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 2024-02-15.

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 6% 6% 4 5% 5% 6% 7% 40%	79019
	Night	13% 6% 2 5% 4 3 4 4 5% 8% 46%	79996
Transparency	Day	42% 6% 10% 18% 18% 7%	78755
	Night	37% 6% 12% 19% 19% 8%	79780
Seeing	Day	14% 9% 19% 25% 22% 11%	13695
	Night	14% 7% 21% 30% 22% 6%	23420

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 4 4 3 4 4 6% 6% 44%	2895
	11th to 20th	Day	12% 5% 2 4 3 2 3 3 4 5% 55%	2976
	21st to end	Day	24% 9% 3 5% 4 2 3 3 4 5% 39%	3476
February	1st to 10th	Day	18% 8% 4 5% 4 3 5% 4 5% 6% 38%	1900
	11th to 20th	Day	19% 6% 2 5% 5% 3 3 4 5% 7% 42%	1788
	21st to end	Day	18% 8% 3 6% 5% 4 5% 4 4 6% 37%	1480
March	1st to 10th	Day	16% 6% 3 5% 5% 3 4 4 5% 6% 42%	1826
	11th to 20th	Day	15% 8% 3 6% 5% 3 5% 4 5% 8% 38%	1980
	21st to end	Day	11% 6% 3 5% 5% 4 5% 4 5% 8% 44%	2312
April	1st to 10th	Day	12% 6% 2 6% 6% 3 5% 5% 5% 8% 43%	2160
	11th to 20th	Day	15% 6% 3 6% 5% 2 5% 5% 5% 6% 41%	2134
	21st to end	Day	9% 4 2 5% 6% 3 5% 5% 5% 8% 48%	2160
May	1st to 10th	Day	11% 5% 2 5% 5% 3 5% 6% 6% 8% 44%	2160
	11th to 20th	Day	9% 6% 3 6% 7% 4 7% 4 6% 8% 40%	2124
	21st to end	Day	9% 4 3 6% 7% 5% 7% 6% 7% 10% 35%	2404
June	1st to 10th	Day	10% 6% 3 6% 7% 5% 8% 9% 7% 9% 30%	2340
	11th to 20th	Day	5% 4 3 7% 8% 5% 7% 7% 8% 11% 35%	2442
	21st to end	Day	7% 6% 3 7% 9% 5% 9% 8% 9% 10% 26%	2432
July	1st to 10th	Day	8% 7% 4 8% 9% 7% 9% 9% 7% 8% 23%	2340
	11th to 20th	Day	9% 6% 3 7% 8% 6% 8% 8% 9% 9% 26%	2268
	21st to end	Day	10% 6% 3 7% 8% 5% 8% 6% 7% 10% 30%	2350
August	1st to 10th	Day	5% 6% 2 8% 8% 5% 8% 8% 8% 10% 33%	2148
	11th to 20th	Day	12% 7% 3 8% 9% 5% 7% 7% 7% 8% 29%	2125
	21st to end	Day	11% 5% 2 7% 8% 5% 8% 7% 7% 8% 31%	2376
September	1st to 10th	Day	9% 7% 3 8% 6% 5% 7% 6% 6% 9% 35%	2160
	11th to 20th	Day	11% 7% 3 6% 6% 4 5% 5% 6% 7% 41%	2080
	21st to end	Day	10% 5% 2 4 5% 3 5% 5% 5% 8% 49%	1980
October	1st to 10th	Day	5% 5% 2 5% 5% 4 4 5% 5% 6% 55%	1952
	11th to 20th	Day	9% 4 2 5% 5% 2 4 5% 5% 7% 53%	1800
	21st to end	Day	10% 4 2 5% 5% 2 4 4 3 5% 55%	1980
November	1st to 10th	Day	11% 4 2 4 5% 2 4 3 4 7% 53%	1858
	11th to 20th	Day	11% 5% 3 3 3 2 4 3 5% 6% 55%	1715
	21st to end	Day	15% 6% 3 5% 5% 3 4 4 4 6% 46%	1710
December	1st to 10th	Day	14% 7% 3 6% 4 2 5% 4 5% 6% 44%	1710
	11th to 20th	Day	16% 9% 4 6% 5% 2 4 3 4 5% 41%	2153
	21st to end	Day	17% 7% 2 6% 5% 3 4 5% 5% 7% 39%	3325
January	1st to 10th	Night	18% 7% 2 4 3 3 3 3 4 6% 47%	4029
	11th to 20th	Night	17% 7% 2 3 3 2 3 3 4 5% 52%	3963
	21st to end	Night	27% 8% 2 4 3 2 3 2 3 5% 41%	4180
February	1st to 10th	Night	18% 7% 3 5% 4 2 4 4 5% 7% 40%	2280
	11th to 20th	Night	21% 8% 3 4 4 3 3 3 4 5% 42%	2153
	21st to end	Night	21% 7% 3 6% 6% 2 4 4 4 6% 36%	1776
March	1st to 10th	Night	15% 6% 3 5% 5% 3 4 4 5% 7% 44%	2147
	11th to 20th	Night	14% 7% 3 6% 6% 4 5% 4 5% 6% 40%	1980
	21st to end	Night	13% 6% 3 3 4 2 4 4 4 6% 52%	2044
April	1st to 10th	Night	13% 6% 2 6% 4 3 4 4 5% 7% 46%	1800
	11th to 20th	Night	14% 5% 3 4 6% 3 4 4 5% 6% 45%	1775
	21st to end	Night	7% 3 2 4 4 2 4 5% 5% 7% 58%	1800
May	1st to 10th	Night	9% 4 2 3 4 2 3 4 4 9% 55%	1800
	11th to 20th	Night	7% 4 2 5% 5% 3 5% 5% 7% 9% 47%	1769
	21st to end	Night	5% 3 2 5% 6% 4 4 5% 7% 11% 47%	1864
June	1st to 10th	Night	7% 5% 3 5% 5% 3 6% 7% 6% 11% 40%	1620
	11th to 20th	Night	3 3 2 4 4 4 6% 7% 8% 12% 47%	1509
	21st to end	Night	7% 5% 2 5% 5% 3 6% 6% 8% 11% 41%	1518
July	1st to 10th	Night	5% 5% 2 6% 6% 3 7% 7% 8% 14% 37%	1620
	11th to 20th	Night	4 4 2 6% 5% 4 6% 7% 10% 14% 37%	1686
	21st to end	Night	4 3 2 6% 6% 4 6% 7% 9% 12% 39%	1968
August	1st to 10th	Night	4 3 2 4 5% 3 5% 7% 9% 14% 45%	1790
	11th to 20th	Night	6% 6% 2 5% 6% 5% 7% 7% 8% 14% 34%	1776
	21st to end	Night	8% 4 2 5% 7% 4 5% 6% 8% 11% 38%	1980
September	1st to 10th	Night	8% 5% 3 6% 6% 4 5% 6% 8% 10% 40%	1800
	11th to 20th	Night	8% 5% 3 5% 5% 3 6% 5% 6% 8% 45%	1862
	21st to end	Night	9% 4 2 5% 6% 3 5% 4 5% 7% 51%	1980
October	1st to 10th	Night	6% 3 2 4 5% 2 4 4 5% 8% 57%	2003
	11th to 20th	Night	8% 4 2 4 5% 2 3 3 4 7% 58%	2160
	21st to end	Night	13% 5% 2 5% 3 2 3 3 3 5% 56%	2376
November	1st to 10th	Night	10% 4 2 4 4 2 4 3 4 5% 59%	2220
	11th to 20th	Night	12% 5% 2 3 3 2 3 4 5% 6% 55%	2460
	21st to end	Night	14% 7% 2 4 4 2 4 4 5% 7% 47%	2470
December	1st to 10th	Night	16% 7% 3 5% 4 2 3 4 5% 6% 46%	2470
	11th to 20th	Night	17% 8% 3 5% 4 3 4 4 5% 5% 42%	3042
	21st to end	Night	19% 9% 3 5% 5% 2 3 4 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	43% 5% 6% 16% 21% 10%	1798
	21st to end	Day	49% 4 7% 15% 17% 9%	1480
March	1st to 10th	Day	43% 4 6% 17% 21% 9%	1826
	11th to 20th	Day	46% 4 9% 14% 19% 8%	1980
	21st to end	Day	39% 5% 10% 18% 22% 7%	2312
April	1st to 10th	Day	41% 4 10% 18% 19% 8%	2160
	11th to 20th	Day	43% 4 8% 16% 21% 8%	2134
	21st to end	Day	35% 5% 9% 23% 24% 5%	2160
May	1st to 10th	Day	36% 5% 10% 21% 22% 6%	2160
	11th to 20th	Day	41% 6% 11% 23% 18% 1	2128
	21st to end	Day	42% 8% 13% 21% 15% 2	2404
June	1st to 10th	Day	47% 8% 13% 22% 9% 2	2340
	11th to 20th	Day	40% 7% 17% 22% 12% 2	2442
	21st to end	Day	48% 10% 16% 19% 6% 1	2432
July	1st to 10th	Day	54% 11% 17% 13% 4 1	2276
	11th to 20th	Day	48% 13% 18% 16% 4 1	2199
	21st to end	Day	45% 9% 20% 19% 6% 1	2219
August	1st to 10th	Day	43% 10% 21% 17% 8%	2136
	11th to 20th	Day	52% 9% 16% 15% 7% 1	2125
	21st to end	Day	47% 9% 16% 15% 10% 2	2376
September	1st to 10th	Day	45% 8% 13% 17% 13% 5%	2160
	11th to 20th	Day	42% 6% 11% 16% 16% 8%	2080
	21st to end	Day	35% 5% 10% 22% 19% 9%	1980
October	1st to 10th	Day	30% 5% 9% 19% 23% 13%	1952
	11th to 20th	Day	31% 5% 6% 16% 29% 13%	1800
	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	46% 3 5% 13% 21% 11%	2154
	21st to end	Night	49% 4 8% 14% 13% 12%	1776
March	1st to 10th	Night	40% 4 9% 15% 22% 10%	2147
	11th to 20th	Night	44% 6% 10% 15% 19% 7%	1980
	21st to end	Night	34% 4 11% 20% 23% 7%	2044
April	1st to 10th	Night	38% 5% 11% 17% 19% 11%	1800
	11th to 20th	Night	40% 4 9% 21% 18% 9%	1775
	21st to end	Night	26% 5% 12% 23% 27% 8%	1800
May	1st to 10th	Night	28% 4 10% 28% 24% 6%	1800
	11th to 20th	Night	31% 7% 18% 23% 18% 3	1770
	21st to end	Night	29% 8% 19% 24% 17% 3	1865
June	1st to 10th	Night	36% 8% 18% 26% 11% 1	1620
	11th to 20th	Night	26% 11% 20% 29% 11% 2	1509
	21st to end	Night	35% 9% 20% 25% 9% 2	1518
July	1st to 10th	Night	35% 14% 25% 20% 6% 1	1575
	11th to 20th	Night	31% 15% 31% 18% 4 1	1632
	21st to end	Night	33% 13% 27% 22% 5% 1	1858
August	1st to 10th	Night	24% 13% 32% 22% 8%	1778
	11th to 20th	Night	35% 16% 23% 17% 6% 2	1776
	21st to end	Night	37% 12% 22% 16% 9% 4	1980
September	1st to 10th	Night	37% 9% 19% 15% 15% 6%	1800
	11th to 20th	Night	35% 7% 18% 17% 14% 9%	1862
	21st to end	Night	33% 6% 11% 24% 15% 11%	1980
October	1st to 10th	Night	27% 6% 10% 19% 22% 16%	2003
	11th to 20th	Night	27% 5% 7% 21% 26% 15%	2160
	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% 15% 4	252
	21st to end	Day	22% 25% 38% 13% 2	198
March	1st to 10th	Day	20% 19% 41% 17% 3	240
	11th to 20th	Day	27% 22% 31% 16% 4 1	240
	21st to end	Day	20% 16% 31% 23% 9% 1	398
April	1st to 10th	Day	15% 16% 33% 25% 11% 1	420
	11th to 20th	Day	23% 14% 28% 22% 12% 2	412
	21st to end	Day	9% 12% 30% 33% 15% 2	420
May	1st to 10th	Day	13% 12% 26% 26% 19% 4	423
	11th to 20th	Day	13% 4 21% 33% 19% 10%	447
	21st to end	Day	7% 3 17% 35% 26% 12%	485
June	1st to 10th	Day	13% 3 12% 32% 24% 16%	450
	11th to 20th	Day	6% 2 11% 36% 31% 15%	552
	21st to end	Day	10% 3 11% 30% 30% 17%	630
July	1st to 10th	Day	6% 4 6% 27% 37% 19%	621
	11th to 20th	Day	9% 2 2 21% 40% 26%	546
	21st to end	Day	11% 3 5% 19% 39% 24%	488
August	1st to 10th	Day	5% 1 6% 22% 34% 32%	446
	11th to 20th	Day	10% 2 4 16% 38% 31%	439
	21st to end	Day	12% 3 9% 19% 34% 24%	495
September	1st to 10th	Day	9% 2 7% 27% 36% 20%	450
	11th to 20th	Day	10% 4 11% 26% 35% 14%	450
	21st to end	Day	13% 2 9% 29% 35% 11%	450
October	1st to 10th	Day	5% 6% 20% 34% 26% 8%	422
	11th to 20th	Day	7% 7% 23% 33% 24% 6%	270
	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% 16% 27% 27% 5%	722
	21st to end	Night	24% 17% 34% 21% 4	592
March	1st to 10th	Night	20% 13% 32% 28% 7%	707
	11th to 20th	Night	18% 9% 33% 31% 8%	540
	21st to end	Night	17% 11% 32% 30% 10%	594
April	1st to 10th	Night	16% 12% 29% 32% 11% 1	540
	11th to 20th	Night	17% 8% 32% 29% 13% 1	533
	21st to end	Night	9% 5% 29% 42% 14% 1	540
May	1st to 10th	Night	11% 6% 24% 40% 18% 1	540
	11th to 20th	Night	9% 3 20% 40% 23% 4	531
	21st to end	Night	6% 1 14% 41% 32% 6%	584
June	1st to 10th	Night	9% 1 10% 36% 36% 8%	540
	11th to 20th	Night	3 2 8% 31% 45% 11%	540
	21st to end	Night	10% 2 6% 27% 40% 15%	540
July	1st to 10th	Night	6% 1 2 22% 50% 19%	525
	11th to 20th	Night	5% 1 3 17% 50% 25%	522
	21st to end	Night	4 1 4 20% 50% 20%	591
August	1st to 10th	Night	4 1 2 18% 47% 28%	537
	11th to 20th	Night	8% 1 3 17% 48% 23%	534
	21st to end	Night	8% 2 5% 22% 41% 22%	594
September	1st to 10th	Night	8% 3 5% 26% 43% 15%	540
	11th to 20th	Night	9% 3 8% 27% 42% 11%	602
	21st to end	Night	10% 2 9% 31% 40% 7%	720
October	1st to 10th	Night	8% 2 18% 36% 33% 4	720
	11th to 20th	Night	10% 4 20% 37% 25% 3	720
	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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