Apsley
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Apsley clear sky chart from 2005-11-06 to 2025-02-25.

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	16% 11% 4 8% 5% 4 4 4 5% 6% 32%	108810
	Night	16% 12% 4 9% 5% 5% 4 3 5% 6% 31%	109844
Transparency	Day	51% 5% 11% 21% 10% 1	112412
	Night	53% 5% 11% 19% 10% 1	114405
Seeing	Day	23% 15% 31% 19% 8% 3	14679
	Night	22% 13% 31% 24% 9% 1	24100

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% 16% 5% 12% 5% 4 5% 3 5% 6% 28%	3597
	11th to 20th	Day	11% 20% 10% 16% 7% 6% 5% 3 4 3 15%	3596
	21st to end	Day	14% 19% 7% 17% 6% 5% 3 3 4 4 19%	4466
February	1st to 10th	Day	13% 17% 6% 14% 8% 4 4 4 4 4 21%	1800
	11th to 20th	Day	13% 14% 7% 13% 7% 5% 4 4 4 4 26%	1753
	21st to end	Day	17% 16% 5% 11% 7% 5% 4 4 4 5% 24%	1515
March	1st to 10th	Day	20% 14% 5% 8% 5% 4 3 3 5% 6% 28%	3340
	11th to 20th	Day	11% 19% 5% 10% 5% 4 4 3 5% 6% 26%	3480
	21st to end	Day	14% 9% 3 6% 4 4 3 4 4 5% 43%	3868
April	1st to 10th	Day	20% 14% 3 7% 5% 4 4 3 5% 5% 30%	2280
	11th to 20th	Day	23% 10% 3 5% 4 5% 3 3 4 5% 36%	2266
	21st to end	Day	23% 11% 3 7% 5% 4 3 3 4 6% 32%	2470
May	1st to 10th	Day	20% 7% 3 7% 4 5% 4 3 5% 7% 34%	4290
	11th to 20th	Day	17% 9% 3 7% 5% 4 4 3 6% 8% 32%	4398
	21st to end	Day	15% 6% 2 6% 4 4 4 4 6% 7% 42%	4837
June	1st to 10th	Day	20% 7% 3 7% 6% 4 4 4 6% 7% 31%	2660
	11th to 20th	Day	16% 6% 2 6% 5% 6% 5% 4 6% 8% 35%	2660
	21st to end	Day	17% 8% 2 5% 5% 5% 5% 4 6% 7% 35%	2660
July	1st to 10th	Day	13% 5% 3 6% 5% 5% 5% 5% 7% 10% 36%	4440
	11th to 20th	Day	12% 5% 2 5% 5% 5% 5% 4 6% 8% 44%	4440
	21st to end	Day	9% 5% 2 6% 5% 4 5% 5% 8% 9% 42%	4762
August	1st to 10th	Day	17% 8% 3 7% 4 5% 5% 5% 6% 8% 34%	4104
	11th to 20th	Day	17% 6% 2 7% 5% 5% 5% 4 6% 8% 33%	4044
	21st to end	Day	11% 5% 2 6% 5% 5% 5% 4 7% 9% 41%	4281
September	1st to 10th	Day	14% 7% 2 6% 5% 4 4 4 6% 8% 40%	2280
	11th to 20th	Day	12% 5% 2 6% 4 4 5% 3 5% 7% 47%	2264
	21st to end	Day	18% 7% 2 6% 5% 5% 4 4 6% 6% 38%	1991
October	1st to 10th	Day	21% 8% 3 8% 5% 3 4 3 5% 7% 33%	1900
	11th to 20th	Day	24% 10% 4 8% 5% 4 5% 3 6% 6% 26%	1900
	21st to end	Day	26% 13% 4 8% 5% 4 4 2 5% 5% 23%	2063
November	1st to 10th	Day	17% 11% 3 7% 5% 4 5% 4 6% 7% 31%	1659
	11th to 20th	Day	16% 14% 5% 8% 5% 4 4 4 6% 6% 28%	1600
	21st to end	Day	21% 19% 5% 9% 5% 6% 5% 4 5% 4 19%	1600
December	1st to 10th	Day	18% 24% 7% 11% 6% 3 4 3 4 6% 15%	2600
	11th to 20th	Day	18% 21% 6% 13% 4 3 3 2 4 3 22%	2986
	21st to end	Day	23% 20% 5% 12% 5% 3 3 2 3 4 21%	3960
January	1st to 10th	Night	15% 17% 7% 12% 6% 5% 4 3 4 5% 22%	6349
	11th to 20th	Night	12% 15% 7% 18% 9% 7% 6% 4 4 5% 14%	6287
	21st to end	Night	14% 15% 8% 15% 6% 5% 4 3 5% 5% 20%	6490
February	1st to 10th	Night	12% 16% 5% 11% 7% 6% 5% 4 6% 6% 21%	2600
	11th to 20th	Night	12% 14% 6% 11% 7% 6% 3 3 5% 5% 28%	2533
	21st to end	Night	15% 15% 4 9% 5% 5% 5% 4 6% 6% 27%	2033
March	1st to 10th	Night	18% 12% 4 8% 5% 5% 4 3 5% 5% 30%	3483
	11th to 20th	Night	13% 17% 6% 10% 5% 4 4 4 5% 5% 27%	3479
	21st to end	Night	15% 7% 3 7% 5% 4 4 4 6% 6% 38%	3764
April	1st to 10th	Night	20% 11% 4 7% 5% 4 4 3 5% 6% 31%	1900
	11th to 20th	Night	17% 9% 3 8% 5% 4 4 3 4 5% 37%	1875
	21st to end	Night	19% 9% 3 7% 5% 5% 4 4 6% 6% 32%	1714
May	1st to 10th	Night	20% 8% 2 7% 4 4 4 4 5% 5% 37%	2651
	11th to 20th	Night	18% 7% 2 6% 5% 5% 4 3 6% 7% 36%	2495
	21st to end	Night	11% 7% 2 6% 4 5% 4 4 5% 7% 46%	2738
June	1st to 10th	Night	16% 7% 3 7% 4 5% 5% 4 6% 8% 37%	1520
	11th to 20th	Night	12% 6% 2 5% 4 6% 5% 4 5% 8% 44%	1520
	21st to end	Night	15% 7% 3 6% 5% 5% 5% 3 5% 6% 39%	1520
July	1st to 10th	Night	9% 5% 2 7% 5% 5% 5% 4 6% 7% 44%	2520
	11th to 20th	Night	15% 6% 2 5% 4 3 3 3 5% 7% 48%	2520
	21st to end	Night	10% 6% 1 5% 3 4 5% 4 6% 8% 49%	2787
August	1st to 10th	Night	12% 5% 1 5% 4 4 5% 4 6% 8% 47%	2825
	11th to 20th	Night	15% 6% 2 5% 4 4 5% 3 5% 7% 44%	2785
	21st to end	Night	11% 5% 2 5% 4 5% 4 3 5% 7% 49%	3341
September	1st to 10th	Night	11% 5% 3 6% 4 4 4 3 7% 8% 45%	1900
	11th to 20th	Night	12% 4 1 4 3 4 5% 3 5% 6% 51%	1913
	21st to end	Night	19% 7% 1 6% 4 3 4 3 5% 5% 42%	2188
October	1st to 10th	Night	18% 7% 3 7% 4 5% 4 3 4 7% 38%	2280
	11th to 20th	Night	22% 10% 3 8% 4 4 4 4 5% 6% 30%	2280
	21st to end	Night	22% 13% 4 7% 5% 4 4 4 5% 5% 26%	2533
November	1st to 10th	Night	18% 10% 3 7% 5% 4 4 3 5% 6% 35%	2637
	11th to 20th	Night	18% 13% 4 7% 5% 4 4 3 5% 7% 30%	2800
	21st to end	Night	20% 14% 4 9% 6% 4 5% 3 5% 7% 22%	2800
December	1st to 10th	Night	21% 19% 6% 11% 5% 4 4 3 5% 6% 16%	4580
	11th to 20th	Night	17% 19% 6% 15% 5% 4 4 3 4 4 18%	5208
	21st to end	Night	24% 19% 5% 13% 5% 4 3 2 3 4 17%	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	56% 5% 9% 20% 9% 1	3598
	11th to 20th	Day	72% 4 9% 8% 6% 1	3626
	21st to end	Day	69% 3 8% 10% 9% 1	4466
February	1st to 10th	Day	66% 5% 9% 14% 5% 1	1800
	11th to 20th	Day	61% 5% 9% 13% 10% 2	1762
	21st to end	Day	63% 5% 9% 13% 9% 1	1515
March	1st to 10th	Day	57% 5% 13% 14% 10% 1	3343
	11th to 20th	Day	56% 6% 12% 16% 8% 1	3480
	21st to end	Day	41% 6% 9% 24% 17% 3	3868
April	1st to 10th	Day	56% 4 8% 19% 10% 3	2280
	11th to 20th	Day	51% 4 8% 21% 15% 1	2266
	21st to end	Day	54% 4 8% 19% 12% 3	2470
May	1st to 10th	Day	49% 5% 10% 23% 12% 2	4302
	11th to 20th	Day	49% 7% 12% 21% 11% 1	4402
	21st to end	Day	40% 6% 13% 22% 15% 3	4837
June	1st to 10th	Day	50% 6% 12% 18% 13% 1	2660
	11th to 20th	Day	44% 6% 12% 24% 13% 2	2660
	21st to end	Day	47% 6% 13% 21% 12% 1	2660
July	1st to 10th	Day	41% 5% 16% 24% 13% 1	4370
	11th to 20th	Day	36% 6% 15% 33% 10%	4365
	21st to end	Day	36% 7% 12% 33% 12%	4610
August	1st to 10th	Day	46% 6% 14% 22% 11%	4091
	11th to 20th	Day	46% 6% 14% 26% 8%	4083
	21st to end	Day	37% 6% 12% 33% 10% 1	4281
September	1st to 10th	Day	42% 6% 12% 28% 12% 1	2280
	11th to 20th	Day	35% 6% 14% 29% 15% 1	2264
	21st to end	Day	45% 7% 14% 24% 10%	1991
October	1st to 10th	Day	45% 5% 11% 28% 10% 1	3160
	11th to 20th	Day	49% 5% 13% 21% 12% 1	3140
	21st to end	Day	57% 5% 13% 19% 6%	3377
November	1st to 10th	Day	51% 5% 15% 20% 8% 1	1659
	11th to 20th	Day	55% 7% 13% 16% 9% 2	1600
	21st to end	Day	66% 7% 11% 11% 5% 1	1600
December	1st to 10th	Day	72% 4 13% 8% 3	2600
	11th to 20th	Day	68% 5% 7% 14% 6%	2986
	21st to end	Day	68% 4 6% 13% 8%	3960
January	1st to 10th	Night	64% 4 8% 16% 8%	6349
	11th to 20th	Night	70% 6% 10% 8% 5% 1	6332
	21st to end	Night	66% 4 9% 8% 12% 1	6490
February	1st to 10th	Night	61% 6% 11% 16% 6% 1	2600
	11th to 20th	Night	58% 4 10% 19% 9% 1	2535
	21st to end	Night	56% 5% 11% 18% 9% 1	2033
March	1st to 10th	Night	55% 5% 10% 19% 10% 1	3493
	11th to 20th	Night	55% 7% 13% 16% 7% 1	3480
	21st to end	Night	41% 7% 11% 25% 15% 2	3764
April	1st to 10th	Night	53% 5% 7% 20% 12% 2	1900
	11th to 20th	Night	48% 5% 9% 21% 15% 2	1875
	21st to end	Night	50% 7% 10% 16% 16% 2	1714
May	1st to 10th	Night	47% 6% 11% 19% 15% 2	2658
	11th to 20th	Night	45% 8% 12% 22% 11% 2	2496
	21st to end	Night	37% 6% 13% 23% 18% 2	2739
June	1st to 10th	Night	43% 6% 14% 23% 14% 1	1520
	11th to 20th	Night	37% 6% 13% 24% 18% 2	1520
	21st to end	Night	44% 5% 15% 22% 13% 1	1520
July	1st to 10th	Night	36% 6% 16% 24% 16% 1	2480
	11th to 20th	Night	36% 5% 12% 34% 14%	2472
	21st to end	Night	33% 6% 16% 32% 13%	2698
August	1st to 10th	Night	36% 5% 14% 26% 17% 1	2814
	11th to 20th	Night	39% 5% 19% 25% 11% 1	2812
	21st to end	Night	33% 6% 13% 34% 11% 2	3341
September	1st to 10th	Night	35% 7% 16% 27% 14% 1	1900
	11th to 20th	Night	33% 5% 14% 33% 14% 1	1913
	21st to end	Night	43% 5% 12% 29% 11%	2188
October	1st to 10th	Night	38% 4 11% 28% 19%	3800
	11th to 20th	Night	46% 5% 14% 26% 9%	3779
	21st to end	Night	51% 5% 14% 22% 7%	4169
November	1st to 10th	Night	49% 6% 13% 23% 9%	2637
	11th to 20th	Night	54% 6% 12% 18% 9% 1	2800
	21st to end	Night	60% 8% 13% 14% 4	2800
December	1st to 10th	Night	68% 6% 11% 12% 3	4580
	11th to 20th	Night	70% 3 10% 11% 6%	5208
	21st to end	Night	72% 4 7% 10% 7%	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	22% 23% 35% 17% 5% 1	200
	11th to 20th	Day	27% 20% 34% 15% 3	207
	21st to end	Day	25% 22% 37% 13% 3 1	330
February	1st to 10th	Day	30% 23% 36% 9% 2	300
	11th to 20th	Day	30% 24% 39% 6% 1	300
	21st to end	Day	35% 19% 36% 10% 1	303
March	1st to 10th	Day	25% 18% 38% 16% 3	460
	11th to 20th	Day	26% 20% 33% 17% 3 1	460
	21st to end	Day	33% 17% 30% 13% 6% 1	506
April	1st to 10th	Day	27% 21% 32% 13% 7%	460
	11th to 20th	Day	34% 21% 20% 15% 8% 2	452
	21st to end	Day	31% 17% 32% 13% 5% 1	460
May	1st to 10th	Day	27% 14% 31% 18% 5% 5%	463
	11th to 20th	Day	21% 14% 35% 19% 8% 3	487
	21st to end	Day	20% 12% 36% 18% 10% 3	529
June	1st to 10th	Day	23% 13% 34% 21% 8% 1	490
	11th to 20th	Day	18% 12% 34% 23% 10% 3	490
	21st to end	Day	21% 13% 34% 20% 9% 2	490
July	1st to 10th	Day	13% 9% 32% 30% 13% 4	485
	11th to 20th	Day	15% 9% 31% 26% 14% 5%	484
	21st to end	Day	12% 10% 33% 30% 11% 4	532
August	1st to 10th	Day	15% 7% 30% 28% 14% 6%	486
	11th to 20th	Day	15% 5% 27% 30% 16% 6%	479
	21st to end	Day	14% 9% 32% 25% 12% 8%	539
September	1st to 10th	Day	15% 11% 31% 24% 13% 6%	490
	11th to 20th	Day	11% 10% 25% 22% 19% 12%	490
	21st to end	Day	19% 14% 26% 25% 11% 6%	490
October	1st to 10th	Day	26% 15% 24% 17% 13% 4	490
	11th to 20th	Day	24% 22% 29% 17% 6% 2	490
	21st to end	Day	31% 20% 28% 16% 4 1	524
November	1st to 10th	Day	28% 17% 31% 16% 7% 1	293
	11th to 20th	Day	30% 23% 33% 10% 4 2	200
	21st to end	Day	30% 20% 36% 13% 2 1	200
December	1st to 10th	Day	32% 20% 30% 17% 2	200
	11th to 20th	Day	36% 21% 28% 13% 2 1	200
	21st to end	Day	33% 20% 33% 11% 3	220
January	1st to 10th	Night	28% 19% 32% 16% 5%	800
	11th to 20th	Night	28% 17% 32% 19% 5%	800
	21st to end	Night	20% 15% 35% 23% 6% 1	880
February	1st to 10th	Night	23% 20% 37% 18% 2	800
	11th to 20th	Night	23% 18% 39% 16% 4	783
	21st to end	Night	24% 22% 31% 20% 3	647
March	1st to 10th	Night	24% 18% 35% 20% 3	760
	11th to 20th	Night	23% 19% 34% 19% 5% 1	760
	21st to end	Night	21% 14% 33% 23% 8% 1	836
April	1st to 10th	Night	26% 15% 32% 22% 5%	760
	11th to 20th	Night	23% 17% 32% 19% 8% 1	751
	21st to end	Night	23% 11% 31% 23% 11% 1	574
May	1st to 10th	Night	23% 6% 32% 26% 12% 1	466
	11th to 20th	Night	16% 10% 35% 30% 9%	374
	21st to end	Night	13% 7% 34% 34% 11%	411
June	1st to 10th	Night	17% 7% 35% 30% 11%	380
	11th to 20th	Night	14% 8% 30% 36% 11% 1	380
	21st to end	Night	17% 7% 29% 34% 12%	380
July	1st to 10th	Night	7% 6% 31% 40% 16% 1	370
	11th to 20th	Night	11% 6% 29% 36% 18% 1	368
	21st to end	Night	11% 3 29% 39% 16% 1	440
August	1st to 10th	Night	14% 2 21% 35% 25% 2	567
	11th to 20th	Night	11% 6% 21% 35% 25% 2	564
	21st to end	Night	13% 4 30% 31% 20% 2	753
September	1st to 10th	Night	13% 7% 31% 30% 17% 3	760
	11th to 20th	Night	13% 4 25% 30% 23% 5%	760
	21st to end	Night	22% 9% 21% 27% 18% 3	760
October	1st to 10th	Night	21% 12% 27% 23% 15% 2	760
	11th to 20th	Night	28% 14% 29% 20% 9% 1	760
	21st to end	Night	29% 14% 32% 20% 5%	836
November	1st to 10th	Night	24% 13% 28% 26% 8%	780
	11th to 20th	Night	26% 16% 33% 17% 6% 2	800
	21st to end	Night	30% 16% 30% 20% 5% 1	800
December	1st to 10th	Night	31% 18% 33% 15% 5%	800
	11th to 20th	Night	25% 19% 36% 17% 4	800
	21st to end	Night	34% 14% 32% 17% 3	880

Page generated at 2025-02-22 06:51:35. Version ac07

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