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 2024-11-04.

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	13% 12% 4 10% 6% 6% 6% 5% 7% 7% 24%	106192
	Night	10% 13% 5% 12% 8% 7% 7% 6% 8% 7% 17%	101394
Transparency	Day	57% 7% 13% 19% 5%	105905
	Night	61% 7% 11% 15% 4	101328
Seeing	Day	18% 5% 20% 30% 20% 7%	15615
	Night	17% 5% 22% 33% 19% 4	24158

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	4 10% 5% 20% 13% 10% 8% 7% 14% 6% 3	1580
	11th to 20th	Day	2 9% 7% 15% 9% 9% 10% 8% 14% 7% 9%	1769
	21st to end	Day	7% 15% 4 11% 8% 8% 9% 7% 14% 10% 7%	2442
February	1st to 10th	Day	2 8% 3 11% 9% 10% 10% 10% 13% 12% 12%	1330
	11th to 20th	Day	4 8% 4 9% 7% 9% 11% 8% 16% 15% 10%	1582
	21st to end	Day	3 10% 4 11% 7% 7% 9% 8% 18% 12% 11%	1413
March	1st to 10th	Day	5% 12% 3 9% 8% 8% 7% 7% 10% 11% 18%	3005
	11th to 20th	Day	7% 18% 5% 12% 9% 9% 8% 9% 10% 6% 9%	3478
	21st to end	Day	3 6% 3 15% 9% 10% 12% 6% 7% 7% 22%	3959
April	1st to 10th	Day	8% 14% 4 10% 8% 10% 11% 7% 7% 6% 15%	2383
	11th to 20th	Day	7% 10% 3 11% 9% 9% 9% 5% 5% 5% 27%	2497
	21st to end	Day	6% 9% 4 12% 12% 10% 7% 5% 6% 8% 21%	2660
May	1st to 10th	Day	21% 20% 6% 14% 7% 4 3 3 3 4 14%	4736
	11th to 20th	Day	16% 15% 5% 10% 6% 5% 4 4 5% 7% 24%	5008
	21st to end	Day	9% 10% 5% 12% 6% 6% 6% 5% 7% 8% 26%	5570
June	1st to 10th	Day	20% 13% 4 7% 4 5% 4 3 5% 7% 28%	3230
	11th to 20th	Day	13% 9% 2 6% 5% 4 6% 4 7% 7% 37%	3230
	21st to end	Day	15% 6% 2 7% 4 4 4 4 5% 8% 41%	3230
July	1st to 10th	Day	13% 8% 3 8% 5% 5% 6% 5% 7% 9% 32%	5380
	11th to 20th	Day	17% 7% 2 6% 4 4 4 4 5% 9% 38%	5323
	21st to end	Day	11% 8% 3 7% 5% 4 5% 5% 7% 9% 37%	5354
August	1st to 10th	Day	17% 5% 2 5% 3 4 4 3 5% 8% 44%	4731
	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% 6% 3 4 3 5%	1474
	11th to 20th	Day	11% 16% 6% 14% 9% 11% 8% 6% 5% 4 11%	1140
	21st to end	Day	8% 17% 7% 15% 11% 8% 7% 6% 5% 6% 10%	1140
December	1st to 10th	Day	5% 16% 5% 16% 10% 10% 9% 4 10% 6% 7%	1102
	11th to 20th	Day	5% 11% 6% 14% 12% 11% 8% 7% 11% 7% 9%	1196
	21st to end	Day	7% 16% 6% 13% 8% 10% 11% 7% 8% 6% 9%	1738
January	1st to 10th	Night	4 10% 5% 16% 13% 11% 11% 8% 12% 6% 4	5366
	11th to 20th	Night	3 11% 6% 18% 10% 8% 9% 7% 12% 7% 8%	5131
	21st to end	Night	7% 15% 5% 11% 8% 8% 8% 7% 14% 9% 8%	5225
February	1st to 10th	Night	3 8% 4 11% 9% 10% 11% 9% 15% 10% 9%	2850
	11th to 20th	Night	3 7% 5% 10% 7% 9% 8% 10% 18% 12% 11%	2483
	21st to end	Night	3 8% 4 11% 8% 8% 10% 11% 16% 10% 10%	2041
March	1st to 10th	Night	4 8% 4 10% 7% 8% 10% 9% 13% 10% 17%	3883
	11th to 20th	Night	6% 15% 5% 14% 12% 10% 8% 5% 9% 5% 11%	3480
	21st to end	Night	4 6% 3 12% 10% 10% 11% 10% 8% 6% 20%	3672
April	1st to 10th	Night	9% 11% 3 9% 8% 7% 8% 7% 9% 7% 22%	1796
	11th to 20th	Night	6% 9% 3 11% 9% 8% 9% 5% 6% 6% 28%	1633
	21st to end	Night	7% 10% 4 10% 10% 9% 5% 4 5% 6% 30%	1520
May	1st to 10th	Night	21% 19% 7% 17% 8% 4 4 3 3 4 13%	2232
	11th to 20th	Night	15% 17% 7% 9% 5% 4 4 3 5% 6% 25%	1897
	21st to end	Night	10% 13% 5% 13% 7% 6% 5% 4 5% 6% 25%	2012
June	1st to 10th	Night	17% 15% 5% 9% 7% 3 3 3 6% 7% 26%	950
	11th to 20th	Night	14% 10% 3 7% 5% 5% 4 5% 7% 6% 35%	950
	21st to end	Night	14% 10% 3 7% 7% 5% 5% 4 4 7% 34%	950
July	1st to 10th	Night	12% 8% 3 7% 6% 4 6% 5% 7% 10% 34%	1580
	11th to 20th	Night	17% 6% 3 5% 3 4 4 5% 6% 9% 37%	1638
	21st to end	Night	13% 10% 4 9% 5% 5% 5% 5% 7% 7% 31%	2209
August	1st to 10th	Night	15% 7% 2 6% 5% 5% 5% 5% 5% 7% 39%	2209
	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% 6% 5% 3 3 3 5%	2674
	11th to 20th	Night	10% 17% 6% 13% 12% 10% 8% 5% 4 4 12%	3040
	21st to end	Night	8% 17% 7% 15% 9% 6% 9% 6% 8% 5% 11%	3040
December	1st to 10th	Night	6% 13% 5% 14% 13% 9% 10% 8% 10% 6% 6%	3074
	11th to 20th	Night	5% 11% 6% 15% 9% 9% 11% 8% 11% 6% 9%	3996
	21st to end	Night	6% 13% 6% 13% 9% 12% 11% 6% 7% 6% 11%	5918

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	71% 7% 13% 8% 1	1580
	11th to 20th	Day	62% 9% 13% 13% 3	1786
	21st to end	Day	60% 8% 7% 15% 9%	2442
February	1st to 10th	Day	54% 7% 13% 21% 5%	1330
	11th to 20th	Day	50% 9% 15% 20% 6%	1591
	21st to end	Day	51% 7% 12% 19% 12%	1413
March	1st to 10th	Day	54% 7% 13% 23% 2	3005
	11th to 20th	Day	68% 5% 8% 12% 8%	3480
	21st to end	Day	58% 5% 8% 19% 8% 1	3959
April	1st to 10th	Day	64% 8% 8% 14% 5%	2383
	11th to 20th	Day	57% 7% 10% 21% 6%	2497
	21st to end	Day	58% 8% 13% 15% 5%	2660
May	1st to 10th	Day	73% 7% 9% 9% 2	4736
	11th to 20th	Day	57% 8% 9% 16% 10%	5012
	21st to end	Day	49% 10% 14% 20% 6%	5570
June	1st to 10th	Day	55% 7% 13% 20% 5%	3230
	11th to 20th	Day	43% 8% 12% 28% 8%	3230
	21st to end	Day	41% 9% 13% 29% 8%	3230
July	1st to 10th	Day	46% 7% 16% 24% 6%	5296
	11th to 20th	Day	44% 7% 18% 30% 2	5230
	21st to end	Day	43% 9% 16% 29% 3	5185
August	1st to 10th	Day	39% 7% 21% 28% 5%	4714
	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	82% 9% 6% 3	1474
	11th to 20th	Day	71% 10% 13% 6% 1	1140
	21st to end	Day	71% 9% 12% 7% 1	1140
December	1st to 10th	Day	70% 7% 10% 11% 3	1102
	11th to 20th	Day	67% 7% 12% 12% 2	1196
	21st to end	Day	69% 6% 9% 16% 1	1738
January	1st to 10th	Night	70% 7% 10% 8% 6%	5366
	11th to 20th	Night	65% 8% 13% 12% 2	5176
	21st to end	Night	62% 6% 8% 17% 7%	5225
February	1st to 10th	Night	55% 8% 14% 17% 6%	2850
	11th to 20th	Night	50% 9% 16% 20% 6%	2484
	21st to end	Night	53% 8% 12% 16% 11%	2041
March	1st to 10th	Night	50% 10% 17% 18% 4	3883
	11th to 20th	Night	68% 5% 8% 13% 6%	3480
	21st to end	Night	56% 7% 9% 17% 10% 1	3672
April	1st to 10th	Night	56% 7% 11% 20% 7% 1	1796
	11th to 20th	Night	52% 8% 10% 21% 8% 1	1633
	21st to end	Night	53% 7% 11% 20% 9%	1520
May	1st to 10th	Night	74% 7% 5% 11% 2	2232
	11th to 20th	Night	58% 7% 7% 13% 14%	1897
	21st to end	Night	56% 8% 8% 23% 5%	2012
June	1st to 10th	Night	57% 8% 9% 21% 5%	950
	11th to 20th	Night	45% 8% 11% 26% 9% 1	950
	21st to end	Night	48% 8% 9% 27% 7%	950
July	1st to 10th	Night	44% 6% 15% 25% 8%	1555
	11th to 20th	Night	43% 7% 19% 28% 3	1608
	21st to end	Night	48% 9% 14% 23% 6%	2139
August	1st to 10th	Night	44% 7% 21% 24% 4	2202
	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	83% 7% 7% 2	2674
	11th to 20th	Night	72% 11% 10% 7% 1	3040
	21st to end	Night	67% 11% 14% 8% 1	3040
December	1st to 10th	Night	67% 9% 12% 10% 2	3074
	11th to 20th	Night	64% 8% 13% 12% 3	3995
	21st to end	Night	67% 8% 8% 15% 2	5918

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	4 3 26% 38% 20% 9%	90
	11th to 20th	Day	5% 7% 26% 43% 17% 1	150
	21st to end	Day	10% 5% 21% 36% 21% 8%	297
February	1st to 10th	Day	4 6% 24% 34% 26% 5%	270
	11th to 20th	Day	6% 3 26% 43% 20% 1	270
	21st to end	Day	3 4 25% 32% 26% 12%	225
March	1st to 10th	Day	10% 6% 16% 29% 26% 13%	270
	11th to 20th	Day	10% 5% 22% 30% 24% 8%	270
	21st to end	Day	6% 7% 22% 33% 20% 12%	378
April	1st to 10th	Day	13% 4 21% 32% 20% 9%	563
	11th to 20th	Day	10% 4 20% 34% 24% 8%	640
	21st to end	Day	7% 5% 22% 32% 26% 9%	650
May	1st to 10th	Day	19% 5% 23% 28% 16% 9%	653
	11th to 20th	Day	22% 6% 23% 32% 14% 4	675
	21st to end	Day	15% 5% 26% 34% 17% 4	734
June	1st to 10th	Day	27% 5% 19% 27% 18% 4	680
	11th to 20th	Day	18% 5% 18% 30% 21% 7%	680
	21st to end	Day	16% 3 18% 33% 24% 7%	680
July	1st to 10th	Day	17% 3 16% 33% 21% 9%	671
	11th to 20th	Day	19% 3 15% 30% 24% 9%	668
	21st to end	Day	16% 4 16% 29% 23% 11%	739
August	1st to 10th	Day	23% 3 16% 28% 22% 8%	676
	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	17% 8% 26% 32% 15% 3	266
	11th to 20th	Day	16% 8% 32% 29% 11% 3	180
	21st to end	Day	21% 9% 20% 24% 20% 7%	180
December	1st to 10th	Day	12% 2 29% 36% 19% 2	162
	11th to 20th	Day	10% 8% 21% 29% 23% 9%	90
	21st to end	Day	2 8% 20% 33% 23% 13%	99
January	1st to 10th	Night	11% 3 22% 37% 21% 6%	950
	11th to 20th	Night	8% 5% 26% 35% 19% 6%	950
	21st to end	Night	8% 7% 23% 34% 22% 5%	1045
February	1st to 10th	Night	6% 5% 24% 38% 23% 5%	950
	11th to 20th	Night	6% 3 21% 42% 22% 5%	929
	21st to end	Night	5% 2 21% 38% 26% 7%	785
March	1st to 10th	Night	8% 4 20% 38% 24% 5%	863
	11th to 20th	Night	11% 4 23% 35% 22% 5%	570
	21st to end	Night	8% 6% 19% 31% 27% 8%	627
April	1st to 10th	Night	14% 4 22% 31% 24% 5%	570
	11th to 20th	Night	9% 3 21% 37% 25% 5%	563
	21st to end	Night	11% 5% 18% 38% 24% 4	570
May	1st to 10th	Night	20% 5% 19% 35% 19% 3	570
	11th to 20th	Night	20% 6% 21% 33% 18% 3	387
	21st to end	Night	19% 5% 27% 35% 13% 1	383
June	1st to 10th	Night	23% 5% 19% 35% 17% 1	190
	11th to 20th	Night	16% 6% 19% 36% 19% 3	190
	21st to end	Night	18% 2 16% 39% 24% 1	190
July	1st to 10th	Night	17% 2 18% 39% 21% 3	185
	11th to 20th	Night	16% 1 14% 44% 22% 3	216
	21st to end	Night	17% 1 14% 35% 28% 5%	497
August	1st to 10th	Night	23% 3 10% 35% 27% 3	567
	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% 8% 27% 27% 13% 2	943
	11th to 20th	Night	18% 7% 25% 36% 12% 3	950
	21st to end	Night	14% 8% 26% 29% 18% 6%	950
December	1st to 10th	Night	10% 5% 26% 40% 16% 3	950
	11th to 20th	Night	12% 6% 20% 32% 23% 7%	950
	21st to end	Night	11% 5% 23% 32% 22% 7%	1045

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