4 4 4 3 5% 5% 42%	3042
	21st to end	Night	18% 9% 3 6% 4 3 3 3 4 5% 40%	4326

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% 4 5% 18% 24% 10%	2896
	11th to 20th	Day	32% 4 4 18% 31% 10%	2976
	21st to end	Day	49% 2 4 14% 21% 9%	3476
February	1st to 10th	Day	48% 4 7% 13% 18% 10%	1900
	11th to 20th	Day	42% 5% 6% 15% 22% 10%	1858
	21st to end	Day	48% 4 6% 16% 17% 9%	1570
March	1st to 10th	Day	55% 4 7% 16% 13% 5%	3174
	11th to 20th	Day	46% 4 8% 18% 18% 5%	3480
	21st to end	Day	44% 5% 9% 14% 19% 8%	4059
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	30% 5% 9% 18% 23% 15%	2060
	11th to 20th	Day	29% 5% 6% 16% 29% 15%	1890
	21st to end	Day	32% 4 4 17% 27% 16%	1980
November	1st to 10th	Day	33% 3 6% 19% 26% 13%	1858
	11th to 20th	Day	32% 4 6% 16% 29% 13%	1715
	21st to end	Day	40% 4 6% 18% 22% 11%	1710
December	1st to 10th	Day	41% 4 8% 20% 20% 7%	1710
	11th to 20th	Day	46% 3 7% 12% 24% 8%	2150
	21st to end	Day	43% 5% 8% 19% 21% 4	3325
January	1st to 10th	Night	40% 3 6% 15% 28% 9%	4029
	11th to 20th	Night	36% 3 4 20% 25% 11%	3963
	21st to end	Night	48% 2 4 15% 24% 7%	4180
February	1st to 10th	Night	44% 4 7% 16% 20% 9%	2280
	11th to 20th	Night	47% 3 5% 13% 21% 12%	2219
	21st to end	Night	48% 4 8% 15% 13% 12%	1884
March	1st to 10th	Night	55% 4 9% 13% 13% 6%	3706
	11th to 20th	Night	45% 6% 9% 15% 19% 6%	3480
	21st to end	Night	40% 4 10% 20% 19% 6%	3572
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	27% 6% 10% 18% 23% 17%	2115
	11th to 20th	Night	25% 5% 7% 21% 26% 17%	2275
	21st to end	Night	32% 4 4 16% 26% 18%	2376
November	1st to 10th	Night	29% 3 7% 19% 25% 16%	2220
	11th to 20th	Night	31% 4 7% 16% 28% 15%	2460
	21st to end	Night	37% 3 7% 20% 20% 13%	2470
December	1st to 10th	Night	39% 4 8% 22% 20% 7%	2470
	11th to 20th	Night	44% 4 6% 12% 22% 12%	3044
	21st to end	Night	46% 4 6% 18% 20% 6%	4326

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% 15% 36% 17% 5% 1	270
	11th to 20th	Day	24% 16% 34% 18% 7% 1	270
	21st to end	Day	26% 25% 34% 13% 2	297
February	1st to 10th	Day	33% 17% 27% 17% 5% 1	270
	11th to 20th	Day	30% 18% 33% 16% 4	270
	21st to end	Day	23% 26% 38% 12% 1	225
March	1st to 10th	Day	22% 19% 39% 16% 4	270
	11th to 20th	Day	27% 20% 31% 18% 4 1	270
	21st to end	Day	20% 17% 32% 22% 8% 1	439
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% 25% 34% 23% 5%	297
	21st to end	Day	19% 11% 19% 30% 18% 3	297
November	1st to 10th	Day	16% 7% 25% 36% 14% 3	270
	11th to 20th	Day	18% 12% 30% 26% 13% 1	270
	21st to end	Day	16% 16% 34% 21% 10% 3	270
December	1st to 10th	Day	21% 12% 34% 24% 8% 1	270
	11th to 20th	Day	26% 16% 34% 18% 5% 1	270
	21st to end	Day	27% 14% 29% 23% 6% 1	297
January	1st to 10th	Night	23% 12% 31% 28% 6%	910
	11th to 20th	Night	21% 12% 31% 30% 7%	858
	21st to end	Night	27% 12% 31% 27% 4	836
February	1st to 10th	Night	22% 13% 33% 27% 5%	760
	11th to 20th	Night	26% 15% 27% 26% 5%	743
	21st to end	Night	24% 18% 35% 21% 4	628
March	1st to 10th	Night	20% 13% 32% 28% 6%	746
	11th to 20th	Night	18% 9% 33% 32% 8%	570
	21st to end	Night	17% 12% 32% 30% 10%	627
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	759
	21st to end	Night	15% 8% 20% 34% 22% 2	792
November	1st to 10th	Night	12% 6% 19% 39% 21% 2	740
	11th to 20th	Night	14% 8% 27% 33% 16% 1	760
	21st to end	Night	17% 8% 25% 34% 14% 1	760
December	1st to 10th	Night	18% 13% 27% 29% 13% 1	760
	11th to 20th	Night	22% 8% 31% 30% 9%	760
	21st to end	Night	26% 11% 31% 26% 7%	836

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