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 2025-02-21.

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%	108430
	Night	17% 13% 4 9% 5% 4 4 3 5% 6% 31%	110232
Transparency	Day	53% 6% 11% 20% 10% 1	112002
	Night	53% 6% 11% 19% 10% 1	114824
Seeing	Day	29% 21% 29% 15% 5% 1	16919
	Night	27% 18% 29% 20% 5%	24814

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% 22% 7% 13% 6% 4 3 2 4 4 25%	3157
	11th to 20th	Day	13% 15% 7% 15% 8% 4 3 2 3 4 26%	3434
	21st to end	Day	11% 10% 5% 10% 5% 4 4 4 5% 7% 35%	4466
February	1st to 10th	Day	15% 14% 4 9% 5% 4 4 4 5% 6% 29%	1800
	11th to 20th	Day	16% 16% 4 9% 5% 6% 3 3 6% 6% 27%	1753
	21st to end	Day	11% 15% 4 7% 6% 4 4 3 5% 5% 35%	1475
March	1st to 10th	Day	22% 19% 5% 7% 4 3 3 2 4 4 26%	3358
	11th to 20th	Day	15% 23% 6% 11% 5% 5% 3 2 4 5% 22%	3480
	21st to end	Day	27% 16% 6% 9% 4 4 3 3 4 4 21%	3816
April	1st to 10th	Day	21% 19% 4 8% 5% 4 3 3 3 5% 25%	2215
	11th to 20th	Day	18% 14% 3 7% 4 4 3 3 5% 6% 32%	2431
	21st to end	Day	22% 13% 3 5% 4 4 4 3 5% 6% 30%	2470
May	1st to 10th	Day	21% 17% 3 8% 5% 4 3 3 4 6% 26%	4106
	11th to 20th	Day	18% 10% 3 9% 5% 5% 4 4 5% 7% 31%	4398
	21st to end	Day	16% 9% 3 8% 6% 5% 5% 5% 7% 8% 28%	4836
June	1st to 10th	Day	26% 10% 3 7% 5% 5% 5% 4 5% 6% 24%	2845
	11th to 20th	Day	21% 9% 3 8% 4 4 5% 4 5% 7% 32%	2850
	21st to end	Day	27% 10% 2 7% 5% 4 4 3 4 6% 27%	2850
July	1st to 10th	Day	16% 8% 2 6% 5% 5% 6% 5% 7% 8% 32%	4760
	11th to 20th	Day	18% 8% 3 7% 4 4 5% 4 6% 8% 33%	4742
	21st to end	Day	15% 7% 3 7% 5% 5% 5% 5% 7% 7% 34%	4570
August	1st to 10th	Day	21% 9% 3 8% 4 4 5% 4 6% 7% 29%	4105
	11th to 20th	Day	14% 7% 3 7% 5% 5% 5% 4 7% 9% 34%	4045
	21st to end	Day	14% 5% 2 5% 4 4 4 3 6% 8% 45%	4478
September	1st to 10th	Day	15% 6% 2 5% 4 3 4 4 5% 7% 43%	2280
	11th to 20th	Day	18% 7% 3 5% 3 3 4 2 5% 5% 45%	2119
	21st to end	Day	20% 7% 3 5% 3 3 3 3 4 6% 43%	2085
October	1st to 10th	Day	19% 8% 3 7% 4 3 3 3 5% 6% 39%	1900
	11th to 20th	Day	20% 11% 2 6% 4 4 4 3 4 7% 36%	1900
	21st to end	Day	19% 12% 3 8% 4 5% 4 2 4 6% 32%	2024
November	1st to 10th	Day	21% 13% 3 7% 4 4 4 2 4 5% 33%	1640
	11th to 20th	Day	19% 14% 4 8% 5% 3 2 4 4 6% 33%	1600
	21st to end	Day	18% 17% 5% 9% 6% 5% 4 2 4 4 27%	1600
December	1st to 10th	Day	17% 15% 5% 9% 8% 4 3 3 4 6% 26%	2610
	11th to 20th	Day	18% 17% 4 9% 5% 3 3 2 3 3 32%	2756
	21st to end	Day	15% 14% 5% 11% 5% 5% 5% 3 4 4 29%	3476
January	1st to 10th	Night	14% 17% 5% 11% 6% 5% 4 3 4 4 28%	6808
	11th to 20th	Night	13% 15% 5% 13% 7% 4 4 3 4 6% 26%	6452
	21st to end	Night	10% 13% 5% 8% 5% 4 4 3 5% 6% 38%	6490
February	1st to 10th	Night	16% 13% 4 10% 6% 4 4 3 5% 6% 30%	2600
	11th to 20th	Night	16% 15% 4 9% 6% 5% 4 4 5% 6% 26%	2533
	21st to end	Night	13% 14% 5% 9% 5% 4 4 3 6% 6% 32%	1976
March	1st to 10th	Night	23% 16% 3 10% 5% 6% 4 2 3 4 24%	3481
	11th to 20th	Night	15% 22% 6% 11% 6% 5% 5% 3 4 4 19%	3479
	21st to end	Night	26% 14% 5% 9% 5% 4 4 3 3 2 24%	3816
April	1st to 10th	Night	20% 15% 4 7% 5% 5% 4 3 4 5% 27%	1964
	11th to 20th	Night	21% 13% 2 6% 5% 3 4 2 5% 6% 32%	1697
	21st to end	Night	20% 14% 3 8% 4 4 5% 3 4 5% 30%	1710
May	1st to 10th	Night	20% 17% 3 6% 4 4 4 3 5% 7% 26%	2820
	11th to 20th	Night	18% 8% 3 6% 4 4 4 4 6% 8% 36%	2506
	21st to end	Night	15% 9% 3 9% 5% 6% 5% 5% 6% 7% 29%	2749
June	1st to 10th	Night	24% 10% 3 7% 5% 4 4 4 6% 7% 26%	1349
	11th to 20th	Night	21% 8% 3 6% 5% 3 4 4 4 7% 35%	1330
	21st to end	Night	24% 9% 3 7% 5% 5% 4 4 5% 5% 29%	1330
July	1st to 10th	Night	15% 10% 3 9% 5% 5% 5% 4 6% 8% 29%	2210
	11th to 20th	Night	19% 9% 3 6% 5% 5% 4 5% 7% 9% 29%	2210
	21st to end	Night	17% 7% 3 7% 5% 4 5% 4 6% 7% 35%	2969
August	1st to 10th	Night	21% 12% 3 7% 4 5% 4 3 5% 7% 28%	2825
	11th to 20th	Night	11% 8% 3 8% 5% 5% 6% 4 6% 9% 35%	2782
	21st to end	Night	13% 6% 2 5% 3 3 3 3 6% 7% 47%	3176
September	1st to 10th	Night	18% 6% 3 5% 3 4 3 4 5% 8% 41%	1900
	11th to 20th	Night	18% 7% 2 5% 4 3 3 3 5% 7% 44%	2059
	21st to end	Night	18% 7% 2 6% 4 3 4 3 4 7% 42%	2093
October	1st to 10th	Night	20% 7% 3 6% 4 4 3 3 5% 6% 39%	2280
	11th to 20th	Night	16% 11% 2 8% 5% 4 4 4 3 7% 38%	2280
	21st to end	Night	23% 11% 2 8% 5% 4 4 2 4 5% 33%	2571
November	1st to 10th	Night	21% 11% 3 6% 4 3 4 4 4 6% 34%	2657
	11th to 20th	Night	18% 14% 5% 8% 5% 4 4 3 5% 5% 30%	2800
	21st to end	Night	19% 16% 4 8% 4 4 4 3 4 5% 28%	2800
December	1st to 10th	Night	15% 13% 5% 11% 5% 4 4 3 5% 6% 29%	4580
	11th to 20th	Night	17% 15% 4 10% 5% 3 3 2 3 4 33%	5448
	21st to end	Night	18% 16% 5% 12% 5% 3 3 3 3 4 29%	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	65% 4 10% 14% 6%	3158
	11th to 20th	Day	63% 4 10% 16% 7% 1	3463
	21st to end	Day	48% 4 10% 16% 19% 3	4466
February	1st to 10th	Day	56% 5% 10% 17% 10% 2	1800
	11th to 20th	Day	59% 4 10% 16% 10% 1	1761
	21st to end	Day	51% 5% 10% 21% 10% 4	1475
March	1st to 10th	Day	62% 4 7% 18% 6% 3	3362
	11th to 20th	Day	65% 5% 9% 13% 7% 1	3480
	21st to end	Day	67% 3 5% 13% 9% 2	3816
April	1st to 10th	Day	63% 5% 7% 13% 10% 2	2215
	11th to 20th	Day	54% 5% 8% 20% 12% 1	2431
	21st to end	Day	55% 5% 9% 16% 12% 3	2470
May	1st to 10th	Day	59% 6% 8% 18% 8% 1	4120
	11th to 20th	Day	51% 7% 14% 20% 8%	4402
	21st to end	Day	50% 9% 13% 17% 10%	4836
June	1st to 10th	Day	58% 7% 9% 17% 8%	2845
	11th to 20th	Day	51% 6% 11% 20% 10% 1	2850
	21st to end	Day	59% 6% 11% 15% 9% 1	2850
July	1st to 10th	Day	48% 7% 15% 20% 10%	4685
	11th to 20th	Day	48% 7% 14% 23% 8%	4661
	21st to end	Day	46% 9% 14% 23% 7% 1	4424
August	1st to 10th	Day	55% 7% 11% 22% 6%	4092
	11th to 20th	Day	45% 8% 13% 25% 9%	4084
	21st to end	Day	36% 6% 14% 26% 18%	4478
September	1st to 10th	Day	39% 8% 13% 26% 14% 1	2280
	11th to 20th	Day	43% 5% 10% 25% 15% 2	2119
	21st to end	Day	43% 4 9% 27% 16% 1	2085
October	1st to 10th	Day	47% 5% 9% 28% 11%	3160
	11th to 20th	Day	49% 4 9% 31% 7% 1	3140
	21st to end	Day	43% 4 14% 21% 17%	3313
November	1st to 10th	Day	56% 4 9% 20% 10% 1	1640
	11th to 20th	Day	55% 4 12% 17% 12% 1	1600
	21st to end	Day	62% 5% 10% 17% 6%	1600
December	1st to 10th	Day	61% 4 11% 20% 3	2610
	11th to 20th	Day	59% 4 7% 17% 10% 3	2755
	21st to end	Day	56% 5% 11% 16% 11% 1	3476
January	1st to 10th	Night	58% 5% 10% 22% 4 1	6808
	11th to 20th	Night	57% 6% 11% 20% 6% 1	6498
	21st to end	Night	47% 5% 7% 21% 17% 3	6490
February	1st to 10th	Night	57% 4 10% 15% 12% 2	2600
	11th to 20th	Night	59% 5% 9% 15% 11% 1	2536
	21st to end	Night	53% 5% 9% 19% 11% 3	1976
March	1st to 10th	Night	64% 3 8% 16% 8% 1	3490
	11th to 20th	Night	66% 6% 8% 11% 8% 1	3480
	21st to end	Night	66% 4 4 14% 9% 2	3816
April	1st to 10th	Night	60% 5% 8% 14% 11% 3	1964
	11th to 20th	Night	53% 4 10% 19% 12% 1	1697
	21st to end	Night	57% 4 9% 15% 14% 1	1710
May	1st to 10th	Night	57% 8% 12% 15% 7% 1	2826
	11th to 20th	Night	44% 10% 16% 23% 7%	2507
	21st to end	Night	51% 9% 15% 18% 6% 1	2750
June	1st to 10th	Night	56% 8% 10% 19% 7% 1	1349
	11th to 20th	Night	49% 8% 10% 22% 9% 1	1330
	21st to end	Night	57% 6% 11% 16% 9% 1	1330
July	1st to 10th	Night	51% 7% 13% 17% 12%	2175
	11th to 20th	Night	49% 9% 14% 22% 7%	2168
	21st to end	Night	45% 10% 12% 21% 11% 1	2875
August	1st to 10th	Night	56% 7% 12% 20% 6%	2813
	11th to 20th	Night	42% 10% 16% 22% 10%	2809
	21st to end	Night	34% 7% 17% 26% 16% 2	3175
September	1st to 10th	Night	41% 6% 10% 29% 13% 1	1900
	11th to 20th	Night	40% 6% 13% 27% 14% 1	2059
	21st to end	Night	44% 5% 10% 25% 13% 2	2093
October	1st to 10th	Night	48% 5% 12% 24% 11%	3800
	11th to 20th	Night	44% 4 11% 27% 14%	3779
	21st to end	Night	48% 4 13% 21% 14%	4232
November	1st to 10th	Night	52% 5% 11% 19% 10% 1	2657
	11th to 20th	Night	56% 6% 11% 17% 9% 1	2800
	21st to end	Night	59% 5% 13% 16% 7%	2800
December	1st to 10th	Night	56% 5% 11% 22% 6%	4580
	11th to 20th	Night	56% 5% 8% 17% 13% 2	5450
	21st to end	Night	60% 3 9% 17% 10% 1	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	34% 26% 32% 9%	200
	11th to 20th	Day	42% 23% 24% 6% 5%	222
	21st to end	Day	23% 25% 33% 15% 4	440
February	1st to 10th	Day	29% 27% 26% 15% 3	400
	11th to 20th	Day	37% 28% 22% 10% 2 1	396
	21st to end	Day	28% 28% 28% 12% 3	337
March	1st to 10th	Day	29% 32% 28% 9% 2	460
	11th to 20th	Day	29% 27% 31% 10% 3 1	460
	21st to end	Day	37% 25% 25% 10% 3 1	506
April	1st to 10th	Day	41% 26% 23% 8% 2	460
	11th to 20th	Day	33% 29% 24% 9% 5%	452
	21st to end	Day	34% 24% 26% 12% 4	460
May	1st to 10th	Day	38% 23% 26% 9% 4	463
	11th to 20th	Day	29% 20% 30% 16% 5% 1	674
	21st to end	Day	28% 21% 31% 15% 4	735
June	1st to 10th	Day	34% 21% 27% 13% 4	680
	11th to 20th	Day	27% 18% 30% 19% 6% 1	680
	21st to end	Day	30% 16% 28% 20% 6%	680
July	1st to 10th	Day	23% 17% 35% 18% 6% 1	670
	11th to 20th	Day	20% 12% 36% 22% 9% 1	668
	21st to end	Day	20% 16% 37% 20% 6% 1	596
August	1st to 10th	Day	20% 14% 41% 20% 5%	486
	11th to 20th	Day	22% 13% 35% 23% 5% 1	479
	21st to end	Day	19% 17% 31% 23% 7% 2	539
September	1st to 10th	Day	16% 17% 39% 20% 6% 1	490
	11th to 20th	Day	27% 17% 24% 22% 8% 2	490
	21st to end	Day	21% 23% 28% 16% 9% 2	487
October	1st to 10th	Day	24% 16% 24% 22% 11% 3	390
	11th to 20th	Day	30% 18% 26% 19% 5% 1	390
	21st to end	Day	29% 26% 27% 12% 6%	429
November	1st to 10th	Day	33% 19% 28% 13% 5% 2	395
	11th to 20th	Day	37% 22% 24% 13% 5%	400
	21st to end	Day	33% 25% 30% 10% 3	400
December	1st to 10th	Day	35% 21% 25% 16% 2 1	400
	11th to 20th	Day	39% 28% 21% 11% 1	285
	21st to end	Day	32% 27% 30% 8% 3	220
January	1st to 10th	Night	30% 24% 26% 18% 3	900
	11th to 20th	Night	29% 23% 25% 18% 6%	843
	21st to end	Night	22% 23% 33% 18% 5%	880
February	1st to 10th	Night	27% 24% 28% 18% 2	800
	11th to 20th	Night	28% 27% 26% 16% 2	783
	21st to end	Night	23% 22% 31% 21% 3	629
March	1st to 10th	Night	27% 23% 32% 16% 2	760
	11th to 20th	Night	26% 21% 31% 17% 4 1	760
	21st to end	Night	28% 21% 31% 14% 5%	836
April	1st to 10th	Night	33% 21% 27% 18% 2	634
	11th to 20th	Night	27% 20% 32% 16% 4	564
	21st to end	Night	31% 18% 29% 17% 5%	570
May	1st to 10th	Night	30% 16% 30% 20% 4	570
	11th to 20th	Night	28% 17% 32% 19% 4	562
	21st to end	Night	22% 17% 40% 19% 2	616
June	1st to 10th	Night	28% 11% 33% 23% 5%	399
	11th to 20th	Night	25% 13% 29% 27% 6%	380
	21st to end	Night	24% 13% 27% 30% 6%	380
July	1st to 10th	Night	19% 11% 37% 26% 6%	370
	11th to 20th	Night	19% 11% 33% 29% 8%	368
	21st to end	Night	19% 11% 34% 30% 5%	608
August	1st to 10th	Night	18% 9% 36% 30% 7%	568
	11th to 20th	Night	20% 11% 29% 31% 9%	562
	21st to end	Night	16% 12% 31% 30% 11%	627
September	1st to 10th	Night	22% 10% 36% 26% 5% 1	570
	11th to 20th	Night	22% 11% 25% 29% 12% 1	729
	21st to end	Night	23% 15% 26% 25% 10% 1	760
October	1st to 10th	Night	23% 13% 28% 26% 10% 1	760
	11th to 20th	Night	23% 18% 31% 19% 8% 1	760
	21st to end	Night	31% 21% 25% 19% 3	836
November	1st to 10th	Night	32% 16% 26% 20% 6% 1	840
	11th to 20th	Night	31% 20% 26% 16% 6% 1	900
	21st to end	Night	32% 21% 25% 18% 3	900
December	1st to 10th	Night	31% 18% 29% 18% 4	900
	11th to 20th	Night	32% 25% 24% 15% 3	900
	21st to end	Night	32% 22% 25% 16% 4 1	990

Page generated at 2025-02-18 04:26:32. Version ac07

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