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 2025-11-18.

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% 10% 4 9% 6% 5% 5% 4 6% 7% 32%	127766
	Night	14% 10% 4 10% 6% 5% 5% 4 6% 6% 30%	122589
Transparency	Day	50% 5% 13% 20% 11% 1	131357
	Night	51% 6% 12% 19% 10% 1	127086
Seeing	Day	18% 8% 28% 25% 14% 6%	16135
	Night	19% 8% 29% 30% 12% 1	25061

By Month

Percentage Cloud Cover NE (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	12% 12% 5% 15% 7% 7% 6% 5% 7% 6% 21%	3597
	11th to 20th	Day	8% 15% 7% 18% 11% 9% 6% 5% 5% 5% 11%	3866
	21st to end	Day	13% 16% 6% 15% 8% 7% 6% 4 5% 5% 14%	4466
February	1st to 10th	Day	12% 15% 6% 14% 9% 7% 6% 4 5% 5% 17%	1800
	11th to 20th	Day	12% 14% 6% 14% 8% 5% 6% 4 5% 6% 21%	1753
	21st to end	Day	14% 15% 5% 12% 7% 6% 5% 4 5% 5% 22%	1531
March	1st to 10th	Day	17% 11% 5% 10% 6% 5% 4 3 5% 6% 29%	4932
	11th to 20th	Day	10% 12% 5% 11% 6% 5% 6% 4 6% 7% 28%	4980
	21st to end	Day	17% 10% 4 12% 5% 5% 4 3 4 5% 31%	5465
April	1st to 10th	Day	16% 14% 4 10% 6% 4 4 3 5% 5% 29%	2360
	11th to 20th	Day	19% 10% 3 6% 4 4 4 4 5% 7% 35%	2472
	21st to end	Day	19% 9% 3 9% 5% 4 3 3 4 7% 34%	2600
May	1st to 10th	Day	16% 8% 3 8% 5% 4 4 4 5% 7% 36%	5888
	11th to 20th	Day	14% 10% 4 8% 5% 5% 4 3 5% 7% 36%	6313
	21st to end	Day	17% 7% 3 7% 4 4 4 3 5% 7% 38%	6949
June	1st to 10th	Day	15% 8% 3 6% 4 5% 4 5% 7% 7% 37%	2800
	11th to 20th	Day	11% 5% 2 6% 4 4 5% 5% 7% 8% 41%	2890
	21st to end	Day	13% 6% 2 6% 5% 6% 5% 3 6% 8% 38%	3000
July	1st to 10th	Day	10% 6% 3 7% 6% 5% 5% 5% 7% 9% 37%	6709
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 4 6% 9% 45%	6360
	21st to end	Day	7% 5% 2 6% 4 5% 4 4 6% 8% 48%	6523
August	1st to 10th	Day	12% 8% 2 7% 4 5% 5% 4 6% 8% 38%	4234
	11th to 20th	Day	12% 7% 3 7% 5% 5% 5% 4 7% 10% 36%	4174
	21st to end	Day	9% 6% 2 6% 5% 4 5% 4 7% 9% 43%	4518
September	1st to 10th	Day	13% 7% 3 7% 4 5% 4 4 6% 8% 41%	2400
	11th to 20th	Day	11% 5% 2 6% 5% 4 5% 3 6% 9% 43%	2275
	21st to end	Day	14% 6% 3 8% 4 5% 5% 5% 6% 7% 35%	2163
October	1st to 10th	Day	17% 9% 3 8% 5% 4 5% 4 6% 7% 35%	2000
	11th to 20th	Day	18% 11% 4 9% 5% 5% 5% 5% 5% 7% 26%	2000
	21st to end	Day	23% 13% 4 10% 6% 4 5% 4 5% 6% 20%	2200
November	1st to 10th	Day	15% 13% 4 9% 5% 6% 5% 4 6% 7% 27%	1746
	11th to 20th	Day	12% 17% 4 12% 7% 5% 6% 5% 6% 6% 21%	1656
	21st to end	Day	18% 16% 6% 12% 7% 6% 5% 3 6% 5% 16%	1600
December	1st to 10th	Day	16% 22% 8% 15% 8% 6% 5% 4 5% 5% 8%	2600
	11th to 20th	Day	15% 18% 5% 11% 7% 6% 5% 4 4 5% 19%	2986
	21st to end	Day	23% 17% 6% 15% 7% 5% 4 3 4 4 11%	3960
January	1st to 10th	Night	12% 14% 7% 17% 7% 5% 5% 4 5% 6% 18%	6349
	11th to 20th	Night	11% 12% 7% 16% 11% 9% 7% 5% 5% 5% 13%	6015
	21st to end	Night	13% 13% 7% 13% 9% 7% 6% 4 5% 6% 17%	6490
February	1st to 10th	Night	11% 17% 5% 12% 8% 6% 6% 5% 5% 5% 19%	2600
	11th to 20th	Night	13% 15% 6% 11% 7% 7% 6% 4 5% 6% 22%	2533
	21st to end	Night	13% 11% 5% 12% 7% 6% 5% 4 6% 6% 26%	2096
March	1st to 10th	Night	15% 10% 5% 11% 6% 6% 5% 4 5% 6% 27%	4908
	11th to 20th	Night	9% 9% 3 9% 5% 5% 6% 5% 8% 8% 33%	4979
	21st to end	Night	18% 8% 4 12% 5% 5% 4 4 5% 5% 30%	5467
April	1st to 10th	Night	18% 11% 3 9% 5% 5% 5% 3 5% 6% 30%	2040
	11th to 20th	Night	17% 9% 3 7% 4 5% 4 5% 4 7% 35%	1894
	21st to end	Night	16% 10% 4 9% 5% 4 4 4 5% 6% 35%	1800
May	1st to 10th	Night	18% 8% 3 7% 5% 4 5% 3 6% 7% 35%	4053
	11th to 20th	Night	11% 6% 3 7% 4 5% 5% 4 6% 8% 40%	3580
	21st to end	Night	17% 6% 2 6% 4 4 4 3 4 7% 42%	3926
June	1st to 10th	Night	13% 5% 3 7% 4 5% 5% 4 5% 8% 41%	1600
	11th to 20th	Night	9% 3 2 7% 6% 5% 6% 5% 6% 9% 42%	1503
	21st to end	Night	10% 6% 3 6% 5% 5% 5% 4 5% 8% 43%	1400
July	1st to 10th	Night	7% 6% 3 7% 4 5% 5% 4 6% 8% 46%	3248
	11th to 20th	Night	10% 5% 2 6% 3 4 4 4 5% 8% 50%	3600
	21st to end	Night	8% 5% 2 5% 4 4 4 3 6% 8% 50%	4327
August	1st to 10th	Night	9% 5% 2 6% 4 5% 4 4 6% 9% 46%	2915
	11th to 20th	Night	13% 5% 2 5% 3 4 3 4 6% 8% 46%	2875
	21st to end	Night	10% 6% 3 6% 4 4 5% 4 6% 8% 46%	3345
September	1st to 10th	Night	10% 5% 2 6% 5% 4 4 4 7% 8% 46%	2000
	11th to 20th	Night	11% 4 2 5% 3 3 3 4 6% 8% 51%	2120
	21st to end	Night	15% 7% 3 7% 5% 4 3 4 5% 6% 41%	2235
October	1st to 10th	Night	16% 7% 3 6% 6% 4 4 4 5% 7% 38%	2400
	11th to 20th	Night	17% 11% 4 7% 5% 5% 5% 4 5% 6% 31%	2400
	21st to end	Night	21% 12% 3 8% 6% 5% 4 4 6% 7% 26%	2640
November	1st to 10th	Night	17% 10% 3 8% 5% 4 5% 4 6% 6% 31%	2768
	11th to 20th	Night	14% 16% 5% 10% 6% 5% 4 5% 5% 6% 23%	2899
	21st to end	Night	18% 15% 5% 10% 6% 6% 5% 4 6% 6% 20%	2800
December	1st to 10th	Night	14% 18% 7% 15% 8% 6% 6% 4 5% 5% 13%	4580
	11th to 20th	Night	14% 16% 5% 14% 7% 6% 6% 4 5% 6% 17%	5208
	21st to end	Night	22% 18% 6% 16% 6% 4 4 3 5% 5% 11%	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	60% 6% 15% 14% 5%	3598
	11th to 20th	Day	73% 5% 11% 7% 4	3900
	21st to end	Day	69% 5% 11% 9% 6%	4466
February	1st to 10th	Day	68% 5% 8% 15% 4	1800
	11th to 20th	Day	63% 4 10% 15% 6% 1	1762
	21st to end	Day	63% 3 10% 16% 7% 1	1531
March	1st to 10th	Day	55% 4 15% 13% 12%	4935
	11th to 20th	Day	53% 5% 14% 19% 7% 2	4980
	21st to end	Day	54% 5% 8% 19% 13% 1	5465
April	1st to 10th	Day	57% 4 9% 19% 10% 1	2360
	11th to 20th	Day	49% 5% 9% 22% 14% 1	2472
	21st to end	Day	51% 4 11% 19% 12% 3	2600
May	1st to 10th	Day	46% 5% 12% 20% 11% 5%	5897
	11th to 20th	Day	48% 5% 11% 16% 18% 1	6317
	21st to end	Day	45% 4 12% 22% 14% 3	6949
June	1st to 10th	Day	43% 6% 12% 23% 15% 1	2800
	11th to 20th	Day	38% 6% 14% 25% 16% 1	2890
	21st to end	Day	42% 6% 13% 24% 14% 1	3000
July	1st to 10th	Day	41% 6% 14% 23% 14% 1	6638
	11th to 20th	Day	33% 6% 12% 31% 16% 1	6285
	21st to end	Day	32% 5% 14% 32% 15% 1	6336
August	1st to 10th	Day	42% 5% 15% 25% 12% 1	4221
	11th to 20th	Day	41% 6% 18% 26% 10%	4213
	21st to end	Day	36% 6% 14% 33% 11% 1	4518
September	1st to 10th	Day	41% 6% 14% 28% 10% 2	2400
	11th to 20th	Day	37% 5% 14% 30% 13% 1	2275
	21st to end	Day	46% 6% 14% 25% 9%	2163
October	1st to 10th	Day	42% 5% 13% 26% 13% 1	3260
	11th to 20th	Day	44% 5% 17% 22% 12%	3240
	21st to end	Day	58% 6% 15% 16% 5%	3538
November	1st to 10th	Day	54% 7% 13% 19% 7% 1	1746
	11th to 20th	Day	61% 7% 12% 13% 7% 1	1656
	21st to end	Day	68% 6% 11% 12% 2	1600
December	1st to 10th	Day	76% 5% 10% 7% 1	2600
	11th to 20th	Day	66% 7% 10% 11% 6%	2986
	21st to end	Day	76% 4 8% 7% 5%	3960
January	1st to 10th	Night	63% 8% 10% 15% 4	6349
	11th to 20th	Night	69% 7% 12% 7% 4	6056
	21st to end	Night	66% 4 12% 9% 8% 1	6490
February	1st to 10th	Night	63% 6% 10% 16% 5%	2600
	11th to 20th	Night	60% 6% 11% 17% 6%	2535
	21st to end	Night	56% 6% 12% 17% 8%	2096
March	1st to 10th	Night	56% 4 11% 17% 10% 2	4918
	11th to 20th	Night	44% 6% 19% 21% 8% 2	4980
	21st to end	Night	51% 8% 9% 18% 13% 2	5467
April	1st to 10th	Night	55% 4 10% 19% 11% 1	2040
	11th to 20th	Night	48% 4 9% 21% 16% 2	1894
	21st to end	Night	49% 5% 10% 18% 15% 2	1800
May	1st to 10th	Night	48% 4 12% 15% 15% 6%	4060
	11th to 20th	Night	42% 5% 14% 23% 15% 2	3581
	21st to end	Night	42% 5% 13% 26% 14% 2	3927
June	1st to 10th	Night	40% 5% 11% 25% 16% 2	1600
	11th to 20th	Night	37% 7% 13% 23% 19% 1	1503
	21st to end	Night	39% 6% 13% 26% 15% 1	1400
July	1st to 10th	Night	36% 5% 14% 26% 16% 2	3211
	11th to 20th	Night	33% 4 12% 31% 19% 1	3552
	21st to end	Night	31% 6% 10% 34% 18% 1	4199
August	1st to 10th	Night	35% 5% 15% 29% 15% 1	2904
	11th to 20th	Night	34% 5% 19% 28% 13% 1	2902
	21st to end	Night	34% 6% 14% 33% 12% 1	3345
September	1st to 10th	Night	34% 7% 16% 30% 12% 1	2000
	11th to 20th	Night	30% 6% 13% 34% 16% 1	2120
	21st to end	Night	45% 4 13% 27% 11% 1	2235
October	1st to 10th	Night	34% 5% 10% 29% 21% 1	3920
	11th to 20th	Night	43% 6% 16% 22% 13%	3899
	21st to end	Night	46% 6% 16% 24% 8%	4252
November	1st to 10th	Night	51% 6% 12% 22% 8% 1	2768
	11th to 20th	Night	58% 6% 13% 14% 7% 1	2899
	21st to end	Night	63% 7% 14% 14% 2	2800
December	1st to 10th	Night	71% 6% 11% 10% 3	4580
	11th to 20th	Night	66% 6% 10% 11% 6%	5208
	21st to end	Night	75% 4 11% 7% 4	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	17% 15% 36% 23% 8% 2	200
	11th to 20th	Day	21% 13% 38% 20% 6% 2	266
	21st to end	Day	21% 13% 37% 22% 6% 1	330
February	1st to 10th	Day	17% 15% 46% 16% 5% 1	300
	11th to 20th	Day	21% 20% 37% 20% 2	300
	21st to end	Day	24% 19% 36% 19% 2	255
March	1st to 10th	Day	21% 10% 38% 22% 8%	500
	11th to 20th	Day	20% 12% 36% 24% 7% 1	500
	21st to end	Day	27% 8% 34% 21% 8% 1	550
April	1st to 10th	Day	24% 13% 34% 20% 8% 2	500
	11th to 20th	Day	30% 10% 27% 17% 12% 4	492
	21st to end	Day	25% 10% 27% 25% 10% 3	500
May	1st to 10th	Day	19% 7% 28% 28% 14% 5%	503
	11th to 20th	Day	17% 7% 25% 26% 18% 7%	527
	21st to end	Day	18% 4 24% 28% 19% 7%	573
June	1st to 10th	Day	17% 6% 27% 29% 18% 3	530
	11th to 20th	Day	14% 7% 25% 31% 19% 4	620
	21st to end	Day	17% 7% 28% 29% 13% 5%	730
July	1st to 10th	Day	11% 4 21% 34% 23% 6%	678
	11th to 20th	Day	12% 4 21% 29% 24% 10%	524
	21st to end	Day	10% 3 20% 38% 19% 10%	576
August	1st to 10th	Day	10% 2 18% 28% 28% 13%	526
	11th to 20th	Day	10% 4 17% 29% 26% 14%	519
	21st to end	Day	14% 3 22% 29% 21% 11%	583
September	1st to 10th	Day	14% 7% 21% 23% 24% 12%	530
	11th to 20th	Day	10% 3 18% 25% 24% 20%	530
	21st to end	Day	15% 6% 26% 24% 20% 8%	530
October	1st to 10th	Day	21% 5% 28% 22% 16% 9%	530
	11th to 20th	Day	21% 13% 33% 22% 9% 2	530
	21st to end	Day	29% 12% 29% 23% 7%	583
November	1st to 10th	Day	25% 17% 30% 17% 10% 1	286
	11th to 20th	Day	24% 16% 36% 17% 6% 1	214
	21st to end	Day	28% 10% 41% 17% 4	200
December	1st to 10th	Day	25% 15% 35% 21% 4 1	200
	11th to 20th	Day	23% 19% 36% 17% 5% 2	200
	21st to end	Day	25% 11% 40% 18% 5%	220
January	1st to 10th	Night	21% 11% 40% 20% 7% 1	800
	11th to 20th	Night	23% 11% 34% 25% 6% 1	800
	21st to end	Night	19% 10% 33% 29% 9% 1	880
February	1st to 10th	Night	20% 11% 39% 26% 4	800
	11th to 20th	Night	20% 12% 37% 26% 5%	783
	21st to end	Night	19% 12% 35% 29% 5%	660
March	1st to 10th	Night	20% 11% 35% 28% 6%	800
	11th to 20th	Night	18% 11% 36% 27% 7% 1	800
	21st to end	Night	21% 9% 32% 28% 10%	880
April	1st to 10th	Night	24% 11% 31% 29% 5%	800
	11th to 20th	Night	20% 8% 33% 28% 10% 2	710
	21st to end	Night	22% 6% 28% 32% 12%	600
May	1st to 10th	Night	19% 4 24% 37% 15% 2	600
	11th to 20th	Night	16% 4 28% 38% 13% 1	399
	21st to end	Night	12% 5% 23% 42% 18% 1	433
June	1st to 10th	Night	14% 4 27% 40% 14% 1	400
	11th to 20th	Night	11% 6% 23% 44% 16% 1	400
	21st to end	Night	13% 4 22% 41% 19% 2	400
July	1st to 10th	Night	7% 4 16% 50% 22% 2	390
	11th to 20th	Night	11% 4 20% 41% 22% 2	388
	21st to end	Night	10% 2 19% 42% 26% 1	608
August	1st to 10th	Night	11% 2 13% 43% 29% 2	597
	11th to 20th	Night	8% 3 13% 39% 33% 4	594
	21st to end	Night	13% 3 21% 35% 26% 3	730
September	1st to 10th	Night	12% 5% 21% 34% 23% 6%	800
	11th to 20th	Night	13% 3 18% 32% 26% 7%	800
	21st to end	Night	18% 7% 19% 32% 20% 4	800
October	1st to 10th	Night	18% 5% 28% 29% 18% 3	800
	11th to 20th	Night	23% 10% 30% 27% 10% 1	800
	21st to end	Night	29% 8% 30% 26% 7%	880
November	1st to 10th	Night	22% 11% 30% 26% 10% 1	820
	11th to 20th	Night	22% 15% 33% 22% 7% 1	829
	21st to end	Night	27% 13% 32% 24% 5%	800
December	1st to 10th	Night	25% 14% 36% 19% 5% 1	800
	11th to 20th	Night	21% 14% 38% 22% 5%	800
	21st to end	Night	29% 10% 34% 23% 4	880

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