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 2025-02-25.

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% 6% 5% 5% 4 7% 8% 34%	112478
	Night	8% 10% 4 10% 6% 6% 5% 4 6% 8% 33%	114773
Transparency	Day	44% 8% 18% 21% 7%	112215
	Night	47% 7% 16% 20% 8% 1	114682
Seeing	Day	13% 6% 23% 31% 20% 7%	16298
	Night	13% 5% 26% 37% 17% 2	24979

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% 6% 10% 9% 6% 6% 5% 8% 8% 24%	3083
	11th to 20th	Day	4 10% 4 11% 7% 6% 5% 7% 8% 9% 30%	3159
	21st to end	Day	8% 12% 4 8% 5% 5% 5% 4 6% 8% 34%	3728
February	1st to 10th	Day	8% 12% 4 11% 8% 7% 7% 4 8% 8% 22%	1600
	11th to 20th	Day	5% 11% 7% 11% 6% 6% 7% 5% 8% 5% 28%	1671
	21st to end	Day	3 12% 6% 10% 9% 7% 6% 5% 6% 7% 29%	1501
March	1st to 10th	Day	4 12% 6% 13% 9% 5% 5% 4 5% 9% 28%	3138
	11th to 20th	Day	5% 9% 5% 10% 6% 4 4 5% 7% 9% 37%	3442
	21st to end	Day	7% 14% 5% 16% 6% 6% 5% 4 6% 7% 23%	3910
April	1st to 10th	Day	7% 12% 4 9% 5% 6% 6% 6% 7% 7% 31%	2370
	11th to 20th	Day	9% 12% 3 8% 5% 7% 6% 5% 6% 8% 33%	2440
	21st to end	Day	11% 11% 3 8% 5% 6% 6% 5% 7% 8% 32%	2478
May	1st to 10th	Day	18% 9% 3 6% 5% 5% 5% 4 6% 7% 32%	4694
	11th to 20th	Day	13% 9% 3 8% 5% 5% 6% 5% 7% 8% 31%	4694
	21st to end	Day	11% 8% 3 9% 7% 6% 6% 5% 7% 9% 27%	5161
June	1st to 10th	Day	15% 7% 3 8% 6% 6% 6% 5% 6% 7% 31%	2878
	11th to 20th	Day	19% 8% 3 8% 6% 5% 6% 5% 7% 8% 25%	3040
	21st to end	Day	9% 6% 2 6% 5% 5% 6% 5% 8% 9% 39%	3040
July	1st to 10th	Day	9% 5% 2 6% 5% 5% 4 4 6% 8% 45%	5060
	11th to 20th	Day	9% 5% 2 5% 4 4 5% 4 6% 9% 46%	4746
	21st to end	Day	13% 5% 2 5% 4 4 4 4 6% 8% 44%	5138
August	1st to 10th	Day	13% 6% 2 6% 4 4 4 3 6% 8% 43%	4539
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 5% 7% 9% 44%	4192
	21st to end	Day	11% 5% 2 7% 5% 5% 4 4 6% 9% 42%	4521
September	1st to 10th	Day	12% 5% 2 8% 5% 5% 5% 4 6% 7% 42%	2419
	11th to 20th	Day	14% 7% 2 5% 5% 5% 4 4 6% 7% 40%	2095
	21st to end	Day	9% 6% 2 7% 5% 4 5% 4 7% 9% 42%	2090
October	1st to 10th	Day	10% 8% 3 9% 5% 5% 5% 4 7% 7% 38%	3305
	11th to 20th	Day	11% 10% 3 8% 5% 4 5% 3 6% 8% 36%	2847
	21st to end	Day	7% 12% 5% 8% 5% 5% 6% 4 6% 7% 34%	3049
November	1st to 10th	Day	10% 14% 5% 9% 8% 6% 6% 4 6% 6% 27%	1565
	11th to 20th	Day	8% 11% 5% 12% 6% 7% 4 4 6% 8% 29%	1400
	21st to end	Day	5% 12% 6% 13% 8% 7% 4 4 7% 7% 28%	1400
December	1st to 10th	Day	9% 15% 5% 12% 8% 7% 6% 3 6% 7% 23%	2290
	11th to 20th	Day	9% 14% 4 11% 6% 7% 7% 6% 6% 9% 21%	2642
	21st to end	Day	4 6% 3 9% 8% 6% 6% 5% 8% 8% 38%	3153
January	1st to 10th	Night	4 11% 5% 16% 9% 7% 6% 3 7% 7% 25%	6841
	11th to 20th	Night	4 10% 3 10% 7% 6% 6% 4 6% 7% 37%	6739
	21st to end	Night	8% 14% 5% 9% 6% 5% 5% 4 6% 7% 32%	7258
February	1st to 10th	Night	6% 12% 5% 12% 10% 7% 7% 4 6% 6% 26%	2800
	11th to 20th	Night	6% 11% 4 10% 7% 8% 6% 5% 7% 7% 29%	2615
	21st to end	Night	4 11% 5% 10% 7% 6% 6% 6% 7% 8% 30%	2043
March	1st to 10th	Night	3 9% 5% 13% 8% 6% 6% 4 6% 10% 29%	3736
	11th to 20th	Night	5% 9% 3 10% 6% 5% 5% 4 7% 10% 35%	3537
	21st to end	Night	8% 12% 6% 16% 7% 6% 5% 4 6% 6% 24%	3741
April	1st to 10th	Night	8% 10% 3 9% 7% 8% 6% 6% 6% 7% 30%	1810
	11th to 20th	Night	10% 12% 4 8% 7% 6% 5% 5% 7% 6% 30%	1691
	21st to end	Night	11% 10% 3 8% 5% 5% 5% 5% 6% 7% 35%	1701
May	1st to 10th	Night	21% 9% 3 8% 5% 4 5% 4 5% 5% 31%	2265
	11th to 20th	Night	12% 11% 3 9% 6% 6% 6% 5% 6% 8% 29%	2199
	21st to end	Night	12% 9% 3 8% 6% 7% 5% 5% 7% 9% 28%	2414
June	1st to 10th	Night	15% 9% 3 7% 5% 5% 7% 4 7% 7% 31%	1297
	11th to 20th	Night	16% 10% 3 9% 6% 5% 7% 6% 8% 9% 22%	1140
	21st to end	Night	9% 6% 3 7% 6% 6% 4 5% 9% 7% 36%	1140
July	1st to 10th	Night	9% 6% 2 6% 4 4 5% 5% 5% 8% 46%	1900
	11th to 20th	Night	8% 4 3 6% 4 5% 6% 6% 6% 11% 42%	2210
	21st to end	Night	14% 6% 2 5% 5% 4 4 4 6% 9% 42%	2414
August	1st to 10th	Night	11% 6% 2 6% 5% 5% 5% 4 6% 7% 43%	2390
	11th to 20th	Night	13% 7% 3 7% 5% 4 4 4 6% 8% 39%	2640
	21st to end	Night	14% 4 2 5% 4 4 4 4 6% 9% 44%	3146
September	1st to 10th	Night	12% 6% 3 6% 3 5% 4 3 5% 8% 45%	1757
	11th to 20th	Night	12% 8% 2 5% 5% 3 5% 4 5% 7% 44%	2085
	21st to end	Night	10% 7% 2 7% 5% 4 5% 3 6% 8% 44%	2090
October	1st to 10th	Night	8% 7% 3 8% 6% 6% 5% 5% 6% 9% 37%	3668
	11th to 20th	Night	10% 7% 3 7% 5% 4 5% 5% 6% 10% 38%	4078
	21st to end	Night	8% 9% 3 9% 5% 5% 6% 5% 7% 10% 34%	4478
November	1st to 10th	Night	13% 15% 5% 9% 7% 5% 5% 4 5% 7% 27%	2730
	11th to 20th	Night	8% 14% 5% 11% 6% 7% 6% 4 6% 6% 27%	3000
	21st to end	Night	6% 14% 5% 13% 7% 5% 6% 4 6% 7% 27%	3000
December	1st to 10th	Night	10% 16% 5% 11% 6% 6% 5% 4 6% 7% 24%	4890
	11th to 20th	Night	10% 15% 5% 12% 7% 7% 6% 5% 6% 7% 19%	5550
	21st to end	Night	3 6% 4 10% 6% 6% 6% 4 7% 8% 41%	7780

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	53% 7% 13% 17% 9% 1	3083
	11th to 20th	Day	47% 7% 13% 13% 19% 2	3180
	21st to end	Day	46% 7% 22% 20% 6%	3728
February	1st to 10th	Day	54% 8% 14% 15% 9%	1600
	11th to 20th	Day	51% 9% 15% 17% 8%	1679
	21st to end	Day	51% 7% 15% 20% 6%	1501
March	1st to 10th	Day	52% 6% 16% 20% 6%	3138
	11th to 20th	Day	40% 10% 20% 21% 9%	3443
	21st to end	Day	56% 8% 16% 17% 3	3910
April	1st to 10th	Day	47% 10% 19% 17% 7% 1	2370
	11th to 20th	Day	47% 7% 16% 22% 8%	2440
	21st to end	Day	48% 7% 17% 20% 6% 1	2478
May	1st to 10th	Day	49% 7% 11% 24% 9%	4694
	11th to 20th	Day	45% 9% 13% 25% 8% 1	4698
	21st to end	Day	47% 10% 19% 15% 8%	5162
June	1st to 10th	Day	49% 8% 15% 20% 7% 1	2878
	11th to 20th	Day	54% 10% 17% 15% 4	3040
	21st to end	Day	38% 11% 23% 21% 6%	3040
July	1st to 10th	Day	34% 8% 17% 28% 12% 1	4981
	11th to 20th	Day	32% 9% 18% 28% 12%	4664
	21st to end	Day	37% 8% 22% 24% 8%	4976
August	1st to 10th	Day	36% 10% 21% 31% 2	4521
	11th to 20th	Day	33% 9% 24% 24% 9%	4234
	21st to end	Day	37% 9% 28% 23% 4	4520
September	1st to 10th	Day	40% 8% 21% 23% 7% 1	2419
	11th to 20th	Day	41% 6% 22% 24% 7%	2095
	21st to end	Day	37% 10% 17% 26% 8% 2	2090
October	1st to 10th	Day	42% 7% 24% 21% 5%	3305
	11th to 20th	Day	45% 6% 16% 26% 7%	2847
	21st to end	Day	46% 7% 18% 26% 3	3049
November	1st to 10th	Day	56% 7% 15% 15% 6% 1	1565
	11th to 20th	Day	52% 7% 17% 18% 5%	1400
	21st to end	Day	53% 8% 15% 18% 5% 1	1400
December	1st to 10th	Day	59% 7% 18% 11% 4 1	2290
	11th to 20th	Day	57% 8% 19% 13% 4	2644
	21st to end	Day	40% 7% 14% 27% 11% 1	3153
January	1st to 10th	Night	55% 6% 12% 15% 10% 1	6841
	11th to 20th	Night	45% 6% 13% 16% 18% 1	6780
	21st to end	Night	50% 6% 21% 18% 6%	7258
February	1st to 10th	Night	56% 7% 12% 15% 9% 1	2800
	11th to 20th	Night	51% 7% 16% 15% 10% 1	2617
	21st to end	Night	48% 8% 15% 20% 8% 1	2043
March	1st to 10th	Night	48% 6% 18% 21% 7%	3736
	11th to 20th	Night	40% 8% 22% 19% 11% 1	3537
	21st to end	Night	55% 9% 13% 16% 6%	3741
April	1st to 10th	Night	47% 10% 14% 18% 10% 1	1810
	11th to 20th	Night	50% 6% 13% 17% 12% 2	1691
	21st to end	Night	45% 8% 10% 23% 13% 1	1701
May	1st to 10th	Night	52% 6% 10% 20% 11% 1	2265
	11th to 20th	Night	47% 10% 12% 20% 10% 1	2199
	21st to end	Night	47% 9% 19% 16% 9% 1	2414
June	1st to 10th	Night	48% 7% 15% 18% 10% 1	1297
	11th to 20th	Night	54% 10% 15% 17% 4	1140
	21st to end	Night	40% 10% 23% 21% 6%	1140
July	1st to 10th	Night	35% 8% 17% 25% 14% 1	1870
	11th to 20th	Night	33% 10% 18% 28% 10%	2169
	21st to end	Night	38% 7% 20% 27% 8%	2337
August	1st to 10th	Night	38% 10% 21% 30% 2	2383
	11th to 20th	Night	41% 9% 19% 21% 10%	2664
	21st to end	Night	35% 8% 25% 26% 5%	3146
September	1st to 10th	Night	37% 8% 17% 28% 8% 2	1757
	11th to 20th	Night	37% 6% 18% 28% 9% 1	2085
	21st to end	Night	38% 7% 15% 26% 12% 2	2090
October	1st to 10th	Night	41% 8% 22% 22% 7%	3668
	11th to 20th	Night	40% 6% 15% 32% 6%	4078
	21st to end	Night	43% 8% 22% 22% 4	4478
November	1st to 10th	Night	57% 6% 12% 19% 5%	2730
	11th to 20th	Night	55% 8% 13% 17% 6%	3000
	21st to end	Night	54% 8% 15% 17% 4 1	3000
December	1st to 10th	Night	58% 7% 15% 14% 6% 1	4890
	11th to 20th	Night	62% 7% 16% 11% 4	5547
	21st to end	Night	39% 7% 15% 26% 12% 2	7780

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% 5% 22% 38% 20% 6%	200
	11th to 20th	Day	10% 8% 29% 31% 17% 6%	200
	21st to end	Day	12% 8% 29% 30% 16% 4	220
February	1st to 10th	Day	11% 9% 24% 35% 15% 8%	200
	11th to 20th	Day	9% 5% 28% 34% 16% 8%	257
	21st to end	Day	4 6% 29% 36% 18% 8%	289
March	1st to 10th	Day	11% 6% 26% 35% 20% 3	460
	11th to 20th	Day	11% 4 24% 37% 18% 7%	460
	21st to end	Day	14% 6% 24% 32% 21% 4	506
April	1st to 10th	Day	10% 6% 29% 35% 17% 4	460
	11th to 20th	Day	14% 6% 21% 34% 18% 7%	452
	21st to end	Day	14% 9% 24% 32% 15% 6%	460
May	1st to 10th	Day	14% 8% 25% 29% 19% 4	620
	11th to 20th	Day	11% 6% 24% 33% 20% 6%	675
	21st to end	Day	15% 5% 20% 31% 20% 9%	734
June	1st to 10th	Day	18% 7% 25% 29% 17% 4	708
	11th to 20th	Day	20% 7% 19% 27% 20% 7%	870
	21st to end	Day	12% 4 17% 29% 28% 10%	870
July	1st to 10th	Day	12% 4 17% 31% 27% 9%	856
	11th to 20th	Day	13% 3 20% 29% 27% 9%	673
	21st to end	Day	12% 3 17% 31% 26% 11%	739
August	1st to 10th	Day	13% 5% 16% 29% 22% 15%	573
	11th to 20th	Day	7% 6% 23% 34% 23% 8%	477
	21st to end	Day	10% 6% 23% 29% 23% 9%	539
September	1st to 10th	Day	17% 8% 25% 25% 17% 8%	490
	11th to 20th	Day	19% 8% 21% 29% 20% 3	490
	21st to end	Day	11% 10% 32% 26% 18% 3	490
October	1st to 10th	Day	17% 6% 28% 26% 18% 5%	383
	11th to 20th	Day	14% 7% 30% 31% 15% 3	300
	21st to end	Day	15% 11% 36% 22% 13% 4	330
November	1st to 10th	Day	19% 4 26% 34% 15% 2	297
	11th to 20th	Day	12% 5% 35% 32% 13% 5%	200
	21st to end	Day	8% 6% 33% 34% 12% 7%	200
December	1st to 10th	Day	8% 9% 26% 39% 15% 5%	200
	11th to 20th	Day	12% 6% 27% 37% 17% 3	200
	21st to end	Day	7% 3 20% 40% 23% 7%	220
January	1st to 10th	Night	11% 4 30% 37% 15% 2	1017
	11th to 20th	Night	12% 7% 29% 37% 13% 2	1000
	21st to end	Night	16% 9% 32% 30% 11% 2	1017
February	1st to 10th	Night	9% 6% 27% 36% 20% 3	800
	11th to 20th	Night	9% 5% 29% 37% 19% 2	784
	21st to end	Night	8% 4 27% 37% 19% 5%	646
March	1st to 10th	Night	8% 5% 27% 38% 17% 4	760
	11th to 20th	Night	12% 4 30% 36% 16% 2	760
	21st to end	Night	14% 5% 25% 36% 18% 2	836
April	1st to 10th	Night	11% 5% 25% 40% 17% 2	670
	11th to 20th	Night	16% 3 26% 36% 16% 2	563
	21st to end	Night	15% 6% 24% 38% 16% 1	561
May	1st to 10th	Night	14% 6% 25% 36% 18% 2	380
	11th to 20th	Night	13% 4 25% 39% 18% 2	374
	21st to end	Night	19% 5% 22% 33% 20% 1	411
June	1st to 10th	Night	19% 7% 21% 38% 14% 2	380
	11th to 20th	Night	19% 6% 18% 32% 22% 3	380
	21st to end	Night	11% 4 15% 41% 25% 5%	380
July	1st to 10th	Night	14% 2 16% 39% 28% 1	370
	11th to 20th	Night	12% 2 18% 40% 24% 3	368
	21st to end	Night	12% 3 13% 41% 27% 4	416
August	1st to 10th	Night	10% 4 14% 36% 30% 6%	378
	11th to 20th	Night	11% 3 19% 40% 25% 2	454
	21st to end	Night	13% 3 19% 42% 22% 1	627
September	1st to 10th	Night	15% 2 19% 39% 22% 2	617
	11th to 20th	Night	13% 5% 18% 42% 20% 1	760
	21st to end	Night	13% 6% 28% 38% 14% 2	760
October	1st to 10th	Night	16% 3 26% 39% 13% 2	760
	11th to 20th	Night	13% 4 28% 39% 13% 2	760
	21st to end	Night	15% 7% 31% 33% 14% 1	845
November	1st to 10th	Night	18% 6% 30% 33% 11% 2	975
	11th to 20th	Night	14% 6% 34% 33% 10% 3	1000
	21st to end	Night	10% 6% 35% 37% 12% 1	1000
December	1st to 10th	Night	12% 7% 31% 32% 14% 4	1000
	11th to 20th	Night	13% 7% 30% 35% 14% 2	1000
	21st to end	Night	10% 3 25% 40% 18% 3	1170

Page generated at 2025-02-22 04:51:38. Version ac07

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