Churchill Northern Studies Centre
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Churchill Northern Studies Centre clear sky chart from 2005-11-06 to 2025-08-05.

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% 11% 4 11% 6% 6% 6% 5% 7% 8% 24%	130423
	Night	9% 12% 5% 13% 8% 7% 8% 6% 8% 7% 17%	126312
Transparency	Day	55% 7% 12% 20% 6%	130086
	Night	61% 7% 11% 15% 5%	126228
Seeing	Day	18% 5% 21% 30% 19% 7%	16581
	Night	16% 5% 22% 33% 19% 4	25098

By Month

Percentage Cloud Cover NW (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	5% 9% 5% 18% 12% 8% 7% 6% 11% 12% 7%	2260
	11th to 20th	Day	2 9% 6% 16% 9% 10% 11% 6% 14% 9% 8%	2547
	21st to end	Day	5% 12% 4 10% 6% 6% 10% 8% 16% 12% 11%	3498
February	1st to 10th	Day	2 7% 3 11% 9% 10% 11% 10% 13% 12% 12%	1400
	11th to 20th	Day	4 8% 3 9% 7% 10% 12% 8% 16% 15% 10%	1668
	21st to end	Day	3 10% 4 11% 7% 7% 9% 7% 19% 12% 12%	1485
March	1st to 10th	Day	3 9% 3 10% 9% 9% 10% 9% 9% 9% 19%	4315
	11th to 20th	Day	5% 15% 5% 13% 9% 10% 11% 8% 8% 6% 10%	4978
	21st to end	Day	3 7% 3 12% 7% 9% 11% 10% 9% 9% 20%	5665
April	1st to 10th	Day	8% 13% 4 10% 8% 9% 11% 7% 7% 6% 16%	2509
	11th to 20th	Day	7% 11% 3 11% 9% 8% 9% 5% 5% 5% 26%	2630
	21st to end	Day	6% 9% 4 12% 12% 10% 7% 5% 6% 8% 20%	2800
May	1st to 10th	Day	17% 17% 5% 14% 7% 5% 4 3 4 5% 17%	6773
	11th to 20th	Day	15% 13% 6% 13% 6% 5% 5% 4 5% 7% 23%	7198
	21st to end	Day	7% 7% 4 10% 5% 6% 6% 5% 8% 9% 34%	8005
June	1st to 10th	Day	20% 14% 4 7% 4 5% 4 3 5% 7% 27%	3400
	11th to 20th	Day	13% 8% 2 6% 5% 4 6% 4 7% 7% 38%	3400
	21st to end	Day	15% 7% 3 7% 4 4 4 4 5% 8% 40%	3400
July	1st to 10th	Day	11% 7% 3 8% 5% 5% 6% 5% 7% 9% 36%	7700
	11th to 20th	Day	14% 6% 2 6% 5% 5% 5% 4 6% 10% 38%	7617
	21st to end	Day	11% 8% 3 7% 5% 5% 5% 5% 7% 8% 36%	7688
August	1st to 10th	Day	17% 5% 2 5% 3 4 4 3 5% 8% 44%	4792
	11th to 20th	Day	18% 10% 2 5% 4 4 4 3 5% 7% 39%	4480
	21st to end	Day	19% 8% 2 7% 4 4 5% 4 5% 8% 32%	4631
September	1st to 10th	Day	24% 12% 3 9% 5% 4 5% 4 5% 7% 22%	2378
	11th to 20th	Day	22% 19% 3 9% 5% 5% 4 3 4 6% 19%	2268
	21st to end	Day	20% 22% 4 8% 5% 5% 4 3 4 5% 21%	1991
October	1st to 10th	Day	17% 23% 7% 11% 5% 4 4 3 4 5% 17%	3160
	11th to 20th	Day	21% 22% 8% 12% 5% 4 4 3 4 4 13%	2860
	21st to end	Day	17% 17% 11% 29% 6% 4 3 2 3 2 7%	2744
November	1st to 10th	Day	13% 21% 8% 22% 10% 7% 5% 3 4 3 5%	1527
	11th to 20th	Day	12% 17% 6% 14% 8% 11% 8% 6% 5% 4 10%	1200
	21st to end	Day	8% 18% 8% 16% 11% 8% 7% 6% 5% 6% 9%	1200
December	1st to 10th	Day	3 13% 6% 21% 11% 10% 9% 5% 10% 5% 7%	1894
	11th to 20th	Day	5% 11% 6% 17% 11% 8% 6% 7% 12% 6% 9%	1876
	21st to end	Day	9% 18% 7% 14% 8% 9% 8% 6% 7% 5% 9%	2486
January	1st to 10th	Night	4 9% 4 17% 12% 10% 9% 6% 10% 9% 9%	7686
	11th to 20th	Night	2 10% 5% 18% 12% 11% 10% 6% 11% 8% 7%	7345
	21st to end	Night	5% 11% 4 10% 7% 8% 10% 9% 16% 9% 11%	7480
February	1st to 10th	Night	2 8% 4 11% 9% 10% 11% 9% 15% 10% 9%	3000
	11th to 20th	Night	3 7% 5% 10% 7% 10% 9% 10% 17% 11% 11%	2617
	21st to end	Night	3 8% 4 11% 8% 8% 10% 11% 15% 10% 12%	2145
March	1st to 10th	Night	3 7% 3 12% 8% 8% 11% 12% 12% 8% 17%	5593
	11th to 20th	Night	5% 13% 4 13% 12% 10% 11% 6% 8% 5% 13%	4980
	21st to end	Night	3 6% 3 10% 8% 9% 10% 14% 11% 8% 18%	5266
April	1st to 10th	Night	9% 11% 3 9% 8% 7% 8% 7% 9% 7% 22%	1890
	11th to 20th	Night	7% 9% 3 11% 9% 8% 9% 5% 6% 6% 28%	1720
	21st to end	Night	8% 10% 4 10% 10% 8% 5% 4 5% 6% 30%	1600
May	1st to 10th	Night	18% 15% 6% 15% 8% 5% 5% 4 4 5% 16%	3192
	11th to 20th	Night	14% 14% 6% 12% 5% 5% 5% 3 5% 6% 25%	2717
	21st to end	Night	7% 9% 4 11% 6% 5% 5% 4 5% 7% 36%	2886
June	1st to 10th	Night	19% 15% 5% 8% 7% 3 4 3 6% 7% 25%	1000
	11th to 20th	Night	14% 9% 3 7% 5% 6% 4 5% 6% 6% 35%	1000
	21st to end	Night	14% 10% 3 7% 7% 5% 5% 4 4 7% 34%	1000
July	1st to 10th	Night	14% 8% 3 7% 6% 5% 6% 6% 7% 10% 30%	2260
	11th to 20th	Night	13% 6% 3 6% 4 5% 6% 5% 7% 10% 35%	2346
	21st to end	Night	14% 9% 3 8% 4 5% 5% 4 7% 7% 35%	3163
August	1st to 10th	Night	15% 7% 2 6% 5% 5% 5% 5% 5% 7% 39%	2237
	11th to 20th	Night	13% 8% 3 7% 4 5% 4 3 5% 8% 38%	2358
	21st to end	Night	17% 7% 2 6% 4 5% 5% 5% 6% 9% 35%	3034
September	1st to 10th	Night	23% 12% 4 8% 5% 6% 5% 4 6% 6% 21%	1801
	11th to 20th	Night	20% 16% 3 9% 6% 5% 4 3 4 6% 24%	1909
	21st to end	Night	20% 19% 3 10% 5% 3 4 3 4 5% 24%	2169
October	1st to 10th	Night	18% 20% 6% 12% 6% 4 5% 3 4 5% 17%	3800
	11th to 20th	Night	18% 20% 8% 10% 4 4 4 3 4 5% 20%	4060
	21st to end	Night	17% 14% 8% 23% 7% 5% 4 3 4 4 10%	4814
November	1st to 10th	Night	14% 22% 9% 19% 10% 7% 5% 3 3 3 5%	2768
	11th to 20th	Night	11% 17% 6% 13% 11% 9% 8% 5% 4 4 11%	3200
	21st to end	Night	8% 17% 7% 16% 9% 6% 8% 6% 7% 5% 10%	3200
December	1st to 10th	Night	3 10% 6% 22% 13% 9% 9% 8% 9% 5% 5%	5290
	11th to 20th	Night	4 11% 7% 16% 8% 9% 10% 9% 13% 5% 7%	6316
	21st to end	Night	10% 15% 5% 15% 9% 10% 9% 5% 7% 5% 11%	8470

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	64% 5% 11% 12% 7%	2260
	11th to 20th	Day	63% 7% 13% 16% 2	2564
	21st to end	Day	53% 6% 8% 23% 9%	3498
February	1st to 10th	Day	54% 7% 13% 21% 5%	1400
	11th to 20th	Day	51% 9% 15% 20% 6%	1677
	21st to end	Day	50% 7% 11% 20% 12%	1485
March	1st to 10th	Day	56% 5% 11% 17% 9% 3	4315
	11th to 20th	Day	70% 4 6% 10% 10%	4980
	21st to end	Day	54% 5% 8% 22% 12%	5665
April	1st to 10th	Day	63% 8% 8% 15% 5%	2509
	11th to 20th	Day	58% 7% 9% 20% 6%	2630
	21st to end	Day	59% 8% 13% 15% 5%	2800
May	1st to 10th	Day	68% 7% 9% 11% 5%	6770
	11th to 20th	Day	58% 7% 9% 18% 8%	7202
	21st to end	Day	41% 8% 11% 21% 17% 1	8005
June	1st to 10th	Day	56% 7% 12% 19% 5%	3400
	11th to 20th	Day	43% 8% 12% 30% 7%	3400
	21st to end	Day	41% 8% 13% 30% 8%	3400
July	1st to 10th	Day	43% 6% 15% 29% 7%	7616
	11th to 20th	Day	42% 6% 18% 31% 2	7524
	21st to end	Day	43% 8% 17% 29% 3	7472
August	1st to 10th	Day	38% 7% 21% 29% 5%	4775
	11th to 20th	Day	46% 6% 15% 22% 12%	4525
	21st to end	Day	49% 8% 19% 22% 2	4630
September	1st to 10th	Day	59% 10% 15% 14% 1	2378
	11th to 20th	Day	67% 7% 14% 11% 1	2268
	21st to end	Day	66% 5% 13% 14% 2	1991
October	1st to 10th	Day	71% 5% 7% 15% 2	3160
	11th to 20th	Day	74% 5% 7% 13% 1	2860
	21st to end	Day	85% 4 5% 5%	2744
November	1st to 10th	Day	83% 8% 6% 3	1527
	11th to 20th	Day	72% 10% 12% 6% 1	1200
	21st to end	Day	72% 9% 12% 7% 1	1200
December	1st to 10th	Day	72% 6% 12% 9% 2	1894
	11th to 20th	Day	66% 5% 11% 16% 1	1876
	21st to end	Day	73% 5% 9% 13%	2486
January	1st to 10th	Night	67% 5% 8% 11% 9%	7686
	11th to 20th	Night	68% 6% 12% 13% 1	7390
	21st to end	Night	55% 5% 8% 23% 8%	7480
February	1st to 10th	Night	55% 8% 14% 18% 6%	3000
	11th to 20th	Night	51% 8% 16% 19% 6%	2618
	21st to end	Night	52% 8% 12% 17% 12%	2145
March	1st to 10th	Night	52% 8% 14% 15% 8% 3	5593
	11th to 20th	Night	69% 4 7% 11% 9%	4980
	21st to end	Night	53% 6% 9% 17% 14%	5266
April	1st to 10th	Night	56% 7% 10% 20% 7% 1	1890
	11th to 20th	Night	53% 7% 10% 20% 8% 1	1720
	21st to end	Night	53% 7% 10% 21% 9%	1600
May	1st to 10th	Night	70% 6% 5% 14% 5%	3192
	11th to 20th	Night	58% 7% 8% 17% 10%	2717
	21st to end	Night	45% 6% 6% 22% 19% 1	2886
June	1st to 10th	Night	58% 8% 9% 21% 5%	1000
	11th to 20th	Night	45% 8% 11% 27% 9% 1	1000
	21st to end	Night	49% 8% 9% 27% 7%	1000
July	1st to 10th	Night	47% 6% 15% 26% 6%	2235
	11th to 20th	Night	43% 6% 17% 30% 4	2316
	21st to end	Night	46% 7% 17% 24% 6%	3075
August	1st to 10th	Night	44% 7% 21% 24% 4	2230
	11th to 20th	Night	44% 6% 16% 23% 11%	2379
	21st to end	Night	45% 7% 22% 23% 3	3034
September	1st to 10th	Night	61% 10% 16% 12% 1	1801
	11th to 20th	Night	62% 7% 14% 15% 2	1909
	21st to end	Night	62% 8% 13% 14% 4	2169
October	1st to 10th	Night	69% 5% 9% 14% 2	3800
	11th to 20th	Night	67% 5% 8% 17% 3	4060
	21st to end	Night	78% 6% 6% 8% 2	4814
November	1st to 10th	Night	84% 7% 7% 2	2768
	11th to 20th	Night	73% 10% 9% 7% 1	3200
	21st to end	Night	68% 10% 14% 7% 1	3200
December	1st to 10th	Night	72% 7% 12% 7% 1	5290
	11th to 20th	Night	67% 6% 12% 13% 2	6315
	21st to end	Night	70% 7% 10% 12% 1	8470

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	5% 3 26% 40% 18% 8%	100
	11th to 20th	Day	5% 7% 26% 43% 16% 2	167
	21st to end	Day	10% 6% 21% 37% 19% 7%	330
February	1st to 10th	Day	4 6% 27% 34% 24% 5%	300
	11th to 20th	Day	5% 3 24% 43% 21% 4	300
	21st to end	Day	3 4 29% 30% 23% 10%	249
March	1st to 10th	Day	9% 6% 17% 30% 27% 12%	300
	11th to 20th	Day	10% 5% 23% 33% 22% 7%	300
	21st to end	Day	7% 6% 21% 33% 21% 12%	415
April	1st to 10th	Day	12% 5% 21% 32% 21% 9%	609
	11th to 20th	Day	11% 5% 20% 33% 23% 8%	690
	21st to end	Day	9% 5% 21% 31% 24% 9%	700
May	1st to 10th	Day	18% 5% 25% 28% 15% 8%	703
	11th to 20th	Day	22% 6% 23% 31% 14% 4	725
	21st to end	Day	14% 5% 26% 34% 17% 3	789
June	1st to 10th	Day	28% 6% 19% 26% 17% 4	730
	11th to 20th	Day	17% 6% 19% 32% 20% 7%	730
	21st to end	Day	16% 2 18% 33% 24% 7%	730
July	1st to 10th	Day	17% 4 18% 33% 20% 9%	721
	11th to 20th	Day	18% 3 15% 31% 24% 9%	718
	21st to end	Day	16% 4 17% 29% 23% 10%	794
August	1st to 10th	Day	22% 3 16% 28% 22% 8%	697
	11th to 20th	Day	24% 2 16% 25% 22% 12%	665
	21st to end	Day	25% 6% 16% 29% 18% 6%	748
September	1st to 10th	Day	31% 7% 21% 26% 11% 4	588
	11th to 20th	Day	29% 5% 23% 22% 17% 3	478
	21st to end	Day	29% 7% 19% 29% 13% 4	300
October	1st to 10th	Day	25% 7% 31% 21% 13% 3	300
	11th to 20th	Day	31% 8% 19% 22% 12% 8%	300
	21st to end	Day	23% 8% 26% 22% 17% 3	330
November	1st to 10th	Day	18% 9% 26% 31% 14% 2	285
	11th to 20th	Day	19% 10% 31% 28% 10% 3	200
	21st to end	Day	21% 8% 20% 25% 20% 8%	200
December	1st to 10th	Day	11% 2 28% 37% 19% 3	180
	11th to 20th	Day	11% 7% 22% 31% 21% 8%	100
	21st to end	Day	3 8% 23% 31% 24% 12%	110
January	1st to 10th	Night	11% 3 23% 37% 20% 6%	1000
	11th to 20th	Night	8% 5% 27% 35% 19% 6%	1000
	21st to end	Night	8% 7% 24% 34% 21% 5%	1100
February	1st to 10th	Night	5% 5% 25% 38% 22% 5%	1000
	11th to 20th	Night	6% 3 20% 42% 23% 6%	979
	21st to end	Night	5% 3 23% 37% 25% 7%	825
March	1st to 10th	Night	8% 4 21% 37% 25% 5%	908
	11th to 20th	Night	11% 4 24% 36% 21% 5%	600
	21st to end	Night	8% 6% 19% 31% 28% 8%	660
April	1st to 10th	Night	13% 4 22% 31% 25% 4	600
	11th to 20th	Night	10% 3 21% 37% 24% 5%	593
	21st to end	Night	12% 5% 18% 37% 24% 4	600
May	1st to 10th	Night	20% 5% 20% 34% 18% 3	600
	11th to 20th	Night	20% 6% 21% 33% 17% 3	407
	21st to end	Night	18% 4 27% 36% 12% 1	403
June	1st to 10th	Night	25% 5% 20% 35% 16% 1	200
	11th to 20th	Night	17% 6% 20% 37% 19% 3	200
	21st to end	Night	18% 2 16% 39% 24% 1	200
July	1st to 10th	Night	17% 2 18% 40% 20% 3	195
	11th to 20th	Night	15% 1 14% 44% 22% 3	228
	21st to end	Night	17% 2 15% 35% 27% 4	523
August	1st to 10th	Night	22% 3 10% 35% 27% 3	579
	11th to 20th	Night	20% 4 16% 31% 26% 3	564
	21st to end	Night	24% 3 17% 34% 19% 3	627
September	1st to 10th	Night	29% 4 23% 29% 13% 2	570
	11th to 20th	Night	31% 6% 21% 27% 13% 2	570
	21st to end	Night	30% 5% 27% 24% 11% 3	649
October	1st to 10th	Night	35% 9% 22% 24% 9% 1	760
	11th to 20th	Night	38% 6% 19% 20% 13% 3	838
	21st to end	Night	29% 7% 23% 27% 11% 3	1045
November	1st to 10th	Night	23% 9% 27% 26% 12% 2	975
	11th to 20th	Night	18% 7% 26% 35% 12% 2	1000
	21st to end	Night	14% 8% 26% 28% 18% 6%	1000
December	1st to 10th	Night	10% 5% 26% 41% 16% 3	1000
	11th to 20th	Night	12% 6% 21% 33% 22% 7%	1000
	21st to end	Night	12% 5% 23% 32% 21% 6%	1100

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