Whitehorse
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Whitehorse clear sky chart from 2005-11-06 to 2025-10-19.

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	7% 10% 4 11% 8% 7% 7% 6% 8% 9% 24%	131316
	Night	4 10% 5% 12% 8% 8% 7% 6% 8% 9% 22%	126985
Transparency	Day	53% 9% 15% 17% 6%	130965
	Night	55% 9% 13% 15% 6% 1	126915
Seeing	Day	16% 13% 26% 26% 15% 4	18479
	Night	11% 10% 28% 30% 18% 3	24491

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	1 6% 4 11% 11% 12% 10% 7% 11% 11% 16%	2003
	11th to 20th	Day	1 11% 5% 14% 7% 8% 7% 8% 13% 9% 17%	2495
	21st to end	Day	3 12% 5% 12% 10% 8% 7% 5% 8% 12% 18%	3010
February	1st to 10th	Day	1 10% 5% 13% 7% 8% 9% 7% 8% 9% 22%	1411
	11th to 20th	Day	3 12% 6% 13% 8% 7% 6% 6% 7% 8% 24%	1560
	21st to end	Day	1 9% 6% 16% 10% 8% 7% 5% 6% 9% 23%	1455
March	1st to 10th	Day	1 7% 4 10% 9% 9% 8% 7% 10% 10% 24%	4424
	11th to 20th	Day	1 9% 5% 13% 10% 10% 8% 6% 7% 8% 23%	4711
	21st to end	Day	1 8% 5% 13% 7% 6% 7% 6% 10% 10% 27%	5918
April	1st to 10th	Day	1 13% 6% 14% 8% 7% 7% 6% 7% 9% 23%	2424
	11th to 20th	Day	2 13% 5% 13% 8% 6% 6% 5% 8% 9% 26%	2766
	21st to end	Day	3 14% 5% 13% 8% 6% 7% 6% 7% 7% 25%	2800
May	1st to 10th	Day	5% 13% 4 11% 7% 7% 7% 4 7% 8% 26%	7115
	11th to 20th	Day	5% 14% 5% 12% 8% 8% 7% 5% 7% 8% 21%	7193
	21st to end	Day	5% 8% 4 10% 8% 8% 8% 6% 8% 8% 28%	8310
June	1st to 10th	Day	12% 10% 3 11% 7% 7% 6% 6% 6% 8% 25%	3600
	11th to 20th	Day	13% 10% 4 11% 8% 6% 6% 6% 6% 8% 22%	3600
	21st to end	Day	8% 9% 4 11% 8% 7% 7% 6% 8% 9% 23%	3600
July	1st to 10th	Day	8% 8% 3 10% 7% 7% 7% 6% 8% 9% 25%	8160
	11th to 20th	Day	10% 10% 4 11% 7% 7% 7% 5% 7% 9% 22%	7700
	21st to end	Day	12% 8% 4 9% 6% 6% 6% 6% 8% 9% 26%	7876
August	1st to 10th	Day	8% 6% 2 7% 5% 5% 5% 4 6% 8% 43%	5112
	11th to 20th	Day	15% 10% 4 8% 5% 5% 5% 4 6% 8% 31%	4542
	21st to end	Day	15% 11% 3 9% 7% 6% 6% 5% 8% 8% 22%	5027
September	1st to 10th	Day	15% 12% 3 11% 6% 7% 6% 5% 6% 6% 23%	2447
	11th to 20th	Day	10% 10% 4 9% 5% 6% 7% 6% 7% 7% 29%	2400
	21st to end	Day	10% 14% 5% 12% 7% 6% 6% 5% 6% 7% 21%	2130
October	1st to 10th	Day	7% 13% 5% 14% 9% 7% 8% 5% 8% 7% 18%	3270
	11th to 20th	Day	5% 12% 6% 15% 8% 6% 7% 5% 6% 8% 22%	2896
	21st to end	Day	8% 15% 5% 13% 9% 8% 7% 6% 8% 7% 14%	2733
November	1st to 10th	Day	4 14% 8% 15% 12% 8% 8% 7% 6% 5% 13%	1372
	11th to 20th	Day	4 15% 9% 16% 11% 7% 7% 7% 6% 8% 10%	1215
	21st to end	Day	2 11% 5% 16% 9% 9% 9% 6% 11% 6% 14%	1075
December	1st to 10th	Day	4 10% 5% 13% 9% 8% 9% 6% 9% 16% 10%	1484
	11th to 20th	Day	1 8% 4 11% 8% 8% 9% 9% 11% 14% 17%	1502
	21st to end	Day	2 11% 5% 14% 11% 8% 9% 7% 10% 9% 12%	1980
January	1st to 10th	Night	2 8% 4 12% 12% 11% 10% 7% 9% 10% 15%	7985
	11th to 20th	Night	2 11% 5% 13% 8% 7% 7% 6% 11% 10% 18%	7402
	21st to end	Night	3 10% 4 13% 10% 10% 7% 6% 8% 10% 19%	7947
February	1st to 10th	Night	3 10% 5% 12% 10% 8% 9% 7% 8% 9% 19%	2989
	11th to 20th	Night	1 11% 5% 15% 10% 9% 7% 6% 7% 8% 21%	2727
	21st to end	Night	2 10% 6% 13% 11% 7% 7% 6% 7% 8% 23%	2174
March	1st to 10th	Night	7% 3 11% 10% 11% 10% 7% 9% 9% 24%	5450
	11th to 20th	Night	2 10% 6% 12% 10% 9% 8% 6% 8% 9% 21%	5258
	21st to end	Night	1 6% 4 13% 8% 7% 7% 6% 9% 10% 28%	5033
April	1st to 10th	Night	1 9% 5% 13% 8% 6% 6% 5% 8% 8% 29%	1975
	11th to 20th	Night	2 8% 4 12% 7% 7% 7% 6% 7% 10% 31%	1583
	21st to end	Night	4 10% 5% 9% 5% 5% 6% 6% 9% 9% 32%	1600
May	1st to 10th	Night	3 10% 4 9% 6% 6% 5% 4 7% 9% 36%	2828
	11th to 20th	Night	5% 10% 4 12% 6% 7% 7% 6% 7% 8% 27%	2702
	21st to end	Night	4 5% 2 8% 6% 7% 7% 5% 8% 10% 37%	2563
June	1st to 10th	Night	8% 9% 3 7% 6% 7% 6% 4 7% 10% 33%	800
	11th to 20th	Night	11% 9% 5% 7% 8% 7% 7% 6% 9% 9% 24%	800
	21st to end	Night	6% 7% 4 10% 6% 7% 6% 8% 7% 11% 28%	800
July	1st to 10th	Night	7% 6% 2 9% 6% 6% 7% 4 8% 10% 35%	1800
	11th to 20th	Night	6% 7% 4 9% 7% 6% 6% 5% 9% 11% 31%	2231
	21st to end	Night	13% 8% 3 8% 6% 6% 7% 5% 9% 8% 28%	2967
August	1st to 10th	Night	7% 4 1 5% 5% 5% 4 3 7% 8% 50%	2040
	11th to 20th	Night	12% 8% 3 7% 4 4 5% 4 7% 9% 35%	2516
	21st to end	Night	12% 10% 4 10% 7% 6% 6% 5% 7% 7% 26%	2882
September	1st to 10th	Night	10% 8% 3 8% 7% 8% 6% 5% 8% 9% 28%	1952
	11th to 20th	Night	7% 7% 4 9% 7% 5% 6% 4 6% 10% 36%	2000
	21st to end	Night	11% 14% 5% 11% 7% 5% 6% 4 7% 8% 23%	2269
October	1st to 10th	Night	8% 13% 4 10% 6% 6% 7% 6% 8% 8% 24%	3920
	11th to 20th	Night	6% 13% 6% 16% 7% 6% 6% 5% 6% 7% 22%	4207
	21st to end	Night	5% 17% 6% 15% 8% 7% 6% 5% 6% 7% 18%	4825
November	1st to 10th	Night	4 13% 6% 17% 10% 10% 8% 6% 7% 7% 14%	2926
	11th to 20th	Night	3 17% 7% 16% 11% 8% 7% 6% 8% 5% 11%	3181
	21st to end	Night	3 13% 6% 16% 11% 8% 8% 7% 9% 7% 13%	3305
December	1st to 10th	Night	3 12% 6% 14% 9% 8% 7% 6% 9% 13% 14%	5682
	11th to 20th	Night	2 9% 4 14% 10% 9% 9% 7% 11% 11% 15%	6690
	21st to end	Night	2 12% 6% 16% 10% 8% 8% 6% 8% 8% 15%	8976

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% 10% 8% 8% 11% 2	2003
	11th to 20th	Day	55% 9% 12% 14% 9%	2512
	21st to end	Day	56% 10% 14% 13% 6% 1	3010
February	1st to 10th	Day	55% 7% 12% 15% 11% 1	1411
	11th to 20th	Day	57% 7% 12% 13% 11%	1568
	21st to end	Day	55% 8% 16% 15% 5%	1455
March	1st to 10th	Day	50% 10% 12% 17% 10% 1	4424
	11th to 20th	Day	53% 9% 13% 13% 11% 2	4711
	21st to end	Day	47% 9% 17% 19% 7% 1	5918
April	1st to 10th	Day	56% 8% 17% 14% 6%	2424
	11th to 20th	Day	53% 7% 15% 18% 7% 1	2766
	21st to end	Day	56% 7% 14% 17% 6%	2800
May	1st to 10th	Day	50% 8% 11% 20% 11%	7115
	11th to 20th	Day	55% 8% 11% 20% 6%	7196
	21st to end	Day	46% 8% 11% 22% 13%	8311
June	1st to 10th	Day	54% 7% 13% 18% 7% 1	3600
	11th to 20th	Day	58% 7% 15% 17% 3	3600
	21st to end	Day	53% 8% 16% 19% 4	3600
July	1st to 10th	Day	47% 9% 21% 19% 3	8071
	11th to 20th	Day	55% 9% 20% 14% 1	7605
	21st to end	Day	48% 11% 27% 12% 1	7655
August	1st to 10th	Day	37% 7% 19% 23% 14% 1	5093
	11th to 20th	Day	50% 6% 18% 20% 5%	4585
	21st to end	Day	54% 9% 19% 17% 1	5026
September	1st to 10th	Day	60% 8% 14% 16% 3	2447
	11th to 20th	Day	51% 9% 17% 20% 3	2400
	21st to end	Day	61% 8% 13% 15% 3	2130
October	1st to 10th	Day	61% 10% 10% 17% 2	3270
	11th to 20th	Day	58% 6% 18% 16% 2	2896
	21st to end	Day	62% 14% 15% 7% 2	2733
November	1st to 10th	Day	72% 8% 11% 8% 2	1372
	11th to 20th	Day	72% 7% 10% 10% 1	1215
	21st to end	Day	67% 8% 12% 8% 5%	1075
December	1st to 10th	Day	60% 18% 12% 8% 2	1484
	11th to 20th	Day	55% 11% 13% 14% 7%	1504
	21st to end	Day	69% 9% 7% 10% 5%	1980
January	1st to 10th	Night	65% 9% 7% 7% 12%	7985
	11th to 20th	Night	54% 9% 11% 13% 10% 3	7447
	21st to end	Night	57% 11% 11% 13% 6% 2	7947
February	1st to 10th	Night	58% 7% 12% 14% 8% 2	2989
	11th to 20th	Night	58% 7% 12% 13% 8% 1	2730
	21st to end	Night	57% 8% 13% 15% 7%	2174
March	1st to 10th	Night	51% 9% 10% 21% 9% 1	5450
	11th to 20th	Night	52% 9% 10% 19% 9% 1	5260
	21st to end	Night	48% 8% 17% 17% 10%	5033
April	1st to 10th	Night	48% 8% 19% 19% 6%	1975
	11th to 20th	Night	47% 6% 17% 22% 8% 1	1583
	21st to end	Night	43% 8% 17% 25% 8%	1600
May	1st to 10th	Night	41% 7% 13% 28% 11% 1	2825
	11th to 20th	Night	46% 11% 13% 17% 11% 1	2702
	21st to end	Night	36% 9% 12% 31% 12%	2563
June	1st to 10th	Night	44% 7% 17% 24% 8%	800
	11th to 20th	Night	51% 9% 16% 20% 4	800
	21st to end	Night	46% 10% 19% 22% 3	800
July	1st to 10th	Night	40% 7% 25% 25% 3	1780
	11th to 20th	Night	43% 9% 28% 18% 3	2204
	21st to end	Night	48% 11% 26% 13% 2	2883
August	1st to 10th	Night	31% 7% 21% 26% 14%	2034
	11th to 20th	Night	43% 7% 20% 23% 6% 1	2539
	21st to end	Night	52% 10% 19% 17% 2	2882
September	1st to 10th	Night	51% 8% 20% 18% 2	1952
	11th to 20th	Night	43% 8% 21% 23% 5%	2000
	21st to end	Night	57% 8% 13% 18% 4	2269
October	1st to 10th	Night	54% 9% 14% 20% 3	3920
	11th to 20th	Night	59% 8% 13% 18% 2	4207
	21st to end	Night	62% 11% 15% 10% 2	4825
November	1st to 10th	Night	69% 9% 12% 8% 3	2926
	11th to 20th	Night	71% 9% 10% 9% 2	3181
	21st to end	Night	66% 8% 12% 12% 4	3305
December	1st to 10th	Night	60% 16% 11% 10% 2	5682
	11th to 20th	Night	57% 11% 13% 14% 4 1	6687
	21st to end	Night	67% 9% 7% 9% 6% 2	8976

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 14% 24% 33% 23% 2	100
	11th to 20th	Day	11% 12% 27% 26% 17% 6%	155
	21st to end	Day	11% 14% 39% 20% 13% 3	225
February	1st to 10th	Day	7% 12% 34% 30% 14% 4	400
	11th to 20th	Day	6% 16% 32% 31% 12% 2	395
	21st to end	Day	11% 14% 35% 28% 10% 3	331
March	1st to 10th	Day	8% 14% 32% 29% 13% 4	400
	11th to 20th	Day	9% 16% 32% 25% 16% 3	400
	21st to end	Day	8% 13% 29% 27% 17% 5%	538
April	1st to 10th	Day	12% 15% 35% 25% 11% 2	519
	11th to 20th	Day	13% 15% 27% 28% 14% 3	690
	21st to end	Day	14% 18% 28% 27% 10% 4	700
May	1st to 10th	Day	20% 22% 29% 21% 7% 2	703
	11th to 20th	Day	15% 15% 28% 24% 14% 5%	725
	21st to end	Day	14% 14% 26% 27% 16% 4	904
June	1st to 10th	Day	17% 11% 23% 28% 17% 4	930
	11th to 20th	Day	19% 8% 23% 25% 21% 5%	930
	21st to end	Day	13% 8% 17% 29% 26% 7%	930
July	1st to 10th	Day	16% 7% 19% 29% 23% 5%	916
	11th to 20th	Day	18% 12% 19% 27% 18% 6%	755
	21st to end	Day	17% 11% 17% 29% 20% 6%	794
August	1st to 10th	Day	17% 11% 21% 28% 17% 5%	726
	11th to 20th	Day	25% 15% 25% 22% 11% 2	715
	21st to end	Day	23% 12% 25% 25% 13% 2	803
September	1st to 10th	Day	25% 12% 25% 19% 14% 4	558
	11th to 20th	Day	18% 15% 23% 24% 15% 4	530
	21st to end	Day	24% 21% 25% 17% 11% 3	483
October	1st to 10th	Day	23% 21% 27% 19% 9% 1	420
	11th to 20th	Day	21% 19% 27% 22% 7% 4	415
	21st to end	Day	16% 13% 30% 27% 11% 3	420
November	1st to 10th	Day	14% 17% 34% 23% 11% 2	200
	11th to 20th	Day	13% 10% 28% 33% 13% 4	200
	21st to end	Day	5% 13% 35% 27% 13% 9%	200
December	1st to 10th	Day	7% 17% 28% 28% 14% 6%	159
	11th to 20th	Day	8% 9% 28% 34% 19% 2	100
	21st to end	Day	6% 13% 30% 27% 16% 7%	110
January	1st to 10th	Night	8% 8% 29% 37% 16% 2	1030
	11th to 20th	Night	11% 10% 38% 26% 13% 2	900
	21st to end	Night	9% 12% 38% 27% 12% 1	990
February	1st to 10th	Night	8% 9% 30% 37% 14% 1	900
	11th to 20th	Night	6% 12% 37% 32% 12% 2	884
	21st to end	Night	7% 10% 35% 32% 13% 2	743
March	1st to 10th	Night	5% 11% 29% 33% 20% 1	817
	11th to 20th	Night	6% 10% 27% 34% 20% 3	724
	21st to end	Night	5% 9% 24% 27% 31% 4	660
April	1st to 10th	Night	6% 8% 27% 33% 23% 4	600
	11th to 20th	Night	6% 9% 24% 30% 26% 5%	593
	21st to end	Night	10% 8% 20% 30% 28% 4	600
May	1st to 10th	Night	12% 10% 25% 28% 21% 4	404
	11th to 20th	Night	9% 11% 24% 29% 24% 3	394
	21st to end	Night	7% 8% 20% 34% 29% 2	377
June	1st to 10th	Night	13% 5% 22% 33% 25% 4	200
	11th to 20th	Night	13% 5% 15% 33% 34% 2	200
	21st to end	Night	10% 4 17% 33% 34% 4	200
July	1st to 10th	Night	12% 2 14% 37% 30% 5%	195
	11th to 20th	Night	16% 7% 14% 29% 29% 5%	219
	21st to end	Night	16% 5% 19% 31% 26% 4	438
August	1st to 10th	Night	15% 7% 20% 32% 22% 5%	446
	11th to 20th	Night	19% 8% 21% 27% 20% 5%	594
	21st to end	Night	23% 5% 21% 30% 18% 4	660
September	1st to 10th	Night	15% 9% 22% 30% 20% 3	600
	11th to 20th	Night	13% 11% 23% 30% 20% 4	600
	21st to end	Night	22% 12% 23% 26% 14% 2	785
October	1st to 10th	Night	20% 12% 28% 24% 15% 1	800
	11th to 20th	Night	21% 13% 23% 27% 13% 3	797
	21st to end	Night	15% 9% 28% 32% 14% 2	946
November	1st to 10th	Night	11% 13% 29% 32% 13% 3	880
	11th to 20th	Night	11% 9% 31% 30% 17% 2	900
	21st to end	Night	10% 10% 34% 28% 15% 3	1005
December	1st to 10th	Night	9% 14% 32% 29% 14% 2	1100
	11th to 20th	Night	10% 9% 33% 31% 14% 3	1100
	21st to end	Night	10% 10% 33% 29% 15% 3	1210

Page generated at 2025-10-15 21:50:30. Version ac07

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