White Rock
Clear Sky Chart
History

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

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	20% 8% 3 7% 4 4 4 3 5% 6% 36%	135851
	Night	25% 9% 3 7% 4 4 4 3 5% 6% 30%	136021
Transparency	Day	32% 37% 11% 13% 6% 1	135560
	Night	37% 36% 9% 12% 5% 1	135890
Seeing	Day	25% 7% 18% 25% 19% 6%	17265
	Night	31% 6% 14% 24% 21% 4	25572

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	38% 12% 4 10% 5% 5% 5% 3 4 5% 9%	4125
	11th to 20th	Day	20% 11% 3 7% 4 5% 4 3 5% 10% 28%	4157
	21st to end	Day	37% 10% 3 7% 4 3 3 2 4 5% 22%	5261
February	1st to 10th	Day	31% 12% 3 9% 5% 4 4 3 5% 5% 18%	1890
	11th to 20th	Day	29% 10% 3 8% 5% 5% 4 3 4 5% 23%	1845
	21st to end	Day	26% 13% 3 7% 5% 4 5% 4 4 5% 22%	1584
March	1st to 10th	Day	24% 13% 3 9% 5% 6% 5% 3 6% 6% 20%	4752
	11th to 20th	Day	22% 11% 3 9% 5% 4 5% 4 6% 6% 25%	5110
	21st to end	Day	26% 13% 4 9% 5% 4 5% 4 6% 6% 19%	5773
April	1st to 10th	Day	24% 11% 3 7% 6% 4 5% 4 5% 7% 24%	2520
	11th to 20th	Day	19% 9% 3 7% 5% 6% 5% 3 6% 6% 30%	2580
	21st to end	Day	21% 9% 3 7% 5% 4 4 4 6% 6% 33%	2794
May	1st to 10th	Day	13% 7% 3 7% 4 3 4 3 4 6% 46%	6460
	11th to 20th	Day	16% 9% 3 7% 4 4 5% 4 6% 9% 33%	6417
	21st to end	Day	20% 8% 3 6% 4 4 5% 4 6% 7% 34%	7137
June	1st to 10th	Day	19% 7% 3 5% 5% 4 5% 4 6% 7% 35%	3001
	11th to 20th	Day	20% 7% 2 8% 5% 4 4 4 5% 7% 33%	3000
	21st to end	Day	13% 7% 2 6% 5% 4 6% 3 6% 7% 41%	3000
July	1st to 10th	Day	8% 3 1 4 3 2 3 2 4 6% 64%	6780
	11th to 20th	Day	4 3 1 3 2 2 2 2 3 5% 73%	6780
	21st to end	Day	8% 3 1 3 2 2 3 3 5% 6% 63%	6930
August	1st to 10th	Day	5% 3 1 3 2 3 2 3 4 7% 68%	4511
	11th to 20th	Day	10% 6% 2 5% 4 4 4 3 5% 7% 49%	4156
	21st to end	Day	14% 7% 3 6% 4 4 5% 3 5% 6% 44%	4542
September	1st to 10th	Day	11% 6% 3 6% 4 4 4 3 5% 7% 49%	2400
	11th to 20th	Day	15% 8% 2 7% 4 4 4 3 4 7% 41%	2400
	21st to end	Day	22% 8% 2 6% 4 3 4 3 5% 7% 35%	2105
October	1st to 10th	Day	19% 7% 2 7% 5% 5% 5% 4 6% 8% 31%	3280
	11th to 20th	Day	33% 7% 2 6% 4 4 4 3 5% 7% 25%	3261
	21st to end	Day	28% 10% 3 10% 6% 5% 5% 3 5% 6% 19%	3291
November	1st to 10th	Day	36% 10% 3 7% 4 5% 4 4 5% 6% 18%	1648
	11th to 20th	Day	35% 11% 3 8% 4 3 3 3 4 5% 21%	1680
	21st to end	Day	35% 12% 3 7% 5% 5% 4 3 4 5% 17%	1680
December	1st to 10th	Day	25% 11% 3 8% 5% 5% 5% 4 6% 7% 20%	2700
	11th to 20th	Day	38% 14% 4 7% 4 5% 4 3 4 4 12%	2726
	21st to end	Day	33% 15% 5% 10% 6% 5% 4 3 4 4 11%	3575
January	1st to 10th	Night	36% 14% 4 10% 5% 4 4 3 4 5% 11%	8770
	11th to 20th	Night	24% 8% 2 5% 4 4 4 4 7% 11% 27%	8738
	21st to end	Night	34% 9% 3 7% 4 3 3 2 4 7% 23%	8920
February	1st to 10th	Night	32% 12% 3 8% 4 4 3 3 5% 5% 21%	2730
	11th to 20th	Night	28% 13% 3 8% 4 4 4 3 4 5% 24%	2662
	21st to end	Night	29% 11% 3 7% 5% 4 4 3 4 6% 24%	2221
March	1st to 10th	Night	22% 14% 3 8% 5% 5% 5% 3 6% 7% 22%	5361
	11th to 20th	Night	25% 10% 3 7% 4 4 5% 4 6% 7% 25%	5109
	21st to end	Night	33% 14% 4 9% 5% 3 4 3 4 5% 16%	5443
April	1st to 10th	Night	26% 11% 4 8% 4 5% 4 4 5% 5% 24%	2100
	11th to 20th	Night	20% 10% 3 8% 5% 4 5% 4 5% 6% 30%	1987
	21st to end	Night	23% 8% 3 8% 5% 5% 4 3 5% 6% 30%	1825
May	1st to 10th	Night	15% 9% 3 6% 3 4 3 2 3 4 48%	3720
	11th to 20th	Night	16% 8% 3 8% 4 4 4 3 6% 7% 36%	3696
	21st to end	Night	16% 9% 3 9% 6% 6% 6% 4 4 7% 30%	3976
June	1st to 10th	Night	20% 7% 3 8% 4 5% 4 4 6% 6% 35%	1400
	11th to 20th	Night	16% 10% 4 8% 6% 5% 5% 5% 6% 6% 31%	1400
	21st to end	Night	14% 7% 2 6% 5% 5% 3 4 5% 8% 43%	1400
July	1st to 10th	Night	8% 3 1 4 3 3 3 3 4 6% 62%	3180
	11th to 20th	Night	3 2 1 3 2 2 2 2 2 5% 78%	3180
	21st to end	Night	5% 4 2 4 3 3 3 2 4 6% 64%	3911
August	1st to 10th	Night	5% 2 1 2 2 2 3 3 4 6% 72%	2637
	11th to 20th	Night	10% 5% 2 6% 4 3 3 3 4 8% 51%	2902
	21st to end	Night	11% 4 2 6% 5% 4 4 3 7% 8% 44%	3370
September	1st to 10th	Night	8% 3 2 4 4 3 3 3 5% 8% 57%	2000
	11th to 20th	Night	18% 6% 2 6% 5% 4 3 2 6% 5% 43%	2000
	21st to end	Night	20% 5% 2 7% 5% 3 4 3 4 6% 39%	2292
October	1st to 10th	Night	18% 7% 2 7% 5% 5% 5% 4 6% 7% 34%	3920
	11th to 20th	Night	29% 7% 3 6% 4 4 4 3 5% 7% 28%	3897
	21st to end	Night	29% 9% 3 8% 4 4 4 3 4 6% 26%	4507
November	1st to 10th	Night	37% 9% 3 7% 5% 5% 5% 3 5% 5% 17%	2870
	11th to 20th	Night	36% 9% 3 6% 4 4 4 3 4 6% 21%	2940
	21st to end	Night	34% 11% 3 9% 5% 4 4 3 4 5% 18%	2940
December	1st to 10th	Night	26% 9% 3 6% 5% 5% 7% 5% 7% 7% 19%	4700
	11th to 20th	Night	41% 13% 4 9% 4 4 4 3 3 4 11%	5694
	21st to end	Night	33% 14% 4 10% 5% 4 4 3 4 4 15%	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	63% 23% 6% 7% 1	4125
	11th to 20th	Day	39% 21% 9% 21% 8% 2	4178
	21st to end	Day	53% 22% 5% 14% 4 1	5262
February	1st to 10th	Day	29% 59% 3 3 3 3	1890
	11th to 20th	Day	28% 58% 4 6% 3	1854
	21st to end	Day	27% 59% 5% 5% 3 1	1584
March	1st to 10th	Day	45% 30% 10% 11% 3	4752
	11th to 20th	Day	43% 29% 9% 16% 3	5110
	21st to end	Day	49% 31% 11% 6% 2	5773
April	1st to 10th	Day	24% 60% 8% 7% 1	2520
	11th to 20th	Day	23% 60% 6% 8% 2 1	2580
	21st to end	Day	21% 59% 7% 9% 4	2794
May	1st to 10th	Day	27% 29% 12% 21% 10%	6460
	11th to 20th	Day	36% 32% 17% 14% 2	6420
	21st to end	Day	35% 30% 19% 14% 2	7138
June	1st to 10th	Day	16% 62% 10% 9% 3	3001
	11th to 20th	Day	19% 62% 7% 8% 3	3000
	21st to end	Day	17% 60% 10% 9% 3	3000
July	1st to 10th	Day	18% 26% 16% 17% 17% 7%	6706
	11th to 20th	Day	9% 25% 17% 28% 19% 2	6699
	21st to end	Day	20% 25% 27% 21% 6% 1	6735
August	1st to 10th	Day	9% 36% 9% 32% 14% 1	4494
	11th to 20th	Day	27% 36% 22% 12% 4 1	4195
	21st to end	Day	30% 36% 12% 12% 9% 1	4541
September	1st to 10th	Day	19% 58% 8% 10% 4 1	2400
	11th to 20th	Day	22% 57% 7% 8% 6% 1	2400
	21st to end	Day	22% 57% 8% 9% 4	2105
October	1st to 10th	Day	36% 37% 12% 14% 2	3280
	11th to 20th	Day	41% 36% 7% 10% 6%	3261
	21st to end	Day	46% 36% 7% 8% 2	3291
November	1st to 10th	Day	32% 54% 7% 6% 1	1648
	11th to 20th	Day	29% 55% 5% 7% 2 2	1680
	21st to end	Day	29% 54% 6% 5% 4 1	1680
December	1st to 10th	Day	40% 36% 6% 14% 3	2700
	11th to 20th	Day	55% 32% 6% 5% 1	2729
	21st to end	Day	59% 29% 7% 4 1	3575
January	1st to 10th	Night	62% 23% 6% 7% 2	8770
	11th to 20th	Night	38% 24% 10% 17% 10% 1	8779
	21st to end	Night	49% 23% 8% 15% 5%	8920
February	1st to 10th	Night	27% 59% 3 4 4 3	2730
	11th to 20th	Night	29% 58% 4 5% 3 1	2664
	21st to end	Night	23% 60% 6% 7% 3 1	2221
March	1st to 10th	Night	43% 31% 11% 10% 6%	5361
	11th to 20th	Night	39% 31% 11% 17% 3	5110
	21st to end	Night	52% 31% 9% 6% 1	5443
April	1st to 10th	Night	26% 59% 5% 9% 1	2100
	11th to 20th	Night	22% 60% 7% 7% 3 1	1987
	21st to end	Night	21% 59% 7% 7% 5%	1825
May	1st to 10th	Night	29% 28% 7% 25% 10%	3717
	11th to 20th	Night	34% 30% 20% 13% 3	3696
	21st to end	Night	39% 31% 14% 14% 1	3976
June	1st to 10th	Night	15% 62% 9% 10% 3 1	1400
	11th to 20th	Night	18% 62% 7% 11% 3	1400
	21st to end	Night	17% 60% 9% 9% 4	1400
July	1st to 10th	Night	18% 25% 15% 21% 15% 6%	3145
	11th to 20th	Night	7% 24% 16% 28% 21% 2	3138
	21st to end	Night	20% 24% 20% 25% 10% 1	3801
August	1st to 10th	Night	7% 35% 9% 26% 21% 1	2629
	11th to 20th	Night	28% 35% 20% 12% 4 1	2929
	21st to end	Night	27% 36% 14% 14% 8% 1	3370
September	1st to 10th	Night	15% 59% 10% 10% 6% 1	2000
	11th to 20th	Night	21% 57% 7% 8% 6% 1	2000
	21st to end	Night	21% 57% 7% 11% 4	2292
October	1st to 10th	Night	35% 38% 14% 11% 2	3920
	11th to 20th	Night	40% 36% 7% 12% 5%	3897
	21st to end	Night	42% 36% 10% 9% 3	4507
November	1st to 10th	Night	33% 54% 6% 5% 2	2870
	11th to 20th	Night	30% 55% 5% 6% 3 2	2940
	21st to end	Night	32% 54% 4 4 5% 1	2940
December	1st to 10th	Night	42% 38% 8% 9% 3	4700
	11th to 20th	Night	56% 32% 5% 5% 1	5690
	21st to end	Night	55% 28% 6% 8% 1	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	45% 8% 16% 17% 9% 6%	220
	11th to 20th	Day	40% 10% 11% 17% 13% 9%	220
	21st to end	Day	46% 6% 13% 13% 16% 6%	265
February	1st to 10th	Day	39% 10% 16% 20% 13% 3	430
	11th to 20th	Day	36% 8% 18% 21% 16% 2	425
	21st to end	Day	34% 11% 21% 23% 8% 3	370
March	1st to 10th	Day	34% 7% 21% 20% 14% 4	540
	11th to 20th	Day	36% 9% 15% 18% 16% 5%	540
	21st to end	Day	35% 9% 20% 21% 12% 2	594
April	1st to 10th	Day	29% 10% 18% 24% 16% 3	540
	11th to 20th	Day	23% 7% 26% 25% 15% 5%	532
	21st to end	Day	25% 8% 26% 23% 14% 5%	540
May	1st to 10th	Day	15% 8% 21% 31% 20% 4	543
	11th to 20th	Day	21% 5% 19% 28% 21% 7%	568
	21st to end	Day	19% 5% 25% 27% 17% 7%	645
June	1st to 10th	Day	20% 5% 23% 30% 19% 4	731
	11th to 20th	Day	19% 6% 25% 28% 19% 3	730
	21st to end	Day	15% 5% 20% 31% 24% 5%	730
July	1st to 10th	Day	13% 3 17% 35% 28% 5%	721
	11th to 20th	Day	8% 4 18% 36% 28% 6%	718
	21st to end	Day	9% 2 13% 33% 34% 9%	605
August	1st to 10th	Day	11% 4 13% 29% 31% 11%	527
	11th to 20th	Day	10% 4 11% 36% 29% 10%	517
	21st to end	Day	13% 5% 19% 29% 24% 10%	583
September	1st to 10th	Day	16% 3 17% 32% 24% 9%	530
	11th to 20th	Day	20% 5% 17% 25% 26% 6%	530
	21st to end	Day	25% 7% 19% 22% 21% 5%	530
October	1st to 10th	Day	27% 6% 18% 21% 20% 8%	530
	11th to 20th	Day	36% 7% 13% 20% 18% 7%	530
	21st to end	Day	36% 12% 16% 19% 12% 5%	439
November	1st to 10th	Day	37% 10% 17% 20% 12% 3	220
	11th to 20th	Day	40% 15% 10% 12% 15% 9%	220
	21st to end	Day	43% 8% 18% 14% 8% 10%	220
December	1st to 10th	Day	45% 9% 13% 9% 11% 13%	220
	11th to 20th	Day	49% 12% 14% 13% 11% 1	220
	21st to end	Day	45% 11% 15% 16% 10% 2	242
January	1st to 10th	Night	48% 8% 15% 17% 9% 3	950
	11th to 20th	Night	42% 6% 12% 21% 16% 3	942
	21st to end	Night	43% 5% 12% 23% 15% 3	924
February	1st to 10th	Night	38% 7% 19% 23% 11% 1	840
	11th to 20th	Night	36% 7% 17% 25% 14% 2	824
	21st to end	Night	37% 7% 21% 20% 12% 2	692
March	1st to 10th	Night	35% 8% 16% 21% 18% 2	840
	11th to 20th	Night	39% 8% 15% 22% 14% 3	840
	21st to end	Night	37% 6% 15% 23% 15% 3	924
April	1st to 10th	Night	34% 6% 12% 27% 17% 3	840
	11th to 20th	Night	24% 8% 17% 29% 17% 5%	739
	21st to end	Night	26% 5% 17% 30% 18% 4	565
May	1st to 10th	Night	18% 4 12% 31% 33% 3	420
	11th to 20th	Night	19% 4 8% 29% 35% 5%	414
	21st to end	Night	18% 4 9% 33% 31% 4	455
June	1st to 10th	Night	23% 4 11% 30% 29% 4	400
	11th to 20th	Night	17% 4 19% 31% 25% 4	400
	21st to end	Night	15% 4 11% 29% 36% 5%	400
July	1st to 10th	Night	12% 2 8% 30% 43% 5%	390
	11th to 20th	Night	9% 2 8% 31% 41% 8%	388
	21st to end	Night	5% 1 6% 28% 50% 10%	438
August	1st to 10th	Night	10% 2 5% 21% 45% 17%	413
	11th to 20th	Night	10% 2 5% 23% 46% 15%	594
	21st to end	Night	12% 3 8% 25% 41% 10%	733
September	1st to 10th	Night	11% 2 6% 34% 38% 10%	800
	11th to 20th	Night	20% 4 12% 19% 34% 12%	800
	21st to end	Night	25% 3 12% 27% 28% 6%	800
October	1st to 10th	Night	24% 5% 13% 25% 27% 7%	800
	11th to 20th	Night	33% 5% 14% 23% 19% 5%	800
	21st to end	Night	38% 6% 15% 25% 13% 4	880
November	1st to 10th	Night	43% 8% 15% 24% 10%	820
	11th to 20th	Night	44% 6% 17% 19% 12% 2	840
	21st to end	Night	41% 6% 15% 21% 13% 3	840
December	1st to 10th	Night	36% 5% 15% 24% 16% 3	840
	11th to 20th	Night	51% 9% 15% 18% 6% 1	942
	21st to end	Night	48% 8% 18% 18% 7% 1	1045

Page generated at 2026-05-28 22:53:31. Version ac07

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