Swain Hill
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Swain Hill 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	17% 11% 4 10% 6% 5% 4 4 5% 6% 28%	113576
	Night	17% 13% 4 11% 6% 5% 4 3 5% 5% 26%	115354
Transparency	Day	51% 8% 12% 18% 10% 1	117118
	Night	53% 9% 12% 16% 9% 1	119976
Seeing	Day	27% 20% 28% 17% 7% 2	15417
	Night	26% 16% 26% 23% 8% 1	24443

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	12% 12% 7% 16% 13% 8% 5% 4 3 4 17%	3597
	11th to 20th	Day	10% 12% 6% 15% 14% 6% 5% 3 4 5% 22%	3570
	21st to end	Day	12% 15% 8% 13% 6% 5% 4 3 4 4 26%	3960
February	1st to 10th	Day	15% 15% 5% 11% 7% 6% 4 4 5% 5% 22%	1755
	11th to 20th	Day	12% 14% 6% 13% 9% 5% 5% 4 5% 6% 23%	1948
	21st to end	Day	14% 14% 5% 12% 7% 4 4 3 6% 5% 26%	1650
March	1st to 10th	Day	19% 15% 5% 16% 5% 5% 4 2 3 4 22%	4476
	11th to 20th	Day	16% 13% 7% 9% 4 4 4 3 4 5% 30%	4701
	21st to end	Day	23% 12% 5% 11% 5% 4 3 3 4 6% 25%	5873
April	1st to 10th	Day	20% 15% 4 9% 5% 4 3 3 3 5% 29%	2400
	11th to 20th	Day	17% 15% 4 7% 5% 4 4 3 4 5% 32%	2391
	21st to end	Day	21% 13% 4 9% 5% 4 4 3 5% 5% 28%	2600
May	1st to 10th	Day	26% 14% 3 8% 5% 4 4 3 4 5% 23%	4236
	11th to 20th	Day	17% 10% 4 10% 6% 6% 5% 4 6% 7% 26%	4353
	21st to end	Day	12% 8% 3 8% 5% 5% 5% 4 7% 8% 35%	4837
June	1st to 10th	Day	21% 9% 3 8% 5% 5% 4 5% 7% 7% 25%	2660
	11th to 20th	Day	18% 9% 2 6% 6% 4 4 4 6% 8% 32%	2660
	21st to end	Day	20% 10% 3 10% 5% 4 5% 4 6% 6% 27%	2660
July	1st to 10th	Day	15% 7% 2 7% 5% 5% 5% 4 7% 10% 32%	4440
	11th to 20th	Day	13% 6% 2 8% 5% 5% 6% 5% 7% 9% 32%	4440
	21st to end	Day	20% 8% 3 7% 4 4 5% 5% 6% 7% 33%	4814
August	1st to 10th	Day	18% 8% 3 8% 5% 5% 5% 5% 6% 7% 30%	4303
	11th to 20th	Day	19% 8% 3 7% 5% 6% 5% 5% 7% 8% 26%	3998
	21st to end	Day	14% 7% 3 6% 4 4 5% 4 6% 9% 38%	4180
September	1st to 10th	Day	14% 6% 2 6% 4 4 4 4 6% 7% 42%	2280
	11th to 20th	Day	15% 6% 2 6% 3 3 3 3 5% 6% 47%	2119
	21st to end	Day	16% 9% 2 6% 4 5% 4 3 6% 6% 39%	2090
October	1st to 10th	Day	22% 11% 3 7% 5% 3 4 3 4 6% 31%	2090
	11th to 20th	Day	16% 12% 4 7% 5% 3 4 3 5% 6% 35%	1800
	21st to end	Day	25% 16% 4 7% 3 4 4 3 4 4 27%	1881
November	1st to 10th	Day	15% 13% 4 8% 4 4 4 3 5% 7% 34%	1761
	11th to 20th	Day	17% 14% 5% 9% 5% 5% 4 4 5% 5% 28%	1800
	21st to end	Day	20% 14% 4 11% 6% 5% 4 4 4 5% 24%	1707
December	1st to 10th	Day	15% 20% 6% 19% 8% 6% 3 2 3 4 14%	2600
	11th to 20th	Day	19% 17% 9% 9% 5% 4 3 3 3 4 25%	2986
	21st to end	Day	20% 15% 5% 11% 5% 4 4 3 4 4 24%	3960
January	1st to 10th	Night	10% 13% 7% 20% 11% 6% 5% 3 4 5% 16%	6349
	11th to 20th	Night	10% 17% 7% 17% 12% 6% 5% 3 5% 4 14%	6315
	21st to end	Night	13% 13% 6% 14% 8% 7% 5% 3 4 4 23%	6996
February	1st to 10th	Night	15% 14% 6% 12% 8% 6% 5% 3 5% 5% 22%	2640
	11th to 20th	Night	11% 13% 6% 13% 8% 6% 5% 4 5% 5% 22%	2338
	21st to end	Night	11% 15% 4 12% 8% 5% 4 4 4 6% 26%	1980
March	1st to 10th	Night	20% 15% 5% 12% 7% 5% 4 3 4 5% 20%	5364
	11th to 20th	Night	15% 16% 5% 10% 5% 4 4 3 4 5% 27%	5257
	21st to end	Night	22% 16% 4 10% 4 3 4 3 4 5% 25%	5056
April	1st to 10th	Night	22% 14% 4 8% 5% 4 3 3 5% 5% 28%	2000
	11th to 20th	Night	19% 13% 4 9% 5% 4 3 4 5% 5% 30%	1956
	21st to end	Night	19% 13% 4 8% 5% 4 3 3 4 6% 32%	1800
May	1st to 10th	Night	27% 12% 3 8% 4 5% 4 3 6% 5% 23%	2924
	11th to 20th	Night	19% 11% 3 8% 6% 5% 6% 4 6% 6% 26%	2576
	21st to end	Night	15% 8% 2 5% 4 4 5% 4 6% 9% 37%	2738
June	1st to 10th	Night	24% 9% 4 7% 4 4 4 4 5% 8% 27%	1520
	11th to 20th	Night	19% 8% 3 6% 5% 5% 5% 4 4 8% 35%	1520
	21st to end	Night	20% 7% 3 7% 5% 5% 4 4 6% 7% 32%	1520
July	1st to 10th	Night	14% 9% 3 6% 5% 6% 6% 4 7% 8% 33%	2520
	11th to 20th	Night	20% 8% 3 7% 3 3 3 3 5% 7% 37%	2520
	21st to end	Night	20% 8% 2 6% 4 4 4 5% 6% 6% 37%	2739
August	1st to 10th	Night	26% 8% 2 6% 4 4 3 3 5% 6% 32%	2594
	11th to 20th	Night	19% 9% 4 8% 5% 4 5% 3 6% 7% 32%	2812
	21st to end	Night	12% 11% 2 6% 4 4 4 3 7% 7% 39%	3476
September	1st to 10th	Night	15% 8% 2 5% 4 3 3 4 4 6% 45%	1900
	11th to 20th	Night	16% 7% 2 5% 4 3 3 3 5% 6% 46%	2061
	21st to end	Night	18% 8% 2 5% 4 4 4 3 5% 6% 41%	2090
October	1st to 10th	Night	22% 9% 2 6% 5% 5% 4 3 5% 6% 33%	2090
	11th to 20th	Night	17% 14% 3 8% 4 4 4 3 5% 6% 32%	2375
	21st to end	Night	22% 16% 4 8% 5% 4 4 3 4 5% 26%	2717
November	1st to 10th	Night	16% 13% 4 10% 4 4 4 3 5% 5% 32%	2537
	11th to 20th	Night	17% 15% 5% 8% 5% 3 4 3 5% 6% 29%	2600
	21st to end	Night	16% 16% 6% 11% 6% 5% 4 3 4 5% 24%	2690
December	1st to 10th	Night	16% 18% 7% 17% 7% 6% 4 3 4 4 13%	4580
	11th to 20th	Night	24% 18% 4 11% 5% 3 3 2 4 4 21%	5208
	21st to end	Night	21% 16% 5% 12% 5% 6% 4 3 4 3 21%	6996

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	53% 16% 11% 16% 4	3598
	11th to 20th	Day	57% 9% 11% 14% 7% 3	3600
	21st to end	Day	55% 9% 8% 15% 11% 2	3960
February	1st to 10th	Day	59% 7% 10% 16% 6% 2	1755
	11th to 20th	Day	56% 9% 12% 14% 8% 1	1957
	21st to end	Day	57% 7% 10% 15% 11% 1	1650
March	1st to 10th	Day	63% 6% 8% 10% 8% 5%	4479
	11th to 20th	Day	51% 10% 13% 16% 8% 2	4701
	21st to end	Day	55% 7% 7% 14% 16% 2	5873
April	1st to 10th	Day	57% 5% 8% 17% 11% 3	2400
	11th to 20th	Day	51% 7% 10% 18% 12% 1	2391
	21st to end	Day	56% 7% 7% 17% 11% 2	2600
May	1st to 10th	Day	60% 7% 9% 16% 6% 1	4250
	11th to 20th	Day	52% 9% 15% 14% 8% 1	4357
	21st to end	Day	39% 11% 14% 25% 11% 1	4837
June	1st to 10th	Day	54% 8% 13% 18% 7%	2660
	11th to 20th	Day	46% 7% 12% 20% 12% 2	2660
	21st to end	Day	54% 9% 11% 17% 8% 1	2660
July	1st to 10th	Day	42% 9% 16% 21% 11%	4370
	11th to 20th	Day	42% 10% 17% 24% 7%	4365
	21st to end	Day	46% 8% 12% 22% 11%	4661
August	1st to 10th	Day	48% 9% 16% 19% 8%	4289
	11th to 20th	Day	49% 10% 16% 20% 5%	4037
	21st to end	Day	35% 11% 18% 22% 13% 1	4180
September	1st to 10th	Day	38% 7% 13% 28% 13% 1	2280
	11th to 20th	Day	37% 6% 10% 30% 15% 2	2119
	21st to end	Day	42% 6% 13% 25% 12% 1	2090
October	1st to 10th	Day	48% 9% 10% 21% 12% 1	3480
	11th to 20th	Day	44% 6% 12% 23% 13% 2	2964
	21st to end	Day	44% 6% 11% 26% 11% 2	3081
November	1st to 10th	Day	47% 6% 14% 20% 12% 1	1761
	11th to 20th	Day	52% 9% 12% 13% 12% 2	1800
	21st to end	Day	56% 9% 16% 15% 4	1707
December	1st to 10th	Day	70% 6% 13% 8% 3	2600
	11th to 20th	Day	59% 7% 8% 14% 11% 1	2986
	21st to end	Day	59% 7% 7% 11% 11% 4	3960
January	1st to 10th	Night	58% 11% 14% 13% 4	6349
	11th to 20th	Night	62% 10% 12% 10% 4 1	6360
	21st to end	Night	54% 11% 9% 12% 11% 2	6996
February	1st to 10th	Night	58% 9% 11% 14% 8% 1	2640
	11th to 20th	Night	55% 11% 10% 15% 9% 1	2340
	21st to end	Night	52% 11% 12% 16% 7% 2	1980
March	1st to 10th	Night	60% 7% 10% 12% 9% 1	5374
	11th to 20th	Night	54% 9% 14% 14% 7% 1	5258
	21st to end	Night	55% 6% 8% 14% 16% 1	5056
April	1st to 10th	Night	55% 6% 9% 18% 10% 2	2000
	11th to 20th	Night	53% 6% 9% 22% 11%	1956
	21st to end	Night	51% 7% 10% 18% 12% 3	1800
May	1st to 10th	Night	60% 8% 9% 15% 7% 1	2930
	11th to 20th	Night	54% 11% 11% 15% 7% 1	2577
	21st to end	Night	39% 11% 17% 19% 12% 2	2739
June	1st to 10th	Night	52% 9% 13% 18% 9%	1520
	11th to 20th	Night	47% 8% 14% 19% 11% 2	1520
	21st to end	Night	49% 7% 15% 19% 9% 1	1520
July	1st to 10th	Night	42% 10% 18% 17% 12%	2480
	11th to 20th	Night	42% 8% 15% 25% 10%	2472
	21st to end	Night	43% 8% 13% 24% 10%	2651
August	1st to 10th	Night	51% 7% 13% 21% 8%	2584
	11th to 20th	Night	47% 9% 16% 18% 9%	2839
	21st to end	Night	33% 13% 21% 21% 12%	3476
September	1st to 10th	Night	37% 9% 14% 28% 13%	1900
	11th to 20th	Night	37% 7% 12% 29% 14% 1	2061
	21st to end	Night	42% 6% 14% 23% 13% 1	2090
October	1st to 10th	Night	46% 9% 15% 20% 9% 1	3480
	11th to 20th	Night	44% 8% 12% 19% 14% 3	3950
	21st to end	Night	47% 7% 11% 25% 10% 1	4467
November	1st to 10th	Night	48% 10% 11% 21% 9% 1	2537
	11th to 20th	Night	51% 7% 12% 18% 11% 2	2600
	21st to end	Night	56% 8% 14% 17% 5%	2690
December	1st to 10th	Night	66% 10% 12% 11% 2	4580
	11th to 20th	Night	60% 9% 8% 10% 12%	5208
	21st to end	Night	61% 8% 8% 13% 8% 3	6996

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	33% 23% 30% 13% 2 1	200
	11th to 20th	Day	38% 25% 26% 9% 3 1	200
	21st to end	Day	25% 31% 28% 12% 4	220
February	1st to 10th	Day	39% 28% 24% 7% 2	303
	11th to 20th	Day	35% 27% 27% 9% 2	496
	21st to end	Day	33% 25% 27% 12% 2	414
March	1st to 10th	Day	31% 24% 30% 12% 3	500
	11th to 20th	Day	33% 28% 24% 11% 3 1	500
	21st to end	Day	36% 22% 25% 10% 5% 2	550
April	1st to 10th	Day	40% 22% 21% 11% 4 1	500
	11th to 20th	Day	34% 28% 20% 12% 5% 1	492
	21st to end	Day	33% 24% 27% 10% 4 2	500
May	1st to 10th	Day	36% 21% 25% 12% 4 2	503
	11th to 20th	Day	22% 23% 32% 15% 6% 2	487
	21st to end	Day	21% 17% 33% 20% 8% 1	529
June	1st to 10th	Day	23% 18% 35% 17% 7%	490
	11th to 20th	Day	21% 18% 32% 20% 8% 1	490
	21st to end	Day	24% 19% 31% 17% 8% 1	490
July	1st to 10th	Day	20% 14% 32% 23% 9% 2	485
	11th to 20th	Day	17% 10% 37% 26% 9% 2	484
	21st to end	Day	17% 13% 36% 23% 8% 2	532
August	1st to 10th	Day	16% 14% 29% 27% 13% 2	486
	11th to 20th	Day	18% 14% 27% 23% 12% 5%	479
	21st to end	Day	13% 16% 30% 24% 12% 5%	539
September	1st to 10th	Day	19% 14% 26% 23% 13% 5%	490
	11th to 20th	Day	18% 15% 24% 20% 16% 7%	490
	21st to end	Day	20% 16% 27% 20% 11% 5%	490
October	1st to 10th	Day	28% 15% 24% 20% 9% 3	490
	11th to 20th	Day	28% 17% 24% 19% 9% 2	437
	21st to end	Day	37% 18% 24% 16% 5%	429
November	1st to 10th	Day	29% 19% 21% 22% 8% 1	395
	11th to 20th	Day	33% 21% 24% 16% 5% 1	400
	21st to end	Day	38% 19% 25% 14% 3	307
December	1st to 10th	Day	30% 26% 27% 13% 5%	200
	11th to 20th	Day	41% 21% 30% 7% 2	200
	21st to end	Day	33% 19% 33% 15% 1	220
January	1st to 10th	Night	25% 25% 30% 18% 3	800
	11th to 20th	Night	30% 21% 28% 19% 3	800
	21st to end	Night	20% 25% 34% 18% 3	880
February	1st to 10th	Night	26% 21% 34% 18% 2	800
	11th to 20th	Night	27% 25% 31% 15% 2	783
	21st to end	Night	25% 22% 30% 20% 3	660
March	1st to 10th	Night	29% 19% 31% 19% 3	800
	11th to 20th	Night	30% 20% 25% 20% 5%	800
	21st to end	Night	30% 19% 26% 18% 6% 1	880
April	1st to 10th	Night	34% 19% 24% 20% 4	800
	11th to 20th	Night	26% 21% 26% 20% 6%	772
	21st to end	Night	28% 22% 27% 20% 4	600
May	1st to 10th	Night	31% 15% 25% 22% 7%	600
	11th to 20th	Night	22% 12% 32% 27% 7%	425
	21st to end	Night	21% 8% 27% 34% 10%	411
June	1st to 10th	Night	23% 10% 25% 33% 8% 1	380
	11th to 20th	Night	22% 10% 24% 32% 12%	380
	21st to end	Night	21% 9% 22% 39% 8%	380
July	1st to 10th	Night	16% 7% 22% 42% 13%	370
	11th to 20th	Night	18% 4 24% 35% 18% 2	368
	21st to end	Night	16% 5% 20% 41% 16% 1	416
August	1st to 10th	Night	19% 4 16% 42% 19%	427
	11th to 20th	Night	18% 7% 25% 32% 17% 2	579
	21st to end	Night	18% 8% 19% 33% 20% 3	836
September	1st to 10th	Night	18% 8% 24% 31% 18% 1	760
	11th to 20th	Night	19% 10% 20% 25% 22% 4	760
	21st to end	Night	21% 13% 21% 29% 15% 2	760
October	1st to 10th	Night	27% 12% 19% 26% 15% 2	760
	11th to 20th	Night	25% 17% 22% 24% 11% 1	760
	21st to end	Night	33% 16% 25% 20% 6%	836
November	1st to 10th	Night	23% 18% 26% 24% 8% 1	780
	11th to 20th	Night	27% 19% 26% 20% 7% 1	800
	21st to end	Night	29% 20% 28% 18% 6% 1	800
December	1st to 10th	Night	31% 21% 29% 16% 3	800
	11th to 20th	Night	33% 19% 28% 16% 4	800
	21st to end	Night	32% 22% 26% 17% 3	880

Page generated at 2025-05-08 09:33:55. Version ac07

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