Tantalus Lookout
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Tantalus Lookout clear sky chart from 2005-11-06 to 2025-11-18.

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	19% 15% 3 7% 4 4 4 3 4 5% 31%	130985
	Night	21% 19% 4 7% 4 3 3 3 4 5% 28%	128090
Transparency	Day	57% 4 13% 16% 8% 2	130688
	Night	60% 4 12% 15% 7% 1	127964
Seeing	Day	30% 13% 26% 21% 7% 1	16769
	Night	33% 8% 24% 24% 11% 1	24838

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	22% 30% 4 7% 3 3 3 2 4 3 19%	3179
	11th to 20th	Day	14% 14% 3 7% 5% 4 4 3 4 6% 37%	3159
	21st to end	Day	32% 20% 4 4 2 2 2 2 2 2 28%	3862
February	1st to 10th	Day	21% 23% 5% 9% 4 3 3 4 4 5% 20%	1800
	11th to 20th	Day	17% 26% 5% 7% 3 3 3 3 3 4 25%	1755
	21st to end	Day	13% 30% 5% 10% 7% 4 3 3 4 3 18%	1487
March	1st to 10th	Day	17% 24% 6% 10% 5% 4 5% 3 4 4 19%	4681
	11th to 20th	Day	13% 27% 5% 10% 5% 3 4 3 3 4 23%	5000
	21st to end	Day	18% 26% 6% 11% 6% 5% 4 3 4 4 13%	5617
April	1st to 10th	Day	21% 25% 4 9% 6% 4 4 3 4 5% 14%	2400
	11th to 20th	Day	19% 24% 4 9% 5% 5% 3 2 4 4 20%	2494
	21st to end	Day	23% 20% 5% 9% 5% 5% 5% 2 3 4 19%	2670
May	1st to 10th	Day	13% 13% 3 8% 6% 5% 5% 3 4 4 34%	6320
	11th to 20th	Day	16% 15% 5% 10% 5% 5% 5% 4 6% 7% 22%	6277
	21st to end	Day	23% 14% 3 8% 5% 5% 5% 4 6% 6% 19%	7173
June	1st to 10th	Day	30% 11% 3 9% 4 3 4 3 4 5% 23%	3000
	11th to 20th	Day	27% 15% 5% 9% 7% 5% 4 3 5% 5% 15%	3000
	21st to end	Day	22% 11% 4 8% 5% 4 4 3 4 5% 29%	3000
July	1st to 10th	Day	10% 4 2 5% 3 3 4 3 4 5% 56%	6780
	11th to 20th	Day	6% 4 1 3 3 3 3 2 4 6% 67%	6780
	21st to end	Day	9% 4 2 4 3 4 3 3 6% 8% 53%	7109
August	1st to 10th	Day	8% 4 1 3 2 2 3 3 5% 8% 62%	4523
	11th to 20th	Day	12% 7% 2 7% 5% 4 5% 4 6% 6% 40%	4156
	21st to end	Day	17% 7% 2 6% 4 4 4 4 6% 7% 39%	4581
September	1st to 10th	Day	15% 5% 2 6% 5% 3 4 3 4 6% 46%	2400
	11th to 20th	Day	22% 8% 3 6% 5% 3 3 2 3 5% 39%	2390
	21st to end	Day	27% 9% 3 7% 3 3 3 2 4 6% 32%	2110
October	1st to 10th	Day	22% 13% 4 9% 5% 5% 5% 3 5% 4 26%	3280
	11th to 20th	Day	34% 12% 3 7% 4 4 4 3 4 4 22%	3261
	21st to end	Day	29% 16% 4 10% 6% 5% 4 3 3 4 15%	3224
November	1st to 10th	Day	36% 18% 3 7% 4 3 3 3 4 4 17%	1648
	11th to 20th	Day	33% 24% 3 6% 3 3 3 2 3 2 18%	1656
	21st to end	Day	29% 22% 3 7% 3 4 3 3 3 4 19%	1600
December	1st to 10th	Day	18% 16% 4 7% 5% 3 3 3 5% 6% 31%	2470
	11th to 20th	Day	31% 28% 5% 9% 4 3 2 2 2 2 12%	2645
	21st to end	Day	29% 26% 5% 9% 5% 4 4 2 3 3 10%	3498
January	1st to 10th	Night	21% 29% 5% 9% 4 4 3 2 3 3 18%	6766
	11th to 20th	Night	17% 13% 3 6% 3 3 4 4 5% 7% 35%	6739
	21st to end	Night	29% 19% 3 4 2 3 2 2 3 4 28%	7091
February	1st to 10th	Night	22% 22% 5% 9% 4 4 4 3 4 5% 18%	2600
	11th to 20th	Night	17% 27% 5% 8% 4 3 3 2 3 4 24%	2532
	21st to end	Night	15% 30% 5% 8% 5% 3 4 2 3 4 20%	2142
March	1st to 10th	Night	18% 24% 5% 10% 5% 4 4 4 4 4 18%	5210
	11th to 20th	Night	12% 30% 6% 9% 4 3 4 2 3 3 23%	4999
	21st to end	Night	22% 28% 5% 9% 5% 3 3 2 3 3 16%	5358
April	1st to 10th	Night	20% 24% 4 9% 6% 6% 4 4 4 3 17%	2000
	11th to 20th	Night	18% 23% 4 6% 4 4 4 3 4 6% 25%	1855
	21st to end	Night	22% 18% 4 8% 5% 4 4 2 4 7% 22%	1730
May	1st to 10th	Night	13% 15% 3 7% 4 3 4 2 3 4 42%	3640
	11th to 20th	Night	16% 17% 4 8% 5% 4 5% 3 5% 7% 27%	3616
	21st to end	Night	22% 15% 3 9% 6% 5% 4 3 4 5% 24%	3699
June	1st to 10th	Night	24% 11% 3 7% 5% 4 4 4 5% 5% 28%	1400
	11th to 20th	Night	25% 13% 3 9% 5% 4 5% 3 6% 5% 22%	1400
	21st to end	Night	21% 10% 4 6% 5% 5% 4 3 5% 5% 33%	1400
July	1st to 10th	Night	11% 5% 1 3 2 2 3 2 4 6% 60%	3180
	11th to 20th	Night	5% 3 1 2 2 2 2 2 3 5% 73%	3180
	21st to end	Night	9% 6% 2 5% 2 3 3 3 5% 7% 54%	3732
August	1st to 10th	Night	7% 4 1 2 1 2 2 2 4 6% 68%	2625
	11th to 20th	Night	18% 8% 2 6% 4 4 4 4 4 6% 39%	2902
	21st to end	Night	15% 9% 3 7% 4 4 4 3 6% 6% 40%	3330
September	1st to 10th	Night	13% 7% 2 6% 2 3 3 4 5% 8% 47%	2000
	11th to 20th	Night	22% 10% 2 5% 3 3 4 2 4 5% 39%	2006
	21st to end	Night	26% 11% 3 7% 4 3 3 2 4 5% 33%	2290
October	1st to 10th	Night	20% 16% 4 10% 5% 4 4 3 3 4 25%	3920
	11th to 20th	Night	30% 15% 4 7% 3 3 3 3 4 4 25%	3897
	21st to end	Night	28% 21% 5% 8% 4 4 3 3 3 4 17%	4573
November	1st to 10th	Night	36% 19% 4 7% 3 3 3 2 4 4 14%	2870
	11th to 20th	Night	32% 24% 4 6% 4 2 3 2 3 4 16%	2898
	21st to end	Night	28% 23% 4 6% 3 4 3 2 4 3 20%	2800
December	1st to 10th	Night	20% 17% 4 6% 5% 3 3 3 5% 6% 27%	4692
	11th to 20th	Night	31% 24% 6% 9% 4 3 3 2 3 3 12%	5560
	21st to end	Night	31% 29% 5% 9% 4 3 2 2 2 2 10%	7458

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	72% 3 10% 12% 3 1	3179
	11th to 20th	Day	52% 4 15% 15% 10% 4	3180
	21st to end	Day	67% 2 4 18% 8% 1	3862
February	1st to 10th	Day	68% 5% 9% 9% 8% 2	1800
	11th to 20th	Day	65% 4 8% 14% 7% 2	1764
	21st to end	Day	72% 3 9% 10% 4 2	1487
March	1st to 10th	Day	70% 4 8% 12% 6%	4681
	11th to 20th	Day	68% 3 6% 18% 4	5000
	21st to end	Day	77% 4 9% 7% 3	5617
April	1st to 10th	Day	74% 6% 11% 8% 1	2400
	11th to 20th	Day	69% 3 8% 11% 7% 1	2494
	21st to end	Day	73% 4 8% 10% 5%	2670
May	1st to 10th	Day	53% 3 14% 19% 10% 1	6320
	11th to 20th	Day	59% 8% 17% 13% 3	6280
	21st to end	Day	64% 7% 14% 12% 3	7174
June	1st to 10th	Day	65% 6% 12% 10% 5% 3	3000
	11th to 20th	Day	72% 5% 11% 9% 3	3000
	21st to end	Day	58% 4 13% 16% 7% 1	3000
July	1st to 10th	Day	32% 4 18% 23% 15% 8%	6706
	11th to 20th	Day	22% 4 17% 35% 20% 2	6699
	21st to end	Day	29% 3 27% 23% 16% 2	6909
August	1st to 10th	Day	23% 4 10% 38% 23% 1	4506
	11th to 20th	Day	44% 4 23% 17% 10% 2	4195
	21st to end	Day	44% 6% 18% 21% 10% 2	4580
September	1st to 10th	Day	40% 6% 14% 24% 12% 4	2400
	11th to 20th	Day	49% 5% 9% 18% 16% 3	2390
	21st to end	Day	54% 6% 13% 16% 9% 2	2110
October	1st to 10th	Day	60% 3 13% 16% 7%	3280
	11th to 20th	Day	68% 4 8% 15% 5%	3261
	21st to end	Day	74% 2 8% 11% 5% 1	3224
November	1st to 10th	Day	72% 5% 9% 8% 5% 1	1648
	11th to 20th	Day	74% 3 7% 7% 6% 2	1656
	21st to end	Day	71% 4 6% 10% 7% 2	1600
December	1st to 10th	Day	59% 3 13% 15% 10% 1	2470
	11th to 20th	Day	82% 2 8% 5% 2 1	2648
	21st to end	Day	80% 3 6% 9% 2	3498
January	1st to 10th	Night	75% 3 6% 10% 6% 1	6766
	11th to 20th	Night	49% 6% 12% 20% 9% 3	6780
	21st to end	Night	63% 3 9% 16% 8% 1	7091
February	1st to 10th	Night	69% 4 9% 9% 8% 2	2600
	11th to 20th	Night	65% 5% 9% 11% 7% 3	2534
	21st to end	Night	69% 4 8% 11% 6% 2	2142
March	1st to 10th	Night	68% 5% 8% 11% 7%	5210
	11th to 20th	Night	67% 4 7% 17% 4	5000
	21st to end	Night	73% 5% 10% 8% 3 1	5358
April	1st to 10th	Night	70% 7% 11% 11% 1	2000
	11th to 20th	Night	62% 6% 10% 15% 7%	1855
	21st to end	Night	64% 5% 12% 13% 6%	1730
May	1st to 10th	Night	47% 5% 16% 23% 8%	3637
	11th to 20th	Night	55% 7% 22% 15% 1	3616
	21st to end	Night	63% 5% 16% 13% 2	3699
June	1st to 10th	Night	60% 6% 18% 9% 6% 3	1400
	11th to 20th	Night	64% 7% 13% 11% 4	1400
	21st to end	Night	53% 9% 14% 17% 6% 2	1400
July	1st to 10th	Night	29% 5% 20% 25% 17% 5%	3145
	11th to 20th	Night	17% 5% 21% 33% 23% 1	3138
	21st to end	Night	30% 3 26% 23% 14% 3	3627
August	1st to 10th	Night	19% 4 12% 39% 25% 1	2617
	11th to 20th	Night	45% 5% 22% 15% 9% 3	2929
	21st to end	Night	45% 5% 19% 22% 8% 2	3330
September	1st to 10th	Night	36% 7% 17% 24% 13% 4	2000
	11th to 20th	Night	47% 5% 12% 18% 14% 4	2006
	21st to end	Night	54% 5% 14% 16% 9% 2	2290
October	1st to 10th	Night	62% 4 14% 14% 6%	3920
	11th to 20th	Night	64% 3 7% 18% 7%	3897
	21st to end	Night	70% 3 7% 15% 5% 1	4573
November	1st to 10th	Night	74% 4 8% 8% 4 1	2870
	11th to 20th	Night	74% 4 8% 8% 6% 2	2898
	21st to end	Night	70% 3 8% 9% 9% 2	2800
December	1st to 10th	Night	60% 4 12% 16% 7% 1	4692
	11th to 20th	Night	78% 3 8% 7% 2 1	5556
	21st to end	Night	81% 2 6% 7% 4 1	7458

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% 12% 25% 17% 8% 1	200
	11th to 20th	Day	38% 10% 24% 24% 6%	200
	21st to end	Day	36% 9% 28% 20% 6% 1	227
February	1st to 10th	Day	36% 17% 27% 16% 5%	400
	11th to 20th	Day	34% 14% 28% 20% 3 1	395
	21st to end	Day	31% 21% 26% 17% 4 1	333
March	1st to 10th	Day	32% 16% 27% 19% 6%	500
	11th to 20th	Day	35% 14% 23% 19% 8% 1	500
	21st to end	Day	37% 14% 27% 14% 7% 1	550
April	1st to 10th	Day	39% 17% 22% 17% 5% 1	500
	11th to 20th	Day	34% 17% 25% 16% 7% 1	492
	21st to end	Day	35% 19% 19% 19% 7% 1	500
May	1st to 10th	Day	28% 16% 26% 24% 6% 1	503
	11th to 20th	Day	30% 9% 24% 24% 12% 1	528
	21st to end	Day	32% 13% 26% 20% 8% 2	687
June	1st to 10th	Day	34% 16% 26% 18% 5% 1	730
	11th to 20th	Day	35% 17% 24% 18% 5% 1	730
	21st to end	Day	27% 12% 29% 25% 5% 1	730
July	1st to 10th	Day	21% 12% 29% 28% 7% 2	721
	11th to 20th	Day	15% 14% 36% 27% 7% 2	718
	21st to end	Day	14% 11% 31% 31% 11% 3	680
August	1st to 10th	Day	15% 9% 27% 31% 13% 5%	527
	11th to 20th	Day	14% 9% 31% 31% 12% 3	517
	21st to end	Day	19% 13% 30% 25% 11% 3	583
September	1st to 10th	Day	20% 15% 29% 24% 9% 2	530
	11th to 20th	Day	29% 11% 27% 22% 9% 2	530
	21st to end	Day	30% 15% 26% 21% 6% 1	530
October	1st to 10th	Day	34% 12% 25% 21% 8% 1	530
	11th to 20th	Day	42% 8% 22% 17% 8% 2	530
	21st to end	Day	42% 13% 23% 16% 5% 1	414
November	1st to 10th	Day	41% 10% 30% 15% 4	220
	11th to 20th	Day	46% 9% 17% 18% 8% 1	214
	21st to end	Day	43% 11% 18% 16% 9% 4	200
December	1st to 10th	Day	39% 9% 18% 18% 15% 2	200
	11th to 20th	Day	43% 12% 22% 19% 4 1	200
	21st to end	Day	35% 10% 37% 15% 3	220
January	1st to 10th	Night	44% 10% 24% 18% 4 1	900
	11th to 20th	Night	38% 8% 29% 21% 4	900
	21st to end	Night	35% 9% 31% 20% 4	915
February	1st to 10th	Night	33% 9% 34% 20% 3	800
	11th to 20th	Night	30% 10% 31% 24% 5%	784
	21st to end	Night	31% 13% 27% 20% 8%	660
March	1st to 10th	Night	33% 11% 24% 22% 10% 1	800
	11th to 20th	Night	38% 11% 22% 22% 6%	800
	21st to end	Night	37% 10% 23% 19% 9% 1	880
April	1st to 10th	Night	35% 8% 21% 27% 9%	800
	11th to 20th	Night	30% 7% 25% 30% 7% 1	667
	21st to end	Night	33% 6% 25% 24% 11%	530
May	1st to 10th	Night	22% 7% 23% 34% 14% 1	400
	11th to 20th	Night	28% 5% 19% 32% 16% 1	394
	21st to end	Night	29% 6% 23% 28% 14%	433
June	1st to 10th	Night	30% 5% 27% 25% 13% 1	400
	11th to 20th	Night	30% 7% 24% 27% 11% 2	400
	21st to end	Night	28% 5% 18% 33% 15% 1	400
July	1st to 10th	Night	16% 4 17% 37% 25% 1	390
	11th to 20th	Night	14% 3 14% 40% 27% 2	388
	21st to end	Night	12% 3 13% 35% 34% 4	438
August	1st to 10th	Night	16% 2 11% 31% 36% 4	401
	11th to 20th	Night	14% 3 10% 38% 32% 3	594
	21st to end	Night	21% 4 15% 34% 25% 2	706
September	1st to 10th	Night	17% 5% 23% 34% 21% 1	800
	11th to 20th	Night	27% 5% 17% 26% 23% 3	800
	21st to end	Night	34% 5% 19% 29% 12% 1	800
October	1st to 10th	Night	29% 6% 24% 28% 12% 1	800
	11th to 20th	Night	39% 5% 22% 23% 10% 1	800
	21st to end	Night	42% 8% 25% 19% 4 1	880
November	1st to 10th	Night	48% 7% 25% 18% 2	820
	11th to 20th	Night	45% 9% 25% 16% 4 1	828
	21st to end	Night	39% 11% 24% 20% 5% 1	800
December	1st to 10th	Night	33% 10% 31% 18% 7%	840
	11th to 20th	Night	45% 10% 29% 14% 2	900
	21st to end	Night	41% 10% 33% 15% 1	990

Page generated at 2025-11-14 21:55:56. Version ac07

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