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 2024-11-05.

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 5% 3 4 4 4 6% 8% 54%	105188
	Night	5% 6% 2 5% 3 3 4 3 5% 7% 57%	102570
Transparency	Day	26% 7% 11% 22% 24% 10%	104932
	Night	24% 6% 11% 24% 25% 9%	102417
Seeing	Day	8% 8% 26% 33% 19% 6%	15599
	Night	7% 4 30% 42% 15% 1	25603

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	1 5% 2 6% 3 3 3 3 6% 8% 60%	2538
	11th to 20th	Day	2 5% 3 5% 4 4 5% 3 6% 7% 56%	2606
	21st to end	Day	2 6% 2 5% 3 3 4 3 5% 8% 59%	3146
February	1st to 10th	Day	3 6% 4 7% 3 4 4 4 5% 7% 53%	1710
	11th to 20th	Day	1 5% 2 7% 5% 4 4 4 5% 9% 56%	1825
	21st to end	Day	3 7% 3 6% 4 3 3 2 5% 6% 57%	1570
March	1st to 10th	Day	4 10% 3 5% 3 3 4 3 5% 7% 54%	3130
	11th to 20th	Day	6% 9% 2 6% 4 4 4 3 4 7% 50%	3446
	21st to end	Day	4 8% 2 4 4 4 4 5% 6% 8% 50%	4062
April	1st to 10th	Day	6% 6% 2 5% 3 3 3 3 6% 8% 55%	2280
	11th to 20th	Day	10% 9% 2 5% 3 3 4 4 5% 7% 47%	2251
	21st to end	Day	10% 7% 2 5% 3 3 4 4 7% 8% 47%	2280
May	1st to 10th	Day	9% 7% 3 7% 4 5% 5% 5% 6% 8% 41%	4014
	11th to 20th	Day	10% 6% 2 5% 4 5% 6% 4 8% 9% 41%	4380
	21st to end	Day	5% 4 1 5% 4 4 5% 5% 7% 9% 51%	4825
June	1st to 10th	Day	5% 4 1 5% 4 4 5% 5% 8% 10% 49%	2660
	11th to 20th	Day	5% 3 1 4 3 4 6% 5% 7% 10% 51%	2660
	21st to end	Day	4 3 1 5% 4 5% 6% 5% 8% 10% 49%	2660
July	1st to 10th	Day	5% 3 1 5% 4 4 4 4 6% 7% 57%	4430
	11th to 20th	Day	3 2 1 4 4 5% 6% 6% 11% 11% 47%	4430
	21st to end	Day	4 3 2 5% 3 4 5% 4 6% 8% 56%	4725
August	1st to 10th	Day	9% 5% 2 6% 5% 6% 6% 5% 9% 10% 38%	4092
	11th to 20th	Day	3 3 1 4 4 5% 6% 5% 8% 9% 51%	3727
	21st to end	Day	3 4 1 5% 4 4 6% 4 7% 10% 51%	4180
September	1st to 10th	Day	5% 3 1 4 3 4 4 3 5% 7% 61%	2280
	11th to 20th	Day	5% 4 1 4 3 2 4 5% 6% 7% 61%	2256
	21st to end	Day	6% 4 2 3 2 3 3 3 3 6% 65%	2090
October	1st to 10th	Day	4 2 2 2 2 2 2 3 5% 77%	3201
	11th to 20th	Day	4 4 2 4 3 3 3 3 5% 7% 62%	3150
	21st to end	Day	5% 6% 1 3 2 2 2 3 3 6% 66%	3314
November	1st to 10th	Day	2 4 1 3 3 2 3 3 6% 8% 65%	1709
	11th to 20th	Day	2 5% 2 4 2 2 3 3 6% 8% 63%	1710
	21st to end	Day	3 6% 2 5% 3 3 2 4 5% 7% 60%	1611
December	1st to 10th	Day	2 6% 2 5% 3 3 4 2 6% 8% 59%	1520
	11th to 20th	Day	2 5% 2 4 3 3 3 2 7% 9% 60%	1926
	21st to end	Day	2 10% 4 7% 4 4 4 3 4 6% 53%	2794
January	1st to 10th	Night	2 8% 3 7% 3 3 3 3 5% 7% 56%	4417
	11th to 20th	Night	1 4 2 4 3 4 4 3 5% 8% 62%	4394
	21st to end	Night	3 7% 3 7% 4 4 4 2 4 5% 56%	4521
February	1st to 10th	Night	2 8% 3 6% 3 4 4 3 5% 6% 56%	2470
	11th to 20th	Night	2 8% 3 7% 5% 4 3 3 4 7% 54%	2240
	21st to end	Night	3 8% 2 7% 4 3 3 2 4 6% 59%	1884
March	1st to 10th	Night	4 9% 2 6% 4 3 3 3 4 6% 57%	3734
	11th to 20th	Night	10% 11% 3 4 3 3 3 3 4 5% 53%	3531
	21st to end	Night	6% 9% 2 5% 4 3 3 3 5% 8% 52%	3582
April	1st to 10th	Night	7% 6% 2 4 4 3 4 3 5% 7% 56%	1900
	11th to 20th	Night	11% 8% 2 5% 3 4 3 3 4 6% 51%	1877
	21st to end	Night	12% 8% 3 5% 3 3 4 3 5% 6% 48%	1900
May	1st to 10th	Night	10% 6% 2 6% 4 4 4 3 6% 7% 47%	2955
	11th to 20th	Night	10% 6% 2 5% 4 3 4 4 5% 9% 47%	2521
	21st to end	Night	8% 5% 2 4 3 3 4 4 6% 6% 56%	2762
June	1st to 10th	Night	6% 3 1 2 3 2 6% 4 6% 10% 58%	1520
	11th to 20th	Night	5% 3 1 3 2 3 4 3 6% 8% 62%	1520
	21st to end	Night	5% 3 1 3 3 3 5% 4 7% 8% 58%	1520
July	1st to 10th	Night	4 3 1 3 4 3 4 4 5% 8% 61%	2540
	11th to 20th	Night	3 3 2 4 4 4 4 4 7% 11% 56%	2540
	21st to end	Night	4 3 1 4 4 4 4 4 8% 8% 57%	2822
August	1st to 10th	Night	8% 4 2 5% 4 4 6% 5% 8% 9% 44%	2847
	11th to 20th	Night	3 2 1 3 4 4 6% 5% 8% 12% 53%	3109
	21st to end	Night	3 3 1 4 4 4 6% 5% 8% 10% 52%	3487
September	1st to 10th	Night	7% 3 1 4 3 3 4 3 6% 7% 59%	1900
	11th to 20th	Night	6% 3 1 3 2 2 2 3 5% 7% 65%	1923
	21st to end	Night	7% 3 1 4 3 2 3 3 3 6% 65%	2090
October	1st to 10th	Night	5% 3 1 2 1 1 2 2 4 4 74%	3765
	11th to 20th	Night	6% 4 1 4 3 3 3 3 5% 7% 63%	3778
	21st to end	Night	6% 6% 1 4 2 2 3 2 3 6% 65%	4212
November	1st to 10th	Night	3 3 1 3 3 2 3 3 5% 7% 67%	2470
	11th to 20th	Night	3 5% 1 5% 3 2 3 4 4 6% 64%	2470
	21st to end	Night	4 6% 2 5% 3 2 3 3 4 6% 62%	2565
December	1st to 10th	Night	2 6% 3 5% 3 3 3 2 5% 8% 61%	2660
	11th to 20th	Night	2 6% 2 5% 3 3 3 3 6% 9% 59%	3271
	21st to end	Night	5% 9% 3 6% 4 4 4 3 6% 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	21% 6% 12% 30% 23% 9%	2539
	11th to 20th	Day	25% 6% 14% 29% 19% 7%	2606
	21st to end	Day	23% 5% 14% 29% 24% 5%	3146
February	1st to 10th	Day	29% 5% 12% 25% 21% 7%	1710
	11th to 20th	Day	24% 6% 9% 28% 27% 6%	1835
	21st to end	Day	26% 5% 9% 22% 27% 11%	1570
March	1st to 10th	Day	28% 4 10% 19% 28% 11%	3130
	11th to 20th	Day	31% 9% 14% 27% 15% 4	3446
	21st to end	Day	27% 8% 16% 28% 17% 4	4062
April	1st to 10th	Day	26% 6% 12% 24% 23% 9%	2280
	11th to 20th	Day	34% 7% 11% 21% 18% 9%	2254
	21st to end	Day	32% 7% 13% 24% 21% 2	2280
May	1st to 10th	Day	35% 9% 13% 22% 15% 6%	4014
	11th to 20th	Day	33% 12% 14% 22% 17% 3	4384
	21st to end	Day	24% 10% 15% 23% 21% 7%	4825
June	1st to 10th	Day	26% 10% 15% 24% 17% 9%	2660
	11th to 20th	Day	24% 8% 12% 19% 23% 14%	2660
	21st to end	Day	25% 7% 12% 21% 23% 12%	2660
July	1st to 10th	Day	24% 6% 8% 16% 29% 17%	4361
	11th to 20th	Day	24% 9% 11% 21% 23% 12%	4355
	21st to end	Day	25% 6% 11% 18% 21% 19%	4576
August	1st to 10th	Day	36% 9% 14% 21% 15% 6%	4078
	11th to 20th	Day	26% 7% 14% 17% 21% 15%	3763
	21st to end	Day	26% 5% 11% 19% 25% 14%	4179
September	1st to 10th	Day	23% 5% 8% 20% 25% 19%	2280
	11th to 20th	Day	22% 6% 7% 20% 28% 17%	2256
	21st to end	Day	21% 4 7% 19% 36% 13%	2090
October	1st to 10th	Day	14% 3 6% 15% 37% 26%	3201
	11th to 20th	Day	23% 4 8% 26% 30% 9%	3150
	21st to end	Day	21% 3 5% 23% 37% 11%	3314
November	1st to 10th	Day	17% 3 10% 25% 29% 15%	1709
	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	26% 6% 10% 28% 26% 4	4417
	11th to 20th	Night	18% 8% 13% 27% 24% 9%	4394
	21st to end	Night	28% 6% 12% 29% 18% 5%	4521
February	1st to 10th	Night	28% 5% 10% 28% 23% 6%	2470
	11th to 20th	Night	30% 6% 7% 28% 23% 6%	2240
	21st to end	Night	27% 4 6% 24% 27% 13%	1884
March	1st to 10th	Night	26% 5% 7% 21% 33% 8%	3734
	11th to 20th	Night	33% 7% 13% 20% 24% 4	3531
	21st to end	Night	28% 8% 12% 32% 17% 3	3582
April	1st to 10th	Night	26% 6% 10% 23% 24% 10%	1900
	11th to 20th	Night	33% 8% 10% 23% 19% 8%	1877
	21st to end	Night	35% 7% 10% 24% 20% 4	1900
May	1st to 10th	Night	32% 11% 12% 21% 15% 9%	2955
	11th to 20th	Night	31% 10% 18% 23% 14% 4	2522
	21st to end	Night	22% 11% 20% 22% 19% 6%	2762
June	1st to 10th	Night	20% 7% 20% 31% 14% 9%	1520
	11th to 20th	Night	20% 5% 15% 20% 24% 16%	1520
	21st to end	Night	20% 7% 12% 23% 28% 10%	1520
July	1st to 10th	Night	20% 6% 12% 19% 32% 11%	2500
	11th to 20th	Night	21% 7% 14% 26% 23% 9%	2492
	21st to end	Night	20% 6% 14% 25% 20% 15%	2734
August	1st to 10th	Night	34% 7% 15% 23% 17% 4	2838
	11th to 20th	Night	20% 9% 20% 20% 20% 11%	3139
	21st to end	Night	24% 6% 14% 22% 27% 6%	3487
September	1st to 10th	Night	23% 6% 9% 22% 23% 17%	1900
	11th to 20th	Night	18% 5% 10% 23% 28% 16%	1923
	21st to end	Night	22% 4 8% 21% 33% 12%	2090
October	1st to 10th	Night	15% 4 6% 19% 32% 24%	3765
	11th to 20th	Night	21% 3 9% 26% 30% 10%	3778
	21st to end	Night	23% 3 5% 23% 34% 13%	4212
November	1st to 10th	Night	18% 5% 8% 25% 29% 15%	2470
	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% 8% 37% 36% 11% 1	270
	21st to end	Day	3 8% 40% 33% 13% 2	297
February	1st to 10th	Day	6% 15% 42% 29% 7% 2	270
	11th to 20th	Day	3 7% 46% 35% 8% 1	270
	21st to end	Day	6% 12% 39% 36% 6%	225
March	1st to 10th	Day	4 11% 34% 36% 12% 2	270
	11th to 20th	Day	13% 14% 29% 29% 13% 2	429
	21st to end	Day	13% 20% 31% 28% 6% 1	506
April	1st to 10th	Day	12% 17% 32% 28% 10% 2	460
	11th to 20th	Day	15% 19% 34% 22% 8% 2	452
	21st to end	Day	14% 14% 31% 27% 12% 1	460
May	1st to 10th	Day	15% 11% 28% 32% 12% 2	586
	11th to 20th	Day	18% 7% 22% 30% 17% 5%	675
	21st to end	Day	9% 9% 25% 31% 20% 6%	734
June	1st to 10th	Day	6% 5% 20% 33% 26% 10%	680
	11th to 20th	Day	6% 8% 26% 35% 19% 6%	680
	21st to end	Day	6% 5% 25% 34% 24% 7%	680
July	1st to 10th	Day	8% 3 16% 30% 33% 10%	671
	11th to 20th	Day	5% 3 15% 31% 37% 10%	668
	21st to end	Day	5% 3 14% 28% 39% 11%	698
August	1st to 10th	Day	7% 4 23% 29% 25% 12%	487
	11th to 20th	Day	4 4 23% 32% 27% 10%	477
	21st to end	Day	7% 3 19% 38% 24% 8%	539
September	1st to 10th	Day	5% 3 20% 35% 27% 10%	490
	11th to 20th	Day	5% 5% 22% 36% 23% 8%	466
	21st to end	Day	6% 6% 22% 34% 22% 9%	300
October	1st to 10th	Day	9% 5% 22% 30% 25% 10%	300
	11th to 20th	Day	4 8% 27% 39% 17% 5%	300
	21st to end	Day	12% 10% 26% 34% 15% 2	330
November	1st to 10th	Day	4 8% 32% 38% 13% 5%	282
	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% 48% 33% 8% 1	927
	21st to end	Night	5% 4 42% 39% 8% 1	836
February	1st to 10th	Night	6% 4 49% 35% 6%	760
	11th to 20th	Night	4 5% 46% 39% 6%	744
	21st to end	Night	7% 7% 44% 39% 3	628
March	1st to 10th	Night	4 5% 43% 39% 9%	760
	11th to 20th	Night	10% 4 32% 42% 11%	760
	21st to end	Night	9% 7% 39% 39% 6%	693
April	1st to 10th	Night	10% 9% 37% 39% 5%	570
	11th to 20th	Night	13% 8% 34% 36% 10%	563
	21st to end	Night	15% 8% 30% 36% 11% 1	570
May	1st to 10th	Night	14% 7% 24% 42% 13% 1	570
	11th to 20th	Night	16% 4 16% 45% 16% 2	561
	21st to end	Night	9% 5% 23% 39% 23% 1	617
June	1st to 10th	Night	5% 4 16% 46% 27% 3	570
	11th to 20th	Night	6% 2 23% 51% 17% 1	570
	21st to end	Night	5% 2 15% 47% 29% 2	570
July	1st to 10th	Night	6% 3 13% 40% 36% 2	555
	11th to 20th	Night	3 1 10% 46% 38% 3	552
	21st to end	Night	5% 2 6% 44% 40% 3	624
August	1st to 10th	Night	7% 2 9% 44% 37% 2	567
	11th to 20th	Night	4 2 14% 47% 32% 2	564
	21st to end	Night	4 1 11% 47% 33% 3	627
September	1st to 10th	Night	6% 3 12% 46% 27% 5%	570
	11th to 20th	Night	6% 2 14% 50% 24% 4	570
	21st to end	Night	7% 2 21% 46% 22% 3	570
October	1st to 10th	Night	11% 4 22% 42% 19% 3	738
	11th to 20th	Night	6% 2 29% 46% 15% 1	760
	21st to end	Night	10% 5% 30% 41% 13% 2	892
November	1st to 10th	Night	4 4 35% 44% 12% 2	950
	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 2024-11-03 00:11:12. Version ac07

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