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 2024-03-23.

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% 5% 7% 6% 7% 7% 22%	77365
	Night	11% 12% 6% 11% 10% 5% 8% 8% 9% 7% 15%	82476
Transparency	Day	58% 8% 13% 17% 4	77033
	Night	62% 8% 11% 14% 4	82389
Seeing	Day	18% 5% 20% 30% 20% 7%	14606
	Night	17% 5% 22% 33% 19% 4	23546

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 9% 5% 17% 16% 8% 9% 8% 14% 6% 3	1580
	11th to 20th	Day	3 9% 7% 12% 12% 7% 10% 10% 13% 8% 9%	1769
	21st to end	Day	7% 14% 5% 10% 10% 5% 9% 9% 14% 11% 7%	2442
February	1st to 10th	Day	2 7% 4 10% 10% 7% 11% 11% 14% 12% 12%	1330
	11th to 20th	Day	4 8% 4 8% 7% 7% 11% 10% 15% 16% 10%	1582
	21st to end	Day	3 9% 5% 9% 9% 5% 9% 9% 18% 12% 11%	1413
March	1st to 10th	Day	6% 11% 4 10% 11% 6% 9% 8% 9% 12% 14%	1815
	11th to 20th	Day	7% 10% 4 8% 10% 4 9% 11% 13% 9% 14%	2088
	21st to end	Day	5% 7% 3 10% 9% 7% 12% 12% 8% 8% 20%	2276
April	1st to 10th	Day	8% 13% 4 9% 8% 7% 12% 10% 7% 6% 15%	2257
	11th to 20th	Day	7% 8% 4 9% 11% 7% 9% 7% 5% 5% 27%	2364
	21st to end	Day	6% 8% 5% 11% 13% 8% 9% 6% 5% 8% 21%	2520
May	1st to 10th	Day	14% 15% 5% 14% 11% 4 4 4 3 5% 20%	2686
	11th to 20th	Day	17% 13% 6% 9% 8% 3 4 5% 5% 5% 24%	2818
	21st to end	Day	14% 10% 7% 9% 7% 4 5% 5% 5% 7% 27%	3135
June	1st to 10th	Day	19% 12% 5% 7% 5% 3 5% 5% 5% 7% 28%	3060
	11th to 20th	Day	14% 8% 3 5% 6% 3 5% 5% 6% 7% 38%	3060
	21st to end	Day	16% 6% 3 6% 4 3 4 4 6% 8% 42%	3060
July	1st to 10th	Day	16% 8% 4 7% 6% 3 5% 5% 6% 8% 31%	3060
	11th to 20th	Day	15% 7% 3 6% 5% 3 5% 4 5% 9% 38%	3029
	21st to end	Day	14% 7% 3 7% 5% 3 4 5% 6% 9% 38%	3059
August	1st to 10th	Day	21% 7% 2 5% 5% 3 4 5% 4 7% 38%	2686
	11th to 20th	Day	21% 8% 3 6% 5% 3 4 4 5% 7% 32%	2555
	21st to end	Day	22% 9% 3 6% 6% 3 4 5% 5% 8% 31%	2631
September	1st to 10th	Day	25% 13% 3 8% 6% 3 4 5% 5% 7% 22%	2253
	11th to 20th	Day	24% 18% 4 8% 6% 4 4 4 4 6% 19%	2149
	21st to end	Day	19% 22% 5% 8% 5% 4 4 4 4 5% 21%	1887
October	1st to 10th	Day	24% 23% 7% 8% 5% 3 3 3 3 5% 15%	1800
	11th to 20th	Day	30% 20% 8% 10% 6% 3 3 3 4 4 10%	1651
	21st to end	Day	24% 21% 11% 15% 7% 3 4 2 3 2 8%	1584
November	1st to 10th	Day	13% 20% 8% 19% 13% 5% 6% 4 4 3 5%	1450
	11th to 20th	Day	12% 15% 6% 13% 10% 7% 9% 8% 5% 4 11%	1140
	21st to end	Day	8% 16% 8% 14% 12% 6% 7% 7% 5% 6% 10%	1140
December	1st to 10th	Day	6% 15% 5% 13% 13% 8% 11% 6% 10% 6% 8%	1102
	11th to 20th	Day	5% 10% 7% 12% 13% 10% 8% 9% 10% 8% 9%	1196
	21st to end	Day	7% 15% 6% 12% 10% 7% 12% 9% 8% 6% 9%	1738
January	1st to 10th	Night	4 8% 6% 13% 15% 8% 11% 11% 12% 7% 4	5366
	11th to 20th	Night	3 10% 7% 16% 12% 6% 9% 9% 13% 7% 8%	5131
	21st to end	Night	7% 13% 6% 10% 10% 6% 8% 9% 14% 9% 8%	5225
February	1st to 10th	Night	3 7% 5% 9% 10% 7% 12% 12% 15% 11% 9%	2850
	11th to 20th	Night	3 6% 5% 9% 9% 6% 9% 11% 18% 12% 11%	2483
	21st to end	Night	3 7% 5% 10% 9% 6% 10% 14% 15% 11% 10%	2041
March	1st to 10th	Night	6% 10% 6% 9% 9% 6% 9% 10% 12% 9% 14%	2383
	11th to 20th	Night	6% 8% 4 8% 10% 7% 10% 9% 12% 8% 17%	2090
	21st to end	Night	6% 7% 4 8% 8% 6% 11% 13% 8% 7% 23%	2124
April	1st to 10th	Night	10% 11% 3 8% 8% 5% 9% 9% 9% 8% 21%	1702
	11th to 20th	Night	6% 7% 3 10% 10% 6% 10% 7% 6% 6% 30%	1547
	21st to end	Night	8% 9% 5% 9% 11% 6% 7% 4 5% 6% 31%	1440
May	1st to 10th	Night	14% 14% 7% 12% 9% 4 4 4 3 7% 22%	1272
	11th to 20th	Night	17% 14% 7% 9% 7% 4 5% 4 5% 6% 22%	1077
	21st to end	Night	15% 13% 7% 10% 5% 4 5% 4 5% 6% 25%	1138
June	1st to 10th	Night	17% 13% 6% 8% 7% 2 4 3 5% 8% 26%	900
	11th to 20th	Night	15% 8% 4 6% 5% 4 4 6% 6% 7% 35%	900
	21st to end	Night	14% 9% 5% 5% 7% 3 5% 6% 4 7% 35%	900
July	1st to 10th	Night	15% 10% 4 8% 7% 3 7% 5% 6% 9% 28%	900
	11th to 20th	Night	15% 6% 4 6% 5% 3 5% 6% 4 8% 37%	930
	21st to end	Night	15% 8% 4 7% 6% 3 5% 5% 7% 7% 34%	1258
August	1st to 10th	Night	18% 9% 3 6% 5% 3 4 7% 5% 6% 34%	1253
	11th to 20th	Night	18% 7% 4 6% 6% 4 5% 4 6% 7% 33%	1345
	21st to end	Night	20% 9% 3 7% 5% 4 5% 5% 6% 7% 31%	1723
September	1st to 10th	Night	24% 11% 4 7% 7% 4 6% 5% 5% 6% 20%	1706
	11th to 20th	Night	21% 15% 4 9% 6% 4 4 4 4 6% 23%	1808
	21st to end	Night	20% 19% 4 8% 6% 2 4 3 4 6% 25%	2054
October	1st to 10th	Night	25% 21% 7% 8% 7% 3 4 3 3 4 16%	2160
	11th to 20th	Night	28% 22% 6% 11% 5% 3 3 3 3 3 12%	2305
	21st to end	Night	24% 17% 8% 15% 9% 4 4 4 3 4 8%	2772
November	1st to 10th	Night	15% 20% 11% 16% 12% 5% 5% 4 3 3 5%	2625
	11th to 20th	Night	10% 16% 6% 11% 13% 7% 9% 6% 4 4 12%	3040
	21st to end	Night	8% 15% 8% 14% 10% 4 8% 8% 8% 6% 11%	3040
December	1st to 10th	Night	6% 11% 6% 12% 15% 7% 10% 10% 10% 6% 6%	3074
	11th to 20th	Night	6% 10% 6% 12% 11% 7% 11% 10% 11% 6% 9%	3996
	21st to end	Night	6% 12% 6% 11% 11% 8% 13% 8% 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	59% 7% 10% 20% 4	1815
	11th to 20th	Day	56% 7% 10% 17% 10%	2090
	21st to end	Day	56% 7% 11% 17% 8% 1	2276
April	1st to 10th	Day	65% 8% 8% 14% 5%	2257
	11th to 20th	Day	56% 7% 10% 21% 6%	2364
	21st to end	Day	58% 8% 13% 16% 5%	2520
May	1st to 10th	Day	68% 5% 10% 14% 3	2686
	11th to 20th	Day	60% 7% 11% 16% 6%	2822
	21st to end	Day	54% 9% 13% 20% 5%	3135
June	1st to 10th	Day	54% 7% 13% 20% 5%	3060
	11th to 20th	Day	43% 9% 13% 28% 8%	3060
	21st to end	Day	41% 9% 13% 29% 8%	3060
July	1st to 10th	Day	48% 10% 16% 22% 4	2976
	11th to 20th	Day	43% 9% 18% 28% 2	2936
	21st to end	Day	43% 12% 16% 27% 2	2889
August	1st to 10th	Day	45% 9% 20% 22% 3	2669
	11th to 20th	Day	51% 9% 18% 20% 2	2555
	21st to end	Day	51% 10% 15% 20% 3	2631
September	1st to 10th	Day	61% 10% 14% 14% 2	2253
	11th to 20th	Day	68% 7% 14% 10% 1	2149
	21st to end	Day	66% 6% 13% 14% 2	1887
October	1st to 10th	Day	73% 6% 8% 10% 3 1	1800
	11th to 20th	Day	79% 7% 7% 6% 1	1651
	21st to end	Day	83% 6% 6% 4 1	1584
November	1st to 10th	Day	82% 9% 6% 3	1450
	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	56% 9% 14% 15% 4	2383
	11th to 20th	Night	55% 7% 11% 17% 10%	2090
	21st to end	Night	52% 8% 11% 19% 8% 1	2124
April	1st to 10th	Night	57% 7% 11% 20% 6% 1	1702
	11th to 20th	Night	51% 8% 11% 21% 8% 1	1547
	21st to end	Night	52% 6% 11% 21% 10%	1440
May	1st to 10th	Night	63% 6% 8% 19% 4	1272
	11th to 20th	Night	61% 7% 9% 15% 8%	1077
	21st to end	Night	59% 7% 9% 20% 5%	1138
June	1st to 10th	Night	56% 8% 10% 22% 5%	900
	11th to 20th	Night	45% 8% 11% 27% 9%	900
	21st to end	Night	47% 8% 9% 28% 7%	900
July	1st to 10th	Night	52% 8% 16% 22% 3	875
	11th to 20th	Night	44% 8% 17% 27% 4	900
	21st to end	Night	45% 11% 15% 27% 3	1188
August	1st to 10th	Night	47% 10% 17% 22% 4	1246
	11th to 20th	Night	51% 7% 18% 22% 2	1345
	21st to end	Night	52% 8% 15% 22% 2	1723
September	1st to 10th	Night	62% 10% 15% 11% 1	1706
	11th to 20th	Night	63% 7% 14% 14% 2	1808
	21st to end	Night	61% 8% 13% 14% 3	2054
October	1st to 10th	Night	73% 6% 9% 8% 3	2160
	11th to 20th	Night	78% 7% 7% 7% 2	2305
	21st to end	Night	80% 9% 5% 4 1	2772
November	1st to 10th	Night	83% 7% 8% 2	2625
	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	7% 7% 23% 33% 20% 11%	346
April	1st to 10th	Day	14% 4 21% 32% 20% 9%	517
	11th to 20th	Day	8% 4 21% 35% 24% 8%	590
	21st to end	Day	8% 4 20% 32% 27% 10%	600
May	1st to 10th	Day	18% 5% 23% 28% 17% 9%	603
	11th to 20th	Day	22% 5% 23% 32% 15% 3	625
	21st to end	Day	16% 5% 26% 32% 17% 4	679
June	1st to 10th	Day	26% 5% 19% 28% 18% 4	630
	11th to 20th	Day	18% 4 18% 32% 20% 7%	630
	21st to end	Day	16% 3 17% 34% 24% 7%	630
July	1st to 10th	Day	18% 3 15% 33% 21% 10%	621
	11th to 20th	Day	19% 3 14% 31% 24% 10%	618
	21st to end	Day	17% 4 15% 29% 24% 11%	684
August	1st to 10th	Day	23% 3 16% 27% 23% 8%	626
	11th to 20th	Day	24% 2 16% 26% 22% 11%	615
	21st to end	Day	25% 6% 15% 28% 19% 7%	693
September	1st to 10th	Day	33% 7% 19% 25% 12% 4	543
	11th to 20th	Day	30% 5% 22% 22% 18% 3	439
	21st to end	Day	26% 7% 19% 29% 14% 4	270
October	1st to 10th	Day	26% 5% 31% 21% 14% 3	270
	11th to 20th	Day	33% 7% 18% 21% 12% 9%	270
	21st to end	Day	24% 6% 24% 24% 19% 3	297
November	1st to 10th	Day	16% 8% 26% 32% 14% 3	257
	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	9% 6% 19% 31% 27% 8%	600
April	1st to 10th	Night	15% 4 21% 30% 25% 4	540
	11th to 20th	Night	8% 2 21% 37% 25% 5%	533
	21st to end	Night	11% 5% 17% 38% 25% 4	540
May	1st to 10th	Night	20% 4 19% 34% 20% 3	540
	11th to 20th	Night	20% 6% 21% 32% 18% 3	367
	21st to end	Night	20% 4 27% 34% 13% 1	363
June	1st to 10th	Night	22% 4 19% 36% 18% 1	180
	11th to 20th	Night	16% 6% 19% 37% 20% 3	180
	21st to end	Night	18% 2 16% 39% 24% 1	180
July	1st to 10th	Night	18% 2 17% 39% 22% 3	175
	11th to 20th	Night	16% 1 14% 44% 23% 3	204
	21st to end	Night	17% 1 14% 34% 29% 5%	470
August	1st to 10th	Night	23% 3 10% 34% 27% 3	537
	11th to 20th	Night	21% 4 16% 32% 25% 3	534
	21st to end	Night	24% 3 16% 34% 20% 3	594
September	1st to 10th	Night	30% 4 23% 28% 14% 2	540
	11th to 20th	Night	31% 6% 20% 27% 13% 2	540
	21st to end	Night	30% 5% 26% 25% 11% 3	614
October	1st to 10th	Night	36% 9% 21% 24% 9% 1	720
	11th to 20th	Night	40% 6% 18% 20% 13% 4	793
	21st to end	Night	30% 7% 22% 27% 11% 3	990
November	1st to 10th	Night	23% 8% 27% 27% 12% 2	925
	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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