Drayton Valley
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Drayton Valley clear sky chart from 2005-11-06 to 2024-12-24.

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	10% 9% 3 8% 5% 5% 5% 4 6% 8% 35%	106761
	Night	10% 11% 4 9% 6% 5% 5% 4 6% 7% 32%	101925
Transparency	Day	45% 8% 17% 21% 8% 1	106500
	Night	48% 8% 14% 20% 8% 1	101832
Seeing	Day	14% 8% 26% 30% 17% 5%	16043
	Night	14% 7% 28% 34% 15% 2	24527

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	5% 14% 5% 12% 7% 5% 5% 4 5% 7% 30%	2219
	11th to 20th	Day	7% 11% 4 9% 5% 5% 6% 7% 6% 9% 31%	2199
	21st to end	Day	8% 11% 4 9% 5% 5% 5% 5% 6% 8% 34%	2739
February	1st to 10th	Day	9% 14% 5% 11% 7% 6% 7% 4 7% 7% 24%	1520
	11th to 20th	Day	6% 15% 5% 11% 8% 5% 6% 5% 7% 6% 28%	1567
	21st to end	Day	4 13% 5% 15% 7% 5% 6% 6% 5% 7% 26%	1527
March	1st to 10th	Day	4 10% 5% 14% 9% 7% 6% 4 6% 6% 29%	3138
	11th to 20th	Day	6% 10% 3 7% 4 4 4 4 6% 10% 43%	3385
	21st to end	Day	7% 16% 7% 13% 5% 6% 5% 4 5% 6% 25%	3974
April	1st to 10th	Day	8% 13% 4 8% 6% 6% 6% 6% 7% 8% 28%	2323
	11th to 20th	Day	12% 13% 4 9% 6% 4 6% 4 7% 7% 29%	2440
	21st to end	Day	11% 10% 3 8% 6% 6% 5% 4 7% 8% 30%	2494
May	1st to 10th	Day	16% 11% 3 8% 6% 5% 5% 4 5% 7% 29%	4566
	11th to 20th	Day	11% 11% 4 9% 6% 5% 7% 4 6% 7% 28%	4694
	21st to end	Day	14% 8% 3 10% 6% 6% 6% 5% 8% 8% 26%	5161
June	1st to 10th	Day	17% 7% 3 7% 5% 5% 5% 4 6% 9% 32%	2855
	11th to 20th	Day	18% 9% 3 9% 7% 7% 5% 5% 7% 7% 24%	3040
	21st to end	Day	10% 5% 2 6% 6% 5% 6% 5% 8% 10% 36%	3040
July	1st to 10th	Day	11% 5% 2 5% 4 4 4 4 6% 8% 48%	5036
	11th to 20th	Day	9% 4 2 5% 4 5% 5% 3 7% 8% 49%	4740
	21st to end	Day	11% 6% 2 6% 4 4 5% 5% 6% 9% 43%	5116
August	1st to 10th	Day	14% 5% 2 7% 4 4 5% 4 6% 8% 42%	4401
	11th to 20th	Day	10% 6% 2 6% 5% 5% 5% 4 7% 9% 41%	4220
	21st to end	Day	12% 6% 3 7% 5% 4 6% 4 6% 9% 40%	4521
September	1st to 10th	Day	13% 6% 3 5% 4 4 5% 4 6% 8% 42%	2356
	11th to 20th	Day	16% 7% 2 6% 3 3 4 3 5% 8% 42%	2123
	21st to end	Day	12% 7% 2 7% 6% 4 4 4 6% 7% 41%	2090
October	1st to 10th	Day	9% 9% 3 8% 5% 4 4 4 5% 7% 40%	3322
	11th to 20th	Day	10% 11% 4 8% 5% 5% 5% 3 6% 8% 34%	2842
	21st to end	Day	7% 12% 4 10% 6% 5% 4 4 5% 7% 37%	3101
November	1st to 10th	Day	11% 16% 5% 11% 7% 5% 5% 4 6% 7% 26%	1597
	11th to 20th	Day	8% 12% 6% 10% 5% 6% 4 3 5% 7% 34%	1400
	21st to end	Day	8% 11% 4 11% 7% 7% 7% 4 6% 8% 26%	1400
December	1st to 10th	Day	9% 14% 5% 11% 7% 5% 5% 4 7% 8% 26%	1400
	11th to 20th	Day	7% 14% 4 11% 6% 5% 6% 4 5% 8% 29%	1752
	21st to end	Day	6% 9% 4 8% 6% 5% 5% 4 7% 8% 37%	2463
January	1st to 10th	Night	4 14% 6% 13% 7% 5% 5% 4 5% 6% 30%	4726
	11th to 20th	Night	6% 12% 4 11% 7% 7% 5% 4 7% 6% 31%	4699
	21st to end	Night	7% 14% 5% 10% 6% 5% 5% 5% 6% 8% 29%	4946
February	1st to 10th	Night	8% 13% 5% 12% 7% 6% 5% 4 5% 6% 28%	2660
	11th to 20th	Night	7% 15% 5% 10% 8% 6% 6% 4 5% 7% 27%	2498
	21st to end	Night	5% 14% 5% 10% 7% 6% 5% 4 6% 7% 30%	1924
March	1st to 10th	Night	4 9% 5% 13% 7% 8% 7% 5% 6% 7% 28%	3736
	11th to 20th	Night	6% 9% 3 7% 5% 5% 7% 6% 7% 9% 36%	3593
	21st to end	Night	7% 17% 7% 15% 7% 6% 5% 4 5% 6% 23%	3675
April	1st to 10th	Night	10% 12% 4 9% 7% 7% 6% 5% 7% 7% 25%	1856
	11th to 20th	Night	11% 12% 4 9% 7% 6% 6% 5% 6% 7% 27%	1691
	21st to end	Night	12% 11% 3 8% 5% 5% 6% 5% 6% 8% 32%	1684
May	1st to 10th	Night	22% 12% 4 7% 4 4 5% 4 5% 5% 27%	2394
	11th to 20th	Night	13% 9% 3 10% 7% 5% 6% 4 7% 8% 29%	2199
	21st to end	Night	18% 9% 4 10% 7% 6% 6% 4 5% 6% 26%	2414
June	1st to 10th	Night	19% 10% 3 8% 5% 5% 4 4 6% 7% 27%	1321
	11th to 20th	Night	20% 8% 3 8% 8% 5% 6% 5% 7% 7% 22%	1140
	21st to end	Night	14% 7% 2 7% 6% 6% 6% 4 8% 9% 32%	1140
July	1st to 10th	Night	11% 7% 2 7% 5% 5% 5% 4 6% 7% 39%	1922
	11th to 20th	Night	9% 5% 2 6% 6% 6% 7% 5% 8% 9% 38%	2220
	21st to end	Night	14% 5% 2 7% 5% 5% 5% 5% 5% 7% 40%	2434
August	1st to 10th	Night	16% 6% 2 6% 4 5% 5% 4 5% 7% 39%	2529
	11th to 20th	Night	14% 8% 2 6% 5% 4 5% 4 6% 9% 37%	2611
	21st to end	Night	14% 6% 2 8% 4 5% 5% 4 6% 8% 39%	3146
September	1st to 10th	Night	12% 5% 2 7% 5% 4 5% 4 5% 7% 43%	1821
	11th to 20th	Night	14% 7% 2 6% 4 4 4 3 5% 7% 43%	2057
	21st to end	Night	12% 6% 2 7% 4 4 4 4 6% 8% 43%	2090
October	1st to 10th	Night	11% 9% 2 7% 5% 5% 5% 4 6% 7% 37%	3652
	11th to 20th	Night	11% 9% 3 8% 5% 5% 5% 4 6% 9% 36%	4086
	21st to end	Night	8% 9% 4 8% 6% 6% 6% 4 6% 8% 35%	4457
November	1st to 10th	Night	14% 15% 5% 9% 5% 4 5% 3 5% 6% 27%	2678
	11th to 20th	Night	10% 14% 5% 10% 6% 6% 5% 4 5% 6% 30%	3000
	21st to end	Night	8% 13% 5% 10% 8% 6% 5% 4 6% 7% 28%	3000
December	1st to 10th	Night	9% 15% 5% 9% 6% 5% 5% 4 5% 7% 31%	3000
	11th to 20th	Night	8% 14% 6% 11% 7% 6% 5% 4 5% 7% 27%	3660
	21st to end	Night	5% 11% 3 9% 5% 5% 5% 4 5% 7% 41%	5266

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	51% 6% 14% 20% 8% 1	2219
	11th to 20th	Day	46% 8% 10% 14% 19% 3	2220
	21st to end	Day	45% 10% 17% 23% 5% 1	2739
February	1st to 10th	Day	55% 8% 14% 14% 8% 1	1520
	11th to 20th	Day	53% 9% 12% 15% 10% 1	1575
	21st to end	Day	56% 7% 12% 16% 9% 1	1527
March	1st to 10th	Day	53% 7% 14% 15% 10%	3138
	11th to 20th	Day	35% 8% 19% 20% 17% 1	3386
	21st to end	Day	56% 8% 13% 17% 6%	3974
April	1st to 10th	Day	50% 10% 15% 17% 7% 1	2323
	11th to 20th	Day	51% 8% 14% 19% 7% 1	2440
	21st to end	Day	48% 10% 16% 19% 6% 1	2494
May	1st to 10th	Day	52% 8% 11% 20% 9%	4566
	11th to 20th	Day	48% 11% 13% 20% 7%	4698
	21st to end	Day	48% 13% 16% 14% 7% 1	5162
June	1st to 10th	Day	47% 9% 18% 19% 6% 1	2855
	11th to 20th	Day	56% 10% 14% 14% 5%	3040
	21st to end	Day	40% 11% 21% 21% 6% 1	3040
July	1st to 10th	Day	33% 7% 15% 30% 13% 1	4957
	11th to 20th	Day	33% 7% 18% 34% 8%	4659
	21st to end	Day	36% 8% 21% 23% 11% 1	4954
August	1st to 10th	Day	39% 8% 21% 30% 2	4384
	11th to 20th	Day	38% 8% 21% 25% 8%	4262
	21st to end	Day	40% 7% 27% 21% 5%	4520
September	1st to 10th	Day	38% 9% 20% 24% 7% 1	2356
	11th to 20th	Day	41% 7% 17% 27% 8%	2123
	21st to end	Day	41% 8% 16% 24% 9% 2	2090
October	1st to 10th	Day	42% 6% 22% 25% 5%	3322
	11th to 20th	Day	47% 6% 14% 23% 10%	2842
	21st to end	Day	47% 5% 16% 26% 6%	3101
November	1st to 10th	Day	56% 7% 15% 16% 5% 1	1597
	11th to 20th	Day	50% 8% 14% 18% 9%	1400
	21st to end	Day	54% 9% 14% 14% 7% 2	1400
December	1st to 10th	Day	55% 8% 13% 15% 7% 2	1400
	11th to 20th	Day	51% 10% 15% 17% 6% 1	1754
	21st to end	Day	40% 8% 14% 19% 16% 3	2463
January	1st to 10th	Night	53% 8% 13% 16% 8% 2	4726
	11th to 20th	Night	51% 6% 10% 14% 15% 4	4740
	21st to end	Night	49% 10% 17% 17% 7% 1	4946
February	1st to 10th	Night	56% 6% 11% 17% 8% 2	2660
	11th to 20th	Night	55% 6% 13% 15% 10% 1	2500
	21st to end	Night	51% 8% 11% 20% 10% 1	1924
March	1st to 10th	Night	49% 10% 13% 19% 9%	3736
	11th to 20th	Night	37% 10% 19% 20% 13% 1	3593
	21st to end	Night	59% 10% 11% 16% 4 1	3675
April	1st to 10th	Night	54% 10% 12% 15% 7% 1	1856
	11th to 20th	Night	52% 9% 13% 18% 7% 2	1691
	21st to end	Night	46% 10% 14% 21% 8% 1	1684
May	1st to 10th	Night	56% 8% 10% 18% 9%	2394
	11th to 20th	Night	47% 11% 15% 19% 7%	2199
	21st to end	Night	56% 11% 14% 11% 7% 1	2414
June	1st to 10th	Night	54% 8% 12% 18% 7% 1	1321
	11th to 20th	Night	55% 11% 15% 15% 3	1140
	21st to end	Night	45% 11% 18% 21% 5% 1	1140
July	1st to 10th	Night	42% 8% 15% 19% 14% 3	1892
	11th to 20th	Night	38% 10% 18% 28% 6%	2178
	21st to end	Night	41% 8% 17% 22% 10% 2	2357
August	1st to 10th	Night	43% 8% 20% 27% 3	2521
	11th to 20th	Night	45% 7% 18% 21% 9% 1	2635
	21st to end	Night	42% 6% 21% 24% 6%	3146
September	1st to 10th	Night	38% 9% 17% 24% 9% 2	1821
	11th to 20th	Night	41% 7% 15% 28% 7% 3	2057
	21st to end	Night	38% 7% 16% 24% 14% 2	2090
October	1st to 10th	Night	45% 5% 18% 24% 7%	3652
	11th to 20th	Night	45% 5% 13% 29% 8%	4086
	21st to end	Night	46% 6% 17% 24% 7%	4457
November	1st to 10th	Night	56% 6% 11% 18% 8%	2678
	11th to 20th	Night	54% 8% 12% 17% 9% 1	3000
	21st to end	Night	54% 7% 15% 17% 6% 2	3000
December	1st to 10th	Night	53% 6% 11% 19% 9% 2	3000
	11th to 20th	Night	55% 8% 15% 17% 6%	3657
	21st to end	Night	40% 7% 13% 23% 14% 2	5266

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	7% 8% 27% 29% 21% 8%	180
	11th to 20th	Day	8% 13% 26% 29% 18% 4	180
	21st to end	Day	14% 10% 28% 31% 12% 5%	198
February	1st to 10th	Day	14% 13% 36% 23% 11% 3	180
	11th to 20th	Day	15% 8% 37% 26% 12% 2	222
	21st to end	Day	10% 9% 32% 31% 14% 4	339
March	1st to 10th	Day	11% 13% 31% 32% 11% 2	460
	11th to 20th	Day	9% 9% 37% 27% 14% 5%	460
	21st to end	Day	16% 11% 31% 25% 15% 2	506
April	1st to 10th	Day	11% 16% 36% 27% 8% 3	460
	11th to 20th	Day	16% 10% 26% 32% 13% 3	452
	21st to end	Day	15% 13% 29% 29% 12% 2	460
May	1st to 10th	Day	17% 13% 25% 28% 15% 3	544
	11th to 20th	Day	12% 9% 23% 31% 20% 4	675
	21st to end	Day	16% 7% 25% 29% 17% 6%	734
June	1st to 10th	Day	16% 8% 30% 29% 15% 2	685
	11th to 20th	Day	18% 7% 23% 29% 17% 5%	870
	21st to end	Day	12% 6% 22% 30% 23% 7%	870
July	1st to 10th	Day	14% 3 19% 35% 22% 7%	845
	11th to 20th	Day	13% 4 20% 31% 25% 6%	668
	21st to end	Day	12% 6% 19% 29% 24% 10%	729
August	1st to 10th	Day	8% 8% 18% 28% 25% 12%	487
	11th to 20th	Day	8% 6% 25% 30% 25% 6%	477
	21st to end	Day	14% 9% 18% 29% 22% 7%	539
September	1st to 10th	Day	16% 7% 27% 24% 21% 6%	490
	11th to 20th	Day	19% 6% 22% 30% 19% 5%	490
	21st to end	Day	13% 11% 29% 28% 14% 4	490
October	1st to 10th	Day	16% 8% 25% 31% 17% 4	426
	11th to 20th	Day	15% 10% 28% 30% 13% 4	300
	21st to end	Day	13% 15% 32% 28% 9% 3	330
November	1st to 10th	Day	18% 7% 30% 31% 11% 2	293
	11th to 20th	Day	12% 7% 37% 33% 9% 3	200
	21st to end	Day	11% 7% 38% 31% 11% 4	200
December	1st to 10th	Day	14% 10% 30% 32% 11% 5%	200
	11th to 20th	Day	11% 13% 32% 29% 13% 3	200
	21st to end	Day	10% 5% 28% 32% 18% 6%	204
January	1st to 10th	Night	11% 8% 32% 33% 13% 2	950
	11th to 20th	Night	13% 8% 31% 35% 11% 1	950
	21st to end	Night	14% 15% 31% 29% 10% 1	881
February	1st to 10th	Night	13% 10% 28% 31% 15% 3	760
	11th to 20th	Night	13% 7% 32% 34% 13% 1	744
	21st to end	Night	10% 5% 30% 34% 17% 4	628
March	1st to 10th	Night	12% 6% 31% 33% 16% 2	760
	11th to 20th	Night	10% 6% 33% 38% 11% 2	760
	21st to end	Night	15% 6% 27% 36% 14% 3	836
April	1st to 10th	Night	14% 10% 32% 31% 13% 1	716
	11th to 20th	Night	15% 6% 32% 33% 14%	563
	21st to end	Night	17% 7% 28% 36% 11% 1	544
May	1st to 10th	Night	21% 8% 22% 33% 15% 1	380
	11th to 20th	Night	14% 5% 29% 34% 17% 1	374
	21st to end	Night	20% 6% 23% 34% 16% 1	411
June	1st to 10th	Night	20% 8% 25% 31% 14% 2	380
	11th to 20th	Night	20% 5% 18% 36% 18% 2	380
	21st to end	Night	16% 5% 20% 34% 23% 2	380
July	1st to 10th	Night	15% 3 16% 45% 20% 1	370
	11th to 20th	Night	14% 2 21% 39% 23% 2	368
	21st to end	Night	13% 3 17% 40% 26% 1	416
August	1st to 10th	Night	16% 3 16% 34% 27% 4	378
	11th to 20th	Night	12% 3 17% 40% 26% 2	438
	21st to end	Night	15% 3 19% 40% 22%	627
September	1st to 10th	Night	14% 2 19% 43% 20% 2	681
	11th to 20th	Night	16% 4 20% 38% 19% 1	760
	21st to end	Night	14% 4 30% 35% 14% 2	760
October	1st to 10th	Night	18% 4 27% 36% 15% 1	760
	11th to 20th	Night	15% 7% 29% 35% 13% 1	760
	21st to end	Night	12% 8% 31% 35% 12% 1	836
November	1st to 10th	Night	20% 8% 29% 30% 10% 2	913
	11th to 20th	Night	14% 9% 36% 29% 10% 2	1000
	21st to end	Night	13% 7% 37% 31% 10% 2	1000
December	1st to 10th	Night	14% 10% 34% 29% 11% 2	1000
	11th to 20th	Night	14% 8% 31% 33% 14% 1	1000
	21st to end	Night	12% 5% 33% 34% 14% 3	1063

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