Hanwell
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hanwell clear sky chart from 2005-11-06 to 2026-06-01.

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	19% 9% 3 7% 5% 4 4 4 5% 6% 33%	131889
	Night	19% 10% 3 7% 5% 4 4 3 5% 6% 33%	131387
Transparency	Day	50% 6% 12% 20% 11% 1	135389
	Night	50% 6% 11% 21% 11% 1	135975
Seeing	Day	28% 18% 30% 18% 6% 1	18321
	Night	26% 15% 31% 22% 6%	26303

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	13% 16% 6% 12% 5% 5% 4 3 5% 5% 26%	4093
	11th to 20th	Day	15% 14% 5% 14% 6% 5% 4 3 4 4 28%	4607
	21st to end	Day	9% 9% 3 9% 4 4 5% 4 7% 8% 37%	5479
February	1st to 10th	Day	15% 12% 4 9% 6% 5% 5% 4 5% 6% 30%	1890
	11th to 20th	Day	16% 12% 3 9% 6% 5% 5% 3 5% 6% 30%	1843
	21st to end	Day	11% 13% 3 7% 6% 5% 5% 4 5% 6% 35%	1697
March	1st to 10th	Day	25% 13% 3 10% 5% 3 3 3 4 5% 27%	4800
	11th to 20th	Day	20% 13% 3 10% 5% 5% 4 4 5% 5% 26%	5090
	21st to end	Day	26% 13% 4 8% 4 4 4 4 4 6% 23%	5587
April	1st to 10th	Day	22% 15% 3 7% 4 5% 3 3 4 6% 29%	2467
	11th to 20th	Day	22% 11% 3 6% 4 3 3 3 5% 7% 33%	2702
	21st to end	Day	26% 9% 2 6% 4 4 3 3 4 6% 32%	2730
May	1st to 10th	Day	26% 11% 3 8% 6% 4 4 3 5% 5% 24%	6016
	11th to 20th	Day	26% 8% 3 6% 4 5% 4 3 5% 6% 29%	6288
	21st to end	Day	22% 9% 3 7% 5% 5% 5% 4 5% 7% 30%	7379
June	1st to 10th	Day	25% 8% 3 7% 5% 4 4 3 5% 7% 28%	3000
	11th to 20th	Day	22% 8% 3 5% 5% 3 4 3 5% 8% 36%	3000
	21st to end	Day	26% 8% 3 6% 5% 4 4 4 5% 6% 27%	3000
July	1st to 10th	Day	17% 7% 2 7% 6% 6% 6% 5% 7% 9% 29%	6820
	11th to 20th	Day	16% 6% 2 6% 4 4 5% 4 6% 8% 39%	6775
	21st to end	Day	14% 7% 2 6% 4 5% 5% 4 6% 8% 40%	6656
August	1st to 10th	Day	20% 7% 3 7% 6% 5% 4 4 5% 6% 34%	4235
	11th to 20th	Day	15% 7% 3 8% 5% 5% 5% 4 6% 8% 33%	4175
	21st to end	Day	15% 5% 2 6% 3 3 5% 4 6% 8% 44%	4675
September	1st to 10th	Day	13% 4 2 5% 4 5% 5% 4 5% 7% 46%	2378
	11th to 20th	Day	17% 6% 2 5% 3 4 4 3 4 5% 48%	2200
	21st to end	Day	18% 6% 2 6% 4 3 3 3 5% 6% 44%	2200
October	1st to 10th	Day	18% 7% 3 6% 4 4 4 3 5% 6% 41%	2049
	11th to 20th	Day	20% 8% 2 6% 4 4 4 3 5% 7% 37%	2000
	21st to end	Day	22% 9% 3 8% 5% 5% 5% 3 5% 5% 31%	2055
November	1st to 10th	Day	21% 8% 2 7% 4 4 5% 2 4 7% 35%	1798
	11th to 20th	Day	21% 8% 2 6% 5% 4 4 3 6% 7% 35%	1680
	21st to end	Day	21% 12% 4 8% 5% 4 5% 3 4 6% 30%	1680
December	1st to 10th	Day	20% 12% 3 7% 3 3 3 3 6% 6% 34%	2604
	11th to 20th	Day	22% 10% 2 8% 5% 4 3 2 4 4 35%	2688
	21st to end	Day	19% 13% 4 8% 4 3 3 3 4 5% 34%	3553
January	1st to 10th	Night	14% 15% 4 9% 6% 4 5% 3 5% 6% 27%	8831
	11th to 20th	Night	19% 13% 4 11% 6% 5% 4 3 5% 5% 26%	8277
	21st to end	Night	8% 11% 4 7% 5% 5% 4 4 6% 7% 38%	8699
February	1st to 10th	Night	14% 13% 3 7% 5% 4 4 3 5% 7% 33%	2730
	11th to 20th	Night	16% 12% 4 8% 5% 4 5% 4 5% 7% 30%	2663
	21st to end	Night	16% 14% 2 6% 6% 5% 5% 4 5% 4 35%	2107
March	1st to 10th	Night	25% 14% 4 8% 5% 5% 4 3 4 3 26%	5259
	11th to 20th	Night	20% 11% 3 8% 4 4 5% 4 6% 7% 28%	5089
	21st to end	Night	30% 11% 3 6% 3 3 3 3 4 5% 28%	5587
April	1st to 10th	Night	25% 12% 2 6% 5% 4 4 3 5% 5% 30%	2151
	11th to 20th	Night	25% 8% 3 6% 3 4 4 2 4 5% 35%	1868
	21st to end	Night	24% 10% 2 6% 4 4 4 3 5% 6% 32%	1890
May	1st to 10th	Night	25% 9% 3 8% 5% 4 4 4 5% 6% 26%	4144
	11th to 20th	Night	24% 8% 2 5% 5% 4 5% 4 5% 6% 32%	3824
	21st to end	Night	19% 8% 2 8% 4 4 5% 3 5% 7% 35%	3802
June	1st to 10th	Night	24% 8% 3 7% 4 5% 6% 3 5% 7% 29%	1400
	11th to 20th	Night	19% 7% 3 6% 4 4 4 4 6% 7% 36%	1400
	21st to end	Night	26% 7% 2 6% 4 5% 4 3 6% 7% 31%	1400
July	1st to 10th	Night	16% 7% 3 8% 5% 5% 6% 5% 7% 8% 31%	3160
	11th to 20th	Night	16% 7% 3 7% 5% 5% 4 4 5% 8% 36%	3199
	21st to end	Night	12% 6% 2 6% 5% 4 5% 5% 7% 7% 41%	4149
August	1st to 10th	Night	18% 7% 3 8% 5% 5% 5% 4 6% 7% 32%	2915
	11th to 20th	Night	13% 7% 3 8% 4 5% 5% 4 7% 8% 36%	2872
	21st to end	Night	14% 4 2 5% 4 4 3 4 5% 7% 47%	3223
September	1st to 10th	Night	15% 5% 2 6% 4 3 3 4 6% 7% 46%	2018
	11th to 20th	Night	16% 6% 2 5% 3 3 3 3 4 6% 49%	2200
	21st to end	Night	18% 6% 1 5% 4 3 4 3 4 6% 45%	2200
October	1st to 10th	Night	19% 7% 2 5% 3 3 3 3 5% 7% 44%	2350
	11th to 20th	Night	18% 8% 3 7% 4 5% 4 3 4 7% 38%	2400
	21st to end	Night	22% 9% 3 8% 5% 4 4 3 4 6% 34%	2784
November	1st to 10th	Night	21% 8% 2 7% 4 4 4 3 4 6% 37%	2720
	11th to 20th	Night	21% 8% 3 6% 5% 4 4 4 5% 6% 35%	2940
	21st to end	Night	23% 10% 3 6% 4 4 4 4 5% 6% 31%	2940
December	1st to 10th	Night	17% 11% 3 8% 4 4 4 3 5% 6% 34%	4788
	11th to 20th	Night	22% 10% 3 6% 4 4 4 3 5% 5% 36%	5741
	21st to end	Night	20% 13% 3 8% 4 4 3 3 4 5% 33%	7667

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	58% 6% 11% 16% 9%	4094
	11th to 20th	Day	59% 4 11% 16% 8% 1	4637
	21st to end	Day	42% 6% 9% 23% 19% 2	5479
February	1st to 10th	Day	53% 6% 11% 18% 10% 2	1890
	11th to 20th	Day	54% 5% 10% 20% 10% 1	1851
	21st to end	Day	48% 6% 11% 22% 11% 2	1697
March	1st to 10th	Day	61% 3 9% 11% 12% 3	4804
	11th to 20th	Day	58% 7% 13% 14% 7% 2	5090
	21st to end	Day	62% 4 6% 17% 10% 2	5587
April	1st to 10th	Day	57% 4 9% 14% 13% 2	2467
	11th to 20th	Day	51% 5% 10% 21% 12% 1	2702
	21st to end	Day	52% 6% 9% 16% 13% 5%	2730
May	1st to 10th	Day	59% 7% 8% 17% 8% 1	6027
	11th to 20th	Day	54% 7% 11% 20% 6% 2	6292
	21st to end	Day	51% 8% 10% 17% 14%	7379
June	1st to 10th	Day	55% 7% 10% 17% 10% 1	3000
	11th to 20th	Day	47% 5% 13% 20% 13% 1	3000
	21st to end	Day	56% 7% 10% 16% 9% 1	3000
July	1st to 10th	Day	49% 7% 16% 19% 9%	6745
	11th to 20th	Day	42% 7% 15% 26% 10% 1	6694
	21st to end	Day	41% 6% 14% 30% 9%	6465
August	1st to 10th	Day	50% 6% 13% 24% 6%	4222
	11th to 20th	Day	48% 7% 14% 23% 7%	4214
	21st to end	Day	38% 6% 14% 26% 16%	4675
September	1st to 10th	Day	36% 7% 13% 28% 15% 1	2378
	11th to 20th	Day	38% 5% 9% 28% 17% 2	2200
	21st to end	Day	41% 5% 10% 26% 17% 1	2200
October	1st to 10th	Day	48% 5% 11% 26% 9%	3335
	11th to 20th	Day	44% 4 13% 29% 9%	3240
	21st to end	Day	42% 4 15% 22% 17%	3292
November	1st to 10th	Day	50% 5% 12% 21% 12% 1	1798
	11th to 20th	Day	48% 6% 14% 18% 13% 2	1680
	21st to end	Day	55% 6% 13% 19% 6%	1680
December	1st to 10th	Day	49% 5% 18% 18% 8%	2604
	11th to 20th	Day	54% 4 10% 18% 10% 4	2688
	21st to end	Day	53% 4 13% 15% 13% 3	3553
January	1st to 10th	Night	54% 7% 11% 19% 8%	8831
	11th to 20th	Night	58% 6% 10% 18% 8% 1	8322
	21st to end	Night	43% 5% 8% 23% 18% 3	8699
February	1st to 10th	Night	51% 5% 12% 18% 12% 1	2730
	11th to 20th	Night	53% 6% 11% 19% 10% 2	2666
	21st to end	Night	51% 5% 9% 20% 12% 3	2107
March	1st to 10th	Night	63% 4 6% 13% 11% 3	5268
	11th to 20th	Night	54% 8% 16% 13% 7% 1	5090
	21st to end	Night	59% 2 6% 19% 12% 1	5587
April	1st to 10th	Night	56% 5% 9% 15% 11% 4	2151
	11th to 20th	Night	51% 5% 9% 19% 15% 1	1868
	21st to end	Night	53% 6% 9% 17% 13% 2	1890
May	1st to 10th	Night	56% 7% 10% 16% 9% 1	4150
	11th to 20th	Night	50% 8% 11% 22% 7% 2	3825
	21st to end	Night	47% 8% 15% 20% 8% 2	3803
June	1st to 10th	Night	53% 8% 11% 20% 8% 1	1400
	11th to 20th	Night	47% 7% 13% 22% 10% 1	1400
	21st to end	Night	53% 7% 11% 20% 8% 2	1400
July	1st to 10th	Night	49% 7% 14% 22% 8%	3125
	11th to 20th	Night	46% 7% 15% 27% 5%	3157
	21st to end	Night	40% 7% 15% 27% 10%	4026
August	1st to 10th	Night	50% 8% 13% 21% 7%	2903
	11th to 20th	Night	44% 8% 18% 23% 6%	2899
	21st to end	Night	36% 7% 16% 24% 16% 2	3223
September	1st to 10th	Night	38% 6% 13% 28% 13% 1	2018
	11th to 20th	Night	37% 5% 13% 29% 16% 1	2200
	21st to end	Night	40% 6% 10% 28% 15% 1	2200
October	1st to 10th	Night	46% 5% 11% 26% 12% 1	3844
	11th to 20th	Night	39% 6% 12% 28% 15%	3899
	21st to end	Night	44% 4 15% 22% 14%	4497
November	1st to 10th	Night	49% 6% 13% 22% 10% 1	2720
	11th to 20th	Night	49% 6% 13% 21% 11% 1	2940
	21st to end	Night	53% 8% 13% 16% 10%	2940
December	1st to 10th	Night	49% 5% 13% 25% 7%	4788
	11th to 20th	Night	51% 4 11% 21% 13% 1	5742
	21st to end	Night	55% 4 10% 18% 10% 3	7667

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	32% 20% 35% 12% 2	220
	11th to 20th	Day	36% 22% 30% 9% 3 1	220
	21st to end	Day	24% 22% 34% 16% 4	365
February	1st to 10th	Day	27% 19% 33% 16% 5%	430
	11th to 20th	Day	28% 24% 31% 12% 4 1	426
	21st to end	Day	26% 25% 29% 17% 2	430
March	1st to 10th	Day	34% 22% 31% 11% 2	540
	11th to 20th	Day	33% 21% 30% 12% 4 1	540
	21st to end	Day	35% 22% 26% 12% 4 1	594
April	1st to 10th	Day	41% 24% 22% 11% 2 1	540
	11th to 20th	Day	35% 22% 27% 12% 3 1	532
	21st to end	Day	32% 20% 31% 12% 4 1	540
May	1st to 10th	Day	36% 20% 28% 12% 4 1	543
	11th to 20th	Day	30% 20% 27% 16% 5% 1	695
	21st to end	Day	28% 17% 34% 15% 6%	845
June	1st to 10th	Day	31% 17% 29% 17% 6% 1	730
	11th to 20th	Day	27% 15% 28% 22% 7% 1	730
	21st to end	Day	29% 13% 31% 20% 6% 1	730
July	1st to 10th	Day	23% 13% 35% 20% 9% 1	720
	11th to 20th	Day	20% 9% 34% 25% 10% 2	699
	21st to end	Day	16% 10% 40% 26% 6% 1	576
August	1st to 10th	Day	18% 10% 36% 24% 10% 2	526
	11th to 20th	Day	17% 12% 28% 28% 12% 3	519
	21st to end	Day	21% 13% 28% 25% 10% 3	583
September	1st to 10th	Day	12% 15% 34% 26% 9% 4	530
	11th to 20th	Day	21% 13% 28% 23% 11% 4	530
	21st to end	Day	21% 17% 30% 19% 9% 4	530
October	1st to 10th	Day	22% 15% 25% 24% 11% 3	446
	11th to 20th	Day	25% 19% 27% 17% 9% 3	420
	21st to end	Day	29% 20% 30% 15% 5%	462
November	1st to 10th	Day	31% 17% 29% 17% 5% 1	425
	11th to 20th	Day	33% 19% 26% 15% 6%	430
	21st to end	Day	32% 22% 28% 15% 3	430
December	1st to 10th	Day	33% 17% 30% 15% 4	383
	11th to 20th	Day	38% 25% 25% 10% 1	220
	21st to end	Day	32% 23% 28% 14% 2	242
January	1st to 10th	Night	27% 20% 30% 19% 3	950
	11th to 20th	Night	30% 16% 30% 20% 4	848
	21st to end	Night	20% 22% 35% 19% 3	924
February	1st to 10th	Night	23% 22% 32% 19% 4	840
	11th to 20th	Night	26% 23% 32% 15% 3	823
	21st to end	Night	25% 19% 32% 21% 3	692
March	1st to 10th	Night	27% 20% 34% 18% 2	840
	11th to 20th	Night	25% 18% 33% 19% 4	840
	21st to end	Night	27% 17% 33% 18% 4	924
April	1st to 10th	Night	35% 18% 30% 15% 2	756
	11th to 20th	Night	30% 18% 32% 16% 4	624
	21st to end	Night	29% 15% 32% 18% 6%	630
May	1st to 10th	Night	28% 15% 31% 23% 3	630
	11th to 20th	Night	30% 15% 32% 19% 4	622
	21st to end	Night	21% 16% 37% 22% 4	578
June	1st to 10th	Night	26% 10% 34% 25% 5%	400
	11th to 20th	Night	21% 12% 32% 29% 7% 1	400
	21st to end	Night	30% 6% 29% 31% 4	400
July	1st to 10th	Night	18% 7% 38% 29% 7%	390
	11th to 20th	Night	20% 5% 30% 35% 10%	388
	21st to end	Night	19% 8% 32% 34% 8%	533
August	1st to 10th	Night	18% 5% 29% 38% 10%	598
	11th to 20th	Night	18% 7% 30% 34% 11%	592
	21st to end	Night	15% 9% 30% 32% 13% 1	660
September	1st to 10th	Night	18% 9% 37% 26% 10% 1	648
	11th to 20th	Night	20% 9% 25% 33% 12% 1	800
	21st to end	Night	21% 13% 29% 27% 10% 1	800
October	1st to 10th	Night	22% 11% 29% 27% 10% 1	800
	11th to 20th	Night	21% 14% 31% 24% 9% 1	800
	21st to end	Night	28% 17% 30% 20% 5%	880
November	1st to 10th	Night	29% 15% 29% 20% 5% 1	848
	11th to 20th	Night	29% 17% 28% 19% 6% 1	950
	21st to end	Night	29% 19% 29% 19% 4	950
December	1st to 10th	Night	33% 15% 29% 19% 4	950
	11th to 20th	Night	31% 22% 28% 16% 2	950
	21st to end	Night	31% 21% 27% 17% 3	1045

Page generated at 2026-05-29 02:53:29. Version ac07

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