Churchill
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Churchill clear sky chart from 2005-11-06 to 2024-12-02.

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%	106336
	Night	10% 12% 5% 12% 8% 7% 7% 6% 8% 7% 18%	101852
Transparency	Day	56% 7% 13% 19% 5%	106048
	Night	62% 7% 11% 15% 4	101787
Seeing	Day	17% 4 14% 30% 25% 10%	15666
	Night	17% 4 18% 33% 22% 5%	24288

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	3 11% 6% 19% 13% 11% 8% 7% 13% 5% 5%	1580
	11th to 20th	Day	2 9% 7% 16% 10% 9% 10% 7% 11% 9% 10%	1744
	21st to end	Day	5% 16% 4 12% 7% 7% 9% 7% 12% 9% 11%	2442
February	1st to 10th	Day	2 8% 4 10% 8% 9% 13% 9% 11% 11% 15%	1330
	11th to 20th	Day	4 8% 4 8% 7% 9% 11% 8% 13% 14% 13%	1578
	21st to end	Day	3 10% 4 12% 6% 8% 8% 8% 16% 11% 13%	1413
March	1st to 10th	Day	5% 12% 3 10% 8% 7% 8% 7% 9% 10% 19%	2993
	11th to 20th	Day	6% 16% 6% 13% 9% 9% 7% 7% 9% 6% 11%	3478
	21st to end	Day	3 6% 4 15% 10% 9% 11% 6% 7% 7% 22%	3936
April	1st to 10th	Day	8% 13% 4 11% 9% 9% 11% 8% 7% 6% 15%	2394
	11th to 20th	Day	7% 10% 3 12% 9% 9% 8% 5% 6% 7% 25%	2489
	21st to end	Day	5% 8% 4 13% 11% 9% 8% 5% 7% 8% 22%	2660
May	1st to 10th	Day	20% 20% 6% 13% 6% 5% 4 3 3 4 15%	4750
	11th to 20th	Day	16% 14% 6% 9% 6% 4 4 3 6% 6% 25%	4985
	21st to end	Day	8% 10% 4 12% 7% 6% 7% 6% 7% 8% 26%	5600
June	1st to 10th	Day	20% 13% 4 7% 4 5% 4 4 5% 6% 28%	3230
	11th to 20th	Day	14% 8% 3 6% 4 5% 5% 4 7% 8% 36%	3230
	21st to end	Day	14% 7% 3 6% 4 4 5% 4 5% 7% 41%	3230
July	1st to 10th	Day	13% 8% 3 8% 5% 5% 6% 5% 7% 9% 30%	5380
	11th to 20th	Day	17% 6% 2 6% 4 4 5% 4 5% 8% 38%	5354
	21st to end	Day	11% 9% 3 8% 5% 4 5% 5% 6% 8% 36%	5342
August	1st to 10th	Day	17% 4 2 5% 4 4 4 3 5% 8% 44%	4731
	11th to 20th	Day	17% 10% 2 6% 4 4 4 4 5% 7% 38%	4491
	21st to end	Day	19% 8% 2 7% 4 4 5% 4 6% 7% 33%	4606
September	1st to 10th	Day	22% 12% 3 9% 6% 4 5% 4 5% 6% 24%	2388
	11th to 20th	Day	21% 19% 4 9% 5% 4 4 3 5% 5% 21%	2256
	21st to end	Day	20% 19% 5% 9% 6% 5% 4 3 4 5% 21%	2001
October	1st to 10th	Day	17% 22% 7% 10% 6% 5% 4 3 4 5% 18%	3160
	11th to 20th	Day	20% 22% 6% 11% 4 5% 4 3 4 5% 16%	2873
	21st to end	Day	15% 18% 8% 26% 7% 5% 3 3 3 3 9%	2744
November	1st to 10th	Day	13% 19% 8% 20% 9% 7% 5% 4 5% 3 6%	1525
	11th to 20th	Day	12% 17% 5% 13% 8% 7% 8% 7% 7% 4 11%	1200
	21st to end	Day	8% 19% 8% 14% 9% 7% 7% 7% 4 6% 12%	1200
December	1st to 10th	Day	5% 15% 5% 15% 11% 9% 8% 5% 11% 7% 9%	1089
	11th to 20th	Day	5% 11% 7% 13% 12% 10% 9% 8% 10% 7% 9%	1196
	21st to end	Day	6% 16% 5% 14% 8% 10% 10% 6% 7% 6% 12%	1738
January	1st to 10th	Night	4 10% 5% 16% 12% 11% 10% 7% 11% 7% 6%	5366
	11th to 20th	Night	3 11% 7% 19% 10% 8% 8% 8% 11% 7% 9%	5152
	21st to end	Night	7% 15% 5% 12% 8% 7% 8% 7% 12% 9% 10%	5225
February	1st to 10th	Night	3 9% 4 11% 10% 9% 11% 9% 15% 10% 11%	2850
	11th to 20th	Night	2 8% 4 10% 9% 9% 8% 8% 17% 11% 13%	2486
	21st to end	Night	3 7% 5% 12% 8% 8% 11% 10% 13% 10% 12%	2041
March	1st to 10th	Night	4 8% 4 11% 7% 8% 10% 9% 10% 9% 20%	3883
	11th to 20th	Night	6% 13% 5% 14% 12% 10% 8% 6% 9% 5% 12%	3480
	21st to end	Night	4 6% 3 12% 10% 10% 11% 9% 7% 6% 22%	3695
April	1st to 10th	Night	9% 12% 3 9% 7% 6% 10% 7% 9% 7% 21%	1783
	11th to 20th	Night	7% 9% 3 11% 8% 8% 10% 4 6% 5% 29%	1642
	21st to end	Night	8% 10% 4 9% 9% 7% 6% 4 7% 6% 31%	1519
May	1st to 10th	Night	21% 21% 6% 15% 8% 4 4 3 4 4 12%	2220
	11th to 20th	Night	15% 17% 6% 10% 5% 4 4 3 3 6% 26%	1921
	21st to end	Night	10% 13% 6% 12% 8% 6% 5% 4 6% 6% 25%	1981
June	1st to 10th	Night	18% 14% 4 10% 4 5% 4 3 5% 7% 26%	950
	11th to 20th	Night	14% 9% 3 8% 4 4 4 4 7% 8% 34%	950
	21st to end	Night	15% 9% 3 8% 7% 4 4 4 5% 5% 36%	950
July	1st to 10th	Night	11% 8% 3 8% 5% 5% 6% 4 8% 10% 34%	1580
	11th to 20th	Night	17% 6% 2 5% 4 5% 4 4 7% 9% 36%	1607
	21st to end	Night	12% 10% 4 8% 6% 5% 5% 4 7% 7% 32%	2219
August	1st to 10th	Night	17% 6% 2 6% 5% 5% 5% 3 5% 8% 39%	2209
	11th to 20th	Night	13% 8% 2 8% 5% 4 6% 3 5% 7% 39%	2335
	21st to end	Night	17% 7% 2 6% 5% 5% 5% 4 6% 9% 34%	3058
September	1st to 10th	Night	24% 10% 4 8% 5% 5% 5% 4 6% 7% 22%	1792
	11th to 20th	Night	21% 16% 4 7% 4 5% 4 3 4 5% 25%	1923
	21st to end	Night	21% 17% 4 8% 5% 3 4 3 4 6% 25%	2160
October	1st to 10th	Night	20% 18% 5% 10% 5% 5% 5% 3 5% 6% 18%	3800
	11th to 20th	Night	18% 20% 8% 9% 4 4 4 3 4 5% 21%	4050
	21st to end	Night	17% 15% 6% 21% 7% 5% 5% 3 4 4 13%	4814
November	1st to 10th	Night	14% 22% 7% 18% 10% 7% 6% 4 4 2 6%	2771
	11th to 20th	Night	11% 17% 6% 13% 8% 8% 7% 6% 7% 4 13%	3200
	21st to end	Night	7% 18% 7% 15% 8% 7% 8% 6% 7% 7% 12%	3200
December	1st to 10th	Night	5% 13% 6% 14% 11% 9% 8% 7% 13% 8% 7%	3126
	11th to 20th	Night	6% 12% 5% 15% 9% 9% 9% 8% 10% 7% 10%	3996
	21st to end	Night	6% 12% 6% 13% 9% 11% 11% 6% 8% 6% 13%	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% 12% 8% 1	1580
	11th to 20th	Day	64% 8% 13% 13% 2	1760
	21st to end	Day	59% 8% 10% 15% 9%	2442
February	1st to 10th	Day	54% 8% 13% 21% 5%	1330
	11th to 20th	Day	51% 8% 15% 20% 6%	1587
	21st to end	Day	51% 8% 11% 18% 11%	1413
March	1st to 10th	Day	54% 7% 14% 22% 3	2993
	11th to 20th	Day	67% 6% 8% 12% 8%	3480
	21st to end	Day	56% 7% 8% 19% 9% 1	3936
April	1st to 10th	Day	62% 9% 10% 13% 6%	2394
	11th to 20th	Day	54% 8% 12% 20% 6%	2489
	21st to end	Day	55% 9% 15% 17% 5%	2660
May	1st to 10th	Day	72% 8% 8% 9% 2	4750
	11th to 20th	Day	56% 7% 10% 16% 10%	4989
	21st to end	Day	49% 12% 13% 20% 6%	5600
June	1st to 10th	Day	56% 6% 13% 19% 5%	3230
	11th to 20th	Day	44% 7% 13% 29% 7%	3230
	21st to end	Day	42% 8% 14% 28% 9%	3230
July	1st to 10th	Day	47% 7% 17% 23% 5%	5296
	11th to 20th	Day	44% 7% 17% 29% 2	5261
	21st to end	Day	44% 8% 17% 28% 3	5173
August	1st to 10th	Day	38% 6% 21% 29% 5%	4714
	11th to 20th	Day	45% 5% 16% 22% 11%	4536
	21st to end	Day	47% 8% 19% 23% 2	4605
September	1st to 10th	Day	59% 10% 15% 15% 1	2388
	11th to 20th	Day	65% 8% 15% 11% 2	2256
	21st to end	Day	65% 6% 13% 14% 2	2001
October	1st to 10th	Day	70% 4 7% 15% 3	3160
	11th to 20th	Day	72% 5% 6% 15% 1	2873
	21st to end	Day	86% 4 4 5%	2744
November	1st to 10th	Day	85% 7% 5% 3	1525
	11th to 20th	Day	76% 9% 8% 6% 1	1200
	21st to end	Day	72% 11% 9% 7% 1	1200
December	1st to 10th	Day	70% 8% 8% 11% 3	1089
	11th to 20th	Day	66% 9% 11% 11% 2	1196
	21st to end	Day	66% 8% 8% 16% 1	1738
January	1st to 10th	Night	70% 7% 9% 8% 6%	5366
	11th to 20th	Night	66% 7% 12% 12% 2	5198
	21st to end	Night	61% 6% 9% 16% 7%	5225
February	1st to 10th	Night	54% 9% 13% 18% 6%	2850
	11th to 20th	Night	50% 9% 16% 19% 6%	2487
	21st to end	Night	54% 11% 9% 15% 10%	2041
March	1st to 10th	Night	52% 9% 18% 17% 4	3883
	11th to 20th	Night	68% 5% 7% 13% 6%	3480
	21st to end	Night	56% 8% 9% 17% 9%	3695
April	1st to 10th	Night	54% 8% 11% 19% 8% 1	1783
	11th to 20th	Night	54% 6% 10% 21% 8% 1	1642
	21st to end	Night	50% 8% 11% 21% 9% 1	1519
May	1st to 10th	Night	75% 6% 6% 11% 2	2220
	11th to 20th	Night	59% 6% 8% 14% 13%	1921
	21st to end	Night	56% 8% 9% 23% 5%	1981
June	1st to 10th	Night	57% 8% 11% 20% 5%	950
	11th to 20th	Night	47% 7% 10% 28% 8% 1	950
	21st to end	Night	47% 7% 11% 27% 8%	950
July	1st to 10th	Night	44% 7% 15% 26% 8%	1555
	11th to 20th	Night	44% 6% 20% 27% 4	1577
	21st to end	Night	50% 8% 14% 22% 6%	2149
August	1st to 10th	Night	45% 5% 21% 23% 5%	2202
	11th to 20th	Night	44% 5% 17% 22% 11%	2356
	21st to end	Night	46% 7% 21% 23% 3	3058
September	1st to 10th	Night	60% 9% 17% 12% 2	1792
	11th to 20th	Night	62% 7% 14% 15% 2	1923
	21st to end	Night	62% 7% 13% 14% 4	2160
October	1st to 10th	Night	70% 5% 9% 14% 3	3800
	11th to 20th	Night	68% 4 8% 16% 3	4050
	21st to end	Night	80% 5% 5% 8% 2	4814
November	1st to 10th	Night	87% 6% 5% 2	2771
	11th to 20th	Night	76% 9% 8% 7% 1	3200
	21st to end	Night	70% 11% 11% 8% 1	3200
December	1st to 10th	Night	67% 10% 10% 10% 2	3126
	11th to 20th	Night	64% 10% 12% 12% 2	3995
	21st to end	Night	66% 9% 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 29% 32% 26% 9%	90
	11th to 20th	Day	8% 6% 21% 42% 20% 3	145
	21st to end	Day	8% 4 16% 38% 27% 7%	297
February	1st to 10th	Day	5% 3 19% 38% 28% 7%	270
	11th to 20th	Day	8% 1 19% 42% 27% 3	270
	21st to end	Day	2 2 17% 35% 29% 15%	225
March	1st to 10th	Day	11% 2 15% 24% 34% 14%	270
	11th to 20th	Day	10% 6% 13% 32% 26% 13%	270
	21st to end	Day	6% 4 18% 30% 25% 16%	368
April	1st to 10th	Day	13% 3 15% 34% 24% 11%	574
	11th to 20th	Day	8% 4 15% 32% 28% 13%	640
	21st to end	Day	8% 3 17% 30% 30% 13%	650
May	1st to 10th	Day	17% 4 18% 28% 23% 10%	653
	11th to 20th	Day	22% 3 15% 33% 22% 5%	675
	21st to end	Day	15% 2 18% 37% 22% 6%	734
June	1st to 10th	Day	28% 4 10% 28% 24% 6%	680
	11th to 20th	Day	17% 3 11% 31% 27% 11%	680
	21st to end	Day	16% 2 8% 31% 32% 11%	680
July	1st to 10th	Day	17% 2 7% 33% 30% 11%	671
	11th to 20th	Day	18% 2 7% 28% 30% 14%	668
	21st to end	Day	17% 3 8% 26% 31% 15%	739
August	1st to 10th	Day	21% 2 10% 28% 28% 11%	676
	11th to 20th	Day	21% 4 9% 25% 23% 17%	665
	21st to end	Day	26% 3 11% 29% 23% 9%	748
September	1st to 10th	Day	30% 6% 15% 27% 16% 6%	598
	11th to 20th	Day	25% 7% 20% 21% 23% 6%	466
	21st to end	Day	28% 5% 15% 28% 20% 5%	300
October	1st to 10th	Day	22% 7% 24% 28% 15% 5%	300
	11th to 20th	Day	31% 7% 17% 21% 17% 7%	300
	21st to end	Day	23% 8% 26% 23% 18% 3	330
November	1st to 10th	Day	18% 7% 26% 28% 18% 4	290
	11th to 20th	Day	21% 8% 26% 31% 11% 4	200
	21st to end	Day	19% 11% 16% 27% 23% 6%	200
December	1st to 10th	Day	12% 3 25% 34% 22% 5%	155
	11th to 20th	Day	9% 9% 18% 28% 30% 7%	90
	21st to end	Day	5% 7% 19% 27% 29% 12%	99
January	1st to 10th	Night	11% 3 21% 36% 23% 6%	950
	11th to 20th	Night	8% 5% 23% 38% 21% 6%	950
	21st to end	Night	9% 6% 20% 35% 25% 5%	1045
February	1st to 10th	Night	6% 4 21% 36% 28% 6%	950
	11th to 20th	Night	6% 2 17% 42% 26% 6%	929
	21st to end	Night	5% 2 18% 38% 29% 8%	785
March	1st to 10th	Night	8% 3 18% 36% 29% 7%	863
	11th to 20th	Night	9% 3 19% 39% 23% 7%	570
	21st to end	Night	8% 5% 15% 32% 30% 9%	627
April	1st to 10th	Night	14% 2 19% 33% 26% 6%	570
	11th to 20th	Night	9% 2 16% 38% 28% 7%	563
	21st to end	Night	10% 5% 16% 33% 30% 6%	570
May	1st to 10th	Night	19% 4 16% 32% 23% 6%	570
	11th to 20th	Night	19% 2 21% 35% 21% 3	398
	21st to end	Night	18% 3 20% 39% 18% 3	365
June	1st to 10th	Night	25% 2 13% 34% 23% 3	190
	11th to 20th	Night	17% 4 9% 38% 28% 4	190
	21st to end	Night	19% 2 9% 36% 31% 4	190
July	1st to 10th	Night	17% 2 9% 42% 25% 4	185
	11th to 20th	Night	17% 2 8% 32% 34% 7%	198
	21st to end	Night	16% 2 9% 30% 34% 9%	507
August	1st to 10th	Night	24% 2 8% 27% 31% 8%	567
	11th to 20th	Night	21% 3 11% 32% 26% 7%	564
	21st to end	Night	26% 4 11% 30% 25% 4	627
September	1st to 10th	Night	31% 3 18% 29% 16% 3	570
	11th to 20th	Night	32% 6% 17% 26% 16% 3	570
	21st to end	Night	33% 5% 20% 25% 14% 3	640
October	1st to 10th	Night	36% 9% 21% 24% 10% 1	760
	11th to 20th	Night	41% 6% 15% 19% 14% 4	850
	21st to end	Night	33% 7% 19% 24% 13% 3	1045
November	1st to 10th	Night	25% 10% 23% 26% 14% 3	975
	11th to 20th	Night	20% 5% 24% 36% 14% 2	1000
	21st to end	Night	13% 7% 25% 29% 20% 6%	1000
December	1st to 10th	Night	11% 4 22% 40% 20% 4	960
	11th to 20th	Night	12% 5% 18% 32% 26% 7%	950
	21st to end	Night	12% 4 21% 33% 24% 7%	1045

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