Westlock
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Westlock clear sky chart from 2005-11-06 to 2026-07-01.

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% 8% 3 8% 6% 5% 5% 4 7% 8% 35%	142845
	Night	8% 10% 4 10% 6% 5% 5% 4 6% 8% 33%	138125
Transparency	Day	44% 8% 18% 23% 7%	142538
	Night	47% 7% 16% 20% 9% 1	138017
Seeing	Day	13% 6% 24% 30% 20% 7%	18229
	Night	13% 5% 27% 36% 16% 2	26627

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	9% 16% 5% 9% 8% 6% 6% 5% 7% 7% 23%	4003
	11th to 20th	Day	3 8% 3 10% 7% 5% 5% 6% 8% 9% 34%	4119
	21st to end	Day	7% 11% 4 9% 6% 5% 6% 4 6% 8% 33%	4829
February	1st to 10th	Day	8% 12% 4 11% 7% 7% 6% 4 8% 9% 23%	1680
	11th to 20th	Day	5% 11% 6% 10% 6% 6% 7% 5% 9% 5% 28%	1757
	21st to end	Day	3 13% 7% 10% 8% 7% 6% 4 6% 7% 29%	1615
March	1st to 10th	Day	5% 9% 5% 11% 8% 7% 7% 5% 7% 9% 28%	4618
	11th to 20th	Day	6% 10% 5% 12% 6% 5% 4 4 6% 8% 33%	5034
	21st to end	Day	8% 14% 5% 15% 6% 6% 5% 4 6% 8% 23%	5734
April	1st to 10th	Day	7% 12% 4 8% 5% 6% 7% 6% 7% 7% 31%	2620
	11th to 20th	Day	9% 11% 3 8% 5% 7% 6% 5% 6% 8% 34%	2700
	21st to end	Day	10% 10% 4 8% 5% 6% 6% 5% 7% 7% 32%	2739
May	1st to 10th	Day	11% 6% 2 6% 5% 5% 6% 4 6% 9% 41%	8691
	11th to 20th	Day	14% 10% 4 9% 6% 6% 6% 5% 7% 8% 27%	8755
	21st to end	Day	10% 6% 3 8% 6% 6% 6% 5% 7% 10% 34%	9649
June	1st to 10th	Day	16% 7% 3 8% 6% 5% 6% 4 7% 8% 31%	3182
	11th to 20th	Day	19% 8% 3 8% 6% 5% 6% 5% 7% 8% 24%	3360
	21st to end	Day	10% 6% 2 6% 5% 4 6% 5% 8% 9% 39%	3360
July	1st to 10th	Day	8% 4 2 6% 5% 5% 5% 5% 7% 9% 44%	7241
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 4 6% 10% 43%	6786
	21st to end	Day	11% 5% 2 5% 4 4 5% 5% 7% 8% 44%	7376
August	1st to 10th	Day	13% 6% 2 6% 4 4 5% 4 6% 8% 43%	4683
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 5% 7% 9% 43%	4328
	21st to end	Day	11% 5% 2 7% 5% 5% 4 4 6% 9% 43%	4664
September	1st to 10th	Day	11% 5% 2 8% 5% 5% 5% 4 6% 7% 42%	2546
	11th to 20th	Day	14% 6% 2 5% 4 5% 4 4 6% 8% 42%	2205
	21st to end	Day	9% 6% 2 7% 5% 4 5% 4 7% 8% 43%	2200
October	1st to 10th	Day	10% 8% 3 9% 5% 5% 5% 4 7% 7% 38%	3409
	11th to 20th	Day	11% 9% 3 8% 5% 5% 5% 3 6% 8% 36%	2937
	21st to end	Day	7% 12% 5% 8% 5% 5% 6% 4 6% 7% 35%	3146
November	1st to 10th	Day	10% 13% 5% 9% 7% 6% 6% 4 6% 7% 27%	1645
	11th to 20th	Day	8% 11% 5% 11% 6% 7% 4 4 6% 8% 30%	1470
	21st to end	Day	5% 13% 6% 12% 7% 7% 4 4 6% 8% 28%	1470
December	1st to 10th	Day	9% 16% 4 12% 8% 7% 6% 3 6% 7% 22%	2360
	11th to 20th	Day	9% 14% 4 11% 6% 7% 7% 6% 7% 9% 20%	2712
	21st to end	Day	4 6% 3 9% 8% 6% 6% 5% 8% 8% 37%	3222
January	1st to 10th	Night	7% 13% 5% 15% 8% 7% 6% 3 6% 7% 23%	8856
	11th to 20th	Night	3 9% 3 9% 6% 6% 6% 4 6% 8% 40%	8779
	21st to end	Night	7% 12% 4 10% 7% 5% 5% 4 6% 7% 30%	9395
February	1st to 10th	Night	6% 11% 5% 12% 10% 7% 7% 4 6% 6% 26%	2940
	11th to 20th	Night	6% 11% 4 10% 7% 7% 6% 5% 7% 7% 29%	2749
	21st to end	Night	4 12% 5% 10% 7% 6% 6% 5% 7% 8% 30%	2187
March	1st to 10th	Night	3 8% 4 12% 8% 6% 6% 5% 6% 10% 32%	5496
	11th to 20th	Night	5% 9% 3 13% 7% 6% 6% 5% 7% 10% 29%	5185
	21st to end	Night	11% 13% 6% 15% 7% 5% 4 4 5% 6% 25%	5481
April	1st to 10th	Night	8% 10% 3 9% 7% 7% 7% 6% 6% 7% 31%	2000
	11th to 20th	Night	10% 12% 4 8% 7% 6% 5% 6% 7% 6% 30%	1871
	21st to end	Night	11% 10% 3 8% 5% 5% 5% 5% 6% 7% 35%	1880
May	1st to 10th	Night	13% 6% 2 7% 4 4 5% 4 6% 8% 41%	4213
	11th to 20th	Night	14% 10% 3 10% 7% 7% 7% 5% 7% 8% 25%	4105
	21st to end	Night	10% 7% 3 7% 5% 5% 5% 4 7% 10% 37%	4522
June	1st to 10th	Night	16% 9% 3 6% 5% 5% 6% 4 6% 7% 31%	1433
	11th to 20th	Night	17% 10% 3 9% 6% 5% 7% 6% 8% 8% 21%	1260
	21st to end	Night	9% 7% 3 7% 6% 5% 4 5% 9% 8% 36%	1260
July	1st to 10th	Night	9% 6% 2 6% 4 4 5% 5% 6% 9% 45%	2720
	11th to 20th	Night	10% 5% 3 7% 5% 5% 6% 5% 5% 9% 39%	3170
	21st to end	Night	12% 6% 2 6% 5% 4 4 4 6% 8% 43%	3467
August	1st to 10th	Night	11% 6% 2 6% 5% 5% 5% 4 6% 8% 43%	2466
	11th to 20th	Night	13% 7% 3 7% 5% 4 4 4 6% 8% 38%	2724
	21st to end	Night	14% 4 2 5% 4 4 4 4 6% 9% 45%	3245
September	1st to 10th	Night	12% 5% 3 6% 3 4 4 3 6% 9% 46%	1850
	11th to 20th	Night	11% 8% 2 5% 4 3 5% 3 6% 7% 46%	2195
	21st to end	Night	10% 7% 2 7% 5% 4 5% 3 6% 8% 44%	2200
October	1st to 10th	Night	8% 7% 3 8% 6% 6% 5% 5% 6% 8% 37%	3784
	11th to 20th	Night	10% 7% 3 7% 5% 4 5% 4 6% 10% 39%	4208
	21st to end	Night	8% 9% 3 9% 5% 5% 6% 5% 6% 10% 34%	4622
November	1st to 10th	Night	13% 15% 5% 9% 7% 5% 5% 4 5% 6% 27%	2870
	11th to 20th	Night	8% 13% 5% 11% 6% 6% 5% 4 6% 6% 29%	3150
	21st to end	Night	5% 13% 5% 13% 7% 5% 6% 4 6% 8% 27%	3150
December	1st to 10th	Night	10% 17% 5% 11% 6% 6% 5% 4 6% 7% 24%	5040
	11th to 20th	Night	10% 15% 5% 12% 7% 7% 6% 5% 7% 7% 19%	5700
	21st to end	Night	3 7% 4 10% 6% 6% 6% 4 7% 8% 41%	7952

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	56% 6% 11% 15% 12%	4003
	11th to 20th	Day	42% 6% 17% 16% 17% 1	4140
	21st to end	Day	46% 8% 22% 19% 5%	4830
February	1st to 10th	Day	53% 8% 15% 16% 9%	1680
	11th to 20th	Day	50% 9% 15% 17% 9%	1765
	21st to end	Day	51% 7% 15% 20% 6%	1615
March	1st to 10th	Day	49% 6% 17% 20% 8%	4618
	11th to 20th	Day	45% 8% 16% 22% 9%	5035
	21st to end	Day	56% 7% 13% 19% 6%	5734
April	1st to 10th	Day	47% 10% 18% 17% 7% 1	2620
	11th to 20th	Day	47% 7% 16% 21% 9% 1	2700
	21st to end	Day	47% 7% 17% 21% 6% 1	2739
May	1st to 10th	Day	38% 6% 13% 29% 13% 1	8691
	11th to 20th	Day	50% 8% 11% 25% 5%	8759
	21st to end	Day	41% 9% 22% 21% 7% 1	9650
June	1st to 10th	Day	48% 8% 15% 20% 8% 1	3182
	11th to 20th	Day	54% 10% 16% 15% 4	3360
	21st to end	Day	38% 10% 23% 22% 6%	3360
July	1st to 10th	Day	33% 9% 21% 27% 9%	7162
	11th to 20th	Day	35% 8% 17% 30% 9%	6704
	21st to end	Day	35% 9% 23% 26% 7%	7169
August	1st to 10th	Day	37% 10% 21% 30% 2	4665
	11th to 20th	Day	34% 9% 24% 24% 9%	4370
	21st to end	Day	36% 8% 27% 24% 4	4663
September	1st to 10th	Day	40% 7% 21% 24% 7% 1	2546
	11th to 20th	Day	39% 6% 21% 25% 8%	2205
	21st to end	Day	37% 10% 17% 27% 8% 2	2200
October	1st to 10th	Day	42% 7% 24% 21% 5%	3409
	11th to 20th	Day	45% 6% 15% 26% 7%	2937
	21st to end	Day	46% 7% 18% 26% 3	3146
November	1st to 10th	Day	55% 7% 15% 16% 7% 1	1645
	11th to 20th	Day	51% 7% 18% 18% 6%	1470
	21st to end	Day	53% 8% 15% 18% 5% 1	1470
December	1st to 10th	Day	60% 7% 17% 11% 4 1	2360
	11th to 20th	Day	57% 8% 18% 13% 4	2714
	21st to end	Day	40% 7% 14% 27% 11% 1	3222
January	1st to 10th	Night	59% 6% 11% 13% 11% 1	8857
	11th to 20th	Night	41% 6% 16% 18% 17% 2	8820
	21st to end	Night	50% 6% 21% 17% 6%	9395
February	1st to 10th	Night	55% 7% 12% 15% 9% 1	2940
	11th to 20th	Night	51% 7% 15% 15% 11% 1	2751
	21st to end	Night	49% 7% 14% 20% 9% 1	2187
March	1st to 10th	Night	44% 6% 19% 22% 8%	5496
	11th to 20th	Night	46% 7% 17% 20% 9%	5185
	21st to end	Night	57% 7% 11% 15% 10%	5481
April	1st to 10th	Night	46% 10% 15% 18% 11% 1	2000
	11th to 20th	Night	49% 7% 13% 17% 13% 2	1871
	21st to end	Night	45% 8% 10% 24% 13% 1	1880
May	1st to 10th	Night	39% 5% 12% 27% 15% 2	4212
	11th to 20th	Night	52% 9% 12% 19% 7%	4105
	21st to end	Night	39% 8% 21% 25% 7%	4526
June	1st to 10th	Night	48% 7% 15% 19% 10% 1	1433
	11th to 20th	Night	55% 10% 14% 17% 4	1260
	21st to end	Night	41% 9% 23% 22% 5%	1260
July	1st to 10th	Night	35% 8% 21% 21% 14%	2690
	11th to 20th	Night	40% 8% 15% 29% 8%	3129
	21st to end	Night	37% 7% 22% 27% 6%	3369
August	1st to 10th	Night	38% 10% 21% 30% 2	2459
	11th to 20th	Night	41% 9% 19% 21% 10%	2748
	21st to end	Night	34% 8% 25% 27% 6%	3245
September	1st to 10th	Night	35% 7% 18% 29% 8% 2	1850
	11th to 20th	Night	36% 6% 18% 29% 11% 1	2195
	21st to end	Night	38% 7% 16% 26% 11% 2	2200
October	1st to 10th	Night	41% 8% 22% 22% 7%	3784
	11th to 20th	Night	40% 6% 14% 32% 7%	4208
	21st to end	Night	43% 8% 22% 23% 4	4622
November	1st to 10th	Night	57% 6% 12% 19% 6%	2870
	11th to 20th	Night	54% 8% 14% 17% 7%	3150
	21st to end	Night	54% 8% 15% 17% 5% 1	3150
December	1st to 10th	Night	59% 7% 15% 14% 5% 1	5040
	11th to 20th	Night	62% 7% 16% 11% 4	5697
	21st to end	Night	39% 7% 15% 26% 12% 2	7952

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% 5% 23% 36% 19% 5%	220
	11th to 20th	Day	10% 10% 30% 30% 16% 5%	220
	21st to end	Day	11% 7% 29% 31% 17% 4	242
February	1st to 10th	Day	11% 8% 25% 35% 15% 7%	220
	11th to 20th	Day	10% 6% 27% 34% 16% 7%	283
	21st to end	Day	4 8% 31% 34% 17% 7%	331
March	1st to 10th	Day	11% 7% 26% 34% 19% 3	540
	11th to 20th	Day	11% 4 24% 36% 17% 6%	540
	21st to end	Day	13% 7% 25% 32% 20% 3	594
April	1st to 10th	Day	9% 6% 27% 35% 17% 4	540
	11th to 20th	Day	14% 8% 21% 33% 18% 6%	532
	21st to end	Day	14% 9% 24% 31% 16% 6%	540
May	1st to 10th	Day	13% 9% 27% 30% 18% 4	716
	11th to 20th	Day	11% 7% 24% 32% 20% 6%	775
	21st to end	Day	15% 5% 21% 31% 20% 8%	844
June	1st to 10th	Day	19% 8% 25% 28% 16% 4	812
	11th to 20th	Day	20% 7% 20% 27% 20% 7%	990
	21st to end	Day	12% 4 18% 29% 27% 10%	990
July	1st to 10th	Day	12% 4 18% 31% 27% 9%	917
	11th to 20th	Day	13% 3 21% 28% 25% 9%	723
	21st to end	Day	11% 3 16% 31% 27% 11%	794
August	1st to 10th	Day	12% 5% 16% 29% 23% 15%	617
	11th to 20th	Day	8% 6% 23% 33% 22% 7%	517
	21st to end	Day	10% 5% 21% 28% 24% 11%	583
September	1st to 10th	Day	15% 8% 24% 25% 18% 9%	530
	11th to 20th	Day	18% 7% 20% 29% 22% 4	530
	21st to end	Day	11% 11% 34% 25% 17% 3	530
October	1st to 10th	Day	16% 6% 28% 26% 19% 5%	417
	11th to 20th	Day	13% 8% 31% 30% 15% 3	330
	21st to end	Day	15% 11% 36% 22% 12% 4	363
November	1st to 10th	Day	19% 5% 29% 31% 14% 2	327
	11th to 20th	Day	11% 5% 35% 32% 14% 5%	220
	21st to end	Day	9% 5% 32% 34% 12% 8%	220
December	1st to 10th	Day	10% 9% 27% 36% 13% 4	220
	11th to 20th	Day	12% 5% 30% 35% 16% 2	220
	21st to end	Day	10% 3 21% 38% 21% 7%	242
January	1st to 10th	Night	12% 4 31% 36% 14% 2	1068
	11th to 20th	Night	12% 8% 30% 36% 12% 2	1050
	21st to end	Night	16% 9% 32% 31% 10% 2	1068
February	1st to 10th	Night	9% 6% 27% 36% 19% 3	840
	11th to 20th	Night	9% 6% 29% 36% 18% 2	824
	21st to end	Night	8% 5% 28% 36% 18% 5%	692
March	1st to 10th	Night	8% 5% 28% 38% 17% 3	840
	11th to 20th	Night	12% 4 30% 37% 16% 2	840
	21st to end	Night	14% 5% 25% 36% 17% 2	924
April	1st to 10th	Night	12% 5% 25% 40% 16% 2	740
	11th to 20th	Night	16% 4 27% 36% 16% 2	623
	21st to end	Night	15% 7% 24% 38% 16% 1	620
May	1st to 10th	Night	13% 5% 26% 37% 17% 2	420
	11th to 20th	Night	13% 4 25% 38% 18% 2	414
	21st to end	Night	18% 5% 22% 33% 20% 1	455
June	1st to 10th	Night	20% 8% 22% 36% 13% 1	420
	11th to 20th	Night	19% 6% 18% 32% 21% 3	420
	21st to end	Night	10% 5% 14% 40% 25% 4	420
July	1st to 10th	Night	13% 3 17% 39% 27% 1	390
	11th to 20th	Night	12% 3 19% 40% 24% 3	388
	21st to end	Night	11% 3 13% 41% 27% 4	438
August	1st to 10th	Night	11% 4 14% 35% 32% 5%	398
	11th to 20th	Night	12% 3 20% 40% 24% 2	478
	21st to end	Night	13% 3 19% 41% 23% 1	660
September	1st to 10th	Night	15% 3 19% 38% 24% 2	650
	11th to 20th	Night	13% 5% 18% 43% 21% 1	800
	21st to end	Night	13% 6% 29% 37% 13% 2	800
October	1st to 10th	Night	16% 4 27% 39% 13% 2	800
	11th to 20th	Night	12% 5% 29% 38% 13% 2	800
	21st to end	Night	15% 7% 32% 32% 13% 1	890
November	1st to 10th	Night	18% 7% 31% 32% 10% 2	1025
	11th to 20th	Night	13% 6% 35% 33% 10% 3	1050
	21st to end	Night	9% 6% 35% 36% 12% 1	1050
December	1st to 10th	Night	13% 7% 32% 31% 14% 3	1050
	11th to 20th	Night	13% 7% 31% 34% 13% 2	1050
	21st to end	Night	11% 3 26% 40% 17% 3	1232

Page generated at 2026-06-28 00:10:44. Version ac07

Copyright © 2026 Attilla Danko.