Cavendish
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cavendish clear sky chart from 2005-11-06 to 2026-04-17.

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 9% 5% 5% 5% 3 5% 6% 29%	132187
	Night	17% 13% 4 11% 6% 5% 5% 3 5% 6% 26%	130109
Transparency	Day	53% 7% 13% 19% 7% 1	135723
	Night	56% 7% 12% 17% 7% 1	134663
Seeing	Day	22% 11% 27% 27% 11% 2	19121
	Night	24% 12% 29% 27% 8% 1	26547

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	13% 22% 8% 19% 7% 7% 5% 3 3 4 9%	4186
	11th to 20th	Day	15% 16% 10% 16% 7% 5% 4 3 3 4 18%	4681
	21st to end	Day	9% 10% 5% 12% 5% 6% 8% 4 5% 7% 29%	5645
February	1st to 10th	Day	16% 14% 5% 9% 6% 5% 5% 4 5% 5% 25%	1890
	11th to 20th	Day	15% 15% 5% 9% 6% 6% 6% 4 5% 6% 25%	1843
	21st to end	Day	10% 13% 4 9% 7% 5% 4 4 5% 7% 32%	1591
March	1st to 10th	Day	18% 13% 5% 12% 6% 8% 4 3 4 5% 25%	4898
	11th to 20th	Day	13% 15% 5% 11% 7% 5% 5% 4 5% 7% 24%	5090
	21st to end	Day	21% 16% 5% 13% 6% 5% 4 3 5% 5% 19%	5666
April	1st to 10th	Day	18% 12% 4 8% 6% 4 4 3 5% 6% 31%	2520
	11th to 20th	Day	19% 10% 3 6% 4 4 4 3 5% 8% 33%	2519
	21st to end	Day	20% 12% 4 7% 4 4 4 4 5% 7% 30%	2600
May	1st to 10th	Day	24% 12% 3 6% 4 4 4 3 4 6% 29%	6245
	11th to 20th	Day	19% 9% 3 7% 5% 4 4 4 4 7% 34%	6318
	21st to end	Day	17% 9% 3 6% 5% 4 5% 4 6% 8% 33%	6948
June	1st to 10th	Day	23% 11% 3 7% 6% 5% 5% 4 5% 7% 26%	2850
	11th to 20th	Day	17% 9% 3 8% 5% 4 4 4 5% 6% 35%	3000
	21st to end	Day	22% 9% 3 8% 5% 4 4 3 5% 7% 30%	3000
July	1st to 10th	Day	13% 9% 4 8% 5% 5% 6% 5% 7% 9% 30%	6820
	11th to 20th	Day	12% 5% 3 6% 4 4 5% 3 6% 8% 43%	6466
	21st to end	Day	10% 6% 3 6% 4 4 5% 4 7% 9% 41%	6927
August	1st to 10th	Day	13% 9% 3 8% 5% 5% 5% 4 6% 6% 35%	4286
	11th to 20th	Day	12% 6% 3 7% 4 5% 5% 4 6% 8% 40%	4175
	21st to end	Day	14% 5% 2 5% 4 4 4 4 6% 8% 44%	4487
September	1st to 10th	Day	15% 6% 2 6% 4 3 4 4 7% 8% 41%	2400
	11th to 20th	Day	17% 9% 2 7% 4 4 5% 2 5% 6% 40%	2370
	21st to end	Day	17% 8% 2 5% 4 4 5% 3 4 7% 40%	2106
October	1st to 10th	Day	18% 8% 3 6% 4 5% 5% 4 5% 8% 35%	2000
	11th to 20th	Day	19% 12% 4 9% 5% 5% 5% 3 5% 7% 25%	2000
	21st to end	Day	19% 15% 4 9% 5% 5% 4 3 4 8% 23%	2140
November	1st to 10th	Day	20% 19% 4 8% 6% 3 5% 3 5% 5% 23%	1671
	11th to 20th	Day	21% 21% 7% 10% 5% 4 4 2 4 5% 16%	1680
	21st to end	Day	18% 16% 8% 13% 7% 5% 4 3 6% 5% 14%	1680
December	1st to 10th	Day	17% 15% 8% 20% 7% 5% 4 2 3 4 16%	2690
	11th to 20th	Day	17% 16% 6% 18% 8% 5% 4 2 2 3 19%	3076
	21st to end	Day	16% 14% 10% 20% 7% 5% 4 2 3 3 15%	3723
January	1st to 10th	Night	15% 17% 6% 18% 7% 6% 5% 3 3 3 14%	8731
	11th to 20th	Night	16% 17% 7% 14% 6% 5% 4 3 3 4 20%	8234
	21st to end	Night	6% 12% 5% 13% 9% 6% 7% 5% 6% 6% 25%	8538
February	1st to 10th	Night	17% 15% 4 10% 5% 4 4 3 6% 6% 26%	2730
	11th to 20th	Night	16% 15% 5% 10% 6% 6% 5% 3 5% 6% 25%	2663
	21st to end	Night	12% 12% 5% 9% 6% 4 6% 3 6% 6% 31%	2214
March	1st to 10th	Night	20% 13% 5% 12% 6% 5% 5% 3 5% 5% 23%	5160
	11th to 20th	Night	18% 15% 5% 11% 6% 5% 4 3 4 5% 23%	5089
	21st to end	Night	24% 14% 4 11% 5% 4 4 4 5% 4 22%	5507
April	1st to 10th	Night	21% 12% 4 8% 6% 5% 4 3 5% 5% 26%	2100
	11th to 20th	Night	20% 13% 3 6% 5% 5% 4 3 4 5% 31%	1959
	21st to end	Night	22% 13% 5% 8% 5% 3 3 3 4 5% 30%	1800
May	1st to 10th	Night	30% 13% 3 8% 5% 4 4 2 4 4 24%	3709
	11th to 20th	Night	18% 9% 3 6% 4 5% 4 4 6% 6% 35%	3596
	21st to end	Night	16% 10% 3 7% 5% 5% 5% 4 6% 8% 33%	3948
June	1st to 10th	Night	25% 11% 3 7% 4 5% 3 3 4 6% 28%	1564
	11th to 20th	Night	17% 7% 4 6% 5% 5% 4 4 5% 7% 38%	1400
	21st to end	Night	23% 10% 4 8% 5% 4 5% 4 6% 5% 29%	1400
July	1st to 10th	Night	14% 8% 3 10% 6% 7% 6% 5% 6% 9% 26%	3160
	11th to 20th	Night	9% 5% 3 8% 5% 5% 6% 5% 8% 9% 36%	3453
	21st to end	Night	14% 7% 3 6% 4 4 4 4 7% 8% 38%	3948
August	1st to 10th	Night	15% 8% 3 8% 5% 6% 5% 4 7% 7% 30%	2850
	11th to 20th	Night	10% 7% 3 8% 5% 5% 5% 4 6% 8% 40%	2872
	21st to end	Night	12% 7% 2 6% 4 4 5% 4 6% 8% 43%	3410
September	1st to 10th	Night	16% 6% 2 7% 4 4 3 3 5% 7% 43%	2000
	11th to 20th	Night	16% 7% 3 6% 3 4 5% 3 5% 6% 42%	2026
	21st to end	Night	17% 7% 3 6% 3 4 4 3 5% 8% 41%	2293
October	1st to 10th	Night	19% 8% 3 6% 4 3 5% 3 4 7% 38%	2400
	11th to 20th	Night	18% 12% 4 9% 5% 3 4 4 5% 6% 30%	2400
	21st to end	Night	24% 13% 3 8% 5% 5% 5% 3 5% 6% 24%	2698
November	1st to 10th	Night	21% 17% 4 8% 5% 4 4 3 5% 5% 24%	2843
	11th to 20th	Night	22% 18% 6% 10% 5% 4 4 3 4 6% 18%	2940
	21st to end	Night	24% 18% 5% 12% 6% 4 4 3 4 5% 15%	2940
December	1st to 10th	Night	14% 15% 6% 16% 9% 6% 5% 3 4 5% 18%	4720
	11th to 20th	Night	17% 16% 6% 17% 7% 5% 4 3 3 4 19%	5353
	21st to end	Night	22% 14% 7% 20% 6% 4 3 2 3 4 15%	7461

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	73% 9% 10% 5% 3	4187
	11th to 20th	Day	67% 9% 9% 9% 6% 1	4710
	21st to end	Day	49% 6% 14% 20% 10% 1	5645
February	1st to 10th	Day	59% 5% 11% 14% 9% 1	1890
	11th to 20th	Day	59% 6% 10% 17% 7% 1	1851
	21st to end	Day	50% 6% 13% 19% 11% 2	1591
March	1st to 10th	Day	61% 5% 9% 17% 7% 1	4902
	11th to 20th	Day	58% 7% 16% 14% 4 1	5090
	21st to end	Day	65% 5% 9% 13% 6% 1	5666
April	1st to 10th	Day	55% 5% 10% 17% 11% 2	2520
	11th to 20th	Day	49% 5% 11% 21% 12% 1	2519
	21st to end	Day	53% 5% 12% 16% 11% 2	2600
May	1st to 10th	Day	56% 6% 12% 16% 9% 1	6257
	11th to 20th	Day	49% 8% 14% 23% 5%	6322
	21st to end	Day	47% 9% 13% 22% 8%	6948
June	1st to 10th	Day	57% 7% 11% 19% 6%	2850
	11th to 20th	Day	49% 6% 12% 23% 9% 1	3000
	21st to end	Day	54% 5% 12% 18% 10% 1	3000
July	1st to 10th	Day	49% 8% 17% 19% 7%	6745
	11th to 20th	Day	38% 7% 18% 31% 7%	6387
	21st to end	Day	38% 9% 21% 27% 6%	6730
August	1st to 10th	Day	49% 8% 13% 25% 6%	4272
	11th to 20th	Day	40% 8% 15% 26% 11%	4214
	21st to end	Day	36% 7% 16% 29% 12%	4487
September	1st to 10th	Day	39% 7% 16% 27% 10%	2400
	11th to 20th	Day	46% 5% 13% 25% 11%	2370
	21st to end	Day	44% 6% 11% 26% 12% 1	2106
October	1st to 10th	Day	47% 8% 14% 24% 7%	3260
	11th to 20th	Day	53% 7% 11% 23% 6%	3240
	21st to end	Day	51% 7% 19% 18% 5%	3445
November	1st to 10th	Day	62% 7% 10% 16% 5%	1671
	11th to 20th	Day	69% 8% 11% 9% 4	1680
	21st to end	Day	67% 11% 13% 8% 2	1680
December	1st to 10th	Day	69% 7% 8% 14% 2	2690
	11th to 20th	Day	69% 6% 10% 7% 7%	3075
	21st to end	Day	72% 7% 10% 8% 3	3723
January	1st to 10th	Night	65% 11% 11% 11% 2	8731
	11th to 20th	Night	64% 9% 9% 12% 5%	8280
	21st to end	Night	49% 10% 14% 15% 11% 1	8538
February	1st to 10th	Night	57% 5% 11% 14% 11% 2	2730
	11th to 20th	Night	60% 5% 10% 16% 8% 1	2666
	21st to end	Night	52% 5% 11% 19% 10% 2	2214
March	1st to 10th	Night	62% 5% 9% 14% 10% 1	5169
	11th to 20th	Night	61% 7% 11% 13% 7% 1	5090
	21st to end	Night	65% 4 8% 14% 9% 1	5507
April	1st to 10th	Night	59% 5% 10% 14% 11% 2	2100
	11th to 20th	Night	55% 4 8% 21% 10% 2	1959
	21st to end	Night	58% 4 8% 15% 13% 2	1800
May	1st to 10th	Night	63% 5% 7% 13% 10% 1	3714
	11th to 20th	Night	47% 7% 14% 22% 9% 1	3597
	21st to end	Night	46% 9% 17% 22% 6%	3949
June	1st to 10th	Night	56% 6% 12% 18% 8%	1564
	11th to 20th	Night	46% 6% 12% 24% 12%	1400
	21st to end	Night	56% 7% 12% 15% 9% 1	1400
July	1st to 10th	Night	52% 8% 15% 18% 7% 1	3125
	11th to 20th	Night	40% 10% 19% 23% 8%	3409
	21st to end	Night	41% 8% 20% 22% 8%	3831
August	1st to 10th	Night	51% 8% 14% 21% 6%	2839
	11th to 20th	Night	40% 9% 17% 23% 10% 1	2899
	21st to end	Night	37% 8% 17% 26% 12%	3409
September	1st to 10th	Night	40% 6% 11% 31% 11%	2000
	11th to 20th	Night	42% 6% 13% 27% 12%	2026
	21st to end	Night	41% 7% 13% 27% 12% 1	2293
October	1st to 10th	Night	49% 6% 15% 22% 7%	3920
	11th to 20th	Night	46% 8% 14% 23% 9%	3899
	21st to end	Night	52% 7% 14% 17% 9%	4346
November	1st to 10th	Night	61% 6% 12% 14% 6%	2843
	11th to 20th	Night	64% 10% 10% 12% 4	2940
	21st to end	Night	67% 10% 11% 8% 3	2940
December	1st to 10th	Night	61% 8% 14% 15% 2	4720
	11th to 20th	Night	65% 8% 9% 12% 5% 1	5355
	21st to end	Night	72% 6% 9% 9% 4	7461

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	26% 20% 33% 16% 4	248
	11th to 20th	Day	25% 18% 30% 20% 7%	430
	21st to end	Day	16% 15% 38% 27% 3	473
February	1st to 10th	Day	26% 16% 32% 20% 4 3	430
	11th to 20th	Day	21% 14% 36% 23% 6%	426
	21st to end	Day	15% 14% 34% 27% 9% 1	355
March	1st to 10th	Day	21% 16% 33% 22% 7%	505
	11th to 20th	Day	18% 16% 32% 24% 9% 1	540
	21st to end	Day	27% 13% 29% 23% 7% 1	594
April	1st to 10th	Day	27% 16% 33% 19% 5%	540
	11th to 20th	Day	24% 12% 32% 23% 8% 1	516
	21st to end	Day	24% 10% 25% 29% 11% 1	500
May	1st to 10th	Day	31% 6% 28% 24% 9% 2	655
	11th to 20th	Day	21% 7% 25% 28% 15% 3	724
	21st to end	Day	21% 7% 29% 31% 12% 1	790
June	1st to 10th	Day	28% 7% 22% 29% 11% 2	730
	11th to 20th	Day	19% 7% 26% 33% 12% 2	730
	21st to end	Day	25% 5% 21% 32% 14% 4	730
July	1st to 10th	Day	18% 4 26% 33% 16% 3	720
	11th to 20th	Day	14% 3 23% 36% 20% 4	718
	21st to end	Day	18% 5% 18% 38% 19% 2	795
August	1st to 10th	Day	13% 3 18% 37% 24% 5%	564
	11th to 20th	Day	16% 4 20% 31% 21% 8%	519
	21st to end	Day	16% 7% 21% 31% 20% 5%	583
September	1st to 10th	Day	14% 6% 28% 33% 15% 4	530
	11th to 20th	Day	21% 9% 23% 29% 16% 2	530
	21st to end	Day	19% 10% 27% 27% 12% 4	478
October	1st to 10th	Day	20% 11% 23% 27% 15% 4	420
	11th to 20th	Day	23% 14% 26% 23% 10% 3	420
	21st to end	Day	25% 15% 31% 20% 8% 2	462
November	1st to 10th	Day	26% 18% 29% 20% 6% 1	425
	11th to 20th	Day	33% 17% 27% 16% 5% 1	430
	21st to end	Day	28% 18% 31% 19% 3 1	430
December	1st to 10th	Day	31% 17% 32% 17% 4	430
	11th to 20th	Day	29% 22% 33% 13% 3	430
	21st to end	Day	22% 21% 35% 18% 3 1	321
January	1st to 10th	Night	26% 15% 34% 20% 5%	950
	11th to 20th	Night	24% 15% 33% 21% 5%	950
	21st to end	Night	16% 19% 35% 22% 7% 1	949
February	1st to 10th	Night	24% 14% 34% 23% 5% 1	840
	11th to 20th	Night	25% 13% 34% 23% 4 1	823
	21st to end	Night	18% 11% 33% 28% 9% 1	692
March	1st to 10th	Night	22% 14% 32% 29% 3	840
	11th to 20th	Night	20% 14% 31% 26% 8% 1	840
	21st to end	Night	26% 13% 27% 23% 9% 1	895
April	1st to 10th	Night	27% 12% 30% 25% 7%	630
	11th to 20th	Night	27% 8% 29% 26% 9% 1	613
	21st to end	Night	27% 8% 25% 29% 10% 1	600
May	1st to 10th	Night	31% 7% 25% 24% 12% 3	600
	11th to 20th	Night	24% 4 25% 31% 14% 2	592
	21st to end	Night	19% 6% 32% 36% 6% 1	649
June	1st to 10th	Night	28% 5% 23% 35% 7% 1	564
	11th to 20th	Night	20% 5% 26% 36% 12% 1	400
	21st to end	Night	26% 4 22% 35% 11% 2	400
July	1st to 10th	Night	18% 5% 26% 36% 13% 2	390
	11th to 20th	Night	14% 4 27% 42% 13% 1	510
	21st to end	Night	18% 5% 22% 39% 16% 1	656
August	1st to 10th	Night	16% 3 26% 36% 19% 1	598
	11th to 20th	Night	15% 5% 22% 38% 19% 1	592
	21st to end	Night	16% 5% 20% 37% 21% 2	660
September	1st to 10th	Night	17% 6% 31% 34% 12% 1	600
	11th to 20th	Night	17% 9% 21% 36% 14% 2	626
	21st to end	Night	20% 10% 29% 29% 11% 2	800
October	1st to 10th	Night	22% 9% 27% 30% 12% 1	800
	11th to 20th	Night	22% 13% 27% 26% 10% 2	800
	21st to end	Night	30% 16% 27% 22% 5%	913
November	1st to 10th	Night	30% 16% 30% 19% 4	930
	11th to 20th	Night	31% 17% 27% 18% 5% 1	950
	21st to end	Night	33% 15% 30% 19% 3 1	950
December	1st to 10th	Night	28% 17% 32% 19% 3	950
	11th to 20th	Night	27% 22% 32% 16% 3 1	950
	21st to end	Night	28% 18% 30% 20% 4 1	1045

Page generated at 2026-04-14 03:44:30. Version ac07

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