Ambersweet Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ambersweet Observatory clear sky chart from 2005-11-06 to 2024-11-02.

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	8% 5% 2 7% 6% 6% 6% 5% 7% 9% 38%	100889
	Night	9% 6% 2 6% 5% 5% 5% 4 7% 9% 42%	98023
Transparency	Day	38% 11% 16% 18% 12% 4	100701
	Night	36% 10% 17% 19% 13% 5%	97880
Seeing	Day	9% 6% 19% 26% 24% 17%	15525
	Night	9% 5% 20% 33% 25% 7%	25584

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	12% 7% 2 7% 5% 4 4 4 5% 6% 44%	2836
	11th to 20th	Day	20% 9% 3 7% 4 4 4 4 5% 6% 35%	2840
	21st to end	Day	11% 6% 2 6% 5% 4 4 4 6% 7% 46%	3200
February	1st to 10th	Day	12% 5% 2 7% 5% 5% 4 4 6% 7% 43%	1900
	11th to 20th	Day	9% 4 2 5% 4 4 4 3 5% 7% 52%	1848
	21st to end	Day	9% 6% 2 6% 5% 4 4 3 5% 7% 49%	1663
March	1st to 10th	Day	7% 8% 3 8% 5% 5% 4 4 7% 10% 39%	3435
	11th to 20th	Day	6% 4 2 5% 4 4 4 4 7% 10% 50%	3480
	21st to end	Day	10% 5% 2 5% 4 4 4 4 6% 7% 49%	3815
April	1st to 10th	Day	7% 3 2 4 4 4 4 4 7% 10% 50%	2090
	11th to 20th	Day	7% 5% 2 6% 6% 5% 6% 5% 7% 9% 42%	2103
	21st to end	Day	5% 3 1 4 4 5% 5% 5% 7% 9% 53%	2280
May	1st to 10th	Day	3 2 1 4 3 4 4 4 6% 10% 58%	4048
	11th to 20th	Day	9% 5% 2 6% 6% 6% 7% 5% 8% 9% 37%	4081
	21st to end	Day	4 3 1 4 4 4 5% 5% 8% 9% 53%	4476
June	1st to 10th	Day	10% 7% 2 8% 7% 7% 8% 6% 8% 10% 26%	2470
	11th to 20th	Day	7% 5% 3 8% 7% 8% 8% 7% 8% 10% 28%	2470
	21st to end	Day	8% 7% 3 9% 8% 6% 7% 6% 7% 11% 29%	2470
July	1st to 10th	Day	7% 5% 3 9% 8% 8% 8% 7% 10% 14% 22%	4120
	11th to 20th	Day	5% 5% 2 9% 8% 8% 9% 6% 10% 13% 25%	4120
	21st to end	Day	5% 5% 3 9% 8% 7% 9% 8% 10% 13% 23%	4466
August	1st to 10th	Day	6% 7% 3 9% 6% 7% 7% 6% 9% 13% 27%	4104
	11th to 20th	Day	4 4 2 7% 5% 7% 8% 6% 10% 13% 35%	3892
	21st to end	Day	7% 6% 3 11% 7% 8% 8% 6% 9% 11% 24%	3828
September	1st to 10th	Day	8% 6% 3 11% 9% 7% 8% 7% 9% 10% 22%	2090
	11th to 20th	Day	10% 8% 3 11% 7% 8% 8% 5% 7% 11% 23%	2090
	21st to end	Day	10% 6% 2 8% 7% 6% 7% 5% 8% 8% 32%	2090
October	1st to 10th	Day	11% 6% 3 8% 6% 6% 7% 5% 6% 9% 33%	2067
	11th to 20th	Day	6% 5% 2 6% 4 5% 5% 4 6% 9% 49%	1900
	21st to end	Day	8% 5% 2 5% 4 4 5% 4 6% 8% 48%	2090
November	1st to 10th	Day	7% 5% 2 6% 4 4 5% 4 6% 8% 49%	1868
	11th to 20th	Day	9% 7% 2 7% 5% 6% 5% 4 5% 7% 43%	1900
	21st to end	Day	10% 7% 2 6% 4 5% 6% 3 5% 9% 43%	1772
December	1st to 10th	Day	7% 5% 2 6% 5% 5% 4 4 7% 8% 48%	1710
	11th to 20th	Day	12% 7% 2 6% 4 5% 4 4 6% 7% 43%	2153
	21st to end	Day	12% 7% 3 8% 6% 4 5% 4 5% 6% 40%	3124
January	1st to 10th	Night	13% 7% 3 7% 5% 4 4 3 5% 7% 42%	4109
	11th to 20th	Night	17% 10% 4 7% 4 3 5% 4 6% 7% 34%	4120
	21st to end	Night	11% 7% 3 7% 5% 4 4 4 7% 8% 41%	4439
February	1st to 10th	Night	12% 7% 3 5% 4 4 4 4 5% 7% 45%	2280
	11th to 20th	Night	10% 5% 2 5% 4 4 4 4 6% 7% 50%	2218
	21st to end	Night	8% 6% 2 6% 4 3 4 3 6% 7% 51%	1789
March	1st to 10th	Night	16% 10% 3 7% 4 4 4 3 4 5% 39%	3418
	11th to 20th	Night	5% 4 2 5% 4 5% 5% 5% 8% 10% 48%	3480
	21st to end	Night	11% 6% 2 6% 4 3 3 3 5% 8% 48%	3817
April	1st to 10th	Night	7% 4 2 4 3 3 4 4 6% 9% 53%	2090
	11th to 20th	Night	7% 5% 2 6% 4 3 4 3 6% 8% 52%	2022
	21st to end	Night	6% 4 1 4 3 4 5% 3 5% 8% 56%	1900
May	1st to 10th	Night	4 3 1 4 4 4 4 3 6% 9% 57%	2861
	11th to 20th	Night	7% 5% 2 5% 4 4 5% 4 7% 10% 46%	2812
	21st to end	Night	4 3 1 4 3 3 4 4 6% 8% 61%	3079
June	1st to 10th	Night	10% 5% 2 7% 5% 5% 6% 5% 8% 12% 35%	1710
	11th to 20th	Night	5% 7% 2 7% 6% 5% 7% 5% 9% 11% 34%	1710
	21st to end	Night	7% 8% 3 8% 7% 6% 6% 5% 9% 10% 31%	1710
July	1st to 10th	Night	7% 6% 3 10% 7% 7% 6% 6% 9% 10% 28%	2840
	11th to 20th	Night	3 6% 3 8% 9% 8% 8% 7% 11% 11% 27%	2840
	21st to end	Night	6% 9% 4 11% 8% 7% 7% 6% 9% 11% 22%	3087
August	1st to 10th	Night	5% 5% 2 8% 7% 6% 7% 5% 8% 12% 35%	2825
	11th to 20th	Night	3 4 2 7% 5% 5% 7% 6% 8% 14% 39%	2900
	21st to end	Night	7% 6% 3 8% 5% 6% 6% 6% 9% 12% 32%	3828
September	1st to 10th	Night	9% 7% 3 8% 6% 6% 6% 6% 8% 11% 31%	2090
	11th to 20th	Night	10% 8% 3 8% 6% 6% 6% 5% 8% 10% 29%	2090
	21st to end	Night	10% 6% 2 7% 5% 6% 7% 6% 6% 8% 37%	2090
October	1st to 10th	Night	11% 7% 3 7% 4 5% 5% 5% 6% 8% 39%	2110
	11th to 20th	Night	5% 5% 1 5% 4 4 4 4 6% 9% 53%	2280
	21st to end	Night	9% 4 2 5% 4 4 4 3 6% 8% 52%	2508
November	1st to 10th	Night	9% 5% 2 5% 4 3 5% 5% 5% 7% 50%	2244
	11th to 20th	Night	9% 7% 3 6% 4 4 5% 4 5% 6% 47%	2280
	21st to end	Night	9% 6% 3 6% 5% 5% 5% 4 5% 8% 45%	2403
December	1st to 10th	Night	8% 5% 2 6% 4 4 5% 3 7% 9% 48%	2470
	11th to 20th	Night	11% 8% 3 5% 4 3 3 2 4 6% 50%	3042
	21st to end	Night	14% 8% 2 7% 5% 5% 4 4 5% 7% 38%	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	40% 6% 8% 17% 20% 9%	2837
	11th to 20th	Day	50% 5% 6% 15% 15% 9%	2840
	21st to end	Day	37% 7% 9% 18% 20% 9%	3200
February	1st to 10th	Day	39% 8% 9% 14% 20% 10%	1900
	11th to 20th	Day	31% 6% 10% 18% 24% 12%	1858
	21st to end	Day	35% 5% 12% 16% 22% 10%	1663
March	1st to 10th	Day	38% 11% 14% 18% 14% 5%	3435
	11th to 20th	Day	28% 7% 13% 25% 21% 6%	3480
	21st to end	Day	32% 7% 11% 26% 15% 8%	3815
April	1st to 10th	Day	28% 9% 14% 22% 19% 9%	2090
	11th to 20th	Day	36% 8% 11% 20% 19% 6%	2103
	21st to end	Day	25% 7% 14% 22% 22% 8%	2280
May	1st to 10th	Day	18% 11% 18% 30% 18% 5%	4085
	11th to 20th	Day	37% 12% 17% 22% 10% 2	4085
	21st to end	Day	24% 10% 16% 33% 17% 1	4489
June	1st to 10th	Day	49% 13% 17% 15% 6%	2470
	11th to 20th	Day	46% 15% 19% 18% 2	2470
	21st to end	Day	49% 14% 21% 13% 3	2470
July	1st to 10th	Day	46% 17% 26% 10% 1	4055
	11th to 20th	Day	44% 18% 31% 7%	4051
	21st to end	Day	43% 19% 28% 9% 1	4324
August	1st to 10th	Day	43% 15% 28% 12% 2	4091
	11th to 20th	Day	35% 16% 24% 19% 5%	3931
	21st to end	Day	48% 15% 21% 15% 1	3828
September	1st to 10th	Day	51% 18% 18% 10% 4	2090
	11th to 20th	Day	51% 16% 19% 11% 3	2090
	21st to end	Day	46% 10% 14% 16% 10% 3	2090
October	1st to 10th	Day	46% 10% 10% 13% 12% 8%	2067
	11th to 20th	Day	32% 7% 12% 21% 23% 6%	1900
	21st to end	Day	31% 8% 10% 17% 23% 11%	2090
November	1st to 10th	Day	33% 8% 12% 19% 18% 10%	1868
	11th to 20th	Day	39% 7% 10% 16% 20% 8%	1900
	21st to end	Day	38% 7% 10% 20% 20% 6%	1772
December	1st to 10th	Day	32% 6% 15% 24% 17% 6%	1710
	11th to 20th	Day	39% 8% 10% 15% 17% 10%	2150
	21st to end	Day	44% 7% 10% 19% 14% 6%	3124
January	1st to 10th	Night	40% 6% 10% 15% 21% 8%	4109
	11th to 20th	Night	48% 6% 8% 18% 14% 5%	4120
	21st to end	Night	38% 9% 11% 15% 16% 10%	4439
February	1st to 10th	Night	37% 6% 11% 16% 17% 13%	2280
	11th to 20th	Night	32% 7% 12% 17% 20% 11%	2219
	21st to end	Night	31% 6% 14% 18% 22% 9%	1789
March	1st to 10th	Night	44% 9% 12% 16% 14% 5%	3418
	11th to 20th	Night	26% 9% 20% 25% 14% 5%	3480
	21st to end	Night	33% 7% 11% 22% 20% 6%	3817
April	1st to 10th	Night	26% 9% 18% 23% 16% 9%	2090
	11th to 20th	Night	30% 8% 12% 25% 18% 7%	2022
	21st to end	Night	24% 8% 14% 25% 21% 7%	1900
May	1st to 10th	Night	18% 15% 20% 29% 14% 5%	2888
	11th to 20th	Night	30% 12% 23% 25% 10% 1	2813
	21st to end	Night	19% 10% 16% 35% 19% 1	3087
June	1st to 10th	Night	39% 13% 21% 22% 5%	1710
	11th to 20th	Night	39% 16% 23% 21% 2	1710
	21st to end	Night	43% 14% 23% 14% 5% 1	1710
July	1st to 10th	Night	44% 16% 28% 10% 1	2795
	11th to 20th	Night	44% 19% 30% 7% 1	2786
	21st to end	Night	50% 17% 24% 8% 1	2988
August	1st to 10th	Night	37% 16% 34% 11% 2	2814
	11th to 20th	Night	31% 15% 28% 21% 5%	2927
	21st to end	Night	41% 16% 27% 14% 2	3828
September	1st to 10th	Night	42% 18% 21% 14% 5%	2090
	11th to 20th	Night	46% 14% 21% 15% 3	2090
	21st to end	Night	41% 11% 18% 18% 10% 2	2090
October	1st to 10th	Night	41% 10% 14% 17% 12% 7%	2110
	11th to 20th	Night	27% 7% 12% 25% 23% 5%	2280
	21st to end	Night	29% 8% 12% 18% 24% 10%	2508
November	1st to 10th	Night	31% 8% 13% 21% 17% 10%	2244
	11th to 20th	Night	35% 8% 11% 18% 19% 9%	2280
	21st to end	Night	36% 6% 11% 21% 19% 7%	2403
December	1st to 10th	Night	32% 6% 16% 27% 13% 6%	2470
	11th to 20th	Night	37% 6% 10% 19% 19% 10%	3044
	21st to end	Night	43% 8% 12% 18% 14% 4	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	11% 11% 43% 25% 7% 2	270
	11th to 20th	Day	8% 14% 38% 25% 11% 3	270
	21st to end	Day	12% 20% 43% 20% 5%	297
February	1st to 10th	Day	13% 14% 36% 21% 14% 2	270
	11th to 20th	Day	11% 8% 40% 26% 11% 3	270
	21st to end	Day	11% 11% 29% 31% 17% 1	318
March	1st to 10th	Day	6% 15% 32% 32% 13% 2	460
	11th to 20th	Day	11% 10% 28% 32% 18% 3	460
	21st to end	Day	8% 11% 29% 29% 20% 4	506
April	1st to 10th	Day	9% 12% 28% 32% 16% 3	460
	11th to 20th	Day	9% 10% 28% 31% 18% 4	452
	21st to end	Day	6% 5% 23% 33% 22% 11%	460
May	1st to 10th	Day	5% 5% 19% 32% 27% 12%	463
	11th to 20th	Day	8% 3 18% 28% 29% 15%	487
	21st to end	Day	6% 2 11% 23% 36% 22%	529
June	1st to 10th	Day	12% 1 11% 24% 31% 21%	490
	11th to 20th	Day	8% 2 10% 23% 28% 29%	490
	21st to end	Day	6% 2 6% 17% 33% 38%	490
July	1st to 10th	Day	7% 1 4 13% 27% 48%	485
	11th to 20th	Day	5% 1 3 12% 32% 46%	484
	21st to end	Day	8% 2 5% 15% 29% 41%	532
August	1st to 10th	Day	6% 4 7% 16% 31% 36%	486
	11th to 20th	Day	6% 1 3 14% 29% 46%	479
	21st to end	Day	9% 3 6% 20% 30% 34%	539
September	1st to 10th	Day	9% 2 5% 17% 34% 32%	490
	11th to 20th	Day	9% 2 8% 26% 33% 23%	490
	21st to end	Day	9% 3 10% 22% 30% 25%	490
October	1st to 10th	Day	14% 5% 19% 31% 24% 8%	490
	11th to 20th	Day	6% 6% 17% 33% 28% 10%	490
	21st to end	Day	8% 5% 22% 36% 24% 4	539
November	1st to 10th	Day	10% 6% 24% 30% 25% 5%	460
	11th to 20th	Day	15% 7% 25% 33% 16% 4	460
	21st to end	Day	11% 9% 30% 31% 17% 2	332
December	1st to 10th	Day	6% 10% 35% 29% 17% 4	270
	11th to 20th	Day	13% 11% 35% 28% 12% 1	270
	21st to end	Day	8% 10% 27% 38% 13% 3	297
January	1st to 10th	Night	12% 10% 34% 31% 12% 1	760
	11th to 20th	Night	11% 9% 28% 36% 14% 1	760
	21st to end	Night	14% 12% 36% 32% 7%	836
February	1st to 10th	Night	15% 9% 30% 35% 12%	760
	11th to 20th	Night	11% 8% 32% 37% 10% 1	743
	21st to end	Night	10% 8% 34% 41% 7%	628
March	1st to 10th	Night	7% 8% 35% 38% 11%	760
	11th to 20th	Night	10% 8% 31% 36% 13% 1	760
	21st to end	Night	8% 8% 31% 39% 13% 1	836
April	1st to 10th	Night	8% 7% 32% 39% 14%	760
	11th to 20th	Night	8% 8% 31% 37% 15% 1	751
	21st to end	Night	7% 4 25% 43% 20% 1	760
May	1st to 10th	Night	4 2 19% 48% 25% 1	605
	11th to 20th	Night	6% 2 14% 44% 30% 4	561
	21st to end	Night	7% 1 10% 35% 41% 6%	617
June	1st to 10th	Night	10% 2 9% 36% 36% 7%	570
	11th to 20th	Night	6% 2 8% 25% 38% 21%	570
	21st to end	Night	8% 2 4 24% 42% 20%	570
July	1st to 10th	Night	8% 1 4 18% 43% 26%	555
	11th to 20th	Night	5% 1 3 14% 47% 31%	552
	21st to end	Night	6% 1 3 16% 50% 24%	624
August	1st to 10th	Night	6% 1 4 16% 49% 24%	567
	11th to 20th	Night	5% 4 14% 44% 32%	583
	21st to end	Night	11% 2 7% 22% 38% 20%	836
September	1st to 10th	Night	11% 2 6% 22% 39% 19%	760
	11th to 20th	Night	12% 4 10% 27% 35% 12%	760
	21st to end	Night	13% 3 8% 28% 39% 9%	760
October	1st to 10th	Night	13% 2 17% 39% 25% 5%	760
	11th to 20th	Night	6% 3 15% 38% 33% 4	760
	21st to end	Night	11% 4 16% 41% 26% 2	836
November	1st to 10th	Night	10% 3 25% 34% 25% 2	748
	11th to 20th	Night	10% 7% 24% 39% 18% 2	760
	21st to end	Night	11% 4 29% 40% 15% 1	760
December	1st to 10th	Night	9% 5% 27% 36% 22% 1	760
	11th to 20th	Night	12% 7% 27% 36% 18% 1	760
	21st to end	Night	12% 7% 25% 39% 16% 1	836

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