Grenville Base Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grenville Base Observatory clear sky chart from 2005-11-06 to 2025-03-12.

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	14% 6% 2 6% 4 4 4 4 6% 7% 42%	108856
	Night	14% 6% 2 5% 4 4 4 3 5% 7% 44%	110152
Transparency	Day	37% 7% 12% 23% 16% 4	112498
	Night	37% 7% 12% 23% 18% 4	114650
Seeing	Day	18% 13% 26% 24% 13% 5%	15333
	Night	17% 11% 30% 29% 12% 1	25037

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	15% 4 1 4 4 4 5% 4 4 5% 50%	3804
	11th to 20th	Day	14% 8% 3 6% 4 4 3 3 4 5% 48%	4060
	21st to end	Day	15% 8% 3 6% 4 4 3 3 5% 6% 44%	4466
February	1st to 10th	Day	14% 7% 2 7% 5% 5% 4 5% 6% 6% 39%	1800
	11th to 20th	Day	19% 8% 2 5% 5% 4 4 3 5% 6% 39%	1781
	21st to end	Day	18% 8% 2 6% 5% 5% 4 3 5% 5% 41%	1650
March	1st to 10th	Day	21% 7% 2 6% 5% 5% 4 3 4 7% 36%	3540
	11th to 20th	Day	10% 5% 2 5% 4 4 4 4 5% 7% 50%	3491
	21st to end	Day	20% 7% 2 6% 4 4 3 3 5% 7% 39%	3838
April	1st to 10th	Day	12% 6% 2 6% 4 4 5% 5% 6% 7% 43%	2280
	11th to 20th	Day	14% 5% 2 6% 5% 3 4 3 4 6% 48%	2254
	21st to end	Day	19% 5% 2 6% 4 4 5% 3 6% 5% 40%	2280
May	1st to 10th	Day	22% 8% 3 6% 4 5% 5% 4 6% 8% 31%	4072
	11th to 20th	Day	23% 6% 2 6% 4 4 4 3 6% 9% 31%	4206
	21st to end	Day	11% 4 2 5% 4 4 5% 4 7% 9% 44%	4823
June	1st to 10th	Day	11% 4 2 6% 5% 4 5% 5% 7% 9% 43%	2660
	11th to 20th	Day	10% 5% 3 6% 4 4 5% 5% 8% 8% 42%	2660
	21st to end	Day	8% 4 2 5% 4 4 5% 6% 7% 11% 44%	2660
July	1st to 10th	Day	6% 4 2 6% 5% 5% 6% 6% 8% 11% 43%	4440
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 5% 8% 11% 39%	4202
	21st to end	Day	11% 6% 2 7% 6% 5% 5% 5% 7% 8% 37%	4466
August	1st to 10th	Day	16% 5% 2 6% 4 5% 6% 5% 8% 10% 34%	4104
	11th to 20th	Day	9% 4 2 6% 5% 5% 6% 6% 8% 9% 41%	3916
	21st to end	Day	11% 4 2 5% 4 5% 5% 4 6% 8% 49%	4180
September	1st to 10th	Day	13% 6% 2 6% 4 4 4 4 4 7% 46%	2280
	11th to 20th	Day	12% 5% 2 5% 4 4 3 4 5% 7% 48%	2279
	21st to end	Day	21% 7% 2 5% 3 4 3 4 5% 5% 39%	1981
October	1st to 10th	Day	17% 5% 2 5% 4 4 3 3 5% 6% 46%	1900
	11th to 20th	Day	16% 4 1 4 4 3 4 3 4 5% 50%	1900
	21st to end	Day	14% 5% 2 4 4 3 3 3 5% 7% 51%	2090
November	1st to 10th	Day	12% 6% 1 4 4 3 3 3 5% 7% 51%	1894
	11th to 20th	Day	15% 6% 1 4 3 3 4 4 5% 6% 50%	1749
	21st to end	Day	15% 5% 2 4 3 4 4 4 5% 7% 47%	1600
December	1st to 10th	Day	11% 5% 2 6% 4 4 4 4 5% 6% 50%	2600
	11th to 20th	Day	18% 8% 2 5% 4 3 4 3 4 6% 42%	2990
	21st to end	Day	18% 7% 3 6% 4 4 5% 3 6% 7% 37%	3960
January	1st to 10th	Night	12% 6% 2 5% 4 3 4 3 5% 6% 51%	6137
	11th to 20th	Night	15% 6% 1 4 3 3 3 3 4 5% 52%	5900
	21st to end	Night	15% 7% 2 6% 3 3 3 2 4 5% 49%	6490
February	1st to 10th	Night	15% 7% 2 7% 4 5% 5% 4 5% 6% 41%	2600
	11th to 20th	Night	19% 7% 2 6% 4 3 4 3 5% 6% 43%	2503
	21st to end	Night	16% 8% 2 6% 4 4 4 3 5% 5% 43%	1980
March	1st to 10th	Night	20% 7% 2 6% 4 3 5% 3 5% 6% 39%	3528
	11th to 20th	Night	11% 6% 2 4 4 3 4 3 5% 7% 50%	3498
	21st to end	Night	14% 8% 2 5% 4 3 4 2 5% 5% 47%	3792
April	1st to 10th	Night	15% 8% 2 5% 5% 5% 4 3 6% 6% 41%	1900
	11th to 20th	Night	13% 6% 2 5% 4 4 4 3 5% 9% 46%	1875
	21st to end	Night	17% 4 2 4 4 4 5% 4 6% 7% 43%	1900
May	1st to 10th	Night	23% 8% 3 7% 4 4 4 4 5% 8% 30%	2851
	11th to 20th	Night	25% 8% 2 7% 5% 4 3 3 4 7% 32%	2686
	21st to end	Night	11% 4 2 5% 4 4 4 4 7% 10% 44%	2732
June	1st to 10th	Night	10% 5% 3 5% 4 5% 4 4 7% 9% 45%	1520
	11th to 20th	Night	12% 5% 3 5% 5% 4 6% 4 6% 8% 43%	1520
	21st to end	Night	8% 5% 2 5% 4 5% 5% 4 7% 10% 44%	1520
July	1st to 10th	Night	8% 5% 2 7% 5% 5% 6% 5% 7% 9% 42%	2520
	11th to 20th	Night	7% 5% 3 7% 6% 5% 6% 5% 7% 11% 39%	2740
	21st to end	Night	9% 8% 4 7% 5% 5% 5% 4 7% 9% 37%	3087
August	1st to 10th	Night	16% 5% 3 7% 5% 6% 5% 4 7% 9% 33%	2825
	11th to 20th	Night	9% 6% 3 8% 5% 7% 5% 4 6% 9% 38%	2879
	21st to end	Night	12% 5% 2 5% 3 3 4 4 5% 8% 49%	3476
September	1st to 10th	Night	14% 5% 2 5% 5% 5% 4 3 6% 5% 45%	1900
	11th to 20th	Night	11% 6% 2 5% 4 4 4 3 5% 7% 50%	1900
	21st to end	Night	21% 7% 3 6% 4 4 3 3 4 5% 40%	2194
October	1st to 10th	Night	19% 6% 2 5% 3 3 3 3 4 5% 47%	2280
	11th to 20th	Night	15% 5% 2 4 3 3 3 3 4 5% 53%	2280
	21st to end	Night	14% 5% 2 5% 3 4 4 3 4 6% 51%	2508
November	1st to 10th	Night	11% 6% 2 5% 3 2 3 3 4 5% 54%	2401
	11th to 20th	Night	14% 6% 2 3 3 3 3 3 5% 6% 50%	2649
	21st to end	Night	14% 4 1 5% 4 4 4 4 6% 6% 49%	2800
December	1st to 10th	Night	10% 5% 2 5% 3 3 3 3 5% 7% 53%	4580
	11th to 20th	Night	20% 6% 2 5% 4 4 4 3 4 7% 41%	5205
	21st to end	Night	16% 7% 2 6% 4 5% 5% 4 6% 8% 36%	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	34% 4 7% 26% 21% 8%	3805
	11th to 20th	Day	38% 5% 8% 21% 17% 11%	4060
	21st to end	Day	41% 6% 9% 27% 15% 3	4466
February	1st to 10th	Day	43% 7% 12% 24% 12% 2	1800
	11th to 20th	Day	44% 4 11% 21% 17% 2	1790
	21st to end	Day	44% 5% 12% 20% 15% 4	1650
March	1st to 10th	Day	46% 6% 11% 20% 13% 4	3540
	11th to 20th	Day	31% 7% 11% 26% 21% 4	3491
	21st to end	Day	44% 7% 11% 19% 15% 3	3838
April	1st to 10th	Day	38% 6% 12% 20% 18% 5%	2280
	11th to 20th	Day	37% 6% 11% 21% 20% 6%	2254
	21st to end	Day	42% 8% 11% 21% 14% 4	2280
May	1st to 10th	Day	49% 10% 12% 19% 10% 1	4110
	11th to 20th	Day	46% 10% 13% 21% 8% 1	4210
	21st to end	Day	31% 11% 16% 23% 17% 2	4837
June	1st to 10th	Day	34% 9% 15% 24% 16% 1	2660
	11th to 20th	Day	36% 10% 14% 24% 14% 2	2660
	21st to end	Day	30% 11% 16% 24% 16% 2	2660
July	1st to 10th	Day	32% 12% 19% 21% 11% 5%	4370
	11th to 20th	Day	35% 10% 16% 28% 10% 1	4130
	21st to end	Day	42% 9% 11% 22% 16% 1	4324
August	1st to 10th	Day	42% 9% 18% 23% 7%	4091
	11th to 20th	Day	34% 11% 13% 27% 14% 1	3955
	21st to end	Day	33% 7% 15% 27% 16% 2	4180
September	1st to 10th	Day	37% 8% 13% 17% 20% 4	2280
	11th to 20th	Day	34% 6% 9% 28% 20% 4	2279
	21st to end	Day	45% 8% 10% 19% 16% 2	1981
October	1st to 10th	Day	36% 5% 7% 17% 29% 5%	3160
	11th to 20th	Day	25% 4 7% 28% 25% 10%	3140
	21st to end	Day	22% 3 11% 30% 27% 8%	3428
November	1st to 10th	Day	32% 4 13% 25% 21% 5%	1894
	11th to 20th	Day	34% 6% 10% 22% 21% 6%	1749
	21st to end	Day	37% 6% 11% 27% 18% 2	1600
December	1st to 10th	Day	33% 6% 12% 27% 18% 5%	2600
	11th to 20th	Day	42% 4 15% 26% 10% 3	2986
	21st to end	Day	42% 8% 15% 21% 14% 1	3960
January	1st to 10th	Night	33% 5% 8% 25% 25% 4	6137
	11th to 20th	Night	33% 4 9% 24% 23% 7%	5900
	21st to end	Night	38% 4 9% 28% 18% 3	6490
February	1st to 10th	Night	42% 7% 14% 22% 14% 2	2600
	11th to 20th	Night	42% 4 10% 21% 17% 5%	2505
	21st to end	Night	42% 5% 10% 21% 16% 5%	1980
March	1st to 10th	Night	43% 7% 11% 24% 12% 3	3528
	11th to 20th	Night	30% 7% 11% 25% 23% 4	3498
	21st to end	Night	39% 5% 9% 22% 19% 5%	3792
April	1st to 10th	Night	41% 7% 9% 19% 19% 4	1900
	11th to 20th	Night	35% 7% 13% 20% 19% 6%	1875
	21st to end	Night	37% 9% 11% 24% 13% 5%	1900
May	1st to 10th	Night	50% 9% 11% 17% 10% 2	2878
	11th to 20th	Night	50% 9% 11% 19% 8% 1	2687
	21st to end	Night	32% 11% 17% 24% 12% 3	2739
June	1st to 10th	Night	33% 10% 14% 25% 17% 2	1520
	11th to 20th	Night	36% 10% 15% 23% 14% 2	1520
	21st to end	Night	33% 8% 17% 22% 18% 3	1520
July	1st to 10th	Night	36% 10% 20% 17% 13% 3	2480
	11th to 20th	Night	36% 11% 19% 22% 11% 1	2690
	21st to end	Night	41% 9% 15% 20% 15% 1	2988
August	1st to 10th	Night	45% 10% 16% 22% 8% 1	2814
	11th to 20th	Night	40% 8% 12% 24% 15% 1	2906
	21st to end	Night	32% 6% 13% 30% 17% 2	3476
September	1st to 10th	Night	39% 8% 10% 19% 19% 5%	1900
	11th to 20th	Night	34% 6% 10% 26% 22% 3	1900
	21st to end	Night	46% 6% 10% 19% 16% 3	2194
October	1st to 10th	Night	40% 5% 11% 20% 21% 4	3800
	11th to 20th	Night	23% 4 11% 22% 31% 8%	3779
	21st to end	Night	22% 4 9% 32% 26% 8%	4120
November	1st to 10th	Night	32% 4 10% 25% 22% 6%	2401
	11th to 20th	Night	35% 5% 8% 24% 22% 6%	2649
	21st to end	Night	35% 5% 12% 24% 20% 4	2800
December	1st to 10th	Night	31% 5% 12% 28% 21% 2	4580
	11th to 20th	Night	43% 5% 12% 19% 16% 4	5208
	21st to end	Night	42% 9% 15% 19% 14% 1	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	32% 25% 28% 13% 2	245
	11th to 20th	Day	32% 25% 27% 14% 1	300
	21st to end	Day	22% 32% 31% 12% 3 1	330
February	1st to 10th	Day	22% 29% 34% 12% 3	300
	11th to 20th	Day	39% 24% 30% 5% 2	328
	21st to end	Day	29% 22% 30% 17% 2	414
March	1st to 10th	Day	24% 24% 29% 20% 3	500
	11th to 20th	Day	23% 24% 30% 16% 5% 2	464
	21st to end	Day	21% 21% 33% 19% 5% 1	506
April	1st to 10th	Day	20% 26% 27% 20% 6% 1	460
	11th to 20th	Day	16% 20% 29% 20% 13% 3	452
	21st to end	Day	23% 16% 32% 18% 10% 2	460
May	1st to 10th	Day	22% 17% 28% 19% 10% 4	463
	11th to 20th	Day	21% 10% 30% 21% 14% 4	487
	21st to end	Day	19% 6% 24% 26% 19% 6%	529
June	1st to 10th	Day	11% 8% 27% 29% 19% 5%	490
	11th to 20th	Day	12% 8% 28% 30% 16% 6%	490
	21st to end	Day	10% 6% 28% 32% 18% 6%	490
July	1st to 10th	Day	12% 4 17% 31% 24% 11%	485
	11th to 20th	Day	5% 3 21% 34% 28% 9%	484
	21st to end	Day	12% 4 20% 33% 23% 8%	532
August	1st to 10th	Day	13% 4 20% 25% 27% 11%	486
	11th to 20th	Day	13% 4 19% 32% 22% 11%	479
	21st to end	Day	9% 3 19% 33% 23% 13%	539
September	1st to 10th	Day	16% 7% 20% 25% 19% 12%	490
	11th to 20th	Day	14% 5% 20% 29% 20% 12%	490
	21st to end	Day	28% 11% 20% 24% 14% 3	490
October	1st to 10th	Day	21% 7% 20% 26% 19% 7%	490
	11th to 20th	Day	17% 15% 23% 28% 14% 2	490
	21st to end	Day	16% 11% 35% 25% 11% 3	539
November	1st to 10th	Day	16% 14% 33% 26% 9% 3	462
	11th to 20th	Day	18% 15% 35% 21% 9% 1	349
	21st to end	Day	22% 23% 30% 19% 6% 1	200
December	1st to 10th	Day	18% 22% 38% 19% 4 1	200
	11th to 20th	Day	21% 24% 30% 20% 6%	200
	21st to end	Day	24% 20% 31% 17% 6% 1	220
January	1st to 10th	Night	21% 20% 38% 18% 3	800
	11th to 20th	Night	22% 16% 34% 24% 4	800
	21st to end	Night	18% 17% 43% 20% 1	880
February	1st to 10th	Night	19% 16% 39% 23% 2	800
	11th to 20th	Night	25% 21% 36% 16% 2	783
	21st to end	Night	25% 17% 38% 17% 3	660
March	1st to 10th	Night	19% 20% 37% 21% 3	800
	11th to 20th	Night	23% 19% 32% 21% 5% 1	767
	21st to end	Night	19% 17% 39% 20% 4	836
April	1st to 10th	Night	18% 20% 36% 22% 4	760
	11th to 20th	Night	16% 13% 35% 27% 8% 1	751
	21st to end	Night	19% 11% 33% 29% 8%	760
May	1st to 10th	Night	22% 8% 26% 32% 12% 1	595
	11th to 20th	Night	20% 6% 27% 34% 12% 1	487
	21st to end	Night	15% 3 22% 39% 20% 1	411
June	1st to 10th	Night	9% 3 26% 41% 21%	380
	11th to 20th	Night	12% 4 23% 41% 19% 1	380
	21st to end	Night	9% 2 23% 49% 17% 1	380
July	1st to 10th	Night	12% 3 18% 38% 26% 4	370
	11th to 20th	Night	6% 1 14% 40% 35% 3	495
	21st to end	Night	9% 3 17% 40% 28% 3	624
August	1st to 10th	Night	11% 2 12% 41% 30% 3	567
	11th to 20th	Night	10% 3 15% 41% 29% 3	619
	21st to end	Night	11% 3 16% 41% 25% 4	836
September	1st to 10th	Night	17% 2 18% 32% 27% 4	760
	11th to 20th	Night	12% 4 18% 35% 28% 3	760
	21st to end	Night	24% 5% 21% 30% 19% 1	760
October	1st to 10th	Night	21% 5% 23% 29% 19% 4	760
	11th to 20th	Night	16% 11% 26% 33% 14% 1	760
	21st to end	Night	16% 12% 32% 28% 12%	836
November	1st to 10th	Night	14% 10% 34% 30% 11% 1	780
	11th to 20th	Night	17% 13% 34% 28% 8% 1	800
	21st to end	Night	17% 15% 33% 28% 8% 1	800
December	1st to 10th	Night	20% 16% 35% 22% 7%	800
	11th to 20th	Night	19% 12% 40% 25% 4	800
	21st to end	Night	20% 14% 33% 30% 3	880

Page generated at 2025-03-09 08:08:48. Version ac07

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