Randolph Center
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Randolph Center clear sky chart from 2005-11-06 to 2024-01-13.

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% 9% 3 7% 6% 4 5% 5% 5% 7% 33%	74782
	Night	15% 8% 4 7% 6% 4 5% 5% 6% 7% 33%	74950
Transparency	Day	51% 4 12% 21% 11% 1	74487
	Night	50% 4 13% 22% 10% 1	74779
Seeing	Day	24% 16% 29% 19% 9% 3	13671
	Night	21% 13% 27% 29% 10% 1	22735

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	11% 11% 5% 13% 8% 3 4 6% 6% 7% 25%	1517
	11th to 20th	Day	14% 11% 5% 9% 8% 4 6% 6% 6% 5% 26%	1462
	21st to end	Day	9% 10% 6% 9% 9% 3 7% 6% 7% 8% 27%	1584
February	1st to 10th	Day	12% 11% 5% 9% 8% 5% 7% 5% 5% 8% 25%	1578
	11th to 20th	Day	10% 10% 5% 11% 10% 5% 6% 5% 5% 7% 26%	1748
	21st to end	Day	14% 12% 5% 9% 7% 4 5% 5% 5% 6% 28%	1480
March	1st to 10th	Day	11% 9% 4 10% 7% 5% 6% 6% 5% 6% 29%	1797
	11th to 20th	Day	15% 12% 4 8% 8% 3 5% 4 6% 6% 30%	1840
	21st to end	Day	15% 10% 4 8% 7% 4 5% 5% 4 6% 32%	2376
April	1st to 10th	Day	18% 11% 4 8% 5% 3 4 3 4 6% 33%	2160
	11th to 20th	Day	16% 8% 3 5% 6% 4 5% 5% 6% 7% 35%	2134
	21st to end	Day	21% 10% 4 8% 5% 4 5% 5% 5% 6% 28%	2322
May	1st to 10th	Day	20% 11% 3 6% 6% 3 5% 4 4 5% 32%	2354
	11th to 20th	Day	18% 7% 3 7% 5% 3 4 5% 4 5% 39%	2478
	21st to end	Day	14% 6% 3 6% 6% 4 6% 6% 6% 7% 38%	2725
June	1st to 10th	Day	20% 7% 3 7% 7% 4 5% 6% 5% 7% 29%	2520
	11th to 20th	Day	17% 7% 2 5% 6% 4 5% 5% 6% 8% 35%	2520
	21st to end	Day	18% 7% 4 7% 6% 3 6% 6% 7% 8% 29%	2520
July	1st to 10th	Day	16% 5% 3 5% 5% 3 6% 6% 6% 7% 38%	2520
	11th to 20th	Day	13% 5% 2 6% 7% 4 5% 6% 7% 9% 34%	2520
	21st to end	Day	15% 4 2 5% 6% 4 5% 7% 7% 9% 36%	2744
August	1st to 10th	Day	12% 5% 2 6% 6% 4 6% 5% 8% 9% 36%	2478
	11th to 20th	Day	13% 7% 3 6% 6% 4 5% 6% 7% 9% 34%	2239
	21st to end	Day	13% 5% 3 5% 5% 4 5% 6% 6% 9% 39%	2376
September	1st to 10th	Day	13% 5% 2 4 5% 3 5% 4 6% 8% 45%	2160
	11th to 20th	Day	14% 6% 3 5% 4 3 4 3 4 7% 47%	2034
	21st to end	Day	15% 7% 2 5% 4 3 4 4 7% 8% 41%	1980
October	1st to 10th	Day	22% 7% 3 8% 7% 3 4 4 4 6% 33%	1940
	11th to 20th	Day	15% 10% 4 7% 6% 3 5% 4 5% 7% 35%	1738
	21st to end	Day	22% 13% 4 6% 6% 3 4 5% 4 5% 27%	1782
November	1st to 10th	Day	12% 8% 3 7% 6% 4 5% 6% 6% 7% 37%	1671
	11th to 20th	Day	16% 10% 3 8% 6% 4 4 5% 7% 6% 31%	1710
	21st to end	Day	16% 11% 4 7% 7% 3 5% 5% 6% 7% 29%	1577
December	1st to 10th	Day	18% 14% 4 9% 9% 5% 4 4 5% 7% 21%	1520
	11th to 20th	Day	20% 12% 3 8% 5% 3 5% 4 5% 7% 27%	1906
	21st to end	Day	20% 14% 5% 10% 7% 4 5% 4 4 4 21%	2772
January	1st to 10th	Night	12% 9% 4 10% 8% 5% 7% 7% 7% 8% 24%	2649
	11th to 20th	Night	13% 10% 5% 10% 8% 5% 7% 6% 5% 6% 23%	2560
	21st to end	Night	9% 7% 6% 10% 8% 4 7% 7% 7% 7% 30%	2772
February	1st to 10th	Night	10% 12% 5% 9% 8% 4 7% 7% 6% 8% 24%	2376
	11th to 20th	Night	9% 9% 5% 9% 8% 5% 7% 7% 8% 7% 27%	2098
	21st to end	Night	12% 9% 4 10% 7% 4 5% 5% 6% 8% 30%	1776
March	1st to 10th	Night	11% 10% 5% 7% 8% 5% 6% 5% 6% 7% 30%	2150
	11th to 20th	Night	13% 10% 4 7% 8% 5% 6% 5% 5% 6% 31%	2115
	21st to end	Night	14% 9% 4 7% 5% 4 4 4 5% 7% 38%	1980
April	1st to 10th	Night	16% 10% 4 7% 6% 4 6% 5% 5% 6% 32%	1800
	11th to 20th	Night	16% 8% 4 6% 5% 4 4 5% 6% 7% 34%	1775
	21st to end	Night	20% 8% 4 6% 6% 3 4 4 5% 7% 33%	1637
May	1st to 10th	Night	20% 10% 4 6% 5% 3 5% 4 5% 8% 30%	1620
	11th to 20th	Night	17% 5% 3 5% 6% 4 5% 4 6% 6% 39%	1415
	21st to end	Night	13% 6% 2 6% 4 4 6% 5% 6% 9% 39%	1550
June	1st to 10th	Night	19% 6% 2 5% 6% 3 6% 6% 6% 9% 33%	1440
	11th to 20th	Night	18% 6% 3 4 4 3 5% 5% 5% 8% 40%	1440
	21st to end	Night	17% 4 3 6% 6% 3 5% 5% 5% 8% 38%	1440
July	1st to 10th	Night	15% 4 2 4 5% 3 5% 4 6% 8% 43%	1440
	11th to 20th	Night	14% 5% 2 3 4 2 4 5% 7% 10% 43%	1440
	21st to end	Night	11% 5% 3 5% 4 2 5% 6% 7% 8% 44%	1574
August	1st to 10th	Night	11% 6% 2 4 5% 3 4 6% 7% 8% 44%	1441
	11th to 20th	Night	12% 5% 3 5% 6% 3 5% 4 5% 8% 43%	1658
	21st to end	Night	13% 6% 2 5% 4 3 4 5% 6% 7% 46%	1980
September	1st to 10th	Night	12% 4 3 4 4 3 4 5% 6% 9% 47%	1800
	11th to 20th	Night	13% 5% 3 5% 5% 3 4 4 5% 6% 49%	1924
	21st to end	Night	13% 6% 3 4 4 3 5% 4 7% 7% 45%	1980
October	1st to 10th	Night	19% 7% 4 6% 5% 3 5% 4 5% 7% 36%	2017
	11th to 20th	Night	16% 7% 3 6% 6% 4 6% 6% 5% 8% 34%	2222
	21st to end	Night	22% 9% 2 6% 6% 3 5% 6% 6% 7% 28%	2574
November	1st to 10th	Night	13% 9% 4 7% 6% 3 5% 5% 5% 6% 35%	2407
	11th to 20th	Night	16% 7% 3 8% 5% 3 5% 6% 6% 7% 33%	2470
	21st to end	Night	15% 8% 4 9% 7% 4 5% 4 6% 8% 30%	2598
December	1st to 10th	Night	17% 12% 6% 10% 8% 4 6% 5% 6% 8% 20%	2660
	11th to 20th	Night	19% 9% 6% 8% 5% 4 5% 5% 5% 8% 26%	3288
	21st to end	Night	20% 13% 6% 14% 7% 3 5% 4 5% 5% 20%	4884

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	57% 3 10% 22% 8%	1518
	11th to 20th	Day	59% 5% 11% 16% 8% 2	1462
	21st to end	Day	54% 3 12% 19% 10% 2	1584
February	1st to 10th	Day	57% 4 12% 18% 7% 2	1578
	11th to 20th	Day	57% 4 11% 18% 8% 2	1757
	21st to end	Day	58% 3 12% 14% 12% 2	1480
March	1st to 10th	Day	56% 3 11% 17% 10% 3	1800
	11th to 20th	Day	55% 2 10% 16% 13% 3	1840
	21st to end	Day	54% 3 7% 18% 14% 4	2376
April	1st to 10th	Day	54% 2 9% 20% 12% 3	2160
	11th to 20th	Day	49% 4 10% 20% 16% 1	2134
	21st to end	Day	58% 3 9% 17% 11% 2	2322
May	1st to 10th	Day	56% 2 7% 21% 12% 2	2354
	11th to 20th	Day	47% 4 8% 23% 16% 3	2482
	21st to end	Day	44% 5% 15% 23% 10% 2	2725
June	1st to 10th	Day	54% 4 14% 18% 9%	2520
	11th to 20th	Day	47% 4 11% 22% 14% 2	2520
	21st to end	Day	53% 5% 13% 19% 9% 1	2520
July	1st to 10th	Day	42% 5% 13% 25% 15% 1	2450
	11th to 20th	Day	44% 5% 16% 26% 9%	2445
	21st to end	Day	43% 4 15% 25% 11% 1	2591
August	1st to 10th	Day	44% 5% 16% 25% 10%	2464
	11th to 20th	Day	46% 4 16% 24% 10%	2239
	21st to end	Day	41% 4 13% 27% 13% 1	2376
September	1st to 10th	Day	37% 4 13% 31% 14% 1	2160
	11th to 20th	Day	39% 3 9% 31% 17% 1	2034
	21st to end	Day	41% 3 14% 28% 12% 2	1980
October	1st to 10th	Day	54% 2 9% 22% 12% 1	1940
	11th to 20th	Day	50% 3 13% 27% 7% 1	1738
	21st to end	Day	61% 3 11% 16% 8% 1	1782
November	1st to 10th	Day	46% 4 13% 23% 13%	1671
	11th to 20th	Day	52% 4 12% 18% 11% 2	1710
	21st to end	Day	55% 3 17% 20% 5% 1	1577
December	1st to 10th	Day	63% 3 14% 15% 5% 1	1520
	11th to 20th	Day	59% 4 10% 18% 8% 1	1906
	21st to end	Day	67% 2 10% 13% 8%	2772
January	1st to 10th	Night	55% 3 13% 20% 8%	2649
	11th to 20th	Night	61% 4 13% 15% 6% 1	2560
	21st to end	Night	52% 4 11% 21% 11% 2	2772
February	1st to 10th	Night	55% 4 14% 18% 8% 1	2376
	11th to 20th	Night	54% 4 9% 21% 11% 1	2100
	21st to end	Night	53% 2 12% 20% 10% 3	1776
March	1st to 10th	Night	54% 3 11% 20% 11% 2	2160
	11th to 20th	Night	54% 3 10% 19% 11% 2	2116
	21st to end	Night	48% 3 10% 21% 16% 3	1980
April	1st to 10th	Night	53% 2 9% 22% 12% 2	1800
	11th to 20th	Night	48% 3 10% 25% 13% 1	1775
	21st to end	Night	52% 4 9% 19% 12% 3	1637
May	1st to 10th	Night	54% 3 7% 19% 15% 2	1620
	11th to 20th	Night	46% 3 11% 23% 15% 2	1416
	21st to end	Night	43% 5% 17% 23% 11% 1	1551
June	1st to 10th	Night	48% 4 15% 21% 12%	1440
	11th to 20th	Night	44% 5% 15% 22% 11% 2	1440
	21st to end	Night	45% 5% 17% 24% 10%	1440
July	1st to 10th	Night	38% 5% 19% 23% 15% 1	1400
	11th to 20th	Night	35% 6% 19% 29% 11%	1392
	21st to end	Night	35% 7% 16% 29% 12% 1	1486
August	1st to 10th	Night	36% 5% 16% 31% 11%	1431
	11th to 20th	Night	41% 4 16% 27% 11%	1658
	21st to end	Night	38% 3 16% 30% 13%	1980
September	1st to 10th	Night	34% 5% 17% 31% 12%	1800
	11th to 20th	Night	37% 3 12% 32% 16% 1	1924
	21st to end	Night	38% 3 14% 27% 14% 3	1980
October	1st to 10th	Night	49% 3 12% 23% 11% 2	2017
	11th to 20th	Night	49% 4 14% 25% 8%	2222
	21st to end	Night	55% 3 13% 19% 9% 1	2574
November	1st to 10th	Night	50% 4 12% 22% 10% 2	2407
	11th to 20th	Night	48% 3 11% 24% 10% 2	2470
	21st to end	Night	53% 4 15% 21% 7%	2598
December	1st to 10th	Night	64% 3 12% 17% 4	2660
	11th to 20th	Night	57% 4 12% 17% 10% 1	3288
	21st to end	Night	68% 2 9% 17% 4	4884

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% 22% 32% 17% 7% 1	180
	11th to 20th	Day	30% 22% 27% 14% 5% 1	166
	21st to end	Day	19% 28% 28% 18% 5% 1	176
February	1st to 10th	Day	36% 25% 28% 9% 2	226
	11th to 20th	Day	30% 22% 31% 14% 3	416
	21st to end	Day	28% 21% 32% 16% 3	346
March	1st to 10th	Day	28% 16% 34% 18% 4	420
	11th to 20th	Day	31% 21% 28% 14% 5%	420
	21st to end	Day	31% 19% 26% 14% 6% 3	462
April	1st to 10th	Day	35% 16% 26% 16% 5% 2	420
	11th to 20th	Day	33% 20% 24% 14% 7% 2	412
	21st to end	Day	33% 17% 30% 10% 8% 2	420
May	1st to 10th	Day	34% 20% 24% 12% 7% 3	423
	11th to 20th	Day	21% 19% 32% 19% 6% 2	447
	21st to end	Day	18% 18% 32% 21% 10% 2	485
June	1st to 10th	Day	24% 14% 38% 15% 8% 1	450
	11th to 20th	Day	23% 14% 31% 21% 9% 2	450
	21st to end	Day	20% 13% 34% 22% 9% 2	450
July	1st to 10th	Day	18% 11% 28% 29% 11% 3	445
	11th to 20th	Day	16% 7% 36% 27% 11% 3	444
	21st to end	Day	14% 10% 38% 25% 11% 2	488
August	1st to 10th	Day	12% 11% 30% 24% 18% 5%	446
	11th to 20th	Day	17% 9% 26% 24% 15% 8%	439
	21st to end	Day	15% 13% 27% 25% 14% 7%	495
September	1st to 10th	Day	16% 12% 24% 26% 14% 8%	450
	11th to 20th	Day	17% 12% 27% 20% 14% 11%	450
	21st to end	Day	20% 10% 29% 22% 12% 7%	450
October	1st to 10th	Day	25% 12% 25% 18% 13% 6%	450
	11th to 20th	Day	22% 16% 27% 23% 11% 2	419
	21st to end	Day	35% 15% 23% 20% 7% 1	396
November	1st to 10th	Day	22% 16% 23% 23% 14% 1	365
	11th to 20th	Day	28% 21% 25% 19% 8% 1	370
	21st to end	Day	31% 16% 27% 18% 7% 1	237
December	1st to 10th	Day	28% 19% 31% 17% 4 2	180
	11th to 20th	Day	32% 20% 29% 17% 2	180
	21st to end	Day	34% 17% 27% 17% 5%	198
January	1st to 10th	Night	20% 18% 35% 23% 4	760
	11th to 20th	Night	25% 17% 28% 26% 4	732
	21st to end	Night	15% 18% 32% 30% 4 1	792
February	1st to 10th	Night	20% 18% 34% 25% 3	720
	11th to 20th	Night	20% 18% 35% 24% 3	703
	21st to end	Night	21% 18% 30% 27% 3	592
March	1st to 10th	Night	23% 14% 34% 27% 3	720
	11th to 20th	Night	24% 18% 27% 24% 6% 1	720
	21st to end	Night	26% 15% 26% 24% 8%	792
April	1st to 10th	Night	25% 17% 30% 23% 5%	720
	11th to 20th	Night	22% 19% 27% 24% 8%	711
	21st to end	Night	29% 13% 29% 23% 7%	557
May	1st to 10th	Night	26% 13% 28% 25% 8%	540
	11th to 20th	Night	18% 12% 29% 34% 6% 1	354
	21st to end	Night	18% 9% 26% 34% 13%	389
June	1st to 10th	Night	21% 7% 28% 32% 12%	360
	11th to 20th	Night	21% 8% 22% 35% 14%	360
	21st to end	Night	17% 4 25% 39% 14% 1	360
July	1st to 10th	Night	17% 3 20% 42% 16% 2	350
	11th to 20th	Night	14% 3 24% 40% 18% 2	348
	21st to end	Night	12% 5% 21% 44% 17% 1	394
August	1st to 10th	Night	14% 2 16% 43% 25%	367
	11th to 20th	Night	15% 6% 19% 37% 20% 3	594
	21st to end	Night	15% 7% 18% 35% 23% 3	792
September	1st to 10th	Night	14% 5% 26% 33% 20% 2	720
	11th to 20th	Night	16% 8% 20% 30% 22% 4	720
	21st to end	Night	16% 8% 25% 32% 18% 2	720
October	1st to 10th	Night	25% 12% 16% 30% 15% 2	720
	11th to 20th	Night	22% 11% 23% 30% 13% 1	720
	21st to end	Night	28% 12% 26% 27% 7%	792
November	1st to 10th	Night	21% 16% 24% 30% 8% 1	740
	11th to 20th	Night	20% 16% 28% 26% 8% 2	760
	21st to end	Night	23% 15% 28% 27% 7%	760
December	1st to 10th	Night	27% 17% 32% 20% 4	760
	11th to 20th	Night	25% 17% 27% 26% 5%	760
	21st to end	Night	26% 17% 30% 23% 3	836

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