Chandler Mountain
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chandler Mountain clear sky chart from 2005-11-06 to 2026-08-15.

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	13% 6% 2 6% 5% 5% 5% 4 6% 8% 40%	143397
	Night	13% 6% 2 6% 4 4 4 3 5% 7% 44%	138519
Transparency	Day	40% 6% 12% 20% 18% 5%	147158
	Night	38% 5% 10% 21% 19% 6%	142818
Seeing	Day	18% 8% 23% 24% 19% 9%	16918
	Night	16% 7% 20% 30% 22% 4	26861

By Month

Percentage Cloud Cover SE (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	21% 10% 4 5% 3 3 3 2 4 4 41%	5258
	11th to 20th	Day	11% 5% 4 6% 3 2 2 2 3 5% 57%	5358
	21st to end	Day	24% 9% 2 5% 4 4 3 3 4 5% 36%	6436
February	1st to 10th	Day	18% 9% 3 6% 4 3 3 4 5% 6% 39%	2100
	11th to 20th	Day	21% 8% 3 6% 4 3 3 3 4 5% 39%	2048
	21st to end	Day	19% 8% 3 7% 4 4 3 3 5% 5% 39%	1730
March	1st to 10th	Day	19% 9% 3 7% 5% 5% 5% 4 4 5% 36%	4580
	11th to 20th	Day	12% 8% 2 6% 4 4 4 3 6% 6% 46%	4944
	21st to end	Day	15% 7% 3 7% 4 4 4 4 5% 6% 42%	5759
April	1st to 10th	Day	12% 6% 2 5% 4 4 5% 4 5% 8% 45%	2520
	11th to 20th	Day	12% 5% 2 6% 5% 4 4 4 5% 5% 49%	2494
	21st to end	Day	9% 5% 2 6% 5% 4 5% 4 6% 8% 46%	2530
May	1st to 10th	Day	16% 7% 3 7% 5% 5% 6% 5% 6% 8% 33%	7616
	11th to 20th	Day	14% 6% 3 7% 5% 5% 5% 4 7% 8% 35%	7626
	21st to end	Day	19% 8% 3 8% 5% 5% 5% 4 6% 6% 31%	8391
June	1st to 10th	Day	11% 6% 3 8% 6% 6% 6% 5% 7% 10% 33%	2730
	11th to 20th	Day	6% 5% 2 6% 5% 6% 6% 6% 9% 10% 40%	2730
	21st to end	Day	6% 5% 3 6% 5% 6% 6% 5% 8% 11% 39%	2730
July	1st to 10th	Day	5% 4 2 7% 7% 7% 7% 6% 9% 13% 32%	7680
	11th to 20th	Day	8% 5% 2 7% 6% 7% 8% 6% 10% 13% 29%	7680
	21st to end	Day	9% 7% 3 9% 6% 7% 7% 6% 9% 13% 25%	8340
August	1st to 10th	Day	5% 3 1 5% 4 4 5% 5% 8% 11% 47%	4040
	11th to 20th	Day	6% 4 2 6% 5% 5% 5% 5% 7% 9% 45%	3897
	21st to end	Day	7% 4 2 5% 5% 4 4 4 5% 8% 51%	4312
September	1st to 10th	Day	10% 5% 2 6% 5% 5% 6% 5% 6% 8% 41%	2325
	11th to 20th	Day	14% 5% 1 7% 5% 5% 5% 4 6% 8% 42%	2200
	21st to end	Day	11% 6% 3 5% 5% 4 4 3 5% 5% 49%	2200
October	1st to 10th	Day	11% 5% 2 4 4 4 5% 3 5% 5% 53%	2200
	11th to 20th	Day	9% 5% 2 5% 3 2 3 3 5% 7% 56%	2200
	21st to end	Day	14% 5% 1 4 3 3 4 3 5% 5% 54%	2063
November	1st to 10th	Day	13% 6% 2 6% 3 4 2 2 4 5% 53%	1852
	11th to 20th	Day	11% 5% 2 5% 4 3 3 3 4 7% 52%	1890
	21st to end	Day	15% 6% 2 6% 4 4 4 4 5% 6% 46%	1890
December	1st to 10th	Day	18% 8% 2 6% 3 2 3 3 4 6% 44%	3020
	11th to 20th	Day	17% 9% 3 7% 5% 4 4 3 4 5% 41%	3463
	21st to end	Day	19% 7% 3 6% 4 4 4 3 5% 5% 39%	4565
January	1st to 10th	Night	18% 9% 3 6% 4 4 3 3 4 5% 41%	7635
	11th to 20th	Night	14% 8% 3 6% 3 3 3 2 4 5% 48%	7583
	21st to end	Night	20% 9% 2 6% 4 3 3 2 4 4 42%	7745
February	1st to 10th	Night	16% 7% 2 7% 4 4 3 3 4 6% 43%	2520
	11th to 20th	Night	21% 8% 2 7% 4 3 2 3 3 5% 41%	2458
	21st to end	Night	19% 6% 2 6% 4 4 4 4 5% 6% 40%	2076
March	1st to 10th	Night	22% 9% 3 7% 4 5% 4 3 4 6% 33%	5493
	11th to 20th	Night	12% 6% 2 5% 3 3 3 4 5% 6% 50%	5234
	21st to end	Night	15% 7% 3 7% 5% 4 4 3 4 4 44%	5454
April	1st to 10th	Night	11% 5% 2 5% 4 4 5% 4 6% 7% 47%	2100
	11th to 20th	Night	13% 5% 2 5% 3 3 4 3 4 7% 51%	2075
	21st to end	Night	10% 5% 2 6% 5% 3 3 4 5% 7% 49%	2088
May	1st to 10th	Night	14% 8% 2 8% 5% 5% 5% 4 6% 7% 36%	5237
	11th to 20th	Night	15% 7% 3 7% 5% 5% 6% 4 6% 7% 36%	5234
	21st to end	Night	16% 8% 3 8% 5% 5% 5% 4 7% 8% 32%	5761
June	1st to 10th	Night	6% 5% 1 7% 5% 6% 6% 4 8% 11% 40%	1890
	11th to 20th	Night	4 4 2 5% 5% 5% 4 4 7% 11% 49%	1890
	21st to end	Night	5% 4 1 6% 4 4 5% 5% 8% 10% 47%	1890
July	1st to 10th	Night	5% 4 2 6% 5% 5% 6% 5% 9% 13% 40%	5278
	11th to 20th	Night	7% 6% 2 6% 4 5% 6% 5% 8% 15% 35%	5277
	21st to end	Night	8% 6% 3 8% 6% 6% 6% 5% 9% 13% 30%	5736
August	1st to 10th	Night	3 3 1 5% 4 3 5% 4 6% 12% 54%	3324
	11th to 20th	Night	6% 4 2 5% 4 4 4 4 5% 9% 53%	3240
	21st to end	Night	6% 4 2 5% 4 4 4 4 5% 8% 55%	3586
September	1st to 10th	Night	8% 5% 2 5% 5% 5% 6% 5% 7% 8% 44%	2054
	11th to 20th	Night	12% 5% 2 5% 5% 4 4 4 5% 8% 45%	2200
	21st to end	Night	8% 5% 2 6% 4 4 4 3 6% 8% 52%	2200
October	1st to 10th	Night	9% 4 3 5% 3 3 3 2 4 6% 59%	2200
	11th to 20th	Night	10% 5% 1 4 3 3 4 3 4 5% 57%	2200
	21st to end	Night	13% 6% 2 4 4 3 3 2 3 4 55%	2770
November	1st to 10th	Night	12% 4 2 5% 4 3 3 3 4 5% 55%	2666
	11th to 20th	Night	12% 6% 2 4 2 3 3 3 4 6% 55%	2730
	21st to end	Night	14% 6% 2 6% 3 4 4 3 4 6% 47%	2730
December	1st to 10th	Night	19% 7% 2 5% 4 3 3 3 4 5% 46%	4380
	11th to 20th	Night	18% 9% 3 6% 4 3 3 2 4 4 43%	4952
	21st to end	Night	21% 6% 3 6% 4 4 3 3 5% 6% 38%	6633

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	48% 3 5% 25% 17% 2	5259
	11th to 20th	Day	31% 3 4 22% 34% 7%	5358
	21st to end	Day	51% 3 5% 15% 18% 8%	6436
February	1st to 10th	Day	45% 2 6% 22% 20% 4	2100
	11th to 20th	Day	48% 3 4 20% 19% 6%	2058
	21st to end	Day	47% 3 6% 14% 22% 7%	1730
March	1st to 10th	Day	49% 4 7% 13% 18% 7%	4580
	11th to 20th	Day	38% 4 6% 18% 21% 14%	4944
	21st to end	Day	42% 4 8% 22% 20% 4	5761
April	1st to 10th	Day	37% 3 12% 23% 19% 7%	2520
	11th to 20th	Day	36% 4 6% 21% 25% 7%	2494
	21st to end	Day	34% 5% 10% 26% 22% 3	2530
May	1st to 10th	Day	46% 7% 12% 20% 13% 2	7651
	11th to 20th	Day	44% 7% 15% 22% 12% 1	7630
	21st to end	Day	51% 7% 9% 18% 14% 1	8405
June	1st to 10th	Day	43% 7% 14% 18% 16% 1	2730
	11th to 20th	Day	34% 7% 18% 24% 16% 1	2730
	21st to end	Day	36% 8% 17% 24% 11% 3	2730
July	1st to 10th	Day	37% 12% 25% 20% 7%	7615
	11th to 20th	Day	40% 13% 28% 15% 3	7611
	21st to end	Day	47% 10% 20% 18% 5%	8156
August	1st to 10th	Day	27% 7% 13% 37% 16% 1	4027
	11th to 20th	Day	31% 7% 17% 26% 15% 2	3933
	21st to end	Day	30% 6% 15% 21% 24% 4	4311
September	1st to 10th	Day	39% 6% 15% 19% 16% 6%	2325
	11th to 20th	Day	40% 5% 12% 21% 19% 3	2200
	21st to end	Day	36% 4 11% 21% 21% 7%	2200
October	1st to 10th	Day	24% 3 5% 14% 36% 17%	3590
	11th to 20th	Day	19% 2 4 17% 33% 25%	3568
	21st to end	Day	25% 3 7% 21% 33% 11%	3299
November	1st to 10th	Day	35% 3 6% 19% 27% 10%	1852
	11th to 20th	Day	33% 3 7% 21% 27% 10%	1890
	21st to end	Day	38% 4 6% 19% 26% 7%	1890
December	1st to 10th	Day	43% 2 6% 18% 26% 5%	3020
	11th to 20th	Day	47% 3 7% 19% 18% 6%	3460
	21st to end	Day	46% 4 6% 25% 17% 2	4565
January	1st to 10th	Night	46% 3 7% 24% 17% 3	7636
	11th to 20th	Night	39% 3 5% 18% 26% 8%	7583
	21st to end	Night	47% 2 3 18% 22% 7%	7745
February	1st to 10th	Night	43% 2 5% 21% 22% 7%	2520
	11th to 20th	Night	47% 2 6% 19% 18% 8%	2459
	21st to end	Night	44% 5% 6% 14% 21% 9%	2076
March	1st to 10th	Night	52% 4 6% 14% 19% 5%	5493
	11th to 20th	Night	32% 5% 7% 17% 23% 16%	5234
	21st to end	Night	43% 3 6% 24% 18% 6%	5454
April	1st to 10th	Night	35% 4 11% 23% 20% 7%	2100
	11th to 20th	Night	34% 4 5% 22% 23% 11%	2075
	21st to end	Night	34% 5% 11% 25% 21% 5%	2088
May	1st to 10th	Night	45% 8% 12% 17% 16% 2	5264
	11th to 20th	Night	45% 7% 13% 21% 12% 1	5235
	21st to end	Night	47% 9% 11% 19% 12% 1	5771
June	1st to 10th	Night	35% 7% 16% 21% 18% 2	1890
	11th to 20th	Night	27% 7% 21% 23% 18% 2	1890
	21st to end	Night	29% 8% 18% 24% 16% 4	1890
July	1st to 10th	Night	33% 11% 28% 20% 7% 1	5235
	11th to 20th	Night	35% 13% 32% 16% 4 1	5226
	21st to end	Night	43% 12% 23% 15% 6%	5606
August	1st to 10th	Night	22% 7% 16% 40% 14% 2	3313
	11th to 20th	Night	28% 7% 18% 30% 15% 3	3270
	21st to end	Night	27% 6% 14% 20% 28% 4	3586
September	1st to 10th	Night	34% 9% 17% 18% 16% 6%	2054
	11th to 20th	Night	38% 7% 10% 21% 21% 4	2200
	21st to end	Night	29% 5% 10% 24% 24% 8%	2200
October	1st to 10th	Night	21% 3 6% 18% 33% 19%	3590
	11th to 20th	Night	18% 2 4 18% 32% 26%	3571
	21st to end	Night	24% 3 3 24% 32% 14%	4471
November	1st to 10th	Night	32% 3 8% 19% 25% 13%	2666
	11th to 20th	Night	32% 2 5% 20% 28% 12%	2730
	21st to end	Night	37% 3 6% 22% 24% 8%	2730
December	1st to 10th	Night	42% 3 5% 21% 26% 4	4380
	11th to 20th	Night	45% 3 7% 17% 24% 4	4954
	21st to end	Night	45% 4 5% 27% 17% 2	6633

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	31% 17% 34% 15% 3	330
	11th to 20th	Day	29% 18% 32% 14% 5% 1	330
	21st to end	Day	28% 18% 37% 12% 3 1	363
February	1st to 10th	Day	26% 18% 35% 15% 5%	330
	11th to 20th	Day	38% 16% 34% 11% 1	330
	21st to end	Day	36% 15% 37% 8% 3	273
March	1st to 10th	Day	29% 18% 33% 15% 4 1	330
	11th to 20th	Day	25% 15% 34% 19% 6% 1	472
	21st to end	Day	21% 15% 34% 24% 6%	594
April	1st to 10th	Day	22% 12% 30% 23% 10% 2	540
	11th to 20th	Day	20% 12% 31% 23% 12% 2	532
	21st to end	Day	14% 12% 29% 27% 15% 2	540
May	1st to 10th	Day	16% 9% 30% 24% 17% 5%	543
	11th to 20th	Day	13% 6% 27% 30% 16% 9%	567
	21st to end	Day	16% 5% 21% 26% 21% 11%	617
June	1st to 10th	Day	15% 2 17% 28% 25% 13%	570
	11th to 20th	Day	9% 4 16% 27% 25% 19%	570
	21st to end	Day	10% 2 12% 27% 31% 19%	570
July	1st to 10th	Day	9% 1 8% 26% 32% 24%	565
	11th to 20th	Day	9% 1 7% 22% 34% 28%	564
	21st to end	Day	8% 3 9% 24% 35% 22%	620
August	1st to 10th	Day	12% 2 9% 26% 28% 23%	566
	11th to 20th	Day	11% 1 7% 27% 36% 18%	535
	21st to end	Day	15% 1 8% 25% 33% 19%	583
September	1st to 10th	Day	10% 2 12% 29% 34% 12%	530
	11th to 20th	Day	16% 3 11% 25% 32% 13%	530
	21st to end	Day	13% 4 16% 31% 28% 8%	530
October	1st to 10th	Day	14% 4 23% 31% 21% 8%	530
	11th to 20th	Day	12% 8% 24% 31% 22% 2	530
	21st to end	Day	19% 9% 24% 32% 14% 2	421
November	1st to 10th	Day	19% 8% 32% 24% 15% 2	330
	11th to 20th	Day	17% 11% 42% 24% 6%	330
	21st to end	Day	17% 17% 34% 22% 8% 2	330
December	1st to 10th	Day	34% 15% 30% 16% 5%	330
	11th to 20th	Day	26% 16% 30% 19% 7% 2	330
	21st to end	Day	29% 15% 30% 21% 5% 1	363
January	1st to 10th	Night	25% 15% 28% 27% 5%	840
	11th to 20th	Night	22% 16% 27% 28% 7%	840
	21st to end	Night	21% 14% 34% 28% 3	924
February	1st to 10th	Night	23% 12% 29% 31% 5%	840
	11th to 20th	Night	28% 14% 29% 27% 3	823
	21st to end	Night	25% 14% 32% 26% 3	692
March	1st to 10th	Night	21% 13% 28% 32% 6%	840
	11th to 20th	Night	19% 11% 31% 30% 9% 1	840
	21st to end	Night	19% 12% 27% 32% 10%	867
April	1st to 10th	Night	15% 11% 28% 32% 14%	630
	11th to 20th	Night	17% 8% 29% 29% 16% 1	623
	21st to end	Night	13% 7% 22% 39% 18% 2	630
May	1st to 10th	Night	12% 5% 23% 34% 23% 3	630
	11th to 20th	Night	13% 3 17% 35% 26% 6%	621
	21st to end	Night	12% 3 10% 33% 36% 7%	683
June	1st to 10th	Night	10% 1 9% 38% 37% 5%	630
	11th to 20th	Night	7% 2 9% 31% 37% 15%	630
	21st to end	Night	8% 6% 30% 45% 10%	630
July	1st to 10th	Night	9% 1 4 20% 48% 18%	615
	11th to 20th	Night	6% 2 4 19% 50% 19%	612
	21st to end	Night	7% 1 6% 25% 45% 16%	690
August	1st to 10th	Night	6% 1 3 28% 47% 14%	627
	11th to 20th	Night	10% 1 5% 25% 47% 12%	606
	21st to end	Night	14% 1 4 29% 41% 11%	660
September	1st to 10th	Night	10% 1 8% 29% 42% 10%	654
	11th to 20th	Night	14% 1 10% 27% 42% 6%	800
	21st to end	Night	11% 2 13% 30% 41% 3	800
October	1st to 10th	Night	11% 3 18% 35% 27% 7%	800
	11th to 20th	Night	12% 6% 20% 36% 25% 2	800
	21st to end	Night	17% 7% 18% 34% 22% 2	880
November	1st to 10th	Night	15% 6% 20% 35% 22% 2	820
	11th to 20th	Night	15% 7% 28% 36% 13% 1	840
	21st to end	Night	18% 11% 24% 35% 12% 1	840
December	1st to 10th	Night	24% 10% 25% 30% 9% 1	840
	11th to 20th	Night	23% 10% 26% 30% 10% 1	840
	21st to end	Night	27% 11% 23% 32% 7%	924

Page generated at 2026-08-12 02:37:15. Version ac07

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