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 2024-11-08.

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% 15% 3 7% 4 4 4 3 4 5% 30%	105532
	Night	23% 19% 4 7% 4 3 3 3 4 5% 27%	102171
Transparency	Day	58% 5% 11% 16% 9% 2	105278
	Night	61% 5% 11% 15% 7% 2	102071
Seeing	Day	30% 13% 26% 21% 8% 1	15449
	Night	33% 8% 24% 24% 11% 1	23535

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	26% 31% 4 5% 3 2 2 2 3 3 19%	2219
	11th to 20th	Day	21% 20% 3 9% 6% 3 3 2 2 4 27%	2199
	21st to end	Day	45% 23% 4 5% 3 2 2 1 2 2 12%	2680
February	1st to 10th	Day	22% 24% 5% 8% 4 3 3 3 3 5% 19%	1710
	11th to 20th	Day	17% 26% 5% 7% 3 4 3 3 3 4 25%	1665
	21st to end	Day	12% 30% 6% 10% 7% 4 3 3 4 3 19%	1415
March	1st to 10th	Day	16% 28% 6% 9% 5% 4 4 3 4 4 18%	3249
	11th to 20th	Day	16% 24% 4 8% 4 2 3 2 3 4 30%	3490
	21st to end	Day	19% 24% 5% 10% 6% 5% 4 3 4 4 17%	3917
April	1st to 10th	Day	21% 26% 4 9% 6% 4 4 3 4 5% 14%	2280
	11th to 20th	Day	20% 25% 4 9% 5% 5% 3 2 4 4 19%	2368
	21st to end	Day	23% 21% 5% 10% 5% 5% 5% 2 3 3 17%	2536
May	1st to 10th	Day	15% 16% 4 9% 5% 5% 4 3 4 4 30%	4420
	11th to 20th	Day	15% 11% 3 8% 5% 5% 5% 5% 7% 9% 26%	4377
	21st to end	Day	28% 18% 4 9% 5% 4 4 3 4 5% 15%	4999
June	1st to 10th	Day	31% 12% 4 9% 5% 4 4 3 4 5% 20%	2850
	11th to 20th	Day	28% 15% 4 9% 6% 5% 4 3 5% 5% 15%	2850
	21st to end	Day	22% 11% 4 8% 5% 4 4 3 4 5% 30%	2850
July	1st to 10th	Day	9% 5% 2 5% 3 2 3 3 4 4 60%	4740
	11th to 20th	Day	8% 4 1 3 2 3 2 2 4 5% 66%	4740
	21st to end	Day	12% 5% 2 5% 3 4 4 4 7% 9% 46%	4955
August	1st to 10th	Day	8% 4 1 3 2 2 3 3 5% 7% 63%	4384
	11th to 20th	Day	12% 7% 2 7% 5% 4 5% 4 6% 7% 40%	4026
	21st to end	Day	17% 7% 2 6% 4 4 4 4 6% 7% 38%	4441
September	1st to 10th	Day	16% 5% 2 6% 4 4 3 3 4 6% 46%	2280
	11th to 20th	Day	23% 8% 3 6% 5% 3 3 2 3 5% 39%	2271
	21st to end	Day	27% 9% 3 6% 3 3 3 2 4 6% 33%	2005
October	1st to 10th	Day	22% 12% 3 9% 5% 5% 5% 3 5% 5% 25%	3180
	11th to 20th	Day	35% 11% 3 7% 4 4 4 3 4 4 21%	3161
	21st to end	Day	29% 16% 4 10% 6% 5% 4 3 3 4 15%	3124
November	1st to 10th	Day	35% 18% 3 6% 4 3 3 3 3 4 18%	1544
	11th to 20th	Day	33% 24% 3 6% 3 2 3 2 3 3 20%	1520
	21st to end	Day	30% 22% 3 6% 3 4 3 3 3 3 20%	1520
December	1st to 10th	Day	24% 23% 4 5% 4 2 2 1 3 3 29%	1440
	11th to 20th	Day	32% 24% 5% 7% 4 3 2 2 2 2 17%	1685
	21st to end	Day	31% 24% 5% 9% 4 3 3 2 3 3 13%	2442
January	1st to 10th	Night	26% 30% 5% 8% 4 3 2 2 3 3 15%	4726
	11th to 20th	Night	25% 17% 3 5% 3 3 4 4 5% 6% 25%	4699
	21st to end	Night	41% 25% 3 5% 3 2 2 1 3 3 11%	4972
February	1st to 10th	Night	23% 23% 5% 8% 4 4 4 3 4 5% 17%	2470
	11th to 20th	Night	18% 27% 5% 8% 4 3 3 2 3 4 24%	2402
	21st to end	Night	14% 30% 5% 8% 5% 3 4 2 4 4 21%	2038
March	1st to 10th	Night	20% 26% 5% 9% 5% 4 4 3 4 3 17%	3622
	11th to 20th	Night	15% 26% 4 6% 3 3 4 2 3 4 30%	3489
	21st to end	Night	19% 26% 5% 9% 5% 4 3 2 3 3 21%	3736
April	1st to 10th	Night	20% 24% 3 8% 6% 6% 4 4 4 3 18%	1900
	11th to 20th	Night	19% 23% 4 6% 4 4 5% 3 4 6% 23%	1762
	21st to end	Night	23% 19% 4 8% 5% 4 3 2 4 7% 21%	1644
May	1st to 10th	Night	14% 16% 3 6% 3 3 4 3 3 4 40%	2540
	11th to 20th	Night	15% 11% 3 6% 5% 4 5% 3 6% 8% 33%	2516
	21st to end	Night	28% 18% 3 7% 4 4 4 3 4 4 21%	2573
June	1st to 10th	Night	25% 11% 3 8% 5% 5% 4 4 5% 5% 25%	1330
	11th to 20th	Night	25% 13% 3 9% 5% 4 5% 3 6% 5% 21%	1330
	21st to end	Night	21% 9% 3 6% 5% 4 4 3 5% 6% 33%	1330
July	1st to 10th	Night	10% 4 1 3 3 3 3 2 4 5% 63%	2220
	11th to 20th	Night	6% 4 1 3 2 2 2 2 3 5% 70%	2220
	21st to end	Night	12% 7% 2 5% 3 4 4 3 5% 6% 49%	2595
August	1st to 10th	Night	7% 4 1 2 1 2 2 2 4 6% 69%	2545
	11th to 20th	Night	17% 8% 2 6% 4 4 4 4 5% 6% 39%	2812
	21st to end	Night	15% 9% 3 7% 4 4 4 3 6% 6% 39%	3228
September	1st to 10th	Night	13% 7% 2 6% 2 3 3 4 5% 8% 47%	1900
	11th to 20th	Night	23% 10% 2 5% 3 3 4 2 4 5% 39%	1905
	21st to end	Night	25% 11% 3 7% 4 3 3 2 4 6% 33%	2175
October	1st to 10th	Night	20% 16% 4 10% 5% 4 4 3 3 4 24%	3800
	11th to 20th	Night	30% 15% 4 7% 3 3 3 3 4 4 24%	3777
	21st to end	Night	28% 21% 5% 8% 4 3 3 3 3 4 18%	4431
November	1st to 10th	Night	36% 19% 4 7% 3 3 3 2 3 4 15%	2694
	11th to 20th	Night	32% 23% 3 5% 4 2 3 2 3 4 18%	2660
	21st to end	Night	29% 23% 4 6% 3 3 3 2 4 3 20%	2660
December	1st to 10th	Night	24% 20% 4 6% 5% 3 3 3 4 4 25%	2736
	11th to 20th	Night	33% 25% 5% 7% 4 3 3 2 2 3 13%	3520
	21st to end	Night	34% 26% 5% 7% 3 3 2 1 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	73% 4 9% 9% 4 1	2219
	11th to 20th	Day	63% 4 11% 10% 7% 5%	2220
	21st to end	Day	82% 2 4 8% 3 1	2680
February	1st to 10th	Day	69% 4 9% 9% 7% 2	1710
	11th to 20th	Day	66% 4 8% 13% 7% 2	1674
	21st to end	Day	71% 3 9% 10% 4 2	1415
March	1st to 10th	Day	70% 5% 9% 10% 6%	3249
	11th to 20th	Day	61% 3 8% 23% 5%	3490
	21st to end	Day	72% 5% 10% 8% 4 1	3917
April	1st to 10th	Day	75% 6% 10% 8% 1	2280
	11th to 20th	Day	71% 4 8% 9% 7% 1	2368
	21st to end	Day	75% 4 7% 9% 5%	2536
May	1st to 10th	Day	57% 5% 15% 17% 6%	4420
	11th to 20th	Day	50% 11% 20% 15% 4	4380
	21st to end	Day	70% 9% 10% 9% 3	5000
June	1st to 10th	Day	68% 6% 11% 9% 4 3	2850
	11th to 20th	Day	73% 5% 10% 9% 3	2850
	21st to end	Day	58% 4 12% 17% 7% 1	2850
July	1st to 10th	Day	29% 3 9% 25% 21% 12%	4666
	11th to 20th	Day	23% 3 11% 38% 22% 3	4659
	21st to end	Day	35% 4 19% 23% 16% 3	4798
August	1st to 10th	Day	23% 4 10% 39% 23% 1	4367
	11th to 20th	Day	44% 4 23% 17% 10% 2	4065
	21st to end	Day	45% 6% 18% 20% 9% 1	4440
September	1st to 10th	Day	41% 6% 13% 24% 12% 5%	2280
	11th to 20th	Day	50% 5% 9% 17% 16% 3	2271
	21st to end	Day	54% 6% 12% 16% 9% 2	2005
October	1st to 10th	Day	60% 3 12% 17% 7%	3180
	11th to 20th	Day	68% 4 9% 15% 5%	3161
	21st to end	Day	74% 2 8% 11% 5% 1	3124
November	1st to 10th	Day	72% 5% 8% 8% 6% 1	1544
	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	77% 3 5% 9% 5% 1	4726
	11th to 20th	Night	58% 6% 12% 13% 7% 4	4740
	21st to end	Night	80% 3 5% 7% 4 1	4972
February	1st to 10th	Night	70% 4 9% 9% 7% 1	2470
	11th to 20th	Night	66% 5% 8% 11% 7% 3	2404
	21st to end	Night	68% 4 8% 12% 6% 2	2038
March	1st to 10th	Night	68% 6% 7% 11% 7%	3622
	11th to 20th	Night	59% 5% 10% 21% 5%	3490
	21st to end	Night	68% 6% 12% 9% 4 1	3736
April	1st to 10th	Night	70% 7% 11% 11% 1	1900
	11th to 20th	Night	63% 6% 9% 14% 7% 1	1762
	21st to end	Night	66% 5% 12% 12% 6%	1644
May	1st to 10th	Night	48% 7% 14% 26% 5%	2540
	11th to 20th	Night	46% 10% 23% 18% 2	2516
	21st to end	Night	66% 7% 14% 10% 3	2573
June	1st to 10th	Night	63% 5% 16% 8% 5% 3	1330
	11th to 20th	Night	64% 7% 13% 11% 4	1330
	21st to end	Night	52% 9% 14% 17% 6% 2	1330
July	1st to 10th	Night	26% 4 12% 28% 24% 7%	2185
	11th to 20th	Night	19% 4 13% 38% 24% 1	2178
	21st to end	Night	35% 4 16% 27% 14% 4	2513
August	1st to 10th	Night	18% 4 12% 39% 26% 1	2537
	11th to 20th	Night	45% 5% 23% 16% 9% 3	2839
	21st to end	Night	46% 5% 19% 22% 7% 1	3228
September	1st to 10th	Night	36% 7% 16% 24% 13% 4	1900
	11th to 20th	Night	48% 5% 12% 17% 14% 4	1905
	21st to end	Night	54% 5% 13% 17% 9% 2	2175
October	1st to 10th	Night	63% 4 14% 14% 6%	3800
	11th to 20th	Night	65% 4 7% 18% 7%	3777
	21st to end	Night	69% 3 7% 15% 5% 1	4431
November	1st to 10th	Night	73% 4 8% 8% 5% 1	2694
	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% 23% 16% 8% 1	180
	11th to 20th	Day	41% 9% 22% 23% 4	180
	21st to end	Day	38% 9% 28% 19% 5%	204
February	1st to 10th	Day	37% 17% 26% 15% 4	370
	11th to 20th	Day	35% 15% 28% 19% 3 1	365
	21st to end	Day	29% 22% 27% 17% 4 1	309
March	1st to 10th	Day	32% 16% 27% 19% 6%	460
	11th to 20th	Day	35% 14% 23% 19% 8% 1	460
	21st to end	Day	37% 15% 25% 14% 7% 1	506
April	1st to 10th	Day	39% 18% 22% 16% 5% 1	460
	11th to 20th	Day	36% 17% 23% 16% 7% 1	452
	21st to end	Day	36% 20% 18% 18% 7%	460
May	1st to 10th	Day	27% 15% 25% 25% 6% 1	463
	11th to 20th	Day	28% 9% 25% 25% 12% 1	488
	21st to end	Day	31% 13% 26% 20% 8% 2	637
June	1st to 10th	Day	36% 16% 25% 17% 6% 1	680
	11th to 20th	Day	36% 16% 24% 17% 6% 1	680
	21st to end	Day	26% 12% 29% 26% 5% 1	680
July	1st to 10th	Day	21% 12% 29% 29% 7% 2	671
	11th to 20th	Day	14% 13% 35% 28% 7% 2	668
	21st to end	Day	14% 11% 31% 31% 11% 3	631
August	1st to 10th	Day	15% 9% 27% 31% 14% 5%	487
	11th to 20th	Day	12% 9% 31% 32% 13% 3	477
	21st to end	Day	20% 14% 32% 22% 11% 2	539
September	1st to 10th	Day	22% 17% 29% 23% 8% 2	490
	11th to 20th	Day	30% 11% 28% 21% 8% 1	490
	21st to end	Day	30% 14% 26% 22% 7% 1	490
October	1st to 10th	Day	34% 12% 24% 21% 8% 1	490
	11th to 20th	Day	43% 7% 22% 17% 8% 2	490
	21st to end	Day	40% 13% 24% 16% 6% 1	380
November	1st to 10th	Day	39% 10% 30% 15% 5%	194
	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% 4 1	850
	11th to 20th	Night	40% 8% 27% 20% 4	850
	21st to end	Night	37% 9% 30% 19% 4	868
February	1st to 10th	Night	35% 9% 33% 20% 3	760
	11th to 20th	Night	31% 10% 32% 24% 4	744
	21st to end	Night	29% 13% 28% 21% 9%	628
March	1st to 10th	Night	33% 11% 25% 21% 10% 1	760
	11th to 20th	Night	39% 10% 22% 22% 6%	760
	21st to end	Night	37% 10% 23% 20% 10% 1	836
April	1st to 10th	Night	35% 8% 21% 26% 10%	760
	11th to 20th	Night	31% 8% 25% 29% 7% 1	634
	21st to end	Night	34% 7% 25% 24% 10%	504
May	1st to 10th	Night	21% 8% 23% 33% 14% 1	380
	11th to 20th	Night	27% 5% 19% 32% 16% 1	374
	21st to end	Night	29% 6% 23% 27% 14%	411
June	1st to 10th	Night	31% 5% 27% 23% 13% 1	380
	11th to 20th	Night	30% 7% 24% 26% 11% 2	380
	21st to end	Night	28% 4 19% 33% 15% 1	380
July	1st to 10th	Night	16% 5% 17% 36% 25% 1	370
	11th to 20th	Night	14% 3 14% 38% 29% 2	368
	21st to end	Night	12% 3 14% 34% 34% 4	416
August	1st to 10th	Night	15% 2 11% 31% 37% 4	381
	11th to 20th	Night	13% 3 9% 38% 33% 3	564
	21st to end	Night	21% 5% 15% 34% 23% 2	670
September	1st to 10th	Night	17% 5% 24% 34% 19% 1	760
	11th to 20th	Night	28% 5% 16% 25% 23% 3	760
	21st to end	Night	33% 5% 18% 30% 13% 1	760
October	1st to 10th	Night	30% 6% 23% 28% 13% 1	760
	11th to 20th	Night	39% 5% 22% 23% 11% 1	760
	21st to end	Night	42% 8% 25% 19% 4 1	836
November	1st to 10th	Night	49% 7% 24% 18% 2	770
	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 2024-11-05 05:15:52. Version ac07

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