Sussex
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sussex clear sky chart from 2005-11-06 to 2026-06-16.

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	18% 12% 4 8% 5% 4 4 3 5% 6% 31%	137662
	Night	18% 13% 4 9% 5% 4 4 3 5% 6% 31%	134510
Transparency	Day	53% 5% 11% 20% 10% 1	141192
	Night	54% 5% 10% 19% 10% 1	139069
Seeing	Day	29% 21% 29% 15% 5% 1	18760
	Night	27% 18% 29% 21% 5%	26506

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	10% 21% 7% 13% 6% 4 4 2 4 4 25%	4093
	11th to 20th	Day	16% 17% 7% 16% 7% 4 3 2 3 3 23%	4476
	21st to end	Day	10% 10% 4 10% 5% 4 4 4 6% 7% 35%	5781
February	1st to 10th	Day	15% 14% 4 9% 5% 5% 5% 4 5% 6% 28%	1890
	11th to 20th	Day	15% 16% 3 9% 5% 6% 3 3 5% 6% 28%	1843
	21st to end	Day	10% 14% 5% 8% 6% 4 4 4 5% 5% 35%	1625
March	1st to 10th	Day	20% 17% 5% 14% 3 3 3 2 4 4 25%	4937
	11th to 20th	Day	17% 18% 5% 9% 4 4 3 2 5% 6% 27%	5090
	21st to end	Day	24% 17% 5% 9% 4 4 3 3 4 5% 21%	5587
April	1st to 10th	Day	21% 18% 5% 8% 5% 4 3 3 4 5% 26%	2449
	11th to 20th	Day	19% 15% 3 8% 4 4 3 3 5% 6% 31%	2690
	21st to end	Day	22% 13% 4 6% 4 4 4 3 5% 6% 31%	2730
May	1st to 10th	Day	26% 16% 3 7% 4 4 3 3 4 6% 23%	7639
	11th to 20th	Day	19% 10% 3 8% 4 4 4 4 5% 7% 31%	8220
	21st to end	Day	22% 10% 3 8% 5% 4 4 4 5% 6% 28%	9059
June	1st to 10th	Day	25% 10% 3 7% 5% 5% 5% 4 5% 7% 26%	3145
	11th to 20th	Day	20% 9% 3 8% 4 4 5% 4 5% 7% 32%	3075
	21st to end	Day	26% 10% 2 7% 5% 4 4 4 4 6% 27%	3000
July	1st to 10th	Day	16% 9% 3 7% 5% 5% 6% 5% 7% 9% 29%	6820
	11th to 20th	Day	16% 8% 3 6% 4 4 4 4 6% 8% 38%	6802
	21st to end	Day	12% 7% 3 7% 5% 4 5% 4 7% 8% 37%	6570
August	1st to 10th	Day	20% 9% 3 8% 4 4 5% 4 6% 7% 31%	4235
	11th to 20th	Day	14% 7% 3 7% 5% 5% 5% 4 7% 8% 35%	4175
	21st to end	Day	14% 5% 2 5% 4 4 5% 3 6% 8% 45%	4619
September	1st to 10th	Day	15% 6% 2 5% 4 3 4 4 6% 8% 43%	2400
	11th to 20th	Day	18% 7% 3 5% 3 3 4 2 5% 5% 46%	2230
	21st to end	Day	19% 6% 3 5% 3 3 3 3 4 6% 44%	2195
October	1st to 10th	Day	18% 8% 3 7% 4 3 3 3 5% 6% 40%	2000
	11th to 20th	Day	20% 11% 2 6% 4 4 4 3 4 7% 35%	2000
	21st to end	Day	19% 12% 3 8% 5% 5% 4 2 4 6% 32%	2130
November	1st to 10th	Day	20% 13% 3 7% 4 4 4 2 4 5% 32%	1724
	11th to 20th	Day	19% 14% 4 7% 5% 3 2 4 4 6% 33%	1680
	21st to end	Day	18% 16% 5% 10% 6% 5% 4 2 4 4 27%	1680
December	1st to 10th	Day	17% 16% 5% 9% 8% 4 3 3 4 6% 26%	2690
	11th to 20th	Day	18% 17% 4 10% 5% 3 3 2 3 3 32%	2830
	21st to end	Day	15% 15% 5% 11% 5% 5% 5% 3 4 4 30%	3553
January	1st to 10th	Night	13% 17% 5% 10% 5% 4 4 3 4 5% 30%	8831
	11th to 20th	Night	17% 17% 5% 12% 6% 4 4 3 4 5% 24%	8414
	21st to end	Night	9% 13% 4 8% 6% 5% 4 3 5% 6% 37%	8400
February	1st to 10th	Night	15% 13% 5% 11% 5% 4 4 3 5% 5% 30%	2730
	11th to 20th	Night	15% 15% 4 9% 6% 5% 4 4 5% 6% 27%	2663
	21st to end	Night	13% 14% 5% 9% 5% 4 4 3 6% 6% 32%	2176
March	1st to 10th	Night	24% 16% 4 11% 5% 5% 3 3 3 4 23%	5122
	11th to 20th	Night	18% 17% 5% 9% 5% 5% 4 3 4 5% 25%	5089
	21st to end	Night	27% 14% 5% 9% 5% 4 4 3 3 3 25%	5587
April	1st to 10th	Night	21% 15% 4 8% 5% 5% 4 3 4 5% 27%	2170
	11th to 20th	Night	22% 13% 2 6% 5% 3 3 3 5% 6% 31%	1877
	21st to end	Night	20% 14% 3 8% 4 4 4 3 5% 6% 30%	1890
May	1st to 10th	Night	26% 16% 4 8% 4 4 4 2 5% 6% 23%	5245
	11th to 20th	Night	20% 10% 3 7% 4 4 4 4 5% 6% 32%	4670
	21st to end	Night	17% 13% 3 8% 5% 5% 4 4 5% 6% 30%	5147
June	1st to 10th	Night	23% 9% 3 8% 4 4 4 4 6% 8% 28%	1491
	11th to 20th	Night	21% 8% 3 6% 5% 4 4 4 5% 7% 35%	1435
	21st to end	Night	24% 9% 3 7% 5% 5% 4 4 5% 5% 29%	1400
July	1st to 10th	Night	17% 10% 3 9% 5% 5% 5% 4 7% 8% 26%	3160
	11th to 20th	Night	16% 8% 3 6% 5% 5% 4 4 6% 9% 33%	3160
	21st to end	Night	15% 6% 3 6% 4 4 5% 4 7% 8% 37%	4257
August	1st to 10th	Night	20% 11% 3 7% 4 5% 4 3 5% 7% 30%	2915
	11th to 20th	Night	11% 7% 3 8% 5% 5% 6% 4 6% 9% 36%	2872
	21st to end	Night	13% 6% 2 5% 4 3 3 4 6% 7% 47%	3277
September	1st to 10th	Night	18% 5% 3 5% 3 4 3 4 5% 8% 42%	2000
	11th to 20th	Night	17% 7% 2 4 4 3 4 3 5% 7% 45%	2168
	21st to end	Night	18% 7% 2 6% 4 3 4 3 5% 7% 42%	2203
October	1st to 10th	Night	19% 7% 3 6% 4 3 3 3 5% 6% 41%	2400
	11th to 20th	Night	16% 11% 2 8% 5% 4 4 3 3 7% 37%	2400
	21st to end	Night	22% 11% 2 8% 5% 4 3 2 4 5% 32%	2707
November	1st to 10th	Night	21% 11% 3 6% 4 3 4 3 4 6% 34%	2793
	11th to 20th	Night	19% 14% 5% 8% 5% 4 4 3 5% 5% 29%	2940
	21st to end	Night	19% 15% 4 9% 5% 4 4 3 4 6% 29%	2940
December	1st to 10th	Night	15% 13% 5% 11% 5% 4 4 3 5% 6% 29%	4720
	11th to 20th	Night	16% 15% 4 11% 5% 3 3 2 3 4 33%	5594
	21st to end	Night	18% 16% 5% 12% 5% 3 3 3 3 4 29%	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	63% 4 10% 14% 8% 1	4094
	11th to 20th	Day	67% 3 8% 14% 7% 1	4505
	21st to end	Day	48% 3 9% 20% 16% 3	5781
February	1st to 10th	Day	56% 5% 10% 18% 10% 2	1890
	11th to 20th	Day	58% 4 10% 17% 10% 1	1851
	21st to end	Day	51% 5% 10% 20% 10% 3	1625
March	1st to 10th	Day	63% 4 7% 14% 11% 3	4941
	11th to 20th	Day	58% 5% 13% 15% 7% 1	5090
	21st to end	Day	66% 3 5% 15% 10% 1	5587
April	1st to 10th	Day	62% 5% 7% 13% 11% 3	2449
	11th to 20th	Day	55% 5% 8% 20% 11% 1	2690
	21st to end	Day	55% 5% 9% 17% 12% 3	2730
May	1st to 10th	Day	63% 5% 9% 15% 9%	7651
	11th to 20th	Day	51% 6% 12% 21% 8% 1	8224
	21st to end	Day	55% 6% 9% 16% 13% 1	9060
June	1st to 10th	Day	57% 7% 10% 18% 9% 1	3145
	11th to 20th	Day	51% 6% 12% 20% 10% 1	3075
	21st to end	Day	58% 6% 10% 15% 9% 1	3000
July	1st to 10th	Day	51% 6% 15% 19% 9%	6745
	11th to 20th	Day	43% 6% 15% 26% 9% 1	6721
	21st to end	Day	43% 8% 15% 27% 8%	6383
August	1st to 10th	Day	54% 6% 11% 22% 7%	4222
	11th to 20th	Day	45% 8% 14% 25% 9%	4214
	21st to end	Day	36% 6% 14% 26% 18%	4619
September	1st to 10th	Day	38% 8% 13% 26% 14% 1	2400
	11th to 20th	Day	42% 5% 10% 26% 15% 2	2230
	21st to end	Day	42% 4 9% 27% 17% 1	2195
October	1st to 10th	Day	46% 5% 9% 29% 11%	3260
	11th to 20th	Day	49% 4 9% 31% 7% 1	3240
	21st to end	Day	44% 4 14% 21% 16%	3419
November	1st to 10th	Day	57% 4 9% 20% 10% 1	1724
	11th to 20th	Day	55% 4 12% 17% 11% 1	1680
	21st to end	Day	62% 5% 11% 17% 6%	1680
December	1st to 10th	Day	61% 4 11% 20% 3	2690
	11th to 20th	Day	59% 4 7% 17% 9% 3	2829
	21st to end	Day	56% 5% 11% 16% 11% 1	3553
January	1st to 10th	Night	57% 5% 9% 21% 7% 1	8831
	11th to 20th	Night	62% 5% 9% 18% 6%	8460
	21st to end	Night	49% 4 8% 20% 16% 3	8400
February	1st to 10th	Night	57% 4 10% 16% 12% 2	2730
	11th to 20th	Night	58% 5% 9% 17% 10% 1	2666
	21st to end	Night	53% 5% 9% 20% 11% 2	2176
March	1st to 10th	Night	66% 2 6% 11% 10% 3	5131
	11th to 20th	Night	60% 6% 13% 13% 8% 1	5090
	21st to end	Night	64% 4 4 16% 10% 2	5587
April	1st to 10th	Night	60% 4 7% 14% 11% 3	2170
	11th to 20th	Night	54% 4 10% 19% 12% 1	1877
	21st to end	Night	57% 4 9% 15% 13% 2	1890
May	1st to 10th	Night	63% 6% 10% 14% 6% 1	5251
	11th to 20th	Night	52% 7% 11% 21% 7% 2	4671
	21st to end	Night	53% 7% 13% 16% 10% 2	5148
June	1st to 10th	Night	54% 7% 10% 20% 8% 1	1491
	11th to 20th	Night	49% 8% 10% 23% 9% 1	1435
	21st to end	Night	57% 6% 11% 15% 9% 1	1400
July	1st to 10th	Night	54% 7% 13% 17% 9%	3125
	11th to 20th	Night	45% 8% 14% 25% 7%	3118
	21st to end	Night	42% 9% 16% 22% 12%	4130
August	1st to 10th	Night	54% 6% 12% 21% 7%	2903
	11th to 20th	Night	41% 10% 17% 22% 10%	2899
	21st to end	Night	34% 7% 17% 26% 15% 2	3276
September	1st to 10th	Night	41% 6% 10% 29% 13% 1	2000
	11th to 20th	Night	39% 6% 13% 27% 14% 1	2168
	21st to end	Night	44% 6% 10% 25% 14% 2	2203
October	1st to 10th	Night	47% 5% 12% 24% 12%	3920
	11th to 20th	Night	45% 4 11% 26% 14%	3899
	21st to end	Night	48% 4 13% 21% 13%	4368
November	1st to 10th	Night	53% 5% 11% 19% 10% 1	2793
	11th to 20th	Night	57% 5% 11% 18% 8% 1	2940
	21st to end	Night	58% 5% 13% 16% 7%	2940
December	1st to 10th	Night	56% 5% 11% 22% 7%	4720
	11th to 20th	Night	56% 5% 8% 17% 13% 1	5596
	21st to end	Night	60% 3 9% 17% 10% 1	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	35% 25% 31% 9%	220
	11th to 20th	Day	43% 23% 24% 5% 4	243
	21st to end	Day	23% 25% 33% 15% 4	473
February	1st to 10th	Day	27% 26% 26% 16% 4 1	430
	11th to 20th	Day	36% 27% 22% 12% 3 1	426
	21st to end	Day	29% 26% 30% 12% 3	391
March	1st to 10th	Day	31% 32% 26% 9% 1	540
	11th to 20th	Day	31% 27% 29% 9% 3 1	540
	21st to end	Day	39% 24% 24% 10% 2 1	594
April	1st to 10th	Day	42% 26% 23% 7% 2	540
	11th to 20th	Day	35% 29% 24% 8% 4	532
	21st to end	Day	33% 25% 25% 12% 5%	540
May	1st to 10th	Day	38% 23% 27% 8% 3	543
	11th to 20th	Day	29% 22% 31% 14% 4 1	774
	21st to end	Day	30% 21% 30% 15% 4	845
June	1st to 10th	Day	32% 21% 28% 14% 5%	780
	11th to 20th	Day	26% 20% 30% 18% 5% 1	755
	21st to end	Day	30% 17% 28% 19% 5%	730
July	1st to 10th	Day	23% 17% 36% 17% 6% 1	720
	11th to 20th	Day	20% 12% 36% 22% 9% 1	718
	21st to end	Day	20% 16% 38% 20% 6% 1	643
August	1st to 10th	Day	18% 14% 41% 21% 6% 1	526
	11th to 20th	Day	21% 13% 34% 24% 6% 2	519
	21st to end	Day	20% 18% 31% 23% 7% 2	583
September	1st to 10th	Day	16% 18% 39% 20% 6% 2	530
	11th to 20th	Day	26% 17% 25% 22% 8% 3	530
	21st to end	Day	21% 24% 29% 16% 9% 2	527
October	1st to 10th	Day	24% 16% 25% 23% 10% 3	420
	11th to 20th	Day	31% 18% 26% 19% 5% 2	420
	21st to end	Day	30% 24% 27% 12% 6% 1	462
November	1st to 10th	Day	35% 19% 27% 12% 5% 2	425
	11th to 20th	Day	37% 21% 24% 13% 5%	430
	21st to end	Day	32% 25% 29% 10% 3	430
December	1st to 10th	Day	34% 21% 26% 16% 2	430
	11th to 20th	Day	38% 29% 22% 11% 1	309
	21st to end	Day	33% 27% 31% 7% 2	242
January	1st to 10th	Night	29% 23% 27% 18% 3	950
	11th to 20th	Night	29% 23% 25% 17% 5%	887
	21st to end	Night	22% 23% 33% 18% 4	924
February	1st to 10th	Night	26% 23% 28% 19% 3	840
	11th to 20th	Night	28% 26% 26% 17% 2	823
	21st to end	Night	23% 22% 32% 20% 3	692
March	1st to 10th	Night	28% 23% 31% 16% 2	840
	11th to 20th	Night	27% 21% 31% 17% 4	840
	21st to end	Night	29% 20% 31% 14% 5%	924
April	1st to 10th	Night	33% 22% 26% 17% 2	700
	11th to 20th	Night	29% 20% 31% 17% 3	624
	21st to end	Night	31% 17% 29% 17% 5%	630
May	1st to 10th	Night	31% 16% 29% 20% 3	630
	11th to 20th	Night	29% 18% 31% 18% 3	622
	21st to end	Night	22% 17% 39% 18% 3	682
June	1st to 10th	Night	28% 11% 34% 24% 4	441
	11th to 20th	Night	25% 13% 30% 26% 6%	410
	21st to end	Night	25% 13% 27% 30% 5%	400
July	1st to 10th	Night	19% 11% 37% 27% 6%	390
	11th to 20th	Night	19% 11% 33% 30% 7%	388
	21st to end	Night	19% 11% 35% 30% 5%	640
August	1st to 10th	Night	18% 9% 36% 31% 7%	598
	11th to 20th	Night	19% 10% 29% 32% 9%	592
	21st to end	Night	16% 12% 32% 30% 11%	660
September	1st to 10th	Night	21% 10% 35% 27% 6% 1	600
	11th to 20th	Night	21% 11% 25% 29% 12% 1	768
	21st to end	Night	22% 15% 27% 26% 10% 1	800
October	1st to 10th	Night	22% 13% 29% 26% 10% 1	800
	11th to 20th	Night	24% 18% 31% 20% 8% 1	800
	21st to end	Night	31% 21% 25% 20% 3	880
November	1st to 10th	Night	33% 16% 25% 19% 6% 1	886
	11th to 20th	Night	31% 19% 26% 17% 6% 1	950
	21st to end	Night	31% 20% 27% 18% 4	950
December	1st to 10th	Night	31% 18% 29% 18% 4	950
	11th to 20th	Night	33% 25% 24% 14% 3	950
	21st to end	Night	32% 22% 26% 16% 4	1045

Page generated at 2026-06-13 03:31:42. Version ac07

Copyright © 2026 Attilla Danko.