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 2025-02-06.

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%	108128
	Night	19% 10% 3 7% 4 4 4 3 5% 6% 34%	110210
Transparency	Day	50% 6% 12% 20% 11% 1	111674
	Night	49% 6% 12% 21% 11% 1	114829
Seeing	Day	27% 17% 30% 18% 6% 1	16524
	Night	25% 15% 31% 22% 6%	24594

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	14% 18% 6% 12% 6% 5% 3 3 4 5% 24%	3157
	11th to 20th	Day	13% 13% 5% 13% 6% 5% 4 3 4 4 31%	3540
	21st to end	Day	10% 9% 3 9% 4 5% 5% 4 6% 7% 38%	4233
February	1st to 10th	Day	16% 12% 4 8% 6% 5% 5% 4 5% 6% 31%	1755
	11th to 20th	Day	16% 13% 3 9% 6% 5% 5% 4 5% 5% 28%	1663
	21st to end	Day	12% 13% 3 7% 6% 5% 5% 4 5% 6% 35%	1539
March	1st to 10th	Day	24% 15% 2 7% 4 3 4 3 4 5% 29%	3265
	11th to 20th	Day	17% 16% 4 11% 6% 6% 4 3 4 5% 23%	3480
	21st to end	Day	28% 13% 4 8% 5% 4 3 3 4 6% 23%	3816
April	1st to 10th	Day	22% 16% 3 7% 4 4 3 3 4 6% 28%	2232
	11th to 20th	Day	21% 10% 2 6% 4 4 3 3 5% 7% 34%	2442
	21st to end	Day	26% 9% 3 6% 4 3 3 3 4 6% 31%	2470
May	1st to 10th	Day	21% 10% 3 7% 6% 5% 4 4 5% 6% 29%	4106
	11th to 20th	Day	20% 8% 3 7% 5% 5% 4 3 5% 7% 32%	4288
	21st to end	Day	18% 7% 3 7% 5% 5% 6% 4 6% 8% 33%	5023
June	1st to 10th	Day	25% 9% 3 8% 5% 4 4 3 5% 6% 28%	2850
	11th to 20th	Day	21% 8% 3 5% 5% 3 4 3 5% 8% 36%	2850
	21st to end	Day	26% 9% 3 7% 5% 4 4 4 5% 6% 27%	2850
July	1st to 10th	Day	15% 6% 2 6% 6% 6% 6% 5% 7% 9% 32%	4760
	11th to 20th	Day	19% 6% 2 6% 4 5% 5% 4 7% 8% 35%	4729
	21st to end	Day	17% 7% 3 6% 5% 5% 5% 4 6% 7% 36%	4628
August	1st to 10th	Day	20% 7% 3 7% 6% 5% 4 4 5% 6% 33%	4105
	11th to 20th	Day	15% 7% 3 9% 5% 6% 5% 4 6% 8% 32%	4045
	21st to end	Day	15% 5% 2 6% 3 4 5% 4 6% 8% 44%	4532
September	1st to 10th	Day	13% 4 2 5% 4 5% 5% 4 5% 7% 46%	2260
	11th to 20th	Day	18% 6% 2 5% 3 4 4 3 4 5% 47%	2090
	21st to end	Day	19% 6% 2 6% 4 3 3 3 5% 6% 43%	2090
October	1st to 10th	Day	18% 7% 3 6% 4 4 4 3 5% 6% 39%	1947
	11th to 20th	Day	20% 8% 2 6% 4 4 4 3 5% 7% 37%	1900
	21st to end	Day	22% 9% 3 8% 5% 5% 5% 3 5% 5% 32%	1952
November	1st to 10th	Day	21% 8% 3 7% 4 4 5% 2 4 7% 36%	1711
	11th to 20th	Day	21% 7% 2 6% 5% 4 4 3 6% 7% 35%	1600
	21st to end	Day	21% 12% 4 8% 4 4 5% 3 3 6% 30%	1600
December	1st to 10th	Day	21% 11% 3 7% 3 3 3 3 6% 7% 33%	2526
	11th to 20th	Day	22% 11% 2 8% 5% 4 3 2 4 4 35%	2618
	21st to end	Day	19% 13% 4 8% 4 3 3 3 4 5% 34%	3476
January	1st to 10th	Night	14% 14% 5% 11% 7% 5% 5% 3 5% 6% 25%	6808
	11th to 20th	Night	16% 11% 4 11% 6% 5% 4 3 5% 6% 30%	6343
	21st to end	Night	10% 11% 4 7% 4 4 3 4 6% 7% 40%	6720
February	1st to 10th	Night	15% 13% 3 7% 5% 4 4 3 5% 7% 32%	2536
	11th to 20th	Night	16% 12% 4 8% 6% 5% 5% 4 5% 7% 29%	2403
	21st to end	Night	15% 14% 2 6% 6% 5% 5% 4 5% 4 34%	1913
March	1st to 10th	Night	24% 14% 3 7% 4 5% 5% 3 4 3 27%	3574
	11th to 20th	Night	20% 14% 4 9% 5% 4 5% 5% 6% 6% 24%	3479
	21st to end	Night	30% 11% 3 6% 4 4 3 3 5% 5% 26%	3816
April	1st to 10th	Night	25% 12% 2 6% 5% 4 4 3 5% 5% 30%	1947
	11th to 20th	Night	23% 8% 3 6% 3 4 4 3 4 6% 36%	1688
	21st to end	Night	25% 11% 2 6% 3 4 4 3 5% 6% 33%	1710
May	1st to 10th	Night	21% 8% 2 7% 5% 4 4 4 6% 7% 31%	2834
	11th to 20th	Night	19% 8% 2 6% 4 5% 5% 4 5% 7% 34%	2611
	21st to end	Night	16% 7% 3 8% 4 4 5% 3 6% 8% 36%	2590
June	1st to 10th	Night	24% 8% 3 7% 4 5% 6% 3 5% 7% 28%	1330
	11th to 20th	Night	19% 7% 3 6% 5% 4 3 4 6% 7% 36%	1330
	21st to end	Night	26% 7% 2 5% 4 5% 5% 3 5% 6% 31%	1330
July	1st to 10th	Night	14% 6% 2 7% 5% 5% 6% 5% 7% 8% 35%	2210
	11th to 20th	Night	19% 7% 2 6% 4 5% 5% 4 6% 8% 35%	2236
	21st to end	Night	15% 7% 2 6% 5% 4 6% 4 7% 6% 38%	2892
August	1st to 10th	Night	19% 7% 3 8% 5% 5% 6% 5% 6% 7% 30%	2825
	11th to 20th	Night	13% 7% 3 9% 5% 5% 5% 4 7% 8% 34%	2782
	21st to end	Night	15% 4 2 5% 4 4 3 4 5% 7% 47%	3124
September	1st to 10th	Night	15% 5% 2 6% 4 3 3 4 6% 7% 45%	1916
	11th to 20th	Night	17% 6% 2 5% 3 3 3 3 4 6% 48%	2090
	21st to end	Night	19% 7% 1 5% 4 3 3 3 4 7% 45%	2090
October	1st to 10th	Night	20% 7% 2 5% 3 3 3 3 5% 7% 42%	2232
	11th to 20th	Night	18% 7% 3 7% 4 5% 4 3 5% 7% 39%	2280
	21st to end	Night	22% 9% 2 7% 4 4 3 3 4 6% 35%	2645
November	1st to 10th	Night	21% 8% 2 7% 4 4 4 3 4 6% 38%	2587
	11th to 20th	Night	21% 8% 2 6% 5% 4 4 4 5% 6% 36%	2800
	21st to end	Night	23% 10% 2 6% 4 4 4 4 6% 6% 31%	2800
December	1st to 10th	Night	17% 11% 3 8% 4 4 4 3 5% 6% 34%	4646
	11th to 20th	Night	22% 10% 3 6% 4 4 4 3 5% 4 37%	5591
	21st to end	Night	20% 13% 3 8% 4 4 3 3 4 5% 32%	7502

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	60% 7% 12% 15% 6%	3158
	11th to 20th	Day	55% 5% 12% 20% 8% 1	3570
	21st to end	Day	43% 7% 10% 19% 20% 2	4233
February	1st to 10th	Day	54% 6% 11% 17% 10% 2	1755
	11th to 20th	Day	56% 5% 11% 17% 10% 1	1671
	21st to end	Day	49% 6% 11% 22% 10% 3	1539
March	1st to 10th	Day	58% 4 10% 15% 8% 4	3269
	11th to 20th	Day	61% 7% 10% 12% 8% 1	3480
	21st to end	Day	62% 4 7% 15% 10% 2	3816
April	1st to 10th	Day	58% 4 9% 14% 12% 2	2232
	11th to 20th	Day	50% 5% 10% 21% 12% 1	2442
	21st to end	Day	52% 6% 9% 15% 13% 5%	2470
May	1st to 10th	Day	51% 9% 9% 21% 8% 1	4120
	11th to 20th	Day	49% 8% 13% 21% 7% 1	4292
	21st to end	Day	46% 10% 12% 18% 14%	5023
June	1st to 10th	Day	56% 8% 10% 16% 10% 1	2850
	11th to 20th	Day	47% 5% 13% 20% 13% 1	2850
	21st to end	Day	57% 7% 10% 15% 9% 1	2850
July	1st to 10th	Day	46% 7% 16% 19% 11%	4685
	11th to 20th	Day	46% 7% 15% 23% 9%	4648
	21st to end	Day	46% 7% 13% 26% 8% 1	4480
August	1st to 10th	Day	52% 6% 13% 24% 6%	4092
	11th to 20th	Day	49% 7% 14% 23% 7%	4084
	21st to end	Day	38% 6% 14% 26% 16%	4532
September	1st to 10th	Day	36% 8% 14% 28% 14% 1	2260
	11th to 20th	Day	39% 6% 9% 28% 16% 2	2090
	21st to end	Day	42% 6% 10% 26% 16% 1	2090
October	1st to 10th	Day	49% 5% 11% 26% 9%	3233
	11th to 20th	Day	43% 4 13% 29% 9%	3140
	21st to end	Day	41% 4 16% 22% 17%	3189
November	1st to 10th	Day	50% 5% 12% 21% 12% 1	1711
	11th to 20th	Day	48% 6% 14% 17% 13% 2	1600
	21st to end	Day	55% 6% 14% 19% 6%	1600
December	1st to 10th	Day	50% 5% 18% 18% 8% 1	2526
	11th to 20th	Day	54% 4 10% 18% 10% 4	2618
	21st to end	Day	53% 4 13% 15% 13% 3	3476
January	1st to 10th	Night	55% 9% 12% 18% 5%	6808
	11th to 20th	Night	53% 7% 11% 21% 7% 1	6388
	21st to end	Night	42% 5% 8% 23% 19% 3	6720
February	1st to 10th	Night	51% 5% 13% 17% 12% 1	2536
	11th to 20th	Night	54% 6% 11% 17% 10% 2	2406
	21st to end	Night	52% 5% 10% 18% 11% 3	1913
March	1st to 10th	Night	60% 5% 8% 17% 9% 1	3583
	11th to 20th	Night	57% 9% 11% 13% 8% 1	3480
	21st to end	Night	61% 3 8% 16% 11% 2	3816
April	1st to 10th	Night	56% 5% 9% 15% 11% 4	1947
	11th to 20th	Night	50% 5% 9% 19% 15% 1	1688
	21st to end	Night	53% 6% 9% 17% 13% 3	1710
May	1st to 10th	Night	47% 10% 14% 20% 8% 1	2840
	11th to 20th	Night	46% 9% 15% 23% 7% 1	2612
	21st to end	Night	43% 10% 17% 20% 9% 1	2591
June	1st to 10th	Night	54% 8% 11% 19% 7% 1	1330
	11th to 20th	Night	47% 7% 13% 22% 11% 1	1330
	21st to end	Night	53% 7% 11% 19% 8% 2	1330
July	1st to 10th	Night	44% 8% 16% 22% 10%	2175
	11th to 20th	Night	46% 7% 15% 25% 6%	2194
	21st to end	Night	44% 8% 13% 26% 9%	2800
August	1st to 10th	Night	52% 9% 13% 20% 6%	2813
	11th to 20th	Night	45% 8% 18% 23% 6%	2809
	21st to end	Night	36% 7% 16% 23% 16% 2	3124
September	1st to 10th	Night	39% 6% 13% 28% 12% 1	1916
	11th to 20th	Night	38% 5% 13% 28% 15% 1	2090
	21st to end	Night	41% 6% 10% 28% 14% 1	2090
October	1st to 10th	Night	47% 5% 11% 25% 12%	3726
	11th to 20th	Night	39% 6% 12% 28% 15%	3779
	21st to end	Night	44% 4 16% 22% 14%	4358
November	1st to 10th	Night	48% 6% 13% 22% 10% 1	2587
	11th to 20th	Night	49% 6% 13% 20% 11% 1	2800
	21st to end	Night	53% 8% 13% 16% 9%	2800
December	1st to 10th	Night	49% 5% 14% 25% 7%	4646
	11th to 20th	Night	50% 4 11% 21% 13% 1	5592
	21st to end	Night	55% 4 10% 18% 10% 3	7502

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% 19% 35% 13% 2	200
	11th to 20th	Day	33% 23% 31% 10% 4 1	200
	21st to end	Day	23% 22% 34% 17% 4	337
February	1st to 10th	Day	30% 18% 33% 15% 4	385
	11th to 20th	Day	28% 24% 31% 12% 4 1	366
	21st to end	Day	27% 24% 29% 19% 2	368
March	1st to 10th	Day	33% 20% 32% 12% 2	460
	11th to 20th	Day	30% 20% 30% 14% 4 1	460
	21st to end	Day	33% 21% 27% 12% 5% 1	506
April	1st to 10th	Day	40% 24% 22% 12% 2 1	460
	11th to 20th	Day	33% 22% 28% 13% 3 1	452
	21st to end	Day	33% 20% 30% 12% 4 1	460
May	1st to 10th	Day	35% 19% 28% 13% 5% 1	463
	11th to 20th	Day	30% 18% 27% 18% 6% 1	603
	21st to end	Day	26% 18% 35% 15% 6%	735
June	1st to 10th	Day	32% 17% 28% 16% 6% 1	680
	11th to 20th	Day	26% 15% 28% 22% 8% 1	680
	21st to end	Day	30% 13% 30% 20% 7% 1	680
July	1st to 10th	Day	22% 12% 36% 20% 9% 1	670
	11th to 20th	Day	21% 9% 34% 24% 10% 2	650
	21st to end	Day	17% 9% 40% 26% 7% 2	532
August	1st to 10th	Day	19% 11% 35% 24% 9% 2	486
	11th to 20th	Day	18% 12% 29% 28% 11% 2	479
	21st to end	Day	21% 12% 27% 26% 10% 3	539
September	1st to 10th	Day	12% 15% 35% 26% 9% 3	490
	11th to 20th	Day	22% 12% 28% 23% 11% 4	490
	21st to end	Day	21% 17% 29% 19% 9% 4	490
October	1st to 10th	Day	22% 15% 25% 23% 12% 3	414
	11th to 20th	Day	25% 19% 26% 18% 9% 3	390
	21st to end	Day	29% 20% 30% 15% 5%	429
November	1st to 10th	Day	31% 16% 29% 18% 5% 1	395
	11th to 20th	Day	34% 20% 26% 14% 6%	400
	21st to end	Day	33% 23% 27% 15% 2 1	400
December	1st to 10th	Day	33% 17% 30% 15% 4	355
	11th to 20th	Day	40% 24% 25% 11% 1	200
	21st to end	Day	30% 24% 28% 15% 2	220
January	1st to 10th	Night	27% 20% 30% 20% 3	900
	11th to 20th	Night	29% 16% 30% 20% 4	807
	21st to end	Night	20% 22% 35% 19% 4	880
February	1st to 10th	Night	25% 22% 30% 19% 4	781
	11th to 20th	Night	26% 22% 33% 15% 3	743
	21st to end	Night	24% 19% 32% 21% 3	628
March	1st to 10th	Night	25% 19% 35% 19% 2	760
	11th to 20th	Night	24% 18% 34% 19% 5%	760
	21st to end	Night	27% 16% 33% 19% 5%	836
April	1st to 10th	Night	35% 17% 31% 15% 2	684
	11th to 20th	Night	29% 18% 32% 16% 5%	564
	21st to end	Night	30% 16% 31% 18% 5%	570
May	1st to 10th	Night	26% 15% 32% 23% 3	570
	11th to 20th	Night	30% 14% 32% 20% 4	562
	21st to end	Night	20% 16% 37% 23% 4	522
June	1st to 10th	Night	26% 10% 34% 25% 5%	380
	11th to 20th	Night	21% 11% 31% 30% 7% 1	380
	21st to end	Night	31% 6% 28% 31% 4	380
July	1st to 10th	Night	18% 7% 39% 29% 7%	370
	11th to 20th	Night	20% 5% 30% 35% 10%	368
	21st to end	Night	20% 7% 31% 34% 8%	506
August	1st to 10th	Night	19% 5% 29% 37% 10%	568
	11th to 20th	Night	19% 8% 29% 34% 11%	562
	21st to end	Night	16% 9% 29% 32% 14% 1	627
September	1st to 10th	Night	18% 9% 38% 25% 10%	615
	11th to 20th	Night	21% 9% 25% 32% 12% 1	760
	21st to end	Night	22% 13% 29% 27% 9% 1	760
October	1st to 10th	Night	23% 10% 29% 27% 10% 1	760
	11th to 20th	Night	21% 15% 31% 24% 9% 1	760
	21st to end	Night	28% 17% 31% 19% 5%	836
November	1st to 10th	Night	28% 15% 30% 21% 6% 1	805
	11th to 20th	Night	29% 17% 28% 19% 6% 1	900
	21st to end	Night	29% 19% 29% 19% 4	900
December	1st to 10th	Night	33% 15% 30% 18% 4	900
	11th to 20th	Night	31% 21% 28% 17% 2	900
	21st to end	Night	31% 21% 27% 18% 3	990

Page generated at 2025-02-03 04:05:53. Version ac07

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