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 2024-02-25.

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	11% 12% 5% 9% 7% 4 5% 5% 5% 6% 30%	78510
	Night	11% 14% 5% 9% 7% 4 5% 4 5% 6% 29%	80738
Transparency	Day	53% 6% 12% 18% 9% 2	78233
	Night	54% 6% 13% 18% 8% 1	80596
Seeing	Day	19% 13% 25% 25% 14% 4	14266
	Night	19% 8% 19% 32% 18% 3	23017

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	11% 19% 10% 18% 12% 5% 5% 3 4 4 9%	2539
	11th to 20th	Day	8% 16% 6% 10% 9% 3 5% 6% 5% 8% 24%	2516
	21st to end	Day	12% 26% 10% 14% 7% 2 4 4 4 4 14%	2794
February	1st to 10th	Day	11% 20% 6% 10% 8% 4 4 4 6% 4 22%	1539
	11th to 20th	Day	8% 17% 8% 14% 7% 3 5% 5% 5% 5% 22%	1667
	21st to end	Day	8% 19% 8% 15% 10% 6% 5% 4 4 4 17%	1521
March	1st to 10th	Day	11% 16% 7% 13% 9% 5% 7% 5% 6% 5% 17%	1800
	11th to 20th	Day	13% 18% 7% 10% 8% 3 5% 4 5% 5% 23%	1856
	21st to end	Day	10% 21% 7% 11% 8% 5% 5% 5% 5% 6% 17%	2366
April	1st to 10th	Day	11% 17% 7% 10% 9% 5% 7% 6% 6% 6% 18%	2160
	11th to 20th	Day	10% 16% 6% 9% 8% 4 6% 6% 5% 5% 23%	2167
	21st to end	Day	11% 15% 5% 9% 8% 4 6% 5% 6% 7% 25%	2438
May	1st to 10th	Day	12% 14% 4 7% 7% 4 4 4 6% 8% 31%	2520
	11th to 20th	Day	13% 12% 4 8% 8% 4 5% 6% 6% 7% 27%	2477
	21st to end	Day	16% 10% 4 9% 9% 5% 6% 5% 6% 7% 25%	2777
June	1st to 10th	Day	21% 9% 4 8% 8% 5% 5% 4 5% 6% 25%	2700
	11th to 20th	Day	20% 10% 4 9% 6% 4 7% 5% 5% 6% 23%	2831
	21st to end	Day	11% 7% 3 8% 7% 4 6% 6% 6% 7% 35%	2880
July	1st to 10th	Day	9% 5% 2 6% 6% 4 6% 5% 6% 7% 44%	2748
	11th to 20th	Day	6% 5% 2 5% 5% 3 5% 4 5% 7% 54%	2520
	21st to end	Day	5% 2 1 3 4 3 4 4 5% 7% 63%	2741
August	1st to 10th	Day	5% 4 2 5% 3 3 4 5% 5% 9% 55%	2507
	11th to 20th	Day	6% 3 2 4 3 3 5% 5% 5% 8% 56%	2475
	21st to end	Day	8% 6% 3 6% 7% 4 5% 5% 6% 8% 42%	2402
September	1st to 10th	Day	9% 6% 2 5% 4 3 4 4 5% 8% 51%	2160
	11th to 20th	Day	11% 6% 3 7% 5% 4 5% 5% 6% 6% 43%	2124
	21st to end	Day	13% 10% 2 7% 5% 3 4 4 5% 7% 39%	1980
October	1st to 10th	Day	12% 12% 4 7% 6% 3 5% 4 5% 6% 38%	1810
	11th to 20th	Day	15% 10% 3 9% 6% 5% 6% 5% 6% 5% 30%	1728
	21st to end	Day	18% 15% 5% 8% 6% 3 4 5% 4 6% 25%	1782
November	1st to 10th	Day	18% 20% 5% 9% 6% 4 5% 5% 5% 5% 18%	1612
	11th to 20th	Day	12% 17% 7% 10% 7% 5% 6% 6% 3 6% 22%	1520
	21st to end	Day	14% 15% 7% 8% 6% 4 6% 6% 6% 6% 20%	1367
December	1st to 10th	Day	10% 13% 6% 10% 8% 6% 6% 7% 6% 8% 20%	1330
	11th to 20th	Day	9% 16% 9% 14% 10% 4 6% 5% 6% 5% 15%	1682
	21st to end	Day	11% 18% 8% 13% 10% 5% 7% 5% 5% 5% 14%	2474
January	1st to 10th	Night	12% 23% 9% 16% 12% 4 4 3 4 3 10%	4406
	11th to 20th	Night	9% 14% 6% 11% 7% 3 4 4 8% 9% 24%	4382
	21st to end	Night	14% 25% 10% 13% 8% 3 4 3 3 4 13%	4862
February	1st to 10th	Night	11% 19% 7% 12% 7% 3 5% 4 6% 5% 21%	2641
	11th to 20th	Night	9% 17% 6% 11% 7% 4 4 4 5% 6% 28%	2399
	21st to end	Night	8% 16% 7% 12% 8% 5% 5% 5% 6% 6% 22%	1829
March	1st to 10th	Night	8% 16% 6% 9% 8% 5% 5% 5% 6% 6% 25%	2160
	11th to 20th	Night	11% 14% 5% 10% 6% 3 4 4 6% 8% 28%	2101
	21st to end	Night	10% 13% 5% 9% 9% 4 7% 5% 5% 7% 26%	1990
April	1st to 10th	Night	7% 13% 4 7% 8% 4 6% 6% 7% 9% 31%	1800
	11th to 20th	Night	9% 12% 4 9% 9% 5% 6% 5% 5% 6% 31%	1744
	21st to end	Night	8% 12% 3 8% 7% 4 6% 6% 5% 8% 33%	1520
May	1st to 10th	Night	9% 10% 4 7% 7% 4 5% 6% 5% 6% 38%	1440
	11th to 20th	Night	10% 8% 3 6% 5% 4 5% 7% 7% 9% 34%	1416
	21st to end	Night	12% 8% 4 7% 6% 4 6% 5% 5% 7% 35%	1498
June	1st to 10th	Night	17% 9% 4 7% 5% 4 5% 4 6% 9% 30%	1260
	11th to 20th	Night	16% 7% 3 7% 5% 4 5% 5% 5% 9% 33%	1123
	21st to end	Night	7% 5% 3 6% 5% 4 6% 6% 7% 7% 44%	1080
July	1st to 10th	Night	4 4 2 5% 5% 3 5% 4 6% 8% 54%	1204
	11th to 20th	Night	5% 4 2 5% 4 2 4 4 6% 9% 55%	1440
	21st to end	Night	4 1 1 3 3 2 3 3 5% 6% 71%	1576
August	1st to 10th	Night	4 3 1 4 4 3 3 4 4 7% 61%	1432
	11th to 20th	Night	4 2 1 4 3 2 3 4 6% 8% 64%	1424
	21st to end	Night	6% 3 2 4 5% 3 5% 5% 7% 6% 53%	1952
September	1st to 10th	Night	5% 5% 2 5% 6% 3 4 4 5% 6% 55%	1800
	11th to 20th	Night	10% 7% 3 6% 5% 3 3 4 5% 8% 47%	1836
	21st to end	Night	13% 9% 2 5% 4 3 4 5% 5% 8% 41%	1980
October	1st to 10th	Night	11% 8% 3 7% 6% 3 6% 5% 5% 6% 41%	2149
	11th to 20th	Night	14% 11% 4 7% 7% 3 4 5% 5% 5% 35%	2231
	21st to end	Night	15% 12% 4 7% 6% 3 5% 4 6% 6% 31%	2574
November	1st to 10th	Night	18% 17% 6% 8% 6% 4 4 4 6% 5% 22%	2466
	11th to 20th	Night	15% 17% 5% 10% 7% 4 5% 4 5% 6% 23%	2660
	21st to end	Night	12% 18% 7% 10% 9% 4 6% 5% 5% 6% 19%	2793
December	1st to 10th	Night	10% 16% 6% 11% 9% 4 6% 5% 5% 6% 23%	2850
	11th to 20th	Night	11% 21% 9% 12% 7% 5% 6% 5% 5% 5% 14%	3510
	21st to end	Night	12% 20% 9% 13% 10% 4 5% 4 4 7% 13%	5210

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	75% 8% 7% 7% 2	2539
	11th to 20th	Day	59% 5% 15% 13% 3 5%	2540
	21st to end	Day	74% 6% 8% 9% 3	2794
February	1st to 10th	Day	64% 5% 13% 12% 4 2	1539
	11th to 20th	Day	62% 7% 12% 12% 6% 1	1676
	21st to end	Day	70% 4 10% 12% 4	1521
March	1st to 10th	Day	66% 6% 11% 12% 5% 1	1800
	11th to 20th	Day	62% 5% 12% 15% 5%	1856
	21st to end	Day	69% 4 14% 10% 3	2366
April	1st to 10th	Day	65% 8% 10% 13% 3	2160
	11th to 20th	Day	60% 6% 11% 16% 6% 1	2167
	21st to end	Day	57% 6% 14% 17% 5%	2438
May	1st to 10th	Day	52% 5% 15% 19% 8% 1	2520
	11th to 20th	Day	55% 7% 13% 20% 5%	2480
	21st to end	Day	58% 7% 14% 18% 4 1	2778
June	1st to 10th	Day	59% 5% 10% 15% 8% 3	2700
	11th to 20th	Day	61% 6% 10% 16% 6%	2831
	21st to end	Day	46% 6% 14% 21% 11% 2	2880
July	1st to 10th	Day	37% 7% 10% 22% 18% 6%	2676
	11th to 20th	Day	31% 6% 10% 25% 22% 6%	2445
	21st to end	Day	23% 5% 9% 27% 29% 8%	2589
August	1st to 10th	Day	28% 6% 12% 27% 22% 6%	2490
	11th to 20th	Day	25% 5% 15% 31% 19% 4	2475
	21st to end	Day	37% 6% 11% 29% 14% 2	2402
September	1st to 10th	Day	33% 5% 15% 30% 15% 3	2160
	11th to 20th	Day	40% 5% 13% 22% 17% 3	2124
	21st to end	Day	45% 5% 18% 19% 11% 2	1980
October	1st to 10th	Day	46% 5% 16% 25% 8%	1810
	11th to 20th	Day	52% 6% 15% 22% 5%	1728
	21st to end	Day	58% 7% 14% 17% 5%	1782
November	1st to 10th	Day	65% 8% 11% 12% 4	1612
	11th to 20th	Day	64% 7% 14% 11% 3 1	1520
	21st to end	Day	62% 8% 13% 12% 5% 1	1367
December	1st to 10th	Day	60% 7% 12% 16% 4	1330
	11th to 20th	Day	70% 7% 10% 9% 4	1684
	21st to end	Day	68% 9% 9% 10% 3	2474
January	1st to 10th	Night	75% 8% 8% 7% 2 1	4406
	11th to 20th	Night	56% 4 12% 17% 6% 6%	4420
	21st to end	Night	74% 6% 8% 7% 3 1	4862
February	1st to 10th	Night	63% 5% 12% 14% 5% 1	2641
	11th to 20th	Night	56% 5% 14% 16% 7% 1	2401
	21st to end	Night	59% 4 12% 16% 7% 1	1829
March	1st to 10th	Night	59% 6% 14% 14% 7%	2160
	11th to 20th	Night	53% 6% 16% 17% 8%	2102
	21st to end	Night	58% 5% 17% 15% 5%	1990
April	1st to 10th	Night	49% 7% 19% 19% 6%	1800
	11th to 20th	Night	52% 6% 16% 17% 8% 1	1744
	21st to end	Night	47% 7% 14% 23% 8%	1520
May	1st to 10th	Night	45% 5% 17% 22% 11% 1	1440
	11th to 20th	Night	43% 5% 21% 22% 9%	1416
	21st to end	Night	48% 6% 18% 23% 5%	1498
June	1st to 10th	Night	49% 6% 13% 20% 9% 3	1260
	11th to 20th	Night	47% 6% 15% 23% 8%	1123
	21st to end	Night	34% 7% 19% 28% 10% 2	1080
July	1st to 10th	Night	27% 5% 14% 30% 19% 5%	1168
	11th to 20th	Night	26% 4 14% 29% 22% 5%	1392
	21st to end	Night	15% 2 13% 30% 33% 7%	1488
August	1st to 10th	Night	24% 5% 11% 30% 24% 7%	1424
	11th to 20th	Night	18% 5% 17% 33% 24% 4	1424
	21st to end	Night	28% 4 16% 33% 16% 3	1952
September	1st to 10th	Night	29% 5% 14% 31% 18% 3	1800
	11th to 20th	Night	36% 4 13% 25% 19% 4	1836
	21st to end	Night	39% 6% 19% 23% 11% 1	1980
October	1st to 10th	Night	42% 7% 17% 26% 8% 1	2149
	11th to 20th	Night	49% 5% 14% 23% 9%	2231
	21st to end	Night	52% 6% 16% 19% 7%	2574
November	1st to 10th	Night	61% 8% 11% 14% 6%	2466
	11th to 20th	Night	63% 6% 13% 12% 5% 1	2660
	21st to end	Night	65% 5% 13% 10% 5% 1	2793
December	1st to 10th	Night	60% 6% 13% 17% 3 1	2850
	11th to 20th	Night	72% 7% 9% 8% 3	3507
	21st to end	Night	69% 7% 10% 11% 3	5210

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	24% 15% 22% 22% 11% 6%	180
	11th to 20th	Day	21% 11% 22% 27% 14% 5%	180
	21st to end	Day	25% 15% 23% 24% 12% 2	198
February	1st to 10th	Day	26% 13% 32% 22% 6% 2	199
	11th to 20th	Day	22% 17% 28% 20% 10% 2	367
	21st to end	Day	19% 13% 29% 26% 12% 1	363
March	1st to 10th	Day	24% 19% 29% 17% 8% 3	420
	11th to 20th	Day	21% 19% 25% 20% 12% 3	420
	21st to end	Day	27% 18% 23% 18% 11% 3	462
April	1st to 10th	Day	26% 16% 29% 20% 8% 1	420
	11th to 20th	Day	21% 21% 24% 19% 10% 5%	412
	21st to end	Day	22% 15% 26% 21% 10% 6%	420
May	1st to 10th	Day	18% 15% 27% 26% 9% 4	423
	11th to 20th	Day	24% 8% 20% 25% 17% 6%	448
	21st to end	Day	21% 11% 19% 24% 19% 5%	539
June	1st to 10th	Day	22% 12% 25% 25% 13% 3	630
	11th to 20th	Day	24% 11% 24% 24% 14% 3	761
	21st to end	Day	13% 10% 23% 28% 21% 5%	810
July	1st to 10th	Day	14% 9% 24% 32% 17% 4	671
	11th to 20th	Day	8% 14% 26% 31% 16% 4	444
	21st to end	Day	3 11% 26% 36% 19% 6%	487
August	1st to 10th	Day	8% 8% 26% 32% 17% 9%	447
	11th to 20th	Day	6% 8% 28% 33% 18% 8%	437
	21st to end	Day	10% 15% 25% 28% 14% 7%	495
September	1st to 10th	Day	14% 12% 30% 24% 15% 5%	450
	11th to 20th	Day	16% 12% 24% 27% 17% 4	450
	21st to end	Day	24% 12% 26% 19% 15% 4	450
October	1st to 10th	Day	20% 11% 26% 24% 13% 5%	450
	11th to 20th	Day	22% 16% 22% 23% 14% 4	378
	21st to end	Day	29% 9% 26% 23% 9% 3	297
November	1st to 10th	Day	30% 14% 20% 25% 10% 1	240
	11th to 20th	Day	23% 14% 23% 21% 14% 4	180
	21st to end	Day	22% 9% 23% 21% 16% 8%	180
December	1st to 10th	Day	22% 7% 18% 22% 22% 9%	180
	11th to 20th	Day	21% 13% 26% 21% 14% 5%	180
	21st to end	Day	25% 6% 18% 35% 12% 4	198
January	1st to 10th	Night	32% 10% 20% 26% 10% 2	760
	11th to 20th	Night	22% 8% 24% 32% 11% 2	760
	21st to end	Night	22% 9% 22% 31% 13% 2	836
February	1st to 10th	Night	24% 9% 22% 31% 11% 3	760
	11th to 20th	Night	20% 10% 21% 35% 13% 1	744
	21st to end	Night	15% 10% 21% 30% 20% 3	610
March	1st to 10th	Night	20% 10% 23% 28% 17% 3	720
	11th to 20th	Night	20% 14% 19% 29% 16% 2	720
	21st to end	Night	19% 10% 20% 29% 18% 3	792
April	1st to 10th	Night	15% 8% 19% 36% 19% 4	720
	11th to 20th	Night	15% 10% 20% 34% 19% 2	676
	21st to end	Night	13% 10% 24% 35% 16% 2	440
May	1st to 10th	Night	14% 6% 18% 36% 24% 2	360
	11th to 20th	Night	15% 5% 13% 38% 25% 4	354
	21st to end	Night	16% 6% 13% 29% 30% 5%	389
June	1st to 10th	Night	21% 9% 15% 34% 19% 3	360
	11th to 20th	Night	19% 6% 16% 38% 18% 4	360
	21st to end	Night	11% 7% 12% 39% 28% 4	360
July	1st to 10th	Night	6% 4 12% 36% 40% 3	350
	11th to 20th	Night	8% 5% 16% 41% 26% 4	348
	21st to end	Night	3 3 14% 39% 36% 5%	394
August	1st to 10th	Night	6% 3 11% 32% 41% 8%	358
	11th to 20th	Night	7% 2 8% 39% 38% 7%	356
	21st to end	Night	8% 6% 14% 38% 31% 3	764
September	1st to 10th	Night	8% 6% 15% 39% 25% 7%	720
	11th to 20th	Night	14% 6% 12% 28% 30% 10%	720
	21st to end	Night	19% 5% 16% 32% 22% 5%	720
October	1st to 10th	Night	17% 7% 15% 33% 25% 3	720
	11th to 20th	Night	22% 8% 18% 31% 19% 2	720
	21st to end	Night	22% 6% 27% 29% 12% 4	792
November	1st to 10th	Night	28% 11% 21% 31% 9% 1	740
	11th to 20th	Night	26% 13% 21% 26% 13% 2	760
	21st to end	Night	25% 11% 19% 30% 12% 3	893
December	1st to 10th	Night	21% 9% 20% 35% 14% 2	950
	11th to 20th	Night	25% 11% 23% 31% 9% 1	950
	21st to end	Night	27% 11% 23% 30% 9% 1	1041

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