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 2026-07-01.

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 3 5% 6% 32%	138331
	Night	16% 12% 4 9% 5% 5% 4 3 5% 6% 31%	134006
Transparency	Day	51% 5% 11% 21% 11% 1	141886
	Night	53% 5% 11% 19% 11% 1	138538
Seeing	Day	23% 15% 32% 19% 8% 3	16450
	Night	22% 13% 31% 24% 9% 1	25722

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	14% 15% 4 12% 6% 4 5% 3 5% 6% 26%	4661
	11th to 20th	Day	10% 24% 10% 16% 7% 7% 5% 3 3 3 13%	4690
	21st to end	Day	11% 16% 7% 18% 8% 7% 5% 3 4 4 18%	5780
February	1st to 10th	Day	13% 17% 6% 14% 8% 4 4 4 4 4 22%	1890
	11th to 20th	Day	13% 14% 7% 13% 7% 5% 4 4 4 4 25%	1843
	21st to end	Day	16% 16% 6% 11% 6% 5% 4 4 4 5% 24%	1631
March	1st to 10th	Day	19% 13% 6% 9% 5% 3 3 3 5% 5% 28%	4905
	11th to 20th	Day	13% 15% 5% 10% 6% 5% 5% 4 6% 7% 26%	5090
	21st to end	Day	20% 12% 4 7% 4 3 3 3 3 4 37%	5668
April	1st to 10th	Day	20% 14% 3 8% 5% 4 4 3 5% 5% 30%	2520
	11th to 20th	Day	23% 10% 3 5% 4 4 3 3 4 4 35%	2508
	21st to end	Day	22% 10% 3 8% 5% 4 3 3 4 6% 32%	2730
May	1st to 10th	Day	21% 8% 3 7% 5% 5% 4 3 5% 7% 34%	7976
	11th to 20th	Day	19% 8% 3 7% 5% 4 4 4 5% 7% 35%	8221
	21st to end	Day	19% 7% 2 6% 4 4 4 4 5% 6% 38%	9060
June	1st to 10th	Day	19% 7% 3 7% 6% 4 4 4 5% 7% 33%	2940
	11th to 20th	Day	16% 6% 2 6% 5% 6% 5% 5% 6% 8% 35%	2940
	21st to end	Day	17% 8% 2 6% 5% 5% 5% 4 6% 8% 35%	2940
July	1st to 10th	Day	14% 6% 3 6% 5% 5% 5% 4 6% 9% 38%	6360
	11th to 20th	Day	11% 5% 2 5% 4 5% 4 4 5% 7% 47%	6360
	21st to end	Day	8% 5% 2 6% 4 4 5% 4 7% 9% 46%	6846
August	1st to 10th	Day	16% 8% 3 6% 4 5% 5% 5% 6% 8% 35%	4234
	11th to 20th	Day	17% 6% 2 7% 5% 5% 5% 4 6% 8% 34%	4174
	21st to end	Day	11% 5% 2 6% 5% 5% 5% 4 7% 9% 41%	4416
September	1st to 10th	Day	13% 7% 2 6% 5% 4 5% 4 6% 8% 41%	2400
	11th to 20th	Day	12% 5% 2 6% 4 4 5% 3 5% 7% 48%	2383
	21st to end	Day	18% 7% 2 7% 5% 5% 4 4 6% 6% 38%	2096
October	1st to 10th	Day	20% 8% 3 8% 5% 3 4 3 5% 7% 35%	2000
	11th to 20th	Day	24% 10% 4 8% 4 4 5% 3 6% 6% 27%	2000
	21st to end	Day	26% 13% 4 8% 5% 4 4 2 5% 5% 24%	2171
November	1st to 10th	Day	17% 12% 3 8% 5% 4 4 3 6% 7% 31%	1744
	11th to 20th	Day	16% 14% 5% 8% 5% 4 4 4 6% 6% 28%	1680
	21st to end	Day	21% 19% 5% 9% 5% 5% 5% 4 5% 4 19%	1680
December	1st to 10th	Day	18% 24% 7% 11% 6% 3 4 3 4 5% 16%	2680
	11th to 20th	Day	18% 21% 6% 13% 4 3 3 3 4 3 22%	3066
	21st to end	Day	23% 20% 5% 12% 5% 3 3 2 3 4 21%	4048
January	1st to 10th	Night	14% 17% 8% 15% 6% 5% 4 3 4 5% 20%	8234
	11th to 20th	Night	11% 19% 7% 18% 9% 7% 5% 3 4 4 14%	8193
	21st to end	Night	11% 14% 7% 14% 8% 5% 4 3 5% 5% 23%	8401
February	1st to 10th	Night	12% 16% 5% 11% 7% 6% 5% 4 6% 6% 23%	2730
	11th to 20th	Night	12% 14% 6% 11% 7% 6% 3 3 5% 5% 27%	2663
	21st to end	Night	14% 15% 4 9% 5% 5% 5% 4 6% 6% 26%	2174
March	1st to 10th	Night	18% 13% 4 10% 5% 5% 4 3 5% 5% 29%	5138
	11th to 20th	Night	10% 13% 5% 9% 5% 5% 5% 5% 6% 7% 29%	5089
	21st to end	Night	20% 8% 3 7% 5% 4 4 4 6% 6% 33%	5506
April	1st to 10th	Night	20% 12% 4 7% 5% 4 4 3 5% 6% 31%	2100
	11th to 20th	Night	17% 10% 3 8% 5% 4 4 3 4 6% 37%	2075
	21st to end	Night	18% 9% 3 7% 5% 5% 4 4 6% 6% 33%	1894
May	1st to 10th	Night	21% 8% 2 6% 4 4 4 3 5% 6% 35%	4923
	11th to 20th	Night	16% 7% 3 6% 4 4 4 3 6% 6% 41%	4639
	21st to end	Night	18% 6% 2 5% 4 4 4 4 5% 6% 41%	5114
June	1st to 10th	Night	15% 6% 3 6% 4 4 4 4 6% 8% 39%	1680
	11th to 20th	Night	13% 6% 2 6% 4 5% 5% 4 5% 8% 43%	1680
	21st to end	Night	14% 6% 3 7% 5% 5% 5% 3 5% 7% 40%	1680
July	1st to 10th	Night	11% 6% 2 7% 5% 5% 5% 5% 7% 8% 40%	3600
	11th to 20th	Night	13% 6% 2 5% 3 3 4 3 5% 7% 50%	3600
	21st to end	Night	9% 5% 2 5% 3 4 5% 3 5% 7% 51%	4002
August	1st to 10th	Night	12% 5% 1 5% 4 4 5% 4 6% 8% 47%	2915
	11th to 20th	Night	15% 6% 2 5% 4 4 5% 3 5% 7% 45%	2875
	21st to end	Night	11% 5% 2 5% 4 4 4 3 5% 7% 49%	3447
September	1st to 10th	Night	11% 5% 3 6% 4 4 4 3 7% 8% 45%	2000
	11th to 20th	Night	12% 4 1 4 3 4 5% 3 5% 6% 52%	2014
	21st to end	Night	19% 7% 1 6% 4 4 4 3 5% 5% 42%	2303
October	1st to 10th	Night	17% 7% 3 7% 4 5% 4 3 4 7% 40%	2400
	11th to 20th	Night	22% 10% 3 8% 4 4 4 4 5% 6% 31%	2400
	21st to end	Night	21% 13% 4 7% 5% 4 5% 4 5% 5% 28%	2667
November	1st to 10th	Night	18% 10% 3 7% 5% 4 4 3 5% 6% 34%	2772
	11th to 20th	Night	18% 13% 4 7% 5% 4 4 3 5% 7% 29%	2940
	21st to end	Night	20% 15% 4 9% 6% 4 5% 3 5% 7% 21%	2940
December	1st to 10th	Night	20% 19% 6% 11% 5% 5% 4 3 5% 6% 16%	4720
	11th to 20th	Night	17% 19% 6% 14% 5% 4 4 3 4 4 19%	5348
	21st to end	Night	24% 19% 5% 13% 5% 4 3 2 3 4 17%	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% 5% 10% 18% 8% 1	4662
	11th to 20th	Day	76% 3 7% 7% 6%	4720
	21st to end	Day	70% 3 8% 10% 8% 1	5780
February	1st to 10th	Day	65% 5% 9% 14% 6% 1	1890
	11th to 20th	Day	62% 5% 8% 14% 10% 1	1852
	21st to end	Day	63% 4 9% 14% 9% 1	1631
March	1st to 10th	Day	58% 4 12% 11% 14% 1	4908
	11th to 20th	Day	56% 5% 14% 16% 7% 2	5090
	21st to end	Day	51% 4 7% 20% 16% 2	5668
April	1st to 10th	Day	56% 4 8% 19% 11% 2	2520
	11th to 20th	Day	52% 4 8% 20% 15% 1	2508
	21st to end	Day	54% 4 8% 19% 11% 3	2730
May	1st to 10th	Day	49% 4 10% 20% 13% 3	7985
	11th to 20th	Day	48% 6% 10% 19% 16% 1	8225
	21st to end	Day	46% 4 10% 24% 13% 2	9061
June	1st to 10th	Day	49% 6% 11% 19% 15% 1	2940
	11th to 20th	Day	44% 6% 11% 24% 12% 2	2940
	21st to end	Day	46% 6% 13% 22% 11% 1	2940
July	1st to 10th	Day	41% 5% 14% 27% 12% 1	6290
	11th to 20th	Day	35% 6% 13% 29% 17%	6285
	21st to end	Day	32% 6% 14% 36% 12%	6649
August	1st to 10th	Day	45% 6% 14% 23% 11%	4221
	11th to 20th	Day	45% 6% 14% 26% 8%	4213
	21st to end	Day	37% 6% 12% 34% 10% 1	4416
September	1st to 10th	Day	41% 6% 12% 29% 12% 1	2400
	11th to 20th	Day	35% 6% 13% 31% 14% 1	2383
	21st to end	Day	45% 6% 14% 24% 11%	2096
October	1st to 10th	Day	44% 5% 11% 29% 11% 1	3260
	11th to 20th	Day	49% 5% 13% 20% 12% 1	3240
	21st to end	Day	57% 5% 13% 19% 7%	3485
November	1st to 10th	Day	52% 5% 14% 20% 8% 1	1744
	11th to 20th	Day	55% 6% 13% 15% 9% 1	1680
	21st to end	Day	66% 7% 11% 11% 6% 1	1680
December	1st to 10th	Day	72% 4 12% 8% 4	2680
	11th to 20th	Day	68% 5% 7% 13% 6%	3066
	21st to end	Day	68% 4 7% 13% 7%	4048
January	1st to 10th	Night	67% 3 8% 14% 7%	8234
	11th to 20th	Night	73% 5% 8% 8% 6% 1	8238
	21st to end	Night	63% 3 9% 11% 13% 1	8401
February	1st to 10th	Night	60% 5% 11% 16% 7% 1	2730
	11th to 20th	Night	59% 4 10% 19% 8% 1	2665
	21st to end	Night	56% 5% 11% 19% 9% 1	2174
March	1st to 10th	Night	57% 4 8% 15% 11% 5%	5148
	11th to 20th	Night	50% 7% 17% 17% 8% 1	5090
	21st to end	Night	48% 5% 10% 21% 14% 2	5506
April	1st to 10th	Night	53% 5% 8% 20% 12% 2	2100
	11th to 20th	Night	48% 5% 9% 21% 15% 1	2075
	21st to end	Night	49% 6% 9% 18% 15% 2	1894
May	1st to 10th	Night	49% 5% 10% 20% 13% 4	4930
	11th to 20th	Night	42% 6% 12% 22% 16% 2	4640
	21st to end	Night	43% 5% 11% 23% 16% 2	5115
June	1st to 10th	Night	42% 6% 13% 23% 16% 1	1680
	11th to 20th	Night	38% 6% 13% 24% 17% 2	1680
	21st to end	Night	44% 5% 15% 22% 13% 1	1680
July	1st to 10th	Night	39% 6% 16% 22% 16% 1	3560
	11th to 20th	Night	34% 4 12% 33% 16%	3552
	21st to end	Night	32% 5% 12% 36% 14%	3884
August	1st to 10th	Night	36% 5% 15% 26% 17% 1	2904
	11th to 20th	Night	38% 5% 18% 25% 12% 1	2902
	21st to end	Night	33% 6% 13% 34% 11% 2	3447
September	1st to 10th	Night	35% 7% 16% 28% 14% 1	2000
	11th to 20th	Night	32% 5% 14% 34% 14% 1	2014
	21st to end	Night	44% 5% 11% 28% 12%	2303
October	1st to 10th	Night	37% 4 11% 28% 19%	3920
	11th to 20th	Night	46% 5% 14% 26% 10%	3899
	21st to end	Night	51% 5% 14% 23% 8%	4303
November	1st to 10th	Night	49% 6% 13% 22% 9%	2772
	11th to 20th	Night	54% 6% 12% 18% 8% 1	2940
	21st to end	Night	61% 8% 12% 14% 4	2940
December	1st to 10th	Night	68% 6% 11% 12% 3	4720
	11th to 20th	Night	69% 3 10% 11% 6%	5348
	21st to end	Night	72% 4 7% 10% 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	22% 23% 35% 16% 4	220
	11th to 20th	Day	29% 20% 34% 14% 3	228
	21st to end	Day	24% 23% 36% 13% 3 1	363
February	1st to 10th	Day	30% 21% 35% 11% 3	330
	11th to 20th	Day	29% 24% 40% 6% 1	330
	21st to end	Day	34% 18% 37% 10% 1	347
March	1st to 10th	Day	26% 18% 37% 15% 3	540
	11th to 20th	Day	30% 20% 31% 16% 2 1	540
	21st to end	Day	34% 17% 29% 13% 6% 1	594
April	1st to 10th	Day	29% 21% 32% 12% 6%	540
	11th to 20th	Day	35% 21% 20% 14% 8% 2	532
	21st to end	Day	31% 16% 33% 14% 5% 1	540
May	1st to 10th	Day	27% 15% 32% 17% 5% 4	543
	11th to 20th	Day	21% 15% 36% 17% 8% 3	567
	21st to end	Day	23% 14% 34% 17% 10% 3	617
June	1st to 10th	Day	22% 14% 33% 21% 8% 1	570
	11th to 20th	Day	18% 13% 35% 22% 9% 3	570
	21st to end	Day	20% 14% 35% 21% 8% 2	570
July	1st to 10th	Day	13% 9% 33% 30% 12% 3	525
	11th to 20th	Day	14% 9% 32% 27% 13% 5%	524
	21st to end	Day	11% 11% 35% 28% 11% 3	576
August	1st to 10th	Day	14% 7% 29% 30% 14% 6%	526
	11th to 20th	Day	15% 5% 28% 30% 16% 6%	519
	21st to end	Day	14% 9% 33% 25% 12% 8%	583
September	1st to 10th	Day	15% 12% 29% 24% 14% 6%	530
	11th to 20th	Day	11% 9% 24% 23% 20% 13%	530
	21st to end	Day	20% 13% 26% 25% 11% 5%	530
October	1st to 10th	Day	25% 15% 24% 19% 14% 4	530
	11th to 20th	Day	24% 23% 30% 16% 5% 2	530
	21st to end	Day	30% 19% 30% 16% 4 1	566
November	1st to 10th	Day	29% 18% 31% 16% 6% 1	318
	11th to 20th	Day	29% 25% 31% 10% 4 1	220
	21st to end	Day	31% 21% 35% 11% 2	220
December	1st to 10th	Day	31% 20% 30% 17% 2	220
	11th to 20th	Day	36% 21% 28% 12% 1 1	220
	21st to end	Day	35% 19% 33% 11% 3	242
January	1st to 10th	Night	27% 19% 32% 17% 5%	840
	11th to 20th	Night	28% 17% 32% 18% 5%	840
	21st to end	Night	20% 15% 35% 23% 6% 1	924
February	1st to 10th	Night	22% 20% 36% 19% 2	840
	11th to 20th	Night	23% 18% 38% 17% 4	823
	21st to end	Night	24% 21% 31% 21% 3	692
March	1st to 10th	Night	25% 18% 36% 19% 3	840
	11th to 20th	Night	23% 19% 34% 19% 4 1	840
	21st to end	Night	22% 14% 33% 22% 8% 1	924
April	1st to 10th	Night	27% 15% 32% 21% 5%	840
	11th to 20th	Night	23% 18% 31% 19% 7% 1	831
	21st to end	Night	23% 12% 32% 24% 10%	634
May	1st to 10th	Night	23% 8% 31% 25% 12% 1	515
	11th to 20th	Night	16% 10% 36% 29% 9%	414
	21st to end	Night	16% 7% 34% 32% 10%	455
June	1st to 10th	Night	17% 7% 35% 31% 11%	420
	11th to 20th	Night	15% 9% 31% 34% 10% 1	420
	21st to end	Night	17% 7% 29% 36% 11%	420
July	1st to 10th	Night	7% 6% 31% 41% 15% 1	390
	11th to 20th	Night	11% 6% 28% 37% 18% 1	388
	21st to end	Night	12% 3 30% 39% 16% 1	464
August	1st to 10th	Night	13% 3 21% 37% 25% 2	597
	11th to 20th	Night	11% 6% 21% 35% 25% 2	594
	21st to end	Night	12% 4 30% 31% 20% 2	793
September	1st to 10th	Night	12% 7% 31% 30% 18% 3	800
	11th to 20th	Night	13% 4 25% 31% 24% 4	800
	21st to end	Night	22% 9% 21% 28% 18% 3	800
October	1st to 10th	Night	20% 11% 27% 24% 16% 2	800
	11th to 20th	Night	27% 14% 30% 20% 8% 1	800
	21st to end	Night	28% 14% 32% 20% 5%	880
November	1st to 10th	Night	25% 13% 28% 26% 8%	820
	11th to 20th	Night	26% 16% 33% 17% 6% 2	840
	21st to end	Night	30% 16% 29% 19% 5%	840
December	1st to 10th	Night	30% 18% 32% 15% 5%	840
	11th to 20th	Night	25% 19% 36% 16% 3	840
	21st to end	Night	34% 14% 32% 17% 3	924

Page generated at 2026-06-28 02:10:43. Version ac07

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