Okanagan Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Okanagan Observatory clear sky chart from 2005-11-06 to 2025-04-26.

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	9% 12% 4 10% 6% 5% 5% 4 6% 6% 33%	117622
	Night	9% 13% 4 10% 6% 5% 5% 4 6% 7% 32%	119690
Transparency	Day	48% 8% 13% 19% 10% 3	117387
	Night	48% 8% 14% 19% 9% 2	119572
Seeing	Day	19% 13% 25% 25% 14% 4	15812
	Night	19% 9% 19% 32% 18% 3	24506

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	8% 21% 8% 20% 9% 7% 4 2 4 5% 12%	3639
	11th to 20th	Day	6% 13% 7% 13% 7% 4 5% 4 6% 9% 27%	3616
	21st to end	Day	9% 23% 7% 14% 6% 3 4 3 4 4 24%	4004
February	1st to 10th	Day	11% 20% 5% 12% 7% 4 4 4 7% 5% 22%	1620
	11th to 20th	Day	7% 18% 8% 15% 5% 5% 5% 4 5% 6% 22%	1758
	21st to end	Day	7% 21% 6% 16% 8% 7% 5% 4 4 4 16%	1650
March	1st to 10th	Day	5% 12% 6% 15% 7% 8% 7% 5% 7% 6% 21%	4518
	11th to 20th	Day	5% 15% 5% 11% 6% 5% 5% 4 6% 6% 33%	4707
	21st to end	Day	10% 19% 5% 12% 7% 7% 6% 4 6% 5% 20%	5959
April	1st to 10th	Day	10% 18% 5% 10% 8% 7% 7% 6% 6% 5% 19%	2400
	11th to 20th	Day	9% 17% 5% 10% 7% 6% 5% 5% 6% 6% 25%	2411
	21st to end	Day	10% 16% 5% 11% 6% 5% 6% 4 7% 6% 26%	2643
May	1st to 10th	Day	7% 12% 4 9% 7% 5% 5% 4 6% 8% 33%	4420
	11th to 20th	Day	8% 10% 3 9% 7% 6% 5% 5% 8% 9% 28%	4377
	21st to end	Day	14% 13% 4 10% 7% 7% 6% 4 6% 7% 21%	4909
June	1st to 10th	Day	21% 10% 3 9% 7% 6% 4 3 5% 6% 26%	2850
	11th to 20th	Day	19% 11% 3 10% 6% 6% 7% 4 5% 5% 23%	2989
	21st to end	Day	11% 7% 3 9% 6% 6% 6% 4 6% 7% 35%	3040
July	1st to 10th	Day	5% 3 2 4 3 4 4 3 5% 6% 61%	4816
	11th to 20th	Day	3 3 1 3 3 3 3 3 4 5% 68%	4420
	21st to end	Day	5% 3 1 4 4 4 5% 4 6% 7% 58%	4790
August	1st to 10th	Day	3 3 1 4 3 4 5% 4 7% 8% 58%	4401
	11th to 20th	Day	5% 3 2 5% 4 5% 6% 4 6% 8% 51%	4330
	21st to end	Day	9% 5% 2 6% 5% 5% 6% 5% 7% 8% 43%	4206
September	1st to 10th	Day	8% 6% 2 6% 3 3 4 3 5% 7% 52%	2280
	11th to 20th	Day	10% 7% 2 7% 4 5% 5% 4 6% 6% 43%	2242
	21st to end	Day	13% 10% 2 7% 4 4 4 4 5% 7% 40%	2090
October	1st to 10th	Day	10% 9% 3 8% 4 4 5% 3 6% 7% 39%	3190
	11th to 20th	Day	15% 12% 4 9% 5% 6% 5% 4 6% 5% 30%	3034
	21st to end	Day	13% 15% 4 8% 6% 5% 5% 4 5% 5% 30%	3101
November	1st to 10th	Day	17% 21% 5% 10% 5% 5% 5% 3 5% 5% 18%	1698
	11th to 20th	Day	12% 19% 6% 12% 5% 6% 6% 4 4 5% 21%	1600
	21st to end	Day	13% 17% 6% 9% 6% 6% 6% 5% 6% 5% 19%	1438
December	1st to 10th	Day	8% 12% 4 9% 6% 6% 5% 5% 7% 8% 30%	2290
	11th to 20th	Day	8% 20% 7% 13% 8% 5% 6% 4 8% 6% 14%	2642
	21st to end	Day	9% 19% 7% 15% 7% 6% 7% 4 7% 6% 13%	3544
January	1st to 10th	Night	9% 22% 8% 18% 9% 6% 5% 3 4 3 12%	6306
	11th to 20th	Night	6% 13% 5% 10% 5% 5% 4 4 9% 9% 30%	6282
	21st to end	Night	10% 20% 7% 14% 6% 4 3 3 3 5% 24%	6952
February	1st to 10th	Night	10% 20% 6% 13% 6% 5% 5% 4 6% 5% 21%	2779
	11th to 20th	Night	8% 17% 5% 12% 6% 5% 4 4 5% 6% 28%	2528
	21st to end	Night	8% 19% 6% 12% 7% 5% 5% 4 6% 7% 22%	1980
March	1st to 10th	Night	4 12% 6% 11% 7% 6% 6% 5% 7% 8% 29%	5376
	11th to 20th	Night	6% 12% 5% 11% 6% 5% 5% 3 6% 7% 36%	5290
	21st to end	Night	8% 14% 5% 12% 7% 6% 7% 5% 6% 7% 22%	5016
April	1st to 10th	Night	7% 14% 3 9% 7% 5% 6% 4 8% 8% 30%	2000
	11th to 20th	Night	8% 12% 3 10% 7% 6% 5% 5% 5% 6% 32%	1940
	21st to end	Night	8% 13% 3 8% 6% 5% 5% 5% 6% 7% 34%	1651
May	1st to 10th	Night	6% 10% 4 8% 5% 4 4 5% 6% 8% 40%	2540
	11th to 20th	Night	6% 6% 2 6% 4 6% 6% 6% 9% 12% 36%	2516
	21st to end	Night	12% 8% 3 7% 5% 6% 7% 6% 7% 8% 30%	2666
June	1st to 10th	Night	16% 9% 3 7% 5% 5% 5% 3 7% 8% 31%	1330
	11th to 20th	Night	15% 9% 3 8% 5% 4 5% 4 5% 9% 33%	1185
	21st to end	Night	7% 6% 2 6% 5% 5% 6% 5% 7% 8% 44%	1140
July	1st to 10th	Night	3 3 1 4 3 3 3 3 5% 6% 68%	2136
	11th to 20th	Night	3 3 1 4 3 2 3 2 4 6% 69%	2540
	21st to end	Night	4 3 1 4 3 3 4 3 5% 6% 65%	2762
August	1st to 10th	Night	2 2 1 4 4 4 4 4 6% 8% 61%	2529
	11th to 20th	Night	4 3 2 5% 3 3 4 4 7% 10% 55%	2497
	21st to end	Night	9% 5% 2 5% 4 4 5% 5% 7% 7% 47%	3456
September	1st to 10th	Night	5% 5% 2 5% 5% 4 4 3 5% 6% 56%	1900
	11th to 20th	Night	9% 7% 2 6% 4 3 4 3 5% 9% 48%	1938
	21st to end	Night	12% 9% 2 6% 3 3 4 4 5% 8% 42%	2090
October	1st to 10th	Night	9% 7% 2 7% 5% 6% 5% 4 5% 7% 42%	3789
	11th to 20th	Night	16% 10% 3 7% 4 4 4 3 6% 7% 36%	3899
	21st to end	Night	11% 13% 5% 9% 6% 4 4 4 6% 5% 34%	4457
November	1st to 10th	Night	17% 18% 6% 10% 5% 5% 4 3 6% 5% 22%	2600
	11th to 20th	Night	14% 18% 5% 11% 6% 6% 4 3 6% 6% 22%	2800
	21st to end	Night	11% 20% 6% 12% 8% 6% 5% 4 5% 6% 18%	2940
December	1st to 10th	Night	8% 12% 4 9% 6% 4 5% 5% 6% 8% 34%	4890
	11th to 20th	Night	9% 19% 7% 15% 7% 7% 6% 4 6% 5% 15%	5550
	21st to end	Night	10% 20% 7% 15% 8% 5% 5% 4 5% 6% 15%	7440

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	70% 9% 10% 9% 2	3639
	11th to 20th	Day	49% 9% 22% 14% 3 3	3640
	21st to end	Day	62% 8% 10% 12% 7%	4004
February	1st to 10th	Day	62% 6% 13% 12% 6% 2	1620
	11th to 20th	Day	61% 7% 12% 13% 7% 1	1767
	21st to end	Day	69% 5% 10% 12% 4	1650
March	1st to 10th	Day	52% 14% 10% 14% 10%	4518
	11th to 20th	Day	47% 8% 16% 24% 5%	4707
	21st to end	Day	62% 9% 11% 11% 6%	5959
April	1st to 10th	Day	62% 9% 12% 13% 4	2400
	11th to 20th	Day	57% 6% 12% 17% 7% 1	2411
	21st to end	Day	56% 7% 13% 17% 6% 1	2643
May	1st to 10th	Day	45% 7% 12% 24% 11% 1	4420
	11th to 20th	Day	47% 6% 16% 24% 7%	4380
	21st to end	Day	59% 8% 12% 18% 3	4910
June	1st to 10th	Day	59% 5% 10% 15% 8% 3	2850
	11th to 20th	Day	61% 7% 10% 16% 6% 1	2989
	21st to end	Day	45% 6% 14% 21% 11% 2	3040
July	1st to 10th	Day	23% 6% 8% 20% 26% 16%	4744
	11th to 20th	Day	19% 4 6% 20% 33% 18%	4345
	21st to end	Day	23% 7% 11% 24% 25% 10%	4639
August	1st to 10th	Day	21% 7% 10% 30% 27% 5%	4384
	11th to 20th	Day	28% 9% 17% 33% 11% 2	4372
	21st to end	Day	33% 10% 18% 26% 12% 1	4205
September	1st to 10th	Day	32% 5% 16% 29% 15% 4	2280
	11th to 20th	Day	39% 6% 13% 23% 16% 2	2242
	21st to end	Day	43% 6% 19% 19% 11% 2	2090
October	1st to 10th	Day	41% 8% 21% 24% 7%	3190
	11th to 20th	Day	52% 7% 11% 26% 5%	3034
	21st to end	Day	52% 10% 15% 18% 5%	3101
November	1st to 10th	Day	65% 8% 11% 12% 4	1698
	11th to 20th	Day	64% 8% 13% 11% 3 1	1600
	21st to end	Day	62% 8% 13% 11% 5% 1	1438
December	1st to 10th	Day	50% 9% 23% 15% 4	2290
	11th to 20th	Day	65% 11% 11% 9% 3	2644
	21st to end	Day	66% 12% 11% 8% 3	3544
January	1st to 10th	Night	70% 10% 9% 9% 2	6306
	11th to 20th	Night	44% 8% 18% 18% 7% 4	6320
	21st to end	Night	61% 7% 11% 13% 8%	6952
February	1st to 10th	Night	62% 6% 11% 14% 5% 2	2779
	11th to 20th	Night	56% 6% 14% 16% 7% 2	2530
	21st to end	Night	60% 4 13% 16% 7% 1	1980
March	1st to 10th	Night	46% 12% 11% 20% 11%	5376
	11th to 20th	Night	43% 8% 16% 24% 8%	5291
	21st to end	Night	57% 9% 13% 14% 7%	5016
April	1st to 10th	Night	50% 7% 20% 18% 6%	2000
	11th to 20th	Night	49% 6% 15% 20% 9% 1	1940
	21st to end	Night	46% 7% 14% 23% 9%	1651
May	1st to 10th	Night	39% 6% 13% 30% 12%	2540
	11th to 20th	Night	34% 5% 25% 28% 8%	2516
	21st to end	Night	46% 8% 19% 24% 3	2666
June	1st to 10th	Night	48% 6% 13% 20% 11% 2	1330
	11th to 20th	Night	47% 6% 15% 23% 9% 1	1185
	21st to end	Night	34% 7% 18% 29% 10% 2	1140
July	1st to 10th	Night	17% 4 10% 30% 24% 16%	2100
	11th to 20th	Night	18% 3 9% 19% 35% 16%	2492
	21st to end	Night	19% 5% 12% 28% 25% 11%	2674
August	1st to 10th	Night	20% 6% 9% 33% 28% 4	2521
	11th to 20th	Night	20% 10% 22% 30% 15% 2	2521
	21st to end	Night	32% 8% 17% 28% 13% 1	3456
September	1st to 10th	Night	28% 5% 14% 31% 18% 4	1900
	11th to 20th	Night	35% 4 13% 25% 18% 4	1938
	21st to end	Night	39% 6% 20% 23% 11% 1	2090
October	1st to 10th	Night	38% 9% 17% 27% 8%	3789
	11th to 20th	Night	46% 6% 13% 25% 10%	3899
	21st to end	Night	48% 9% 17% 21% 5%	4457
November	1st to 10th	Night	61% 9% 11% 14% 5%	2600
	11th to 20th	Night	63% 7% 12% 12% 5% 1	2800
	21st to end	Night	66% 6% 13% 10% 5% 1	2940
December	1st to 10th	Night	46% 9% 20% 20% 4	4890
	11th to 20th	Night	69% 9% 11% 9% 2	5547
	21st to end	Night	65% 10% 11% 11% 3	7440

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	23% 15% 25% 21% 11% 6%	200
	11th to 20th	Day	21% 12% 23% 27% 14% 5%	200
	21st to end	Day	24% 14% 25% 24% 12% 1	220
February	1st to 10th	Day	23% 12% 33% 22% 6% 3	220
	11th to 20th	Day	20% 17% 31% 20% 10% 3	398
	21st to end	Day	22% 14% 29% 24% 11%	414
March	1st to 10th	Day	23% 18% 31% 19% 7% 3	500
	11th to 20th	Day	20% 20% 25% 20% 12% 4	500
	21st to end	Day	26% 19% 22% 19% 11% 3	550
April	1st to 10th	Day	26% 16% 29% 21% 7% 1	500
	11th to 20th	Day	20% 20% 26% 20% 9% 5%	492
	21st to end	Day	21% 15% 25% 22% 11% 6%	481
May	1st to 10th	Day	18% 15% 28% 25% 10% 4	463
	11th to 20th	Day	23% 8% 22% 25% 16% 6%	488
	21st to end	Day	20% 11% 22% 24% 18% 5%	586
June	1st to 10th	Day	22% 12% 25% 25% 13% 3	680
	11th to 20th	Day	24% 13% 23% 23% 14% 3	819
	21st to end	Day	14% 10% 24% 27% 20% 5%	870
July	1st to 10th	Day	13% 10% 25% 32% 17% 4	723
	11th to 20th	Day	8% 13% 27% 32% 15% 5%	484
	21st to end	Day	4 10% 27% 36% 18% 5%	531
August	1st to 10th	Day	7% 8% 27% 31% 17% 9%	487
	11th to 20th	Day	6% 8% 27% 33% 18% 8%	477
	21st to end	Day	11% 14% 25% 28% 14% 7%	539
September	1st to 10th	Day	13% 11% 29% 25% 16% 5%	490
	11th to 20th	Day	16% 12% 25% 27% 17% 4	490
	21st to end	Day	23% 13% 27% 19% 14% 4	490
October	1st to 10th	Day	20% 12% 27% 24% 13% 5%	490
	11th to 20th	Day	23% 16% 21% 23% 14% 4	414
	21st to end	Day	28% 11% 27% 23% 8% 3	330
November	1st to 10th	Day	31% 15% 21% 23% 9% 1	266
	11th to 20th	Day	26% 17% 22% 20% 13% 4	200
	21st to end	Day	20% 11% 26% 21% 16% 8%	200
December	1st to 10th	Day	21% 7% 19% 23% 23% 9%	200
	11th to 20th	Day	23% 14% 26% 20% 13% 5%	200
	21st to end	Day	25% 7% 20% 34% 11% 3	220
January	1st to 10th	Night	31% 11% 20% 26% 10% 2	800
	11th to 20th	Night	21% 9% 26% 31% 11% 2	800
	21st to end	Night	21% 10% 23% 32% 12% 2	880
February	1st to 10th	Night	23% 10% 23% 31% 11% 3	800
	11th to 20th	Night	20% 10% 21% 35% 13% 1	784
	21st to end	Night	17% 12% 21% 29% 19% 2	660
March	1st to 10th	Night	18% 10% 23% 29% 18% 2	800
	11th to 20th	Night	19% 14% 20% 30% 16% 2	800
	21st to end	Night	19% 10% 21% 29% 17% 3	880
April	1st to 10th	Night	16% 8% 20% 35% 18% 3	800
	11th to 20th	Night	15% 10% 21% 34% 18% 2	752
	21st to end	Night	13% 11% 23% 36% 16% 2	479
May	1st to 10th	Night	13% 7% 17% 36% 24% 2	380
	11th to 20th	Night	14% 5% 14% 38% 25% 4	374
	21st to end	Night	16% 6% 13% 30% 29% 5%	411
June	1st to 10th	Night	20% 8% 16% 34% 19% 2	380
	11th to 20th	Night	19% 7% 16% 37% 17% 3	380
	21st to end	Night	11% 7% 13% 39% 27% 3	380
July	1st to 10th	Night	5% 4 12% 36% 40% 2	370
	11th to 20th	Night	8% 5% 15% 41% 27% 4	368
	21st to end	Night	4 3 13% 40% 35% 5%	416
August	1st to 10th	Night	5% 2 11% 33% 41% 8%	378
	11th to 20th	Night	7% 2 8% 38% 38% 7%	376
	21st to end	Night	8% 6% 14% 38% 30% 3	807
September	1st to 10th	Night	8% 6% 14% 39% 26% 6%	760
	11th to 20th	Night	14% 6% 12% 28% 30% 10%	760
	21st to end	Night	19% 7% 16% 32% 21% 5%	760
October	1st to 10th	Night	16% 8% 16% 33% 24% 3	760
	11th to 20th	Night	22% 8% 18% 31% 19% 2	760
	21st to end	Night	22% 6% 27% 29% 12% 4	836
November	1st to 10th	Night	28% 11% 22% 30% 8% 1	780
	11th to 20th	Night	27% 13% 21% 25% 13% 2	800
	21st to end	Night	24% 11% 21% 29% 12% 3	940
December	1st to 10th	Night	20% 9% 20% 35% 14% 2	1000
	11th to 20th	Night	25% 11% 24% 30% 9% 1	1000
	21st to end	Night	27% 12% 23% 29% 8% 1	1095

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