Chandler
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chandler clear sky chart from 2005-11-06 to 2026-03-18.

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% 10% 3 7% 5% 4 5% 4 5% 7% 33%	131226
	Night	16% 10% 3 8% 5% 4 4 3 5% 6% 35%	130492
Transparency	Day	48% 4 14% 22% 10% 1	134689
	Night	51% 4 12% 21% 10% 1	135120
Seeing	Day	21% 4 18% 33% 19% 5%	18353
	Night	22% 5% 24% 31% 15% 2	26030

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% 17% 5% 9% 4 4 4 4 4 6% 30%	4093
	11th to 20th	Day	12% 14% 7% 11% 5% 4 6% 5% 7% 7% 23%	4084
	21st to end	Day	4 8% 4 10% 6% 4 4 5% 7% 7% 42%	5347
February	1st to 10th	Day	9% 12% 5% 9% 6% 5% 5% 4 4 6% 35%	1890
	11th to 20th	Day	11% 15% 4 10% 6% 4 4 3 6% 5% 31%	1843
	21st to end	Day	8% 11% 5% 7% 5% 6% 5% 5% 6% 6% 36%	1557
March	1st to 10th	Day	17% 12% 4 9% 5% 4 4 3 5% 6% 31%	4900
	11th to 20th	Day	16% 18% 5% 10% 5% 4 4 3 4 5% 24%	5057
	21st to end	Day	22% 14% 3 8% 5% 4 4 3 4 5% 27%	5466
April	1st to 10th	Day	20% 11% 3 8% 5% 4 5% 3 5% 5% 30%	2389
	11th to 20th	Day	18% 10% 3 7% 6% 4 5% 3 5% 7% 32%	2562
	21st to end	Day	24% 10% 3 8% 5% 4 4 3 5% 5% 31%	2600
May	1st to 10th	Day	22% 12% 4 9% 6% 5% 4 3 4 6% 25%	6242
	11th to 20th	Day	22% 10% 3 7% 4 4 4 3 5% 5% 31%	6318
	21st to end	Day	23% 9% 3 7% 4 4 4 3 4 6% 32%	7259
June	1st to 10th	Day	23% 9% 2 7% 5% 5% 4 3 6% 6% 29%	3000
	11th to 20th	Day	17% 7% 2 6% 6% 4 4 5% 6% 8% 34%	3000
	21st to end	Day	26% 8% 3 7% 5% 4 5% 4 6% 7% 25%	3000
July	1st to 10th	Day	12% 6% 3 7% 5% 5% 7% 5% 9% 9% 33%	6820
	11th to 20th	Day	11% 6% 2 6% 5% 5% 6% 5% 7% 9% 37%	6820
	21st to end	Day	13% 6% 2 6% 4 4 4 4 6% 8% 43%	7155
August	1st to 10th	Day	15% 6% 2 6% 5% 5% 6% 5% 7% 9% 35%	4305
	11th to 20th	Day	15% 6% 2 6% 5% 5% 5% 4 7% 7% 38%	4175
	21st to end	Day	12% 5% 2 5% 4 5% 4 3 6% 8% 45%	4624
September	1st to 10th	Day	16% 6% 2 6% 4 5% 4 3 6% 8% 41%	2400
	11th to 20th	Day	18% 5% 2 5% 4 4 4 3 5% 6% 43%	2302
	21st to end	Day	18% 6% 2 6% 4 4 5% 3 5% 6% 42%	2153
October	1st to 10th	Day	16% 7% 2 6% 4 5% 4 3 5% 7% 41%	2000
	11th to 20th	Day	20% 10% 2 6% 5% 4 4 4 4 7% 34%	2000
	21st to end	Day	22% 9% 3 7% 5% 4 4 3 6% 7% 31%	2060
November	1st to 10th	Day	20% 8% 3 6% 5% 5% 5% 4 6% 7% 33%	1645
	11th to 20th	Day	21% 9% 3 7% 4 5% 5% 4 5% 5% 30%	1680
	21st to end	Day	20% 12% 3 8% 5% 5% 5% 3 5% 6% 28%	1680
December	1st to 10th	Day	14% 13% 3 7% 4 5% 5% 3 5% 5% 35%	2559
	11th to 20th	Day	20% 10% 3 9% 4 5% 5% 3 4 6% 32%	2688
	21st to end	Day	16% 12% 4 10% 5% 4 3 3 3 5% 35%	3553
January	1st to 10th	Night	12% 17% 5% 10% 5% 5% 5% 3 5% 6% 26%	8831
	11th to 20th	Night	15% 12% 5% 10% 5% 5% 5% 4 5% 6% 28%	8831
	21st to end	Night	4 8% 3 9% 5% 4 4 3 6% 6% 46%	8826
February	1st to 10th	Night	8% 12% 5% 9% 5% 5% 5% 4 6% 5% 36%	2730
	11th to 20th	Night	13% 13% 4 9% 6% 5% 5% 3 5% 4 33%	2663
	21st to end	Night	11% 12% 5% 7% 6% 5% 4 4 5% 6% 35%	2249
March	1st to 10th	Night	17% 13% 4 8% 5% 5% 4 4 4 6% 30%	5148
	11th to 20th	Night	20% 15% 5% 11% 5% 4 4 4 4 4 23%	5057
	21st to end	Night	24% 14% 4 8% 4 3 3 2 3 5% 28%	5466
April	1st to 10th	Night	22% 11% 3 8% 6% 3 4 2 5% 5% 31%	2010
	11th to 20th	Night	19% 9% 3 8% 4 3 4 3 6% 7% 35%	1783
	21st to end	Night	24% 10% 3 6% 4 3 3 2 4 5% 36%	1800
May	1st to 10th	Night	20% 11% 4 8% 4 4 4 3 4 5% 33%	3713
	11th to 20th	Night	21% 8% 3 6% 3 4 4 3 4 6% 38%	3596
	21st to end	Night	19% 9% 3 7% 5% 4 4 3 5% 7% 33%	3679
June	1st to 10th	Night	24% 8% 3 6% 4 4 3 3 6% 6% 32%	1400
	11th to 20th	Night	19% 6% 3 6% 4 5% 4 3 6% 7% 37%	1400
	21st to end	Night	26% 7% 3 6% 5% 4 4 5% 5% 5% 30%	1400
July	1st to 10th	Night	15% 9% 3 6% 5% 4 5% 3 6% 9% 35%	3160
	11th to 20th	Night	13% 5% 2 5% 5% 5% 5% 5% 7% 9% 39%	3160
	21st to end	Night	10% 6% 2 6% 4 4 4 4 6% 6% 48%	3673
August	1st to 10th	Night	15% 6% 3 8% 5% 5% 5% 4 5% 7% 36%	2844
	11th to 20th	Night	14% 6% 3 7% 4 5% 5% 3 6% 7% 42%	2872
	21st to end	Night	13% 5% 2 5% 4 4 4 3 5% 7% 48%	3274
September	1st to 10th	Night	17% 6% 2 4 4 3 4 2 4 6% 46%	2000
	11th to 20th	Night	16% 5% 2 4 3 4 3 4 5% 5% 48%	2095
	21st to end	Night	17% 6% 2 5% 3 4 3 3 5% 5% 47%	2246
October	1st to 10th	Night	18% 7% 2 6% 4 4 4 4 4 6% 42%	2400
	11th to 20th	Night	19% 9% 3 6% 4 4 4 3 5% 5% 37%	2400
	21st to end	Night	23% 8% 2 6% 4 4 4 3 5% 5% 36%	2779
November	1st to 10th	Night	20% 8% 3 7% 5% 4 5% 4 5% 6% 36%	2873
	11th to 20th	Night	21% 10% 3 8% 4 4 4 3 5% 6% 32%	2940
	21st to end	Night	21% 12% 3 7% 4 4 4 4 5% 6% 29%	2940
December	1st to 10th	Night	14% 12% 4 7% 5% 4 3 3 4 6% 40%	4846
	11th to 20th	Night	21% 10% 3 7% 4 3 4 3 5% 6% 35%	5741
	21st to end	Night	17% 11% 5% 11% 5% 4 4 3 5% 5% 30%	7667

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	58% 3 11% 18% 10%	4094
	11th to 20th	Day	58% 5% 13% 15% 9%	4110
	21st to end	Day	40% 4 12% 24% 19%	5347
February	1st to 10th	Day	51% 3 11% 20% 12% 2	1890
	11th to 20th	Day	54% 3 10% 20% 12% 1	1851
	21st to end	Day	46% 4 9% 24% 14% 3	1557
March	1st to 10th	Day	54% 2 13% 18% 13% 1	4904
	11th to 20th	Day	60% 3 11% 15% 8% 4	5057
	21st to end	Day	59% 3 6% 21% 10% 1	5466
April	1st to 10th	Day	52% 3 8% 19% 16% 2	2389
	11th to 20th	Day	48% 4 10% 24% 13% 1	2562
	21st to end	Day	53% 2 11% 18% 12% 3	2600
May	1st to 10th	Day	57% 3 10% 21% 9%	6254
	11th to 20th	Day	53% 4 14% 22% 6% 1	6322
	21st to end	Day	51% 4 12% 23% 9% 1	7259
June	1st to 10th	Day	54% 3 12% 21% 9% 1	3000
	11th to 20th	Day	46% 5% 13% 25% 10% 1	3000
	21st to end	Day	55% 5% 13% 19% 8%	3000
July	1st to 10th	Day	42% 7% 19% 22% 10%	6745
	11th to 20th	Day	40% 6% 22% 25% 7%	6739
	21st to end	Day	36% 6% 21% 28% 9%	6951
August	1st to 10th	Day	41% 7% 22% 22% 8%	4291
	11th to 20th	Day	41% 6% 20% 24% 8% 1	4214
	21st to end	Day	35% 5% 19% 26% 15%	4624
September	1st to 10th	Day	40% 4 17% 28% 11%	2400
	11th to 20th	Day	41% 3 15% 29% 11% 1	2302
	21st to end	Day	42% 4 12% 29% 12% 1	2153
October	1st to 10th	Day	42% 4 17% 27% 9% 1	3260
	11th to 20th	Day	50% 5% 13% 25% 7% 1	3240
	21st to end	Day	43% 4 17% 23% 13% 1	3303
November	1st to 10th	Day	49% 4 15% 20% 11% 1	1645
	11th to 20th	Day	55% 4 13% 18% 9% 1	1680
	21st to end	Day	59% 4 13% 15% 9%	1680
December	1st to 10th	Day	52% 5% 9% 25% 9%	2559
	11th to 20th	Day	54% 3 11% 19% 12% 1	2688
	21st to end	Day	55% 3 11% 21% 7% 2	3553
January	1st to 10th	Night	62% 3 11% 20% 4	8831
	11th to 20th	Night	59% 5% 10% 17% 9% 1	8880
	21st to end	Night	42% 3 12% 23% 19% 1	8826
February	1st to 10th	Night	50% 4 10% 19% 15% 3	2730
	11th to 20th	Night	55% 3 9% 20% 12% 1	2666
	21st to end	Night	50% 4 8% 21% 15% 2	2249
March	1st to 10th	Night	57% 3 8% 18% 11% 2	5157
	11th to 20th	Night	65% 3 7% 14% 9% 2	5058
	21st to end	Night	60% 2 6% 15% 14% 2	5466
April	1st to 10th	Night	56% 3 9% 15% 15% 3	2010
	11th to 20th	Night	51% 3 8% 23% 13% 2	1783
	21st to end	Night	54% 2 8% 18% 16% 2	1800
May	1st to 10th	Night	57% 3 7% 19% 11% 3	3718
	11th to 20th	Night	50% 3 9% 26% 11% 1	3597
	21st to end	Night	51% 5% 12% 22% 10% 1	3680
June	1st to 10th	Night	54% 3 12% 21% 10%	1400
	11th to 20th	Night	47% 5% 13% 21% 12% 2	1400
	21st to end	Night	55% 5% 14% 16% 9% 1	1400
July	1st to 10th	Night	46% 7% 17% 21% 9%	3125
	11th to 20th	Night	39% 8% 19% 29% 4	3118
	21st to end	Night	37% 8% 21% 25% 10%	3564
August	1st to 10th	Night	49% 7% 16% 22% 7%	2833
	11th to 20th	Night	40% 6% 19% 26% 8%	2899
	21st to end	Night	34% 6% 19% 28% 13%	3273
September	1st to 10th	Night	41% 4 15% 31% 9% 1	2000
	11th to 20th	Night	38% 4 16% 28% 13% 1	2095
	21st to end	Night	40% 4 12% 27% 15% 1	2246
October	1st to 10th	Night	47% 4 13% 27% 8% 1	3920
	11th to 20th	Night	43% 3 12% 30% 12%	3899
	21st to end	Night	45% 6% 16% 22% 11%	4489
November	1st to 10th	Night	50% 4 15% 21% 9% 1	2873
	11th to 20th	Night	55% 3 11% 21% 9% 1	2940
	21st to end	Night	59% 4 13% 16% 8%	2940
December	1st to 10th	Night	49% 4 12% 25% 11%	4846
	11th to 20th	Night	53% 4 9% 23% 10% 1	5742
	21st to end	Night	58% 3 13% 19% 6%	7667

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	27% 7% 20% 32% 11% 3	220
	11th to 20th	Day	25% 6% 25% 24% 14% 6%	220
	21st to end	Day	11% 3 31% 35% 15% 4	379
February	1st to 10th	Day	18% 4 27% 33% 13% 5%	430
	11th to 20th	Day	16% 3 29% 34% 14% 3	426
	21st to end	Day	15% 3 25% 35% 19% 3	355
March	1st to 10th	Day	19% 5% 23% 36% 14% 2	529
	11th to 20th	Day	19% 4 25% 28% 18% 6%	528
	21st to end	Day	24% 5% 20% 34% 14% 3	550
April	1st to 10th	Day	27% 5% 20% 33% 14% 1	500
	11th to 20th	Day	23% 5% 18% 36% 16% 3	492
	21st to end	Day	25% 6% 16% 30% 18% 4	500
May	1st to 10th	Day	27% 3 12% 35% 20% 4	652
	11th to 20th	Day	23% 4 15% 28% 23% 6%	724
	21st to end	Day	24% 3 14% 39% 18% 3	790
June	1st to 10th	Day	28% 3 12% 31% 22% 4	730
	11th to 20th	Day	19% 2 15% 36% 24% 5%	730
	21st to end	Day	26% 3 9% 36% 21% 4	730
July	1st to 10th	Day	19% 2 11% 38% 25% 5%	720
	11th to 20th	Day	13% 2 10% 41% 31% 3	718
	21st to end	Day	19% 3 9% 35% 27% 7%	795
August	1st to 10th	Day	12% 2 9% 33% 35% 9%	570
	11th to 20th	Day	15% 2 6% 35% 32% 10%	519
	21st to end	Day	14% 2 12% 35% 28% 9%	583
September	1st to 10th	Day	17% 2 16% 39% 21% 5%	530
	11th to 20th	Day	21% 3 17% 34% 20% 5%	530
	21st to end	Day	18% 4 22% 33% 17% 6%	503
October	1st to 10th	Day	16% 3 21% 36% 21% 3	420
	11th to 20th	Day	26% 4 23% 27% 15% 6%	420
	21st to end	Day	26% 4 27% 30% 12% 2	462
November	1st to 10th	Day	25% 7% 27% 29% 11% 2	425
	11th to 20th	Day	27% 7% 27% 25% 10% 3	430
	21st to end	Day	25% 10% 31% 25% 7% 2	430
December	1st to 10th	Day	23% 7% 27% 31% 10% 2	351
	11th to 20th	Day	22% 8% 36% 23% 9% 1	220
	21st to end	Day	19% 10% 28% 26% 12% 4	242
January	1st to 10th	Night	26% 6% 33% 25% 10% 1	950
	11th to 20th	Night	22% 8% 31% 28% 10% 1	950
	21st to end	Night	16% 7% 35% 30% 12% 1	954
February	1st to 10th	Night	17% 5% 34% 32% 11% 2	840
	11th to 20th	Night	22% 4 28% 35% 10% 2	823
	21st to end	Night	18% 4 24% 37% 15% 2	692
March	1st to 10th	Night	19% 4 29% 35% 11% 1	840
	11th to 20th	Night	22% 5% 27% 32% 13% 2	829
	21st to end	Night	23% 6% 26% 33% 11% 2	880
April	1st to 10th	Night	30% 3 23% 32% 10% 1	610
	11th to 20th	Night	24% 5% 22% 32% 16% 2	594
	21st to end	Night	29% 3 17% 32% 15% 3	600
May	1st to 10th	Night	28% 4 12% 33% 20% 3	600
	11th to 20th	Night	23% 4 11% 33% 24% 5%	592
	21st to end	Night	20% 3 20% 38% 17% 2	536
June	1st to 10th	Night	29% 3 11% 32% 24% 1	400
	11th to 20th	Night	21% 4 9% 37% 25% 5%	400
	21st to end	Night	28% 3 8% 38% 21% 3	400
July	1st to 10th	Night	19% 3 9% 37% 27% 5%	390
	11th to 20th	Night	16% 2 11% 38% 29% 3	388
	21st to end	Night	19% 3 7% 39% 29% 3	537
August	1st to 10th	Night	15% 2 8% 38% 33% 5%	598
	11th to 20th	Night	17% 2 8% 36% 31% 6%	592
	21st to end	Night	15% 3 10% 38% 28% 6%	660
September	1st to 10th	Night	21% 4 19% 35% 18% 4	600
	11th to 20th	Night	19% 3 19% 34% 23% 2	695
	21st to end	Night	19% 4 26% 33% 17% 2	800
October	1st to 10th	Night	20% 4 27% 33% 15% 1	800
	11th to 20th	Night	24% 6% 23% 32% 14% 2	800
	21st to end	Night	27% 5% 30% 26% 12% 1	905
November	1st to 10th	Night	24% 8% 32% 27% 8% 1	930
	11th to 20th	Night	26% 11% 28% 25% 8% 2	950
	21st to end	Night	28% 7% 30% 27% 7% 1	950
December	1st to 10th	Night	26% 8% 34% 24% 8% 1	950
	11th to 20th	Night	26% 12% 35% 21% 5% 1	950
	21st to end	Night	25% 8% 33% 23% 9% 1	1045

Page generated at 2026-03-15 02:23:53. Version ac07

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