Porter
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Porter clear sky chart from 2005-11-06 to 2026-02-01.

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	15% 9% 3 7% 5% 4 5% 4 6% 7% 35%	127562
	Night	15% 9% 3 8% 5% 4 4 4 5% 7% 36%	124475
Transparency	Day	47% 5% 10% 22% 14% 2	131247
	Night	47% 5% 10% 22% 14% 2	128859
Seeing	Day	20% 12% 26% 22% 14% 6%	16551
	Night	20% 10% 24% 30% 16% 2	26682

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	17% 19% 6% 11% 7% 4 4 3 4 5% 21%	4146
	11th to 20th	Day	15% 18% 8% 12% 6% 3 4 3 3 3 26%	4150
	21st to end	Day	20% 15% 4 10% 7% 5% 5% 3 4 4 24%	4565
February	1st to 10th	Day	22% 13% 5% 9% 5% 3 3 2 4 5% 30%	1800
	11th to 20th	Day	21% 13% 4 9% 4 4 4 3 4 6% 27%	1913
	21st to end	Day	21% 13% 3 7% 4 3 4 2 4 5% 32%	1650
March	1st to 10th	Day	19% 14% 3 7% 5% 4 4 3 4 6% 32%	4792
	11th to 20th	Day	13% 7% 3 6% 4 4 4 3 6% 7% 42%	4980
	21st to end	Day	19% 8% 3 7% 5% 4 5% 4 6% 7% 33%	5465
April	1st to 10th	Day	16% 10% 3 7% 5% 4 5% 3 5% 7% 35%	2200
	11th to 20th	Day	16% 8% 3 6% 4 4 4 3 5% 7% 39%	2517
	21st to end	Day	18% 8% 3 6% 5% 5% 4 4 6% 6% 35%	2600
May	1st to 10th	Day	19% 8% 3 8% 5% 5% 6% 5% 7% 8% 27%	5862
	11th to 20th	Day	19% 8% 3 7% 5% 5% 4 4 5% 7% 32%	5861
	21st to end	Day	16% 5% 2 7% 5% 5% 4 5% 7% 8% 35%	6433
June	1st to 10th	Day	11% 5% 3 6% 4 5% 6% 5% 7% 9% 40%	2600
	11th to 20th	Day	8% 6% 2 7% 5% 6% 7% 5% 7% 9% 39%	2600
	21st to end	Day	10% 5% 2 4 5% 5% 7% 5% 7% 10% 40%	2600
July	1st to 10th	Day	8% 6% 3 7% 4 4 5% 4 6% 8% 44%	5900
	11th to 20th	Day	10% 6% 2 8% 6% 5% 6% 5% 7% 10% 34%	5900
	21st to end	Day	12% 7% 3 8% 6% 6% 5% 5% 7% 9% 33%	6424
August	1st to 10th	Day	9% 6% 2 7% 5% 5% 6% 5% 8% 10% 37%	4234
	11th to 20th	Day	8% 6% 2 7% 5% 6% 6% 4 7% 10% 39%	4174
	21st to end	Day	6% 3 2 5% 4 4 5% 4 6% 8% 54%	4581
September	1st to 10th	Day	14% 5% 2 5% 4 4 4 4 6% 6% 47%	2200
	11th to 20th	Day	10% 4 2 6% 5% 4 4 4 5% 7% 49%	2200
	21st to end	Day	18% 7% 2 6% 5% 4 4 3 5% 5% 42%	2200
October	1st to 10th	Day	12% 5% 2 5% 3 4 3 4 6% 7% 50%	2120
	11th to 20th	Day	15% 6% 2 6% 4 4 4 3 4 6% 47%	2000
	21st to end	Day	20% 6% 2 6% 4 4 4 3 4 7% 40%	2200
November	1st to 10th	Day	12% 6% 3 4 4 3 4 3 5% 6% 50%	1867
	11th to 20th	Day	17% 12% 3 6% 5% 4 3 3 5% 6% 38%	1890
	21st to end	Day	17% 11% 4 7% 5% 3 5% 4 4 6% 34%	1890
December	1st to 10th	Day	20% 12% 4 9% 5% 5% 3 3 4 5% 30%	3020
	11th to 20th	Day	24% 13% 5% 8% 4 4 4 3 5% 6% 24%	3463
	21st to end	Day	20% 17% 4 11% 5% 5% 5% 3 5% 5% 21%	4565
January	1st to 10th	Night	17% 16% 5% 15% 6% 5% 4 3 5% 5% 19%	6019
	11th to 20th	Night	18% 14% 6% 12% 6% 4 4 3 4 5% 24%	6030
	21st to end	Night	21% 11% 4 8% 5% 4 4 3 4 5% 31%	6633
February	1st to 10th	Night	22% 13% 4 8% 5% 4 4 3 4 5% 29%	2601
	11th to 20th	Night	22% 11% 3 9% 5% 4 4 3 5% 4 29%	2368
	21st to end	Night	21% 14% 4 8% 4 3 3 3 4 5% 30%	1980
March	1st to 10th	Night	19% 11% 2 6% 4 4 3 3 4 6% 37%	5061
	11th to 20th	Night	12% 8% 2 5% 4 3 4 4 5% 8% 43%	4980
	21st to end	Night	17% 9% 3 8% 5% 5% 4 4 4 5% 34%	5467
April	1st to 10th	Night	16% 10% 3 7% 4 4 3 4 6% 6% 37%	2200
	11th to 20th	Night	14% 8% 2 6% 5% 3 4 4 6% 8% 40%	1847
	21st to end	Night	16% 7% 2 8% 4 4 5% 3 5% 7% 38%	1800
May	1st to 10th	Night	22% 9% 3 8% 5% 5% 5% 4 5% 7% 28%	4033
	11th to 20th	Night	17% 6% 2 7% 5% 4 5% 4 6% 9% 35%	4032
	21st to end	Night	15% 6% 3 7% 5% 5% 5% 4 6% 8% 36%	4421
June	1st to 10th	Night	10% 5% 2 5% 4 3 5% 5% 6% 9% 46%	1800
	11th to 20th	Night	8% 5% 2 6% 5% 4 5% 5% 7% 10% 43%	1800
	21st to end	Night	9% 5% 1 6% 3 4 5% 6% 7% 10% 45%	1800
July	1st to 10th	Night	9% 6% 2 6% 4 4 4 4 7% 8% 44%	4058
	11th to 20th	Night	10% 8% 3 7% 5% 4 5% 4 6% 10% 38%	4057
	21st to end	Night	10% 7% 3 6% 6% 5% 6% 5% 7% 8% 38%	4427
August	1st to 10th	Night	8% 7% 3 8% 4 6% 6% 5% 7% 9% 38%	2915
	11th to 20th	Night	10% 6% 3 8% 5% 4 5% 4 6% 7% 44%	2875
	21st to end	Night	5% 3 1 5% 4 4 4 4 7% 9% 53%	3280
September	1st to 10th	Night	12% 4 2 5% 3 4 4 3 5% 6% 52%	2200
	11th to 20th	Night	10% 4 2 5% 3 4 4 3 4 6% 55%	2200
	21st to end	Night	16% 7% 2 5% 3 4 5% 3 4 6% 44%	2200
October	1st to 10th	Night	11% 5% 2 6% 4 3 3 2 6% 6% 52%	2280
	11th to 20th	Night	14% 6% 2 5% 4 3 4 3 5% 7% 47%	2400
	21st to end	Night	21% 6% 2 5% 4 4 3 3 3 6% 43%	2640
November	1st to 10th	Night	12% 7% 2 5% 4 4 3 3 4 6% 50%	2646
	11th to 20th	Night	17% 8% 3 6% 5% 4 4 3 6% 7% 38%	2730
	21st to end	Night	19% 11% 3 6% 5% 4 4 2 4 6% 36%	2730
December	1st to 10th	Night	18% 11% 5% 10% 5% 4 4 3 4 5% 32%	4380
	11th to 20th	Night	18% 14% 6% 9% 5% 4 4 3 4 5% 27%	4952
	21st to end	Night	18% 16% 4 10% 6% 5% 5% 4 5% 6% 21%	6633

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	66% 4 9% 15% 6%	4147
	11th to 20th	Day	65% 3 6% 15% 11% 1	4150
	21st to end	Day	64% 4 7% 18% 6% 1	4565
February	1st to 10th	Day	59% 3 10% 18% 9% 1	1800
	11th to 20th	Day	59% 4 10% 17% 10% 1	1923
	21st to end	Day	55% 3 8% 22% 10% 2	1650
March	1st to 10th	Day	54% 4 6% 22% 10% 4	4792
	11th to 20th	Day	40% 4 7% 27% 18% 3	4980
	21st to end	Day	49% 5% 8% 23% 13% 2	5465
April	1st to 10th	Day	48% 5% 11% 21% 12% 3	2200
	11th to 20th	Day	44% 5% 9% 21% 17% 4	2517
	21st to end	Day	47% 6% 12% 20% 12% 3	2600
May	1st to 10th	Day	53% 5% 12% 18% 10% 2	5897
	11th to 20th	Day	51% 5% 10% 23% 10% 1	5865
	21st to end	Day	43% 7% 13% 20% 14% 3	6447
June	1st to 10th	Day	38% 8% 13% 23% 16% 1	2600
	11th to 20th	Day	40% 9% 13% 20% 16% 2	2600
	21st to end	Day	36% 10% 15% 22% 15% 2	2600
July	1st to 10th	Day	38% 6% 11% 27% 17% 2	5835
	11th to 20th	Day	44% 6% 13% 25% 11% 1	5831
	21st to end	Day	47% 5% 12% 19% 15% 2	6240
August	1st to 10th	Day	39% 7% 14% 29% 11%	4221
	11th to 20th	Day	38% 5% 10% 29% 16% 1	4213
	21st to end	Day	28% 5% 14% 29% 22% 3	4581
September	1st to 10th	Day	36% 6% 9% 21% 22% 6%	2200
	11th to 20th	Day	34% 5% 11% 26% 22% 2	2200
	21st to end	Day	44% 5% 9% 21% 19% 1	2200
October	1st to 10th	Day	26% 3 7% 29% 33% 2	3458
	11th to 20th	Day	30% 3 10% 30% 22% 4	3240
	21st to end	Day	33% 2 11% 30% 22% 2	3538
November	1st to 10th	Day	36% 5% 9% 29% 19% 3	1867
	11th to 20th	Day	48% 3 8% 21% 16% 3	1890
	21st to end	Day	51% 5% 9% 21% 13% 1	1890
December	1st to 10th	Day	59% 3 8% 23% 7%	3020
	11th to 20th	Day	62% 4 10% 18% 6%	3460
	21st to end	Day	65% 5% 11% 14% 5%	4565
January	1st to 10th	Night	67% 5% 8% 15% 5%	6019
	11th to 20th	Night	63% 4 8% 14% 8% 3	6030
	21st to end	Night	56% 4 9% 21% 9% 1	6633
February	1st to 10th	Night	58% 4 10% 18% 9% 1	2601
	11th to 20th	Night	58% 4 8% 17% 12% 1	2369
	21st to end	Night	57% 4 9% 17% 10% 3	1980
March	1st to 10th	Night	48% 4 6% 26% 14% 2	5061
	11th to 20th	Night	38% 5% 8% 29% 19% 1	4980
	21st to end	Night	52% 4 7% 22% 13% 2	5467
April	1st to 10th	Night	47% 6% 8% 20% 16% 3	2200
	11th to 20th	Night	42% 5% 11% 22% 15% 6%	1847
	21st to end	Night	45% 5% 10% 21% 17% 2	1800
May	1st to 10th	Night	56% 6% 12% 14% 10% 3	4060
	11th to 20th	Night	46% 7% 13% 21% 11% 1	4033
	21st to end	Night	43% 7% 15% 22% 10% 3	4429
June	1st to 10th	Night	33% 8% 11% 25% 19% 3	1800
	11th to 20th	Night	34% 9% 15% 19% 20% 3	1800
	21st to end	Night	31% 9% 16% 22% 20% 3	1800
July	1st to 10th	Night	37% 6% 10% 25% 19% 3	4015
	11th to 20th	Night	43% 5% 16% 23% 12% 1	4006
	21st to end	Night	42% 6% 12% 22% 13% 4	4297
August	1st to 10th	Night	41% 6% 14% 27% 10% 1	2904
	11th to 20th	Night	38% 6% 9% 24% 22% 1	2902
	21st to end	Night	27% 4 13% 31% 23% 3	3280
September	1st to 10th	Night	32% 6% 12% 22% 21% 7%	2200
	11th to 20th	Night	30% 5% 10% 25% 26% 3	2200
	21st to end	Night	40% 6% 10% 19% 23% 1	2200
October	1st to 10th	Night	28% 3 7% 26% 33% 4	3722
	11th to 20th	Night	29% 3 8% 27% 25% 8%	3899
	21st to end	Night	33% 3 6% 26% 27% 5%	4252
November	1st to 10th	Night	36% 4 9% 24% 22% 4	2646
	11th to 20th	Night	45% 4 9% 21% 17% 4	2730
	21st to end	Night	50% 4 9% 23% 12% 1	2730
December	1st to 10th	Night	56% 3 11% 23% 7%	4380
	11th to 20th	Night	60% 3 12% 18% 6% 1	4954
	21st to end	Night	64% 5% 10% 15% 6%	6633

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	30% 20% 34% 10% 5% 1	330
	11th to 20th	Day	33% 26% 30% 9% 2	330
	21st to end	Day	29% 25% 31% 12% 3	363
February	1st to 10th	Day	33% 21% 29% 13% 4	300
	11th to 20th	Day	44% 19% 29% 5% 1 1	300
	21st to end	Day	31% 24% 36% 8% 1	249
March	1st to 10th	Day	28% 20% 30% 16% 5%	430
	11th to 20th	Day	26% 19% 34% 15% 6% 1	500
	21st to end	Day	31% 14% 31% 20% 4	550
April	1st to 10th	Day	24% 19% 28% 20% 8% 1	500
	11th to 20th	Day	24% 15% 31% 19% 9% 2	492
	21st to end	Day	25% 14% 32% 19% 8% 2	500
May	1st to 10th	Day	23% 15% 31% 17% 9% 4	503
	11th to 20th	Day	18% 12% 26% 24% 14% 6%	527
	21st to end	Day	14% 9% 25% 24% 21% 7%	573
June	1st to 10th	Day	13% 7% 24% 27% 21% 7%	530
	11th to 20th	Day	10% 7% 23% 32% 19% 9%	530
	21st to end	Day	12% 5% 27% 25% 21% 10%	530
July	1st to 10th	Day	14% 4 16% 25% 20% 21%	525
	11th to 20th	Day	9% 4 21% 30% 24% 12%	524
	21st to end	Day	15% 2 17% 27% 24% 15%	576
August	1st to 10th	Day	12% 3 18% 29% 27% 12%	526
	11th to 20th	Day	12% 5% 15% 29% 25% 13%	519
	21st to end	Day	9% 5% 16% 34% 24% 14%	583
September	1st to 10th	Day	14% 5% 15% 26% 26% 14%	530
	11th to 20th	Day	10% 2 13% 28% 31% 16%	530
	21st to end	Day	22% 8% 18% 25% 21% 6%	530
October	1st to 10th	Day	14% 8% 22% 28% 22% 8%	530
	11th to 20th	Day	16% 10% 30% 25% 14% 5%	530
	21st to end	Day	24% 13% 30% 23% 10% 1	583
November	1st to 10th	Day	20% 13% 34% 23% 8% 2	345
	11th to 20th	Day	29% 14% 28% 21% 6% 2	330
	21st to end	Day	24% 17% 38% 16% 4 1	330
December	1st to 10th	Day	27% 20% 34% 14% 5% 1	330
	11th to 20th	Day	28% 19% 33% 15% 4 1	330
	21st to end	Day	32% 17% 33% 13% 5%	363
January	1st to 10th	Night	26% 18% 31% 20% 4	840
	11th to 20th	Night	31% 17% 29% 20% 2	840
	21st to end	Night	27% 15% 32% 23% 3	924
February	1st to 10th	Night	30% 11% 31% 27% 1	801
	11th to 20th	Night	31% 17% 31% 20% 2	783
	21st to end	Night	29% 15% 34% 21% 1	660
March	1st to 10th	Night	26% 14% 28% 28% 5%	800
	11th to 20th	Night	25% 15% 30% 22% 8% 1	800
	21st to end	Night	23% 14% 31% 25% 6%	880
April	1st to 10th	Night	22% 16% 30% 24% 8%	800
	11th to 20th	Night	18% 12% 33% 25% 12% 1	634
	21st to end	Night	19% 9% 32% 31% 9% 1	600
May	1st to 10th	Night	19% 8% 27% 34% 12% 1	600
	11th to 20th	Night	17% 7% 26% 30% 19% 1	591
	21st to end	Night	10% 5% 20% 33% 29% 4	650
June	1st to 10th	Night	9% 4 19% 42% 24% 2	600
	11th to 20th	Night	10% 3 18% 40% 27% 3	600
	21st to end	Night	11% 3 18% 35% 30% 3	600
July	1st to 10th	Night	11% 3 12% 33% 33% 8%	585
	11th to 20th	Night	8% 3 15% 33% 36% 5%	582
	21st to end	Night	11% 2 13% 33% 36% 5%	657
August	1st to 10th	Night	9% 4 8% 40% 36% 3	597
	11th to 20th	Night	10% 2 11% 36% 33% 8%	594
	21st to end	Night	10% 1 13% 34% 36% 7%	680
September	1st to 10th	Night	14% 2 12% 35% 33% 5%	800
	11th to 20th	Night	11% 3 12% 33% 35% 6%	800
	21st to end	Night	19% 4 18% 33% 22% 3	800
October	1st to 10th	Night	13% 5% 18% 34% 27% 3	800
	11th to 20th	Night	16% 8% 26% 33% 16% 2	800
	21st to end	Night	23% 10% 23% 30% 13% 1	880
November	1st to 10th	Night	15% 9% 25% 33% 16% 1	820
	11th to 20th	Night	22% 15% 26% 28% 9% 1	840
	21st to end	Night	25% 10% 25% 31% 8% 1	840
December	1st to 10th	Night	29% 13% 28% 23% 6%	840
	11th to 20th	Night	24% 13% 29% 28% 5%	840
	21st to end	Night	29% 14% 25% 29% 3	924

Page generated at 2026-01-29 01:21:43. Version ac07

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