Sleaford Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sleaford Observatory clear sky chart from 2005-11-06 to 2025-09-04.

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	9% 8% 3 8% 6% 5% 5% 4 6% 8% 37%	130007
	Night	7% 9% 4 10% 6% 6% 5% 4 6% 8% 35%	127404
Transparency	Day	45% 5% 16% 25% 9% 1	129709
	Night	46% 5% 15% 23% 9% 1	127283
Seeing	Day	13% 8% 27% 31% 17% 5%	17014
	Night	12% 6% 30% 35% 15% 2	26208

By Month

Percentage Cloud Cover NW (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	6% 15% 5% 15% 9% 8% 7% 5% 6% 7% 19%	3060
	11th to 20th	Day	3 7% 4 10% 9% 12% 8% 6% 9% 8% 25%	3159
	21st to end	Day	6% 12% 5% 13% 7% 6% 6% 4 6% 8% 28%	3847
February	1st to 10th	Day	7% 11% 4 12% 9% 6% 9% 7% 7% 7% 20%	1650
	11th to 20th	Day	7% 11% 4 12% 8% 6% 7% 5% 7% 8% 27%	1755
	21st to end	Day	4 10% 6% 12% 11% 8% 5% 6% 7% 8% 22%	1540
March	1st to 10th	Day	6% 11% 5% 16% 9% 7% 7% 4 5% 8% 22%	4500
	11th to 20th	Day	6% 13% 6% 16% 8% 5% 4 4 5% 7% 27%	4877
	21st to end	Day	10% 11% 3 9% 5% 5% 6% 4 7% 8% 33%	5824
April	1st to 10th	Day	6% 10% 3 8% 6% 5% 5% 5% 7% 9% 36%	2400
	11th to 20th	Day	13% 11% 4 8% 6% 4 5% 4 6% 8% 31%	2450
	21st to end	Day	12% 10% 4 9% 6% 4 4 4 6% 7% 35%	2800
May	1st to 10th	Day	12% 10% 3 7% 3 4 4 4 5% 6% 42%	6340
	11th to 20th	Day	12% 8% 3 9% 6% 6% 6% 5% 7% 8% 31%	6298
	21st to end	Day	11% 9% 2 6% 4 4 4 3 6% 8% 44%	7497
June	1st to 10th	Day	13% 8% 2 7% 5% 5% 5% 5% 7% 9% 34%	3200
	11th to 20th	Day	21% 8% 3 7% 5% 5% 5% 5% 7% 8% 26%	3200
	21st to end	Day	12% 5% 3 7% 5% 5% 5% 6% 8% 9% 36%	3200
July	1st to 10th	Day	7% 4 2 5% 4 5% 5% 5% 7% 9% 48%	7179
	11th to 20th	Day	10% 5% 2 5% 4 5% 5% 4 7% 10% 43%	6780
	21st to end	Day	5% 4 2 4 3 3 5% 4 7% 9% 54%	7126
August	1st to 10th	Day	10% 6% 2 6% 4 4 4 5% 6% 8% 44%	4552
	11th to 20th	Day	7% 6% 2 6% 4 4 4 4 6% 10% 47%	4460
	21st to end	Day	12% 5% 2 4 3 3 4 3 5% 8% 52%	4514
September	1st to 10th	Day	9% 5% 2 6% 4 5% 4 4 6% 8% 47%	2317
	11th to 20th	Day	13% 9% 3 7% 5% 4 3 4 5% 7% 41%	2280
	21st to end	Day	10% 5% 2 5% 4 4 4 4 6% 8% 49%	2029
October	1st to 10th	Day	9% 8% 3 6% 4 3 4 4 6% 9% 45%	3170
	11th to 20th	Day	7% 6% 2 7% 5% 5% 5% 6% 8% 13% 37%	3039
	21st to end	Day	7% 9% 4 9% 6% 6% 6% 5% 6% 7% 35%	3045
November	1st to 10th	Day	16% 15% 5% 10% 5% 5% 4 3 6% 6% 27%	1568
	11th to 20th	Day	7% 16% 6% 13% 10% 7% 4 4 5% 4 23%	1600
	21st to end	Day	7% 13% 7% 14% 6% 6% 6% 4 6% 7% 24%	1403
December	1st to 10th	Day	6% 11% 5% 11% 11% 8% 7% 5% 6% 8% 22%	2097
	11th to 20th	Day	7% 12% 6% 16% 9% 9% 8% 4 5% 5% 19%	2259
	21st to end	Day	3 8% 4 13% 8% 6% 6% 4 7% 6% 36%	2992
January	1st to 10th	Night	3 12% 5% 15% 9% 8% 7% 5% 7% 9% 20%	6881
	11th to 20th	Night	3 8% 4 14% 10% 9% 7% 5% 8% 8% 24%	6739
	21st to end	Night	4 10% 4 11% 8% 7% 6% 5% 6% 8% 29%	7123
February	1st to 10th	Night	7% 12% 5% 13% 7% 7% 7% 5% 6% 9% 24%	2766
	11th to 20th	Night	4 10% 5% 10% 7% 5% 7% 4 7% 7% 34%	2532
	21st to end	Night	3 9% 3 9% 7% 6% 7% 6% 8% 11% 31%	2090
March	1st to 10th	Night	5% 12% 5% 13% 7% 7% 7% 5% 7% 9% 23%	5374
	11th to 20th	Night	6% 10% 4 13% 7% 6% 5% 5% 7% 8% 30%	5115
	21st to end	Night	8% 10% 4 10% 6% 5% 5% 5% 7% 8% 32%	5134
April	1st to 10th	Night	5% 10% 3 8% 5% 4 4 4 7% 8% 43%	2000
	11th to 20th	Night	10% 13% 3 7% 6% 6% 6% 3 6% 7% 33%	1898
	21st to end	Night	12% 9% 3 8% 5% 4 5% 4 6% 9% 34%	1600
May	1st to 10th	Night	13% 8% 3 7% 5% 4 4 3 5% 5% 43%	3640
	11th to 20th	Night	13% 9% 3 9% 6% 5% 6% 5% 6% 7% 31%	3618
	21st to end	Night	12% 8% 2 6% 4 4 4 4 5% 7% 43%	3368
June	1st to 10th	Night	10% 6% 3 7% 4 3 4 4 8% 10% 42%	1200
	11th to 20th	Night	16% 9% 2 8% 5% 5% 5% 4 6% 7% 33%	1200
	21st to end	Night	10% 5% 2 5% 5% 5% 4 4 8% 8% 44%	1200
July	1st to 10th	Night	8% 5% 2 4 3 5% 4 3 7% 11% 46%	2764
	11th to 20th	Night	9% 4 1 4 3 4 5% 5% 7% 10% 49%	3180
	21st to end	Night	4 3 2 4 3 3 4 3 6% 9% 59%	3729
August	1st to 10th	Night	7% 4 2 6% 3 4 4 4 6% 8% 53%	2608
	11th to 20th	Night	5% 4 2 4 4 5% 5% 5% 7% 11% 48%	2598
	21st to end	Night	7% 5% 2 5% 3 3 4 3 6% 7% 54%	3395
September	1st to 10th	Night	9% 5% 2 5% 3 4 4 4 7% 7% 50%	1930
	11th to 20th	Night	11% 5% 2 7% 4 4 4 4 6% 8% 45%	1900
	21st to end	Night	8% 6% 2 4 3 4 4 3 5% 8% 51%	2148
October	1st to 10th	Night	10% 8% 2 5% 4 4 4 3 5% 7% 49%	3800
	11th to 20th	Night	6% 6% 2 7% 5% 4 5% 4 6% 8% 48%	3857
	21st to end	Night	8% 8% 3 7% 5% 5% 5% 4 6% 7% 41%	4513
November	1st to 10th	Night	14% 15% 4 10% 5% 5% 4 4 5% 7% 27%	2730
	11th to 20th	Night	7% 14% 6% 13% 8% 6% 5% 4 6% 6% 26%	2800
	21st to end	Night	7% 12% 5% 13% 7% 5% 5% 4 6% 8% 28%	2995
December	1st to 10th	Night	7% 13% 6% 15% 9% 8% 6% 4 6% 6% 20%	5082
	11th to 20th	Night	5% 14% 6% 13% 7% 8% 7% 5% 7% 7% 20%	5933
	21st to end	Night	2 7% 3 13% 10% 6% 5% 3 5% 5% 40%	7964

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 9% 15% 7%	3060
	11th to 20th	Day	54% 4 13% 18% 10%	3180
	21st to end	Day	54% 6% 19% 16% 5%	3847
February	1st to 10th	Day	59% 8% 12% 13% 9%	1650
	11th to 20th	Day	52% 6% 13% 18% 10% 1	1764
	21st to end	Day	56% 6% 10% 20% 8%	1540
March	1st to 10th	Day	61% 5% 15% 12% 8%	4500
	11th to 20th	Day	57% 6% 13% 17% 7%	4878
	21st to end	Day	48% 5% 11% 21% 15%	5824
April	1st to 10th	Day	42% 7% 17% 22% 12% 1	2400
	11th to 20th	Day	51% 6% 13% 19% 9% 1	2450
	21st to end	Day	47% 7% 13% 23% 9% 1	2800
May	1st to 10th	Day	42% 3 11% 26% 16% 2	6340
	11th to 20th	Day	48% 5% 13% 25% 8% 1	6302
	21st to end	Day	38% 5% 12% 28% 16% 1	7497
June	1st to 10th	Day	44% 6% 14% 23% 11% 1	3200
	11th to 20th	Day	53% 7% 13% 19% 7% 1	3200
	21st to end	Day	42% 9% 18% 24% 7%	3200
July	1st to 10th	Day	32% 4 26% 30% 8%	7101
	11th to 20th	Day	37% 4 15% 38% 5%	6699
	21st to end	Day	28% 4 21% 37% 10%	6926
August	1st to 10th	Day	38% 5% 20% 30% 8%	4536
	11th to 20th	Day	34% 4 20% 36% 6%	4502
	21st to end	Day	32% 4 20% 36% 8%	4513
September	1st to 10th	Day	34% 7% 17% 28% 12% 2	2317
	11th to 20th	Day	43% 6% 16% 26% 8% 1	2280
	21st to end	Day	33% 6% 16% 32% 13% 2	2029
October	1st to 10th	Day	37% 4 24% 27% 9%	3170
	11th to 20th	Day	38% 4 19% 31% 8% 1	3039
	21st to end	Day	47% 4 19% 26% 4	3045
November	1st to 10th	Day	59% 6% 13% 16% 6%	1568
	11th to 20th	Day	64% 6% 13% 13% 4	1600
	21st to end	Day	60% 5% 15% 16% 4	1403
December	1st to 10th	Day	59% 5% 16% 13% 7%	2097
	11th to 20th	Day	65% 5% 16% 9% 4	2260
	21st to end	Day	47% 5% 10% 21% 14% 3	2992
January	1st to 10th	Night	59% 5% 12% 19% 5%	6881
	11th to 20th	Night	56% 5% 13% 16% 10%	6780
	21st to end	Night	51% 7% 20% 18% 4	7123
February	1st to 10th	Night	56% 6% 11% 18% 9% 1	2766
	11th to 20th	Night	46% 7% 13% 22% 13%	2533
	21st to end	Night	43% 6% 16% 24% 10%	2090
March	1st to 10th	Night	56% 5% 18% 16% 6%	5374
	11th to 20th	Night	50% 6% 15% 20% 9%	5115
	21st to end	Night	49% 4 10% 23% 14%	5134
April	1st to 10th	Night	38% 5% 16% 25% 14% 2	2000
	11th to 20th	Night	49% 5% 11% 20% 12% 2	1898
	21st to end	Night	46% 6% 13% 21% 13% 1	1600
May	1st to 10th	Night	43% 5% 9% 22% 18% 3	3640
	11th to 20th	Night	50% 5% 11% 20% 13% 1	3618
	21st to end	Night	40% 5% 10% 26% 18% 1	3368
June	1st to 10th	Night	35% 7% 14% 29% 13% 2	1200
	11th to 20th	Night	48% 8% 13% 19% 10% 1	1200
	21st to end	Night	35% 9% 20% 26% 10%	1200
July	1st to 10th	Night	31% 5% 25% 34% 5%	2734
	11th to 20th	Night	32% 3 15% 44% 6%	3138
	21st to end	Night	23% 3 22% 39% 11% 1	3624
August	1st to 10th	Night	30% 4 23% 31% 11% 1	2600
	11th to 20th	Night	30% 5% 20% 35% 10%	2622
	21st to end	Night	29% 4 18% 36% 13%	3395
September	1st to 10th	Night	31% 8% 15% 28% 14% 4	1930
	11th to 20th	Night	37% 7% 15% 29% 11% 1	1900
	21st to end	Night	30% 6% 13% 32% 17% 2	2148
October	1st to 10th	Night	36% 4 17% 35% 8%	3800
	11th to 20th	Night	35% 3 14% 34% 13% 1	3857
	21st to end	Night	42% 4 18% 30% 6%	4513
November	1st to 10th	Night	58% 6% 12% 17% 7% 1	2730
	11th to 20th	Night	59% 7% 15% 15% 4	2800
	21st to end	Night	54% 8% 16% 18% 4	2995
December	1st to 10th	Night	65% 5% 13% 12% 4	5082
	11th to 20th	Night	62% 5% 17% 13% 4	5931
	21st to end	Night	46% 4 9% 24% 14% 3	7964

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	12% 8% 30% 31% 14% 6%	200
	11th to 20th	Day	12% 7% 33% 31% 17% 2	200
	21st to end	Day	12% 9% 31% 31% 15% 2	297
February	1st to 10th	Day	11% 9% 33% 34% 11% 3	300
	11th to 20th	Day	9% 5% 29% 35% 17% 5%	300
	21st to end	Day	6% 6% 29% 40% 17% 2	249
March	1st to 10th	Day	13% 7% 29% 35% 12% 3	300
	11th to 20th	Day	12% 8% 34% 28% 16% 2	443
	21st to end	Day	15% 5% 28% 34% 15% 3	550
April	1st to 10th	Day	9% 9% 32% 31% 14% 5%	500
	11th to 20th	Day	17% 11% 30% 28% 12% 3	506
	21st to end	Day	14% 8% 29% 30% 15% 3	700
May	1st to 10th	Day	12% 8% 25% 32% 18% 5%	703
	11th to 20th	Day	13% 7% 26% 31% 18% 6%	725
	21st to end	Day	16% 9% 21% 29% 19% 6%	789
June	1st to 10th	Day	15% 10% 28% 29% 13% 4	730
	11th to 20th	Day	23% 9% 26% 26% 12% 4	730
	21st to end	Day	14% 6% 23% 28% 22% 6%	730
July	1st to 10th	Day	9% 5% 22% 35% 22% 7%	721
	11th to 20th	Day	11% 6% 24% 30% 22% 8%	718
	21st to end	Day	8% 5% 24% 31% 22% 9%	794
August	1st to 10th	Day	10% 6% 22% 31% 22% 9%	726
	11th to 20th	Day	9% 6% 21% 34% 23% 7%	705
	21st to end	Day	12% 7% 25% 32% 20% 5%	583
September	1st to 10th	Day	14% 10% 27% 30% 17% 3	503
	11th to 20th	Day	18% 9% 26% 35% 11% 2	490
	21st to end	Day	15% 8% 29% 30% 16% 2	329
October	1st to 10th	Day	15% 5% 31% 23% 22% 4	300
	11th to 20th	Day	12% 10% 33% 28% 16% 2	300
	21st to end	Day	11% 9% 42% 23% 12% 3	330
November	1st to 10th	Day	19% 8% 29% 31% 8% 4	300
	11th to 20th	Day	14% 10% 37% 31% 6% 2	300
	21st to end	Day	10% 8% 34% 35% 9% 3	300
December	1st to 10th	Day	7% 9% 28% 36% 15% 5%	243
	11th to 20th	Day	12% 7% 37% 36% 8% 1	200
	21st to end	Day	11% 3 29% 34% 17% 6%	220
January	1st to 10th	Night	10% 7% 35% 33% 13% 2	1000
	11th to 20th	Night	12% 7% 39% 32% 10% 2	1000
	21st to end	Night	10% 8% 39% 32% 9% 1	1100
February	1st to 10th	Night	12% 6% 30% 35% 15% 2	966
	11th to 20th	Night	7% 7% 31% 38% 14% 4	784
	21st to end	Night	6% 5% 27% 35% 24% 3	660
March	1st to 10th	Night	11% 6% 31% 37% 13% 2	800
	11th to 20th	Night	10% 8% 33% 35% 14% 1	800
	21st to end	Night	14% 6% 26% 38% 12% 3	732
April	1st to 10th	Night	8% 7% 30% 39% 15% 2	600
	11th to 20th	Night	16% 7% 31% 34% 11% 1	593
	21st to end	Night	17% 8% 28% 36% 11% 1	600
May	1st to 10th	Night	13% 7% 28% 35% 17% 1	600
	11th to 20th	Night	13% 7% 30% 34% 15% 1	591
	21st to end	Night	16% 7% 24% 34% 16% 3	518
June	1st to 10th	Night	12% 7% 28% 36% 16% 2	400
	11th to 20th	Night	19% 7% 25% 30% 17% 2	400
	21st to end	Night	13% 5% 20% 42% 18% 2	400
July	1st to 10th	Night	8% 3 22% 39% 25% 3	390
	11th to 20th	Night	12% 4 21% 40% 21% 2	388
	21st to end	Night	4 4 20% 45% 23% 4	548
August	1st to 10th	Night	7% 2 22% 38% 28% 3	597
	11th to 20th	Night	8% 4 18% 43% 25% 3	594
	21st to end	Night	12% 5% 25% 36% 21% 1	660
September	1st to 10th	Night	11% 3 27% 39% 19% 2	579
	11th to 20th	Night	15% 4 24% 39% 17% 1	570
	21st to end	Night	11% 7% 32% 37% 12% 2	657
October	1st to 10th	Night	19% 5% 26% 36% 13% 2	760
	11th to 20th	Night	10% 6% 34% 34% 14% 2	801
	21st to end	Night	16% 8% 32% 33% 11% 1	1045
November	1st to 10th	Night	21% 6% 31% 32% 9% 1	975
	11th to 20th	Night	13% 8% 34% 32% 11% 2	1000
	21st to end	Night	11% 9% 38% 32% 9% 1	1000
December	1st to 10th	Night	10% 6% 33% 34% 15% 3	1000
	11th to 20th	Night	12% 9% 33% 35% 11% 1	1000
	21st to end	Night	9% 5% 29% 36% 16% 4	1100

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