Writing-on-Stone Provincial Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Writing-on-Stone Provincial Park clear sky chart from 2005-11-06 to 2026-10-14.

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	8% 7% 3 6% 4 4 5% 4 6% 8% 46%	159083
	Night	6% 8% 3 7% 4 4 5% 4 6% 8% 44%	150036
Transparency	Day	23% 34% 10% 19% 11% 3	153733
	Night	24% 35% 11% 18% 9% 2	146397
Seeing	Day	12% 10% 27% 28% 17% 5%	18077
	Night	11% 10% 33% 32% 13% 1	27202

By Month

Percentage Cloud Cover SW (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 15% 5% 9% 6% 5% 4 4 7% 9% 31%	4124
	11th to 20th	Day	4 10% 5% 8% 5% 6% 8% 5% 6% 7% 36%	4177
	21st to end	Day	2 9% 5% 9% 5% 5% 5% 3 5% 7% 46%	5187
February	1st to 10th	Day	6% 13% 4 11% 7% 5% 5% 4 6% 8% 30%	1863
	11th to 20th	Day	7% 14% 4 9% 7% 6% 5% 5% 7% 6% 30%	1845
	21st to end	Day	4 10% 5% 11% 7% 5% 5% 4 5% 8% 36%	1607
March	1st to 10th	Day	4 9% 3 8% 4 5% 4 4 6% 9% 45%	4792
	11th to 20th	Day	5% 7% 3 9% 5% 5% 6% 4 6% 9% 41%	5110
	21st to end	Day	8% 15% 6% 12% 6% 5% 5% 5% 7% 7% 24%	5607
April	1st to 10th	Day	9% 11% 3 8% 7% 5% 5% 5% 6% 7% 34%	2532
	11th to 20th	Day	10% 13% 4 9% 5% 4 4 3 5% 8% 33%	2697
	21st to end	Day	12% 9% 3 8% 5% 4 5% 4 6% 8% 36%	2730
May	1st to 10th	Day	13% 8% 2 5% 3 4 4 4 6% 8% 42%	7864
	11th to 20th	Day	12% 8% 3 7% 5% 5% 6% 5% 8% 9% 33%	8391
	21st to end	Day	13% 6% 2 5% 4 5% 6% 4 6% 9% 39%	9648
June	1st to 10th	Day	15% 6% 2 6% 4 5% 5% 4 7% 7% 38%	3150
	11th to 20th	Day	15% 6% 2 6% 6% 5% 5% 5% 6% 8% 35%	3150
	21st to end	Day	8% 4 1 4 4 4 5% 4 7% 9% 50%	3150
July	1st to 10th	Day	5% 3 1 3 3 3 4 4 5% 8% 62%	8819
	11th to 20th	Day	7% 4 1 4 3 3 4 4 6% 9% 56%	8820
	21st to end	Day	4 3 1 3 2 2 3 3 4 7% 68%	9444
August	1st to 10th	Day	7% 4 2 5% 3 4 4 3 6% 9% 53%	7668
	11th to 20th	Day	7% 3 1 4 4 4 5% 4 7% 10% 51%	7565
	21st to end	Day	7% 3 1 4 3 3 3 3 4 6% 62%	8374
September	1st to 10th	Day	11% 4 2 4 3 3 3 3 5% 7% 55%	2596
	11th to 20th	Day	12% 5% 2 5% 4 3 4 3 5% 7% 49%	2310
	21st to end	Day	10% 5% 2 4 3 3 3 3 5% 7% 54%	2310
October	1st to 10th	Day	7% 6% 2 6% 3 4 4 4 6% 8% 51%	3438
	11th to 20th	Day	5% 6% 2 6% 4 4 4 4 6% 8% 51%	3280
	21st to end	Day	6% 8% 2 6% 4 3 4 4 6% 9% 46%	3205
November	1st to 10th	Day	13% 13% 3 7% 5% 4 4 4 5% 7% 35%	1673
	11th to 20th	Day	4 10% 3 9% 5% 5% 5% 5% 6% 8% 40%	1680
	21st to end	Day	5% 12% 4 8% 7% 5% 5% 3 5% 8% 38%	1627
December	1st to 10th	Day	4 11% 4 9% 6% 4 6% 4 5% 8% 38%	2360
	11th to 20th	Day	4 11% 4 9% 5% 7% 5% 4 6% 9% 35%	2715
	21st to end	Day	5% 10% 4 8% 5% 4 4 4 5% 7% 42%	3575
January	1st to 10th	Night	4 13% 5% 9% 5% 4 4 4 6% 9% 36%	8768
	11th to 20th	Night	3 12% 4 9% 4 5% 6% 4 6% 7% 39%	8820
	21st to end	Night	3 8% 4 9% 5% 5% 5% 4 6% 8% 43%	9017
February	1st to 10th	Night	6% 13% 4 10% 6% 5% 5% 4 6% 7% 32%	2756
	11th to 20th	Night	5% 12% 3 10% 7% 6% 5% 5% 6% 7% 33%	2662
	21st to end	Night	5% 11% 5% 10% 6% 6% 5% 4 5% 8% 35%	2197
March	1st to 10th	Night	3 9% 5% 9% 6% 5% 5% 4 6% 9% 40%	5321
	11th to 20th	Night	3 6% 3 7% 6% 5% 6% 5% 7% 9% 42%	5110
	21st to end	Night	12% 16% 5% 10% 5% 4 4 4 6% 9% 26%	5610
April	1st to 10th	Night	9% 11% 4 7% 5% 5% 5% 4 6% 7% 37%	2082
	11th to 20th	Night	10% 11% 3 8% 4 4 4 5% 6% 8% 36%	1871
	21st to end	Night	12% 11% 2 7% 5% 4 7% 4 5% 8% 36%	1890
May	1st to 10th	Night	15% 8% 2 5% 3 4 4 3 5% 7% 42%	5083
	11th to 20th	Night	9% 6% 3 7% 6% 5% 5% 5% 8% 11% 33%	4485
	21st to end	Night	12% 5% 2 6% 5% 5% 5% 4 6% 8% 41%	4525
June	1st to 10th	Night	12% 6% 2 5% 4 4 4 5% 6% 11% 41%	1470
	11th to 20th	Night	15% 4 2 5% 5% 4 6% 4 6% 8% 41%	1470
	21st to end	Night	8% 4 1 3 3 3 4 4 6% 9% 54%	1470
July	1st to 10th	Night	5% 2 1 3 2 3 3 3 5% 8% 66%	4140
	11th to 20th	Night	7% 4 1 5% 3 3 3 3 6% 8% 58%	4140
	21st to end	Night	4 3 1 4 3 3 3 2 5% 7% 65%	4616
August	1st to 10th	Night	8% 4 1 5% 4 4 5% 3 5% 8% 53%	5146
	11th to 20th	Night	7% 4 1 5% 4 4 5% 4 8% 12% 46%	5291
	21st to end	Night	7% 3 1 4 3 2 3 3 5% 7% 62%	5842
September	1st to 10th	Night	10% 4 2 4 3 4 3 3 6% 7% 54%	2019
	11th to 20th	Night	11% 6% 2 4 4 3 3 4 6% 6% 50%	2310
	21st to end	Night	8% 6% 2 5% 3 3 3 4 5% 7% 53%	2310
October	1st to 10th	Night	7% 5% 2 5% 4 4 3 3 5% 7% 53%	3973
	11th to 20th	Night	6% 5% 2 6% 4 4 4 4 6% 7% 51%	3934
	21st to end	Night	5% 6% 3 6% 4 5% 5% 4 6% 8% 48%	4594
November	1st to 10th	Night	11% 12% 4 8% 5% 4 4 3 6% 7% 36%	2824
	11th to 20th	Night	5% 9% 3 8% 6% 5% 6% 4 6% 8% 40%	2940
	21st to end	Night	4 12% 4 8% 5% 4 4 4 6% 9% 40%	2991
December	1st to 10th	Night	4 11% 3 7% 5% 6% 7% 5% 7% 8% 37%	5040
	11th to 20th	Night	4 10% 5% 11% 6% 6% 5% 4 6% 8% 35%	5696
	21st to end	Night	3 10% 4 8% 4 5% 5% 4 6% 8% 44%	7623

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	39% 24% 13% 18% 5% 1	4125
	11th to 20th	Day	35% 25% 15% 13% 10% 2	4177
	21st to end	Day	28% 24% 14% 24% 9%	5187
February	1st to 10th	Day	25% 59% 4 6% 5% 1	1863
	11th to 20th	Day	21% 60% 5% 8% 5% 1	1854
	21st to end	Day	19% 59% 6% 10% 5% 1	1607
March	1st to 10th	Day	22% 30% 9% 20% 17%	4792
	11th to 20th	Day	26% 32% 12% 19% 11%	5110
	21st to end	Day	43% 32% 12% 8% 4	5607
April	1st to 10th	Day	19% 60% 7% 10% 4	2532
	11th to 20th	Day	17% 61% 5% 12% 5% 1	2700
	21st to end	Day	19% 59% 4 12% 5%	2730
May	1st to 10th	Day	29% 25% 8% 22% 13% 4	7864
	11th to 20th	Day	35% 28% 9% 19% 9% 1	8395
	21st to end	Day	26% 27% 9% 24% 13%	9650
June	1st to 10th	Day	13% 61% 6% 13% 6% 1	3150
	11th to 20th	Day	18% 60% 6% 10% 6% 1	3150
	21st to end	Day	15% 59% 6% 14% 5% 1	3150
July	1st to 10th	Day	15% 22% 13% 21% 22% 7%	8746
	11th to 20th	Day	19% 24% 12% 20% 16% 9%	8739
	21st to end	Day	13% 21% 8% 19% 20% 19%	9233
August	1st to 10th	Day	16% 28% 9% 22% 17% 7%	6071
	11th to 20th	Day	21% 30% 15% 26% 7%	5964
	21st to end	Day	17% 28% 6% 31% 16% 2	6569
September	1st to 10th	Day	13% 55% 8% 15% 6% 2	2596
	11th to 20th	Day	19% 56% 5% 14% 5% 1	2310
	21st to end	Day	14% 55% 6% 15% 9% 2	2310
October	1st to 10th	Day	18% 36% 8% 24% 12% 2	3438
	11th to 20th	Day	15% 37% 9% 27% 12%	3280
	21st to end	Day	19% 36% 7% 26% 11%	3205
November	1st to 10th	Day	26% 55% 6% 10% 2 1	1673
	11th to 20th	Day	16% 57% 9% 13% 4 1	1680
	21st to end	Day	19% 56% 9% 11% 6%	1627
December	1st to 10th	Day	30% 36% 11% 20% 2 1	2360
	11th to 20th	Day	30% 34% 15% 18% 2	2714
	21st to end	Day	28% 27% 10% 18% 15% 2	3575
January	1st to 10th	Night	35% 24% 14% 19% 8% 1	8769
	11th to 20th	Night	33% 23% 16% 15% 11% 1	8820
	21st to end	Night	27% 26% 15% 24% 7%	9017
February	1st to 10th	Night	23% 59% 5% 6% 6% 1	2756
	11th to 20th	Night	20% 59% 5% 8% 6% 1	2663
	21st to end	Night	19% 59% 7% 9% 6% 1	2197
March	1st to 10th	Night	27% 30% 14% 17% 11% 1	5321
	11th to 20th	Night	24% 33% 12% 23% 8%	5110
	21st to end	Night	43% 30% 14% 9% 3	5610
April	1st to 10th	Night	18% 60% 7% 11% 4	2082
	11th to 20th	Night	13% 61% 6% 11% 6% 1	1871
	21st to end	Night	20% 61% 3 11% 4 1	1890
May	1st to 10th	Night	31% 25% 8% 22% 10% 3	5081
	11th to 20th	Night	32% 29% 10% 19% 9%	4486
	21st to end	Night	25% 27% 10% 27% 10% 1	4526
June	1st to 10th	Night	13% 59% 5% 15% 7%	1470
	11th to 20th	Night	16% 62% 5% 11% 6% 1	1470
	21st to end	Night	13% 59% 5% 15% 6% 1	1470
July	1st to 10th	Night	13% 23% 13% 24% 18% 10%	4105
	11th to 20th	Night	20% 23% 11% 23% 14% 9%	4098
	21st to end	Night	16% 20% 10% 17% 17% 19%	4516
August	1st to 10th	Night	15% 28% 7% 20% 21% 10%	4068
	11th to 20th	Night	25% 33% 18% 19% 5% 1	4168
	21st to end	Night	18% 27% 6% 33% 14% 2	4578
September	1st to 10th	Night	15% 55% 7% 14% 7% 2	2019
	11th to 20th	Night	16% 55% 7% 15% 6% 2	2310
	21st to end	Night	16% 55% 6% 13% 9% 1	2310
October	1st to 10th	Night	19% 36% 6% 23% 13% 3	3973
	11th to 20th	Night	17% 37% 7% 24% 13% 1	3934
	21st to end	Night	20% 37% 5% 26% 12% 1	4594
November	1st to 10th	Night	24% 54% 7% 11% 4 1	2824
	11th to 20th	Night	19% 56% 8% 10% 7%	2940
	21st to end	Night	19% 55% 9% 11% 6%	2991
December	1st to 10th	Night	29% 38% 13% 14% 6%	5040
	11th to 20th	Night	32% 35% 19% 11% 3 1	5697
	21st to end	Night	26% 28% 8% 21% 13% 4	7623

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	10% 15% 34% 20% 13% 8%	220
	11th to 20th	Day	10% 15% 33% 30% 10% 3	220
	21st to end	Day	10% 13% 44% 23% 7% 2	242
February	1st to 10th	Day	16% 16% 34% 25% 7% 2	316
	11th to 20th	Day	9% 12% 38% 30% 8% 3	330
	21st to end	Day	11% 16% 34% 25% 12% 2	323
March	1st to 10th	Day	11% 15% 36% 22% 13% 2	540
	11th to 20th	Day	12% 16% 31% 26% 10% 4	540
	21st to end	Day	14% 14% 33% 24% 12% 2	594
April	1st to 10th	Day	15% 14% 32% 28% 10% 1	540
	11th to 20th	Day	15% 15% 31% 25% 11% 2	532
	21st to end	Day	17% 13% 23% 26% 18% 3	540
May	1st to 10th	Day	16% 12% 21% 27% 19% 5%	627
	11th to 20th	Day	17% 10% 20% 28% 19% 6%	775
	21st to end	Day	18% 6% 19% 29% 20% 7%	844
June	1st to 10th	Day	16% 13% 25% 25% 16% 5%	780
	11th to 20th	Day	17% 11% 31% 26% 11% 3	780
	21st to end	Day	10% 6% 26% 32% 21% 5%	780
July	1st to 10th	Day	7% 5% 20% 34% 28% 6%	771
	11th to 20th	Day	8% 5% 23% 37% 23% 4	768
	21st to end	Day	4 5% 22% 35% 26% 8%	806
August	1st to 10th	Day	7% 5% 20% 34% 28% 6%	567
	11th to 20th	Day	9% 4 15% 34% 27% 11%	557
	21st to end	Day	7% 5% 25% 30% 24% 8%	627
September	1st to 10th	Day	11% 8% 25% 27% 21% 8%	570
	11th to 20th	Day	14% 8% 23% 30% 19% 8%	570
	21st to end	Day	14% 11% 29% 26% 16% 5%	570
October	1st to 10th	Day	13% 7% 28% 27% 19% 6%	402
	11th to 20th	Day	14% 12% 31% 22% 17% 5%	339
	21st to end	Day	18% 14% 34% 22% 11% 1	363
November	1st to 10th	Day	24% 9% 36% 22% 8%	330
	11th to 20th	Day	8% 15% 43% 23% 9% 2	330
	21st to end	Day	12% 15% 35% 25% 9% 4	302
December	1st to 10th	Day	12% 17% 42% 17% 11% 1	220
	11th to 20th	Day	13% 16% 40% 17% 12% 1	220
	21st to end	Day	12% 10% 33% 27% 12% 5%	242
January	1st to 10th	Night	10% 13% 40% 26% 9% 1	1050
	11th to 20th	Night	10% 15% 40% 25% 10% 1	1050
	21st to end	Night	8% 14% 40% 28% 8% 2	934
February	1st to 10th	Night	13% 13% 33% 29% 11% 2	840
	11th to 20th	Night	11% 13% 35% 31% 11% 1	824
	21st to end	Night	10% 11% 33% 31% 13% 2	692
March	1st to 10th	Night	8% 11% 35% 31% 14% 1	840
	11th to 20th	Night	9% 13% 35% 32% 10% 1	840
	21st to end	Night	14% 12% 34% 28% 11% 1	924
April	1st to 10th	Night	13% 11% 34% 31% 11% 1	712
	11th to 20th	Night	13% 10% 38% 28% 10% 1	623
	21st to end	Night	18% 10% 29% 30% 13%	630
May	1st to 10th	Night	13% 8% 27% 33% 17% 1	630
	11th to 20th	Night	11% 7% 24% 39% 17% 1	549
	21st to end	Night	22% 6% 16% 36% 16% 3	455
June	1st to 10th	Night	16% 8% 27% 33% 15% 1	420
	11th to 20th	Night	16% 10% 33% 30% 11%	420
	21st to end	Night	10% 5% 21% 46% 17%	420
July	1st to 10th	Night	7% 4 23% 46% 20%	410
	11th to 20th	Night	9% 2 24% 46% 17%	408
	21st to end	Night	6% 3 22% 46% 23% 1	460
August	1st to 10th	Night	8% 3 21% 43% 23% 2	577
	11th to 20th	Night	7% 5% 21% 43% 23% 1	624
	21st to end	Night	10% 4 24% 43% 19%	693
September	1st to 10th	Night	12% 3 25% 40% 19% 2	631
	11th to 20th	Night	13% 4 21% 37% 24% 2	840
	21st to end	Night	11% 8% 28% 34% 16% 2	840
October	1st to 10th	Night	15% 7% 25% 37% 16% 1	840
	11th to 20th	Night	13% 9% 33% 30% 12% 3	812
	21st to end	Night	11% 14% 37% 29% 9% 1	880
November	1st to 10th	Night	17% 11% 39% 26% 6%	979
	11th to 20th	Night	10% 16% 40% 25% 8% 1	1050
	21st to end	Night	9% 11% 41% 30% 8% 1	1050
December	1st to 10th	Night	10% 12% 35% 32% 8% 2	1050
	11th to 20th	Night	11% 11% 40% 27% 9% 2	1050
	21st to end	Night	12% 12% 40% 27% 9% 1	1155

Page generated at 2026-10-11 03:06:26. Version ac07

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