Prevost
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Prevost clear sky chart from 2005-11-06 to 2026-03-18.

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	16% 10% 4 9% 5% 5% 5% 4 5% 6% 32%	131288
	Night	15% 11% 4 9% 5% 4 4 3 5% 6% 33%	130234
Transparency	Day	51% 5% 12% 20% 10% 1	134733
	Night	52% 5% 11% 20% 11% 1	134876
Seeing	Day	24% 16% 32% 19% 7% 2	16138
	Night	22% 13% 32% 25% 8% 1	25253

By Month

Percentage Cloud Cover NE (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	11% 18% 7% 14% 5% 5% 4 3 4 4 24%	4661
	11th to 20th	Day	10% 17% 8% 17% 7% 4 4 3 3 4 22%	4650
	21st to end	Day	7% 13% 7% 14% 8% 6% 6% 4 5% 5% 25%	5124
February	1st to 10th	Day	12% 16% 4 11% 7% 5% 4 4 4 5% 28%	1852
	11th to 20th	Day	10% 17% 5% 12% 6% 5% 5% 3 6% 5% 27%	1927
	21st to end	Day	15% 15% 5% 11% 6% 5% 4 3 4 4 27%	1730
March	1st to 10th	Day	18% 16% 6% 11% 6% 4 3 3 4 4 25%	4576
	11th to 20th	Day	10% 14% 4 11% 5% 4 4 3 5% 7% 32%	4812
	21st to end	Day	18% 12% 3 8% 4 4 4 3 4 5% 34%	5941
April	1st to 10th	Day	20% 13% 4 8% 5% 4 3 3 4 5% 32%	2400
	11th to 20th	Day	21% 9% 3 7% 5% 3 4 3 5% 6% 35%	2374
	21st to end	Day	21% 10% 3 8% 5% 4 4 3 5% 5% 31%	2552
May	1st to 10th	Day	26% 10% 3 8% 6% 5% 4 3 4 5% 24%	6341
	11th to 20th	Day	21% 8% 3 8% 5% 4 4 3 6% 8% 30%	6318
	21st to end	Day	23% 8% 2 6% 5% 5% 5% 4 5% 6% 30%	6949
June	1st to 10th	Day	21% 6% 3 8% 6% 5% 5% 4 6% 7% 29%	2800
	11th to 20th	Day	15% 7% 2 7% 6% 5% 5% 4 7% 7% 35%	2800
	21st to end	Day	21% 9% 2 8% 5% 6% 5% 4 6% 7% 28%	2800
July	1st to 10th	Day	15% 6% 3 8% 6% 6% 6% 5% 6% 8% 31%	6360
	11th to 20th	Day	13% 5% 2 5% 4 5% 6% 5% 7% 8% 39%	6360
	21st to end	Day	10% 5% 2 5% 4 4 5% 4 6% 8% 46%	6926
August	1st to 10th	Day	14% 5% 2 7% 5% 6% 6% 5% 7% 8% 33%	4563
	11th to 20th	Day	14% 6% 3 7% 5% 5% 5% 5% 6% 8% 36%	4150
	21st to end	Day	16% 5% 2 7% 4 4 5% 4 6% 7% 39%	4312
September	1st to 10th	Day	14% 6% 1 5% 4 4 4 5% 7% 8% 42%	2400
	11th to 20th	Day	13% 5% 2 6% 3 4 4 3 5% 6% 49%	2330
	21st to end	Day	18% 6% 2 6% 3 4 4 3 5% 7% 41%	2200
October	1st to 10th	Day	20% 7% 3 6% 4 4 4 3 4 8% 38%	2040
	11th to 20th	Day	20% 10% 3 8% 5% 4 4 5% 5% 6% 30%	1966
	21st to end	Day	24% 11% 3 7% 5% 4 4 3 5% 6% 29%	1980
November	1st to 10th	Day	14% 11% 3 8% 5% 4 5% 4 6% 7% 33%	1851
	11th to 20th	Day	17% 12% 4 9% 5% 4 4 4 6% 6% 29%	1769
	21st to end	Day	21% 13% 4 9% 4 5% 5% 3 5% 6% 24%	1680
December	1st to 10th	Day	18% 17% 8% 12% 5% 4 4 3 4 4 20%	2680
	11th to 20th	Day	17% 16% 6% 10% 5% 3 4 3 4 4 28%	3066
	21st to end	Day	20% 16% 5% 9% 4 4 4 3 4 4 26%	4048
January	1st to 10th	Night	11% 18% 7% 13% 6% 4 4 3 5% 5% 24%	8234
	11th to 20th	Night	9% 16% 8% 18% 8% 5% 4 3 4 5% 21%	8235
	21st to end	Night	8% 12% 5% 14% 8% 6% 5% 4 5% 6% 28%	9057
February	1st to 10th	Night	11% 13% 6% 12% 7% 6% 5% 4 5% 6% 28%	2767
	11th to 20th	Night	11% 13% 4 11% 6% 4 5% 4 5% 6% 31%	2578
	21st to end	Night	13% 13% 5% 9% 7% 5% 5% 3 5% 5% 31%	2076
March	1st to 10th	Night	19% 15% 5% 10% 5% 4 4 2 4 4 29%	5484
	11th to 20th	Night	11% 15% 4 8% 4 4 5% 4 5% 8% 32%	5297
	21st to end	Night	15% 10% 4 9% 6% 5% 4 4 5% 5% 34%	4990
April	1st to 10th	Night	22% 11% 3 7% 5% 4 3 2 5% 6% 32%	2000
	11th to 20th	Night	19% 7% 3 7% 5% 4 4 4 4 6% 37%	1975
	21st to end	Night	21% 8% 3 7% 4 4 4 3 5% 5% 36%	1847
May	1st to 10th	Night	24% 9% 3 9% 5% 4 5% 4 5% 6% 26%	3643
	11th to 20th	Night	21% 8% 3 7% 5% 4 3 3 5% 7% 34%	3575
	21st to end	Night	22% 8% 3 7% 5% 5% 5% 3 5% 6% 30%	3926
June	1st to 10th	Night	19% 7% 3 6% 5% 4 5% 4 5% 8% 33%	1600
	11th to 20th	Night	17% 7% 2 6% 4 4 5% 3 5% 7% 39%	1600
	21st to end	Night	20% 7% 1 6% 5% 4 4 3 7% 7% 37%	1600
July	1st to 10th	Night	14% 5% 3 7% 6% 5% 6% 5% 7% 7% 36%	3600
	11th to 20th	Night	14% 6% 2 5% 4 4 4 4 5% 8% 45%	3600
	21st to end	Night	10% 5% 1 5% 4 5% 5% 4 6% 7% 48%	3926
August	1st to 10th	Night	13% 6% 2 6% 5% 5% 5% 4 6% 8% 41%	2586
	11th to 20th	Night	14% 7% 3 6% 3 4 4 3 5% 6% 44%	2863
	21st to end	Night	17% 5% 2 6% 4 4 4 3 5% 6% 43%	3586
September	1st to 10th	Night	10% 5% 2 6% 4 4 4 4 5% 7% 48%	2000
	11th to 20th	Night	12% 5% 2 5% 3 3 3 3 5% 7% 51%	2070
	21st to end	Night	17% 5% 2 5% 3 3 3 3 5% 6% 46%	2200
October	1st to 10th	Night	17% 7% 3 6% 4 4 3 3 4 5% 43%	2356
	11th to 20th	Night	19% 7% 2 7% 4 4 4 3 5% 6% 38%	2432
	21st to end	Night	20% 10% 3 7% 5% 4 4 3 5% 7% 32%	2860
November	1st to 10th	Night	17% 10% 3 7% 5% 5% 4 3 6% 7% 32%	2667
	11th to 20th	Night	16% 11% 3 8% 5% 5% 4 4 5% 6% 34%	2846
	21st to end	Night	18% 14% 5% 8% 5% 4 4 4 5% 6% 28%	2940
December	1st to 10th	Night	15% 14% 6% 10% 6% 5% 5% 4 6% 7% 22%	4720
	11th to 20th	Night	22% 13% 5% 10% 5% 4 4 3 3 4 28%	5348
	21st to end	Night	21% 18% 4 8% 4 3 3 2 3 4 29%	7150

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	62% 5% 9% 17% 6% 1	4662
	11th to 20th	Day	65% 3 7% 13% 9% 2	4680
	21st to end	Day	59% 5% 8% 14% 13% 1	5124
February	1st to 10th	Day	59% 3 8% 19% 8% 2	1852
	11th to 20th	Day	58% 3 10% 16% 12% 1	1936
	21st to end	Day	60% 4 8% 14% 11% 3	1730
March	1st to 10th	Day	64% 3 8% 11% 11% 3	4579
	11th to 20th	Day	51% 4 16% 14% 12% 2	4812
	21st to end	Day	53% 3 6% 16% 19% 2	5941
April	1st to 10th	Day	55% 3 8% 18% 13% 3	2400
	11th to 20th	Day	50% 3 8% 22% 14% 2	2374
	21st to end	Day	54% 5% 8% 17% 13% 3	2552
May	1st to 10th	Day	62% 4 7% 18% 8% 1	6352
	11th to 20th	Day	52% 5% 15% 17% 8% 2	6322
	21st to end	Day	53% 5% 11% 18% 12% 1	6949
June	1st to 10th	Day	53% 5% 10% 18% 12% 2	2800
	11th to 20th	Day	46% 6% 12% 21% 12% 3	2800
	21st to end	Day	55% 6% 11% 18% 9% 1	2800
July	1st to 10th	Day	47% 7% 14% 24% 8%	6290
	11th to 20th	Day	38% 7% 15% 30% 10%	6285
	21st to end	Day	34% 6% 17% 33% 10%	6728
August	1st to 10th	Day	45% 6% 19% 22% 8%	4549
	11th to 20th	Day	43% 7% 16% 28% 6%	4192
	21st to end	Day	42% 7% 16% 21% 14% 1	4312
September	1st to 10th	Day	38% 6% 14% 29% 13%	2400
	11th to 20th	Day	35% 4 14% 33% 13% 1	2330
	21st to end	Day	42% 4 14% 26% 13% 1	2200
October	1st to 10th	Day	43% 5% 15% 26% 10%	3326
	11th to 20th	Day	44% 5% 15% 27% 9%	3182
	21st to end	Day	44% 4 22% 19% 10% 1	3180
November	1st to 10th	Day	49% 5% 14% 21% 10% 1	1851
	11th to 20th	Day	53% 5% 12% 17% 11% 1	1769
	21st to end	Day	59% 6% 13% 14% 7% 1	1680
December	1st to 10th	Day	69% 4 9% 13% 5%	2680
	11th to 20th	Day	60% 3 8% 17% 11%	3066
	21st to end	Day	62% 4 9% 16% 9%	4048
January	1st to 10th	Night	63% 4 10% 15% 6% 1	8234
	11th to 20th	Night	66% 5% 9% 15% 6% 1	8280
	21st to end	Night	57% 5% 9% 14% 14% 1	9057
February	1st to 10th	Night	57% 4 9% 20% 10% 1	2767
	11th to 20th	Night	53% 5% 10% 20% 11% 1	2580
	21st to end	Night	54% 4 9% 19% 13% 2	2076
March	1st to 10th	Night	60% 3 8% 13% 12% 4	5494
	11th to 20th	Night	50% 6% 13% 16% 12% 3	5298
	21st to end	Night	53% 4 6% 21% 13% 3	4990
April	1st to 10th	Night	54% 3 8% 19% 15% 2	2000
	11th to 20th	Night	48% 4 8% 22% 15% 2	1975
	21st to end	Night	49% 4 8% 21% 15% 3	1847
May	1st to 10th	Night	58% 4 10% 18% 7% 2	3649
	11th to 20th	Night	49% 6% 13% 19% 11% 2	3576
	21st to end	Night	54% 5% 10% 17% 13% 1	3927
June	1st to 10th	Night	48% 6% 11% 21% 14% 1	1600
	11th to 20th	Night	45% 5% 11% 23% 14% 3	1600
	21st to end	Night	47% 6% 13% 20% 13% 1	1600
July	1st to 10th	Night	44% 7% 18% 22% 9%	3560
	11th to 20th	Night	36% 6% 18% 31% 8%	3552
	21st to end	Night	33% 7% 16% 31% 13%	3809
August	1st to 10th	Night	41% 6% 15% 28% 9%	2576
	11th to 20th	Night	41% 5% 18% 28% 8%	2887
	21st to end	Night	41% 6% 14% 24% 13% 1	3586
September	1st to 10th	Night	34% 6% 15% 29% 16% 1	2000
	11th to 20th	Night	34% 4 15% 34% 12% 1	2070
	21st to end	Night	39% 5% 12% 29% 15%	2200
October	1st to 10th	Night	42% 5% 14% 25% 13% 1	3850
	11th to 20th	Night	41% 5% 13% 29% 12%	3955
	21st to end	Night	43% 5% 19% 19% 13% 2	4610
November	1st to 10th	Night	50% 5% 12% 23% 8% 1	2667
	11th to 20th	Night	51% 4 13% 20% 11% 2	2846
	21st to end	Night	57% 5% 11% 18% 8% 1	2940
December	1st to 10th	Night	61% 5% 12% 16% 5%	4720
	11th to 20th	Night	60% 4 7% 17% 11% 1	5348
	21st to end	Night	61% 3 6% 15% 14%	7150

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	18% 23% 38% 18% 3	220
	11th to 20th	Day	31% 17% 36% 11% 4 1	220
	21st to end	Day	20% 21% 38% 14% 5%	242
February	1st to 10th	Day	30% 24% 34% 10% 2	311
	11th to 20th	Day	23% 24% 39% 13% 2	414
	21st to end	Day	27% 20% 34% 16% 3	446
March	1st to 10th	Day	29% 22% 31% 15% 3	540
	11th to 20th	Day	27% 22% 33% 14% 3 1	528
	21st to end	Day	31% 21% 27% 15% 5% 1	550
April	1st to 10th	Day	30% 19% 31% 14% 5% 1	500
	11th to 20th	Day	34% 19% 25% 14% 7% 1	492
	21st to end	Day	30% 20% 30% 15% 4 1	500
May	1st to 10th	Day	27% 17% 36% 15% 4 1	503
	11th to 20th	Day	24% 18% 31% 20% 5% 1	527
	21st to end	Day	25% 19% 30% 19% 6% 1	573
June	1st to 10th	Day	24% 17% 35% 17% 6% 1	530
	11th to 20th	Day	19% 17% 35% 19% 9% 1	530
	21st to end	Day	26% 12% 34% 19% 7% 2	530
July	1st to 10th	Day	14% 12% 37% 25% 9% 2	525
	11th to 20th	Day	15% 12% 36% 27% 8% 1	524
	21st to end	Day	13% 11% 36% 27% 11% 1	576
August	1st to 10th	Day	14% 9% 31% 31% 13% 2	526
	11th to 20th	Day	13% 7% 31% 29% 13% 6%	519
	21st to end	Day	18% 11% 30% 23% 12% 6%	583
September	1st to 10th	Day	18% 8% 36% 23% 11% 5%	530
	11th to 20th	Day	11% 10% 28% 27% 18% 6%	530
	21st to end	Day	22% 12% 28% 23% 11% 5%	530
October	1st to 10th	Day	23% 17% 28% 20% 11% 2	530
	11th to 20th	Day	23% 17% 32% 21% 6% 1	511
	21st to end	Day	32% 14% 31% 18% 5%	462
November	1st to 10th	Day	30% 17% 30% 17% 4 1	425
	11th to 20th	Day	27% 19% 32% 18% 5%	309
	21st to end	Day	32% 20% 30% 14% 4	220
December	1st to 10th	Day	26% 17% 34% 18% 5% 1	220
	11th to 20th	Day	30% 21% 36% 10% 2	220
	21st to end	Day	34% 20% 26% 14% 5% 1	242
January	1st to 10th	Night	23% 16% 35% 21% 4	840
	11th to 20th	Night	24% 17% 31% 22% 5%	840
	21st to end	Night	16% 16% 38% 23% 6%	924
February	1st to 10th	Night	17% 17% 38% 23% 5%	840
	11th to 20th	Night	21% 18% 39% 18% 4	823
	21st to end	Night	21% 15% 36% 26% 2	692
March	1st to 10th	Night	25% 18% 33% 22% 3	840
	11th to 20th	Night	25% 16% 35% 18% 5%	829
	21st to end	Night	23% 14% 34% 23% 5%	880
April	1st to 10th	Night	28% 15% 32% 22% 4	800
	11th to 20th	Night	25% 14% 33% 23% 5% 1	791
	21st to end	Night	27% 15% 33% 17% 7%	647
May	1st to 10th	Night	26% 13% 32% 22% 5% 2	423
	11th to 20th	Night	23% 11% 28% 28% 8% 1	394
	21st to end	Night	21% 9% 39% 24% 7%	433
June	1st to 10th	Night	23% 11% 34% 25% 8%	400
	11th to 20th	Night	23% 9% 29% 29% 10%	400
	21st to end	Night	23% 6% 28% 29% 14% 1	400
July	1st to 10th	Night	10% 8% 33% 35% 12% 1	390
	11th to 20th	Night	11% 6% 27% 42% 14%	388
	21st to end	Night	12% 6% 29% 40% 12% 2	438
August	1st to 10th	Night	14% 5% 21% 42% 17% 1	398
	11th to 20th	Night	14% 4 26% 33% 21% 2	579
	21st to end	Night	15% 7% 27% 32% 16% 2	880
September	1st to 10th	Night	13% 7% 30% 37% 13% 1	800
	11th to 20th	Night	15% 6% 26% 31% 21% 1	800
	21st to end	Night	19% 8% 26% 30% 15% 2	800
October	1st to 10th	Night	21% 11% 27% 29% 11% 1	800
	11th to 20th	Night	24% 12% 30% 25% 8% 1	800
	21st to end	Night	27% 12% 33% 24% 5%	880
November	1st to 10th	Night	25% 15% 29% 23% 7% 1	820
	11th to 20th	Night	25% 14% 32% 21% 8% 1	840
	21st to end	Night	28% 17% 29% 20% 5% 1	840
December	1st to 10th	Night	27% 16% 32% 19% 6%	840
	11th to 20th	Night	28% 18% 31% 18% 4 1	840
	21st to end	Night	32% 16% 30% 18% 4	924

Page generated at 2026-03-15 02:23:51. Version ac07

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