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 2024-01-12.

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	23% 15% 4 6% 5% 3 4 4 4 5% 28%	74728
	Night	25% 18% 4 6% 5% 2 4 3 4 5% 26%	75007
Transparency	Day	61% 5% 11% 14% 7% 1	74414
	Night	62% 5% 11% 13% 7% 2	74839
Seeing	Day	30% 13% 26% 21% 8% 1	14278
	Night	33% 8% 24% 24% 11% 1	22563

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	34% 25% 4 4 4 2 3 3 3 2 17%	1329
	11th to 20th	Day	28% 22% 4 7% 5% 2 3 3 3 5% 18%	1269
	21st to end	Day	23% 25% 5% 7% 5% 2 4 3 3 3 20%	1526
February	1st to 10th	Day	23% 24% 5% 7% 4 2 4 4 3 5% 20%	1620
	11th to 20th	Day	18% 26% 5% 7% 4 2 3 3 4 4 24%	1575
	21st to end	Day	12% 28% 7% 8% 8% 2 3 4 4 3 20%	1333
March	1st to 10th	Day	22% 22% 5% 8% 6% 3 4 4 4 4 18%	1859
	11th to 20th	Day	25% 29% 4 7% 5% 3 2 2 2 4 17%	1980
	21st to end	Day	24% 25% 5% 8% 6% 3 4 4 3 4 15%	2232
April	1st to 10th	Day	22% 25% 6% 7% 6% 3 4 4 4 5% 14%	2160
	11th to 20th	Day	23% 23% 6% 8% 7% 4 4 3 3 3 17%	2241
	21st to end	Day	24% 19% 6% 9% 6% 4 5% 4 3 3 18%	2402
May	1st to 10th	Day	20% 16% 4 9% 7% 3 4 4 4 4 25%	2520
	11th to 20th	Day	19% 13% 5% 8% 7% 4 4 5% 6% 7% 23%	2477
	21st to end	Day	27% 12% 4 6% 6% 3 5% 4 4 6% 23%	2825
June	1st to 10th	Day	32% 11% 4 7% 6% 3 4 4 4 5% 20%	2700
	11th to 20th	Day	29% 14% 5% 8% 8% 4 4 4 4 5% 15%	2700
	21st to end	Day	23% 10% 4 7% 6% 3 4 3 4 5% 31%	2700
July	1st to 10th	Day	16% 7% 3 6% 5% 2 4 4 4 6% 42%	2700
	11th to 20th	Day	14% 6% 3 4 5% 3 4 4 6% 7% 45%	2700
	21st to end	Day	12% 5% 2 4 3 2 4 4 5% 7% 51%	2836
August	1st to 10th	Day	13% 6% 2 5% 4 2 3 4 5% 7% 51%	2503
	11th to 20th	Day	10% 6% 2 4 4 2 4 5% 5% 6% 54%	2298
	21st to end	Day	16% 7% 3 6% 5% 3 4 4 5% 8% 38%	2533
September	1st to 10th	Day	17% 6% 2 6% 5% 3 3 4 4 6% 45%	2160
	11th to 20th	Day	24% 7% 3 5% 5% 3 3 3 3 5% 39%	2152
	21st to end	Day	27% 9% 3 6% 4 2 3 2 4 5% 34%	1900
October	1st to 10th	Day	25% 11% 3 5% 4 2 3 3 4 4 36%	1800
	11th to 20th	Day	34% 11% 3 5% 4 3 3 4 4 6% 25%	1800
	21st to end	Day	38% 12% 3 5% 4 2 2 3 2 5% 25%	1803
November	1st to 10th	Day	37% 17% 3 5% 5% 2 3 4 3 4 18%	1488
	11th to 20th	Day	35% 22% 3 5% 3 2 3 2 3 3 20%	1520
	21st to end	Day	31% 21% 3 5% 3 3 3 4 3 3 20%	1520
December	1st to 10th	Day	24% 22% 5% 5% 5% 2 2 2 2 3 29%	1440
	11th to 20th	Day	32% 23% 5% 6% 5% 2 2 2 2 2 17%	1685
	21st to end	Day	31% 23% 5% 7% 6% 2 3 3 3 3 13%	2442
January	1st to 10th	Night	31% 24% 4 7% 4 2 3 2 3 3 16%	2836
	11th to 20th	Night	31% 20% 4 6% 4 2 4 4 4 5% 17%	2730
	21st to end	Night	23% 24% 4 7% 5% 3 3 3 4 6% 19%	2826
February	1st to 10th	Night	24% 22% 5% 8% 4 3 4 3 4 5% 18%	2340
	11th to 20th	Night	19% 27% 5% 7% 4 2 3 3 3 4 23%	2272
	21st to end	Night	15% 26% 6% 8% 6% 2 4 4 3 4 21%	1922
March	1st to 10th	Night	23% 23% 4 6% 5% 2 4 4 4 4 19%	2098
	11th to 20th	Night	24% 30% 4 5% 4 3 4 3 3 4 17%	1979
	21st to end	Night	25% 24% 5% 8% 6% 2 3 3 4 4 16%	2124
April	1st to 10th	Night	21% 22% 4 8% 6% 4 5% 4 5% 3 18%	1800
	11th to 20th	Night	21% 21% 5% 6% 5% 3 4 4 4 5% 22%	1669
	21st to end	Night	24% 17% 5% 7% 5% 3 4 3 4 7% 22%	1558
May	1st to 10th	Night	18% 13% 2 6% 6% 2 5% 5% 5% 6% 32%	1440
	11th to 20th	Night	19% 13% 3 6% 6% 3 5% 5% 6% 8% 26%	1416
	21st to end	Night	23% 10% 3 6% 5% 3 5% 5% 5% 6% 30%	1447
June	1st to 10th	Night	26% 10% 4 7% 6% 3 5% 5% 5% 5% 25%	1260
	11th to 20th	Night	26% 13% 3 8% 6% 4 4 4 5% 6% 21%	1260
	21st to end	Night	21% 8% 3 5% 5% 3 5% 5% 5% 6% 34%	1260
July	1st to 10th	Night	16% 5% 2 4 4 3 4 4 4 6% 47%	1260
	11th to 20th	Night	11% 6% 3 4 4 2 3 4 5% 7% 50%	1260
	21st to end	Night	9% 5% 2 3 3 2 3 3 5% 6% 59%	1479
August	1st to 10th	Night	12% 6% 2 3 3 2 3 3 4 5% 58%	1434
	11th to 20th	Night	8% 5% 1 3 4 3 4 4 4 6% 58%	1602
	21st to end	Night	14% 7% 3 5% 5% 4 4 4 7% 9% 39%	1822
September	1st to 10th	Night	14% 7% 2 5% 3 2 3 5% 5% 8% 46%	1800
	11th to 20th	Night	24% 8% 2 4 4 2 4 3 4 5% 40%	1804
	21st to end	Night	25% 11% 3 6% 5% 2 3 3 4 6% 34%	2060
October	1st to 10th	Night	24% 11% 3 5% 5% 3 4 4 3 4 34%	2160
	11th to 20th	Night	33% 12% 3 5% 4 2 3 3 4 4 26%	2160
	21st to end	Night	33% 15% 4 6% 4 2 3 3 3 4 22%	2549
November	1st to 10th	Night	38% 18% 4 5% 4 2 3 3 3 4 14%	2590
	11th to 20th	Night	34% 22% 3 5% 4 2 3 3 3 4 18%	2660
	21st to end	Night	30% 21% 4 5% 4 3 3 3 3 3 21%	2660
December	1st to 10th	Night	25% 18% 5% 5% 5% 2 4 3 4 4 25%	2736
	11th to 20th	Night	33% 24% 6% 6% 4 2 3 3 2 3 13%	3520
	21st to end	Night	34% 25% 5% 6% 4 2 2 2 2 3 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	76% 3 8% 7% 4 2	1329
	11th to 20th	Day	70% 5% 9% 11% 3 3	1269
	21st to end	Day	71% 3 7% 13% 5% 2	1526
February	1st to 10th	Day	69% 4 9% 8% 7% 2	1620
	11th to 20th	Day	66% 4 8% 13% 7% 1	1584
	21st to end	Day	70% 3 9% 11% 4 2	1333
March	1st to 10th	Day	70% 5% 9% 11% 5% 1	1859
	11th to 20th	Day	77% 3 6% 9% 4 1	1980
	21st to end	Day	76% 4 9% 9% 2	2232
April	1st to 10th	Day	75% 6% 10% 8% 1	2160
	11th to 20th	Day	73% 3 8% 8% 7%	2241
	21st to end	Day	75% 4 7% 9% 5%	2402
May	1st to 10th	Day	65% 5% 13% 12% 5% 1	2520
	11th to 20th	Day	61% 7% 12% 14% 5%	2480
	21st to end	Day	63% 7% 10% 14% 5%	2826
June	1st to 10th	Day	68% 6% 11% 9% 4 3	2700
	11th to 20th	Day	74% 5% 10% 8% 3	2700
	21st to end	Day	57% 4 13% 17% 7% 1	2700
July	1st to 10th	Day	46% 6% 14% 20% 12% 2	2626
	11th to 20th	Day	40% 5% 17% 26% 11% 1	2619
	21st to end	Day	32% 6% 15% 26% 17% 4	2678
August	1st to 10th	Day	35% 7% 15% 26% 15% 3	2486
	11th to 20th	Day	31% 6% 18% 24% 17% 3	2298
	21st to end	Day	44% 7% 17% 24% 7% 2	2533
September	1st to 10th	Day	42% 6% 12% 24% 11% 4	2160
	11th to 20th	Day	50% 5% 8% 17% 17% 3	2152
	21st to end	Day	53% 6% 12% 17% 10% 2	1900
October	1st to 10th	Day	54% 4 13% 16% 12%	1800
	11th to 20th	Day	63% 5% 11% 15% 6%	1800
	21st to end	Day	66% 2 10% 12% 8% 1	1803
November	1st to 10th	Day	71% 5% 8% 8% 6% 1	1488
	11th to 20th	Day	73% 3 8% 7% 7% 2	1520
	21st to end	Day	70% 4 6% 10% 8% 2	1520
December	1st to 10th	Day	64% 3 9% 12% 10% 1	1440
	11th to 20th	Day	76% 4 9% 6% 3 2	1688
	21st to end	Day	78% 4 5% 9% 3 1	2442
January	1st to 10th	Night	76% 3 6% 10% 4 2	2836
	11th to 20th	Night	69% 6% 9% 9% 4 2	2730
	21st to end	Night	67% 5% 8% 11% 7% 2	2826
February	1st to 10th	Night	69% 4 9% 10% 7% 1	2340
	11th to 20th	Night	67% 5% 8% 11% 7% 2	2274
	21st to end	Night	67% 4 8% 12% 6% 2	1922
March	1st to 10th	Night	70% 5% 8% 12% 5% 1	2098
	11th to 20th	Night	74% 4 8% 9% 5%	1980
	21st to end	Night	72% 5% 12% 9% 2	2124
April	1st to 10th	Night	70% 7% 11% 12% 1	1800
	11th to 20th	Night	65% 6% 9% 14% 6%	1669
	21st to end	Night	65% 5% 12% 12% 6%	1558
May	1st to 10th	Night	52% 9% 17% 17% 4 1	1440
	11th to 20th	Night	55% 8% 17% 16% 3	1416
	21st to end	Night	55% 7% 16% 17% 4	1447
June	1st to 10th	Night	63% 5% 17% 8% 4 3	1260
	11th to 20th	Night	65% 8% 13% 10% 4	1260
	21st to end	Night	50% 9% 15% 18% 6% 2	1260
July	1st to 10th	Night	40% 7% 18% 21% 12% 2	1225
	11th to 20th	Night	34% 7% 22% 27% 10%	1218
	21st to end	Night	25% 6% 16% 33% 16% 4	1397
August	1st to 10th	Night	29% 6% 17% 29% 17% 2	1426
	11th to 20th	Night	27% 8% 22% 23% 15% 5%	1602
	21st to end	Night	41% 7% 19% 24% 6% 2	1822
September	1st to 10th	Night	37% 7% 17% 24% 13% 3	1800
	11th to 20th	Night	47% 5% 11% 18% 15% 4	1804
	21st to end	Night	53% 5% 13% 17% 9% 2	2060
October	1st to 10th	Night	54% 6% 12% 17% 10% 1	2160
	11th to 20th	Night	62% 5% 10% 15% 8%	2160
	21st to end	Night	67% 4 8% 13% 8% 1	2549
November	1st to 10th	Night	74% 4 8% 8% 5% 1	2590
	11th to 20th	Night	72% 4 8% 8% 6% 2	2660
	21st to end	Night	69% 3 8% 9% 9% 2	2660
December	1st to 10th	Night	63% 5% 10% 12% 8% 2	2736
	11th to 20th	Night	77% 5% 7% 6% 4 2	3516
	21st to end	Night	78% 2 5% 8% 5% 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	40% 13% 22% 16% 8% 1	180
	11th to 20th	Day	42% 9% 23% 23% 4	163
	21st to end	Day	32% 9% 30% 21% 6% 1	182
February	1st to 10th	Day	39% 18% 26% 14% 3	340
	11th to 20th	Day	36% 16% 27% 19% 3	335
	21st to end	Day	28% 22% 27% 17% 5% 1	281
March	1st to 10th	Day	32% 17% 28% 18% 5%	420
	11th to 20th	Day	37% 14% 23% 17% 8%	420
	21st to end	Day	39% 14% 26% 14% 7% 1	462
April	1st to 10th	Day	40% 18% 20% 17% 5%	420
	11th to 20th	Day	38% 17% 21% 17% 7% 1	412
	21st to end	Day	36% 19% 18% 19% 7%	420
May	1st to 10th	Day	28% 16% 26% 24% 6% 1	423
	11th to 20th	Day	27% 8% 25% 26% 13% 2	448
	21st to end	Day	30% 11% 27% 21% 9% 2	587
June	1st to 10th	Day	36% 16% 24% 17% 6% 1	630
	11th to 20th	Day	35% 17% 24% 17% 6% 1	630
	21st to end	Day	25% 12% 29% 27% 6% 1	630
July	1st to 10th	Day	22% 11% 28% 29% 8% 2	621
	11th to 20th	Day	16% 14% 34% 27% 7% 1	618
	21st to end	Day	12% 10% 31% 32% 12% 3	582
August	1st to 10th	Day	16% 9% 26% 31% 14% 4	447
	11th to 20th	Day	11% 9% 31% 32% 14% 3	437
	21st to end	Day	19% 14% 32% 23% 11% 2	495
September	1st to 10th	Day	23% 18% 31% 22% 6% 1	450
	11th to 20th	Day	31% 10% 27% 22% 8% 1	450
	21st to end	Day	28% 14% 27% 22% 7% 1	450
October	1st to 10th	Day	34% 12% 24% 21% 9% 1	450
	11th to 20th	Day	42% 8% 23% 17% 8% 2	450
	21st to end	Day	41% 13% 24% 15% 5% 1	347
November	1st to 10th	Day	38% 11% 31% 15% 5%	180
	11th to 20th	Day	44% 10% 17% 18% 9% 2	180
	21st to end	Day	44% 11% 18% 14% 9% 3	180
December	1st to 10th	Day	41% 9% 19% 18% 12% 1	180
	11th to 20th	Day	43% 12% 22% 20% 3 1	180
	21st to end	Day	32% 10% 41% 15% 2 1	198
January	1st to 10th	Night	45% 10% 23% 17% 3 1	850
	11th to 20th	Night	41% 7% 27% 21% 4	810
	21st to end	Night	34% 10% 32% 20% 4	820
February	1st to 10th	Night	35% 9% 34% 20% 3	720
	11th to 20th	Night	31% 9% 32% 24% 4	704
	21st to end	Night	28% 13% 28% 22% 8%	592
March	1st to 10th	Night	33% 12% 25% 20% 10% 1	720
	11th to 20th	Night	40% 11% 22% 21% 7%	720
	21st to end	Night	37% 10% 23% 20% 10% 1	792
April	1st to 10th	Night	35% 7% 21% 26% 10%	720
	11th to 20th	Night	32% 7% 23% 29% 7% 1	601
	21st to end	Night	34% 6% 25% 25% 10%	478
May	1st to 10th	Night	22% 8% 23% 33% 14% 1	360
	11th to 20th	Night	28% 4 18% 32% 17% 1	354
	21st to end	Night	29% 6% 23% 28% 15%	389
June	1st to 10th	Night	31% 5% 26% 23% 14% 1	360
	11th to 20th	Night	30% 7% 24% 26% 11% 2	360
	21st to end	Night	27% 4 19% 33% 16% 1	360
July	1st to 10th	Night	17% 5% 15% 36% 26% 1	350
	11th to 20th	Night	15% 3 15% 37% 27% 2	348
	21st to end	Night	11% 3 14% 34% 35% 4	394
August	1st to 10th	Night	16% 2 12% 29% 36% 5%	360
	11th to 20th	Night	12% 2 9% 40% 33% 4	534
	21st to end	Night	21% 5% 15% 33% 24% 2	634
September	1st to 10th	Night	18% 5% 24% 33% 19%	720
	11th to 20th	Night	27% 5% 16% 25% 23% 3	720
	21st to end	Night	32% 5% 18% 30% 13% 1	720
October	1st to 10th	Night	29% 6% 22% 28% 13% 1	720
	11th to 20th	Night	39% 5% 21% 24% 11% 1	720
	21st to end	Night	42% 8% 25% 20% 4 1	792
November	1st to 10th	Night	49% 7% 24% 18% 2	740
	11th to 20th	Night	45% 9% 26% 16% 4 1	760
	21st to end	Night	40% 12% 24% 19% 5% 1	760
December	1st to 10th	Night	34% 10% 32% 18% 7%	796
	11th to 20th	Night	45% 10% 30% 14% 2	850
	21st to end	Night	40% 10% 33% 15% 1	935

Page generated at 2024-01-09 13:24:23. Version ac07

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