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 2025-05-26.

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% 8% 3 7% 4 4 5% 4 6% 8% 44%	117446
	Night	7% 9% 3 7% 5% 5% 5% 4 6% 8% 42%	120613
Transparency	Day	21% 43% 8% 16% 9% 2	117169
	Night	24% 41% 9% 16% 8% 2	120481
Seeing	Day	13% 10% 27% 28% 17% 5%	16129
	Night	12% 10% 33% 32% 13% 1	25513

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	5% 19% 6% 11% 6% 5% 4 4 6% 6% 28%	3179
	11th to 20th	Day	5% 11% 4 9% 6% 6% 10% 5% 6% 6% 33%	3203
	21st to end	Day	2 10% 5% 7% 4 4 5% 3 5% 7% 48%	4004
February	1st to 10th	Day	6% 14% 5% 11% 7% 6% 5% 4 5% 8% 30%	1774
	11th to 20th	Day	7% 15% 5% 10% 7% 6% 5% 5% 7% 6% 29%	1755
	21st to end	Day	4 11% 5% 12% 7% 5% 5% 3 5% 8% 36%	1533
March	1st to 10th	Day	4 9% 3 8% 4 5% 4 4 6% 9% 45%	4688
	11th to 20th	Day	5% 7% 3 9% 5% 5% 6% 4 6% 9% 41%	5000
	21st to end	Day	9% 15% 6% 12% 6% 5% 5% 5% 7% 7% 24%	5486
April	1st to 10th	Day	9% 11% 3 8% 7% 5% 5% 4 6% 7% 34%	2411
	11th to 20th	Day	11% 14% 4 9% 5% 4 4 3 5% 8% 33%	2567
	21st to end	Day	12% 8% 3 9% 5% 5% 5% 4 6% 8% 36%	2600
May	1st to 10th	Day	21% 13% 3 6% 4 3 4 4 5% 6% 31%	4372
	11th to 20th	Day	11% 7% 2 7% 5% 4 6% 5% 8% 9% 36%	4641
	21st to end	Day	17% 8% 3 7% 5% 5% 6% 4 6% 7% 31%	5239
June	1st to 10th	Day	14% 7% 2 6% 5% 5% 5% 4 7% 7% 39%	2850
	11th to 20th	Day	16% 6% 2 6% 6% 5% 5% 5% 6% 8% 35%	2850
	21st to end	Day	7% 4 1 4 4 4 5% 4 7% 9% 51%	2850
July	1st to 10th	Day	5% 3 1 4 4 3 4 4 6% 8% 58%	4740
	11th to 20th	Day	4 2 1 3 2 3 3 3 5% 9% 65%	4740
	21st to end	Day	2 1 1 2 2 3 3 3 5% 7% 71%	5063
August	1st to 10th	Day	8% 4 2 4 3 4 5% 3 6% 10% 51%	4159
	11th to 20th	Day	5% 3 1 4 3 4 5% 4 6% 9% 55%	4028
	21st to end	Day	7% 3 1 3 3 3 3 3 5% 6% 60%	4521
September	1st to 10th	Day	11% 4 2 4 3 3 3 3 5% 6% 56%	2348
	11th to 20th	Day	12% 5% 2 5% 4 3 5% 3 5% 7% 49%	2090
	21st to end	Day	11% 5% 2 5% 3 3 3 3 5% 7% 53%	2090
October	1st to 10th	Day	7% 6% 2 6% 3 4 4 4 6% 8% 50%	3234
	11th to 20th	Day	4 6% 2 6% 4 4 4 4 6% 8% 52%	3150
	21st to end	Day	6% 8% 3 6% 5% 3 4 4 7% 10% 46%	3106
November	1st to 10th	Day	13% 13% 3 7% 5% 4 4 4 5% 6% 35%	1592
	11th to 20th	Day	5% 10% 3 9% 5% 5% 5% 5% 6% 8% 40%	1600
	21st to end	Day	5% 12% 4 8% 6% 5% 5% 3 5% 9% 37%	1550
December	1st to 10th	Day	4 11% 4 9% 6% 4 6% 4 5% 8% 38%	2290
	11th to 20th	Day	4 11% 4 10% 5% 7% 5% 4 6% 9% 35%	2645
	21st to end	Day	5% 10% 4 8% 5% 4 4 4 5% 7% 42%	3498
January	1st to 10th	Night	5% 17% 6% 9% 5% 4 4 4 6% 8% 32%	6766
	11th to 20th	Night	4 15% 4 10% 4 6% 6% 4 6% 7% 34%	6780
	21st to end	Night	3 8% 4 9% 5% 5% 5% 4 5% 7% 45%	6975
February	1st to 10th	Night	7% 13% 5% 10% 6% 5% 5% 4 6% 7% 32%	2625
	11th to 20th	Night	5% 13% 4 10% 7% 6% 5% 5% 6% 7% 32%	2532
	21st to end	Night	6% 12% 5% 10% 6% 6% 5% 4 5% 8% 34%	2095
March	1st to 10th	Night	3 8% 5% 9% 6% 5% 5% 4 6% 9% 40%	5205
	11th to 20th	Night	3 6% 3 7% 6% 5% 6% 5% 7% 9% 43%	5000
	21st to end	Night	12% 16% 5% 10% 4 4 4 4 6% 9% 26%	5489
April	1st to 10th	Night	9% 11% 4 7% 5% 5% 5% 4 6% 7% 38%	1983
	11th to 20th	Night	10% 11% 3 8% 4 4 5% 4 7% 7% 36%	1781
	21st to end	Night	12% 10% 2 7% 5% 5% 7% 4 5% 8% 36%	1800
May	1st to 10th	Night	20% 10% 3 6% 4 4 5% 3 5% 6% 34%	2818
	11th to 20th	Night	9% 5% 2 7% 5% 5% 5% 4 7% 10% 40%	2475
	21st to end	Night	17% 8% 2 7% 5% 6% 5% 5% 5% 7% 33%	2452
June	1st to 10th	Night	11% 6% 2 4 4 5% 5% 5% 7% 10% 41%	1330
	11th to 20th	Night	16% 4 2 5% 5% 5% 5% 4 5% 8% 41%	1330
	21st to end	Night	7% 4 1 3 3 3 4 4 7% 9% 55%	1330
July	1st to 10th	Night	5% 2 1 3 3 4 3 4 7% 9% 59%	2220
	11th to 20th	Night	4 2 1 3 2 2 2 2 5% 8% 68%	2220
	21st to end	Night	2 2 1 3 3 3 3 3 5% 7% 68%	2473
August	1st to 10th	Night	9% 3 1 4 3 3 4 3 6% 8% 55%	2776
	11th to 20th	Night	4 3 1 4 4 4 4 4 8% 12% 50%	2811
	21st to end	Night	10% 4 1 4 3 3 3 3 4 6% 58%	3146
September	1st to 10th	Night	10% 4 2 4 3 4 3 3 6% 7% 54%	1827
	11th to 20th	Night	11% 7% 2 5% 4 3 3 4 6% 6% 51%	2090
	21st to end	Night	9% 6% 2 5% 3 3 3 4 5% 7% 51%	2090
October	1st to 10th	Night	7% 5% 2 6% 4 4 3 3 5% 7% 53%	3737
	11th to 20th	Night	6% 5% 2 6% 4 4 4 4 6% 8% 51%	3778
	21st to end	Night	5% 6% 3 6% 4 5% 5% 4 7% 7% 47%	4451
November	1st to 10th	Night	11% 12% 4 8% 5% 4 4 3 6% 6% 36%	2686
	11th to 20th	Night	5% 10% 3 8% 6% 5% 6% 4 6% 8% 40%	2800
	21st to end	Night	5% 12% 4 8% 5% 4 4 4 6% 9% 39%	2848
December	1st to 10th	Night	4 11% 3 7% 5% 6% 7% 5% 7% 8% 38%	4890
	11th to 20th	Night	3 10% 5% 12% 6% 6% 5% 4 7% 8% 35%	5546
	21st to end	Night	3 10% 4 8% 4 5% 5% 4 6% 8% 44%	7458

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	45% 29% 8% 15% 2 1	3179
	11th to 20th	Day	36% 31% 10% 10% 12% 1	3203
	21st to end	Day	23% 30% 13% 26% 8%	4004
February	1st to 10th	Day	25% 62% 3 6% 4 1	1774
	11th to 20th	Day	21% 63% 5% 7% 3	1764
	21st to end	Day	18% 61% 6% 9% 5% 1	1533
March	1st to 10th	Day	22% 31% 9% 20% 17%	4688
	11th to 20th	Day	26% 32% 12% 19% 11%	5000
	21st to end	Day	43% 33% 13% 8% 4	5486
April	1st to 10th	Day	19% 63% 5% 9% 4	2411
	11th to 20th	Day	16% 64% 4 10% 5% 1	2570
	21st to end	Day	18% 62% 4 11% 5%	2600
May	1st to 10th	Day	36% 40% 5% 15% 3 1	4372
	11th to 20th	Day	25% 44% 7% 17% 6%	4645
	21st to end	Day	26% 45% 7% 16% 5%	5240
June	1st to 10th	Day	10% 66% 6% 12% 5% 1	2850
	11th to 20th	Day	17% 66% 5% 7% 4 1	2850
	21st to end	Day	13% 65% 5% 12% 4 1	2850
July	1st to 10th	Day	15% 39% 9% 16% 17% 5%	4666
	11th to 20th	Day	8% 37% 7% 23% 17% 7%	4659
	21st to end	Day	6% 36% 5% 14% 21% 18%	4903
August	1st to 10th	Day	19% 39% 6% 19% 15% 2	4143
	11th to 20th	Day	15% 41% 10% 25% 9% 1	4067
	21st to end	Day	15% 38% 6% 26% 14% 2	4520
September	1st to 10th	Day	12% 60% 7% 12% 6% 3	2348
	11th to 20th	Day	17% 61% 5% 11% 4 1	2090
	21st to end	Day	15% 60% 5% 13% 6% 1	2090
October	1st to 10th	Day	18% 38% 7% 23% 12% 2	3234
	11th to 20th	Day	14% 38% 9% 27% 13%	3150
	21st to end	Day	19% 37% 6% 26% 10%	3106
November	1st to 10th	Day	25% 57% 5% 10% 2 1	1592
	11th to 20th	Day	15% 59% 9% 11% 5% 1	1600
	21st to end	Day	17% 58% 8% 10% 6%	1550
December	1st to 10th	Day	29% 37% 10% 20% 2 1	2290
	11th to 20th	Day	30% 35% 15% 18% 2	2644
	21st to end	Day	28% 28% 10% 17% 15% 2	3498
January	1st to 10th	Night	38% 29% 12% 16% 4 1	6766
	11th to 20th	Night	36% 29% 11% 12% 12% 1	6780
	21st to end	Night	24% 31% 13% 24% 7%	6975
February	1st to 10th	Night	22% 62% 4 5% 5% 1	2625
	11th to 20th	Night	20% 62% 5% 7% 5% 1	2533
	21st to end	Night	19% 61% 6% 7% 6% 1	2095
March	1st to 10th	Night	27% 31% 13% 17% 11% 1	5205
	11th to 20th	Night	24% 33% 12% 23% 8%	5000
	21st to end	Night	43% 31% 14% 9% 3	5489
April	1st to 10th	Night	16% 63% 6% 10% 4	1983
	11th to 20th	Night	13% 64% 5% 11% 6% 1	1781
	21st to end	Night	19% 64% 3 10% 4 1	1800
May	1st to 10th	Night	35% 42% 6% 13% 4 1	2818
	11th to 20th	Night	22% 45% 9% 18% 6%	2476
	21st to end	Night	25% 44% 9% 17% 5%	2452
June	1st to 10th	Night	10% 65% 5% 14% 4	1330
	11th to 20th	Night	15% 67% 4 9% 5% 1	1330
	21st to end	Night	11% 65% 5% 12% 7% 1	1330
July	1st to 10th	Night	13% 39% 9% 19% 16% 4	2185
	11th to 20th	Night	8% 37% 7% 24% 14% 9%	2178
	21st to end	Night	9% 35% 4 15% 21% 15%	2396
August	1st to 10th	Night	17% 39% 6% 19% 14% 6%	2768
	11th to 20th	Night	16% 42% 14% 21% 7% 1	2838
	21st to end	Night	19% 37% 6% 27% 9% 2	3146
September	1st to 10th	Night	13% 60% 6% 12% 6% 3	1827
	11th to 20th	Night	14% 61% 5% 13% 5% 2	2090
	21st to end	Night	16% 60% 6% 10% 8%	2090
October	1st to 10th	Night	19% 38% 5% 22% 13% 3	3737
	11th to 20th	Night	16% 38% 7% 24% 13% 1	3778
	21st to end	Night	20% 38% 5% 25% 11% 1	4451
November	1st to 10th	Night	23% 57% 6% 10% 3 1	2686
	11th to 20th	Night	18% 58% 7% 10% 7%	2800
	21st to end	Night	18% 58% 8% 10% 6%	2848
December	1st to 10th	Night	28% 39% 13% 14% 6%	4890
	11th to 20th	Night	31% 36% 19% 11% 3 1	5547
	21st to end	Night	25% 28% 8% 21% 13% 4	7458

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	11% 13% 34% 21% 15% 9%	200
	11th to 20th	Day	9% 12% 32% 33% 11% 3	200
	21st to end	Day	11% 12% 44% 23% 7% 3	220
February	1st to 10th	Day	16% 16% 34% 24% 7% 2	287
	11th to 20th	Day	9% 13% 36% 31% 8% 3	300
	21st to end	Day	11% 15% 34% 26% 12% 2	297
March	1st to 10th	Day	11% 16% 36% 22% 13% 2	500
	11th to 20th	Day	11% 15% 32% 28% 11% 4	500
	21st to end	Day	13% 14% 33% 25% 13% 2	550
April	1st to 10th	Day	15% 15% 33% 27% 9% 1	500
	11th to 20th	Day	15% 15% 32% 24% 11% 2	492
	21st to end	Day	17% 12% 22% 27% 19% 2	500
May	1st to 10th	Day	17% 11% 21% 26% 19% 5%	583
	11th to 20th	Day	17% 9% 20% 28% 20% 6%	725
	21st to end	Day	19% 6% 18% 29% 21% 7%	760
June	1st to 10th	Day	15% 13% 24% 26% 17% 5%	680
	11th to 20th	Day	19% 12% 31% 25% 11% 3	680
	21st to end	Day	9% 7% 26% 32% 21% 5%	680
July	1st to 10th	Day	7% 4 19% 33% 28% 7%	671
	11th to 20th	Day	7% 5% 23% 38% 23% 4	668
	21st to end	Day	3 5% 21% 36% 27% 8%	700
August	1st to 10th	Day	7% 6% 19% 34% 28% 6%	487
	11th to 20th	Day	8% 4 16% 34% 27% 11%	477
	21st to end	Day	8% 5% 25% 30% 23% 8%	539
September	1st to 10th	Day	11% 8% 25% 27% 21% 7%	490
	11th to 20th	Day	14% 8% 24% 31% 16% 6%	490
	21st to end	Day	16% 11% 28% 26% 14% 5%	490
October	1st to 10th	Day	14% 8% 28% 27% 17% 7%	338
	11th to 20th	Day	13% 11% 32% 22% 17% 6%	300
	21st to end	Day	18% 14% 32% 23% 11% 2	330
November	1st to 10th	Day	23% 10% 36% 23% 9%	300
	11th to 20th	Day	8% 15% 44% 23% 8% 1	300
	21st to end	Day	11% 15% 36% 24% 9% 4	275
December	1st to 10th	Day	12% 18% 42% 16% 12% 2	200
	11th to 20th	Day	13% 15% 41% 18% 13% 1	200
	21st to end	Day	13% 11% 30% 27% 13% 6%	220
January	1st to 10th	Night	11% 13% 40% 26% 10% 1	1000
	11th to 20th	Night	10% 13% 40% 26% 11% 1	1000
	21st to end	Night	9% 14% 40% 28% 8% 2	890
February	1st to 10th	Night	13% 14% 32% 29% 11% 2	800
	11th to 20th	Night	11% 13% 35% 30% 11% 1	784
	21st to end	Night	10% 10% 33% 31% 13% 2	660
March	1st to 10th	Night	8% 11% 35% 31% 14% 1	800
	11th to 20th	Night	9% 12% 35% 33% 10% 1	800
	21st to end	Night	14% 12% 33% 28% 11% 1	880
April	1st to 10th	Night	13% 11% 34% 30% 11% 1	678
	11th to 20th	Night	13% 10% 38% 28% 10% 1	593
	21st to end	Night	17% 10% 30% 30% 14%	600
May	1st to 10th	Night	14% 8% 28% 32% 18% 1	600
	11th to 20th	Night	11% 7% 24% 40% 18% 1	522
	21st to end	Night	23% 6% 17% 36% 15% 3	422
June	1st to 10th	Night	15% 8% 28% 33% 15% 1	380
	11th to 20th	Night	17% 11% 33% 28% 11%	380
	21st to end	Night	9% 4 22% 45% 19%	380
July	1st to 10th	Night	7% 4 23% 46% 21% 1	370
	11th to 20th	Night	9% 2 24% 46% 18% 1	368
	21st to end	Night	5% 3 21% 47% 23% 1	416
August	1st to 10th	Night	8% 3 22% 42% 24% 2	521
	11th to 20th	Night	7% 4 21% 43% 24% 1	564
	21st to end	Night	10% 4 24% 42% 20%	627
September	1st to 10th	Night	12% 2 25% 40% 19% 2	571
	11th to 20th	Night	13% 4 22% 37% 22% 1	760
	21st to end	Night	12% 8% 29% 33% 16% 2	760
October	1st to 10th	Night	15% 7% 24% 35% 17% 1	760
	11th to 20th	Night	13% 9% 34% 29% 12% 3	760
	21st to end	Night	11% 13% 37% 29% 9% 1	836
November	1st to 10th	Night	17% 11% 39% 26% 7%	931
	11th to 20th	Night	10% 16% 41% 24% 8% 1	1000
	21st to end	Night	9% 11% 41% 30% 8% 1	1000
December	1st to 10th	Night	10% 12% 35% 33% 9% 2	1000
	11th to 20th	Night	10% 11% 40% 28% 9% 2	1000
	21st to end	Night	12% 12% 39% 27% 9% 2	1100

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