Western Head
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Western Head clear sky chart from 2005-11-06 to 2026-04-02.

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	19% 9% 3 8% 5% 5% 5% 4 5% 7% 29%	132205
	Night	19% 10% 3 9% 5% 4 4 4 5% 6% 30%	129751
Transparency	Day	52% 7% 12% 18% 10% 1	135797
	Night	54% 6% 11% 18% 10% 1	134246
Seeing	Day	23% 10% 27% 29% 11% 1	19083
	Night	24% 10% 28% 28% 10% 1	26801

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	15% 11% 4 15% 6% 6% 5% 4 5% 6% 23%	4690
	11th to 20th	Day	16% 10% 5% 13% 8% 7% 7% 5% 8% 7% 14%	5111
	21st to end	Day	15% 9% 3 9% 7% 6% 6% 5% 8% 8% 25%	5781
February	1st to 10th	Day	18% 13% 4 9% 5% 5% 6% 5% 6% 8% 21%	1890
	11th to 20th	Day	17% 8% 3 9% 7% 6% 5% 4 6% 6% 27%	1843
	21st to end	Day	14% 9% 4 7% 6% 5% 6% 4 5% 8% 32%	1611
March	1st to 10th	Day	21% 11% 4 9% 6% 6% 5% 4 5% 5% 25%	5040
	11th to 20th	Day	16% 12% 4 7% 5% 4 4 3 6% 7% 32%	5090
	21st to end	Day	26% 10% 3 8% 5% 5% 4 3 5% 6% 24%	5587
April	1st to 10th	Day	22% 12% 3 8% 5% 4 4 3 5% 6% 28%	2365
	11th to 20th	Day	20% 9% 3 5% 4 4 4 4 6% 7% 35%	2469
	21st to end	Day	21% 7% 3 7% 5% 4 4 4 5% 7% 35%	2600
May	1st to 10th	Day	24% 10% 4 9% 6% 5% 5% 4 6% 6% 21%	5886
	11th to 20th	Day	25% 11% 4 8% 4 3 3 3 5% 6% 28%	6268
	21st to end	Day	18% 10% 3 8% 5% 4 5% 4 5% 8% 31%	6948
June	1st to 10th	Day	24% 10% 2 6% 5% 5% 4 4 4 8% 29%	2800
	11th to 20th	Day	19% 9% 3 7% 4 4 4 4 5% 6% 35%	2870
	21st to end	Day	24% 10% 4 7% 5% 5% 5% 4 4 6% 27%	3000
July	1st to 10th	Day	19% 10% 4 9% 6% 5% 5% 4 6% 8% 25%	6592
	11th to 20th	Day	12% 9% 4 10% 7% 7% 6% 4 6% 10% 25%	6360
	21st to end	Day	15% 8% 2 7% 5% 4 4 4 6% 10% 35%	6477
August	1st to 10th	Day	20% 10% 3 8% 5% 5% 6% 4 5% 8% 26%	4235
	11th to 20th	Day	15% 9% 3 8% 5% 6% 5% 5% 6% 8% 31%	4175
	21st to end	Day	8% 5% 2 5% 4 5% 4 3 5% 8% 49%	4518
September	1st to 10th	Day	14% 7% 2 5% 4 4 4 3 5% 8% 43%	2400
	11th to 20th	Day	17% 7% 2 5% 3 3 4 4 4 7% 44%	2270
	21st to end	Day	17% 7% 2 5% 5% 4 4 3 4 6% 43%	2168
October	1st to 10th	Day	18% 5% 2 5% 4 4 4 4 5% 8% 42%	2000
	11th to 20th	Day	18% 7% 3 6% 4 4 5% 4 5% 6% 38%	2000
	21st to end	Day	21% 7% 2 6% 5% 5% 5% 3 6% 7% 33%	2200
November	1st to 10th	Day	19% 9% 3 6% 5% 5% 3 4 5% 6% 34%	1765
	11th to 20th	Day	21% 9% 3 6% 5% 5% 6% 5% 6% 6% 29%	1680
	21st to end	Day	20% 9% 3 9% 6% 4 5% 4 5% 7% 28%	1680
December	1st to 10th	Day	24% 9% 2 7% 5% 5% 5% 4 6% 6% 26%	2690
	11th to 20th	Day	29% 10% 3 6% 4 4 4 4 5% 6% 25%	3076
	21st to end	Day	21% 11% 4 9% 4 4 4 3 4 6% 30%	4070
January	1st to 10th	Night	14% 13% 5% 15% 7% 6% 6% 5% 6% 7% 18%	8234
	11th to 20th	Night	20% 11% 5% 11% 6% 6% 6% 4 5% 7% 19%	7777
	21st to end	Night	13% 11% 4 12% 6% 6% 5% 4 6% 7% 24%	8400
February	1st to 10th	Night	19% 11% 3 9% 6% 4 5% 5% 6% 7% 24%	2730
	11th to 20th	Night	17% 9% 3 9% 6% 5% 5% 5% 7% 7% 26%	2663
	21st to end	Night	14% 9% 4 8% 5% 5% 5% 4 5% 7% 34%	2191
March	1st to 10th	Night	24% 11% 4 10% 7% 5% 4 3 4 5% 25%	5020
	11th to 20th	Night	18% 13% 4 9% 4 4 4 4 6% 7% 27%	5089
	21st to end	Night	30% 10% 3 7% 3 3 3 3 4 6% 28%	5587
April	1st to 10th	Night	26% 11% 3 7% 5% 3 3 3 4 5% 30%	2057
	11th to 20th	Night	25% 7% 3 5% 5% 4 4 2 5% 7% 34%	1878
	21st to end	Night	23% 9% 3 6% 4 4 4 3 5% 6% 34%	1800
May	1st to 10th	Night	26% 12% 3 9% 5% 5% 4 3 4 5% 24%	4054
	11th to 20th	Night	26% 10% 3 7% 3 4 4 3 5% 5% 30%	3641
	21st to end	Night	17% 14% 4 8% 5% 4 4 3 4 5% 32%	3948
June	1st to 10th	Night	25% 9% 3 6% 4 4 3 3 5% 7% 31%	1600
	11th to 20th	Night	21% 9% 2 6% 4 5% 4 3 4 6% 36%	1540
	21st to end	Night	27% 10% 3 8% 5% 3 4 3 5% 5% 27%	1400
July	1st to 10th	Night	19% 12% 4 11% 6% 3 4 3 5% 7% 25%	3334
	11th to 20th	Night	16% 9% 4 12% 7% 5% 5% 4 5% 6% 26%	3620
	21st to end	Night	17% 9% 3 7% 4 4 4 3 5% 8% 35%	4374
August	1st to 10th	Night	17% 9% 4 11% 6% 4 3 2 4 6% 34%	2915
	11th to 20th	Night	15% 10% 4 8% 4 4 4 4 5% 8% 34%	2872
	21st to end	Night	10% 5% 3 5% 3 4 4 3 5% 8% 51%	3378
September	1st to 10th	Night	14% 7% 2 6% 5% 4 4 4 6% 7% 41%	2000
	11th to 20th	Night	17% 7% 2 6% 3 3 3 3 5% 5% 46%	2127
	21st to end	Night	19% 8% 2 6% 5% 4 4 2 5% 6% 39%	2230
October	1st to 10th	Night	19% 6% 2 5% 3 4 3 3 4 7% 44%	2400
	11th to 20th	Night	18% 7% 3 5% 3 4 4 4 5% 7% 40%	2400
	21st to end	Night	23% 8% 1 5% 4 4 4 3 5% 7% 36%	2640
November	1st to 10th	Night	18% 8% 3 7% 4 3 4 3 6% 7% 37%	2749
	11th to 20th	Night	19% 9% 3 8% 4 5% 5% 5% 7% 7% 29%	2940
	21st to end	Night	20% 9% 3 6% 5% 5% 5% 4 6% 7% 30%	2940
December	1st to 10th	Night	16% 10% 5% 10% 5% 5% 5% 4 6% 7% 27%	4720
	11th to 20th	Night	24% 10% 3 7% 4 4 4 3 5% 6% 30%	5353
	21st to end	Night	24% 9% 3 8% 6% 4 4 3 4 5% 31%	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	62% 6% 13% 15% 5%	4691
	11th to 20th	Day	63% 9% 12% 11% 5%	5144
	21st to end	Day	53% 7% 11% 15% 12% 2	5781
February	1st to 10th	Day	60% 7% 12% 13% 7% 1	1890
	11th to 20th	Day	56% 7% 12% 17% 7% 1	1851
	21st to end	Day	48% 8% 12% 20% 10% 2	1611
March	1st to 10th	Day	58% 6% 11% 13% 9% 3	5044
	11th to 20th	Day	49% 9% 16% 17% 8% 1	5090
	21st to end	Day	60% 4 7% 17% 12% 1	5587
April	1st to 10th	Day	55% 5% 9% 15% 13% 2	2365
	11th to 20th	Day	48% 5% 13% 20% 13% 1	2469
	21st to end	Day	48% 6% 12% 19% 13% 2	2600
May	1st to 10th	Day	61% 8% 12% 13% 5%	5897
	11th to 20th	Day	55% 7% 10% 19% 8% 1	6272
	21st to end	Day	49% 11% 12% 17% 11% 1	6948
June	1st to 10th	Day	54% 7% 10% 18% 10%	2800
	11th to 20th	Day	49% 7% 11% 19% 12% 2	2870
	21st to end	Day	59% 7% 9% 13% 10% 1	3000
July	1st to 10th	Day	58% 8% 10% 12% 10% 1	6521
	11th to 20th	Day	54% 9% 11% 18% 7%	6284
	21st to end	Day	44% 9% 13% 23% 11% 1	6292
August	1st to 10th	Day	57% 8% 9% 15% 10% 1	4222
	11th to 20th	Day	47% 8% 16% 21% 7% 1	4214
	21st to end	Day	32% 7% 12% 28% 20% 1	4518
September	1st to 10th	Day	40% 6% 11% 24% 16% 2	2400
	11th to 20th	Day	41% 6% 10% 24% 17% 2	2270
	21st to end	Day	43% 7% 9% 22% 17% 1	2168
October	1st to 10th	Day	41% 6% 12% 26% 12% 1	3260
	11th to 20th	Day	36% 6% 10% 23% 24% 1	3240
	21st to end	Day	42% 6% 14% 24% 11% 4	3538
November	1st to 10th	Day	50% 6% 12% 22% 9% 1	1765
	11th to 20th	Day	54% 9% 12% 16% 8% 1	1680
	21st to end	Day	56% 7% 14% 18% 5% 1	1680
December	1st to 10th	Day	57% 6% 13% 19% 4	2690
	11th to 20th	Day	60% 6% 10% 14% 8% 1	3075
	21st to end	Day	56% 7% 13% 16% 7% 1	4070
January	1st to 10th	Night	63% 7% 12% 13% 4	8234
	11th to 20th	Night	63% 8% 10% 14% 5%	7819
	21st to end	Night	57% 8% 11% 13% 10% 1	8400
February	1st to 10th	Night	59% 9% 10% 13% 7% 1	2730
	11th to 20th	Night	55% 9% 12% 15% 9% 1	2666
	21st to end	Night	50% 8% 13% 18% 9% 1	2191
March	1st to 10th	Night	63% 4 8% 11% 13% 1	5029
	11th to 20th	Night	55% 7% 14% 16% 7% 1	5090
	21st to end	Night	58% 4 7% 18% 11% 1	5587
April	1st to 10th	Night	56% 5% 9% 14% 12% 3	2057
	11th to 20th	Night	50% 5% 10% 20% 13% 2	1878
	21st to end	Night	51% 4 10% 20% 12% 3	1800
May	1st to 10th	Night	63% 6% 8% 14% 8% 1	4060
	11th to 20th	Night	54% 7% 8% 21% 7% 3	3642
	21st to end	Night	54% 7% 10% 18% 9% 1	3949
June	1st to 10th	Night	55% 5% 10% 21% 9%	1600
	11th to 20th	Night	50% 5% 12% 18% 13% 1	1540
	21st to end	Night	61% 6% 10% 12% 12% 1	1400
July	1st to 10th	Night	60% 6% 9% 14% 9% 1	3296
	11th to 20th	Night	59% 7% 6% 15% 11% 2	3572
	21st to end	Night	47% 7% 10% 21% 14% 1	4245
August	1st to 10th	Night	54% 6% 9% 21% 10% 1	2903
	11th to 20th	Night	47% 8% 16% 21% 7%	2899
	21st to end	Night	32% 6% 13% 28% 21%	3377
September	1st to 10th	Night	41% 6% 10% 25% 15% 2	2000
	11th to 20th	Night	41% 5% 8% 26% 18% 2	2127
	21st to end	Night	47% 5% 8% 23% 15% 1	2230
October	1st to 10th	Night	44% 5% 12% 25% 12% 2	3920
	11th to 20th	Night	33% 5% 12% 27% 22%	3899
	21st to end	Night	41% 5% 9% 26% 17% 1	4252
November	1st to 10th	Night	49% 7% 13% 20% 11% 1	2749
	11th to 20th	Night	54% 8% 13% 17% 8%	2940
	21st to end	Night	55% 9% 15% 15% 6%	2940
December	1st to 10th	Night	55% 6% 11% 22% 5%	4720
	11th to 20th	Night	56% 6% 12% 15% 11% 1	5355
	21st to end	Night	57% 5% 12% 16% 9% 1	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	27% 17% 34% 18% 5%	430
	11th to 20th	Day	27% 16% 33% 18% 5%	430
	21st to end	Day	19% 16% 37% 25% 4	473
February	1st to 10th	Day	25% 13% 34% 23% 5%	430
	11th to 20th	Day	22% 20% 34% 21% 2	426
	21st to end	Day	17% 15% 36% 27% 5%	382
March	1st to 10th	Day	22% 15% 38% 21% 4	540
	11th to 20th	Day	20% 16% 33% 25% 6% 1	540
	21st to end	Day	25% 12% 33% 23% 6% 1	594
April	1st to 10th	Day	31% 13% 31% 21% 4	504
	11th to 20th	Day	22% 10% 36% 27% 5%	492
	21st to end	Day	21% 8% 29% 30% 10% 1	500
May	1st to 10th	Day	26% 8% 31% 28% 7% 1	503
	11th to 20th	Day	22% 6% 27% 32% 11% 1	700
	21st to end	Day	25% 7% 26% 32% 9% 2	790
June	1st to 10th	Day	26% 6% 22% 35% 10% 1	730
	11th to 20th	Day	22% 5% 21% 38% 13% 1	730
	21st to end	Day	30% 5% 18% 34% 12% 2	730
July	1st to 10th	Day	23% 5% 20% 36% 15% 1	720
	11th to 20th	Day	23% 5% 14% 39% 17% 2	718
	21st to end	Day	21% 4 16% 39% 18% 1	602
August	1st to 10th	Day	17% 6% 16% 37% 21% 2	526
	11th to 20th	Day	20% 6% 18% 29% 22% 4	519
	21st to end	Day	15% 4 21% 35% 20% 4	583
September	1st to 10th	Day	19% 3 19% 39% 19% 2	530
	11th to 20th	Day	21% 6% 21% 28% 19% 5%	530
	21st to end	Day	20% 6% 23% 31% 17% 3	511
October	1st to 10th	Day	19% 5% 26% 30% 18% 2	420
	11th to 20th	Day	18% 13% 27% 26% 14% 2	420
	21st to end	Day	24% 10% 29% 26% 9% 1	462
November	1st to 10th	Day	20% 13% 30% 27% 10%	425
	11th to 20th	Day	24% 19% 28% 22% 6% 1	430
	21st to end	Day	25% 14% 33% 22% 5%	430
December	1st to 10th	Day	31% 13% 31% 21% 4	430
	11th to 20th	Day	29% 23% 29% 16% 2 1	430
	21st to end	Day	23% 18% 34% 20% 4 1	473
January	1st to 10th	Night	24% 17% 35% 21% 4	950
	11th to 20th	Night	26% 15% 32% 22% 5%	865
	21st to end	Night	20% 16% 39% 20% 5%	924
February	1st to 10th	Night	27% 16% 32% 22% 3	840
	11th to 20th	Night	22% 18% 38% 20% 2	823
	21st to end	Night	18% 12% 39% 24% 6%	692
March	1st to 10th	Night	24% 13% 38% 22% 3	840
	11th to 20th	Night	23% 11% 32% 28% 7%	840
	21st to end	Night	24% 12% 33% 24% 6%	924
April	1st to 10th	Night	33% 7% 29% 24% 6%	650
	11th to 20th	Night	28% 6% 33% 27% 7% 1	594
	21st to end	Night	26% 6% 25% 31% 10% 2	600
May	1st to 10th	Night	29% 5% 25% 31% 8% 2	600
	11th to 20th	Night	27% 4 25% 32% 11% 2	592
	21st to end	Night	24% 4 22% 36% 12% 1	649
June	1st to 10th	Night	29% 5% 19% 32% 14% 1	600
	11th to 20th	Night	26% 4 19% 39% 13%	540
	21st to end	Night	30% 4 16% 34% 16% 1	400
July	1st to 10th	Night	24% 4 14% 42% 16% 1	457
	11th to 20th	Night	20% 5% 15% 43% 15% 1	582
	21st to end	Night	23% 5% 12% 41% 18% 1	656
August	1st to 10th	Night	17% 3 15% 39% 23% 3	598
	11th to 20th	Night	19% 3 18% 36% 23% 2	592
	21st to end	Night	17% 3 19% 36% 20% 4	660
September	1st to 10th	Night	18% 5% 19% 37% 19% 2	600
	11th to 20th	Night	21% 4 21% 31% 19% 4	727
	21st to end	Night	23% 6% 23% 31% 14% 4	800
October	1st to 10th	Night	23% 5% 25% 32% 13% 2	800
	11th to 20th	Night	21% 10% 29% 26% 13% 2	800
	21st to end	Night	27% 11% 28% 27% 7%	880
November	1st to 10th	Night	22% 11% 30% 28% 8% 1	881
	11th to 20th	Night	25% 17% 28% 24% 7%	950
	21st to end	Night	24% 16% 32% 20% 7% 1	950
December	1st to 10th	Night	29% 13% 32% 21% 5% 1	950
	11th to 20th	Night	26% 21% 33% 16% 3	950
	21st to end	Night	26% 16% 34% 19% 5%	1045

Page generated at 2026-03-30 03:18:46. Version ac07

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