Port Hilford
Clear Sky Chart
History

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

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	22% 11% 4 8% 5% 4 4 3 5% 6% 27%	127785
	Night	23% 12% 4 7% 4 4 4 3 5% 6% 29%	123793
Transparency	Day	55% 8% 11% 17% 8% 1	131294
	Night	56% 7% 11% 18% 8% 1	128378
Seeing	Day	27% 11% 26% 26% 9% 1	19103
	Night	28% 11% 28% 24% 8% 1	26709

By Month

Percentage Cloud Cover NE (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	13% 14% 5% 14% 6% 5% 5% 4 5% 7% 21%	3624
	11th to 20th	Day	14% 13% 6% 13% 6% 4 5% 3 4 5% 25%	3591
	21st to end	Day	14% 11% 4 7% 5% 4 4 4 6% 8% 33%	4307
February	1st to 10th	Day	22% 12% 3 8% 5% 4 5% 4 5% 6% 26%	1800
	11th to 20th	Day	18% 12% 4 9% 6% 6% 5% 4 5% 5% 28%	1753
	21st to end	Day	14% 13% 4 7% 7% 4 4 3 6% 6% 32%	1609
March	1st to 10th	Day	22% 12% 4 10% 5% 4 4 3 3 5% 27%	4626
	11th to 20th	Day	18% 13% 5% 9% 5% 4 4 3 6% 6% 27%	4980
	21st to end	Day	30% 10% 3 10% 5% 5% 4 3 4 5% 21%	5666
April	1st to 10th	Day	26% 11% 3 8% 3 4 3 3 5% 6% 27%	2400
	11th to 20th	Day	26% 10% 3 6% 4 4 3 3 5% 7% 30%	2374
	21st to end	Day	25% 11% 3 6% 4 4 4 4 5% 5% 30%	2544
May	1st to 10th	Day	29% 12% 4 8% 5% 4 4 3 4 6% 21%	6335
	11th to 20th	Day	25% 11% 3 6% 5% 5% 4 3 4 6% 28%	6318
	21st to end	Day	23% 13% 3 7% 4 5% 4 3 5% 6% 26%	6948
June	1st to 10th	Day	30% 10% 3 7% 5% 4 4 4 5% 6% 23%	2800
	11th to 20th	Day	24% 10% 3 6% 5% 5% 4 4 5% 6% 29%	2800
	21st to end	Day	34% 13% 4 6% 4 4 4 3 4 5% 20%	2800
July	1st to 10th	Day	23% 13% 4 9% 6% 5% 6% 4 5% 7% 18%	6360
	11th to 20th	Day	23% 14% 4 8% 5% 4 4 4 5% 7% 23%	6360
	21st to end	Day	18% 9% 3 7% 5% 5% 4 4 5% 8% 32%	6927
August	1st to 10th	Day	24% 11% 4 8% 4 4 5% 3 5% 8% 24%	4351
	11th to 20th	Day	20% 8% 3 7% 5% 4 4 4 7% 9% 29%	4175
	21st to end	Day	12% 7% 2 6% 4 4 5% 3 5% 8% 44%	4327
September	1st to 10th	Day	20% 7% 3 6% 5% 4 4 3 6% 7% 37%	2400
	11th to 20th	Day	20% 8% 3 6% 3 3 4 3 6% 8% 36%	2400
	21st to end	Day	21% 6% 2 6% 5% 5% 4 4 4 6% 36%	2112
October	1st to 10th	Day	17% 9% 3 6% 4 5% 4 4 6% 7% 35%	2000
	11th to 20th	Day	20% 8% 3 6% 4 5% 5% 4 6% 7% 32%	2000
	21st to end	Day	23% 8% 2 7% 5% 5% 5% 3 6% 6% 30%	2097
November	1st to 10th	Day	23% 9% 2 8% 4 4 4 3 5% 6% 30%	1805
	11th to 20th	Day	24% 11% 3 7% 5% 4 5% 4 5% 7% 26%	1680
	21st to end	Day	18% 11% 3 7% 5% 4 5% 4 7% 7% 28%	1680
December	1st to 10th	Day	18% 12% 4 9% 5% 4 4 3 5% 6% 30%	2690
	11th to 20th	Day	28% 11% 3 6% 4 4 4 3 4 4 28%	3076
	21st to end	Day	20% 10% 6% 10% 4 4 4 3 5% 6% 28%	4070
January	1st to 10th	Night	14% 15% 5% 10% 5% 5% 5% 4 5% 7% 25%	6364
	11th to 20th	Night	18% 12% 6% 13% 6% 5% 5% 4 4 5% 22%	6314
	21st to end	Night	13% 11% 5% 8% 4 4 4 3 5% 7% 35%	6654
February	1st to 10th	Night	22% 11% 5% 8% 5% 5% 4 4 5% 6% 26%	2600
	11th to 20th	Night	19% 11% 4 9% 6% 5% 4 3 5% 6% 28%	2533
	21st to end	Night	13% 13% 5% 8% 5% 5% 5% 4 5% 6% 31%	2020
March	1st to 10th	Night	24% 12% 4 9% 5% 4 5% 3 4 6% 24%	5210
	11th to 20th	Night	23% 14% 4 6% 3 3 3 2 4 6% 31%	4979
	21st to end	Night	32% 12% 5% 9% 4 3 2 2 3 4 24%	5265
April	1st to 10th	Night	28% 14% 3 6% 4 4 4 2 4 6% 27%	2000
	11th to 20th	Night	24% 10% 2 6% 4 3 4 3 6% 6% 31%	1976
	21st to end	Night	28% 9% 2 5% 4 4 4 3 5% 5% 29%	1854
May	1st to 10th	Night	32% 14% 4 7% 3 4 3 4 4 6% 19%	3625
	11th to 20th	Night	28% 13% 3 6% 3 3 3 3 4 6% 27%	3596
	21st to end	Night	21% 17% 4 7% 3 3 4 3 4 6% 26%	3948
June	1st to 10th	Night	32% 12% 3 8% 4 3 4 2 4 6% 23%	1600
	11th to 20th	Night	27% 9% 3 6% 4 3 3 3 6% 6% 31%	1600
	21st to end	Night	36% 13% 3 6% 4 4 3 2 4 5% 21%	1600
July	1st to 10th	Night	27% 15% 5% 8% 4 3 4 3 5% 7% 18%	3619
	11th to 20th	Night	28% 16% 4 6% 4 3 3 3 4 5% 24%	3620
	21st to end	Night	23% 13% 3 5% 4 3 4 3 5% 7% 29%	3948
August	1st to 10th	Night	27% 13% 3 7% 3 3 3 3 5% 5% 28%	2787
	11th to 20th	Night	20% 11% 3 7% 4 4 4 4 5% 9% 29%	2872
	21st to end	Night	17% 6% 2 5% 3 3 4 3 5% 8% 46%	3570
September	1st to 10th	Night	20% 8% 2 5% 4 4 4 3 4 7% 38%	2000
	11th to 20th	Night	21% 8% 3 5% 3 3 3 3 5% 7% 40%	2000
	21st to end	Night	21% 9% 3 6% 4 3 3 3 5% 6% 37%	2285
October	1st to 10th	Night	19% 6% 2 5% 4 3 5% 3 5% 7% 41%	2400
	11th to 20th	Night	21% 7% 3 4 4 3 4 4 5% 7% 38%	2400
	21st to end	Night	25% 9% 2 6% 4 5% 4 3 5% 6% 31%	2741
November	1st to 10th	Night	22% 10% 3 6% 5% 3 4 3 5% 7% 31%	2710
	11th to 20th	Night	23% 10% 3 7% 4 4 4 3 5% 7% 28%	2940
	21st to end	Night	21% 12% 2 7% 4 5% 5% 4 5% 7% 28%	2940
December	1st to 10th	Night	19% 10% 4 7% 5% 4 4 3 5% 6% 34%	4720
	11th to 20th	Night	27% 9% 4 7% 4 3 3 3 5% 5% 30%	5353
	21st to end	Night	25% 11% 4 8% 3 3 4 3 5% 6% 28%	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	59% 12% 10% 15% 3	3625
	11th to 20th	Day	55% 8% 14% 17% 6%	3620
	21st to end	Day	48% 8% 9% 17% 16% 3	4307
February	1st to 10th	Day	58% 7% 12% 11% 11% 1	1800
	11th to 20th	Day	57% 7% 12% 16% 7% 1	1761
	21st to end	Day	50% 6% 11% 18% 11% 3	1609
March	1st to 10th	Day	56% 5% 9% 14% 11% 4	4630
	11th to 20th	Day	51% 11% 16% 17% 4 1	4980
	21st to end	Day	62% 5% 9% 17% 6% 1	5666
April	1st to 10th	Day	54% 7% 11% 16% 11% 2	2400
	11th to 20th	Day	52% 5% 11% 19% 12% 1	2374
	21st to end	Day	55% 7% 11% 17% 9% 1	2544
May	1st to 10th	Day	65% 7% 8% 14% 5%	6347
	11th to 20th	Day	55% 8% 13% 17% 7%	6322
	21st to end	Day	54% 11% 10% 18% 6%	6948
June	1st to 10th	Day	61% 8% 10% 14% 5%	2800
	11th to 20th	Day	55% 6% 11% 18% 9% 1	2800
	21st to end	Day	70% 6% 6% 10% 7% 1	2800
July	1st to 10th	Day	67% 7% 9% 9% 7%	6290
	11th to 20th	Day	62% 8% 7% 16% 7%	6284
	21st to end	Day	49% 7% 14% 21% 8%	6730
August	1st to 10th	Day	60% 8% 10% 15% 6%	4337
	11th to 20th	Day	48% 10% 15% 22% 5%	4214
	21st to end	Day	38% 6% 14% 28% 14%	4327
September	1st to 10th	Day	46% 6% 13% 23% 11% 1	2400
	11th to 20th	Day	44% 7% 14% 24% 10% 1	2400
	21st to end	Day	46% 7% 11% 22% 13% 1	2112
October	1st to 10th	Day	47% 8% 14% 22% 9% 1	3260
	11th to 20th	Day	37% 7% 12% 29% 15%	3240
	21st to end	Day	47% 7% 15% 20% 10% 2	3367
November	1st to 10th	Day	53% 8% 13% 19% 7%	1805
	11th to 20th	Day	57% 8% 13% 15% 6% 1	1680
	21st to end	Day	55% 10% 14% 14% 6% 1	1680
December	1st to 10th	Day	56% 6% 11% 21% 7%	2690
	11th to 20th	Day	59% 6% 11% 12% 13%	3075
	21st to end	Day	59% 6% 15% 12% 8% 1	4070
January	1st to 10th	Night	60% 11% 13% 12% 3	6364
	11th to 20th	Night	61% 9% 10% 15% 4	6360
	21st to end	Night	51% 8% 9% 16% 13% 2	6654
February	1st to 10th	Night	61% 7% 10% 12% 8% 1	2600
	11th to 20th	Night	59% 7% 11% 14% 7% 1	2536
	21st to end	Night	55% 6% 10% 18% 9% 1	2020
March	1st to 10th	Night	61% 5% 8% 13% 11% 2	5219
	11th to 20th	Night	55% 8% 12% 17% 7% 1	4980
	21st to end	Night	65% 5% 6% 18% 6% 1	5265
April	1st to 10th	Night	59% 5% 10% 14% 10% 2	2000
	11th to 20th	Night	51% 7% 10% 20% 11% 2	1976
	21st to end	Night	55% 5% 10% 18% 11% 1	1854
May	1st to 10th	Night	67% 6% 7% 13% 6% 1	3630
	11th to 20th	Night	57% 7% 10% 20% 6%	3597
	21st to end	Night	58% 8% 9% 20% 4	3949
June	1st to 10th	Night	64% 6% 8% 16% 6%	1600
	11th to 20th	Night	55% 6% 8% 18% 13%	1600
	21st to end	Night	67% 7% 8% 13% 6% 1	1600
July	1st to 10th	Night	68% 5% 8% 12% 6% 1	3581
	11th to 20th	Night	64% 5% 8% 15% 8% 1	3572
	21st to end	Night	53% 7% 13% 21% 7%	3831
August	1st to 10th	Night	60% 7% 11% 18% 5%	2776
	11th to 20th	Night	51% 9% 15% 20% 4	2899
	21st to end	Night	36% 7% 16% 28% 13%	3569
September	1st to 10th	Night	45% 7% 13% 23% 12% 1	2000
	11th to 20th	Night	44% 6% 12% 24% 13% 1	2000
	21st to end	Night	48% 6% 9% 25% 12% 1	2285
October	1st to 10th	Night	46% 6% 14% 22% 11%	3920
	11th to 20th	Night	36% 6% 13% 28% 17%	3899
	21st to end	Night	47% 7% 12% 22% 11%	4427
November	1st to 10th	Night	52% 8% 13% 18% 8%	2710
	11th to 20th	Night	55% 8% 13% 14% 8% 1	2940
	21st to end	Night	58% 8% 14% 14% 6%	2940
December	1st to 10th	Night	51% 5% 12% 22% 9%	4720
	11th to 20th	Night	57% 7% 9% 16% 10% 2	5355
	21st to end	Night	60% 6% 12% 16% 7%	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	25% 20% 33% 18% 4	403
	11th to 20th	Day	26% 17% 31% 17% 8% 1	400
	21st to end	Day	22% 16% 38% 19% 5%	440
February	1st to 10th	Day	27% 17% 33% 19% 4	400
	11th to 20th	Day	24% 23% 29% 21% 3	396
	21st to end	Day	20% 15% 34% 24% 5% 1	331
March	1st to 10th	Day	22% 14% 35% 24% 4	431
	11th to 20th	Day	18% 16% 35% 25% 6%	500
	21st to end	Day	25% 16% 33% 19% 6% 1	550
April	1st to 10th	Day	30% 13% 31% 22% 3	500
	11th to 20th	Day	29% 10% 33% 20% 7% 1	492
	21st to end	Day	26% 10% 25% 30% 8% 1	500
May	1st to 10th	Day	32% 7% 29% 24% 5% 2	693
	11th to 20th	Day	26% 6% 24% 32% 11% 2	724
	21st to end	Day	30% 8% 21% 31% 8% 1	790
June	1st to 10th	Day	35% 5% 22% 27% 10%	730
	11th to 20th	Day	26% 6% 21% 35% 11% 1	730
	21st to end	Day	41% 6% 13% 29% 11% 1	730
July	1st to 10th	Day	31% 6% 16% 34% 13%	720
	11th to 20th	Day	32% 7% 15% 31% 14% 1	718
	21st to end	Day	31% 5% 16% 32% 15% 1	795
August	1st to 10th	Day	22% 3 18% 37% 15% 3	603
	11th to 20th	Day	22% 7% 20% 31% 17% 3	519
	21st to end	Day	19% 7% 22% 30% 19% 2	583
September	1st to 10th	Day	18% 8% 24% 30% 17% 2	530
	11th to 20th	Day	24% 7% 23% 31% 14% 2	530
	21st to end	Day	24% 5% 25% 29% 13% 4	445
October	1st to 10th	Day	18% 8% 26% 32% 14% 3	420
	11th to 20th	Day	21% 10% 27% 26% 14% 2	420
	21st to end	Day	26% 12% 34% 20% 8% 1	462
November	1st to 10th	Day	21% 14% 33% 24% 6% 2	425
	11th to 20th	Day	28% 21% 28% 16% 6%	430
	21st to end	Day	25% 17% 34% 19% 4 1	430
December	1st to 10th	Day	32% 15% 28% 22% 3	430
	11th to 20th	Day	30% 26% 27% 16% 2	430
	21st to end	Day	23% 20% 34% 17% 6% 1	473
January	1st to 10th	Night	27% 20% 33% 17% 3	906
	11th to 20th	Night	27% 17% 34% 18% 5%	900
	21st to end	Night	21% 18% 40% 16% 5%	915
February	1st to 10th	Night	27% 16% 35% 19% 3	800
	11th to 20th	Night	25% 20% 37% 16% 2	783
	21st to end	Night	19% 14% 41% 19% 7%	660
March	1st to 10th	Night	24% 15% 37% 20% 4	800
	11th to 20th	Night	24% 13% 29% 24% 10% 1	800
	21st to end	Night	27% 12% 29% 26% 5%	795
April	1st to 10th	Night	34% 10% 29% 22% 6%	600
	11th to 20th	Night	27% 8% 29% 27% 9% 1	594
	21st to end	Night	31% 6% 28% 28% 8% 1	600
May	1st to 10th	Night	37% 4 24% 26% 9% 2	600
	11th to 20th	Night	30% 6% 22% 27% 14% 1	592
	21st to end	Night	29% 6% 25% 31% 9% 1	649
June	1st to 10th	Night	39% 5% 19% 28% 9%	600
	11th to 20th	Night	31% 4 17% 37% 10% 1	600
	21st to end	Night	38% 6% 17% 28% 10% 1	600
July	1st to 10th	Night	31% 5% 14% 34% 15% 1	586
	11th to 20th	Night	34% 4 15% 31% 16%	582
	21st to end	Night	35% 5% 14% 31% 14%	656
August	1st to 10th	Night	27% 5% 13% 36% 17% 3	598
	11th to 20th	Night	24% 4 17% 34% 19% 2	592
	21st to end	Night	22% 3 20% 32% 21% 2	660
September	1st to 10th	Night	23% 4 24% 31% 17% 2	600
	11th to 20th	Night	23% 5% 22% 32% 17% 1	600
	21st to end	Night	25% 6% 25% 31% 10% 2	730
October	1st to 10th	Night	23% 7% 27% 30% 11% 2	800
	11th to 20th	Night	24% 10% 30% 25% 11% 1	800
	21st to end	Night	29% 13% 31% 21% 5%	936
November	1st to 10th	Night	27% 12% 33% 23% 4 1	930
	11th to 20th	Night	28% 18% 28% 19% 7%	950
	21st to end	Night	28% 16% 33% 18% 4	950
December	1st to 10th	Night	31% 16% 31% 16% 5%	950
	11th to 20th	Night	31% 20% 33% 13% 3	950
	21st to end	Night	29% 18% 32% 18% 4	1045

Page generated at 2025-12-30 02:14:08. Version ac07

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