UBC Liquid-Mirror Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the UBC Liquid-Mirror Observatory clear sky chart from 2005-11-06 to 2024-01-15.

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	32% 9% 3 5% 4 3 3 3 4 5% 29%	74767
	Night	35% 9% 2 4 4 2 3 3 4 5% 29%	75008
Transparency	Day	60% 6% 11% 15% 7% 1	74457
	Night	61% 6% 11% 14% 7% 2	74836
Seeing	Day	34% 11% 21% 23% 9% 2	14158
	Night	39% 7% 19% 22% 12% 1	22544

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	51% 11% 2 4 4 2 2 3 3 3 14%	1329
	11th to 20th	Day	42% 10% 3 5% 4 2 4 3 3 3 20%	1297
	21st to end	Day	41% 13% 3 4 3 2 3 3 4 4 22%	1584
February	1st to 10th	Day	42% 12% 3 5% 4 2 4 3 3 5% 18%	1620
	11th to 20th	Day	39% 11% 3 5% 5% 2 3 3 3 4 21%	1575
	21st to end	Day	29% 19% 4 5% 5% 2 4 4 3 3 20%	1360
March	1st to 10th	Day	37% 15% 3 5% 4 3 4 4 3 4 19%	1840
	11th to 20th	Day	42% 13% 3 5% 4 2 3 3 3 4 17%	1980
	21st to end	Day	38% 14% 3 5% 5% 3 4 4 3 4 16%	2248
April	1st to 10th	Day	37% 12% 4 6% 5% 3 4 4 4 5% 17%	2160
	11th to 20th	Day	32% 12% 3 6% 5% 4 5% 4 4 5% 20%	2201
	21st to end	Day	34% 10% 4 7% 5% 2 4 4 4 5% 21%	2412
May	1st to 10th	Day	28% 8% 3 5% 5% 3 5% 4 4 5% 30%	2520
	11th to 20th	Day	26% 8% 3 6% 5% 3 5% 4 5% 7% 28%	2477
	21st to end	Day	31% 9% 3 6% 5% 3 4 4 3 5% 27%	2749
June	1st to 10th	Day	33% 9% 2 5% 5% 3 4 4 4 6% 25%	2700
	11th to 20th	Day	37% 8% 4 6% 5% 3 4 3 3 6% 19%	2700
	21st to end	Day	25% 7% 2 5% 4 3 5% 4 4 6% 34%	2700
July	1st to 10th	Day	19% 5% 2 4 4 2 4 4 5% 5% 46%	2700
	11th to 20th	Day	18% 5% 2 4 4 2 4 3 5% 7% 47%	2700
	21st to end	Day	15% 3 1 2 2 2 3 3 4 6% 59%	2771
August	1st to 10th	Day	17% 5% 2 3 4 2 3 3 4 6% 51%	2507
	11th to 20th	Day	13% 5% 2 4 2 2 3 4 5% 7% 54%	2303
	21st to end	Day	21% 6% 3 3 4 3 4 4 4 7% 42%	2503
September	1st to 10th	Day	21% 7% 3 5% 3 2 3 3 3 6% 44%	2160
	11th to 20th	Day	25% 8% 3 4 4 2 3 3 3 5% 39%	2160
	21st to end	Day	32% 8% 2 4 3 2 3 3 3 5% 36%	1899
October	1st to 10th	Day	31% 7% 3 4 4 2 3 4 4 5% 34%	1800
	11th to 20th	Day	39% 7% 3 4 3 3 3 3 4 5% 25%	1800
	21st to end	Day	43% 8% 2 4 4 2 3 3 3 6% 23%	1832
November	1st to 10th	Day	49% 9% 2 5% 4 2 4 2 3 4 16%	1488
	11th to 20th	Day	47% 11% 2 3 4 1 2 3 2 3 22%	1520
	21st to end	Day	45% 9% 3 4 4 2 3 3 4 4 19%	1520
December	1st to 10th	Day	38% 12% 3 4 3 2 2 4 3 3 26%	1520
	11th to 20th	Day	45% 14% 3 4 4 3 3 2 3 3 16%	1690
	21st to end	Day	49% 14% 3 6% 4 2 3 3 3 3 10%	2442
January	1st to 10th	Night	50% 12% 2 4 3 2 2 2 3 3 16%	2836
	11th to 20th	Night	45% 9% 2 4 3 2 2 3 3 5% 22%	2748
	21st to end	Night	40% 13% 3 5% 3 2 3 3 3 4 22%	2768
February	1st to 10th	Night	42% 10% 3 4 4 3 3 3 3 4 20%	2340
	11th to 20th	Night	36% 15% 3 5% 5% 2 3 2 3 4 22%	2272
	21st to end	Night	32% 12% 3 6% 4 2 3 3 3 5% 25%	1895
March	1st to 10th	Night	36% 12% 3 5% 3 2 4 3 4 4 23%	2119
	11th to 20th	Night	41% 12% 2 5% 5% 2 4 2 3 4 20%	1979
	21st to end	Night	40% 12% 3 5% 4 2 4 3 4 4 19%	2107
April	1st to 10th	Night	34% 12% 3 6% 5% 3 4 3 4 6% 21%	1800
	11th to 20th	Night	29% 12% 3 5% 5% 3 4 3 4 6% 26%	1705
	21st to end	Night	33% 9% 3 6% 4 3 4 4 4 5% 23%	1548
May	1st to 10th	Night	24% 8% 3 5% 4 3 3 4 4 5% 37%	1440
	11th to 20th	Night	22% 6% 3 5% 4 3 5% 6% 6% 6% 34%	1416
	21st to end	Night	26% 5% 2 5% 5% 2 4 5% 5% 7% 34%	1521
June	1st to 10th	Night	28% 8% 3 5% 6% 4 4 4 5% 6% 28%	1260
	11th to 20th	Night	28% 9% 4 6% 5% 3 4 4 5% 7% 25%	1260
	21st to end	Night	21% 4 2 5% 4 3 3 5% 5% 6% 42%	1260
July	1st to 10th	Night	15% 5% 2 5% 5% 2 3 3 4 5% 51%	1260
	11th to 20th	Night	12% 3 2 4 3 3 3 3 4 6% 57%	1260
	21st to end	Night	9% 2 1 2 3 2 3 4 4 4 66%	1545
August	1st to 10th	Night	12% 3 2 3 3 2 2 3 4 6% 60%	1432
	11th to 20th	Night	9% 3 1 3 3 2 2 2 5% 8% 62%	1594
	21st to end	Night	16% 4 2 4 4 3 4 4 7% 8% 45%	1852
September	1st to 10th	Night	16% 3 2 3 4 2 4 4 4 6% 52%	1800
	11th to 20th	Night	24% 6% 2 5% 4 2 3 3 3 5% 42%	1800
	21st to end	Night	29% 7% 2 4 3 2 3 3 3 5% 38%	2055
October	1st to 10th	Night	27% 7% 2 5% 4 3 3 3 4 4 38%	2160
	11th to 20th	Night	38% 7% 2 4 4 2 3 3 3 5% 28%	2160
	21st to end	Night	40% 9% 2 4 4 2 3 3 3 4 27%	2522
November	1st to 10th	Night	48% 8% 3 4 4 3 3 3 3 4 18%	2590
	11th to 20th	Night	45% 9% 2 4 3 2 2 3 3 4 23%	2660
	21st to end	Night	44% 9% 2 5% 4 2 3 3 2 3 22%	2660
December	1st to 10th	Night	37% 10% 3 4 3 2 3 3 3 4 27%	2660
	11th to 20th	Night	51% 13% 3 4 4 2 3 2 3 3 14%	3510
	21st to end	Night	51% 13% 3 5% 4 2 2 2 2 2 14%	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	78% 3 6% 7% 4 1	1329
	11th to 20th	Day	72% 5% 6% 10% 4 2	1297
	21st to end	Day	70% 3 8% 13% 5% 1	1584
February	1st to 10th	Day	74% 4 6% 8% 6% 2	1620
	11th to 20th	Day	70% 5% 6% 10% 7% 1	1584
	21st to end	Day	71% 4 8% 11% 5% 2	1360
March	1st to 10th	Day	73% 5% 8% 10% 4	1840
	11th to 20th	Day	75% 4 5% 10% 5% 1	1980
	21st to end	Day	74% 5% 8% 10% 2	2248
April	1st to 10th	Day	73% 6% 11% 8% 2	2160
	11th to 20th	Day	69% 6% 6% 10% 7% 1	2201
	21st to end	Day	68% 7% 9% 9% 6% 1	2412
May	1st to 10th	Day	59% 8% 12% 15% 6% 1	2520
	11th to 20th	Day	57% 7% 14% 16% 5%	2480
	21st to end	Day	62% 6% 14% 16% 2	2750
June	1st to 10th	Day	62% 7% 13% 12% 4 2	2700
	11th to 20th	Day	71% 7% 11% 8% 3	2700
	21st to end	Day	53% 7% 14% 16% 7% 1	2700
July	1st to 10th	Day	43% 7% 13% 24% 11% 2	2626
	11th to 20th	Day	39% 7% 17% 27% 9% 2	2619
	21st to end	Day	29% 6% 16% 33% 13% 3	2617
August	1st to 10th	Day	36% 8% 14% 28% 11% 2	2490
	11th to 20th	Day	32% 8% 18% 26% 14% 3	2303
	21st to end	Day	45% 7% 16% 25% 6% 1	2503
September	1st to 10th	Day	44% 6% 12% 22% 11% 4	2160
	11th to 20th	Day	52% 5% 9% 18% 14% 3	2160
	21st to end	Day	54% 7% 13% 15% 9% 2	1899
October	1st to 10th	Day	55% 5% 12% 16% 12%	1800
	11th to 20th	Day	63% 6% 10% 16% 5% 1	1800
	21st to end	Day	66% 5% 9% 11% 8% 1	1832
November	1st to 10th	Day	75% 5% 7% 9% 3 1	1488
	11th to 20th	Day	72% 3 7% 8% 7% 2	1520
	21st to end	Day	71% 4 5% 10% 8% 2	1520
December	1st to 10th	Day	67% 5% 6% 10% 11% 1	1520
	11th to 20th	Day	74% 6% 6% 10% 2 2	1693
	21st to end	Day	80% 7% 4 6% 2 1	2442
January	1st to 10th	Night	77% 3 4 11% 4 1	2836
	11th to 20th	Night	69% 4 9% 11% 6% 2	2748
	21st to end	Night	69% 3 7% 12% 7% 1	2768
February	1st to 10th	Night	71% 5% 8% 9% 5% 3	2340
	11th to 20th	Night	70% 5% 6% 10% 7% 2	2274
	21st to end	Night	65% 4 8% 14% 7% 2	1895
March	1st to 10th	Night	66% 5% 12% 12% 5%	2119
	11th to 20th	Night	71% 5% 8% 10% 6% 1	1980
	21st to end	Night	71% 5% 13% 8% 2	2107
April	1st to 10th	Night	68% 6% 13% 11% 3	1800
	11th to 20th	Night	63% 6% 9% 14% 6% 2	1705
	21st to end	Night	64% 7% 11% 11% 6% 1	1548
May	1st to 10th	Night	50% 7% 18% 17% 6% 1	1440
	11th to 20th	Night	51% 9% 18% 16% 6%	1416
	21st to end	Night	52% 7% 19% 17% 5%	1521
June	1st to 10th	Night	59% 5% 17% 13% 4 3	1260
	11th to 20th	Night	61% 8% 16% 11% 3	1260
	21st to end	Night	45% 7% 19% 18% 8% 3	1260
July	1st to 10th	Night	38% 7% 12% 25% 14% 3	1225
	11th to 20th	Night	31% 6% 21% 28% 13% 1	1218
	21st to end	Night	21% 8% 13% 36% 17% 5%	1459
August	1st to 10th	Night	27% 7% 17% 29% 17% 3	1424
	11th to 20th	Night	22% 7% 24% 27% 13% 6%	1594
	21st to end	Night	37% 8% 19% 26% 9% 2	1852
September	1st to 10th	Night	36% 8% 14% 24% 15% 4	1800
	11th to 20th	Night	47% 4 12% 17% 16% 4	1800
	21st to end	Night	51% 7% 13% 18% 9% 2	2055
October	1st to 10th	Night	53% 6% 12% 18% 11% 1	2160
	11th to 20th	Night	62% 5% 11% 15% 7% 1	2160
	21st to end	Night	64% 6% 7% 13% 9% 1	2522
November	1st to 10th	Night	73% 5% 7% 10% 4 1	2590
	11th to 20th	Night	70% 4 8% 8% 7% 2	2660
	21st to end	Night	71% 4 7% 7% 9% 1	2660
December	1st to 10th	Night	64% 5% 8% 12% 9% 2	2660
	11th to 20th	Night	78% 5% 6% 6% 4 2	3506
	21st to end	Night	78% 6% 5% 7% 4 1	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	51% 17% 13% 10% 8% 2	180
	11th to 20th	Day	56% 12% 11% 16% 5%	168
	21st to end	Day	52% 9% 16% 18% 5%	176
February	1st to 10th	Day	51% 14% 17% 14% 3 1	340
	11th to 20th	Day	46% 14% 19% 16% 4 1	335
	21st to end	Day	44% 12% 27% 12% 5%	294
March	1st to 10th	Day	46% 13% 18% 17% 5%	420
	11th to 20th	Day	45% 12% 17% 18% 7% 1	420
	21st to end	Day	48% 14% 18% 15% 5% 1	462
April	1st to 10th	Day	44% 14% 18% 17% 6% 1	420
	11th to 20th	Day	36% 16% 21% 18% 7% 2	412
	21st to end	Day	35% 16% 24% 17% 8% 1	420
May	1st to 10th	Day	29% 12% 26% 24% 6% 1	423
	11th to 20th	Day	31% 9% 20% 27% 10% 3	448
	21st to end	Day	30% 14% 24% 19% 9% 3	511
June	1st to 10th	Day	33% 12% 23% 23% 8% 1	630
	11th to 20th	Day	34% 15% 23% 21% 5% 2	630
	21st to end	Day	27% 9% 21% 28% 12% 2	630
July	1st to 10th	Day	21% 9% 26% 31% 10% 3	621
	11th to 20th	Day	17% 12% 24% 33% 11% 3	618
	21st to end	Day	11% 8% 24% 38% 13% 6%	517
August	1st to 10th	Day	19% 8% 20% 34% 13% 6%	447
	11th to 20th	Day	14% 5% 24% 35% 17% 5%	437
	21st to end	Day	23% 10% 23% 30% 12% 2	495
September	1st to 10th	Day	25% 11% 25% 27% 10% 1	450
	11th to 20th	Day	30% 9% 19% 26% 14% 2	450
	21st to end	Day	32% 10% 20% 25% 10% 3	450
October	1st to 10th	Day	34% 7% 22% 22% 12% 2	450
	11th to 20th	Day	42% 8% 16% 19% 13% 2	450
	21st to end	Day	45% 10% 20% 15% 9% 2	356
November	1st to 10th	Day	50% 8% 21% 14% 7%	180
	11th to 20th	Day	54% 6% 12% 11% 13% 3	180
	21st to end	Day	51% 11% 15% 8% 9% 5%	180
December	1st to 10th	Day	52% 9% 13% 11% 12% 2	180
	11th to 20th	Day	58% 9% 14% 13% 2 2	180
	21st to end	Day	54% 9% 21% 14% 2 1	198
January	1st to 10th	Night	56% 9% 19% 12% 3	850
	11th to 20th	Night	50% 6% 22% 20% 2	814
	21st to end	Night	48% 8% 22% 18% 4	792
February	1st to 10th	Night	48% 11% 24% 15% 3	720
	11th to 20th	Night	45% 9% 24% 18% 4	704
	21st to end	Night	40% 10% 26% 18% 6% 1	592
March	1st to 10th	Night	41% 10% 21% 19% 8%	720
	11th to 20th	Night	47% 8% 22% 18% 4	720
	21st to end	Night	47% 7% 19% 20% 6% 1	792
April	1st to 10th	Night	41% 7% 21% 22% 8% 1	720
	11th to 20th	Night	36% 9% 26% 20% 8% 1	637
	21st to end	Night	39% 6% 23% 23% 8% 1	468
May	1st to 10th	Night	28% 5% 19% 30% 18%	360
	11th to 20th	Night	23% 3 13% 35% 23% 2	354
	21st to end	Night	29% 4 15% 29% 23% 1	389
June	1st to 10th	Night	32% 5% 14% 32% 16% 1	360
	11th to 20th	Night	30% 6% 16% 29% 16% 2	360
	21st to end	Night	22% 6% 13% 35% 23% 2	360
July	1st to 10th	Night	15% 5% 10% 34% 31% 5%	350
	11th to 20th	Night	14% 4 11% 32% 33% 6%	348
	21st to end	Night	9% 2 7% 35% 43% 4	394
August	1st to 10th	Night	13% 1 7% 25% 46% 7%	358
	11th to 20th	Night	9% 3 6% 37% 40% 5%	526
	21st to end	Night	18% 3 13% 36% 28% 2	664
September	1st to 10th	Night	17% 3 18% 35% 25% 1	720
	11th to 20th	Night	27% 5% 14% 26% 25% 4	720
	21st to end	Night	32% 7% 12% 34% 14% 1	720
October	1st to 10th	Night	32% 6% 19% 28% 14% 1	720
	11th to 20th	Night	42% 6% 20% 22% 11% 1	720
	21st to end	Night	44% 8% 23% 18% 6% 1	792
November	1st to 10th	Night	54% 8% 22% 13% 2	740
	11th to 20th	Night	52% 8% 22% 14% 4 1	760
	21st to end	Night	49% 11% 18% 17% 5%	760
December	1st to 10th	Night	42% 9% 25% 20% 4	760
	11th to 20th	Night	58% 9% 22% 9% 2	845
	21st to end	Night	56% 10% 25% 8% 1	935

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