Westminster
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Westminster clear sky chart from 2005-11-06 to 2024-01-12.

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	5% 5% 2 4 4 3 4 5% 6% 8% 53%	74766
	Night	5% 5% 2 4 4 2 4 4 5% 7% 58%	74968
Transparency	Day	26% 7% 11% 23% 23% 10%	74473
	Night	25% 6% 11% 25% 25% 9%	74784
Seeing	Day	8% 8% 26% 33% 19% 6%	14428
	Night	7% 4 30% 42% 16% 1	24586

By Month

Percentage Cloud Cover SW (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	2 5% 2 3 3 2 3 4 5% 9% 63%	1518
	11th to 20th	Day	3 7% 4 3 3 2 3 2 4 6% 61%	1486
	21st to end	Day	1 6% 3 5% 4 2 4 4 5% 9% 57%	1782
February	1st to 10th	Day	2 6% 4 7% 3 2 4 4 5% 8% 55%	1620
	11th to 20th	Day	1 4 2 6% 5% 3 4 5% 5% 9% 55%	1727
	21st to end	Day	3 7% 3 6% 5% 2 4 3 6% 6% 55%	1480
March	1st to 10th	Day	3 6% 2 5% 4 2 4 4 5% 10% 54%	1800
	11th to 20th	Day	6% 7% 3 4 4 2 4 4 4 9% 52%	1950
	21st to end	Day	6% 7% 2 4 4 3 4 6% 7% 8% 49%	2312
April	1st to 10th	Day	6% 6% 2 4 3 2 3 4 6% 9% 55%	2160
	11th to 20th	Day	10% 8% 2 5% 4 3 4 5% 5% 7% 48%	2131
	21st to end	Day	10% 6% 2 5% 4 2 4 5% 7% 8% 46%	2160
May	1st to 10th	Day	12% 6% 2 5% 5% 3 5% 6% 7% 8% 41%	2276
	11th to 20th	Day	14% 7% 2 4 5% 3 5% 5% 7% 9% 39%	2470
	21st to end	Day	7% 4 2 5% 5% 3 6% 6% 7% 10% 45%	2724
June	1st to 10th	Day	5% 3 1 4 5% 3 5% 6% 8% 10% 49%	2520
	11th to 20th	Day	6% 3 1 3 4 3 6% 7% 7% 9% 51%	2520
	21st to end	Day	4 3 1 4 5% 4 5% 7% 8% 10% 50%	2520
July	1st to 10th	Day	6% 3 1 5% 5% 3 6% 7% 9% 10% 46%	2520
	11th to 20th	Day	4 3 1 4 4 3 5% 6% 9% 10% 52%	2520
	21st to end	Day	5% 4 2 5% 5% 4 5% 6% 8% 11% 47%	2704
August	1st to 10th	Day	5% 4 2 5% 6% 4 7% 7% 9% 11% 42%	2328
	11th to 20th	Day	4 2 1 3 4 4 5% 6% 7% 9% 54%	2124
	21st to end	Day	4 3 1 3 5% 3 5% 6% 7% 10% 52%	2376
September	1st to 10th	Day	5% 3 1 3 4 2 5% 4 5% 7% 61%	2160
	11th to 20th	Day	5% 3 1 3 4 1 3 5% 6% 7% 61%	2138
	21st to end	Day	6% 4 2 3 3 2 3 4 3 6% 65%	1980
October	1st to 10th	Day	7% 4 1 4 4 2 4 3 4 7% 62%	1818
	11th to 20th	Day	4 3 1 3 3 2 3 4 4 6% 67%	1800
	21st to end	Day	6% 7% 2 3 3 2 2 3 3 5% 64%	1908
November	1st to 10th	Day	2 4 1 3 3 2 2 3 5% 8% 66%	1673
	11th to 20th	Day	2 5% 2 4 3 2 3 3 6% 8% 63%	1710
	21st to end	Day	3 5% 3 5% 3 2 3 4 5% 7% 60%	1611
December	1st to 10th	Day	2 5% 2 5% 4 2 3 3 5% 8% 59%	1520
	11th to 20th	Day	2 4 3 4 3 2 4 3 6% 9% 60%	1926
	21st to end	Day	3 9% 4 6% 5% 3 4 4 4 6% 53%	2794
January	1st to 10th	Night	1 5% 3 5% 3 2 3 3 4 7% 63%	2647
	11th to 20th	Night	2 4 4 4 3 2 3 3 4 7% 63%	2519
	21st to end	Night	3 7% 3 6% 5% 2 4 3 4 6% 58%	2574
February	1st to 10th	Night	2 8% 3 5% 3 3 4 4 4 7% 57%	2340
	11th to 20th	Night	2 6% 4 6% 6% 3 4 4 4 7% 54%	2119
	21st to end	Night	3 8% 3 6% 5% 2 3 3 4 6% 57%	1776
March	1st to 10th	Night	3 5% 3 5% 4 2 3 4 5% 8% 57%	2160
	11th to 20th	Night	7% 6% 3 3 4 2 3 3 4 5% 60%	2008
	21st to end	Night	7% 9% 3 4 4 2 3 3 5% 9% 53%	2043
April	1st to 10th	Night	7% 6% 3 4 4 2 4 4 5% 7% 56%	1800
	11th to 20th	Night	10% 8% 2 4 3 3 4 4 4 7% 52%	1777
	21st to end	Night	12% 7% 3 5% 4 3 3 4 5% 7% 48%	1800
May	1st to 10th	Night	12% 5% 2 6% 5% 3 5% 5% 6% 7% 45%	1683
	11th to 20th	Night	14% 7% 2 5% 4 3 4 4 4 7% 46%	1421
	21st to end	Night	9% 5% 2 3 3 3 3 5% 6% 7% 55%	1552
June	1st to 10th	Night	6% 3 1 2 3 2 4 6% 6% 11% 58%	1440
	11th to 20th	Night	5% 3 1 3 3 2 5% 4 5% 8% 62%	1440
	21st to end	Night	5% 2 2 2 3 2 5% 5% 7% 8% 59%	1440
July	1st to 10th	Night	6% 3 2 4 5% 3 5% 4 6% 11% 52%	1440
	11th to 20th	Night	2 3 2 2 4 2 4 5% 7% 11% 59%	1440
	21st to end	Night	5% 3 2 3 4 3 5% 6% 9% 10% 50%	1611
August	1st to 10th	Night	4 4 2 5% 5% 3 5% 6% 7% 9% 51%	1611
	11th to 20th	Night	5% 3 1 2 4 2 5% 4 6% 10% 58%	1774
	21st to end	Night	3 4 1 3 4 3 5% 6% 6% 8% 57%	1980
September	1st to 10th	Night	7% 3 2 4 3 2 4 4 6% 7% 59%	1800
	11th to 20th	Night	7% 2 1 2 3 1 3 4 5% 7% 65%	1821
	21st to end	Night	7% 3 1 3 4 2 3 3 4 6% 65%	1980
October	1st to 10th	Night	9% 5% 2 3 2 2 3 3 5% 6% 61%	2139
	11th to 20th	Night	4 3 1 3 3 2 3 3 4 6% 69%	2160
	21st to end	Night	7% 7% 2 4 3 2 3 2 4 6% 61%	2429
November	1st to 10th	Night	3 3 1 3 3 2 2 3 5% 7% 68%	2405
	11th to 20th	Night	3 5% 2 4 3 1 3 4 4 6% 65%	2470
	21st to end	Night	4 6% 3 4 3 2 3 4 3 6% 62%	2565
December	1st to 10th	Night	2 6% 3 4 4 3 3 3 5% 8% 61%	2660
	11th to 20th	Night	2 5% 3 4 4 2 3 4 5% 9% 59%	3271
	21st to end	Night	5% 9% 3 5% 5% 3 4 4 5% 6% 52%	4873

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	19% 5% 13% 30% 25% 8%	1519
	11th to 20th	Day	26% 4 8% 31% 24% 7%	1486
	21st to end	Day	25% 4 11% 31% 21% 7%	1782
February	1st to 10th	Day	29% 5% 12% 26% 20% 8%	1620
	11th to 20th	Day	25% 5% 9% 28% 27% 6%	1737
	21st to end	Day	27% 5% 9% 23% 27% 9%	1480
March	1st to 10th	Day	25% 6% 11% 25% 26% 8%	1800
	11th to 20th	Day	30% 4 12% 24% 24% 7%	1950
	21st to end	Day	30% 7% 14% 23% 19% 7%	2312
April	1st to 10th	Day	26% 6% 13% 24% 23% 9%	2160
	11th to 20th	Day	34% 7% 11% 22% 19% 8%	2134
	21st to end	Day	32% 7% 12% 24% 21% 3	2160
May	1st to 10th	Day	37% 8% 14% 22% 15% 3	2276
	11th to 20th	Day	40% 9% 14% 23% 11% 3	2474
	21st to end	Day	32% 9% 15% 23% 14% 7%	2724
June	1st to 10th	Day	26% 9% 16% 23% 17% 9%	2520
	11th to 20th	Day	25% 8% 12% 18% 23% 15%	2520
	21st to end	Day	25% 8% 12% 21% 24% 11%	2520
July	1st to 10th	Day	29% 8% 13% 19% 19% 11%	2451
	11th to 20th	Day	24% 9% 11% 22% 23% 11%	2445
	21st to end	Day	29% 9% 16% 23% 15% 7%	2554
August	1st to 10th	Day	31% 11% 15% 22% 15% 7%	2313
	11th to 20th	Day	24% 7% 12% 21% 23% 13%	2124
	21st to end	Day	25% 7% 11% 22% 25% 11%	2376
September	1st to 10th	Day	23% 5% 8% 20% 24% 20%	2160
	11th to 20th	Day	22% 6% 7% 20% 28% 17%	2138
	21st to end	Day	21% 4 7% 20% 36% 12%	1980
October	1st to 10th	Day	24% 5% 10% 19% 29% 13%	1818
	11th to 20th	Day	18% 5% 8% 23% 31% 16%	1800
	21st to end	Day	24% 4 8% 25% 26% 14%	1908
November	1st to 10th	Day	17% 3 10% 25% 29% 16%	1673
	11th to 20th	Day	19% 5% 8% 27% 33% 9%	1710
	21st to end	Day	24% 3 8% 22% 29% 13%	1611
December	1st to 10th	Day	23% 5% 12% 28% 23% 9%	1520
	11th to 20th	Day	20% 8% 9% 25% 32% 6%	1924
	21st to end	Day	29% 8% 9% 20% 26% 8%	2794
January	1st to 10th	Night	22% 5% 9% 29% 28% 7%	2647
	11th to 20th	Night	21% 5% 9% 27% 30% 8%	2519
	21st to end	Night	29% 4 9% 27% 24% 7%	2574
February	1st to 10th	Night	27% 5% 10% 29% 23% 6%	2340
	11th to 20th	Night	30% 6% 7% 28% 23% 6%	2119
	21st to end	Night	28% 4 6% 25% 26% 12%	1776
March	1st to 10th	Night	25% 5% 9% 24% 29% 8%	2160
	11th to 20th	Night	26% 5% 9% 23% 30% 6%	2008
	21st to end	Night	29% 6% 11% 26% 23% 5%	2043
April	1st to 10th	Night	27% 6% 11% 24% 24% 10%	1800
	11th to 20th	Night	32% 7% 10% 24% 19% 8%	1777
	21st to end	Night	35% 6% 9% 24% 21% 5%	1800
May	1st to 10th	Night	35% 10% 12% 22% 16% 4	1683
	11th to 20th	Night	36% 9% 19% 20% 13% 3	1422
	21st to end	Night	26% 9% 18% 25% 14% 8%	1552
June	1st to 10th	Night	20% 7% 20% 31% 14% 8%	1440
	11th to 20th	Night	20% 5% 15% 19% 24% 17%	1440
	21st to end	Night	19% 7% 12% 24% 29% 9%	1440
July	1st to 10th	Night	25% 9% 15% 23% 21% 7%	1400
	11th to 20th	Night	18% 8% 12% 26% 23% 12%	1392
	21st to end	Night	24% 9% 18% 28% 15% 5%	1522
August	1st to 10th	Night	28% 9% 18% 25% 14% 6%	1602
	11th to 20th	Night	20% 7% 12% 26% 25% 11%	1774
	21st to end	Night	23% 7% 12% 22% 27% 9%	1980
September	1st to 10th	Night	23% 6% 9% 21% 23% 17%	1800
	11th to 20th	Night	18% 6% 11% 23% 27% 16%	1821
	21st to end	Night	22% 4 8% 22% 33% 11%	1980
October	1st to 10th	Night	24% 6% 11% 19% 28% 12%	2139
	11th to 20th	Night	17% 4 11% 22% 29% 18%	2160
	21st to end	Night	26% 4 8% 23% 27% 13%	2429
November	1st to 10th	Night	17% 5% 9% 25% 29% 16%	2405
	11th to 20th	Night	20% 4 7% 27% 33% 9%	2470
	21st to end	Night	24% 4 9% 21% 32% 11%	2565
December	1st to 10th	Night	23% 5% 12% 26% 25% 9%	2660
	11th to 20th	Night	22% 6% 9% 28% 31% 5%	3272
	21st to end	Night	28% 8% 10% 23% 25% 5%	4873

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	4 10% 27% 43% 12% 4	270
	11th to 20th	Day	7% 7% 35% 39% 12%	243
	21st to end	Day	4 9% 44% 34% 8% 2	264
February	1st to 10th	Day	5% 16% 44% 27% 6% 2	240
	11th to 20th	Day	4 7% 46% 34% 8% 2	240
	21st to end	Day	6% 11% 38% 39% 7%	198
March	1st to 10th	Day	5% 11% 34% 36% 12% 2	240
	11th to 20th	Day	13% 14% 30% 30% 13% 1	390
	21st to end	Day	13% 21% 32% 27% 6% 1	462
April	1st to 10th	Day	11% 17% 32% 28% 10% 2	420
	11th to 20th	Day	16% 19% 35% 22% 7% 2	412
	21st to end	Day	15% 15% 32% 25% 12% 1	420
May	1st to 10th	Day	15% 9% 28% 33% 12% 2	539
	11th to 20th	Day	19% 7% 22% 29% 17% 6%	625
	21st to end	Day	10% 9% 24% 30% 20% 7%	679
June	1st to 10th	Day	6% 5% 20% 32% 26% 11%	630
	11th to 20th	Day	7% 7% 26% 35% 20% 6%	630
	21st to end	Day	6% 5% 23% 33% 25% 7%	630
July	1st to 10th	Day	8% 3 14% 30% 35% 11%	621
	11th to 20th	Day	5% 3 15% 30% 37% 10%	618
	21st to end	Day	5% 2 14% 28% 39% 11%	646
August	1st to 10th	Day	6% 4 23% 30% 25% 12%	447
	11th to 20th	Day	4 3 22% 32% 28% 10%	437
	21st to end	Day	7% 3 19% 38% 25% 8%	495
September	1st to 10th	Day	6% 4 21% 35% 26% 8%	450
	11th to 20th	Day	5% 4 22% 36% 24% 9%	428
	21st to end	Day	6% 6% 22% 35% 21% 9%	270
October	1st to 10th	Day	9% 6% 23% 28% 25% 10%	270
	11th to 20th	Day	4 8% 27% 40% 16% 5%	270
	21st to end	Day	10% 11% 26% 33% 16% 2	297
November	1st to 10th	Day	4 8% 31% 37% 14% 5%	270
	11th to 20th	Day	8% 9% 29% 40% 11% 3	270
	21st to end	Day	8% 9% 31% 36% 14% 2	270
December	1st to 10th	Day	7% 9% 33% 35% 13% 1	270
	11th to 20th	Day	5% 7% 40% 41% 7% 1	270
	21st to end	Day	5% 9% 38% 39% 7% 2	297
January	1st to 10th	Night	4 5% 41% 39% 10% 2	950
	11th to 20th	Night	4 6% 47% 34% 9% 1	886
	21st to end	Night	5% 4 43% 40% 7% 1	792
February	1st to 10th	Night	6% 4 50% 36% 5%	720
	11th to 20th	Night	4 4 47% 38% 7%	704
	21st to end	Night	7% 6% 43% 40% 3	592
March	1st to 10th	Night	4 4 44% 38% 9%	720
	11th to 20th	Night	9% 4 34% 42% 11%	720
	21st to end	Night	9% 7% 39% 38% 6%	657
April	1st to 10th	Night	10% 8% 38% 39% 5%	540
	11th to 20th	Night	12% 7% 34% 37% 10%	533
	21st to end	Night	15% 7% 29% 36% 12% 1	540
May	1st to 10th	Night	14% 6% 22% 43% 14% 1	540
	11th to 20th	Night	16% 4 16% 44% 17% 2	531
	21st to end	Night	10% 5% 22% 39% 23% 2	584
June	1st to 10th	Night	5% 4 15% 46% 28% 3	540
	11th to 20th	Night	6% 2 22% 50% 18% 1	540
	21st to end	Night	5% 2 14% 47% 30% 2	540
July	1st to 10th	Night	6% 3 11% 40% 38% 2	525
	11th to 20th	Night	2 1 10% 46% 38% 3	522
	21st to end	Night	5% 2 5% 44% 41% 3	591
August	1st to 10th	Night	6% 2 9% 44% 37% 2	537
	11th to 20th	Night	4 2 12% 48% 32% 2	534
	21st to end	Night	4 1 11% 46% 35% 3	594
September	1st to 10th	Night	6% 3 12% 47% 26% 6%	540
	11th to 20th	Night	7% 2 13% 50% 25% 4	540
	21st to end	Night	7% 1 20% 47% 22% 3	540
October	1st to 10th	Night	12% 4 22% 41% 19% 3	699
	11th to 20th	Night	5% 3 30% 46% 15% 2	720
	21st to end	Night	10% 5% 30% 40% 14% 2	845
November	1st to 10th	Night	3 4 35% 44% 12% 2	925
	11th to 20th	Night	5% 6% 37% 44% 7% 1	950
	21st to end	Night	7% 4 35% 44% 11% 1	950
December	1st to 10th	Night	4 5% 41% 42% 8%	950
	11th to 20th	Night	5% 6% 43% 39% 7% 1	950
	21st to end	Night	7% 6% 42% 37% 7% 1	1045

Page generated at 2024-01-09 05:14:44. Version ac07

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