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 2024-12-09.

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% 3 8% 5% 5% 5% 4 6% 8% 34%	101526
	Night	15% 10% 4 8% 5% 4 4 4 5% 7% 35%	98228
Transparency	Day	45% 7% 14% 21% 11% 1	105168
	Night	47% 6% 12% 21% 12% 1	102725
Seeing	Day	16% 10% 28% 26% 15% 5%	15302
	Night	19% 10% 28% 29% 13% 2	24861

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	16% 16% 7% 11% 6% 6% 5% 4 5% 5% 18%	2527
	11th to 20th	Day	15% 18% 6% 14% 8% 7% 6% 4 5% 4 12%	2720
	21st to end	Day	27% 18% 8% 12% 5% 3 3 3 4 4 14%	3124
February	1st to 10th	Day	16% 16% 4 12% 6% 6% 4 3 5% 5% 23%	1710
	11th to 20th	Day	14% 13% 4 11% 6% 6% 6% 3 5% 7% 25%	1746
	21st to end	Day	17% 11% 4 11% 7% 4 5% 3 6% 6% 27%	1570
March	1st to 10th	Day	18% 13% 5% 9% 6% 4 4 3 5% 5% 28%	3272
	11th to 20th	Day	12% 10% 4 10% 5% 5% 5% 4 5% 6% 33%	3480
	21st to end	Day	18% 12% 4 9% 6% 4 4 3 4 6% 30%	3815
April	1st to 10th	Day	14% 12% 4 6% 5% 4 4 5% 6% 7% 33%	2146
	11th to 20th	Day	17% 9% 3 7% 5% 4 5% 4 5% 8% 34%	2442
	21st to end	Day	18% 7% 3 8% 5% 5% 4 3 5% 6% 35%	2470
May	1st to 10th	Day	16% 9% 3 7% 4 4 4 4 5% 7% 36%	4082
	11th to 20th	Day	17% 8% 3 6% 4 5% 4 4 6% 9% 35%	4081
	21st to end	Day	8% 6% 2 6% 4 5% 4 4 6% 9% 46%	4500
June	1st to 10th	Day	9% 7% 3 5% 5% 5% 5% 5% 7% 9% 40%	2710
	11th to 20th	Day	10% 4 2 7% 5% 5% 4 5% 7% 9% 42%	2850
	21st to end	Day	9% 5% 2 5% 5% 4 5% 5% 7% 9% 42%	2828
July	1st to 10th	Day	10% 6% 2 6% 5% 4 5% 4 7% 10% 41%	4440
	11th to 20th	Day	7% 4 2 4 4 4 5% 4 7% 11% 48%	4321
	21st to end	Day	6% 5% 2 6% 5% 5% 6% 5% 6% 9% 44%	4466
August	1st to 10th	Day	11% 7% 2 6% 4 3 5% 4 6% 9% 43%	4104
	11th to 20th	Day	8% 5% 3 7% 5% 5% 6% 4 8% 11% 39%	4044
	21st to end	Day	8% 5% 2 6% 4 4 6% 5% 8% 9% 42%	4532
September	1st to 10th	Day	10% 7% 2 6% 4 4 6% 4 6% 9% 43%	2141
	11th to 20th	Day	9% 5% 2 5% 4 4 4 3 8% 11% 46%	2090
	21st to end	Day	16% 8% 3 6% 5% 4 4 4 6% 7% 38%	2090
October	1st to 10th	Day	11% 6% 3 7% 5% 5% 4 5% 6% 7% 42%	1995
	11th to 20th	Day	17% 8% 2 7% 5% 4 4 4 6% 8% 34%	1900
	21st to end	Day	20% 9% 3 7% 5% 4 5% 4 6% 6% 31%	2003
November	1st to 10th	Day	11% 9% 3 9% 4 5% 6% 4 5% 8% 36%	1762
	11th to 20th	Day	15% 12% 3 8% 5% 5% 5% 3 5% 7% 32%	1796
	21st to end	Day	18% 11% 5% 11% 6% 5% 5% 4 6% 6% 22%	1600
December	1st to 10th	Day	15% 15% 6% 13% 7% 5% 6% 5% 5% 6% 18%	1584
	11th to 20th	Day	16% 16% 4 10% 6% 5% 6% 4 6% 6% 20%	1910
	21st to end	Day	26% 19% 7% 13% 4 4 4 3 5% 5% 10%	2675
January	1st to 10th	Night	15% 15% 6% 11% 5% 5% 5% 3 5% 5% 24%	4428
	11th to 20th	Night	15% 18% 8% 13% 6% 5% 5% 3 4 5% 18%	4238
	21st to end	Night	26% 18% 7% 10% 4 4 4 2 4 4 17%	4532
February	1st to 10th	Night	17% 15% 4 10% 6% 6% 5% 3 4 5% 24%	2470
	11th to 20th	Night	16% 14% 5% 9% 6% 4 6% 3 6% 6% 26%	2318
	21st to end	Night	18% 12% 4 9% 6% 4 5% 4 4 5% 29%	1884
March	1st to 10th	Night	17% 13% 4 8% 5% 5% 4 3 4 5% 32%	3580
	11th to 20th	Night	10% 8% 3 6% 6% 5% 5% 4 7% 7% 39%	3480
	21st to end	Night	16% 9% 3 8% 4 3 4 3 5% 6% 38%	3817
April	1st to 10th	Night	15% 12% 3 7% 4 5% 3 4 5% 7% 36%	2048
	11th to 20th	Night	17% 8% 2 7% 5% 5% 4 4 6% 8% 35%	1687
	21st to end	Night	16% 8% 3 6% 5% 4 4 4 5% 7% 38%	1710
May	1st to 10th	Night	17% 8% 3 8% 5% 5% 5% 3 6% 7% 34%	2813
	11th to 20th	Night	14% 8% 2 8% 5% 6% 5% 4 6% 8% 34%	2812
	21st to end	Night	10% 5% 2 6% 4 4 4 3 6% 8% 48%	3050
June	1st to 10th	Night	11% 5% 2 5% 5% 4 5% 4 6% 9% 44%	1465
	11th to 20th	Night	9% 5% 2 5% 5% 5% 5% 4 6% 9% 45%	1330
	21st to end	Night	10% 5% 2 5% 5% 4 4 3 5% 9% 47%	1345
July	1st to 10th	Night	8% 4 1 5% 3 4 3 3 7% 11% 50%	2530
	11th to 20th	Night	8% 4 1 5% 4 3 4 4 6% 10% 51%	2644
	21st to end	Night	5% 4 1 5% 5% 5% 5% 4 7% 10% 47%	3087
August	1st to 10th	Night	12% 5% 2 6% 4 4 3 4 6% 9% 45%	2825
	11th to 20th	Night	8% 6% 3 7% 5% 4 5% 4 5% 7% 47%	2785
	21st to end	Night	7% 5% 2 6% 4 5% 5% 4 6% 8% 47%	3124
September	1st to 10th	Night	9% 5% 2 6% 4 4 4 4 7% 8% 47%	2014
	11th to 20th	Night	8% 5% 2 4 4 3 4 3 5% 7% 54%	2090
	21st to end	Night	14% 7% 2 7% 4 4 4 3 5% 6% 42%	2090
October	1st to 10th	Night	12% 6% 3 6% 4 4 4 3 5% 6% 47%	2183
	11th to 20th	Night	17% 8% 3 8% 5% 4 4 4 4 7% 36%	2280
	21st to end	Night	20% 9% 3 8% 5% 5% 4 4 5% 6% 32%	2593
November	1st to 10th	Night	14% 8% 3 7% 4 4 4 4 5% 6% 42%	2536
	11th to 20th	Night	17% 10% 3 8% 5% 4 5% 4 6% 7% 30%	2600
	21st to end	Night	20% 14% 4 8% 5% 4 4 4 6% 6% 25%	2800
December	1st to 10th	Night	18% 14% 5% 10% 6% 5% 5% 4 5% 6% 21%	2779
	11th to 20th	Night	15% 15% 5% 10% 6% 5% 5% 4 6% 5% 25%	3285
	21st to end	Night	27% 20% 6% 11% 5% 4 4 3 4 4 12%	4976

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	66% 8% 11% 10% 5%	2528
	11th to 20th	Day	74% 6% 10% 8% 2	2720
	21st to end	Day	74% 5% 10% 8% 4	3124
February	1st to 10th	Day	61% 6% 11% 16% 6%	1710
	11th to 20th	Day	58% 7% 11% 16% 7% 1	1755
	21st to end	Day	56% 6% 13% 17% 6% 2	1570
March	1st to 10th	Day	56% 6% 12% 20% 4 1	3272
	11th to 20th	Day	49% 7% 17% 20% 7% 1	3480
	21st to end	Day	51% 9% 13% 18% 9% 1	3815
April	1st to 10th	Day	47% 7% 15% 18% 12% 1	2146
	11th to 20th	Day	47% 7% 12% 20% 12% 2	2442
	21st to end	Day	47% 7% 13% 19% 12% 2	2470
May	1st to 10th	Day	45% 8% 12% 24% 11% 2	4120
	11th to 20th	Day	44% 8% 19% 20% 8% 1	4085
	21st to end	Day	33% 8% 13% 19% 20% 7%	4514
June	1st to 10th	Day	37% 7% 14% 27% 13% 2	2710
	11th to 20th	Day	36% 8% 14% 23% 17% 2	2850
	21st to end	Day	35% 8% 16% 25% 15% 2	2828
July	1st to 10th	Day	38% 6% 12% 25% 17% 2	4371
	11th to 20th	Day	28% 8% 14% 34% 15% 1	4246
	21st to end	Day	33% 9% 17% 18% 21% 2	4324
August	1st to 10th	Day	36% 6% 15% 28% 14% 1	4091
	11th to 20th	Day	36% 8% 14% 33% 9% 1	4083
	21st to end	Day	34% 8% 15% 30% 12% 1	4531
September	1st to 10th	Day	36% 8% 16% 25% 14% 1	2141
	11th to 20th	Day	31% 7% 14% 30% 16% 1	2090
	21st to end	Day	43% 7% 13% 24% 13% 1	2090
October	1st to 10th	Day	30% 6% 14% 29% 19% 1	3320
	11th to 20th	Day	37% 6% 14% 26% 17%	3140
	21st to end	Day	42% 6% 16% 22% 13% 1	3279
November	1st to 10th	Day	45% 8% 12% 23% 10% 2	1762
	11th to 20th	Day	50% 6% 16% 18% 9% 1	1796
	21st to end	Day	60% 7% 13% 15% 5%	1600
December	1st to 10th	Day	64% 9% 12% 12% 4	1584
	11th to 20th	Day	61% 8% 15% 12% 4	1906
	21st to end	Day	76% 5% 11% 6% 2	2675
January	1st to 10th	Night	60% 6% 14% 15% 5%	4428
	11th to 20th	Night	68% 8% 14% 9% 2	4238
	21st to end	Night	71% 5% 9% 12% 3	4532
February	1st to 10th	Night	61% 6% 12% 15% 5% 1	2470
	11th to 20th	Night	57% 6% 11% 17% 9%	2320
	21st to end	Night	57% 5% 12% 17% 8% 1	1884
March	1st to 10th	Night	53% 5% 14% 21% 7%	3580
	11th to 20th	Night	39% 7% 18% 20% 15% 1	3480
	21st to end	Night	45% 6% 12% 23% 13% 1	3817
April	1st to 10th	Night	48% 5% 12% 20% 12% 2	2048
	11th to 20th	Night	45% 5% 13% 20% 15% 2	1687
	21st to end	Night	44% 6% 12% 19% 15% 4	1710
May	1st to 10th	Night	49% 6% 10% 18% 14% 3	2840
	11th to 20th	Night	45% 8% 12% 23% 12% 2	2813
	21st to end	Night	31% 8% 13% 22% 20% 6%	3057
June	1st to 10th	Night	36% 6% 12% 27% 16% 3	1465
	11th to 20th	Night	35% 8% 13% 25% 16% 3	1330
	21st to end	Night	33% 6% 12% 27% 21%	1345
July	1st to 10th	Night	28% 6% 12% 30% 18% 6%	2490
	11th to 20th	Night	28% 6% 14% 31% 19% 2	2596
	21st to end	Night	30% 8% 16% 22% 22% 2	2988
August	1st to 10th	Night	35% 7% 13% 27% 16% 2	2814
	11th to 20th	Night	35% 7% 10% 32% 15% 2	2812
	21st to end	Night	33% 7% 12% 31% 16% 1	3124
September	1st to 10th	Night	33% 7% 14% 26% 17% 3	2014
	11th to 20th	Night	29% 6% 12% 31% 20% 2	2090
	21st to end	Night	42% 6% 13% 25% 14% 1	2090
October	1st to 10th	Night	27% 5% 10% 30% 27% 1	3638
	11th to 20th	Night	45% 5% 10% 21% 18% 1	3779
	21st to end	Night	41% 5% 13% 27% 14% 1	4267
November	1st to 10th	Night	42% 6% 13% 24% 14% 1	2536
	11th to 20th	Night	51% 6% 14% 18% 10% 1	2600
	21st to end	Night	58% 6% 14% 18% 5%	2800
December	1st to 10th	Night	62% 8% 13% 13% 4	2779
	11th to 20th	Night	58% 8% 15% 14% 5%	3288
	21st to end	Night	76% 5% 10% 7% 2	4976

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	23% 19% 35% 16% 7% 1	270
	11th to 20th	Day	22% 19% 41% 13% 4 1	270
	21st to end	Day	25% 16% 29% 25% 4	297
February	1st to 10th	Day	23% 20% 34% 18% 4 1	270
	11th to 20th	Day	22% 17% 38% 20% 3	270
	21st to end	Day	22% 20% 39% 19% 1	225
March	1st to 10th	Day	23% 17% 35% 21% 4	357
	11th to 20th	Day	19% 13% 36% 24% 7% 1	460
	21st to end	Day	25% 12% 33% 25% 5% 1	506
April	1st to 10th	Day	17% 12% 38% 25% 7% 2	460
	11th to 20th	Day	24% 12% 31% 18% 10% 5%	452
	21st to end	Day	21% 13% 34% 20% 10% 2	460
May	1st to 10th	Day	22% 9% 30% 25% 12% 3	463
	11th to 20th	Day	17% 8% 28% 26% 16% 5%	487
	21st to end	Day	12% 6% 21% 33% 21% 7%	529
June	1st to 10th	Day	11% 4 31% 34% 16% 4	540
	11th to 20th	Day	10% 6% 28% 36% 17% 3	680
	21st to end	Day	11% 6% 29% 34% 16% 4	680
July	1st to 10th	Day	10% 4 22% 30% 26% 9%	671
	11th to 20th	Day	10% 4 25% 31% 20% 9%	601
	21st to end	Day	8% 4 18% 33% 29% 8%	532
August	1st to 10th	Day	9% 4 18% 28% 26% 14%	486
	11th to 20th	Day	11% 3 18% 32% 24% 12%	479
	21st to end	Day	8% 3 19% 31% 27% 12%	539
September	1st to 10th	Day	11% 4 23% 28% 22% 11%	490
	11th to 20th	Day	8% 3 18% 28% 31% 12%	490
	21st to end	Day	17% 10% 22% 26% 18% 8%	490
October	1st to 10th	Day	12% 8% 25% 27% 20% 7%	490
	11th to 20th	Day	17% 15% 30% 26% 10% 2	490
	21st to end	Day	23% 15% 34% 19% 6% 1	452
November	1st to 10th	Day	13% 17% 35% 23% 10% 2	300
	11th to 20th	Day	21% 20% 31% 21% 6% 2	297
	21st to end	Day	22% 17% 42% 17% 3	200
December	1st to 10th	Day	19% 21% 33% 18% 8% 1	196
	11th to 20th	Day	25% 20% 33% 18% 4	180
	21st to end	Day	25% 23% 32% 17% 3	243
January	1st to 10th	Night	23% 15% 36% 19% 6% 1	950
	11th to 20th	Night	27% 15% 33% 21% 4	826
	21st to end	Night	23% 13% 32% 27% 6%	836
February	1st to 10th	Night	26% 12% 35% 23% 4	760
	11th to 20th	Night	23% 13% 32% 25% 6%	743
	21st to end	Night	23% 15% 37% 22% 2	628
March	1st to 10th	Night	19% 11% 37% 26% 7%	760
	11th to 20th	Night	16% 11% 41% 25% 6% 1	760
	21st to end	Night	21% 10% 32% 27% 10%	836
April	1st to 10th	Night	19% 13% 33% 30% 5%	727
	11th to 20th	Night	20% 9% 35% 27% 7% 1	563
	21st to end	Night	18% 9% 32% 31% 9%	570
May	1st to 10th	Night	19% 7% 27% 35% 10% 2	570
	11th to 20th	Night	19% 5% 25% 34% 15% 2	561
	21st to end	Night	11% 4 23% 36% 24% 2	600
June	1st to 10th	Night	12% 4 26% 42% 15% 1	380
	11th to 20th	Night	11% 3 20% 46% 19%	380
	21st to end	Night	11% 4 22% 47% 15% 1	395
July	1st to 10th	Night	8% 3 13% 41% 29% 5%	555
	11th to 20th	Night	10% 3 17% 39% 27% 5%	552
	21st to end	Night	7% 1 16% 44% 29% 3	624
August	1st to 10th	Night	8% 2 14% 41% 28% 7%	567
	11th to 20th	Night	7% 2 15% 38% 33% 5%	564
	21st to end	Night	10% 2 15% 37% 32% 4	627
September	1st to 10th	Night	12% 5% 15% 34% 29% 5%	728
	11th to 20th	Night	10% 4 16% 31% 32% 6%	760
	21st to end	Night	15% 6% 22% 32% 19% 6%	760
October	1st to 10th	Night	14% 7% 24% 31% 20% 4	760
	11th to 20th	Night	20% 12% 28% 26% 12% 2	760
	21st to end	Night	23% 14% 29% 28% 5%	836
November	1st to 10th	Night	17% 12% 30% 26% 13% 1	780
	11th to 20th	Night	24% 15% 32% 22% 6% 1	800
	21st to end	Night	27% 15% 31% 21% 7%	800
December	1st to 10th	Night	25% 16% 34% 18% 6% 1	795
	11th to 20th	Night	24% 15% 36% 22% 4	760
	21st to end	Night	30% 12% 34% 21% 3	988

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