UBC Forest Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the UBC Forest Site clear sky chart from 2005-11-06 to 2026-02-16.

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	28% 10% 3 6% 4 4 4 3 4 5% 30%	134533
	Night	31% 11% 3 6% 3 3 3 2 4 4 29%	134985
Transparency	Day	57% 6% 12% 16% 7% 1	134242
	Night	60% 5% 11% 15% 8% 1	134854
Seeing	Day	34% 11% 21% 23% 9% 2	16830
	Night	39% 7% 19% 22% 12% 1	25204

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	46% 16% 3 7% 4 4 3 2 3 2 9%	4125
	11th to 20th	Day	23% 10% 3 6% 3 4 3 2 4 4 39%	4156
	21st to end	Day	44% 8% 2 5% 2 2 3 2 2 3 26%	5186
February	1st to 10th	Day	37% 12% 3 7% 4 4 4 2 4 5% 17%	1890
	11th to 20th	Day	35% 13% 3 7% 3 4 3 2 4 4 21%	1801
	21st to end	Day	29% 19% 4 8% 5% 4 4 2 3 3 19%	1517
March	1st to 10th	Day	31% 19% 5% 9% 4 4 4 2 3 4 15%	4623
	11th to 20th	Day	29% 18% 4 8% 4 4 2 2 3 3 23%	5000
	21st to end	Day	35% 14% 4 8% 5% 4 4 3 4 5% 14%	5659
April	1st to 10th	Day	34% 13% 3 8% 6% 5% 4 4 5% 5% 14%	2400
	11th to 20th	Day	27% 12% 3 7% 6% 5% 4 4 4 5% 22%	2449
	21st to end	Day	31% 12% 3 8% 5% 4 4 3 5% 5% 21%	2680
May	1st to 10th	Day	20% 10% 3 7% 3 3 4 3 4 6% 37%	6320
	11th to 20th	Day	26% 11% 3 8% 5% 5% 5% 4 5% 6% 23%	6277
	21st to end	Day	28% 10% 3 7% 4 4 5% 4 5% 7% 22%	6985
June	1st to 10th	Day	28% 9% 3 6% 5% 4 4 3 5% 6% 27%	3000
	11th to 20th	Day	32% 11% 3 8% 6% 5% 4 3 4 5% 18%	3000
	21st to end	Day	23% 9% 3 7% 4 5% 4 3 5% 6% 32%	3000
July	1st to 10th	Day	12% 4 1 5% 3 3 3 3 4 5% 56%	6780
	11th to 20th	Day	7% 3 1 3 3 2 2 2 4 5% 67%	6780
	21st to end	Day	14% 4 1 4 3 2 3 3 5% 8% 53%	6932
August	1st to 10th	Day	9% 4 2 4 2 2 3 2 4 6% 62%	4541
	11th to 20th	Day	21% 7% 2 6% 3 4 4 3 5% 6% 38%	4161
	21st to end	Day	22% 7% 2 6% 4 3 4 3 5% 6% 38%	4537
September	1st to 10th	Day	19% 6% 2 5% 3 3 3 3 4 7% 44%	2400
	11th to 20th	Day	23% 8% 3 6% 4 3 3 2 5% 5% 38%	2400
	21st to end	Day	29% 8% 2 6% 3 3 4 2 5% 5% 33%	2110
October	1st to 10th	Day	29% 8% 3 7% 5% 4 4 3 5% 6% 26%	3280
	11th to 20th	Day	41% 7% 2 5% 4 4 4 4 4 5% 21%	3261
	21st to end	Day	40% 13% 4 8% 4 4 3 3 3 5% 14%	3280
November	1st to 10th	Day	44% 10% 3 6% 4 3 4 3 3 3 15%	1648
	11th to 20th	Day	46% 11% 2 6% 3 3 2 1 2 4 20%	1680
	21st to end	Day	44% 12% 3 5% 3 3 3 3 4 4 18%	1680
December	1st to 10th	Day	29% 11% 3 6% 4 4 4 4 5% 5% 26%	2700
	11th to 20th	Day	49% 15% 3 6% 3 3 2 1 3 3 11%	2720
	21st to end	Day	48% 16% 4 8% 4 3 3 2 3 3 7%	3575
January	1st to 10th	Night	44% 15% 4 7% 4 3 3 2 3 3 13%	8770
	11th to 20th	Night	26% 7% 2 4 3 3 3 3 5% 7% 38%	8739
	21st to end	Night	41% 8% 2 5% 2 2 3 2 3 3 29%	8977
February	1st to 10th	Night	36% 13% 3 7% 4 4 3 3 3 5% 19%	2730
	11th to 20th	Night	33% 15% 3 7% 5% 3 3 2 3 4 23%	2597
	21st to end	Night	33% 12% 3 7% 4 3 3 3 5% 4 23%	2112
March	1st to 10th	Night	29% 19% 3 7% 4 3 3 2 4 4 21%	5258
	11th to 20th	Night	31% 17% 3 7% 4 3 3 2 3 4 24%	4999
	21st to end	Night	43% 14% 3 6% 3 3 2 2 3 3 16%	5315
April	1st to 10th	Night	32% 13% 3 8% 5% 5% 3 3 5% 6% 20%	2000
	11th to 20th	Night	24% 11% 3 8% 5% 4 4 4 5% 5% 27%	1897
	21st to end	Night	29% 10% 3 8% 4 4 4 3 5% 6% 23%	1720
May	1st to 10th	Night	18% 10% 3 6% 3 3 2 2 3 4 46%	3640
	11th to 20th	Night	21% 11% 3 7% 3 4 4 4 6% 6% 32%	3616
	21st to end	Night	25% 10% 3 8% 4 5% 4 4 4 6% 27%	3886
June	1st to 10th	Night	25% 9% 2 6% 5% 4 4 3 5% 5% 30%	1400
	11th to 20th	Night	23% 10% 4 8% 5% 5% 4 4 5% 7% 25%	1400
	21st to end	Night	19% 8% 3 6% 4 4 3 4 6% 5% 39%	1400
July	1st to 10th	Night	11% 3 2 4 3 2 2 2 4 5% 60%	3180
	11th to 20th	Night	4 2 1 2 2 2 2 2 3 4 77%	3180
	21st to end	Night	8% 3 2 4 3 2 3 3 5% 6% 61%	3896
August	1st to 10th	Night	7% 2 1 3 2 1 2 2 5% 7% 68%	2609
	11th to 20th	Night	15% 5% 2 5% 4 3 3 3 5% 7% 47%	2894
	21st to end	Night	18% 6% 2 6% 4 3 4 3 6% 6% 42%	3375
September	1st to 10th	Night	13% 5% 2 4 3 3 4 3 4 6% 52%	2000
	11th to 20th	Night	23% 6% 2 6% 4 4 3 2 3 5% 42%	2000
	21st to end	Night	28% 8% 2 7% 3 4 2 3 4 5% 36%	2285
October	1st to 10th	Night	27% 12% 2 7% 4 4 4 3 5% 5% 27%	3920
	11th to 20th	Night	38% 9% 2 5% 3 3 3 3 4 5% 26%	3897
	21st to end	Night	38% 14% 3 7% 3 3 3 2 3 3 21%	4520
November	1st to 10th	Night	46% 9% 3 6% 4 3 3 2 3 4 16%	2870
	11th to 20th	Night	44% 11% 2 5% 3 3 2 2 3 3 21%	2940
	21st to end	Night	41% 11% 3 7% 4 3 4 2 2 3 20%	2940
December	1st to 10th	Night	30% 11% 3 5% 3 4 3 3 4 6% 27%	4700
	11th to 20th	Night	48% 14% 3 7% 3 3 3 2 3 3 11%	5700
	21st to end	Night	49% 16% 3 7% 3 3 2 2 2 2 10%	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	83% 4 5% 6% 2	4125
	11th to 20th	Day	50% 4 9% 22% 12% 3	4177
	21st to end	Day	67% 2 5% 17% 7% 1	5187
February	1st to 10th	Day	73% 4 7% 8% 6% 2	1890
	11th to 20th	Day	69% 5% 6% 12% 7% 2	1810
	21st to end	Day	73% 4 8% 10% 4 2	1517
March	1st to 10th	Day	71% 9% 7% 10% 4	4623
	11th to 20th	Day	69% 5% 6% 17% 3	5000
	21st to end	Day	73% 7% 9% 9% 2	5659
April	1st to 10th	Day	73% 7% 11% 7% 2	2400
	11th to 20th	Day	65% 6% 8% 11% 8% 2	2449
	21st to end	Day	66% 7% 10% 10% 6%	2680
May	1st to 10th	Day	50% 6% 14% 23% 7%	6320
	11th to 20th	Day	61% 10% 14% 12% 2	6280
	21st to end	Day	60% 9% 14% 15% 2	6986
June	1st to 10th	Day	60% 7% 14% 13% 5% 2	3000
	11th to 20th	Day	70% 7% 12% 9% 3	3000
	21st to end	Day	54% 7% 14% 16% 7% 1	3000
July	1st to 10th	Day	33% 5% 17% 22% 17% 6%	6706
	11th to 20th	Day	22% 5% 18% 34% 20% 2	6699
	21st to end	Day	31% 6% 27% 25% 9% 1	6737
August	1st to 10th	Day	25% 6% 12% 40% 16% 1	4524
	11th to 20th	Day	46% 8% 19% 18% 8% 1	4200
	21st to end	Day	49% 6% 15% 20% 9% 1	4536
September	1st to 10th	Day	43% 7% 13% 22% 11% 4	2400
	11th to 20th	Day	51% 6% 10% 18% 13% 2	2400
	21st to end	Day	55% 7% 13% 15% 9% 2	2110
October	1st to 10th	Day	60% 5% 13% 15% 7%	3280
	11th to 20th	Day	67% 4 10% 12% 7% 1	3261
	21st to end	Day	74% 5% 6% 10% 5% 1	3280
November	1st to 10th	Day	75% 5% 7% 9% 3 1	1648
	11th to 20th	Day	74% 4 7% 8% 6% 2	1680
	21st to end	Day	72% 4 5% 9% 8% 1	1680
December	1st to 10th	Day	62% 5% 8% 15% 10%	2700
	11th to 20th	Day	81% 5% 5% 7% 1 1	2723
	21st to end	Day	83% 6% 5% 5% 2	3575
January	1st to 10th	Night	80% 3 5% 8% 3	8770
	11th to 20th	Night	46% 5% 11% 18% 17% 3	8780
	21st to end	Night	63% 3 7% 17% 10% 1	8977
February	1st to 10th	Night	71% 5% 8% 8% 6% 3	2730
	11th to 20th	Night	68% 5% 6% 11% 7% 2	2599
	21st to end	Night	67% 4 8% 14% 6% 2	2112
March	1st to 10th	Night	67% 5% 9% 11% 7%	5258
	11th to 20th	Night	66% 5% 9% 15% 5%	5000
	21st to end	Night	74% 5% 12% 7% 2	5315
April	1st to 10th	Night	68% 7% 13% 10% 2	2000
	11th to 20th	Night	60% 6% 10% 15% 7% 2	1897
	21st to end	Night	63% 7% 12% 12% 5% 1	1720
May	1st to 10th	Night	46% 4 15% 24% 11% 1	3637
	11th to 20th	Night	53% 10% 19% 15% 3	3616
	21st to end	Night	58% 8% 16% 16% 2	3886
June	1st to 10th	Night	57% 5% 17% 13% 6% 3	1400
	11th to 20th	Night	60% 8% 15% 13% 3	1400
	21st to end	Night	47% 7% 19% 17% 7% 3	1400
July	1st to 10th	Night	27% 5% 18% 29% 14% 7%	3145
	11th to 20th	Night	15% 4 22% 35% 24% 1	3138
	21st to end	Night	24% 5% 23% 30% 15% 2	3786
August	1st to 10th	Night	19% 4 16% 38% 21% 2	2601
	11th to 20th	Night	36% 7% 27% 19% 8% 4	2921
	21st to end	Night	44% 7% 17% 21% 10% 1	3375
September	1st to 10th	Night	35% 8% 15% 23% 14% 5%	2000
	11th to 20th	Night	46% 5% 13% 17% 15% 4	2000
	21st to end	Night	52% 6% 14% 18% 8% 2	2285
October	1st to 10th	Night	60% 6% 14% 14% 6%	3920
	11th to 20th	Night	63% 4 9% 14% 9% 1	3897
	21st to end	Night	71% 4 6% 12% 6%	4520
November	1st to 10th	Night	74% 5% 7% 10% 3 1	2870
	11th to 20th	Night	72% 4 8% 8% 6% 2	2940
	21st to end	Night	72% 4 7% 7% 8% 1	2940
December	1st to 10th	Night	60% 5% 11% 17% 6% 1	4700
	11th to 20th	Night	81% 4 5% 7% 2 1	5696
	21st to end	Night	81% 5% 5% 6% 3 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	51% 15% 13% 13% 6% 2	220
	11th to 20th	Day	47% 11% 15% 19% 8%	220
	21st to end	Day	50% 7% 19% 17% 6% 1	242
February	1st to 10th	Day	47% 13% 18% 17% 4 1	430
	11th to 20th	Day	45% 14% 18% 18% 4 1	411
	21st to end	Day	45% 12% 26% 11% 5%	349
March	1st to 10th	Day	46% 12% 17% 19% 5% 1	500
	11th to 20th	Day	44% 12% 16% 20% 7% 1	500
	21st to end	Day	48% 14% 18% 14% 5% 1	550
April	1st to 10th	Day	44% 13% 19% 17% 6% 1	500
	11th to 20th	Day	33% 15% 25% 18% 7% 2	492
	21st to end	Day	35% 15% 22% 17% 8% 2	500
May	1st to 10th	Day	29% 13% 25% 24% 8% 1	503
	11th to 20th	Day	32% 10% 20% 25% 10% 3	528
	21st to end	Day	31% 14% 24% 19% 9% 3	603
June	1st to 10th	Day	31% 12% 25% 22% 9% 1	730
	11th to 20th	Day	35% 14% 22% 21% 6% 2	730
	21st to end	Day	28% 9% 21% 28% 12% 3	730
July	1st to 10th	Day	20% 9% 26% 32% 10% 3	721
	11th to 20th	Day	15% 11% 25% 34% 11% 4	718
	21st to end	Day	13% 7% 24% 38% 12% 6%	607
August	1st to 10th	Day	19% 7% 20% 34% 13% 7%	527
	11th to 20th	Day	16% 5% 23% 34% 16% 5%	517
	21st to end	Day	22% 9% 22% 31% 13% 3	583
September	1st to 10th	Day	23% 11% 22% 27% 13% 4	530
	11th to 20th	Day	29% 9% 19% 26% 15% 2	530
	21st to end	Day	33% 10% 20% 25% 10% 3	530
October	1st to 10th	Day	34% 7% 22% 23% 12% 2	530
	11th to 20th	Day	43% 8% 16% 19% 12% 2	530
	21st to end	Day	47% 10% 18% 15% 8% 1	427
November	1st to 10th	Day	50% 9% 20% 15% 6%	220
	11th to 20th	Day	57% 7% 12% 11% 10% 3	220
	21st to end	Day	54% 10% 15% 8% 9% 5%	220
December	1st to 10th	Day	51% 9% 10% 13% 15% 2	220
	11th to 20th	Day	61% 10% 13% 12% 2 2	220
	21st to end	Day	55% 9% 19% 14% 2	242
January	1st to 10th	Night	55% 10% 19% 13% 3	950
	11th to 20th	Night	46% 6% 23% 21% 3	942
	21st to end	Night	47% 8% 21% 19% 5%	924
February	1st to 10th	Night	46% 11% 24% 15% 4	840
	11th to 20th	Night	44% 9% 24% 18% 3	804
	21st to end	Night	42% 11% 25% 17% 5%	660
March	1st to 10th	Night	41% 11% 21% 19% 8% 1	800
	11th to 20th	Night	47% 8% 22% 20% 4	800
	21st to end	Night	47% 8% 19% 19% 6%	880
April	1st to 10th	Night	41% 7% 21% 23% 8% 1	800
	11th to 20th	Night	34% 9% 26% 22% 8% 1	709
	21st to end	Night	38% 7% 22% 23% 9% 1	520
May	1st to 10th	Night	28% 5% 19% 31% 17%	400
	11th to 20th	Night	25% 4 13% 35% 21% 2	394
	21st to end	Night	28% 5% 16% 29% 21% 1	433
June	1st to 10th	Night	31% 5% 16% 31% 16% 1	400
	11th to 20th	Night	30% 6% 15% 29% 17% 2	400
	21st to end	Night	24% 6% 12% 34% 22% 2	400
July	1st to 10th	Night	14% 5% 11% 35% 31% 4	390
	11th to 20th	Night	12% 3 11% 35% 32% 6%	388
	21st to end	Night	9% 2 8% 34% 43% 4	438
August	1st to 10th	Night	14% 1 7% 25% 46% 7%	398
	11th to 20th	Night	11% 4 7% 35% 38% 5%	586
	21st to end	Night	18% 3 13% 35% 30% 2	738
September	1st to 10th	Night	17% 3 17% 34% 28% 2	800
	11th to 20th	Night	26% 5% 13% 26% 26% 4	800
	21st to end	Night	33% 6% 12% 35% 13% 1	800
October	1st to 10th	Night	32% 7% 19% 28% 13% 1	800
	11th to 20th	Night	42% 6% 19% 22% 10% 1	800
	21st to end	Night	46% 8% 23% 17% 5% 1	880
November	1st to 10th	Night	54% 8% 23% 13% 2	820
	11th to 20th	Night	54% 8% 21% 13% 3	840
	21st to end	Night	49% 11% 18% 17% 5%	840
December	1st to 10th	Night	42% 9% 23% 20% 5%	840
	11th to 20th	Night	60% 10% 20% 8% 2	945
	21st to end	Night	56% 11% 24% 9% 1	1045

Page generated at 2026-02-12 22:42:12. Version ac07

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