Bourne Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bourne Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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	14% 6% 2 6% 4 4 5% 4 6% 7% 41%	131125
	Night	15% 7% 2 6% 4 4 4 3 5% 7% 43%	130057
Transparency	Day	40% 7% 11% 20% 18% 5%	134912
	Night	39% 6% 10% 20% 20% 6%	134339
Seeing	Day	18% 11% 22% 23% 17% 8%	16552
	Night	17% 9% 22% 29% 19% 3	26654

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	22% 11% 2 5% 3 3 2 2 3 4 42%	5258
	11th to 20th	Day	12% 7% 3 4 3 3 2 2 3 4 57%	5280
	21st to end	Day	25% 10% 3 6% 3 3 3 2 4 4 37%	6002
February	1st to 10th	Day	17% 9% 3 6% 3 4 4 3 5% 6% 40%	2100
	11th to 20th	Day	25% 7% 1 5% 3 2 4 3 5% 6% 39%	2048
	21st to end	Day	22% 7% 3 6% 4 4 3 3 3 5% 40%	1730
March	1st to 10th	Day	23% 7% 3 7% 4 4 4 3 5% 5% 34%	4629
	11th to 20th	Day	12% 6% 2 5% 4 4 4 3 4 7% 50%	4980
	21st to end	Day	17% 6% 2 6% 4 4 4 3 4 5% 44%	5465
April	1st to 10th	Day	14% 6% 2 5% 4 4 5% 4 6% 7% 44%	2268
	11th to 20th	Day	12% 6% 3 6% 4 4 4 3 5% 6% 46%	2374
	21st to end	Day	12% 4 2 5% 4 4 5% 3 6% 8% 47%	2560
May	1st to 10th	Day	16% 7% 3 7% 4 5% 5% 4 6% 9% 35%	5862
	11th to 20th	Day	19% 8% 3 7% 5% 6% 6% 4 7% 8% 28%	5861
	21st to end	Day	15% 7% 2 7% 4 4 5% 4 6% 7% 39%	6433
June	1st to 10th	Day	10% 7% 3 7% 6% 6% 6% 5% 8% 10% 34%	2600
	11th to 20th	Day	6% 4 1 6% 5% 5% 6% 6% 9% 12% 39%	2600
	21st to end	Day	8% 4 2 6% 5% 6% 7% 7% 8% 10% 39%	2600
July	1st to 10th	Day	7% 5% 2 6% 6% 6% 6% 6% 8% 11% 35%	5900
	11th to 20th	Day	7% 4 2 6% 6% 6% 8% 6% 9% 13% 33%	5900
	21st to end	Day	11% 7% 3 9% 7% 7% 7% 6% 8% 10% 26%	6424
August	1st to 10th	Day	7% 5% 2 6% 5% 6% 6% 6% 8% 11% 38%	4234
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 4 7% 11% 41%	3916
	21st to end	Day	8% 4 2 6% 5% 4 5% 4 6% 8% 48%	4312
September	1st to 10th	Day	11% 6% 3 7% 5% 5% 4 4 7% 8% 40%	2214
	11th to 20th	Day	14% 7% 2 5% 4 4 4 4 6% 8% 41%	2200
	21st to end	Day	13% 6% 2 5% 4 5% 4 4 5% 8% 45%	2200
October	1st to 10th	Day	12% 5% 2 4 3 4 3 3 3 5% 55%	2200
	11th to 20th	Day	13% 5% 2 3 2 3 3 3 4 6% 57%	2090
	21st to end	Day	15% 5% 2 4 3 3 3 2 4 4 55%	2200
November	1st to 10th	Day	13% 5% 3 4 3 3 3 2 3 4 56%	1857
	11th to 20th	Day	14% 4 2 4 3 3 4 3 5% 6% 53%	1890
	21st to end	Day	15% 6% 2 5% 3 4 4 4 5% 6% 46%	1890
December	1st to 10th	Day	21% 6% 1 4 3 3 3 3 5% 5% 45%	3020
	11th to 20th	Day	18% 8% 3 7% 5% 4 4 3 4 4 40%	3463
	21st to end	Day	22% 7% 3 4 3 3 4 4 5% 7% 38%	4565
January	1st to 10th	Night	19% 7% 3 5% 3 3 3 3 5% 6% 43%	7635
	11th to 20th	Night	15% 8% 3 6% 3 3 3 2 4 5% 49%	7680
	21st to end	Night	22% 10% 2 6% 3 3 3 2 3 4 42%	8161
February	1st to 10th	Night	16% 7% 3 6% 4 3 3 4 5% 6% 43%	2520
	11th to 20th	Night	22% 7% 2 6% 5% 3 3 3 3 4 41%	2458
	21st to end	Night	22% 6% 2 6% 4 3 4 3 5% 6% 39%	2076
March	1st to 10th	Night	22% 9% 2 6% 5% 4 4 3 5% 5% 34%	5267
	11th to 20th	Night	11% 5% 2 5% 3 3 4 3 6% 7% 51%	4980
	21st to end	Night	15% 7% 2 6% 4 4 4 3 5% 6% 44%	5467
April	1st to 10th	Night	12% 5% 1 5% 4 4 4 5% 6% 7% 48%	2148
	11th to 20th	Night	13% 7% 2 4 3 3 4 4 5% 6% 50%	1975
	21st to end	Night	10% 6% 2 6% 4 3 4 3 6% 7% 49%	1838
May	1st to 10th	Night	13% 7% 3 8% 5% 4 5% 4 6% 8% 36%	4033
	11th to 20th	Night	21% 8% 2 6% 6% 5% 5% 4 7% 7% 30%	4032
	21st to end	Night	13% 6% 2 6% 4 4 4 4 7% 9% 40%	4421
June	1st to 10th	Night	9% 5% 2 7% 6% 6% 6% 5% 8% 10% 36%	1800
	11th to 20th	Night	5% 4 2 6% 5% 4 4 6% 8% 12% 44%	1800
	21st to end	Night	6% 5% 2 5% 5% 4 6% 6% 7% 11% 43%	1800
July	1st to 10th	Night	9% 6% 2 8% 6% 5% 6% 5% 7% 10% 35%	4058
	11th to 20th	Night	7% 5% 3 8% 5% 6% 7% 5% 10% 12% 34%	4057
	21st to end	Night	12% 8% 3 9% 6% 6% 5% 5% 6% 10% 30%	4427
August	1st to 10th	Night	8% 5% 2 7% 5% 5% 5% 5% 8% 12% 39%	2915
	11th to 20th	Night	10% 7% 2 7% 5% 4 4 3 5% 8% 45%	3128
	21st to end	Night	8% 5% 2 5% 4 4 4 3 6% 8% 51%	3586
September	1st to 10th	Night	9% 6% 3 6% 5% 5% 5% 4 7% 8% 40%	2165
	11th to 20th	Night	14% 6% 2 6% 4 4 4 4 6% 7% 44%	2200
	21st to end	Night	13% 5% 2 5% 3 4 5% 4 5% 6% 48%	2200
October	1st to 10th	Night	12% 4 1 5% 2 3 3 3 5% 6% 56%	2200
	11th to 20th	Night	10% 5% 2 4 4 3 3 2 4 5% 59%	2309
	21st to end	Night	14% 5% 2 4 3 3 2 3 4 5% 56%	2640
November	1st to 10th	Night	14% 5% 2 4 3 3 2 2 4 6% 55%	2656
	11th to 20th	Night	14% 5% 2 4 3 3 3 3 4 6% 54%	2730
	21st to end	Night	17% 6% 2 4 4 3 3 4 5% 6% 46%	2730
December	1st to 10th	Night	19% 7% 2 5% 3 3 3 2 4 5% 48%	4380
	11th to 20th	Night	20% 8% 2 6% 3 3 4 2 5% 5% 43%	4952
	21st to end	Night	22% 7% 3 6% 3 3 4 3 4 5% 41%	6633

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	48% 3 5% 21% 18% 4	5259
	11th to 20th	Day	32% 3 5% 17% 31% 11%	5280
	21st to end	Day	52% 2 5% 15% 19% 8%	6002
February	1st to 10th	Day	44% 4 7% 19% 21% 4	2100
	11th to 20th	Day	46% 4 6% 19% 19% 6%	2058
	21st to end	Day	47% 3 7% 14% 21% 8%	1730
March	1st to 10th	Day	51% 4 7% 12% 18% 8%	4629
	11th to 20th	Day	34% 4 6% 15% 24% 16%	4980
	21st to end	Day	43% 4 7% 23% 20% 4	5465
April	1st to 10th	Day	39% 5% 10% 22% 19% 5%	2268
	11th to 20th	Day	37% 4 8% 19% 24% 8%	2374
	21st to end	Day	34% 6% 11% 27% 19% 3	2560
May	1st to 10th	Day	44% 8% 13% 19% 14% 2	5897
	11th to 20th	Day	50% 9% 13% 18% 8% 1	5865
	21st to end	Day	41% 8% 10% 21% 17% 2	6447
June	1st to 10th	Day	42% 9% 16% 21% 10% 2	2600
	11th to 20th	Day	32% 11% 18% 27% 11% 1	2600
	21st to end	Day	35% 12% 17% 21% 13% 3	2600
July	1st to 10th	Day	36% 14% 19% 24% 7% 1	5835
	11th to 20th	Day	36% 15% 27% 17% 4 1	5831
	21st to end	Day	49% 12% 17% 17% 4	6240
August	1st to 10th	Day	36% 9% 15% 32% 9%	4221
	11th to 20th	Day	33% 9% 18% 24% 13% 2	3954
	21st to end	Day	31% 9% 13% 24% 20% 3	4311
September	1st to 10th	Day	38% 10% 14% 16% 18% 4	2214
	11th to 20th	Day	40% 8% 11% 20% 19% 2	2200
	21st to end	Day	37% 6% 13% 17% 21% 6%	2200
October	1st to 10th	Day	25% 4 6% 14% 37% 13%	3590
	11th to 20th	Day	20% 2 5% 16% 29% 27%	3382
	21st to end	Day	29% 3 5% 22% 28% 12%	3538
November	1st to 10th	Day	34% 2 8% 16% 31% 9%	1857
	11th to 20th	Day	33% 3 7% 19% 28% 10%	1890
	21st to end	Day	38% 4 7% 19% 26% 7%	1890
December	1st to 10th	Day	42% 4 5% 17% 26% 7%	3020
	11th to 20th	Day	48% 3 7% 17% 19% 6%	3460
	21st to end	Day	45% 4 9% 25% 15% 2	4565
January	1st to 10th	Night	41% 4 9% 20% 22% 4	7636
	11th to 20th	Night	39% 3 4 19% 27% 8%	7680
	21st to end	Night	49% 2 4 15% 22% 8%	8161
February	1st to 10th	Night	41% 4 7% 19% 22% 7%	2520
	11th to 20th	Night	48% 3 5% 18% 19% 7%	2459
	21st to end	Night	46% 4 8% 16% 18% 9%	2076
March	1st to 10th	Night	50% 5% 8% 13% 18% 6%	5267
	11th to 20th	Night	31% 4 8% 16% 26% 15%	4980
	21st to end	Night	41% 4 6% 22% 20% 8%	5467
April	1st to 10th	Night	35% 6% 10% 20% 24% 6%	2148
	11th to 20th	Night	34% 4 8% 21% 23% 10%	1975
	21st to end	Night	34% 5% 11% 27% 19% 4	1838
May	1st to 10th	Night	42% 10% 15% 20% 12% 1	4060
	11th to 20th	Night	50% 8% 15% 18% 8%	4033
	21st to end	Night	37% 9% 13% 24% 16% 2	4429
June	1st to 10th	Night	38% 12% 16% 21% 12% 2	1800
	11th to 20th	Night	30% 10% 21% 25% 14% 1	1800
	21st to end	Night	32% 10% 19% 21% 14% 4	1800
July	1st to 10th	Night	42% 11% 17% 23% 7%	4015
	11th to 20th	Night	37% 15% 25% 18% 4 1	4006
	21st to end	Night	49% 11% 18% 17% 5% 1	4297
August	1st to 10th	Night	36% 9% 18% 30% 7%	2904
	11th to 20th	Night	37% 9% 15% 24% 15% 1	3156
	21st to end	Night	30% 8% 13% 22% 25% 3	3586
September	1st to 10th	Night	39% 9% 13% 18% 17% 5%	2165
	11th to 20th	Night	38% 8% 13% 21% 20% 2	2200
	21st to end	Night	35% 7% 12% 19% 19% 8%	2200
October	1st to 10th	Night	23% 5% 9% 15% 37% 11%	3590
	11th to 20th	Night	18% 2 5% 18% 33% 23%	3756
	21st to end	Night	25% 3 4 23% 29% 16%	4252
November	1st to 10th	Night	33% 4 7% 18% 27% 11%	2656
	11th to 20th	Night	33% 3 5% 18% 29% 11%	2730
	21st to end	Night	38% 4 6% 21% 25% 7%	2730
December	1st to 10th	Night	40% 3 6% 18% 27% 7%	4380
	11th to 20th	Night	43% 4 8% 16% 23% 5%	4954
	21st to end	Night	46% 3 7% 27% 15% 2	6633

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	33% 20% 35% 11% 2	330
	11th to 20th	Day	32% 24% 25% 15% 4	330
	21st to end	Day	31% 24% 34% 9% 3 1	363
February	1st to 10th	Day	26% 23% 34% 14% 3	330
	11th to 20th	Day	40% 24% 24% 11% 1	330
	21st to end	Day	34% 22% 32% 9% 2 1	273
March	1st to 10th	Day	31% 17% 31% 16% 5%	357
	11th to 20th	Day	26% 21% 27% 20% 6% 1	500
	21st to end	Day	23% 18% 35% 17% 6% 1	550
April	1st to 10th	Day	25% 14% 29% 22% 8% 2	500
	11th to 20th	Day	16% 17% 33% 21% 13% 1	492
	21st to end	Day	19% 10% 27% 26% 14% 3	500
May	1st to 10th	Day	18% 12% 28% 23% 13% 5%	503
	11th to 20th	Day	17% 8% 24% 28% 15% 9%	527
	21st to end	Day	15% 7% 18% 31% 19% 10%	573
June	1st to 10th	Day	12% 3 20% 28% 24% 12%	530
	11th to 20th	Day	7% 5% 16% 27% 27% 18%	530
	21st to end	Day	9% 4 15% 30% 26% 15%	530
July	1st to 10th	Day	11% 3 8% 28% 30% 22%	525
	11th to 20th	Day	8% 2 10% 20% 36% 24%	524
	21st to end	Day	7% 3 8% 26% 34% 22%	576
August	1st to 10th	Day	14% 4 14% 24% 25% 19%	526
	11th to 20th	Day	12% 5% 8% 29% 29% 17%	519
	21st to end	Day	14% 3 10% 25% 29% 19%	583
September	1st to 10th	Day	10% 6% 15% 30% 26% 13%	530
	11th to 20th	Day	15% 6% 13% 25% 29% 12%	530
	21st to end	Day	14% 6% 17% 29% 24% 10%	530
October	1st to 10th	Day	16% 4 22% 33% 18% 8%	530
	11th to 20th	Day	15% 11% 21% 29% 21% 4	530
	21st to end	Day	20% 10% 26% 26% 15% 3	583
November	1st to 10th	Day	19% 13% 29% 24% 13% 3	335
	11th to 20th	Day	19% 18% 35% 18% 9% 1	330
	21st to end	Day	19% 15% 37% 20% 8% 1	330
December	1st to 10th	Day	32% 22% 27% 14% 3 1	330
	11th to 20th	Day	26% 16% 34% 19% 4 1	330
	21st to end	Day	32% 16% 31% 16% 4 1	363
January	1st to 10th	Night	26% 17% 30% 22% 5%	840
	11th to 20th	Night	24% 17% 30% 23% 6%	840
	21st to end	Night	24% 16% 33% 25% 3	924
February	1st to 10th	Night	21% 15% 30% 29% 5%	840
	11th to 20th	Night	28% 17% 30% 22% 2	823
	21st to end	Night	25% 16% 33% 23% 3	692
March	1st to 10th	Night	21% 16% 30% 28% 5%	809
	11th to 20th	Night	19% 14% 33% 25% 8% 1	800
	21st to end	Night	21% 13% 29% 29% 7%	880
April	1st to 10th	Night	16% 14% 29% 29% 11%	748
	11th to 20th	Night	16% 10% 28% 32% 13% 1	593
	21st to end	Night	12% 8% 26% 35% 18% 1	600
May	1st to 10th	Night	11% 9% 23% 35% 22% 1	600
	11th to 20th	Night	16% 6% 20% 32% 23% 4	591
	21st to end	Night	13% 4 14% 33% 31% 6%	650
June	1st to 10th	Night	10% 3 11% 39% 33% 5%	600
	11th to 20th	Night	6% 2 14% 30% 36% 13%	600
	21st to end	Night	6% 2 9% 35% 39% 8%	600
July	1st to 10th	Night	10% 3 8% 22% 46% 12%	585
	11th to 20th	Night	7% 2 7% 22% 48% 14%	582
	21st to end	Night	8% 2 6% 26% 46% 11%	657
August	1st to 10th	Night	12% 3 6% 30% 39% 10%	597
	11th to 20th	Night	12% 3 9% 27% 40% 9%	594
	21st to end	Night	15% 3 10% 27% 36% 9%	660
September	1st to 10th	Night	10% 4 12% 31% 34% 9%	765
	11th to 20th	Night	16% 2 13% 30% 36% 4	800
	21st to end	Night	14% 3 16% 33% 31% 3	800
October	1st to 10th	Night	13% 6% 18% 34% 23% 6%	800
	11th to 20th	Night	12% 7% 24% 34% 20% 3	800
	21st to end	Night	17% 9% 20% 36% 16% 2	880
November	1st to 10th	Night	17% 7% 24% 34% 18% 2	820
	11th to 20th	Night	17% 11% 28% 30% 13% 1	840
	21st to end	Night	20% 13% 26% 28% 12% 1	840
December	1st to 10th	Night	23% 15% 28% 26% 7%	840
	11th to 20th	Night	24% 12% 28% 27% 8% 1	840
	21st to end	Night	29% 13% 26% 27% 6%	924

Page generated at 2026-02-28 01:40:21. Version ac07

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