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 2026-03-03.

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	11% 11% 4 11% 7% 6% 7% 5% 7% 8% 24%	134495
	Night	9% 11% 5% 13% 8% 7% 8% 7% 8% 7% 17%	135381
Transparency	Day	55% 6% 12% 20% 6%	134157
	Night	62% 6% 11% 15% 5%	135299
Seeing	Day	17% 4 15% 30% 25% 9%	17168
	Night	17% 4 19% 33% 22% 5%	26012

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 8% 5% 17% 12% 10% 7% 6% 11% 10% 11%	2930
	11th to 20th	Day	3 8% 6% 15% 10% 10% 10% 6% 13% 10% 8%	3300
	21st to end	Day	3 12% 5% 11% 7% 7% 11% 9% 14% 9% 12%	4532
February	1st to 10th	Day	2 8% 4 10% 8% 10% 13% 9% 10% 11% 14%	1470
	11th to 20th	Day	4 9% 4 9% 7% 10% 11% 7% 12% 14% 13%	1751
	21st to end	Day	3 10% 4 12% 6% 8% 8% 8% 16% 11% 14%	1557
March	1st to 10th	Day	4 9% 3 11% 8% 8% 11% 9% 8% 9% 19%	4321
	11th to 20th	Day	5% 14% 5% 13% 9% 10% 12% 7% 8% 6% 12%	4978
	21st to end	Day	2 7% 3 11% 8% 8% 11% 9% 10% 10% 21%	5642
April	1st to 10th	Day	7% 13% 4 12% 8% 9% 11% 7% 7% 6% 16%	2520
	11th to 20th	Day	7% 11% 4 12% 9% 8% 7% 5% 6% 7% 24%	2622
	21st to end	Day	6% 9% 4 13% 11% 10% 8% 5% 6% 8% 21%	2800
May	1st to 10th	Day	16% 17% 5% 14% 7% 5% 5% 3 4 5% 19%	6800
	11th to 20th	Day	15% 12% 6% 12% 6% 5% 5% 3 6% 7% 24%	7161
	21st to end	Day	7% 7% 3 10% 6% 5% 6% 5% 7% 8% 35%	8048
June	1st to 10th	Day	21% 13% 4 7% 4 4 4 3 5% 6% 27%	3400
	11th to 20th	Day	14% 8% 3 6% 4 5% 5% 4 7% 8% 37%	3400
	21st to end	Day	14% 7% 3 6% 5% 4 5% 4 5% 7% 40%	3400
July	1st to 10th	Day	11% 7% 3 8% 5% 5% 6% 5% 7% 9% 34%	7700
	11th to 20th	Day	14% 6% 2 7% 4 5% 5% 5% 6% 9% 36%	7661
	21st to end	Day	11% 8% 3 7% 5% 5% 5% 5% 7% 9% 36%	7676
August	1st to 10th	Day	16% 4 2 5% 4 4 4 3 6% 8% 44%	4881
	11th to 20th	Day	17% 10% 3 6% 5% 4 4 4 5% 7% 37%	4636
	21st to end	Day	19% 8% 3 7% 4 4 5% 4 6% 7% 34%	4751
September	1st to 10th	Day	21% 12% 3 9% 6% 5% 5% 4 5% 6% 25%	2513
	11th to 20th	Day	21% 18% 4 9% 5% 4 4 3 5% 6% 21%	2375
	21st to end	Day	20% 18% 5% 8% 6% 5% 4 3 4 6% 21%	2106
October	1st to 10th	Day	17% 21% 7% 10% 5% 5% 4 3 4 6% 19%	3260
	11th to 20th	Day	20% 22% 6% 10% 4 5% 4 3 4 5% 16%	2964
	21st to end	Day	15% 18% 8% 26% 7% 5% 3 3 3 3 9%	2832
November	1st to 10th	Day	13% 19% 8% 19% 10% 8% 5% 4 5% 3 6%	1603
	11th to 20th	Day	13% 17% 5% 13% 9% 7% 7% 7% 7% 4 11%	1260
	21st to end	Day	7% 18% 8% 14% 9% 7% 7% 7% 4 6% 12%	1260
December	1st to 10th	Day	3 11% 6% 20% 12% 9% 8% 6% 11% 7% 7%	1918
	11th to 20th	Day	5% 10% 7% 18% 11% 7% 7% 8% 9% 6% 12%	1926
	21st to end	Day	8% 20% 6% 15% 8% 8% 8% 5% 7% 5% 11%	2541
January	1st to 10th	Night	3 8% 5% 18% 12% 10% 9% 6% 9% 8% 11%	9964
	11th to 20th	Night	3 10% 6% 18% 10% 10% 9% 9% 12% 8% 6%	9606
	21st to end	Night	4 10% 4 11% 7% 8% 11% 11% 13% 8% 12%	9681
February	1st to 10th	Night	3 9% 4 11% 9% 9% 11% 8% 15% 10% 11%	3150
	11th to 20th	Night	2 8% 4 10% 9% 10% 9% 8% 16% 11% 12%	2752
	21st to end	Night	3 7% 5% 11% 8% 8% 12% 10% 13% 10% 14%	2249
March	1st to 10th	Night	3 6% 3 12% 8% 9% 11% 11% 9% 8% 19%	5620
	11th to 20th	Night	4 11% 5% 14% 12% 10% 11% 7% 8% 5% 14%	4980
	21st to end	Night	3 6% 3 10% 8% 9% 12% 13% 9% 7% 18%	5289
April	1st to 10th	Night	8% 12% 3 10% 7% 5% 10% 7% 8% 7% 22%	1877
	11th to 20th	Night	7% 9% 3 12% 8% 8% 10% 4 6% 5% 28%	1729
	21st to end	Night	8% 10% 4 9% 9% 7% 5% 4 6% 6% 31%	1599
May	1st to 10th	Night	19% 16% 5% 13% 8% 6% 5% 4 4 4 15%	3180
	11th to 20th	Night	14% 14% 6% 13% 5% 5% 4 3 4 6% 26%	2755
	21st to end	Night	7% 9% 4 10% 7% 5% 5% 4 5% 7% 36%	2841
June	1st to 10th	Night	20% 14% 4 10% 4 5% 4 3 5% 6% 25%	1000
	11th to 20th	Night	14% 9% 3 8% 5% 4 4 5% 7% 7% 34%	1000
	21st to end	Night	15% 9% 3 8% 7% 4 5% 4 5% 5% 36%	1000
July	1st to 10th	Night	12% 8% 3 8% 5% 5% 6% 5% 8% 10% 30%	2260
	11th to 20th	Night	13% 5% 2 6% 5% 6% 5% 6% 8% 9% 34%	2301
	21st to end	Night	12% 9% 3 7% 5% 5% 5% 4 7% 7% 34%	3187
August	1st to 10th	Night	17% 6% 2 7% 5% 5% 5% 3 5% 8% 39%	2279
	11th to 20th	Night	13% 8% 2 8% 5% 5% 6% 3 6% 7% 39%	2410
	21st to end	Night	16% 7% 2 6% 5% 5% 5% 5% 6% 9% 34%	3155
September	1st to 10th	Night	23% 10% 4 7% 5% 5% 5% 4 6% 7% 23%	1887
	11th to 20th	Night	21% 16% 4 7% 4 5% 4 3 4 5% 25%	2024
	21st to end	Night	21% 16% 4 8% 5% 4 4 3 4 6% 25%	2274
October	1st to 10th	Night	19% 18% 5% 10% 5% 5% 5% 3 6% 6% 18%	3920
	11th to 20th	Night	18% 20% 8% 9% 4 4 4 3 4 5% 21%	4179
	21st to end	Night	17% 15% 6% 20% 7% 5% 5% 3 4 4 13%	4968
November	1st to 10th	Night	13% 21% 7% 18% 9% 7% 6% 4 4 3 6%	2913
	11th to 20th	Night	11% 18% 6% 13% 9% 8% 7% 6% 6% 3 13%	3360
	21st to end	Night	7% 18% 7% 15% 8% 7% 7% 6% 7% 7% 11%	3360
December	1st to 10th	Night	3 10% 6% 22% 12% 8% 7% 7% 12% 7% 5%	5489
	11th to 20th	Night	4 12% 7% 16% 9% 8% 9% 9% 11% 6% 10%	6486
	21st to end	Night	9% 15% 6% 15% 8% 9% 9% 5% 7% 5% 13%	8657

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	65% 4 10% 14% 6%	2930
	11th to 20th	Day	63% 5% 12% 18% 1	3316
	21st to end	Day	58% 5% 10% 19% 7%	4532
February	1st to 10th	Day	55% 7% 13% 20% 4	1470
	11th to 20th	Day	53% 8% 14% 20% 5%	1760
	21st to end	Day	50% 8% 11% 20% 11%	1557
March	1st to 10th	Day	54% 5% 12% 17% 9% 3	4321
	11th to 20th	Day	68% 4 6% 10% 11%	4980
	21st to end	Day	50% 5% 9% 23% 13%	5642
April	1st to 10th	Day	62% 8% 10% 15% 5%	2520
	11th to 20th	Day	55% 8% 12% 20% 6%	2622
	21st to end	Day	55% 9% 14% 17% 5%	2800
May	1st to 10th	Day	66% 7% 9% 12% 6%	6797
	11th to 20th	Day	58% 6% 11% 18% 8%	7165
	21st to end	Day	42% 9% 10% 22% 16% 1	8048
June	1st to 10th	Day	57% 6% 12% 19% 6%	3400
	11th to 20th	Day	43% 7% 13% 31% 6%	3400
	21st to end	Day	42% 7% 13% 28% 8%	3400
July	1st to 10th	Day	44% 6% 16% 28% 6%	7616
	11th to 20th	Day	43% 6% 18% 31% 2	7568
	21st to end	Day	43% 7% 18% 28% 4	7460
August	1st to 10th	Day	38% 6% 21% 29% 5%	4864
	11th to 20th	Day	46% 5% 16% 22% 11%	4681
	21st to end	Day	47% 8% 19% 24% 2	4750
September	1st to 10th	Day	58% 9% 16% 16% 1	2513
	11th to 20th	Day	64% 8% 16% 11% 1	2375
	21st to end	Day	64% 6% 14% 14% 2	2106
October	1st to 10th	Day	69% 4 8% 15% 3	3260
	11th to 20th	Day	71% 5% 7% 15% 1	2964
	21st to end	Day	86% 4 5% 5%	2832
November	1st to 10th	Day	85% 7% 5% 3	1603
	11th to 20th	Day	77% 9% 8% 6% 1	1260
	21st to end	Day	74% 10% 9% 7% 1	1260
December	1st to 10th	Day	74% 6% 9% 9% 2	1918
	11th to 20th	Day	67% 6% 11% 15% 1	1926
	21st to end	Day	71% 7% 9% 13% 1	2541
January	1st to 10th	Night	67% 5% 8% 13% 8%	9965
	11th to 20th	Night	69% 5% 10% 15% 2	9652
	21st to end	Night	59% 5% 10% 20% 6%	9682
February	1st to 10th	Night	55% 9% 13% 18% 5%	3150
	11th to 20th	Night	53% 9% 15% 17% 6%	2753
	21st to end	Night	53% 10% 9% 16% 11%	2249
March	1st to 10th	Night	54% 7% 14% 15% 8% 3	5620
	11th to 20th	Night	69% 4 7% 11% 9%	4980
	21st to end	Night	53% 6% 10% 17% 13%	5289
April	1st to 10th	Night	54% 8% 11% 19% 8% 1	1877
	11th to 20th	Night	55% 6% 9% 21% 8% 1	1729
	21st to end	Night	51% 8% 11% 21% 8% 1	1599
May	1st to 10th	Night	70% 5% 6% 14% 5%	3180
	11th to 20th	Night	59% 6% 9% 17% 10%	2755
	21st to end	Night	46% 6% 7% 22% 19% 1	2841
June	1st to 10th	Night	58% 8% 10% 20% 4	1000
	11th to 20th	Night	47% 7% 10% 28% 8% 1	1000
	21st to end	Night	48% 7% 11% 27% 8%	1000
July	1st to 10th	Night	48% 7% 15% 25% 6%	2235
	11th to 20th	Night	43% 5% 18% 29% 5%	2271
	21st to end	Night	47% 7% 17% 22% 6%	3098
August	1st to 10th	Night	45% 5% 21% 23% 5%	2272
	11th to 20th	Night	44% 5% 17% 23% 11%	2431
	21st to end	Night	46% 6% 22% 23% 3	3155
September	1st to 10th	Night	60% 9% 17% 14% 1	1887
	11th to 20th	Night	62% 7% 14% 15% 2	2024
	21st to end	Night	61% 7% 14% 14% 4	2274
October	1st to 10th	Night	69% 5% 9% 14% 3	3920
	11th to 20th	Night	69% 4 8% 16% 3	4179
	21st to end	Night	80% 5% 5% 8% 2	4968
November	1st to 10th	Night	86% 6% 5% 2	2913
	11th to 20th	Night	76% 9% 7% 6% 1	3360
	21st to end	Night	71% 10% 10% 8% 1	3360
December	1st to 10th	Night	73% 8% 10% 9% 1	5489
	11th to 20th	Night	67% 7% 11% 12% 2	6485
	21st to end	Night	69% 7% 10% 12% 1	8657

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% 28% 35% 23% 9%	110
	11th to 20th	Day	7% 5% 22% 45% 18% 3	179
	21st to end	Day	8% 5% 19% 37% 25% 6%	363
February	1st to 10th	Day	5% 4 22% 37% 26% 5%	330
	11th to 20th	Day	8% 1 18% 40% 27% 5%	330
	21st to end	Day	2 3 19% 34% 29% 12%	273
March	1st to 10th	Day	10% 2 15% 25% 34% 15%	306
	11th to 20th	Day	9% 6% 14% 35% 25% 11%	300
	21st to end	Day	6% 4 17% 29% 28% 16%	405
April	1st to 10th	Day	12% 3 16% 33% 25% 12%	620
	11th to 20th	Day	9% 4 15% 31% 28% 13%	690
	21st to end	Day	9% 3 18% 29% 29% 13%	700
May	1st to 10th	Day	16% 4 19% 29% 22% 10%	703
	11th to 20th	Day	22% 3 15% 33% 21% 5%	725
	21st to end	Day	14% 2 17% 37% 24% 6%	789
June	1st to 10th	Day	29% 4 10% 28% 23% 6%	730
	11th to 20th	Day	16% 3 11% 31% 28% 10%	730
	21st to end	Day	16% 2 8% 31% 32% 12%	730
July	1st to 10th	Day	16% 2 8% 34% 30% 11%	721
	11th to 20th	Day	18% 2 8% 27% 32% 14%	718
	21st to end	Day	16% 3 9% 26% 31% 14%	794
August	1st to 10th	Day	20% 2 9% 28% 30% 12%	726
	11th to 20th	Day	21% 4 9% 24% 25% 17%	715
	21st to end	Day	25% 3 10% 29% 23% 10%	803
September	1st to 10th	Day	29% 5% 15% 28% 18% 6%	643
	11th to 20th	Day	24% 7% 19% 21% 23% 6%	505
	21st to end	Day	27% 5% 15% 29% 19% 5%	330
October	1st to 10th	Day	21% 7% 25% 28% 15% 5%	330
	11th to 20th	Day	30% 7% 18% 21% 17% 8%	330
	21st to end	Day	23% 7% 25% 25% 17% 2	363
November	1st to 10th	Day	18% 6% 27% 29% 16% 4	319
	11th to 20th	Day	22% 7% 25% 30% 12% 3	220
	21st to end	Day	17% 9% 16% 29% 23% 6%	220
December	1st to 10th	Day	11% 4 25% 36% 19% 5%	187
	11th to 20th	Day	8% 7% 20% 31% 27% 6%	110
	21st to end	Day	5% 7% 21% 31% 26% 10%	121
January	1st to 10th	Night	10% 3 21% 36% 22% 6%	1050
	11th to 20th	Night	8% 4 24% 38% 20% 6%	1050
	21st to end	Night	8% 6% 21% 36% 24% 5%	1155
February	1st to 10th	Night	6% 4 22% 37% 27% 5%	1050
	11th to 20th	Night	6% 2 17% 41% 27% 7%	1029
	21st to end	Night	5% 2 19% 38% 29% 8%	865
March	1st to 10th	Night	7% 3 17% 36% 29% 7%	919
	11th to 20th	Night	9% 3 19% 40% 22% 7%	600
	21st to end	Night	8% 5% 15% 33% 31% 9%	660
April	1st to 10th	Night	13% 2 19% 33% 26% 7%	600
	11th to 20th	Night	9% 2 16% 37% 28% 7%	593
	21st to end	Night	11% 4 17% 33% 29% 6%	600
May	1st to 10th	Night	19% 4 18% 32% 22% 5%	600
	11th to 20th	Night	19% 2 21% 34% 21% 3	419
	21st to end	Night	17% 3 19% 40% 19% 3	384
June	1st to 10th	Night	27% 2 13% 33% 23% 3	200
	11th to 20th	Night	18% 4 10% 38% 27% 4	200
	21st to end	Night	19% 2 9% 36% 31% 5%	200
July	1st to 10th	Night	17% 3 9% 41% 26% 4	195
	11th to 20th	Night	16% 2 9% 33% 34% 7%	209
	21st to end	Night	16% 2 10% 30% 33% 9%	534
August	1st to 10th	Night	23% 2 8% 27% 32% 8%	597
	11th to 20th	Night	21% 3 10% 32% 27% 7%	594
	21st to end	Night	26% 3 11% 30% 25% 4	660
September	1st to 10th	Night	30% 4 18% 29% 17% 3	600
	11th to 20th	Night	31% 6% 17% 27% 16% 3	600
	21st to end	Night	33% 5% 20% 26% 14% 3	674
October	1st to 10th	Night	35% 9% 21% 25% 10% 1	800
	11th to 20th	Night	40% 6% 16% 19% 14% 4	895
	21st to end	Night	32% 7% 20% 25% 13% 3	1100
November	1st to 10th	Night	24% 10% 24% 26% 13% 3	1025
	11th to 20th	Night	20% 5% 24% 35% 14% 2	1050
	21st to end	Night	13% 8% 24% 30% 20% 6%	1050
December	1st to 10th	Night	10% 4 24% 40% 20% 3	1050
	11th to 20th	Night	11% 5% 19% 33% 25% 7%	1050
	21st to end	Night	12% 3 21% 34% 23% 7%	1155

Page generated at 2026-02-28 00:40:22. Version ac07

Copyright © 2026 Attilla Danko.