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 2026-04-17.

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 9% 6% 5% 5% 4 6% 8% 35%	136382
	Night	10% 11% 4 10% 6% 5% 5% 4 6% 7% 32%	134493
Transparency	Day	45% 7% 16% 22% 9% 1	136077
	Night	49% 7% 15% 20% 9% 1	134377
Seeing	Day	14% 8% 26% 29% 17% 5%	17722
	Night	14% 7% 29% 34% 15% 2	26281

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	7% 14% 5% 13% 6% 6% 5% 4 5% 6% 29%	4125
	11th to 20th	Day	5% 8% 3 9% 6% 5% 5% 5% 6% 9% 39%	4119
	21st to end	Day	7% 12% 4 10% 6% 6% 5% 4 5% 7% 34%	5103
February	1st to 10th	Day	8% 13% 5% 11% 7% 6% 7% 4 7% 7% 25%	1680
	11th to 20th	Day	6% 14% 5% 11% 7% 5% 6% 5% 7% 5% 28%	1737
	21st to end	Day	4 14% 5% 15% 7% 5% 7% 6% 5% 7% 25%	1683
March	1st to 10th	Day	4 8% 4 12% 8% 7% 6% 5% 7% 8% 32%	4618
	11th to 20th	Day	7% 12% 4 10% 5% 4 4 3 5% 9% 38%	4962
	21st to end	Day	10% 17% 6% 12% 5% 5% 5% 4 6% 6% 23%	5826
April	1st to 10th	Day	8% 13% 4 8% 6% 6% 6% 6% 7% 8% 28%	2568
	11th to 20th	Day	12% 13% 4 8% 6% 4 6% 4 7% 7% 29%	2649
	21st to end	Day	11% 10% 3 8% 6% 6% 5% 4 7% 9% 30%	2625
May	1st to 10th	Day	12% 9% 3 8% 6% 6% 5% 4 5% 7% 36%	6522
	11th to 20th	Day	13% 11% 4 10% 7% 6% 6% 5% 6% 8% 25%	6734
	21st to end	Day	11% 6% 2 8% 5% 5% 5% 4 7% 8% 37%	7405
June	1st to 10th	Day	16% 7% 3 7% 5% 5% 6% 4 6% 9% 33%	3005
	11th to 20th	Day	18% 9% 3 9% 6% 7% 5% 5% 7% 7% 24%	3200
	21st to end	Day	10% 6% 2 6% 6% 5% 6% 5% 8% 10% 36%	3200
July	1st to 10th	Day	9% 5% 2 5% 4 4 5% 4 7% 9% 46%	7202
	11th to 20th	Day	11% 5% 2 7% 5% 5% 5% 4 7% 8% 41%	6780
	21st to end	Day	11% 6% 2 6% 5% 5% 5% 5% 6% 9% 40%	7340
August	1st to 10th	Day	13% 5% 2 7% 4 4 5% 4 6% 9% 42%	4541
	11th to 20th	Day	10% 6% 2 7% 5% 5% 5% 4 7% 9% 41%	4356
	21st to end	Day	12% 6% 3 6% 5% 4 5% 4 6% 9% 41%	4664
September	1st to 10th	Day	12% 6% 3 5% 4 4 5% 4 6% 8% 42%	2480
	11th to 20th	Day	15% 6% 2 5% 3 3 4 3 5% 9% 43%	2235
	21st to end	Day	12% 7% 2 7% 6% 4 4 4 5% 7% 41%	2200
October	1st to 10th	Day	9% 9% 3 8% 5% 4 4 4 5% 7% 40%	3426
	11th to 20th	Day	10% 11% 4 8% 5% 5% 5% 4 6% 8% 34%	2932
	21st to end	Day	6% 12% 4 10% 6% 5% 4 4 5% 7% 37%	3200
November	1st to 10th	Day	11% 15% 5% 10% 7% 5% 5% 4 6% 7% 26%	1678
	11th to 20th	Day	7% 11% 6% 10% 5% 6% 4 3 5% 7% 35%	1470
	21st to end	Day	7% 12% 4 11% 7% 7% 7% 4 6% 8% 26%	1470
December	1st to 10th	Day	11% 17% 5% 11% 7% 5% 5% 4 7% 8% 21%	2360
	11th to 20th	Day	9% 15% 4 11% 7% 6% 6% 4 5% 7% 26%	2712
	21st to end	Day	5% 8% 3 9% 6% 6% 5% 4 7% 8% 39%	3575
January	1st to 10th	Night	7% 15% 6% 14% 8% 6% 5% 3 4 6% 25%	8768
	11th to 20th	Night	3 10% 4 10% 7% 6% 5% 4 6% 7% 38%	8779
	21st to end	Night	7% 12% 5% 10% 6% 5% 5% 5% 7% 8% 31%	9134
February	1st to 10th	Night	8% 13% 5% 12% 7% 7% 5% 4 5% 6% 27%	2940
	11th to 20th	Night	7% 14% 5% 11% 7% 6% 6% 4 6% 7% 27%	2768
	21st to end	Night	5% 14% 5% 11% 7% 6% 5% 4 6% 7% 30%	2120
March	1st to 10th	Night	4 9% 4 11% 6% 6% 6% 5% 6% 8% 34%	5496
	11th to 20th	Night	5% 12% 4 10% 7% 6% 7% 6% 7% 8% 28%	5256
	21st to end	Night	11% 17% 6% 13% 6% 5% 4 3 5% 6% 23%	5387
April	1st to 10th	Night	10% 13% 4 9% 7% 7% 6% 5% 6% 7% 27%	2051
	11th to 20th	Night	11% 13% 4 9% 7% 6% 6% 5% 6% 7% 27%	1835
	21st to end	Night	12% 11% 3 8% 5% 4 6% 5% 6% 8% 32%	1772
May	1st to 10th	Night	18% 10% 3 7% 4 4 5% 4 5% 6% 35%	3438
	11th to 20th	Night	14% 9% 4 12% 7% 6% 6% 4 6% 7% 25%	3159
	21st to end	Night	14% 8% 3 9% 5% 5% 5% 4 5% 6% 37%	3470
June	1st to 10th	Night	19% 10% 3 8% 5% 5% 4 4 6% 7% 27%	1390
	11th to 20th	Night	20% 8% 3 8% 8% 5% 6% 5% 7% 7% 22%	1200
	21st to end	Night	13% 7% 2 7% 6% 6% 6% 4 8% 9% 32%	1200
July	1st to 10th	Night	10% 7% 2 6% 5% 5% 5% 5% 7% 8% 40%	2756
	11th to 20th	Night	13% 6% 2 7% 5% 6% 7% 4 7% 8% 34%	3180
	21st to end	Night	14% 6% 2 7% 5% 5% 5% 4 5% 7% 41%	3501
August	1st to 10th	Night	16% 6% 2 6% 4 5% 5% 4 6% 7% 38%	2609
	11th to 20th	Night	14% 8% 2 7% 5% 4 5% 4 6% 9% 36%	2694
	21st to end	Night	14% 6% 2 7% 4 4 4 4 6% 8% 41%	3245
September	1st to 10th	Night	11% 5% 2 7% 5% 4 5% 4 6% 7% 44%	1917
	11th to 20th	Night	14% 7% 2 6% 4 4 4 3 5% 7% 45%	2165
	21st to end	Night	12% 6% 2 7% 4 4 4 4 7% 9% 42%	2200
October	1st to 10th	Night	11% 9% 2 7% 5% 5% 5% 4 6% 8% 37%	3768
	11th to 20th	Night	11% 9% 3 8% 5% 5% 5% 4 6% 9% 36%	4216
	21st to end	Night	8% 9% 4 8% 6% 6% 6% 4 6% 8% 36%	4600
November	1st to 10th	Night	14% 15% 5% 9% 5% 4 5% 3 5% 6% 28%	2816
	11th to 20th	Night	9% 13% 5% 10% 6% 6% 5% 4 5% 6% 30%	3150
	21st to end	Night	8% 12% 5% 10% 8% 6% 5% 4 7% 7% 28%	3150
December	1st to 10th	Night	12% 17% 5% 9% 6% 6% 5% 3 5% 6% 26%	5040
	11th to 20th	Night	10% 16% 6% 13% 7% 5% 5% 3 5% 7% 22%	5700
	21st to end	Night	4 10% 3 10% 5% 5% 6% 4 5% 7% 40%	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	55% 4 11% 16% 13% 1	4125
	11th to 20th	Day	41% 5% 14% 18% 18% 3	4140
	21st to end	Day	49% 6% 16% 24% 6%	5104
February	1st to 10th	Day	54% 7% 14% 14% 10% 1	1680
	11th to 20th	Day	52% 9% 12% 16% 11%	1745
	21st to end	Day	57% 6% 12% 16% 9% 1	1683
March	1st to 10th	Day	47% 6% 16% 17% 14%	4618
	11th to 20th	Day	43% 7% 13% 21% 16% 1	4963
	21st to end	Day	59% 7% 10% 15% 8%	5826
April	1st to 10th	Day	50% 10% 15% 17% 7% 1	2568
	11th to 20th	Day	51% 8% 14% 19% 7% 1	2649
	21st to end	Day	48% 9% 15% 20% 6% 1	2625
May	1st to 10th	Day	47% 7% 9% 22% 15% 1	6522
	11th to 20th	Day	53% 9% 14% 19% 5%	6738
	21st to end	Day	40% 10% 19% 23% 8% 1	7406
June	1st to 10th	Day	46% 8% 17% 20% 7% 1	3005
	11th to 20th	Day	56% 10% 15% 14% 4	3200
	21st to end	Day	40% 11% 21% 22% 6% 1	3200
July	1st to 10th	Day	33% 6% 19% 30% 11% 1	7123
	11th to 20th	Day	40% 6% 18% 30% 6%	6699
	21st to end	Day	40% 7% 21% 24% 8% 1	7133
August	1st to 10th	Day	39% 8% 21% 30% 2	4524
	11th to 20th	Day	38% 8% 21% 25% 8%	4398
	21st to end	Day	39% 7% 26% 22% 5%	4663
September	1st to 10th	Day	38% 8% 20% 26% 7% 1	2480
	11th to 20th	Day	39% 7% 17% 28% 9%	2235
	21st to end	Day	41% 8% 16% 24% 9% 2	2200
October	1st to 10th	Day	42% 6% 22% 25% 5%	3426
	11th to 20th	Day	47% 6% 14% 23% 10%	2932
	21st to end	Day	46% 5% 16% 26% 7%	3200
November	1st to 10th	Day	56% 6% 15% 17% 6% 1	1678
	11th to 20th	Day	49% 8% 15% 18% 10%	1470
	21st to end	Day	54% 8% 14% 15% 7% 2	1470
December	1st to 10th	Day	61% 5% 17% 11% 4 1	2360
	11th to 20th	Day	57% 7% 16% 14% 5% 1	2714
	21st to end	Day	40% 6% 13% 24% 14% 2	3575
January	1st to 10th	Night	61% 5% 10% 14% 10% 1	8769
	11th to 20th	Night	45% 4 14% 19% 15% 3	8820
	21st to end	Night	48% 7% 20% 19% 6%	9134
February	1st to 10th	Night	56% 6% 11% 17% 8% 1	2940
	11th to 20th	Night	54% 6% 12% 15% 12% 1	2770
	21st to end	Night	51% 7% 11% 20% 10% 1	2120
March	1st to 10th	Night	45% 7% 16% 21% 11%	5496
	11th to 20th	Night	48% 8% 14% 18% 10% 1	5256
	21st to end	Night	60% 8% 8% 17% 7%	5387
April	1st to 10th	Night	53% 9% 13% 16% 8% 1	2051
	11th to 20th	Night	52% 9% 13% 18% 7% 2	1835
	21st to end	Night	46% 10% 14% 21% 8% 1	1772
May	1st to 10th	Night	48% 6% 9% 19% 15% 3	3435
	11th to 20th	Night	54% 9% 16% 16% 5%	3159
	21st to end	Night	46% 8% 14% 21% 8% 1	3470
June	1st to 10th	Night	53% 8% 12% 18% 7% 1	1390
	11th to 20th	Night	56% 10% 16% 15% 3	1200
	21st to end	Night	45% 11% 17% 22% 5% 1	1200
July	1st to 10th	Night	40% 6% 20% 18% 13% 2	2726
	11th to 20th	Night	45% 8% 16% 26% 5%	3138
	21st to end	Night	42% 7% 21% 22% 7% 1	3403
August	1st to 10th	Night	43% 7% 20% 27% 3	2601
	11th to 20th	Night	45% 7% 18% 21% 9% 1	2718
	21st to end	Night	41% 6% 21% 25% 7%	3245
September	1st to 10th	Night	37% 9% 17% 26% 9% 2	1917
	11th to 20th	Night	39% 7% 14% 28% 9% 2	2165
	21st to end	Night	38% 6% 17% 24% 13% 2	2200
October	1st to 10th	Night	44% 5% 19% 24% 7%	3768
	11th to 20th	Night	45% 5% 13% 28% 9% 1	4216
	21st to end	Night	45% 6% 17% 24% 7%	4600
November	1st to 10th	Night	56% 6% 11% 18% 8% 1	2816
	11th to 20th	Night	53% 7% 13% 17% 9% 1	3150
	21st to end	Night	53% 7% 15% 17% 6% 2	3150
December	1st to 10th	Night	60% 5% 13% 16% 5% 1	5040
	11th to 20th	Night	61% 6% 15% 14% 5%	5697
	21st to end	Night	42% 6% 11% 25% 14% 2	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% 7% 29% 30% 19% 7%	220
	11th to 20th	Day	9% 13% 29% 30% 15% 4	220
	21st to end	Day	14% 10% 29% 31% 13% 4	242
February	1st to 10th	Day	12% 11% 35% 26% 14% 3	220
	11th to 20th	Day	14% 7% 36% 28% 12% 2	272
	21st to end	Day	9% 10% 33% 32% 13% 4	399
March	1st to 10th	Day	11% 12% 30% 33% 12% 2	540
	11th to 20th	Day	10% 8% 36% 28% 14% 5%	540
	21st to end	Day	16% 11% 30% 25% 15% 2	594
April	1st to 10th	Day	12% 15% 33% 27% 10% 3	540
	11th to 20th	Day	17% 11% 26% 30% 13% 3	517
	21st to end	Day	15% 12% 29% 29% 13% 2	500
May	1st to 10th	Day	16% 13% 26% 27% 16% 3	588
	11th to 20th	Day	12% 9% 23% 31% 21% 4	725
	21st to end	Day	16% 6% 25% 29% 18% 6%	789
June	1st to 10th	Day	15% 9% 30% 29% 15% 2	735
	11th to 20th	Day	18% 7% 23% 29% 18% 5%	930
	21st to end	Day	12% 6% 22% 31% 23% 8%	930
July	1st to 10th	Day	14% 3 20% 35% 21% 6%	904
	11th to 20th	Day	14% 4 20% 31% 25% 6%	718
	21st to end	Day	12% 5% 18% 29% 25% 10%	783
August	1st to 10th	Day	8% 8% 19% 27% 26% 12%	527
	11th to 20th	Day	9% 6% 26% 31% 23% 5%	517
	21st to end	Day	13% 9% 17% 28% 23% 10%	583
September	1st to 10th	Day	15% 7% 25% 24% 22% 7%	530
	11th to 20th	Day	18% 5% 20% 30% 21% 6%	530
	21st to end	Day	13% 12% 30% 27% 14% 4	530
October	1st to 10th	Day	15% 7% 25% 31% 17% 4	462
	11th to 20th	Day	15% 10% 28% 31% 13% 4	330
	21st to end	Day	12% 15% 34% 27% 9% 2	363
November	1st to 10th	Day	18% 7% 31% 30% 11% 2	322
	11th to 20th	Day	11% 6% 38% 32% 9% 3	220
	21st to end	Day	10% 6% 37% 30% 12% 5%	220
December	1st to 10th	Day	15% 10% 30% 30% 10% 4	220
	11th to 20th	Day	10% 12% 34% 29% 12% 3	220
	21st to end	Day	13% 5% 24% 33% 18% 6%	242
January	1st to 10th	Night	12% 8% 32% 33% 13% 2	1050
	11th to 20th	Night	12% 9% 34% 33% 10% 1	1050
	21st to end	Night	14% 14% 32% 29% 9% 1	973
February	1st to 10th	Night	13% 9% 27% 32% 15% 3	840
	11th to 20th	Night	13% 7% 32% 34% 13% 1	824
	21st to end	Night	10% 5% 33% 33% 16% 3	692
March	1st to 10th	Night	12% 6% 31% 34% 15% 2	840
	11th to 20th	Night	10% 5% 33% 38% 12% 2	840
	21st to end	Night	15% 6% 27% 35% 13% 2	924
April	1st to 10th	Night	13% 10% 31% 31% 14% 1	791
	11th to 20th	Night	15% 6% 33% 33% 13%	611
	21st to end	Night	17% 6% 28% 37% 11% 1	572
May	1st to 10th	Night	21% 8% 22% 34% 15% 1	400
	11th to 20th	Night	14% 5% 29% 34% 17% 1	394
	21st to end	Night	20% 6% 23% 34% 16% 1	433
June	1st to 10th	Night	20% 9% 25% 31% 14% 2	400
	11th to 20th	Night	21% 5% 18% 36% 18% 3	400
	21st to end	Night	16% 5% 19% 35% 23% 2	400
July	1st to 10th	Night	15% 3 17% 44% 20% 1	390
	11th to 20th	Night	14% 2 20% 39% 23% 2	388
	21st to end	Night	13% 3 16% 39% 27% 1	438
August	1st to 10th	Night	16% 3 16% 34% 27% 4	398
	11th to 20th	Night	11% 3 17% 41% 26% 2	461
	21st to end	Night	14% 3 19% 40% 23%	660
September	1st to 10th	Night	14% 2 18% 42% 22% 2	717
	11th to 20th	Night	16% 4 19% 39% 20% 1	800
	21st to end	Night	14% 4 31% 35% 14% 2	800
October	1st to 10th	Night	17% 5% 27% 36% 15% 1	800
	11th to 20th	Night	15% 7% 30% 35% 13% 1	800
	21st to end	Night	13% 8% 33% 34% 12% 1	880
November	1st to 10th	Night	20% 8% 30% 31% 10% 1	960
	11th to 20th	Night	13% 9% 36% 29% 11% 2	1050
	21st to end	Night	13% 7% 36% 32% 10% 2	1050
December	1st to 10th	Night	14% 10% 35% 29% 11% 1	1050
	11th to 20th	Night	14% 8% 31% 33% 13% 1	1050
	21st to end	Night	12% 5% 32% 34% 14% 2	1155

Page generated at 2026-04-14 00:44:25. Version ac07

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