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 2024-01-16.

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	18% 9% 3 6% 6% 3 5% 5% 5% 7% 33%	74705
	Night	18% 9% 4 6% 6% 3 4 4 5% 6% 35%	75105
Transparency	Day	49% 4 13% 22% 10% 1	74394
	Night	52% 3 12% 21% 10% 1	74952
Seeing	Day	22% 3 17% 34% 19% 4	15465
	Night	23% 5% 24% 31% 14% 2	23145

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	15% 14% 5% 8% 6% 3 6% 4 4 4 30%	1327
	11th to 20th	Day	17% 12% 5% 7% 6% 4 5% 5% 5% 4 30%	1295
	21st to end	Day	10% 8% 4 7% 6% 4 4 5% 6% 8% 39%	1647
February	1st to 10th	Day	10% 12% 5% 7% 7% 4 5% 5% 4 6% 34%	1620
	11th to 20th	Day	11% 15% 4 9% 8% 3 5% 4 6% 5% 30%	1573
	21st to end	Day	9% 10% 6% 6% 5% 4 6% 6% 6% 6% 36%	1332
March	1st to 10th	Day	13% 12% 5% 8% 5% 3 5% 4 5% 6% 33%	1917
	11th to 20th	Day	13% 10% 5% 6% 7% 4 6% 5% 5% 6% 33%	1980
	21st to end	Day	14% 12% 4 7% 6% 3 5% 4 4 4 35%	2178
April	1st to 10th	Day	21% 10% 3 7% 6% 3 5% 4 5% 6% 29%	2149
	11th to 20th	Day	18% 9% 3 6% 6% 3 5% 4 5% 7% 34%	2302
	21st to end	Day	24% 9% 3 6% 6% 3 4 4 5% 6% 31%	2340
May	1st to 10th	Day	23% 10% 3 6% 6% 4 5% 4 4 6% 30%	2473
	11th to 20th	Day	22% 8% 4 6% 5% 4 5% 5% 4 5% 32%	2478
	21st to end	Day	20% 7% 3 6% 6% 3 5% 5% 4 7% 35%	2839
June	1st to 10th	Day	24% 9% 3 7% 6% 4 4 4 5% 6% 29%	2700
	11th to 20th	Day	19% 7% 2 5% 6% 3 5% 5% 5% 8% 34%	2700
	21st to end	Day	26% 8% 3 6% 6% 3 5% 6% 6% 7% 24%	2700
July	1st to 10th	Day	17% 6% 3 6% 5% 4 7% 6% 8% 9% 28%	2700
	11th to 20th	Day	13% 6% 3 6% 6% 4 6% 7% 6% 8% 36%	2700
	21st to end	Day	18% 7% 3 7% 5% 3 4 6% 6% 7% 33%	2833
August	1st to 10th	Day	15% 5% 2 6% 6% 4 6% 6% 8% 9% 33%	2367
	11th to 20th	Day	16% 7% 3 5% 7% 3 6% 5% 7% 8% 34%	2304
	21st to end	Day	14% 6% 3 6% 6% 5% 6% 6% 6% 8% 36%	2552
September	1st to 10th	Day	17% 6% 2 6% 5% 4 4 4 5% 8% 40%	2160
	11th to 20th	Day	20% 5% 3 4 5% 3 4 4 5% 6% 42%	2072
	21st to end	Day	19% 6% 2 5% 5% 3 4 4 6% 6% 41%	1939
October	1st to 10th	Day	16% 7% 2 5% 5% 4 5% 3 5% 7% 40%	1800
	11th to 20th	Day	21% 10% 2 6% 5% 2 4 5% 4 7% 34%	1800
	21st to end	Day	23% 9% 3 6% 5% 2 4 5% 5% 7% 30%	1855
November	1st to 10th	Day	19% 7% 3 5% 5% 4 5% 5% 5% 6% 35%	1485
	11th to 20th	Day	21% 9% 3 7% 5% 3 6% 6% 5% 6% 31%	1520
	21st to end	Day	19% 11% 4 7% 6% 4 5% 4 4 7% 29%	1520
December	1st to 10th	Day	20% 11% 3 7% 5% 4 5% 4 5% 5% 31%	1449
	11th to 20th	Day	17% 11% 4 9% 7% 4 6% 4 4 5% 29%	1668
	21st to end	Day	16% 11% 4 8% 6% 2 4 4 4 5% 37%	2431
January	1st to 10th	Night	14% 14% 5% 8% 7% 5% 5% 4 5% 6% 28%	2838
	11th to 20th	Night	15% 12% 4 7% 6% 4 5% 5% 5% 5% 33%	2787
	21st to end	Night	11% 9% 4 6% 6% 3 4 5% 6% 7% 39%	2706
February	1st to 10th	Night	9% 11% 5% 8% 6% 4 5% 5% 5% 6% 36%	2340
	11th to 20th	Night	14% 13% 4 8% 7% 4 5% 4 5% 4 32%	2273
	21st to end	Night	12% 11% 5% 7% 6% 4 5% 5% 5% 6% 33%	1924
March	1st to 10th	Night	14% 12% 5% 8% 5% 3 5% 4 5% 5% 33%	2030
	11th to 20th	Night	15% 11% 4 7% 6% 4 5% 5% 6% 5% 33%	1979
	21st to end	Night	15% 10% 4 7% 6% 3 4 4 5% 5% 36%	2178
April	1st to 10th	Night	23% 10% 4 7% 7% 3 4 3 5% 5% 30%	1810
	11th to 20th	Night	19% 8% 3 7% 5% 3 4 4 5% 7% 36%	1603
	21st to end	Night	25% 9% 3 6% 4 2 3 3 4 5% 35%	1620
May	1st to 10th	Night	23% 9% 4 6% 5% 3 3 4 3 6% 34%	1486
	11th to 20th	Night	19% 7% 4 6% 4 3 5% 4 5% 6% 38%	1416
	21st to end	Night	17% 7% 2 5% 6% 4 5% 4 6% 9% 35%	1449
June	1st to 10th	Night	25% 8% 3 5% 4 3 4 4 5% 6% 33%	1260
	11th to 20th	Night	20% 6% 3 5% 5% 4 5% 4 5% 7% 37%	1260
	21st to end	Night	26% 6% 3 5% 6% 3 5% 5% 6% 5% 30%	1260
July	1st to 10th	Night	16% 7% 2 5% 5% 4 5% 5% 6% 9% 36%	1260
	11th to 20th	Night	16% 5% 2 5% 6% 3 5% 5% 5% 8% 40%	1260
	21st to end	Night	15% 7% 3 6% 5% 3 4 7% 7% 8% 35%	1466
August	1st to 10th	Night	15% 5% 3 5% 5% 3 5% 6% 5% 8% 40%	1569
	11th to 20th	Night	17% 6% 3 6% 5% 4 5% 4 6% 8% 38%	1596
	21st to end	Night	13% 7% 3 5% 5% 4 4 5% 5% 7% 43%	1804
September	1st to 10th	Night	17% 7% 3 4 4 2 4 4 4 6% 45%	1800
	11th to 20th	Night	17% 6% 3 4 4 3 4 4 4 5% 47%	1885
	21st to end	Night	17% 5% 2 4 4 3 3 4 5% 6% 47%	2020
October	1st to 10th	Night	19% 7% 2 5% 4 3 4 5% 4 6% 41%	2160
	11th to 20th	Night	21% 8% 3 6% 5% 3 4 4 5% 6% 36%	2160
	21st to end	Night	24% 8% 2 5% 5% 3 3 4 5% 5% 37%	2500
November	1st to 10th	Night	21% 7% 3 6% 5% 3 5% 4 5% 5% 36%	2593
	11th to 20th	Night	21% 9% 4 6% 5% 3 4 4 5% 6% 33%	2660
	21st to end	Night	20% 12% 4 6% 5% 3 4 5% 5% 6% 29%	2660
December	1st to 10th	Night	21% 12% 3 6% 6% 2 5% 4 4 6% 31%	2726
	11th to 20th	Night	23% 11% 4 7% 6% 3 5% 3 4 5% 28%	3531
	21st to end	Night	17% 10% 4 9% 6% 3 5% 4 5% 5% 31%	5236

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	61% 3 9% 16% 11% 1	1328
	11th to 20th	Day	58% 4 9% 17% 12%	1295
	21st to end	Day	44% 4 9% 26% 16% 1	1647
February	1st to 10th	Day	52% 3 11% 21% 11% 2	1620
	11th to 20th	Day	55% 3 9% 21% 11% 1	1581
	21st to end	Day	45% 5% 9% 24% 14% 3	1332
March	1st to 10th	Day	50% 3 11% 23% 12% 2	1921
	11th to 20th	Day	49% 3 11% 20% 13% 4	1980
	21st to end	Day	51% 3 10% 20% 13% 4	2178
April	1st to 10th	Day	52% 3 8% 19% 16% 2	2149
	11th to 20th	Day	47% 4 10% 24% 13% 1	2302
	21st to end	Day	53% 2 11% 18% 12% 3	2340
May	1st to 10th	Day	55% 4 8% 19% 13% 1	2473
	11th to 20th	Day	52% 2 11% 23% 11% 2	2482
	21st to end	Day	47% 6% 13% 22% 10% 2	2839
June	1st to 10th	Day	56% 3 11% 21% 9% 1	2700
	11th to 20th	Day	46% 5% 12% 26% 10% 1	2700
	21st to end	Day	56% 5% 13% 19% 8%	2700
July	1st to 10th	Day	46% 5% 19% 21% 8%	2625
	11th to 20th	Day	41% 4 18% 26% 11%	2619
	21st to end	Day	47% 5% 17% 24% 7%	2675
August	1st to 10th	Day	43% 5% 21% 26% 5%	2353
	11th to 20th	Day	45% 5% 21% 22% 6% 1	2304
	21st to end	Day	41% 4 19% 27% 9%	2552
September	1st to 10th	Day	42% 4 16% 27% 11%	2160
	11th to 20th	Day	43% 3 12% 29% 12% 1	2072
	21st to end	Day	43% 4 12% 28% 12% 1	1939
October	1st to 10th	Day	43% 3 15% 27% 11% 2	1800
	11th to 20th	Day	48% 3 17% 24% 7% 1	1800
	21st to end	Day	52% 3 14% 21% 9% 1	1855
November	1st to 10th	Day	48% 3 16% 20% 11% 1	1485
	11th to 20th	Day	54% 4 14% 19% 9% 1	1520
	21st to end	Day	58% 4 13% 16% 9%	1520
December	1st to 10th	Day	56% 4 12% 20% 7%	1449
	11th to 20th	Day	59% 3 10% 19% 9%	1668
	21st to end	Day	54% 2 13% 19% 9% 3	2431
January	1st to 10th	Night	61% 4 11% 16% 7% 1	2838
	11th to 20th	Night	55% 5% 12% 18% 9% 1	2787
	21st to end	Night	47% 3 10% 24% 15% 1	2706
February	1st to 10th	Night	50% 3 10% 19% 14% 4	2340
	11th to 20th	Night	56% 3 9% 20% 11% 1	2276
	21st to end	Night	52% 4 7% 21% 15% 2	1924
March	1st to 10th	Night	54% 4 7% 21% 12% 2	2039
	11th to 20th	Night	53% 2 8% 22% 13% 2	1980
	21st to end	Night	52% 2 11% 19% 13% 3	2178
April	1st to 10th	Night	57% 3 9% 15% 14% 2	1810
	11th to 20th	Night	50% 3 8% 24% 14% 2	1603
	21st to end	Night	54% 2 8% 18% 16% 2	1620
May	1st to 10th	Night	54% 3 8% 21% 13% 2	1486
	11th to 20th	Night	49% 3 9% 26% 12% 1	1417
	21st to end	Night	47% 5% 13% 22% 12% 2	1450
June	1st to 10th	Night	53% 3 12% 22% 10%	1260
	11th to 20th	Night	47% 5% 13% 22% 12% 1	1260
	21st to end	Night	55% 5% 15% 16% 9% 1	1260
July	1st to 10th	Night	44% 5% 18% 25% 7%	1225
	11th to 20th	Night	43% 5% 15% 28% 9% 1	1218
	21st to end	Night	48% 7% 15% 22% 9%	1385
August	1st to 10th	Night	44% 6% 19% 24% 7%	1558
	11th to 20th	Night	44% 5% 18% 24% 8%	1596
	21st to end	Night	41% 4 18% 28% 9%	1804
September	1st to 10th	Night	43% 4 14% 30% 9% 1	1800
	11th to 20th	Night	40% 4 13% 28% 13% 1	1885
	21st to end	Night	40% 4 13% 27% 15% 1	2020
October	1st to 10th	Night	45% 3 12% 26% 12% 2	2160
	11th to 20th	Night	49% 3 14% 25% 9%	2160
	21st to end	Night	51% 3 13% 22% 10% 1	2500
November	1st to 10th	Night	51% 3 15% 20% 10% 2	2593
	11th to 20th	Night	54% 3 12% 21% 9% 1	2660
	21st to end	Night	59% 4 14% 15% 8%	2660
December	1st to 10th	Night	57% 3 13% 18% 7%	2726
	11th to 20th	Night	63% 2 8% 18% 8% 1	3532
	21st to end	Night	58% 2 14% 19% 7%	5236

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% 6% 19% 34% 11% 3	180
	11th to 20th	Day	28% 4 24% 27% 12% 5%	170
	21st to end	Day	14% 3 29% 34% 15% 4	293
February	1st to 10th	Day	21% 4 28% 35% 11% 2	340
	11th to 20th	Day	17% 3 30% 35% 14% 2	336
	21st to end	Day	15% 1 24% 36% 21% 3	280
March	1st to 10th	Day	20% 5% 20% 38% 14% 3	412
	11th to 20th	Day	16% 4 25% 30% 19% 6%	420
	21st to end	Day	23% 4 21% 34% 15% 4	462
April	1st to 10th	Day	27% 5% 20% 33% 14% 1	420
	11th to 20th	Day	23% 4 18% 36% 16% 4	412
	21st to end	Day	26% 4 15% 30% 20% 5%	420
May	1st to 10th	Day	28% 3 11% 34% 19% 4	556
	11th to 20th	Day	23% 4 15% 29% 24% 6%	624
	21st to end	Day	23% 3 15% 40% 16% 3	680
June	1st to 10th	Day	31% 3 12% 31% 20% 3	630
	11th to 20th	Day	21% 2 13% 36% 23% 6%	630
	21st to end	Day	28% 3 8% 34% 23% 5%	630
July	1st to 10th	Day	21% 1 10% 38% 25% 5%	620
	11th to 20th	Day	14% 1 9% 40% 32% 3	618
	21st to end	Day	20% 2 7% 34% 28% 8%	685
August	1st to 10th	Day	13% 2 8% 32% 35% 9%	486
	11th to 20th	Day	16% 2 6% 37% 30% 10%	439
	21st to end	Day	15% 2 12% 35% 28% 8%	495
September	1st to 10th	Day	19% 1 14% 39% 21% 5%	450
	11th to 20th	Day	24% 3 17% 35% 16% 5%	450
	21st to end	Day	20% 4 22% 33% 17% 4	429
October	1st to 10th	Day	16% 3 22% 36% 21% 2	360
	11th to 20th	Day	27% 3 23% 27% 15% 5%	360
	21st to end	Day	27% 4 27% 29% 11% 2	396
November	1st to 10th	Day	24% 7% 26% 30% 11% 2	365
	11th to 20th	Day	28% 6% 27% 26% 10% 3	370
	21st to end	Day	24% 8% 32% 26% 7% 1	370
December	1st to 10th	Day	24% 6% 26% 32% 10% 2	299
	11th to 20th	Day	23% 6% 37% 24% 8% 2	180
	21st to end	Day	19% 11% 30% 26% 11% 3	198
January	1st to 10th	Night	25% 5% 35% 26% 9% 1	850
	11th to 20th	Night	23% 7% 32% 29% 8% 1	829
	21st to end	Night	18% 5% 34% 29% 13% 1	814
February	1st to 10th	Night	18% 4 35% 32% 9% 1	720
	11th to 20th	Night	23% 3 29% 34% 9% 2	703
	21st to end	Night	19% 4 24% 35% 15% 3	592
March	1st to 10th	Night	19% 3 29% 35% 12% 2	720
	11th to 20th	Night	20% 4 29% 33% 13% 2	720
	21st to end	Night	21% 5% 26% 34% 11% 2	792
April	1st to 10th	Night	31% 3 23% 32% 10% 1	550
	11th to 20th	Night	24% 4 23% 31% 15% 2	534
	21st to end	Night	30% 3 16% 33% 14% 4	540
May	1st to 10th	Night	28% 3 12% 34% 20% 2	540
	11th to 20th	Night	23% 4 11% 34% 23% 5%	532
	21st to end	Night	20% 2 22% 39% 15% 2	482
June	1st to 10th	Night	30% 3 11% 33% 23% 1	360
	11th to 20th	Night	22% 4 9% 36% 24% 6%	360
	21st to end	Night	28% 2 8% 36% 23% 4	360
July	1st to 10th	Night	19% 3 9% 37% 27% 5%	350
	11th to 20th	Night	17% 2 11% 38% 29% 4	348
	21st to end	Night	20% 2 7% 38% 30% 3	483
August	1st to 10th	Night	15% 2 7% 39% 32% 5%	538
	11th to 20th	Night	18% 2 8% 36% 29% 6%	532
	21st to end	Night	16% 3 10% 38% 28% 6%	594
September	1st to 10th	Night	21% 4 19% 36% 16% 3	540
	11th to 20th	Night	21% 3 20% 34% 20% 2	625
	21st to end	Night	19% 4 26% 34% 16% 1	720
October	1st to 10th	Night	21% 3 27% 32% 15% 1	720
	11th to 20th	Night	25% 6% 23% 31% 13% 2	720
	21st to end	Night	28% 5% 30% 25% 11% 1	812
November	1st to 10th	Night	25% 7% 31% 27% 9% 1	830
	11th to 20th	Night	26% 9% 28% 25% 9% 2	850
	21st to end	Night	27% 7% 31% 27% 7% 1	850
December	1st to 10th	Night	28% 8% 32% 24% 8% 1	850
	11th to 20th	Night	26% 12% 36% 21% 5% 1	850
	21st to end	Night	26% 8% 34% 23% 8% 1	935

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