Porter West Preserve
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Porter West Preserve clear sky chart from 2005-11-06 to 2026-04-17.

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	14% 9% 3 7% 5% 4 5% 4 6% 7% 37%	131129
	Night	14% 10% 3 8% 5% 4 4 3 5% 6% 37%	131221
Transparency	Day	45% 5% 11% 23% 14% 2	134735
	Night	46% 4 10% 22% 15% 2	135688
Seeing	Day	19% 14% 29% 21% 12% 4	17440
	Night	19% 13% 28% 25% 13% 2	27355

By Month

Percentage Cloud Cover SE (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	14% 13% 6% 10% 4 4 4 3 5% 7% 29%	4691
	11th to 20th	Day	10% 12% 5% 14% 7% 5% 4 3 5% 5% 30%	4710
	21st to end	Day	18% 14% 5% 11% 7% 5% 4 3 4 4 26%	5535
February	1st to 10th	Day	18% 12% 5% 8% 4 4 4 4 5% 6% 30%	2100
	11th to 20th	Day	16% 11% 5% 9% 4 4 4 3 5% 7% 32%	2048
	21st to end	Day	15% 14% 5% 9% 5% 3 4 3 4 6% 34%	1730
March	1st to 10th	Day	20% 13% 4 7% 3 3 3 2 3 4 38%	4580
	11th to 20th	Day	11% 10% 2 6% 4 3 4 3 5% 8% 41%	4854
	21st to end	Day	16% 11% 4 8% 5% 5% 4 3 6% 7% 32%	5911
April	1st to 10th	Day	16% 9% 3 8% 4 4 5% 3 5% 7% 36%	2520
	11th to 20th	Day	16% 9% 3 6% 4 4 4 3 5% 6% 38%	2446
	21st to end	Day	20% 7% 3 7% 5% 4 4 3 5% 6% 36%	2525
May	1st to 10th	Day	19% 10% 3 8% 5% 4 5% 4 6% 9% 27%	5862
	11th to 20th	Day	19% 9% 3 7% 4 4 5% 4 5% 6% 34%	5861
	21st to end	Day	13% 8% 3 8% 5% 5% 5% 5% 7% 9% 32%	6589
June	1st to 10th	Day	12% 6% 2 6% 4 4 5% 4 6% 8% 43%	2800
	11th to 20th	Day	9% 6% 2 5% 5% 5% 5% 5% 6% 8% 43%	2800
	21st to end	Day	11% 6% 2 5% 5% 5% 5% 5% 8% 8% 42%	2800
July	1st to 10th	Day	12% 5% 2 6% 4 4 4 4 7% 12% 41%	6360
	11th to 20th	Day	8% 6% 2 8% 5% 5% 6% 5% 8% 11% 37%	6360
	21st to end	Day	10% 7% 3 8% 5% 5% 7% 5% 8% 10% 32%	6749
August	1st to 10th	Day	9% 4 2 5% 3 4 4 5% 6% 10% 49%	4234
	11th to 20th	Day	10% 4 2 6% 5% 5% 6% 5% 8% 11% 38%	3883
	21st to end	Day	5% 4 2 5% 3 4 5% 3 5% 10% 54%	4312
September	1st to 10th	Day	12% 5% 2 6% 4 4 4 4 6% 8% 47%	2388
	11th to 20th	Day	9% 6% 2 6% 4 3 5% 3 7% 7% 50%	2200
	21st to end	Day	13% 5% 2 5% 4 4 4 2 6% 6% 50%	2200
October	1st to 10th	Day	11% 4 2 6% 3 3 5% 3 4 7% 52%	2200
	11th to 20th	Day	13% 6% 2 6% 4 4 4 3 5% 7% 45%	2055
	21st to end	Day	20% 9% 2 7% 4 3 4 3 4 5% 40%	1980
November	1st to 10th	Day	10% 6% 2 5% 4 3 4 4 5% 6% 52%	1852
	11th to 20th	Day	16% 10% 4 6% 4 2 3 3 4 7% 40%	1890
	21st to end	Day	19% 11% 3 8% 4 3 3 3 3 6% 37%	1890
December	1st to 10th	Day	13% 12% 4 7% 4 3 3 2 4 5% 43%	3020
	11th to 20th	Day	16% 14% 6% 9% 4 5% 5% 3 4 5% 29%	3124
	21st to end	Day	24% 19% 7% 8% 5% 4 4 3 4 4 17%	4070
January	1st to 10th	Night	14% 14% 7% 11% 5% 5% 4 3 4 6% 26%	8232
	11th to 20th	Night	11% 14% 6% 14% 7% 6% 5% 3 4 5% 25%	8280
	21st to end	Night	17% 12% 5% 8% 5% 5% 4 3 5% 5% 31%	8663
February	1st to 10th	Night	18% 12% 5% 9% 5% 4 4 4 5% 5% 27%	2520
	11th to 20th	Night	16% 12% 4 9% 6% 4 3 4 4 6% 32%	2458
	21st to end	Night	19% 12% 4 9% 4 4 4 3 5% 6% 33%	2076
March	1st to 10th	Night	21% 15% 3 5% 4 3 3 2 3 5% 36%	5493
	11th to 20th	Night	13% 7% 2 6% 4 4 4 3 4 8% 45%	5325
	21st to end	Night	16% 9% 3 8% 5% 6% 5% 3 5% 6% 33%	5289
April	1st to 10th	Night	15% 10% 2 6% 5% 3 4 3 5% 7% 40%	2100
	11th to 20th	Night	16% 9% 3 6% 3 4 3 4 6% 6% 40%	2035
	21st to end	Night	16% 7% 2 7% 4 4 5% 3 7% 8% 37%	1873
May	1st to 10th	Night	19% 11% 3 8% 4 4 5% 3 5% 7% 29%	4033
	11th to 20th	Night	16% 8% 3 6% 4 4 4 4 7% 8% 35%	4032
	21st to end	Night	14% 7% 2 7% 6% 4 5% 4 6% 8% 36%	4255
June	1st to 10th	Night	10% 6% 2 5% 4 4 4 3 6% 10% 46%	1600
	11th to 20th	Night	9% 5% 3 5% 4 4 4 4 6% 9% 48%	1600
	21st to end	Night	6% 3 2 4 3 5% 6% 5% 7% 10% 49%	1600
July	1st to 10th	Night	9% 5% 2 6% 4 4 5% 4 6% 8% 46%	3618
	11th to 20th	Night	7% 5% 2 6% 4 4 5% 4 8% 10% 44%	3617
	21st to end	Night	8% 6% 2 8% 5% 5% 5% 5% 8% 9% 38%	4115
August	1st to 10th	Night	9% 5% 2 6% 4 4 4 3 5% 8% 51%	2915
	11th to 20th	Night	7% 5% 2 6% 4 4 5% 4 7% 9% 47%	3161
	21st to end	Night	4 3 1 4 4 4 5% 4 5% 8% 58%	3586
September	1st to 10th	Night	10% 6% 2 5% 4 3 4 4 4 6% 52%	2000
	11th to 20th	Night	7% 4 2 5% 3 4 3 3 5% 7% 56%	2190
	21st to end	Night	13% 5% 2 6% 3 4 3 3 5% 6% 50%	2200
October	1st to 10th	Night	8% 4 1 5% 4 3 3 3 6% 6% 57%	2200
	11th to 20th	Night	16% 7% 2 4 3 3 4 3 4 5% 50%	2341
	21st to end	Night	19% 9% 2 6% 4 3 3 3 5% 6% 41%	2860
November	1st to 10th	Night	11% 8% 2 6% 4 3 3 3 4 7% 49%	2666
	11th to 20th	Night	14% 9% 3 6% 5% 3 4 3 5% 7% 41%	2730
	21st to end	Night	19% 13% 3 7% 4 3 4 3 4 6% 35%	2730
December	1st to 10th	Night	17% 12% 3 8% 4 3 3 3 4 4 39%	4380
	11th to 20th	Night	18% 13% 5% 10% 4 4 4 3 5% 5% 29%	5298
	21st to end	Night	27% 19% 6% 9% 5% 4 4 2 3 4 18%	7150

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	55% 3 12% 22% 8% 1	4692
	11th to 20th	Day	57% 2 10% 17% 12% 2	4710
	21st to end	Day	63% 2 8% 17% 5% 5%	5535
February	1st to 10th	Day	55% 4 9% 22% 9% 2	2100
	11th to 20th	Day	53% 4 9% 22% 11% 2	2058
	21st to end	Day	53% 4 8% 23% 10% 3	1730
March	1st to 10th	Day	53% 3 5% 19% 16% 5%	4580
	11th to 20th	Day	41% 4 9% 31% 14% 2	4854
	21st to end	Day	52% 4 13% 20% 10% 1	5913
April	1st to 10th	Day	47% 5% 13% 20% 14% 1	2520
	11th to 20th	Day	46% 4 11% 25% 12% 3	2446
	21st to end	Day	49% 4 10% 23% 11% 3	2525
May	1st to 10th	Day	52% 7% 12% 19% 10%	5897
	11th to 20th	Day	49% 5% 10% 24% 11%	5865
	21st to end	Day	45% 7% 13% 20% 14% 2	6603
June	1st to 10th	Day	38% 6% 11% 25% 17% 3	2800
	11th to 20th	Day	37% 6% 13% 23% 19% 3	2800
	21st to end	Day	36% 8% 15% 19% 18% 4	2800
July	1st to 10th	Day	36% 6% 10% 28% 19% 2	6291
	11th to 20th	Day	38% 9% 13% 25% 14% 1	6285
	21st to end	Day	44% 8% 14% 21% 11% 1	6555
August	1st to 10th	Day	30% 7% 14% 33% 16% 1	4221
	11th to 20th	Day	36% 6% 13% 28% 15% 1	3920
	21st to end	Day	26% 5% 14% 33% 17% 4	4311
September	1st to 10th	Day	35% 6% 10% 25% 20% 4	2388
	11th to 20th	Day	32% 5% 11% 28% 21% 3	2200
	21st to end	Day	35% 4 9% 25% 25% 2	2200
October	1st to 10th	Day	22% 4 7% 30% 36% 2	3590
	11th to 20th	Day	30% 3 9% 27% 29% 2	3323
	21st to end	Day	39% 3 8% 24% 25% 2	3180
November	1st to 10th	Day	33% 4 11% 30% 18% 5%	1852
	11th to 20th	Day	46% 3 9% 22% 16% 4	1890
	21st to end	Day	51% 3 9% 22% 15% 1	1890
December	1st to 10th	Day	46% 3 10% 27% 14%	3020
	11th to 20th	Day	59% 3 8% 21% 8% 1	3121
	21st to end	Day	70% 3 10% 11% 5%	4070
January	1st to 10th	Night	59% 3 10% 19% 7%	8233
	11th to 20th	Night	63% 2 9% 12% 12% 3	8280
	21st to end	Night	56% 3 10% 21% 9% 1	8663
February	1st to 10th	Night	58% 5% 11% 18% 7% 1	2520
	11th to 20th	Night	53% 4 10% 20% 12% 1	2459
	21st to end	Night	54% 4 10% 20% 11% 2	2076
March	1st to 10th	Night	53% 3 5% 23% 12% 4	5493
	11th to 20th	Night	39% 4 10% 25% 21% 1	5325
	21st to end	Night	52% 4 9% 26% 8% 1	5289
April	1st to 10th	Night	44% 4 10% 26% 14% 2	2100
	11th to 20th	Night	44% 4 11% 20% 17% 5%	2035
	21st to end	Night	44% 5% 10% 24% 13% 3	1873
May	1st to 10th	Night	53% 7% 11% 19% 8% 2	4060
	11th to 20th	Night	45% 6% 12% 25% 11% 1	4033
	21st to end	Night	44% 7% 14% 20% 12% 3	4263
June	1st to 10th	Night	35% 5% 10% 25% 22% 4	1600
	11th to 20th	Night	33% 4 11% 25% 21% 6%	1600
	21st to end	Night	27% 8% 15% 24% 20% 5%	1600
July	1st to 10th	Night	35% 6% 10% 26% 20% 2	3580
	11th to 20th	Night	32% 8% 16% 27% 15% 1	3572
	21st to end	Night	39% 8% 15% 25% 11% 2	3996
August	1st to 10th	Night	31% 5% 14% 36% 12% 2	2904
	11th to 20th	Night	32% 6% 15% 28% 18% 1	3190
	21st to end	Night	24% 7% 13% 31% 22% 4	3586
September	1st to 10th	Night	34% 5% 9% 26% 20% 6%	2000
	11th to 20th	Night	27% 4 9% 27% 29% 5%	2190
	21st to end	Night	36% 4 10% 21% 24% 5%	2200
October	1st to 10th	Night	19% 4 7% 21% 46% 3	3590
	11th to 20th	Night	26% 3 7% 26% 36% 3	3812
	21st to end	Night	36% 3 7% 23% 26% 5%	4610
November	1st to 10th	Night	36% 4 9% 25% 22% 4	2666
	11th to 20th	Night	44% 3 9% 23% 18% 3	2730
	21st to end	Night	51% 3 10% 22% 13% 1	2730
December	1st to 10th	Night	50% 2 11% 23% 13%	4380
	11th to 20th	Night	57% 5% 10% 18% 10%	5300
	21st to end	Night	72% 3 8% 12% 5%	7150

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	28% 23% 30% 15% 3	330
	11th to 20th	Day	34% 26% 31% 8% 1	330
	21st to end	Day	28% 30% 30% 10% 1 1	363
February	1st to 10th	Day	29% 26% 33% 10% 2	330
	11th to 20th	Day	32% 28% 30% 8% 2 1	330
	21st to end	Day	30% 25% 37% 7%	273
March	1st to 10th	Day	30% 25% 35% 8% 2	330
	11th to 20th	Day	29% 24% 33% 9% 3	434
	21st to end	Day	31% 22% 31% 13% 3 1	594
April	1st to 10th	Day	26% 21% 32% 14% 6% 1	540
	11th to 20th	Day	26% 23% 24% 19% 7% 1	516
	21st to end	Day	27% 19% 32% 13% 7% 1	500
May	1st to 10th	Day	24% 15% 34% 19% 6% 2	503
	11th to 20th	Day	24% 12% 31% 20% 8% 5%	527
	21st to end	Day	14% 12% 31% 22% 14% 6%	638
June	1st to 10th	Day	12% 9% 33% 26% 16% 4	730
	11th to 20th	Day	13% 6% 28% 32% 17% 5%	730
	21st to end	Day	13% 8% 32% 24% 18% 5%	730
July	1st to 10th	Day	13% 4 21% 27% 24% 12%	721
	11th to 20th	Day	7% 5% 28% 32% 20% 8%	718
	21st to end	Day	11% 4 23% 34% 21% 7%	719
August	1st to 10th	Day	10% 5% 18% 32% 25% 9%	526
	11th to 20th	Day	13% 5% 25% 29% 22% 7%	519
	21st to end	Day	8% 4 19% 34% 25% 11%	583
September	1st to 10th	Day	10% 6% 20% 34% 24% 5%	530
	11th to 20th	Day	8% 3 19% 30% 27% 12%	530
	21st to end	Day	17% 10% 22% 23% 21% 7%	530
October	1st to 10th	Day	9% 9% 29% 31% 16% 6%	530
	11th to 20th	Day	17% 19% 33% 18% 10% 3	430
	21st to end	Day	33% 14% 38% 14% 1	363
November	1st to 10th	Day	13% 23% 35% 22% 5% 1	330
	11th to 20th	Day	26% 24% 30% 14% 5% 1	330
	21st to end	Day	30% 14% 38% 14% 4	330
December	1st to 10th	Day	22% 27% 33% 14% 3 1	330
	11th to 20th	Day	28% 21% 36% 12% 3	330
	21st to end	Day	36% 20% 33% 10% 1 1	363
January	1st to 10th	Night	24% 23% 31% 18% 3	1050
	11th to 20th	Night	27% 24% 34% 13% 2	1050
	21st to end	Night	24% 22% 34% 18% 2	1083
February	1st to 10th	Night	26% 19% 35% 18% 2	840
	11th to 20th	Night	24% 25% 32% 17% 2	823
	21st to end	Night	27% 20% 35% 16% 1	692
March	1st to 10th	Night	28% 19% 31% 20% 2	840
	11th to 20th	Night	24% 19% 32% 19% 6%	840
	21st to end	Night	26% 17% 30% 22% 5%	793
April	1st to 10th	Night	21% 15% 33% 25% 5% 1	630
	11th to 20th	Night	19% 17% 30% 24% 9%	611
	21st to end	Night	21% 12% 37% 24% 7% 1	600
May	1st to 10th	Night	21% 10% 34% 26% 10%	600
	11th to 20th	Night	16% 9% 27% 29% 15% 3	591
	21st to end	Night	12% 6% 26% 34% 20% 3	650
June	1st to 10th	Night	11% 7% 25% 35% 22% 1	600
	11th to 20th	Night	9% 6% 23% 43% 19% 1	600
	21st to end	Night	7% 3 27% 37% 24% 2	600
July	1st to 10th	Night	10% 2 15% 33% 31% 8%	585
	11th to 20th	Night	7% 4 18% 34% 32% 5%	582
	21st to end	Night	9% 2 15% 39% 31% 4	657
August	1st to 10th	Night	8% 2 13% 39% 32% 6%	597
	11th to 20th	Night	7% 3 14% 38% 35% 4	594
	21st to end	Night	6% 1 17% 36% 35% 6%	660
September	1st to 10th	Night	13% 4 15% 35% 29% 4	600
	11th to 20th	Night	7% 4 19% 30% 34% 5%	790
	21st to end	Night	15% 9% 20% 30% 24% 4	800
October	1st to 10th	Night	11% 7% 24% 32% 23% 4	800
	11th to 20th	Night	18% 12% 28% 26% 14% 2	800
	21st to end	Night	23% 16% 31% 23% 7%	880
November	1st to 10th	Night	16% 13% 34% 26% 11% 1	820
	11th to 20th	Night	21% 19% 30% 22% 7%	840
	21st to end	Night	26% 19% 26% 23% 6%	840
December	1st to 10th	Night	26% 18% 33% 19% 4	840
	11th to 20th	Night	27% 17% 31% 21% 5% 1	1022
	21st to end	Night	33% 18% 31% 15% 3	1155

Page generated at 2026-04-14 02:44:29. Version ac07

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