Cathead Bay
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cathead Bay clear sky chart from 2005-11-06 to 2025-05-11.

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% 12% 5% 11% 5% 4 4 3 5% 6% 32%	113145
	Night	15% 15% 6% 13% 6% 4 4 3 4 5% 25%	115955
Transparency	Day	52% 7% 12% 19% 10% 1	116673
	Night	61% 6% 10% 15% 7% 1	120590
Seeing	Day	17% 8% 22% 30% 19% 4	16318
	Night	22% 9% 25% 29% 14% 2	26231

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	11% 23% 15% 30% 6% 4 2 1 2 2 4	3617
	11th to 20th	Day	12% 17% 9% 30% 16% 7% 2 2 2 1 2	3589
	21st to end	Day	12% 20% 9% 24% 7% 6% 6% 2 3 3 9%	3982
February	1st to 10th	Day	10% 19% 7% 18% 15% 6% 3 2 5% 4 11%	1690
	11th to 20th	Day	13% 16% 6% 16% 11% 6% 5% 3 4 5% 16%	1943
	21st to end	Day	12% 16% 7% 13% 8% 5% 4 4 4 6% 21%	1650
March	1st to 10th	Day	13% 9% 3 12% 7% 4 4 4 4 6% 33%	4478
	11th to 20th	Day	11% 11% 4 12% 6% 6% 5% 4 6% 6% 28%	4834
	21st to end	Day	21% 10% 6% 14% 5% 5% 4 3 4 4 24%	5689
April	1st to 10th	Day	17% 12% 3 9% 6% 4 5% 3 5% 6% 30%	2400
	11th to 20th	Day	18% 11% 3 6% 4 3 4 3 5% 6% 37%	2477
	21st to end	Day	14% 8% 3 8% 5% 4 5% 4 5% 7% 35%	2600
May	1st to 10th	Day	13% 8% 3 6% 4 4 4 3 5% 6% 45%	4238
	11th to 20th	Day	9% 6% 3 7% 5% 4 4 3 5% 7% 47%	4266
	21st to end	Day	11% 6% 2 5% 4 4 5% 4 6% 9% 44%	4837
June	1st to 10th	Day	13% 5% 2 5% 4 4 4 4 7% 9% 44%	2760
	11th to 20th	Day	11% 5% 2 6% 4 4 4 4 7% 9% 44%	2850
	21st to end	Day	13% 7% 2 6% 4 4 4 4 5% 8% 43%	2747
July	1st to 10th	Day	16% 8% 2 5% 3 3 4 4 5% 8% 42%	4440
	11th to 20th	Day	7% 4 1 4 3 3 4 3 6% 9% 56%	4440
	21st to end	Day	8% 5% 2 5% 3 4 4 3 6% 9% 52%	4814
August	1st to 10th	Day	7% 5% 2 6% 5% 5% 5% 4 7% 9% 45%	4275
	11th to 20th	Day	13% 5% 2 5% 3 4 3 3 5% 7% 50%	4044
	21st to end	Day	13% 6% 3 6% 4 4 4 3 4 6% 47%	4344
September	1st to 10th	Day	14% 7% 2 6% 3 4 4 4 5% 7% 46%	2261
	11th to 20th	Day	14% 5% 2 6% 4 4 4 3 5% 8% 46%	2090
	21st to end	Day	18% 7% 2 5% 4 3 5% 3 6% 8% 37%	2090
October	1st to 10th	Day	19% 9% 2 8% 5% 4 4 3 5% 7% 35%	2090
	11th to 20th	Day	24% 16% 4 10% 5% 5% 4 3 5% 5% 20%	1787
	21st to end	Day	26% 18% 4 10% 5% 5% 4 3 5% 4 17%	1881
November	1st to 10th	Day	18% 21% 4 7% 5% 4 3 2 4 6% 25%	1762
	11th to 20th	Day	15% 25% 8% 14% 5% 4 4 3 4 4 15%	1800
	21st to end	Day	21% 25% 8% 14% 5% 3 3 2 3 3 12%	1635
December	1st to 10th	Day	17% 32% 13% 17% 5% 4 2 1 1 1 5%	2280
	11th to 20th	Day	15% 29% 9% 23% 7% 4 3 2 2 2 5%	2621
	21st to end	Day	21% 23% 10% 15% 8% 5% 3 2 3 3 6%	3844
January	1st to 10th	Night	12% 22% 11% 33% 7% 3 2 1 1 2 6%	6348
	11th to 20th	Night	12% 20% 10% 27% 17% 5% 2 1 1 1 4	6316
	21st to end	Night	12% 20% 10% 21% 9% 8% 4 2 2 3 9%	6996
February	1st to 10th	Night	12% 19% 6% 19% 13% 6% 4 2 3 4 12%	2709
	11th to 20th	Night	12% 16% 6% 17% 10% 5% 4 3 4 4 18%	2340
	21st to end	Night	16% 17% 6% 13% 7% 5% 4 3 4 5% 21%	1980
March	1st to 10th	Night	17% 11% 3 11% 5% 3 3 2 4 6% 35%	5362
	11th to 20th	Night	10% 14% 5% 9% 5% 4 4 4 6% 8% 31%	5124
	21st to end	Night	20% 11% 5% 12% 5% 4 4 3 5% 6% 26%	5284
April	1st to 10th	Night	18% 13% 3 10% 5% 4 5% 3 5% 5% 29%	2000
	11th to 20th	Night	19% 10% 3 7% 5% 5% 4 4 5% 5% 33%	1872
	21st to end	Night	16% 10% 3 9% 4 5% 5% 3 5% 6% 36%	1800
May	1st to 10th	Night	16% 7% 3 7% 4 4 4 3 5% 6% 40%	2923
	11th to 20th	Night	12% 7% 3 7% 5% 5% 6% 4 6% 7% 39%	2636
	21st to end	Night	19% 6% 2 5% 3 4 4 3 6% 7% 42%	2750
June	1st to 10th	Night	14% 6% 2 7% 5% 4 5% 3 5% 7% 42%	1416
	11th to 20th	Night	11% 6% 2 6% 5% 5% 5% 5% 6% 7% 41%	1330
	21st to end	Night	12% 8% 2 6% 4 4 4 3 5% 8% 45%	1426
July	1st to 10th	Night	14% 7% 3 6% 4 4 5% 3 6% 7% 41%	2530
	11th to 20th	Night	7% 5% 2 7% 4 4 4 4 5% 7% 53%	2530
	21st to end	Night	9% 5% 2 6% 4 4 4 4 6% 8% 48%	2750
August	1st to 10th	Night	11% 5% 3 8% 4 5% 4 4 5% 8% 42%	2657
	11th to 20th	Night	13% 5% 2 5% 3 3 4 3 6% 8% 48%	2785
	21st to end	Night	12% 6% 2 6% 4 4 4 3 5% 7% 47%	3309
September	1st to 10th	Night	13% 8% 2 6% 4 4 3 2 5% 7% 47%	1900
	11th to 20th	Night	15% 6% 2 6% 3 3 4 3 5% 6% 45%	2089
	21st to end	Night	19% 6% 3 7% 4 4 4 4 6% 8% 36%	2090
October	1st to 10th	Night	19% 10% 3 7% 5% 4 4 3 4 6% 35%	2090
	11th to 20th	Night	24% 13% 4 9% 5% 5% 4 3 4 6% 23%	2390
	21st to end	Night	27% 16% 4 10% 5% 5% 3 3 4 4 19%	2717
November	1st to 10th	Night	18% 18% 4 9% 5% 4 3 2 4 6% 26%	2536
	11th to 20th	Night	18% 25% 6% 12% 5% 4 4 2 3 4 17%	2600
	21st to end	Night	19% 28% 8% 12% 4 4 4 3 3 3 12%	2758
December	1st to 10th	Night	15% 30% 15% 16% 6% 4 3 2 2 2 7%	4910
	11th to 20th	Night	14% 26% 10% 25% 6% 3 2 2 2 2 8%	5586
	21st to end	Night	24% 21% 10% 15% 6% 4 3 2 3 3 8%	7116

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	90% 4 3 3	3618
	11th to 20th	Day	92% 4 2 2	3620
	21st to end	Day	83% 6% 8% 3	3982
February	1st to 10th	Day	77% 5% 8% 8% 1	1690
	11th to 20th	Day	71% 7% 10% 9% 4	1953
	21st to end	Day	61% 9% 10% 14% 5%	1650
March	1st to 10th	Day	51% 6% 16% 16% 10% 1	4480
	11th to 20th	Day	53% 6% 16% 18% 7% 1	4834
	21st to end	Day	62% 7% 10% 15% 6%	5689
April	1st to 10th	Day	53% 6% 11% 20% 10%	2400
	11th to 20th	Day	47% 6% 10% 21% 14% 2	2477
	21st to end	Day	46% 8% 12% 17% 15% 3	2600
May	1st to 10th	Day	37% 8% 12% 23% 19% 2	4250
	11th to 20th	Day	34% 8% 12% 28% 16% 2	4270
	21st to end	Day	33% 9% 14% 22% 19% 2	4837
June	1st to 10th	Day	35% 8% 14% 25% 15% 2	2760
	11th to 20th	Day	35% 8% 14% 24% 17% 2	2850
	21st to end	Day	38% 7% 15% 25% 13% 1	2747
July	1st to 10th	Day	40% 7% 15% 24% 13% 1	4371
	11th to 20th	Day	24% 7% 16% 39% 13% 1	4365
	21st to end	Day	29% 8% 13% 31% 19% 1	4662
August	1st to 10th	Day	33% 8% 16% 30% 12% 1	4260
	11th to 20th	Day	34% 6% 14% 32% 13% 1	4083
	21st to end	Day	37% 7% 15% 31% 10% 1	4343
September	1st to 10th	Day	37% 8% 14% 26% 14% 1	2261
	11th to 20th	Day	36% 8% 17% 28% 11%	2090
	21st to end	Day	43% 8% 15% 24% 10%	2090
October	1st to 10th	Day	34% 7% 11% 31% 17%	3480
	11th to 20th	Day	57% 6% 10% 20% 8%	2938
	21st to end	Day	62% 6% 15% 15% 2	3081
November	1st to 10th	Day	61% 6% 10% 17% 6%	1762
	11th to 20th	Day	73% 6% 9% 8% 3	1800
	21st to end	Day	78% 6% 10% 5% 1	1635
December	1st to 10th	Day	90% 3 3 4	2280
	11th to 20th	Day	89% 4 4 2 1	2621
	21st to end	Day	83% 7% 5% 3 2	3844
January	1st to 10th	Night	87% 5% 4 3	6348
	11th to 20th	Night	91% 3 3 2	6360
	21st to end	Night	82% 6% 5% 6% 1	6996
February	1st to 10th	Night	78% 5% 8% 7% 3	2709
	11th to 20th	Night	70% 6% 9% 12% 3	2341
	21st to end	Night	64% 7% 11% 13% 5%	1980
March	1st to 10th	Night	50% 5% 17% 19% 8%	5373
	11th to 20th	Night	49% 7% 18% 19% 7% 1	5125
	21st to end	Night	57% 7% 12% 14% 8%	5284
April	1st to 10th	Night	56% 7% 8% 17% 11% 1	2000
	11th to 20th	Night	50% 6% 10% 19% 13% 2	1872
	21st to end	Night	49% 7% 9% 18% 14% 3	1800
May	1st to 10th	Night	42% 8% 11% 20% 16% 3	2930
	11th to 20th	Night	40% 10% 9% 26% 14% 1	2637
	21st to end	Night	40% 7% 13% 20% 14% 6%	2751
June	1st to 10th	Night	39% 8% 11% 26% 14% 2	1416
	11th to 20th	Night	39% 9% 11% 22% 15% 3	1330
	21st to end	Night	38% 8% 11% 26% 15% 2	1426
July	1st to 10th	Night	41% 6% 14% 23% 13% 3	2490
	11th to 20th	Night	28% 8% 14% 30% 18% 2	2482
	21st to end	Night	33% 8% 14% 24% 20% 1	2662
August	1st to 10th	Night	38% 8% 16% 24% 13% 1	2648
	11th to 20th	Night	34% 6% 17% 28% 14% 1	2812
	21st to end	Night	36% 7% 15% 26% 16% 1	3309
September	1st to 10th	Night	37% 7% 13% 23% 18% 1	1900
	11th to 20th	Night	38% 7% 15% 27% 12%	2089
	21st to end	Night	46% 8% 15% 22% 9%	2090
October	1st to 10th	Night	43% 6% 11% 30% 11%	3480
	11th to 20th	Night	53% 5% 11% 19% 11%	3978
	21st to end	Night	60% 6% 12% 18% 4	4467
November	1st to 10th	Night	60% 7% 11% 16% 6% 1	2536
	11th to 20th	Night	73% 5% 8% 8% 5% 1	2600
	21st to end	Night	79% 6% 8% 6% 1	2758
December	1st to 10th	Night	87% 4 4 5% 1	4910
	11th to 20th	Night	85% 4 7% 3 1	5585
	21st to end	Night	80% 7% 7% 4 1	7116

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	20% 16% 30% 26% 7% 1	300
	11th to 20th	Day	22% 15% 33% 21% 7% 2	300
	21st to end	Day	22% 11% 31% 28% 8%	330
February	1st to 10th	Day	18% 13% 40% 21% 6% 1	300
	11th to 20th	Day	14% 18% 36% 23% 9%	300
	21st to end	Day	17% 14% 42% 23% 4	249
March	1st to 10th	Day	16% 12% 33% 30% 9%	300
	11th to 20th	Day	15% 10% 30% 32% 11% 2	432
	21st to end	Day	25% 8% 26% 29% 11% 1	550
April	1st to 10th	Day	22% 10% 27% 29% 10% 1	500
	11th to 20th	Day	23% 11% 25% 27% 11% 3	492
	21st to end	Day	17% 7% 23% 34% 17% 2	500
May	1st to 10th	Day	18% 5% 24% 35% 16% 3	503
	11th to 20th	Day	12% 3 24% 37% 20% 4	594
	21st to end	Day	12% 3 18% 36% 26% 5%	734
June	1st to 10th	Day	12% 3 23% 36% 23% 4	680
	11th to 20th	Day	14% 3 18% 37% 24% 5%	680
	21st to end	Day	16% 5% 13% 38% 24% 4	680
July	1st to 10th	Day	9% 3 14% 37% 30% 8%	671
	11th to 20th	Day	10% 4 16% 32% 30% 8%	668
	21st to end	Day	8% 3 13% 37% 32% 8%	739
August	1st to 10th	Day	10% 2 10% 36% 31% 10%	593
	11th to 20th	Day	12% 3 11% 30% 32% 11%	479
	21st to end	Day	16% 6% 13% 31% 26% 8%	539
September	1st to 10th	Day	16% 7% 17% 27% 25% 9%	490
	11th to 20th	Day	14% 5% 14% 26% 31% 9%	490
	21st to end	Day	17% 9% 16% 25% 25% 8%	490
October	1st to 10th	Day	23% 7% 26% 22% 17% 5%	490
	11th to 20th	Day	27% 13% 27% 21% 11% 1	362
	21st to end	Day	32% 11% 27% 22% 7% 1	330
November	1st to 10th	Day	23% 15% 28% 22% 11% 1	300
	11th to 20th	Day	23% 17% 34% 21% 4	300
	21st to end	Day	30% 18% 30% 17% 5%	253
December	1st to 10th	Day	35% 14% 26% 18% 7% 2	200
	11th to 20th	Day	26% 26% 28% 16% 4 1	200
	21st to end	Day	32% 20% 32% 13% 2 1	300
January	1st to 10th	Night	23% 14% 34% 22% 6% 1	1000
	11th to 20th	Night	26% 14% 31% 23% 6% 1	1000
	21st to end	Night	23% 10% 32% 27% 7% 1	1100
February	1st to 10th	Night	19% 14% 32% 27% 7% 1	909
	11th to 20th	Night	19% 14% 33% 26% 6%	783
	21st to end	Night	23% 11% 30% 29% 5% 1	660
March	1st to 10th	Night	19% 7% 32% 30% 11% 1	800
	11th to 20th	Night	15% 9% 34% 32% 10% 1	800
	21st to end	Night	20% 7% 26% 34% 12% 1	800
April	1st to 10th	Night	23% 7% 29% 32% 8% 1	600
	11th to 20th	Night	22% 8% 22% 34% 13% 1	593
	21st to end	Night	20% 6% 23% 36% 13% 2	600
May	1st to 10th	Night	17% 4 23% 37% 20% 1	600
	11th to 20th	Night	15% 5% 20% 39% 20% 1	563
	21st to end	Night	15% 2 18% 36% 26% 4	617
June	1st to 10th	Night	15% 3 17% 39% 24% 2	466
	11th to 20th	Night	11% 2 19% 40% 26% 3	380
	21st to end	Night	14% 3 19% 34% 29% 2	476
July	1st to 10th	Night	12% 2 11% 39% 32% 5%	555
	11th to 20th	Night	10% 3 13% 35% 32% 7%	552
	21st to end	Night	10% 1 14% 37% 34% 4	624
August	1st to 10th	Night	10% 2 14% 38% 33% 3	567
	11th to 20th	Night	13% 3 12% 30% 35% 8%	564
	21st to end	Night	15% 4 14% 37% 27% 2	627
September	1st to 10th	Night	15% 2 18% 33% 26% 5%	570
	11th to 20th	Night	17% 4 16% 31% 25% 7%	759
	21st to end	Night	22% 5% 18% 28% 22% 6%	760
October	1st to 10th	Night	23% 8% 22% 27% 17% 3	760
	11th to 20th	Night	31% 12% 22% 26% 8% 1	760
	21st to end	Night	35% 11% 22% 23% 7% 1	836
November	1st to 10th	Night	32% 11% 23% 26% 8% 1	780
	11th to 20th	Night	31% 14% 31% 19% 5%	800
	21st to end	Night	32% 16% 27% 21% 4	870
December	1st to 10th	Night	28% 20% 30% 18% 4	1000
	11th to 20th	Night	27% 17% 33% 18% 5% 1	1000
	21st to end	Night	33% 12% 32% 19% 3	1100

Page generated at 2025-05-08 09:33:54. Version ac07

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