Tilbury
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Tilbury clear sky chart from 2005-11-06 to 2026-04-02.

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	13% 9% 4 8% 5% 5% 5% 4 6% 8% 34%	131040
	Night	14% 10% 4 8% 5% 5% 5% 4 5% 7% 34%	130841
Transparency	Day	46% 7% 13% 21% 12% 1	134638
	Night	48% 6% 12% 20% 12% 1	135313
Seeing	Day	16% 10% 29% 26% 14% 5%	16986
	Night	18% 10% 28% 29% 13% 2	26641

By Month

Percentage Cloud Cover SE (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% 14% 6% 14% 7% 6% 5% 5% 6% 6% 19%	4691
	11th to 20th	Day	12% 17% 7% 15% 9% 6% 5% 4 5% 5% 16%	5082
	21st to end	Day	18% 15% 6% 12% 6% 5% 5% 4 5% 6% 18%	5780
February	1st to 10th	Day	15% 15% 4 12% 6% 6% 4 3 5% 5% 24%	1890
	11th to 20th	Day	13% 13% 5% 11% 6% 5% 6% 3 5% 7% 25%	1936
	21st to end	Day	16% 11% 4 12% 7% 4 5% 3 6% 6% 26%	1730
March	1st to 10th	Day	15% 12% 6% 10% 6% 4 4 3 5% 6% 30%	4807
	11th to 20th	Day	11% 10% 4 9% 5% 5% 5% 3 5% 6% 35%	5090
	21st to end	Day	17% 12% 4 9% 5% 5% 4 3 4 6% 30%	5586
April	1st to 10th	Day	14% 12% 4 7% 5% 4 4 5% 6% 7% 32%	2271
	11th to 20th	Day	17% 9% 3 7% 5% 4 5% 4 5% 8% 34%	2572
	21st to end	Day	18% 7% 3 8% 5% 5% 4 3 5% 6% 36%	2600
May	1st to 10th	Day	18% 11% 3 7% 4 4 4 4 4 6% 34%	5862
	11th to 20th	Day	16% 8% 3 6% 4 5% 5% 4 6% 9% 35%	5861
	21st to end	Day	14% 7% 3 7% 4 4 4 4 6% 8% 41%	6471
June	1st to 10th	Day	10% 6% 3 5% 5% 5% 5% 5% 7% 9% 41%	2853
	11th to 20th	Day	10% 5% 2 7% 5% 5% 5% 5% 7% 9% 41%	3000
	21st to end	Day	9% 5% 2 5% 5% 4 5% 5% 7% 9% 42%	2977
July	1st to 10th	Day	9% 5% 2 6% 4 4 5% 4 7% 10% 44%	6360
	11th to 20th	Day	6% 5% 2 6% 4 4 5% 4 7% 10% 47%	6185
	21st to end	Day	7% 5% 2 6% 5% 5% 5% 5% 6% 9% 45%	6424
August	1st to 10th	Day	11% 6% 2 5% 4 3 5% 4 6% 9% 43%	4234
	11th to 20th	Day	8% 5% 3 7% 5% 5% 6% 4 8% 11% 39%	4174
	21st to end	Day	8% 5% 2 6% 4 4 6% 5% 8% 9% 43%	4675
September	1st to 10th	Day	9% 6% 2 6% 4 4 6% 4 6% 8% 43%	2253
	11th to 20th	Day	8% 5% 2 5% 4 4 4 3 8% 11% 47%	2200
	21st to end	Day	16% 7% 3 6% 5% 4 4 4 6% 7% 39%	2200
October	1st to 10th	Day	11% 5% 2 6% 4 5% 4 5% 6% 7% 43%	2100
	11th to 20th	Day	17% 7% 2 7% 5% 4 4 4 6% 8% 35%	2000
	21st to end	Day	19% 8% 3 7% 5% 4 5% 4 6% 6% 31%	2108
November	1st to 10th	Day	12% 9% 3 8% 4 4 6% 4 6% 8% 36%	1852
	11th to 20th	Day	14% 12% 3 8% 5% 5% 5% 3 5% 7% 32%	1886
	21st to end	Day	17% 11% 5% 11% 7% 5% 5% 4 6% 6% 22%	1680
December	1st to 10th	Day	15% 13% 5% 12% 7% 6% 7% 5% 6% 6% 20%	2680
	11th to 20th	Day	20% 20% 4 9% 5% 5% 5% 4 6% 6% 17%	3070
	21st to end	Day	25% 18% 6% 13% 5% 5% 4 3 4 5% 12%	3900
January	1st to 10th	Night	14% 14% 5% 14% 6% 5% 5% 4 5% 5% 22%	8232
	11th to 20th	Night	9% 16% 7% 12% 7% 7% 6% 4 5% 6% 21%	7882
	21st to end	Night	16% 15% 6% 10% 5% 5% 5% 3 5% 6% 26%	8401
February	1st to 10th	Night	16% 14% 4 10% 6% 5% 5% 3 5% 5% 25%	2730
	11th to 20th	Night	15% 14% 4 9% 6% 5% 6% 3 6% 6% 26%	2568
	21st to end	Night	18% 13% 4 9% 5% 4 5% 4 4 5% 28%	2076
March	1st to 10th	Night	15% 12% 4 8% 5% 5% 4 4 5% 5% 34%	5265
	11th to 20th	Night	9% 7% 3 6% 5% 5% 6% 4 7% 8% 40%	5090
	21st to end	Night	15% 9% 3 9% 4 4 4 3 5% 6% 37%	5588
April	1st to 10th	Night	16% 12% 3 7% 4 5% 3 4 5% 7% 35%	2167
	11th to 20th	Night	16% 8% 2 7% 5% 5% 4 3 6% 8% 35%	1777
	21st to end	Night	16% 8% 2 6% 5% 4 4 4 5% 7% 39%	1800
May	1st to 10th	Night	20% 9% 4 8% 5% 4 5% 3 5% 5% 31%	4033
	11th to 20th	Night	14% 9% 3 8% 5% 5% 5% 3 5% 7% 36%	4032
	21st to end	Night	14% 6% 2 7% 4 4 4 3 6% 8% 42%	4378
June	1st to 10th	Night	11% 5% 3 5% 5% 4 4 4 6% 9% 45%	1542
	11th to 20th	Night	10% 5% 2 5% 5% 5% 5% 4 6% 9% 44%	1400
	21st to end	Night	10% 5% 2 5% 5% 4 4 3 5% 9% 47%	1416
July	1st to 10th	Night	7% 5% 2 6% 4 4 4 4 7% 11% 47%	3618
	11th to 20th	Night	7% 5% 2 6% 4 4 4 5% 7% 10% 46%	3783
	21st to end	Night	7% 5% 2 5% 4 4 4 3 6% 9% 50%	4427
August	1st to 10th	Night	11% 5% 2 6% 4 4 3 4 5% 9% 46%	2915
	11th to 20th	Night	8% 6% 3 7% 5% 4 5% 4 5% 7% 48%	2875
	21st to end	Night	7% 5% 2 6% 4 5% 5% 4 6% 8% 48%	3223
September	1st to 10th	Night	9% 5% 2 5% 4 4 4 4 7% 7% 47%	2121
	11th to 20th	Night	8% 5% 2 4 4 3 4 3 5% 7% 56%	2200
	21st to end	Night	13% 7% 2 7% 4 4 4 3 5% 6% 42%	2200
October	1st to 10th	Night	12% 6% 3 6% 4 4 4 3 5% 6% 48%	2298
	11th to 20th	Night	17% 8% 3 8% 5% 4 4 4 4 7% 37%	2400
	21st to end	Night	19% 9% 3 8% 5% 5% 4 4 5% 6% 33%	2730
November	1st to 10th	Night	14% 8% 3 7% 4 4 4 4 5% 6% 42%	2666
	11th to 20th	Night	16% 10% 3 8% 5% 4 5% 4 6% 7% 31%	2730
	21st to end	Night	20% 14% 4 9% 5% 4 5% 4 5% 6% 25%	2940
December	1st to 10th	Night	15% 13% 5% 10% 6% 6% 6% 5% 7% 7% 22%	4720
	11th to 20th	Night	17% 17% 6% 11% 6% 5% 5% 3 5% 5% 22%	5345
	21st to end	Night	25% 19% 6% 11% 5% 4 4 3 4 4 15%	7273

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% 9% 11% 12% 4 1	4692
	11th to 20th	Day	70% 6% 7% 9% 9%	5082
	21st to end	Day	66% 4 10% 13% 7%	5780
February	1st to 10th	Day	60% 6% 10% 17% 6%	1890
	11th to 20th	Day	59% 6% 11% 16% 7% 1	1945
	21st to end	Day	57% 6% 13% 17% 6% 2	1730
March	1st to 10th	Day	54% 5% 14% 18% 8% 1	4807
	11th to 20th	Day	47% 6% 16% 22% 8% 1	5090
	21st to end	Day	52% 7% 12% 18% 10% 1	5586
April	1st to 10th	Day	48% 7% 14% 18% 12% 1	2271
	11th to 20th	Day	47% 7% 11% 21% 12% 2	2572
	21st to end	Day	47% 7% 13% 20% 12% 2	2600
May	1st to 10th	Day	50% 7% 9% 20% 11% 3	5897
	11th to 20th	Day	44% 8% 16% 20% 10% 2	5865
	21st to end	Day	39% 7% 11% 20% 17% 5%	6486
June	1st to 10th	Day	37% 7% 14% 27% 13% 2	2853
	11th to 20th	Day	36% 8% 14% 23% 16% 2	3000
	21st to end	Day	34% 8% 15% 25% 15% 2	2977
July	1st to 10th	Day	35% 6% 12% 29% 17% 2	6291
	11th to 20th	Day	30% 8% 14% 31% 17% 1	6110
	21st to end	Day	32% 7% 16% 22% 21% 2	6240
August	1st to 10th	Day	36% 7% 15% 28% 14% 1	4221
	11th to 20th	Day	36% 8% 14% 34% 8% 1	4213
	21st to end	Day	34% 8% 15% 31% 12% 1	4674
September	1st to 10th	Day	35% 8% 16% 26% 14% 1	2253
	11th to 20th	Day	30% 7% 15% 32% 16% 1	2200
	21st to end	Day	42% 6% 13% 25% 13% 1	2200
October	1st to 10th	Day	30% 6% 14% 30% 20% 1	3425
	11th to 20th	Day	37% 6% 13% 26% 17%	3240
	21st to end	Day	42% 6% 16% 22% 14% 1	3384
November	1st to 10th	Day	45% 8% 12% 23% 10% 2	1852
	11th to 20th	Day	50% 6% 16% 19% 8% 1	1886
	21st to end	Day	60% 7% 13% 15% 5%	1680
December	1st to 10th	Day	59% 11% 14% 12% 3	2680
	11th to 20th	Day	65% 7% 12% 12% 4	3066
	21st to end	Day	74% 4 9% 8% 5%	3900
January	1st to 10th	Night	60% 8% 11% 14% 6% 1	8233
	11th to 20th	Night	63% 7% 11% 11% 7% 1	7882
	21st to end	Night	59% 5% 9% 17% 9%	8401
February	1st to 10th	Night	60% 6% 11% 16% 5% 1	2730
	11th to 20th	Night	57% 6% 11% 16% 9%	2570
	21st to end	Night	57% 5% 12% 17% 8% 1	2076
March	1st to 10th	Night	51% 5% 14% 20% 10% 1	5265
	11th to 20th	Night	39% 6% 16% 26% 13% 1	5090
	21st to end	Night	47% 6% 11% 20% 16% 1	5588
April	1st to 10th	Night	49% 5% 12% 19% 12% 2	2167
	11th to 20th	Night	45% 5% 13% 20% 15% 2	1777
	21st to end	Night	43% 6% 12% 20% 16% 4	1800
May	1st to 10th	Night	54% 5% 9% 14% 13% 5%	4060
	11th to 20th	Night	45% 7% 11% 25% 10% 3	4033
	21st to end	Night	38% 7% 13% 21% 16% 5%	4385
June	1st to 10th	Night	36% 6% 11% 27% 17% 3	1542
	11th to 20th	Night	36% 7% 13% 26% 15% 3	1400
	21st to end	Night	33% 6% 12% 26% 21% 1	1416
July	1st to 10th	Night	30% 7% 14% 27% 18% 5%	3580
	11th to 20th	Night	31% 7% 15% 28% 18% 2	3738
	21st to end	Night	31% 6% 12% 21% 26% 2	4297
August	1st to 10th	Night	35% 6% 13% 27% 17% 2	2904
	11th to 20th	Night	34% 7% 10% 32% 15% 1	2902
	21st to end	Night	33% 8% 12% 31% 16% 1	3223
September	1st to 10th	Night	33% 7% 13% 28% 16% 3	2121
	11th to 20th	Night	28% 6% 12% 33% 19% 2	2200
	21st to end	Night	42% 6% 13% 24% 15% 1	2200
October	1st to 10th	Night	26% 5% 10% 31% 28% 1	3753
	11th to 20th	Night	44% 5% 10% 21% 18% 1	3899
	21st to end	Night	41% 5% 12% 27% 14% 1	4404
November	1st to 10th	Night	42% 5% 13% 24% 14% 1	2666
	11th to 20th	Night	50% 6% 14% 19% 10% 1	2730
	21st to end	Night	58% 6% 13% 17% 5%	2940
December	1st to 10th	Night	56% 10% 17% 14% 3	4720
	11th to 20th	Night	62% 7% 12% 13% 6%	5348
	21st to end	Night	72% 5% 10% 9% 3	7273

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	21% 17% 39% 16% 5% 1	330
	11th to 20th	Day	22% 21% 40% 13% 4 1	330
	21st to end	Day	24% 18% 29% 23% 6%	363
February	1st to 10th	Day	21% 19% 35% 19% 5% 1	330
	11th to 20th	Day	22% 19% 35% 20% 4	330
	21st to end	Day	21% 18% 42% 18% 1	273
March	1st to 10th	Day	22% 16% 38% 19% 4	427
	11th to 20th	Day	21% 15% 35% 22% 6% 1	540
	21st to end	Day	24% 14% 34% 23% 4 1	594
April	1st to 10th	Day	19% 13% 37% 23% 7% 1	504
	11th to 20th	Day	23% 13% 31% 18% 10% 4	492
	21st to end	Day	21% 13% 34% 21% 10% 2	500
May	1st to 10th	Day	23% 10% 30% 24% 11% 2	503
	11th to 20th	Day	17% 9% 29% 26% 15% 4	527
	21st to end	Day	13% 7% 23% 32% 19% 6%	573
June	1st to 10th	Day	12% 4 31% 33% 15% 3	583
	11th to 20th	Day	10% 6% 29% 36% 16% 3	730
	21st to end	Day	11% 6% 28% 36% 16% 4	730
July	1st to 10th	Day	10% 4 22% 30% 25% 9%	721
	11th to 20th	Day	10% 4 25% 31% 21% 9%	647
	21st to end	Day	9% 4 18% 33% 29% 8%	576
August	1st to 10th	Day	9% 4 17% 30% 26% 14%	526
	11th to 20th	Day	11% 4 17% 33% 24% 12%	519
	21st to end	Day	8% 3 19% 31% 26% 13%	583
September	1st to 10th	Day	11% 4 23% 28% 22% 12%	530
	11th to 20th	Day	7% 3 16% 27% 32% 14%	530
	21st to end	Day	17% 9% 21% 25% 19% 9%	530
October	1st to 10th	Day	12% 8% 25% 27% 20% 8%	530
	11th to 20th	Day	16% 14% 32% 26% 9% 2	530
	21st to end	Day	22% 16% 35% 19% 6% 1	491
November	1st to 10th	Day	15% 16% 35% 22% 9% 2	330
	11th to 20th	Day	20% 22% 31% 20% 6% 2	327
	21st to end	Day	23% 18% 40% 16% 3	220
December	1st to 10th	Day	21% 21% 31% 19% 8%	220
	11th to 20th	Day	26% 20% 33% 17% 4	220
	21st to end	Day	26% 22% 33% 16% 3	297
January	1st to 10th	Night	23% 16% 36% 19% 6% 1	1050
	11th to 20th	Night	26% 17% 34% 20% 4	912
	21st to end	Night	21% 14% 33% 26% 5%	924
February	1st to 10th	Night	25% 12% 34% 24% 4	840
	11th to 20th	Night	22% 14% 34% 24% 6%	823
	21st to end	Night	23% 15% 38% 22% 2	692
March	1st to 10th	Night	19% 12% 37% 26% 6%	840
	11th to 20th	Night	17% 13% 40% 25% 5% 1	840
	21st to end	Night	21% 12% 33% 26% 9%	924
April	1st to 10th	Night	20% 13% 33% 29% 5%	770
	11th to 20th	Night	19% 10% 35% 28% 7% 1	593
	21st to end	Night	18% 9% 33% 32% 9%	600
May	1st to 10th	Night	20% 7% 27% 35% 10% 2	600
	11th to 20th	Night	18% 6% 26% 33% 15% 2	591
	21st to end	Night	11% 4 23% 37% 23% 2	632
June	1st to 10th	Night	12% 4 27% 42% 15% 1	400
	11th to 20th	Night	12% 3 20% 47% 19%	400
	21st to end	Night	11% 4 22% 46% 17% 1	416
July	1st to 10th	Night	8% 3 13% 42% 30% 5%	585
	11th to 20th	Night	10% 3 17% 39% 27% 5%	582
	21st to end	Night	7% 2 15% 44% 29% 3	657
August	1st to 10th	Night	8% 2 14% 41% 29% 7%	597
	11th to 20th	Night	7% 2 14% 38% 34% 5%	594
	21st to end	Night	10% 2 15% 37% 33% 4	660
September	1st to 10th	Night	12% 5% 14% 33% 30% 6%	767
	11th to 20th	Night	10% 4 15% 31% 34% 7%	800
	21st to end	Night	15% 6% 21% 32% 20% 6%	800
October	1st to 10th	Night	14% 7% 23% 32% 21% 4	800
	11th to 20th	Night	20% 12% 29% 27% 12% 2	800
	21st to end	Night	22% 14% 30% 29% 5%	880
November	1st to 10th	Night	17% 13% 31% 26% 13% 1	820
	11th to 20th	Night	23% 16% 32% 22% 6% 1	840
	21st to end	Night	27% 15% 32% 21% 6%	840
December	1st to 10th	Night	25% 16% 34% 19% 6% 1	840
	11th to 20th	Night	24% 15% 37% 21% 3	840
	21st to end	Night	30% 12% 33% 22% 3	1092

Page generated at 2026-03-30 02:18:42. Version ac07

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