University of British Columbia
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the University of British Columbia clear sky chart from 2005-11-06 to 2024-12-09.

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	21% 8% 3 6% 4 4 4 3 5% 6% 36%	105785
	Night	26% 9% 3 7% 4 4 4 3 4 5% 31%	102563
Transparency	Day	26% 49% 8% 11% 5% 1	105533
	Night	30% 48% 7% 9% 4 1	102461
Seeing	Day	22% 3 10% 25% 29% 11%	15503
	Night	30% 4 11% 24% 23% 8%	23682

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	42% 12% 3 8% 5% 4 4 3 5% 4 11%	2219
	11th to 20th	Day	29% 13% 3 6% 4 4 3 2 3 7% 26%	2199
	21st to end	Day	53% 11% 3 6% 3 3 3 2 3 3 9%	2775
February	1st to 10th	Day	33% 9% 4 7% 5% 5% 4 5% 5% 6% 17%	1710
	11th to 20th	Day	27% 10% 3 8% 6% 5% 5% 3 4 6% 24%	1665
	21st to end	Day	24% 11% 3 9% 5% 5% 5% 3 4 6% 24%	1418
March	1st to 10th	Day	23% 13% 3 9% 6% 5% 5% 5% 6% 6% 18%	3261
	11th to 20th	Day	21% 9% 3 7% 5% 4 4 3 5% 7% 33%	3490
	21st to end	Day	24% 13% 4 8% 5% 4 3 3 4 5% 27%	3908
April	1st to 10th	Day	23% 11% 3 8% 5% 5% 6% 4 6% 6% 23%	2280
	11th to 20th	Day	20% 9% 3 7% 5% 5% 5% 4 6% 6% 31%	2359
	21st to end	Day	20% 10% 4 7% 5% 4 4 4 5% 6% 31%	2515
May	1st to 10th	Day	13% 8% 2 7% 4 3 3 3 5% 5% 46%	4420
	11th to 20th	Day	11% 6% 2 7% 5% 5% 4 4 7% 10% 40%	4377
	21st to end	Day	23% 9% 2 7% 5% 5% 5% 4 6% 7% 29%	4941
June	1st to 10th	Day	18% 7% 2 7% 5% 4 4 4 6% 8% 35%	2850
	11th to 20th	Day	16% 8% 3 7% 6% 4 5% 4 6% 7% 33%	2850
	21st to end	Day	13% 6% 2 7% 4 4 5% 4 6% 7% 41%	2850
July	1st to 10th	Day	7% 4 1 3 2 2 3 2 3 5% 66%	4740
	11th to 20th	Day	5% 3 1 3 2 2 2 2 4 5% 72%	4740
	21st to end	Day	11% 4 2 4 3 3 3 3 5% 6% 56%	4902
August	1st to 10th	Day	5% 3 1 3 2 2 2 2 4 6% 69%	4345
	11th to 20th	Day	10% 6% 2 7% 4 4 4 3 5% 6% 48%	4026
	21st to end	Day	15% 5% 2 7% 4 4 4 3 5% 7% 45%	4437
September	1st to 10th	Day	11% 5% 3 6% 4 4 4 3 5% 7% 50%	2280
	11th to 20th	Day	17% 7% 3 8% 5% 4 4 4 5% 7% 39%	2265
	21st to end	Day	22% 5% 2 6% 4 4 4 3 5% 6% 37%	2009
October	1st to 10th	Day	21% 7% 3 8% 5% 5% 5% 4 5% 7% 30%	3180
	11th to 20th	Day	35% 8% 3 5% 4 4 4 3 4 6% 25%	3161
	21st to end	Day	28% 13% 4 8% 5% 5% 3 3 5% 6% 20%	3177
November	1st to 10th	Day	38% 11% 2 6% 4 4 4 3 4 5% 19%	1568
	11th to 20th	Day	35% 11% 3 7% 5% 3 4 3 4 5% 21%	1600
	21st to end	Day	38% 11% 3 5% 5% 4 3 4 5% 4 18%	1600
December	1st to 10th	Day	31% 12% 3 6% 5% 4 5% 3 4 5% 23%	1541
	11th to 20th	Day	35% 13% 4 11% 7% 4 4 3 3 3 14%	1685
	21st to end	Day	40% 12% 4 8% 4 4 3 3 3 3 12%	2442
January	1st to 10th	Night	38% 12% 3 8% 5% 4 4 3 4 4 15%	4726
	11th to 20th	Night	32% 9% 3 6% 3 3 4 3 4 7% 24%	4699
	21st to end	Night	49% 11% 3 7% 3 3 3 2 3 4 12%	4867
February	1st to 10th	Night	32% 11% 4 8% 4 5% 4 4 4 6% 20%	2470
	11th to 20th	Night	27% 13% 3 7% 4 4 3 2 4 6% 26%	2402
	21st to end	Night	28% 11% 2 8% 5% 4 4 3 5% 5% 26%	2036
March	1st to 10th	Night	27% 12% 3 8% 5% 4 4 3 4 5% 22%	3612
	11th to 20th	Night	22% 8% 2 6% 5% 4 4 3 5% 6% 36%	3489
	21st to end	Night	29% 13% 4 9% 5% 4 3 3 4 4 21%	3744
April	1st to 10th	Night	25% 11% 4 7% 6% 5% 5% 4 5% 6% 23%	1900
	11th to 20th	Night	21% 9% 3 8% 5% 5% 4 4 5% 6% 29%	1767
	21st to end	Night	20% 9% 4 9% 6% 5% 4 3 7% 6% 26%	1663
May	1st to 10th	Night	15% 7% 3 5% 4 4 4 3 4 5% 47%	2540
	11th to 20th	Night	11% 5% 2 5% 3 5% 4 4 6% 9% 46%	2516
	21st to end	Night	20% 10% 4 11% 6% 5% 4 3 4 5% 28%	2618
June	1st to 10th	Night	21% 7% 3 8% 5% 5% 4 3 6% 5% 34%	1330
	11th to 20th	Night	15% 9% 3 8% 5% 4 6% 5% 6% 8% 31%	1330
	21st to end	Night	11% 6% 2 6% 5% 5% 6% 4 6% 9% 40%	1330
July	1st to 10th	Night	6% 4 1 3 3 3 3 3 4 6% 65%	2220
	11th to 20th	Night	3 3 1 3 2 1 2 2 3 4 75%	2220
	21st to end	Night	8% 5% 2 6% 4 3 4 3 5% 5% 54%	2635
August	1st to 10th	Night	4 2 1 2 2 2 2 2 3 7% 73%	2584
	11th to 20th	Night	12% 5% 2 5% 3 3 4 3 4 7% 50%	2812
	21st to end	Night	13% 6% 3 6% 4 4 4 4 6% 7% 44%	3232
September	1st to 10th	Night	8% 4 2 4 3 4 4 4 5% 6% 56%	1900
	11th to 20th	Night	18% 6% 2 6% 3 3 4 3 5% 6% 43%	1913
	21st to end	Night	20% 7% 2 7% 4 4 4 3 4 6% 39%	2171
October	1st to 10th	Night	20% 9% 4 9% 4 4 5% 4 5% 6% 31%	3800
	11th to 20th	Night	29% 8% 3 7% 4 3 4 3 5% 6% 28%	3777
	21st to end	Night	30% 10% 3 7% 4 4 3 3 4 5% 27%	4379
November	1st to 10th	Night	37% 10% 3 7% 4 4 4 3 4 5% 18%	2730
	11th to 20th	Night	35% 10% 3 5% 4 5% 4 3 4 4 23%	2800
	21st to end	Night	36% 11% 3 7% 6% 4 3 2 4 5% 20%	2800
December	1st to 10th	Night	31% 11% 3 7% 5% 4 3 3 4 6% 23%	2817
	11th to 20th	Night	44% 13% 4 7% 4 3 3 3 3 4 13%	3520
	21st to end	Night	41% 14% 4 7% 4 3 3 2 3 3 16%	5214

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	45% 42% 6% 3 2 1	2219
	11th to 20th	Day	37% 38% 11% 8% 3 3	2220
	21st to end	Day	55% 40% 2 2	2775
February	1st to 10th	Day	25% 65% 4 2 2 2	1710
	11th to 20th	Day	26% 65% 3 5% 1 1	1674
	21st to end	Day	21% 67% 5% 5% 2 1	1418
March	1st to 10th	Day	33% 49% 10% 5% 2	3261
	11th to 20th	Day	25% 45% 10% 18% 2	3490
	21st to end	Day	36% 46% 11% 5% 2	3908
April	1st to 10th	Day	23% 68% 4 5% 1	2280
	11th to 20th	Day	21% 67% 4 5% 2 1	2359
	21st to end	Day	20% 67% 4 6% 3	2515
May	1st to 10th	Day	20% 44% 12% 14% 9%	4420
	11th to 20th	Day	19% 49% 16% 13% 3	4380
	21st to end	Day	32% 49% 11% 7% 1	4942
June	1st to 10th	Day	17% 68% 6% 7% 2	2850
	11th to 20th	Day	16% 68% 5% 7% 4	2850
	21st to end	Day	16% 65% 7% 9% 3	2850
July	1st to 10th	Day	12% 37% 5% 17% 21% 8%	4666
	11th to 20th	Day	7% 37% 8% 28% 19% 1	4659
	21st to end	Day	21% 40% 14% 17% 8% 1	4747
August	1st to 10th	Day	8% 38% 7% 33% 13% 1	4328
	11th to 20th	Day	25% 40% 21% 10% 4	4065
	21st to end	Day	27% 41% 11% 12% 8%	4436
September	1st to 10th	Day	18% 63% 5% 10% 3 1	2280
	11th to 20th	Day	22% 61% 4 6% 6% 1	2265
	21st to end	Day	21% 60% 7% 8% 5%	2009
October	1st to 10th	Day	36% 40% 12% 11% 1	3180
	11th to 20th	Day	40% 38% 7% 11% 4	3161
	21st to end	Day	43% 39% 6% 9% 2	3177
November	1st to 10th	Day	30% 58% 5% 5% 2	1568
	11th to 20th	Day	27% 59% 4 6% 3 2	1600
	21st to end	Day	27% 59% 5% 4 4 1	1600
December	1st to 10th	Day	27% 59% 4 7% 3	1541
	11th to 20th	Day	40% 52% 4 3 1 1	1688
	21st to end	Day	50% 41% 5% 3 1 1	2442
January	1st to 10th	Night	44% 42% 6% 5% 2	4726
	11th to 20th	Night	33% 40% 12% 7% 6% 1	4740
	21st to end	Night	54% 40% 3 3	4867
February	1st to 10th	Night	24% 66% 2 3 2 2	2470
	11th to 20th	Night	25% 65% 3 4 2	2404
	21st to end	Night	19% 66% 6% 6% 3 1	2036
March	1st to 10th	Night	36% 46% 8% 6% 4	3612
	11th to 20th	Night	20% 47% 12% 19% 2	3490
	21st to end	Night	40% 44% 8% 6% 1	3744
April	1st to 10th	Night	21% 67% 4 7% 2	1900
	11th to 20th	Night	20% 66% 4 6% 3 1	1767
	21st to end	Night	20% 66% 4 5% 5%	1663
May	1st to 10th	Night	20% 44% 9% 19% 8%	2540
	11th to 20th	Night	15% 47% 20% 14% 4	2516
	21st to end	Night	35% 44% 12% 7% 2	2618
June	1st to 10th	Night	16% 66% 6% 8% 2 1	1330
	11th to 20th	Night	15% 66% 7% 9% 4	1330
	21st to end	Night	15% 66% 8% 8% 4	1330
July	1st to 10th	Night	10% 36% 4 23% 21% 6%	2185
	11th to 20th	Night	6% 36% 6% 31% 20% 1	2178
	21st to end	Night	23% 36% 11% 19% 10%	2551
August	1st to 10th	Night	5% 37% 8% 29% 19% 1	2576
	11th to 20th	Night	27% 39% 20% 10% 3 1	2839
	21st to end	Night	26% 40% 13% 13% 8%	3232
September	1st to 10th	Night	14% 63% 8% 9% 5%	1900
	11th to 20th	Night	20% 62% 5% 6% 6% 1	1913
	21st to end	Night	19% 61% 7% 10% 4	2171
October	1st to 10th	Night	35% 42% 12% 10% 2	3800
	11th to 20th	Night	40% 39% 6% 13% 3	3777
	21st to end	Night	39% 40% 8% 10% 3	4379
November	1st to 10th	Night	30% 58% 6% 4 2	2730
	11th to 20th	Night	29% 57% 4 6% 3 2	2800
	21st to end	Night	29% 58% 4 4 5% 1	2800
December	1st to 10th	Night	28% 59% 4 6% 3	2817
	11th to 20th	Night	42% 51% 3 2 2	3516
	21st to end	Night	47% 40% 5% 6% 2	5214

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	39% 4 13% 22% 13% 8%	180
	11th to 20th	Day	43% 4 15% 17% 16% 4	180
	21st to end	Day	47% 4 8% 21% 16% 4	231
February	1st to 10th	Day	37% 5% 12% 26% 17% 2	370
	11th to 20th	Day	33% 4 13% 30% 18% 3	365
	21st to end	Day	29% 3 14% 30% 16% 8%	310
March	1st to 10th	Day	30% 3 12% 28% 21% 7%	460
	11th to 20th	Day	29% 3 13% 22% 25% 7%	460
	21st to end	Day	33% 4 14% 24% 18% 7%	506
April	1st to 10th	Day	27% 4 16% 25% 22% 7%	460
	11th to 20th	Day	26% 3 12% 31% 19% 9%	452
	21st to end	Day	22% 4 11% 33% 23% 7%	460
May	1st to 10th	Day	15% 2 10% 29% 35% 9%	463
	11th to 20th	Day	15% 2 5% 27% 35% 16%	488
	21st to end	Day	16% 3 7% 28% 32% 14%	605
June	1st to 10th	Day	18% 2 8% 31% 33% 9%	680
	11th to 20th	Day	17% 2 10% 29% 32% 10%	680
	21st to end	Day	13% 3 8% 25% 38% 14%	680
July	1st to 10th	Day	11% 1 4 27% 40% 17%	671
	11th to 20th	Day	6% 2 6% 23% 47% 16%	668
	21st to end	Day	6% 2 4 19% 49% 21%	604
August	1st to 10th	Day	10% 1 4 17% 42% 25%	487
	11th to 20th	Day	7% 2 4 23% 44% 19%	477
	21st to end	Day	11% 3 10% 25% 40% 12%	539
September	1st to 10th	Day	12% 2 7% 32% 35% 11%	490
	11th to 20th	Day	19% 5% 8% 24% 30% 13%	490
	21st to end	Day	22% 2 13% 26% 27% 10%	490
October	1st to 10th	Day	26% 4 9% 26% 23% 12%	490
	11th to 20th	Day	36% 4 10% 18% 23% 9%	490
	21st to end	Day	33% 6% 15% 25% 16% 4	403
November	1st to 10th	Day	41% 5% 14% 24% 13% 4	200
	11th to 20th	Day	36% 4 15% 19% 19% 8%	200
	21st to end	Day	41% 7% 13% 19% 13% 9%	200
December	1st to 10th	Day	40% 3 14% 15% 17% 11%	196
	11th to 20th	Day	37% 7% 12% 26% 17% 2	180
	21st to end	Day	42% 5% 7% 33% 9% 4	198
January	1st to 10th	Night	47% 6% 11% 19% 13% 4	850
	11th to 20th	Night	42% 4 10% 20% 22% 2	850
	21st to end	Night	40% 4 11% 24% 16% 5%	854
February	1st to 10th	Night	35% 6% 13% 27% 16% 3	760
	11th to 20th	Night	34% 3 14% 28% 18% 3	744
	21st to end	Night	31% 4 16% 27% 17% 5%	628
March	1st to 10th	Night	33% 4 14% 24% 20% 5%	760
	11th to 20th	Night	34% 5% 14% 23% 18% 7%	760
	21st to end	Night	34% 5% 11% 23% 19% 7%	836
April	1st to 10th	Night	30% 3 11% 28% 22% 6%	760
	11th to 20th	Night	24% 5% 15% 29% 21% 7%	639
	21st to end	Night	25% 4 10% 31% 24% 6%	523
May	1st to 10th	Night	17% 2 11% 29% 31% 10%	380
	11th to 20th	Night	18% 2 9% 22% 36% 14%	374
	21st to end	Night	19% 4 6% 25% 37% 9%	411
June	1st to 10th	Night	22% 2 8% 28% 32% 7%	380
	11th to 20th	Night	15% 7% 11% 33% 26% 9%	380
	21st to end	Night	13% 1 5% 30% 41% 10%	380
July	1st to 10th	Night	12% 2 6% 24% 40% 16%	370
	11th to 20th	Night	7% 1 7% 26% 43% 17%	368
	21st to end	Night	6% 3 4 20% 49% 19%	416
August	1st to 10th	Night	8% 1 3 17% 44% 26%	407
	11th to 20th	Night	8% 2 23% 46% 20%	564
	21st to end	Night	13% 2 9% 22% 39% 16%	674
September	1st to 10th	Night	10% 3 6% 27% 36% 19%	760
	11th to 20th	Night	21% 3 8% 20% 28% 20%	760
	21st to end	Night	24% 3 11% 23% 28% 10%	760
October	1st to 10th	Night	22% 2 10% 28% 25% 13%	760
	11th to 20th	Night	33% 3 11% 23% 22% 9%	760
	21st to end	Night	37% 5% 12% 25% 16% 6%	836
November	1st to 10th	Night	41% 6% 11% 25% 16% 1	780
	11th to 20th	Night	38% 5% 16% 23% 13% 5%	800
	21st to end	Night	41% 4 12% 22% 16% 5%	800
December	1st to 10th	Night	35% 5% 12% 24% 20% 4	813
	11th to 20th	Night	49% 5% 11% 23% 12% 1	850
	21st to end	Night	46% 6% 16% 20% 10% 2	935

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