White City
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the White City 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	9% 5% 2 5% 5% 4 5% 6% 7% 9% 42%	74844
	Night	9% 5% 2 5% 5% 3 5% 5% 6% 9% 47%	74977
Transparency	Day	37% 6% 15% 20% 16% 7%	74572
	Night	34% 6% 15% 20% 18% 8%	74768
Seeing	Day	10% 7% 21% 27% 21% 13%	13787
	Night	10% 5% 18% 29% 28% 9%	23830

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	12% 5% 2 6% 6% 3 4 5% 5% 8% 43%	1706
	11th to 20th	Day	10% 4 3 4 5% 3 4 5% 5% 8% 49%	1780
	21st to end	Day	15% 6% 2 6% 5% 3 5% 5% 5% 6% 43%	1980
February	1st to 10th	Day	14% 7% 3 6% 6% 3 4 4 4 7% 44%	1800
	11th to 20th	Day	13% 4 2 4 4 2 5% 4 4 8% 50%	1748
	21st to end	Day	14% 6% 3 5% 4 3 4 4 5% 7% 45%	1480
March	1st to 10th	Day	6% 4 1 4 4 2 4 5% 5% 9% 54%	1822
	11th to 20th	Day	9% 4 2 4 4 3 4 4 5% 8% 52%	1980
	21st to end	Day	9% 3 2 4 3 3 4 4 4 8% 55%	2178
April	1st to 10th	Day	10% 4 2 4 5% 2 6% 5% 7% 9% 48%	2052
	11th to 20th	Day	11% 6% 3 5% 4 3 4 4 5% 8% 48%	2134
	21st to end	Day	5% 3 2 3 3 2 5% 4 5% 9% 58%	2160
May	1st to 10th	Day	5% 2 2 4 4 2 3 4 6% 9% 59%	2311
	11th to 20th	Day	5% 5% 2 5% 6% 4 6% 5% 7% 10% 46%	2301
	21st to end	Day	5% 5% 2 4 5% 4 5% 6% 7% 11% 47%	2531
June	1st to 10th	Day	8% 7% 3 7% 7% 4 6% 7% 8% 12% 31%	2340
	11th to 20th	Day	6% 5% 3 6% 6% 4 7% 7% 9% 13% 34%	2340
	21st to end	Day	7% 5% 3 7% 7% 5% 8% 9% 10% 11% 29%	2340
July	1st to 10th	Day	8% 5% 3 7% 8% 4 9% 9% 10% 11% 25%	2340
	11th to 20th	Day	11% 6% 3 7% 9% 5% 7% 8% 10% 12% 23%	2340
	21st to end	Day	6% 5% 2 8% 8% 6% 8% 10% 9% 12% 26%	2463
August	1st to 10th	Day	8% 5% 3 8% 7% 6% 7% 8% 9% 13% 26%	2148
	11th to 20th	Day	12% 6% 3 7% 7% 5% 8% 7% 9% 13% 24%	2125
	21st to end	Day	9% 7% 3 6% 7% 5% 7% 8% 8% 11% 28%	2376
September	1st to 10th	Day	7% 6% 3 8% 7% 5% 7% 8% 11% 13% 26%	1994
	11th to 20th	Day	7% 6% 3 6% 8% 6% 7% 8% 8% 9% 32%	1980
	21st to end	Day	9% 5% 3 5% 5% 4 6% 7% 6% 8% 43%	1980
October	1st to 10th	Day	8% 6% 2 6% 6% 3 5% 5% 5% 7% 47%	1980
	11th to 20th	Day	7% 3 2 3 4 3 4 3 6% 8% 59%	1980
	21st to end	Day	7% 5% 3 5% 5% 3 4 3 5% 6% 54%	2008
November	1st to 10th	Day	8% 4 1 4 3 2 4 4 4 8% 57%	1740
	11th to 20th	Day	8% 4 3 5% 4 2 3 3 4 7% 56%	1710
	21st to end	Day	9% 5% 2 5% 5% 3 5% 6% 5% 7% 48%	1710
December	1st to 10th	Day	12% 5% 2 4 5% 2 5% 4 6% 7% 46%	1710
	11th to 20th	Day	14% 8% 3 5% 4 3 4 4 5% 6% 43%	2153
	21st to end	Day	18% 9% 3 5% 5% 2 4 4 4 6% 42%	3124
January	1st to 10th	Night	13% 7% 3 5% 4 3 4 4 6% 6% 44%	2459
	11th to 20th	Night	10% 6% 3 4 4 3 4 5% 5% 7% 49%	2304
	21st to end	Night	16% 8% 3 5% 4 3 4 4 4 6% 42%	2376
February	1st to 10th	Night	15% 6% 2 4 4 3 4 4 5% 6% 47%	2160
	11th to 20th	Night	14% 6% 2 5% 4 2 3 4 4 5% 51%	2098
	21st to end	Night	14% 6% 4 5% 5% 3 5% 4 4 6% 45%	1776
March	1st to 10th	Night	10% 5% 2 4 3 3 4 4 5% 9% 51%	2137
	11th to 20th	Night	10% 4 2 5% 5% 3 4 5% 6% 9% 47%	1980
	21st to end	Night	8% 4 2 4 4 2 4 3 5% 8% 56%	2178
April	1st to 10th	Night	8% 4 3 5% 5% 3 4 4 6% 7% 50%	1920
	11th to 20th	Night	8% 5% 3 5% 5% 3 4 5% 7% 7% 49%	1775
	21st to end	Night	5% 3 2 3 4 2 5% 4 5% 8% 59%	1800
May	1st to 10th	Night	5% 3 1 4 3 2 3 4 5% 8% 62%	1648
	11th to 20th	Night	5% 3 2 3 5% 4 4 6% 5% 9% 53%	1592
	21st to end	Night	4 3 2 3 4 2 5% 6% 6% 12% 52%	1744
June	1st to 10th	Night	6% 5% 3 5% 7% 4 5% 7% 7% 10% 42%	1620
	11th to 20th	Night	4 3 2 4 5% 4 4 6% 7% 12% 48%	1620
	21st to end	Night	6% 4 2 5% 6% 3 5% 6% 7% 14% 43%	1620
July	1st to 10th	Night	5% 4 2 6% 6% 4 5% 6% 7% 12% 41%	1620
	11th to 20th	Night	4 5% 2 4 6% 5% 6% 7% 10% 13% 39%	1620
	21st to end	Night	3 4 2 4 5% 3 6% 6% 10% 15% 42%	1850
August	1st to 10th	Night	4 4 1 4 5% 3 5% 6% 9% 14% 44%	1790
	11th to 20th	Night	5% 4 2 6% 6% 4 7% 7% 9% 15% 36%	1776
	21st to end	Night	8% 5% 2 6% 6% 4 6% 7% 8% 11% 38%	1980
September	1st to 10th	Night	5% 5% 2 4 7% 4 7% 8% 8% 12% 38%	1948
	11th to 20th	Night	7% 6% 3 6% 7% 4 6% 6% 8% 9% 37%	1980
	21st to end	Night	6% 4 3 4 4 3 5% 5% 7% 10% 48%	1980
October	1st to 10th	Night	5% 4 2 5% 5% 4 5% 6% 5% 8% 52%	1980
	11th to 20th	Night	6% 3 2 4 3 2 4 3 5% 7% 60%	1980
	21st to end	Night	7% 4 2 5% 4 3 5% 5% 5% 6% 52%	2348
November	1st to 10th	Night	7% 4 2 5% 4 3 4 4 5% 6% 57%	2334
	11th to 20th	Night	9% 5% 2 5% 4 2 4 4 5% 6% 54%	2470
	21st to end	Night	11% 5% 2 5% 5% 2 4 3 5% 8% 50%	2470
December	1st to 10th	Night	11% 5% 3 5% 4 3 4 4 4 7% 49%	2470
	11th to 20th	Night	14% 5% 3 5% 4 3 4 4 5% 7% 46%	3042
	21st to end	Night	17% 9% 3 6% 5% 3 4 4 5% 5% 40%	4532

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	39% 3 10% 18% 18% 12%	1707
	11th to 20th	Day	34% 4 10% 18% 23% 11%	1780
	21st to end	Day	43% 3 9% 14% 22% 9%	1980
February	1st to 10th	Day	42% 4 8% 17% 20% 10%	1800
	11th to 20th	Day	36% 3 7% 20% 22% 12%	1758
	21st to end	Day	40% 4 10% 18% 20% 9%	1480
March	1st to 10th	Day	28% 5% 9% 23% 26% 9%	1822
	11th to 20th	Day	30% 5% 17% 20% 18% 10%	1980
	21st to end	Day	29% 3 14% 27% 19% 8%	2178
April	1st to 10th	Day	33% 5% 13% 20% 18% 10%	2052
	11th to 20th	Day	35% 3 9% 23% 21% 9%	2134
	21st to end	Day	24% 4 11% 31% 21% 9%	2160
May	1st to 10th	Day	22% 4 12% 29% 23% 9%	2311
	11th to 20th	Day	31% 6% 20% 21% 18% 3	2305
	21st to end	Day	28% 6% 17% 27% 19% 2	2531
June	1st to 10th	Day	42% 8% 20% 24% 6%	2340
	11th to 20th	Day	36% 8% 23% 25% 8%	2340
	21st to end	Day	43% 9% 23% 19% 6% 1	2340
July	1st to 10th	Day	45% 13% 26% 15% 1	2275
	11th to 20th	Day	48% 10% 25% 15% 2	2271
	21st to end	Day	42% 11% 28% 18% 2	2325
August	1st to 10th	Day	44% 11% 25% 17% 3	2136
	11th to 20th	Day	49% 11% 26% 12% 3	2125
	21st to end	Day	45% 11% 22% 16% 6% 1	2376
September	1st to 10th	Day	42% 11% 23% 15% 6% 2	1994
	11th to 20th	Day	44% 9% 19% 16% 8% 3	1980
	21st to end	Day	35% 7% 14% 21% 15% 8%	1980
October	1st to 10th	Day	36% 5% 10% 19% 18% 12%	1980
	11th to 20th	Day	24% 4 7% 25% 30% 11%	1980
	21st to end	Day	31% 5% 6% 17% 25% 16%	2008
November	1st to 10th	Day	28% 3 9% 22% 22% 17%	1740
	11th to 20th	Day	29% 4 8% 17% 30% 13%	1710
	21st to end	Day	34% 4 8% 24% 21% 9%	1710
December	1st to 10th	Day	37% 2 13% 20% 20% 7%	1710
	11th to 20th	Day	42% 3 8% 15% 20% 11%	2150
	21st to end	Day	46% 3 6% 20% 18% 6%	3124
January	1st to 10th	Night	40% 4 9% 15% 20% 12%	2459
	11th to 20th	Night	35% 3 8% 19% 24% 12%	2304
	21st to end	Night	44% 4 6% 14% 23% 10%	2376
February	1st to 10th	Night	39% 3 8% 16% 23% 11%	2160
	11th to 20th	Night	38% 3 8% 17% 20% 14%	2099
	21st to end	Night	41% 4 9% 18% 17% 12%	1776
March	1st to 10th	Night	31% 4 10% 23% 23% 9%	2137
	11th to 20th	Night	34% 5% 12% 22% 18% 10%	1980
	21st to end	Night	28% 3 13% 25% 21% 11%	2178
April	1st to 10th	Night	32% 5% 13% 19% 20% 11%	1920
	11th to 20th	Night	33% 3 9% 23% 24% 8%	1775
	21st to end	Night	24% 5% 9% 29% 25% 9%	1800
May	1st to 10th	Night	21% 4 13% 24% 26% 11%	1648
	11th to 20th	Night	27% 6% 19% 25% 21% 3	1593
	21st to end	Night	25% 5% 19% 26% 21% 5%	1745
June	1st to 10th	Night	35% 6% 21% 30% 8%	1620
	11th to 20th	Night	27% 9% 27% 28% 8% 1	1620
	21st to end	Night	31% 7% 29% 21% 10% 1	1620
July	1st to 10th	Night	34% 10% 32% 21% 2	1575
	11th to 20th	Night	32% 14% 34% 16% 4	1566
	21st to end	Night	27% 11% 34% 25% 2 1	1747
August	1st to 10th	Night	27% 11% 36% 21% 5%	1778
	11th to 20th	Night	34% 11% 35% 17% 2	1776
	21st to end	Night	36% 11% 27% 20% 5% 1	1980
September	1st to 10th	Night	36% 9% 28% 17% 8% 3	1948
	11th to 20th	Night	40% 10% 20% 19% 10% 2	1980
	21st to end	Night	31% 7% 17% 21% 20% 6%	1980
October	1st to 10th	Night	30% 6% 13% 18% 21% 12%	1980
	11th to 20th	Night	24% 4 7% 24% 30% 11%	1980
	21st to end	Night	32% 5% 6% 17% 24% 16%	2348
November	1st to 10th	Night	29% 3 7% 22% 21% 18%	2334
	11th to 20th	Night	32% 4 7% 18% 26% 14%	2470
	21st to end	Night	34% 4 9% 20% 20% 12%	2470
December	1st to 10th	Night	36% 3 12% 22% 20% 7%	2470
	11th to 20th	Night	39% 3 8% 16% 21% 13%	3044
	21st to end	Night	48% 3 6% 22% 17% 4	4532

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	11% 20% 44% 21% 3	270
	11th to 20th	Day	13% 18% 33% 28% 8% 1	257
	21st to end	Day	20% 21% 43% 13% 3	264
February	1st to 10th	Day	16% 16% 44% 20% 4	240
	11th to 20th	Day	15% 17% 40% 21% 5% 1	240
	21st to end	Day	11% 13% 50% 20% 6% 1	198
March	1st to 10th	Day	11% 18% 41% 25% 5%	262
	11th to 20th	Day	13% 13% 35% 25% 13% 1	420
	21st to end	Day	11% 11% 36% 26% 14% 1	462
April	1st to 10th	Day	14% 13% 30% 28% 12% 3	420
	11th to 20th	Day	14% 12% 29% 25% 17% 3	412
	21st to end	Day	7% 8% 30% 32% 19% 4	420
May	1st to 10th	Day	6% 7% 29% 31% 19% 8%	423
	11th to 20th	Day	7% 4 18% 34% 23% 15%	447
	21st to end	Day	7% 2 12% 31% 27% 20%	485
June	1st to 10th	Day	11% 4 12% 30% 24% 20%	450
	11th to 20th	Day	8% 3 11% 22% 29% 26%	450
	21st to end	Day	7% 2 8% 26% 31% 25%	450
July	1st to 10th	Day	8% 3 5% 17% 31% 37%	445
	11th to 20th	Day	8% 2 3 16% 34% 37%	444
	21st to end	Day	4 1 7% 20% 39% 29%	488
August	1st to 10th	Day	8% 2 4 21% 33% 32%	446
	11th to 20th	Day	11% 1 3 15% 36% 33%	439
	21st to end	Day	8% 4 7% 22% 33% 25%	495
September	1st to 10th	Day	7% 4 6% 26% 33% 24%	450
	11th to 20th	Day	9% 3 8% 35% 27% 18%	450
	21st to end	Day	8% 2 11% 28% 35% 16%	450
October	1st to 10th	Day	8% 4 24% 39% 18% 7%	450
	11th to 20th	Day	6% 4 17% 36% 29% 8%	450
	21st to end	Day	9% 6% 24% 36% 20% 4	495
November	1st to 10th	Day	11% 5% 26% 38% 16% 4	338
	11th to 20th	Day	9% 11% 33% 37% 8% 2	270
	21st to end	Day	8% 15% 34% 30% 10% 2	270
December	1st to 10th	Day	15% 15% 31% 25% 10% 3	270
	11th to 20th	Day	18% 14% 35% 26% 6%	270
	21st to end	Day	18% 11% 36% 26% 8% 1	297
January	1st to 10th	Night	16% 12% 31% 29% 11% 1	760
	11th to 20th	Night	13% 11% 28% 34% 14% 1	744
	21st to end	Night	20% 9% 32% 29% 9% 1	792
February	1st to 10th	Night	17% 10% 26% 35% 12% 1	720
	11th to 20th	Night	17% 9% 30% 32% 13% 1	703
	21st to end	Night	18% 7% 28% 37% 9%	592
March	1st to 10th	Night	11% 8% 28% 38% 14%	720
	11th to 20th	Night	12% 6% 27% 34% 20% 1	720
	21st to end	Night	9% 7% 29% 36% 20%	792
April	1st to 10th	Night	11% 6% 24% 37% 21% 1	660
	11th to 20th	Night	11% 6% 22% 35% 25% 1	533
	21st to end	Night	6% 4 22% 35% 31% 2	540
May	1st to 10th	Night	7% 3 17% 36% 31% 7%	540
	11th to 20th	Night	6% 2 15% 33% 37% 7%	531
	21st to end	Night	5% 1 7% 29% 46% 11%	584
June	1st to 10th	Night	8% 2 6% 25% 47% 12%	540
	11th to 20th	Night	4 2 5% 19% 47% 23%	540
	21st to end	Night	7% 2 5% 15% 49% 23%	540
July	1st to 10th	Night	5% 3 2 13% 45% 32%	525
	11th to 20th	Night	6% 1 2 8% 48% 36%	522
	21st to end	Night	3 2 2 11% 51% 32%	591
August	1st to 10th	Night	4 1 1 10% 49% 36%	537
	11th to 20th	Night	5% 1 3 9% 44% 38%	534
	21st to end	Night	10% 2 4 15% 45% 23%	594
September	1st to 10th	Night	7% 2 4 18% 45% 23%	688
	11th to 20th	Night	9% 3 7% 24% 45% 12%	720
	21st to end	Night	9% 2 8% 28% 42% 11%	720
October	1st to 10th	Night	6% 3 19% 34% 33% 6%	720
	11th to 20th	Night	7% 4 15% 34% 35% 6%	720
	21st to end	Night	10% 5% 18% 37% 27% 2	792
November	1st to 10th	Night	9% 4 20% 42% 23% 3	740
	11th to 20th	Night	11% 8% 26% 34% 20% 2	760
	21st to end	Night	13% 6% 24% 38% 17% 1	760
December	1st to 10th	Night	13% 8% 27% 32% 19% 2	760
	11th to 20th	Night	14% 8% 29% 34% 14% 1	760
	21st to end	Night	19% 8% 27% 32% 14%	836

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