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 2024-12-02.

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	14% 6% 2 6% 4 5% 5% 4 6% 7% 40%	101216
	Night	15% 7% 2 6% 4 4 4 4 5% 7% 43%	98373
Transparency	Day	39% 7% 11% 19% 18% 5%	105047
	Night	38% 6% 10% 19% 20% 6%	102680
Seeing	Day	18% 10% 22% 23% 17% 9%	14946
	Night	17% 9% 22% 29% 20% 3	24981

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	20% 12% 2 5% 4 4 3 3 4 5% 37%	2836
	11th to 20th	Day	16% 7% 2 5% 4 4 3 3 4 4 48%	2840
	21st to end	Day	28% 9% 2 4 2 3 3 2 4 4 39%	3234
February	1st to 10th	Day	18% 9% 3 6% 3 4 4 3 5% 6% 38%	1900
	11th to 20th	Day	25% 7% 1 5% 3 2 4 3 5% 5% 40%	1848
	21st to end	Day	24% 7% 2 6% 4 4 3 3 3 5% 39%	1570
March	1st to 10th	Day	27% 7% 3 7% 3 3 4 3 5% 5% 34%	3207
	11th to 20th	Day	15% 6% 2 5% 4 4 5% 3 4 6% 47%	3480
	21st to end	Day	18% 6% 2 7% 5% 4 4 3 5% 6% 40%	3815
April	1st to 10th	Day	13% 6% 2 5% 4 4 5% 4 6% 6% 44%	2154
	11th to 20th	Day	13% 6% 3 6% 4 4 4 3 6% 6% 45%	2254
	21st to end	Day	12% 4 2 5% 4 4 5% 3 6% 8% 49%	2432
May	1st to 10th	Day	13% 6% 2 7% 4 4 5% 3 5% 9% 41%	4082
	11th to 20th	Day	19% 7% 2 6% 4 5% 5% 4 6% 7% 35%	4081
	21st to end	Day	9% 5% 2 7% 5% 4 5% 4 7% 9% 42%	4476
June	1st to 10th	Day	10% 6% 3 7% 6% 6% 5% 5% 7% 10% 34%	2470
	11th to 20th	Day	6% 4 1 5% 5% 5% 6% 6% 9% 12% 41%	2470
	21st to end	Day	8% 4 2 6% 5% 6% 7% 6% 8% 9% 38%	2470
July	1st to 10th	Day	8% 6% 2 7% 7% 7% 7% 7% 9% 12% 28%	4120
	11th to 20th	Day	9% 5% 2 7% 7% 7% 7% 5% 7% 11% 33%	4120
	21st to end	Day	13% 9% 4 10% 8% 8% 8% 6% 7% 8% 19%	4466
August	1st to 10th	Day	6% 5% 2 6% 5% 5% 5% 6% 8% 11% 39%	4104
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 4 7% 11% 42%	3794
	21st to end	Day	8% 4 2 6% 5% 4 5% 4 6% 7% 49%	4180
September	1st to 10th	Day	11% 6% 3 7% 5% 5% 4 4 7% 8% 40%	2104
	11th to 20th	Day	15% 7% 2 6% 4 4 4 4 6% 8% 40%	2090
	21st to end	Day	13% 6% 2 5% 4 4 4 4 4 7% 46%	2090
October	1st to 10th	Day	12% 4 2 4 3 4 3 3 3 5% 56%	2090
	11th to 20th	Day	13% 5% 2 3 2 3 3 3 4 6% 56%	1986
	21st to end	Day	15% 4 2 5% 3 3 3 2 4 4 55%	2090
November	1st to 10th	Day	13% 6% 3 4 3 3 3 2 3 4 55%	1767
	11th to 20th	Day	15% 4 2 4 3 3 4 3 5% 6% 52%	1800
	21st to end	Day	15% 6% 2 5% 3 4 3 4 5% 6% 46%	1800
December	1st to 10th	Day	23% 7% 2 5% 3 4 3 4 6% 6% 38%	1719
	11th to 20th	Day	19% 6% 2 7% 5% 3 4 2 4 5% 43%	2153
	21st to end	Day	23% 8% 4 5% 3 4 4 4 6% 7% 34%	3124
January	1st to 10th	Night	19% 9% 2 5% 3 3 4 3 5% 7% 40%	4109
	11th to 20th	Night	15% 9% 3 6% 4 3 3 3 4 5% 44%	4120
	21st to end	Night	27% 12% 2 4 3 2 2 2 3 4 39%	4404
February	1st to 10th	Night	17% 7% 2 6% 4 4 3 4 5% 6% 42%	2280
	11th to 20th	Night	22% 7% 2 5% 5% 3 3 3 3 4 42%	2218
	21st to end	Night	23% 6% 2 6% 4 3 4 3 5% 5% 38%	1884
March	1st to 10th	Night	25% 8% 3 6% 5% 4 3 3 5% 5% 35%	3644
	11th to 20th	Night	12% 5% 2 4 3 4 4 3 6% 8% 49%	3480
	21st to end	Night	17% 6% 3 6% 5% 4 4 3 5% 6% 42%	3817
April	1st to 10th	Night	12% 5% 1 5% 4 4 4 5% 6% 7% 48%	2041
	11th to 20th	Night	13% 7% 2 4 3 3 4 4 5% 6% 49%	1875
	21st to end	Night	9% 5% 2 5% 4 3 4 3 6% 7% 50%	1746
May	1st to 10th	Night	12% 6% 3 7% 5% 4 5% 4 5% 8% 41%	2813
	11th to 20th	Night	18% 7% 2 5% 6% 4 4 4 6% 8% 37%	2812
	21st to end	Night	7% 4 2 6% 4 5% 4 4 8% 11% 45%	3079
June	1st to 10th	Night	9% 5% 2 7% 6% 6% 6% 5% 8% 10% 35%	1710
	11th to 20th	Night	4 4 2 6% 5% 4 4 6% 7% 12% 45%	1710
	21st to end	Night	7% 5% 2 5% 5% 4 6% 6% 8% 11% 43%	1710
July	1st to 10th	Night	11% 7% 3 9% 6% 5% 6% 5% 7% 10% 31%	2840
	11th to 20th	Night	7% 4 3 7% 5% 6% 6% 5% 10% 12% 35%	2840
	21st to end	Night	14% 9% 4 10% 8% 6% 6% 6% 7% 10% 23%	3087
August	1st to 10th	Night	7% 5% 2 7% 5% 5% 5% 5% 8% 12% 40%	2825
	11th to 20th	Night	10% 7% 3 7% 5% 4 4 3 5% 8% 46%	3030
	21st to end	Night	8% 5% 1 5% 4 4 4 3 6% 8% 52%	3476
September	1st to 10th	Night	9% 6% 3 6% 5% 5% 5% 4 7% 8% 41%	2057
	11th to 20th	Night	14% 6% 2 6% 4 4 4 4 6% 7% 43%	2090
	21st to end	Night	13% 5% 2 5% 3 4 5% 4 5% 6% 49%	2090
October	1st to 10th	Night	12% 4 1 5% 2 3 3 3 4 6% 57%	2090
	11th to 20th	Night	11% 5% 2 4 4 3 3 2 4 5% 58%	2193
	21st to end	Night	13% 5% 1 4 2 3 2 3 4 5% 56%	2508
November	1st to 10th	Night	14% 5% 2 4 3 3 3 2 4 6% 55%	2526
	11th to 20th	Night	15% 5% 2 4 3 3 3 3 4 6% 53%	2600
	21st to end	Night	17% 6% 2 4 4 3 3 4 5% 6% 46%	2600
December	1st to 10th	Night	20% 8% 2 5% 5% 4 4 3 5% 6% 39%	2495
	11th to 20th	Night	18% 8% 2 5% 3 3 3 2 5% 5% 46%	3042
	21st to end	Night	22% 9% 3 6% 4 4 4 3 5% 6% 34%	4532

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	49% 4 6% 15% 22% 5%	2837
	11th to 20th	Day	40% 4 7% 21% 23% 6%	2840
	21st to end	Day	49% 3 5% 16% 22% 5%	3234
February	1st to 10th	Day	46% 4 7% 18% 20% 4	1900
	11th to 20th	Day	45% 4 6% 19% 19% 6%	1858
	21st to end	Day	49% 4 7% 13% 20% 7%	1570
March	1st to 10th	Day	52% 4 10% 12% 13% 9%	3207
	11th to 20th	Day	38% 5% 7% 18% 24% 9%	3480
	21st to end	Day	45% 4 8% 20% 19% 3	3815
April	1st to 10th	Day	39% 5% 10% 21% 20% 5%	2154
	11th to 20th	Day	39% 4 9% 17% 24% 8%	2254
	21st to end	Day	33% 6% 11% 27% 20% 3	2432
May	1st to 10th	Day	39% 8% 13% 21% 18% 2	4120
	11th to 20th	Day	45% 9% 11% 23% 11% 1	4085
	21st to end	Day	34% 9% 14% 22% 20% 1	4489
June	1st to 10th	Day	42% 9% 16% 20% 10% 2	2470
	11th to 20th	Day	31% 10% 19% 27% 12% 1	2470
	21st to end	Day	36% 11% 17% 21% 12% 3	2470
July	1st to 10th	Day	41% 15% 20% 18% 5% 1	4055
	11th to 20th	Day	41% 12% 21% 18% 6% 1	4051
	21st to end	Day	59% 11% 13% 12% 5%	4324
August	1st to 10th	Day	35% 9% 15% 32% 9%	4091
	11th to 20th	Day	33% 9% 18% 24% 14% 2	3832
	21st to end	Day	31% 9% 12% 24% 20% 3	4179
September	1st to 10th	Day	39% 10% 13% 16% 18% 4	2104
	11th to 20th	Day	41% 8% 10% 19% 20% 2	2090
	21st to end	Day	37% 5% 12% 17% 22% 7%	2090
October	1st to 10th	Day	24% 4 6% 14% 38% 13%	3480
	11th to 20th	Day	21% 2 5% 15% 29% 28%	3278
	21st to end	Day	28% 4 6% 22% 28% 13%	3428
November	1st to 10th	Day	34% 2 7% 16% 31% 9%	1767
	11th to 20th	Day	34% 3 7% 17% 28% 10%	1800
	21st to end	Day	38% 4 7% 18% 26% 7%	1800
December	1st to 10th	Day	47% 5% 6% 20% 17% 6%	1719
	11th to 20th	Day	45% 3 6% 16% 23% 7%	2150
	21st to end	Day	48% 4 10% 18% 18% 3	3124
January	1st to 10th	Night	43% 4 7% 16% 26% 5%	4109
	11th to 20th	Night	42% 4 6% 22% 18% 8%	4120
	21st to end	Night	52% 2 5% 13% 23% 5%	4404
February	1st to 10th	Night	42% 4 8% 18% 21% 7%	2280
	11th to 20th	Night	47% 3 5% 18% 20% 7%	2219
	21st to end	Night	47% 4 8% 15% 18% 8%	1884
March	1st to 10th	Night	51% 4 10% 14% 14% 6%	3644
	11th to 20th	Night	32% 4 10% 17% 26% 10%	3480
	21st to end	Night	43% 4 7% 19% 20% 7%	3817
April	1st to 10th	Night	35% 5% 10% 19% 25% 6%	2041
	11th to 20th	Night	35% 4 8% 20% 22% 10%	1875
	21st to end	Night	32% 5% 10% 28% 20% 5%	1746
May	1st to 10th	Night	38% 10% 14% 21% 16% 2	2840
	11th to 20th	Night	42% 8% 14% 23% 12% 1	2813
	21st to end	Night	29% 10% 15% 26% 18% 2	3087
June	1st to 10th	Night	38% 12% 16% 20% 12% 2	1710
	11th to 20th	Night	29% 10% 21% 25% 14% 1	1710
	21st to end	Night	33% 10% 19% 20% 14% 4	1710
July	1st to 10th	Night	46% 11% 18% 18% 6% 1	2795
	11th to 20th	Night	36% 14% 21% 21% 6% 2	2786
	21st to end	Night	55% 12% 15% 13% 5% 1	2988
August	1st to 10th	Night	34% 9% 18% 31% 7%	2814
	11th to 20th	Night	36% 8% 15% 25% 15% 1	3058
	21st to end	Night	30% 8% 13% 22% 25% 3	3476
September	1st to 10th	Night	39% 8% 13% 18% 17% 5%	2057
	11th to 20th	Night	39% 7% 12% 19% 20% 2	2090
	21st to end	Night	35% 7% 12% 19% 20% 8%	2090
October	1st to 10th	Night	23% 5% 9% 15% 37% 12%	3480
	11th to 20th	Night	18% 2 5% 17% 33% 23%	3640
	21st to end	Night	24% 3 4 24% 29% 17%	4120
November	1st to 10th	Night	33% 4 7% 18% 26% 12%	2526
	11th to 20th	Night	34% 3 5% 18% 29% 11%	2600
	21st to end	Night	38% 4 6% 20% 25% 6%	2600
December	1st to 10th	Night	46% 4 8% 19% 17% 5%	2495
	11th to 20th	Night	40% 4 7% 16% 27% 6%	3044
	21st to end	Night	50% 3 8% 20% 16% 2	4532

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	30% 21% 36% 11% 2	270
	11th to 20th	Day	33% 20% 25% 17% 5%	270
	21st to end	Day	28% 25% 35% 9% 3 1	297
February	1st to 10th	Day	30% 22% 32% 13% 3	270
	11th to 20th	Day	40% 24% 24% 10% 1	270
	21st to end	Day	36% 23% 31% 8% 1	225
March	1st to 10th	Day	30% 15% 33% 16% 6%	318
	11th to 20th	Day	25% 21% 27% 20% 7% 1	460
	21st to end	Day	23% 17% 34% 18% 6% 1	506
April	1st to 10th	Day	25% 14% 28% 22% 9% 2	460
	11th to 20th	Day	16% 17% 31% 22% 13% 1	452
	21st to end	Day	19% 10% 28% 26% 14% 3	460
May	1st to 10th	Day	18% 11% 29% 23% 14% 5%	463
	11th to 20th	Day	16% 7% 23% 29% 16% 10%	487
	21st to end	Day	13% 6% 17% 33% 21% 11%	529
June	1st to 10th	Day	12% 3 19% 29% 24% 12%	490
	11th to 20th	Day	7% 5% 16% 26% 28% 19%	490
	21st to end	Day	10% 4 17% 28% 26% 15%	490
July	1st to 10th	Day	11% 3 7% 27% 29% 22%	485
	11th to 20th	Day	9% 2 10% 20% 35% 24%	484
	21st to end	Day	8% 3 8% 25% 32% 23%	532
August	1st to 10th	Day	12% 4 14% 24% 26% 21%	486
	11th to 20th	Day	12% 5% 8% 30% 29% 16%	479
	21st to end	Day	14% 3 9% 25% 30% 19%	539
September	1st to 10th	Day	10% 7% 14% 29% 26% 14%	490
	11th to 20th	Day	16% 7% 13% 24% 28% 11%	490
	21st to end	Day	14% 6% 17% 29% 24% 11%	490
October	1st to 10th	Day	16% 4 21% 33% 19% 8%	490
	11th to 20th	Day	15% 11% 22% 28% 19% 4	490
	21st to end	Day	19% 9% 25% 27% 17% 3	539
November	1st to 10th	Day	19% 13% 28% 24% 12% 4	305
	11th to 20th	Day	21% 18% 34% 17% 9% 1	300
	21st to end	Day	18% 15% 37% 20% 9% 1	300
December	1st to 10th	Day	32% 21% 27% 16% 3 1	273
	11th to 20th	Day	26% 16% 37% 17% 3 1	270
	21st to end	Day	34% 17% 28% 16% 5%	297
January	1st to 10th	Night	26% 16% 31% 21% 5%	760
	11th to 20th	Night	24% 17% 30% 23% 7%	760
	21st to end	Night	24% 16% 32% 25% 3	836
February	1st to 10th	Night	22% 14% 30% 29% 5%	760
	11th to 20th	Night	27% 17% 31% 23% 2	743
	21st to end	Night	25% 16% 34% 22% 3	628
March	1st to 10th	Night	21% 15% 31% 29% 5%	760
	11th to 20th	Night	19% 14% 33% 25% 9% 1	760
	21st to end	Night	21% 13% 29% 29% 8%	836
April	1st to 10th	Night	16% 14% 28% 29% 12%	711
	11th to 20th	Night	17% 10% 28% 31% 13% 1	563
	21st to end	Night	12% 8% 26% 35% 18% 1	570
May	1st to 10th	Night	10% 9% 23% 34% 23% 1	570
	11th to 20th	Night	16% 5% 19% 33% 24% 4	561
	21st to end	Night	12% 3 13% 34% 32% 6%	617
June	1st to 10th	Night	10% 2 10% 39% 34% 5%	570
	11th to 20th	Night	5% 2 14% 30% 35% 13%	570
	21st to end	Night	6% 2 9% 36% 38% 9%	570
July	1st to 10th	Night	10% 3 8% 21% 45% 12%	555
	11th to 20th	Night	7% 2 7% 22% 48% 14%	552
	21st to end	Night	8% 2 6% 26% 45% 12%	624
August	1st to 10th	Night	10% 2 6% 29% 41% 11%	567
	11th to 20th	Night	12% 3 9% 28% 40% 9%	564
	21st to end	Night	15% 2 9% 26% 37% 9%	627
September	1st to 10th	Night	10% 4 12% 29% 35% 10%	727
	11th to 20th	Night	16% 2 14% 29% 34% 4	760
	21st to end	Night	14% 3 17% 32% 32% 3	760
October	1st to 10th	Night	13% 5% 18% 34% 24% 6%	760
	11th to 20th	Night	13% 7% 24% 34% 19% 3	760
	21st to end	Night	16% 8% 20% 36% 17% 2	836
November	1st to 10th	Night	16% 7% 24% 33% 19% 2	780
	11th to 20th	Night	17% 12% 27% 30% 13% 1	800
	21st to end	Night	20% 13% 26% 28% 13% 1	800
December	1st to 10th	Night	23% 16% 27% 26% 8%	768
	11th to 20th	Night	24% 12% 29% 27% 8% 1	760
	21st to end	Night	29% 13% 25% 26% 6%	836

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