Skywalker Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Skywalker Observatory clear sky chart from 2005-11-06 to 2024-11-05.

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	9% 8% 3 7% 5% 4 5% 4 5% 7% 44%	105156
	Night	11% 10% 3 7% 5% 4 4 4 6% 7% 40%	102473
Transparency	Day	40% 4 11% 22% 16% 7%	104924
	Night	44% 4 11% 21% 15% 6%	102351
Seeing	Day	15% 13% 26% 28% 15% 3	14716
	Night	16% 7% 17% 36% 22% 3	23810

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	17% 21% 7% 12% 6% 6% 6% 3 5% 5% 14%	2539
	11th to 20th	Day	20% 12% 3 10% 7% 6% 6% 3 5% 6% 21%	2516
	21st to end	Day	27% 22% 7% 10% 6% 5% 4 2 3 5% 11%	2794
February	1st to 10th	Day	14% 14% 4 11% 6% 6% 5% 4 6% 7% 24%	1543
	11th to 20th	Day	12% 14% 4 8% 6% 4 5% 4 6% 7% 30%	1739
	21st to end	Day	14% 13% 4 8% 5% 4 5% 5% 6% 6% 29%	1570
March	1st to 10th	Day	11% 12% 3 8% 5% 6% 5% 4 6% 8% 31%	3138
	11th to 20th	Day	9% 8% 2 6% 4 3 3 3 5% 7% 50%	3255
	21st to end	Day	11% 11% 4 10% 6% 6% 6% 5% 7% 7% 28%	4165
April	1st to 10th	Day	12% 10% 3 10% 7% 6% 6% 5% 7% 8% 28%	2280
	11th to 20th	Day	8% 9% 3 8% 6% 6% 5% 4 6% 7% 38%	2294
	21st to end	Day	8% 9% 3 8% 6% 6% 6% 4 7% 9% 36%	2473
May	1st to 10th	Day	7% 7% 3 7% 5% 4 5% 4 7% 9% 43%	4420
	11th to 20th	Day	5% 6% 2 6% 5% 5% 5% 5% 7% 11% 43%	4377
	21st to end	Day	7% 8% 4 8% 6% 7% 6% 6% 7% 9% 33%	4813
June	1st to 10th	Day	14% 7% 3 7% 5% 5% 5% 3 6% 7% 39%	2812
	11th to 20th	Day	10% 6% 3 8% 6% 5% 6% 5% 6% 7% 38%	2850
	21st to end	Day	5% 4 2 5% 4 4 5% 3 6% 7% 55%	2826
July	1st to 10th	Day	2 2 1 2 2 3 3 2 5% 5% 74%	4420
	11th to 20th	Day	2 2 1 2 2 2 2 2 3 4 78%	4420
	21st to end	Day	2 1 1 2 2 2 2 2 4 5% 76%	4790
August	1st to 10th	Day	3 2 1 4 2 3 3 3 4 6% 69%	4401
	11th to 20th	Day	2 2 1 4 3 3 4 3 5% 8% 66%	4216
	21st to end	Day	3 4 1 5% 4 5% 5% 3 4 6% 61%	4212
September	1st to 10th	Day	3 3 1 3 3 3 4 3 4 6% 66%	2280
	11th to 20th	Day	5% 5% 2 5% 5% 4 5% 4 6% 7% 52%	2204
	21st to end	Day	7% 4 2 5% 4 3 5% 4 6% 8% 51%	2090
October	1st to 10th	Day	6% 5% 2 5% 4 3 3 3 5% 7% 56%	3286
	11th to 20th	Day	7% 4 2 6% 5% 5% 7% 5% 6% 8% 44%	3010
	21st to end	Day	13% 7% 3 7% 5% 5% 5% 4 5% 6% 40%	3101
November	1st to 10th	Day	19% 11% 4 9% 5% 4 5% 4 5% 7% 28%	1710
	11th to 20th	Day	18% 14% 4 9% 5% 5% 4 4 5% 5% 29%	1561
	21st to end	Day	16% 18% 5% 9% 7% 5% 4 4 5% 6% 20%	1428
December	1st to 10th	Day	14% 17% 6% 11% 8% 5% 5% 3 5% 6% 20%	1330
	11th to 20th	Day	24% 21% 5% 12% 5% 5% 5% 4 4 5% 10%	1682
	21st to end	Day	14% 21% 8% 11% 6% 5% 6% 4 5% 6% 14%	2611
January	1st to 10th	Night	21% 21% 7% 10% 5% 5% 4 3 5% 5% 15%	4406
	11th to 20th	Night	18% 15% 3 10% 5% 5% 4 4 6% 8% 21%	4382
	21st to end	Night	24% 23% 8% 9% 5% 5% 4 2 3 4 12%	4862
February	1st to 10th	Night	14% 16% 4 10% 5% 5% 5% 4 6% 7% 25%	2634
	11th to 20th	Night	11% 12% 5% 8% 5% 6% 5% 4 6% 6% 32%	2327
	21st to end	Night	10% 11% 4 7% 5% 5% 4 4 8% 8% 34%	1884
March	1st to 10th	Night	7% 10% 3 7% 5% 5% 6% 4 7% 9% 36%	3736
	11th to 20th	Night	9% 8% 2 5% 4 3 3 3 4 6% 52%	3720
	21st to end	Night	10% 10% 3 9% 7% 7% 6% 6% 8% 8% 26%	3488
April	1st to 10th	Night	9% 9% 3 8% 6% 6% 5% 4 6% 8% 36%	1900
	11th to 20th	Night	8% 9% 3 9% 7% 5% 5% 3 5% 6% 41%	1837
	21st to end	Night	6% 8% 3 8% 6% 5% 5% 4 7% 7% 41%	1705
May	1st to 10th	Night	8% 6% 2 7% 6% 5% 5% 5% 7% 8% 41%	2540
	11th to 20th	Night	4 4 2 5% 4 4 4 4 7% 9% 52%	2516
	21st to end	Night	8% 6% 2 8% 5% 5% 5% 4 7% 10% 40%	2762
June	1st to 10th	Night	10% 6% 3 5% 5% 4 4 4 6% 9% 44%	1363
	11th to 20th	Night	7% 6% 3 6% 4 5% 5% 5% 8% 8% 43%	1330
	21st to end	Night	4 4 1 4 4 4 4 3 5% 7% 60%	1348
July	1st to 10th	Night	1 1 1 2 2 3 3 3 4 6% 74%	2540
	11th to 20th	Night	1 1 2 1 1 2 2 3 4 82%	2540
	21st to end	Night	1 1 1 1 1 2 2 2 3 5% 80%	2762
August	1st to 10th	Night	3 3 1 3 2 2 3 3 4 7% 70%	2529
	11th to 20th	Night	2 3 1 4 3 3 3 3 7% 8% 64%	2615
	21st to end	Night	3 2 1 5% 3 4 4 4 5% 6% 63%	3461
September	1st to 10th	Night	2 2 1 3 2 3 3 3 5% 8% 66%	1900
	11th to 20th	Night	6% 4 2 5% 4 4 4 4 6% 8% 53%	1976
	21st to end	Night	8% 4 1 4 4 3 3 4 6% 8% 54%	2090
October	1st to 10th	Night	6% 3 1 4 2 3 3 4 6% 9% 59%	3693
	11th to 20th	Night	9% 6% 2 6% 3 4 4 4 6% 9% 47%	3922
	21st to end	Night	13% 7% 3 5% 4 4 5% 3 6% 7% 42%	4457
November	1st to 10th	Night	18% 12% 3 7% 4 5% 5% 5% 7% 8% 27%	2469
	11th to 20th	Night	17% 12% 3 7% 6% 5% 6% 5% 6% 6% 28%	2617
	21st to end	Night	18% 13% 4 11% 6% 6% 4 4 5% 5% 24%	2730
December	1st to 10th	Night	13% 16% 4 12% 6% 6% 4 4 6% 6% 23%	2850
	11th to 20th	Night	21% 20% 8% 12% 6% 5% 3 4 4 4 12%	3510
	21st to end	Night	19% 20% 6% 11% 6% 5% 6% 4 5% 5% 12%	5072

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% 10% 4	2539
	11th to 20th	Day	65% 4 10% 15% 4 3	2540
	21st to end	Day	77% 3 10% 7% 3	2794
February	1st to 10th	Day	60% 5% 16% 11% 7% 1	1543
	11th to 20th	Day	53% 5% 15% 18% 9% 1	1748
	21st to end	Day	53% 6% 13% 16% 11% 1	1570
March	1st to 10th	Day	51% 4 10% 21% 13% 1	3138
	11th to 20th	Day	36% 3 18% 35% 8%	3255
	21st to end	Day	56% 3 17% 16% 7% 1	4165
April	1st to 10th	Day	54% 5% 13% 19% 8% 1	2280
	11th to 20th	Day	45% 5% 14% 20% 13% 3	2294
	21st to end	Day	46% 4 11% 24% 11% 3	2473
May	1st to 10th	Day	39% 3 6% 31% 20% 1	4420
	11th to 20th	Day	33% 4 9% 33% 19% 2	4380
	21st to end	Day	47% 2 15% 25% 10% 1	4814
June	1st to 10th	Day	45% 3 9% 22% 17% 4	2812
	11th to 20th	Day	43% 5% 10% 23% 14% 4	2850
	21st to end	Day	28% 3 11% 28% 24% 6%	2826
July	1st to 10th	Day	14% 2 6% 17% 27% 34%	4351
	11th to 20th	Day	12% 2 4 12% 24% 45%	4345
	21st to end	Day	12% 2 3 22% 35% 26%	4639
August	1st to 10th	Day	17% 3 6% 23% 36% 15%	4384
	11th to 20th	Day	17% 4 9% 39% 26% 5%	4258
	21st to end	Day	25% 3 9% 32% 26% 5%	4211
September	1st to 10th	Day	21% 2 10% 30% 28% 9%	2280
	11th to 20th	Day	31% 4 12% 26% 22% 5%	2204
	21st to end	Day	32% 5% 17% 29% 12% 4	2090
October	1st to 10th	Day	28% 4 15% 36% 17%	3286
	11th to 20th	Day	37% 6% 18% 23% 16%	3010
	21st to end	Day	42% 6% 16% 22% 13% 1	3101
November	1st to 10th	Day	57% 6% 14% 16% 5% 1	1710
	11th to 20th	Day	57% 6% 15% 15% 5% 2	1561
	21st to end	Day	63% 5% 14% 14% 4 1	1428
December	1st to 10th	Day	65% 4 12% 13% 6%	1330
	11th to 20th	Day	76% 4 12% 6% 3	1684
	21st to end	Day	69% 3 13% 10% 5%	2611
January	1st to 10th	Night	72% 3 9% 12% 3	4406
	11th to 20th	Night	62% 3 13% 16% 6% 2	4420
	21st to end	Night	75% 3 11% 8% 2 1	4862
February	1st to 10th	Night	57% 6% 15% 13% 7% 1	2634
	11th to 20th	Night	52% 5% 15% 17% 10% 1	2329
	21st to end	Night	47% 5% 14% 20% 12% 1	1884
March	1st to 10th	Night	44% 3 12% 26% 15% 1	3736
	11th to 20th	Night	35% 2 16% 33% 13% 1	3721
	21st to end	Night	54% 3 14% 21% 7% 1	3488
April	1st to 10th	Night	46% 4 13% 24% 12% 1	1900
	11th to 20th	Night	44% 3 9% 23% 18% 2	1837
	21st to end	Night	41% 3 10% 25% 18% 3	1705
May	1st to 10th	Night	41% 3 5% 26% 23% 2	2540
	11th to 20th	Night	28% 4 8% 35% 23% 3	2516
	21st to end	Night	39% 3 15% 29% 13% 1	2762
June	1st to 10th	Night	37% 4 8% 26% 19% 6%	1363
	11th to 20th	Night	37% 5% 9% 24% 19% 5%	1330
	21st to end	Night	25% 4 7% 29% 27% 8%	1348
July	1st to 10th	Night	12% 3 5% 17% 30% 33%	2500
	11th to 20th	Night	8% 2 4 12% 20% 53%	2492
	21st to end	Night	9% 2 2 18% 35% 35%	2674
August	1st to 10th	Night	15% 3 5% 22% 33% 22%	2521
	11th to 20th	Night	19% 3 6% 34% 31% 7%	2639
	21st to end	Night	22% 4 9% 26% 30% 9%	3461
September	1st to 10th	Night	18% 3 6% 28% 30% 15%	1900
	11th to 20th	Night	29% 4 9% 24% 25% 8%	1976
	21st to end	Night	29% 3 13% 33% 16% 6%	2090
October	1st to 10th	Night	23% 4 11% 41% 20% 1	3693
	11th to 20th	Night	34% 5% 14% 25% 20% 1	3922
	21st to end	Night	41% 5% 12% 29% 10% 2	4457
November	1st to 10th	Night	53% 7% 17% 15% 7% 1	2469
	11th to 20th	Night	54% 5% 16% 15% 7% 3	2617
	21st to end	Night	60% 4 14% 14% 7%	2730
December	1st to 10th	Night	60% 5% 13% 17% 4	2850
	11th to 20th	Night	74% 5% 11% 8% 3	3507
	21st to end	Night	71% 3 12% 9% 5%	5072

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	32% 8% 21% 27% 11% 2	180
	11th to 20th	Day	30% 11% 18% 24% 13% 4	180
	21st to end	Day	34% 5% 26% 23% 11% 1	198
February	1st to 10th	Day	23% 16% 30% 22% 9%	203
	11th to 20th	Day	22% 13% 29% 24% 10% 1	401
	21st to end	Day	18% 18% 30% 23% 11% 1	382
March	1st to 10th	Day	21% 19% 29% 19% 10% 2	460
	11th to 20th	Day	20% 15% 22% 25% 17% 2	460
	21st to end	Day	20% 21% 27% 21% 10% 1	506
April	1st to 10th	Day	21% 17% 30% 22% 9% 2	460
	11th to 20th	Day	15% 27% 24% 22% 9% 2	452
	21st to end	Day	15% 17% 28% 27% 10% 3	460
May	1st to 10th	Day	11% 15% 31% 27% 13% 2	463
	11th to 20th	Day	16% 14% 26% 27% 14% 3	488
	21st to end	Day	12% 14% 26% 27% 15% 5%	529
June	1st to 10th	Day	19% 12% 21% 28% 16% 3	642
	11th to 20th	Day	13% 16% 26% 28% 14% 3	680
	21st to end	Day	7% 11% 22% 33% 22% 5%	656
July	1st to 10th	Day	4 10% 33% 37% 14% 2	486
	11th to 20th	Day	5% 11% 29% 33% 16% 5%	484
	21st to end	Day	4 9% 26% 37% 19% 5%	531
August	1st to 10th	Day	6% 8% 21% 38% 21% 6%	487
	11th to 20th	Day	4 5% 23% 35% 25% 7%	477
	21st to end	Day	6% 12% 27% 28% 20% 7%	539
September	1st to 10th	Day	6% 8% 29% 29% 22% 6%	490
	11th to 20th	Day	8% 12% 27% 29% 20% 4	490
	21st to end	Day	12% 14% 29% 24% 16% 5%	490
October	1st to 10th	Day	12% 12% 28% 27% 17% 3	490
	11th to 20th	Day	17% 10% 26% 25% 18% 3	403
	21st to end	Day	22% 10% 28% 27% 10% 3	330
November	1st to 10th	Day	24% 11% 23% 29% 12%	282
	11th to 20th	Day	28% 15% 18% 21% 16% 3	199
	21st to end	Day	27% 7% 21% 28% 15% 3	180
December	1st to 10th	Day	23% 12% 24% 21% 16% 4	180
	11th to 20th	Day	28% 13% 24% 26% 7% 2	180
	21st to end	Day	29% 12% 21% 28% 10%	198
January	1st to 10th	Night	31% 8% 20% 27% 12% 1	760
	11th to 20th	Night	28% 7% 19% 29% 14% 2	760
	21st to end	Night	24% 5% 17% 37% 15% 1	836
February	1st to 10th	Night	23% 7% 23% 32% 13% 2	760
	11th to 20th	Night	18% 8% 21% 34% 18% 1	744
	21st to end	Night	17% 8% 18% 36% 18% 4	628
March	1st to 10th	Night	18% 9% 18% 33% 19% 2	760
	11th to 20th	Night	17% 9% 18% 33% 21% 2	760
	21st to end	Night	16% 8% 19% 37% 16% 3	836
April	1st to 10th	Night	14% 7% 18% 36% 22% 3	760
	11th to 20th	Night	12% 8% 24% 36% 17% 3	709
	21st to end	Night	10% 10% 20% 37% 21% 2	565
May	1st to 10th	Night	9% 8% 21% 34% 27% 3	380
	11th to 20th	Night	9% 9% 14% 31% 31% 7%	374
	21st to end	Night	11% 6% 17% 34% 26% 7%	411
June	1st to 10th	Night	11% 7% 12% 42% 25% 3	380
	11th to 20th	Night	10% 7% 17% 42% 21% 4	380
	21st to end	Night	7% 5% 12% 41% 33% 3	380
July	1st to 10th	Night	3 3 10% 41% 39% 4	370
	11th to 20th	Night	2 3 14% 44% 36% 1	368
	21st to end	Night	1 3 8% 40% 43% 5%	416
August	1st to 10th	Night	3 3 11% 32% 44% 8%	378
	11th to 20th	Night	2 5% 11% 33% 40% 10%	442
	21st to end	Night	3 5% 12% 39% 35% 5%	812
September	1st to 10th	Night	4 4 13% 42% 32% 6%	760
	11th to 20th	Night	7% 5% 12% 31% 38% 7%	760
	21st to end	Night	11% 4 16% 37% 28% 4	760
October	1st to 10th	Night	13% 5% 13% 39% 29% 2	760
	11th to 20th	Night	14% 7% 16% 33% 27% 3	760
	21st to end	Night	19% 7% 15% 40% 16% 3	836
November	1st to 10th	Night	26% 9% 12% 41% 12%	760
	11th to 20th	Night	23% 9% 19% 33% 15% 2	760
	21st to end	Night	25% 7% 14% 33% 19% 2	830
December	1st to 10th	Night	22% 7% 22% 35% 14% 1	950
	11th to 20th	Night	29% 7% 18% 34% 11% 1	950
	21st to end	Night	29% 9% 16% 37% 8% 1	955

Page generated at 2024-11-02 23:11:11. Version ac07

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