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 2024-01-15.

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% 11% 5% 9% 8% 4 7% 6% 7% 7% 23%	74480
	Night	11% 12% 5% 10% 9% 5% 8% 8% 8% 7% 17%	75310
Transparency	Day	57% 8% 13% 18% 4	74131
	Night	62% 8% 11% 15% 4	75178
Seeing	Day	18% 4 14% 30% 25% 10%	14400
	Night	17% 4 18% 32% 23% 5%	23229

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	6% 13% 6% 13% 12% 7% 9% 10% 11% 7% 9%	950
	11th to 20th	Day	4 10% 5% 10% 9% 7% 10% 8% 15% 12% 11%	1030
	21st to end	Day	5% 12% 4 11% 10% 6% 10% 11% 12% 12% 7%	1386
February	1st to 10th	Day	2 7% 5% 7% 9% 6% 14% 12% 11% 12% 16%	1260
	11th to 20th	Day	4 7% 5% 7% 8% 6% 11% 10% 13% 16% 13%	1493
	21st to end	Day	4 10% 4 11% 7% 6% 8% 10% 16% 10% 14%	1332
March	1st to 10th	Day	6% 10% 5% 11% 10% 6% 7% 10% 9% 11% 14%	1719
	11th to 20th	Day	7% 9% 5% 8% 9% 5% 9% 10% 11% 9% 17%	1978
	21st to end	Day	5% 7% 4 9% 9% 6% 12% 11% 8% 7% 21%	2245
April	1st to 10th	Day	8% 12% 4 10% 9% 7% 12% 10% 7% 6% 15%	2268
	11th to 20th	Day	7% 9% 4 10% 11% 7% 8% 7% 6% 7% 26%	2356
	21st to end	Day	6% 7% 4 11% 13% 7% 9% 7% 6% 8% 23%	2520
May	1st to 10th	Day	14% 14% 6% 13% 10% 4 5% 4 4 5% 20%	2700
	11th to 20th	Day	17% 12% 6% 9% 7% 3 5% 4 6% 6% 24%	2809
	21st to end	Day	13% 11% 6% 8% 7% 3 5% 6% 5% 7% 28%	3152
June	1st to 10th	Day	19% 12% 5% 6% 5% 4 5% 5% 5% 6% 29%	3060
	11th to 20th	Day	14% 7% 3 5% 5% 3 6% 5% 7% 8% 37%	3060
	21st to end	Day	14% 6% 3 5% 5% 3 5% 5% 5% 7% 41%	3060
July	1st to 10th	Day	16% 9% 4 7% 5% 3 6% 6% 6% 9% 30%	3060
	11th to 20th	Day	16% 6% 3 6% 5% 3 5% 6% 6% 8% 37%	3047
	21st to end	Day	14% 8% 3 6% 5% 3 5% 5% 6% 9% 37%	3047
August	1st to 10th	Day	21% 6% 2 5% 5% 3 5% 5% 5% 7% 37%	2686
	11th to 20th	Day	20% 8% 3 6% 6% 3 4 5% 6% 7% 31%	2566
	21st to end	Day	22% 8% 3 7% 4 2 5% 5% 5% 7% 31%	2620
September	1st to 10th	Day	24% 12% 3 7% 8% 3 5% 5% 5% 6% 23%	2263
	11th to 20th	Day	22% 18% 4 8% 6% 3 4 4 5% 5% 20%	2138
	21st to end	Day	19% 19% 6% 7% 7% 4 4 4 4 6% 21%	1896
October	1st to 10th	Day	24% 20% 7% 7% 6% 4 4 3 3 5% 18%	1800
	11th to 20th	Day	29% 20% 7% 10% 5% 2 4 3 4 5% 10%	1650
	21st to end	Day	22% 22% 8% 16% 9% 3 3 2 3 3 10%	1584
November	1st to 10th	Day	14% 17% 9% 18% 11% 6% 6% 5% 5% 3 6%	1448
	11th to 20th	Day	12% 15% 6% 12% 9% 6% 8% 9% 7% 4 11%	1140
	21st to end	Day	8% 16% 9% 12% 9% 6% 8% 9% 4 6% 13%	1140
December	1st to 10th	Day	5% 14% 5% 13% 13% 7% 9% 6% 12% 7% 9%	1083
	11th to 20th	Day	5% 10% 7% 11% 13% 9% 9% 10% 9% 8% 9%	1196
	21st to end	Day	6% 15% 6% 12% 9% 7% 11% 8% 7% 6% 12%	1738
January	1st to 10th	Night	8% 11% 6% 14% 12% 7% 9% 9% 12% 6% 7%	3216
	11th to 20th	Night	4 10% 5% 10% 9% 6% 8% 11% 17% 10% 9%	3019
	21st to end	Night	5% 11% 5% 10% 9% 6% 11% 11% 13% 9% 9%	2970
February	1st to 10th	Night	3 7% 4 9% 11% 7% 11% 11% 14% 11% 12%	2700
	11th to 20th	Night	3 7% 4 9% 10% 7% 8% 10% 17% 13% 14%	2351
	21st to end	Night	4 6% 6% 11% 10% 5% 11% 13% 12% 10% 13%	1924
March	1st to 10th	Night	6% 10% 6% 9% 9% 6% 9% 11% 9% 9% 17%	2259
	11th to 20th	Night	6% 8% 4 8% 9% 6% 11% 10% 10% 7% 20%	1980
	21st to end	Night	6% 7% 3 7% 8% 6% 11% 11% 8% 7% 25%	2111
April	1st to 10th	Night	9% 11% 4 8% 8% 4 10% 10% 8% 8% 21%	1690
	11th to 20th	Night	6% 7% 4 10% 9% 6% 11% 6% 6% 6% 30%	1555
	21st to end	Night	8% 8% 5% 8% 10% 6% 5% 5% 6% 6% 32%	1440
May	1st to 10th	Night	15% 13% 7% 12% 10% 2 4 4 5% 6% 22%	1260
	11th to 20th	Night	17% 12% 6% 10% 7% 5% 5% 4 4 6% 23%	1087
	21st to end	Night	15% 13% 7% 9% 7% 3 4 4 5% 6% 26%	1121
June	1st to 10th	Night	17% 12% 6% 9% 5% 3 5% 4 5% 8% 26%	900
	11th to 20th	Night	14% 8% 4 7% 5% 3 4 6% 7% 8% 34%	900
	21st to end	Night	15% 8% 4 6% 7% 3 4 5% 5% 6% 37%	900
July	1st to 10th	Night	14% 9% 4 8% 6% 3 7% 5% 7% 8% 29%	900
	11th to 20th	Night	14% 7% 3 5% 5% 5% 5% 6% 6% 7% 38%	913
	21st to end	Night	14% 9% 3 6% 6% 3 5% 4 7% 8% 35%	1268
August	1st to 10th	Night	21% 7% 2 5% 6% 2 4 5% 5% 8% 34%	1253
	11th to 20th	Night	18% 8% 3 8% 7% 3 5% 5% 5% 6% 32%	1335
	21st to end	Night	21% 9% 3 6% 6% 3 5% 6% 5% 7% 30%	1733
September	1st to 10th	Night	24% 9% 5% 6% 7% 4 5% 5% 6% 6% 22%	1697
	11th to 20th	Night	23% 15% 4 6% 5% 4 5% 5% 4 5% 24%	1821
	21st to end	Night	21% 16% 4 6% 6% 3 4 4 4 6% 26%	2046
October	1st to 10th	Night	25% 19% 6% 8% 6% 4 4 3 4 4 18%	2160
	11th to 20th	Night	28% 21% 6% 9% 6% 3 4 3 3 3 13%	2309
	21st to end	Night	24% 19% 7% 12% 9% 4 5% 4 3 4 9%	2772
November	1st to 10th	Night	14% 20% 9% 15% 12% 5% 6% 6% 4 3 6%	2629
	11th to 20th	Night	10% 16% 7% 11% 9% 6% 9% 8% 7% 4 14%	3040
	21st to end	Night	6% 15% 9% 13% 9% 5% 8% 9% 7% 7% 12%	3040
December	1st to 10th	Night	6% 11% 6% 12% 13% 7% 8% 9% 13% 8% 7%	3097
	11th to 20th	Night	6% 10% 5% 13% 11% 6% 9% 10% 10% 8% 11%	3996
	21st to end	Night	6% 12% 6% 12% 10% 8% 12% 8% 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	67% 9% 12% 10% 2	950
	11th to 20th	Day	55% 7% 16% 17% 4	1030
	21st to end	Day	60% 8% 10% 16% 6%	1386
February	1st to 10th	Day	52% 8% 14% 21% 5%	1260
	11th to 20th	Day	51% 8% 15% 21% 5%	1502
	21st to end	Day	52% 8% 12% 17% 10%	1332
March	1st to 10th	Day	58% 8% 10% 20% 4	1719
	11th to 20th	Day	55% 7% 11% 18% 9%	1980
	21st to end	Day	54% 8% 12% 17% 8% 1	2245
April	1st to 10th	Day	63% 8% 10% 14% 5%	2268
	11th to 20th	Day	54% 8% 12% 20% 6%	2356
	21st to end	Day	55% 9% 15% 17% 5%	2520
May	1st to 10th	Day	66% 6% 10% 15% 4	2700
	11th to 20th	Day	57% 7% 13% 17% 6%	2813
	21st to end	Day	52% 8% 14% 21% 6%	3152
June	1st to 10th	Day	56% 6% 13% 20% 5%	3060
	11th to 20th	Day	44% 7% 14% 29% 6%	3060
	21st to end	Day	41% 8% 14% 28% 9%	3060
July	1st to 10th	Day	49% 9% 17% 22% 4	2976
	11th to 20th	Day	43% 8% 18% 28% 2	2954
	21st to end	Day	44% 10% 18% 26% 3	2877
August	1st to 10th	Day	45% 9% 22% 21% 3	2669
	11th to 20th	Day	50% 8% 20% 21% 2	2566
	21st to end	Day	50% 10% 16% 21% 3	2620
September	1st to 10th	Day	60% 10% 14% 15% 1	2263
	11th to 20th	Day	66% 8% 15% 10% 1	2138
	21st to end	Day	65% 6% 13% 13% 2	1896
October	1st to 10th	Day	72% 5% 8% 10% 4 1	1800
	11th to 20th	Day	80% 7% 6% 7% 1	1650
	21st to end	Day	85% 5% 5% 4 1	1584
November	1st to 10th	Day	85% 7% 5% 3	1448
	11th to 20th	Day	75% 10% 9% 6% 1	1140
	21st to end	Day	72% 11% 9% 8% 1	1140
December	1st to 10th	Day	70% 8% 8% 11% 3	1083
	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	69% 7% 11% 11% 3	3216
	11th to 20th	Night	55% 9% 15% 18% 3	3019
	21st to end	Night	60% 7% 9% 18% 6%	2970
February	1st to 10th	Night	54% 9% 13% 18% 6%	2700
	11th to 20th	Night	49% 9% 17% 19% 6%	2352
	21st to end	Night	55% 11% 9% 15% 10%	1924
March	1st to 10th	Night	58% 8% 14% 15% 4	2259
	11th to 20th	Night	54% 7% 11% 18% 10% 1	1980
	21st to end	Night	52% 8% 12% 19% 9% 1	2111
April	1st to 10th	Night	55% 8% 11% 19% 7%	1690
	11th to 20th	Night	53% 6% 10% 22% 8% 1	1555
	21st to end	Night	50% 7% 12% 21% 9% 1	1440
May	1st to 10th	Night	63% 5% 10% 19% 4	1260
	11th to 20th	Night	61% 6% 10% 15% 8%	1087
	21st to end	Night	58% 6% 10% 21% 5%	1121
June	1st to 10th	Night	56% 8% 11% 21% 4	900
	11th to 20th	Night	46% 7% 10% 28% 8%	900
	21st to end	Night	46% 7% 11% 27% 8%	900
July	1st to 10th	Night	50% 10% 15% 22% 3	875
	11th to 20th	Night	45% 7% 18% 27% 4	883
	21st to end	Night	44% 11% 15% 27% 3	1198
August	1st to 10th	Night	49% 8% 17% 22% 4	1246
	11th to 20th	Night	50% 7% 19% 22% 2	1335
	21st to end	Night	53% 8% 14% 23% 2	1733
September	1st to 10th	Night	62% 9% 16% 12% 2	1697
	11th to 20th	Night	63% 7% 14% 14% 2	1821
	21st to end	Night	61% 7% 14% 14% 4	2046
October	1st to 10th	Night	74% 5% 8% 9% 3 1	2160
	11th to 20th	Night	80% 5% 7% 6% 2	2309
	21st to end	Night	83% 7% 4 4 1	2772
November	1st to 10th	Night	86% 7% 5% 2	2629
	11th to 20th	Night	75% 10% 8% 7% 1	3040
	21st to end	Night	69% 11% 11% 9% 1	3040
December	1st to 10th	Night	67% 11% 10% 10% 2	3097
	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	9% 5% 22% 40% 21% 2	134
	21st to end	Day	9% 3 13% 39% 29% 8%	264
February	1st to 10th	Day	5% 3 17% 40% 29% 6%	240
	11th to 20th	Day	8% 17% 44% 28% 3	240
	21st to end	Day	2 2 16% 34% 30% 16%	198
March	1st to 10th	Day	10% 2 15% 24% 34% 15%	240
	11th to 20th	Day	11% 4 14% 32% 25% 14%	240
	21st to end	Day	7% 4 19% 30% 25% 15%	331
April	1st to 10th	Day	14% 2 15% 33% 24% 12%	528
	11th to 20th	Day	7% 3 15% 34% 29% 13%	590
	21st to end	Day	8% 3 16% 30% 30% 14%	600
May	1st to 10th	Day	16% 3 18% 28% 23% 11%	603
	11th to 20th	Day	22% 3 15% 33% 22% 4	625
	21st to end	Day	16% 3 19% 36% 22% 6%	679
June	1st to 10th	Day	27% 3 10% 29% 25% 6%	630
	11th to 20th	Day	17% 3 11% 31% 27% 11%	630
	21st to end	Day	16% 2 8% 32% 31% 11%	630
July	1st to 10th	Day	18% 2 7% 31% 30% 12%	621
	11th to 20th	Day	18% 2 8% 28% 30% 15%	618
	21st to end	Day	17% 3 9% 25% 31% 14%	684
August	1st to 10th	Day	21% 2 10% 28% 28% 11%	626
	11th to 20th	Day	21% 4 10% 26% 24% 16%	615
	21st to end	Day	26% 3 11% 28% 22% 10%	693
September	1st to 10th	Day	32% 6% 15% 25% 16% 5%	553
	11th to 20th	Day	26% 7% 20% 20% 22% 5%	428
	21st to end	Day	25% 5% 14% 29% 21% 6%	270
October	1st to 10th	Day	23% 6% 24% 27% 16% 5%	270
	11th to 20th	Day	33% 7% 16% 20% 17% 8%	270
	21st to end	Day	25% 7% 23% 24% 19% 3	297
November	1st to 10th	Day	18% 7% 26% 27% 18% 5%	261
	11th to 20th	Day	19% 7% 26% 33% 12% 4	180
	21st to end	Day	18% 11% 16% 27% 22% 7%	180
December	1st to 10th	Day	12% 3 25% 35% 21% 5%	153
	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% 4 23% 38% 21% 6%	923
	21st to end	Night	8% 5% 18% 36% 26% 6%	990
February	1st to 10th	Night	5% 3 21% 36% 28% 6%	900
	11th to 20th	Night	6% 2 17% 42% 27% 6%	879
	21st to end	Night	5% 2 17% 38% 29% 9%	740
March	1st to 10th	Night	8% 3 17% 35% 29% 7%	819
	11th to 20th	Night	9% 3 19% 38% 24% 7%	540
	21st to end	Night	9% 5% 15% 32% 30% 9%	594
April	1st to 10th	Night	14% 2 18% 33% 26% 6%	540
	11th to 20th	Night	8% 2 17% 38% 29% 7%	533
	21st to end	Night	11% 4 15% 33% 30% 6%	540
May	1st to 10th	Night	18% 4 16% 33% 23% 6%	540
	11th to 20th	Night	20% 2 21% 33% 21% 3	377
	21st to end	Night	18% 3 21% 38% 18% 3	346
June	1st to 10th	Night	24% 2 12% 36% 23% 3	180
	11th to 20th	Night	17% 4 9% 38% 28% 4	180
	21st to end	Night	18% 2 9% 37% 29% 4	180
July	1st to 10th	Night	18% 2 10% 40% 26% 4	175
	11th to 20th	Night	16% 2 9% 32% 34% 7%	187
	21st to end	Night	16% 2 10% 30% 33% 9%	480
August	1st to 10th	Night	24% 2 8% 27% 31% 8%	537
	11th to 20th	Night	22% 4 11% 31% 26% 7%	534
	21st to end	Night	27% 3 11% 29% 25% 4	594
September	1st to 10th	Night	32% 3 18% 28% 16% 3	540
	11th to 20th	Night	33% 6% 17% 26% 15% 3	540
	21st to end	Night	33% 5% 20% 25% 14% 3	606
October	1st to 10th	Night	36% 9% 21% 23% 10% 1	720
	11th to 20th	Night	42% 6% 14% 18% 15% 5%	805
	21st to end	Night	34% 7% 18% 24% 14% 3	990
November	1st to 10th	Night	25% 9% 22% 26% 14% 3	925
	11th to 20th	Night	18% 5% 23% 37% 14% 2	950
	21st to end	Night	13% 8% 25% 29% 20% 6%	950
December	1st to 10th	Night	11% 4 23% 39% 20% 4	950
	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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