Prospect
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Prospect 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	17% 7% 2 6% 4 4 4 4 5% 7% 39%	130728
	Night	18% 8% 3 6% 4 4 4 3 5% 6% 41%	130417
Transparency	Day	42% 7% 12% 21% 15% 3	134257
	Night	43% 5% 12% 21% 16% 3	134975
Seeing	Day	21% 18% 32% 20% 8% 2	16615
	Night	21% 15% 32% 24% 7% 1	25779

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% 8% 3 9% 5% 5% 4 4 5% 6% 41%	4661
	11th to 20th	Day	16% 8% 3 6% 4 4 4 4 4 5% 43%	4650
	21st to end	Day	16% 9% 3 5% 4 3 3 3 5% 6% 43%	5286
February	1st to 10th	Day	16% 7% 3 7% 4 5% 5% 3 5% 5% 39%	1954
	11th to 20th	Day	15% 7% 3 5% 4 5% 4 4 6% 7% 40%	2048
	21st to end	Day	21% 9% 2 5% 4 3 3 3 4 5% 39%	1730
March	1st to 10th	Day	23% 9% 3 5% 3 4 3 2 4 5% 40%	4496
	11th to 20th	Day	14% 7% 2 6% 4 4 4 4 6% 7% 41%	4666
	21st to end	Day	20% 8% 2 6% 4 4 4 4 5% 7% 36%	5970
April	1st to 10th	Day	20% 8% 3 6% 4 4 4 3 6% 7% 36%	2400
	11th to 20th	Day	20% 6% 2 6% 4 3 4 3 5% 7% 41%	2374
	21st to end	Day	21% 5% 3 5% 4 4 4 3 5% 5% 41%	2505
May	1st to 10th	Day	31% 8% 2 6% 4 3 4 3 5% 8% 26%	5886
	11th to 20th	Day	21% 8% 3 7% 4 4 5% 4 5% 7% 33%	6152
	21st to end	Day	22% 6% 2 5% 4 4 4 4 5% 7% 35%	6949
June	1st to 10th	Day	18% 5% 2 6% 5% 5% 4 5% 7% 9% 34%	2800
	11th to 20th	Day	17% 7% 2 6% 4 4 5% 4 7% 9% 35%	2800
	21st to end	Day	14% 6% 2 7% 5% 4 5% 4 6% 9% 38%	2800
July	1st to 10th	Day	14% 7% 2 7% 5% 6% 6% 4 6% 9% 33%	6360
	11th to 20th	Day	12% 6% 3 7% 5% 5% 6% 5% 8% 11% 33%	6360
	21st to end	Day	15% 7% 2 6% 4 4 5% 3 6% 8% 40%	6926
August	1st to 10th	Day	20% 6% 3 7% 5% 5% 5% 4 7% 8% 31%	4406
	11th to 20th	Day	11% 6% 2 7% 4 5% 4 4 6% 8% 42%	3858
	21st to end	Day	9% 5% 2 6% 4 5% 5% 4 7% 9% 45%	4312
September	1st to 10th	Day	13% 6% 2 6% 4 4 4 4 4 7% 44%	2400
	11th to 20th	Day	12% 6% 2 5% 3 4 3 4 5% 7% 50%	2265
	21st to end	Day	19% 6% 3 6% 4 4 4 3 5% 6% 40%	2200
October	1st to 10th	Day	20% 7% 2 5% 3 4 3 3 4 6% 43%	2161
	11th to 20th	Day	15% 6% 2 5% 4 3 3 4 6% 7% 46%	1988
	21st to end	Day	20% 6% 2 6% 3 4 4 3 5% 6% 41%	1980
November	1st to 10th	Day	13% 6% 2 5% 5% 4 4 2 4 7% 48%	1851
	11th to 20th	Day	17% 5% 2 4 3 3 3 2 6% 6% 48%	1890
	21st to end	Day	18% 8% 2 5% 4 4 4 3 6% 6% 42%	1850
December	1st to 10th	Day	16% 6% 2 6% 4 4 3 3 5% 7% 45%	2680
	11th to 20th	Day	21% 7% 2 5% 3 3 2 2 4 5% 46%	3066
	21st to end	Day	19% 10% 3 7% 4 3 3 2 4 5% 39%	4048
January	1st to 10th	Night	15% 7% 2 6% 4 4 4 3 5% 6% 44%	8234
	11th to 20th	Night	15% 9% 3 7% 4 4 4 3 4 5% 42%	8235
	21st to end	Night	16% 8% 3 6% 4 4 4 3 4 5% 42%	8877
February	1st to 10th	Night	16% 9% 3 5% 4 4 5% 3 5% 5% 41%	2664
	11th to 20th	Night	17% 8% 3 6% 4 4 4 3 5% 6% 40%	2458
	21st to end	Night	19% 8% 3 7% 5% 4 3 3 5% 5% 40%	2076
March	1st to 10th	Night	20% 9% 3 5% 3 3 3 3 4 5% 41%	5389
	11th to 20th	Night	16% 9% 3 6% 4 4 4 3 4 6% 41%	5275
	21st to end	Night	21% 10% 2 6% 3 3 4 3 5% 5% 38%	4962
April	1st to 10th	Night	20% 8% 2 6% 4 4 4 4 6% 6% 37%	2000
	11th to 20th	Night	20% 6% 2 6% 4 3 4 3 6% 6% 40%	1975
	21st to end	Night	19% 8% 3 6% 4 3 3 3 4 5% 42%	1894
May	1st to 10th	Night	29% 11% 3 7% 4 3 3 3 4 7% 26%	4054
	11th to 20th	Night	20% 10% 4 7% 4 4 4 4 5% 7% 33%	3769
	21st to end	Night	23% 9% 2 6% 4 3 4 4 5% 6% 34%	3926
June	1st to 10th	Night	18% 7% 2 6% 5% 4 4 3 5% 6% 39%	1600
	11th to 20th	Night	17% 7% 2 5% 4 4 4 4 6% 8% 40%	1600
	21st to end	Night	13% 6% 3 6% 4 4 5% 3 5% 9% 42%	1600
July	1st to 10th	Night	16% 7% 2 7% 4 4 4 5% 7% 8% 37%	3600
	11th to 20th	Night	17% 9% 3 8% 4 4 5% 4 6% 8% 32%	3600
	21st to end	Night	14% 7% 3 6% 4 4 4 4 5% 7% 43%	3926
August	1st to 10th	Night	21% 8% 3 7% 5% 4 5% 4 6% 6% 31%	2710
	11th to 20th	Night	14% 7% 2 8% 5% 4 4 4 5% 7% 41%	3186
	21st to end	Night	11% 4 2 4 3 3 3 3 5% 9% 53%	3586
September	1st to 10th	Night	14% 6% 3 6% 4 3 5% 3 5% 6% 45%	2000
	11th to 20th	Night	13% 6% 2 5% 3 3 4 3 4 6% 52%	2134
	21st to end	Night	18% 7% 2 6% 3 4 5% 3 4 5% 44%	2200
October	1st to 10th	Night	19% 6% 3 5% 4 4 3 3 5% 5% 43%	2235
	11th to 20th	Night	16% 5% 2 6% 3 3 4 3 3 6% 49%	2410
	21st to end	Night	21% 7% 2 5% 3 3 3 3 4 5% 42%	2860
November	1st to 10th	Night	12% 6% 2 5% 4 3 4 3 4 6% 49%	2667
	11th to 20th	Night	18% 6% 2 4 3 4 3 3 4 5% 47%	2730
	21st to end	Night	16% 6% 2 5% 4 4 3 3 5% 6% 46%	2767
December	1st to 10th	Night	17% 7% 3 6% 3 3 3 3 4 4 48%	4720
	11th to 20th	Night	25% 7% 2 4 3 3 4 3 3 4 42%	5348
	21st to end	Night	19% 10% 3 7% 3 3 3 3 3 5% 41%	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	41% 5% 11% 25% 15% 3	4662
	11th to 20th	Day	42% 4 10% 20% 15% 9%	4680
	21st to end	Day	40% 6% 10% 25% 18% 1	5286
February	1st to 10th	Day	45% 6% 12% 19% 15% 2	1954
	11th to 20th	Day	41% 6% 12% 22% 16% 3	2057
	21st to end	Day	46% 5% 12% 20% 14% 3	1730
March	1st to 10th	Day	46% 5% 12% 16% 12% 10%	4499
	11th to 20th	Day	36% 9% 17% 26% 11% 2	4666
	21st to end	Day	46% 5% 10% 14% 20% 6%	5970
April	1st to 10th	Day	46% 6% 11% 19% 15% 3	2400
	11th to 20th	Day	42% 6% 12% 20% 17% 3	2374
	21st to end	Day	43% 6% 10% 20% 16% 4	2505
May	1st to 10th	Day	55% 8% 14% 13% 8% 1	5897
	11th to 20th	Day	48% 8% 12% 18% 12% 1	6156
	21st to end	Day	45% 8% 12% 21% 13% 1	6949
June	1st to 10th	Day	43% 9% 12% 23% 12% 1	2800
	11th to 20th	Day	42% 9% 15% 20% 12% 1	2800
	21st to end	Day	41% 9% 14% 22% 14% 1	2800
July	1st to 10th	Day	46% 9% 11% 19% 14% 1	6290
	11th to 20th	Day	41% 8% 18% 22% 10% 1	6285
	21st to end	Day	42% 6% 11% 22% 19% 1	6728
August	1st to 10th	Day	48% 8% 15% 20% 9% 1	4392
	11th to 20th	Day	38% 8% 14% 29% 11% 1	3894
	21st to end	Day	34% 6% 15% 26% 17% 2	4312
September	1st to 10th	Day	39% 8% 11% 22% 19% 2	2400
	11th to 20th	Day	33% 7% 11% 23% 24% 2	2265
	21st to end	Day	44% 7% 10% 20% 17% 3	2200
October	1st to 10th	Day	37% 5% 11% 18% 24% 6%	3525
	11th to 20th	Day	28% 5% 10% 24% 30% 4	3216
	21st to end	Day	32% 4 13% 24% 23% 3	3180
November	1st to 10th	Day	37% 5% 12% 22% 21% 2	1851
	11th to 20th	Day	36% 5% 11% 23% 21% 3	1890
	21st to end	Day	42% 4 13% 26% 12% 3	1850
December	1st to 10th	Day	40% 5% 15% 27% 12% 1	2680
	11th to 20th	Day	41% 3 15% 19% 18% 3	3066
	21st to end	Day	47% 5% 8% 19% 15% 4	4048
January	1st to 10th	Night	40% 4 12% 24% 18% 3	8234
	11th to 20th	Night	44% 3 13% 19% 16% 6%	8280
	21st to end	Night	44% 4 10% 20% 18% 4	8877
February	1st to 10th	Night	44% 6% 13% 23% 13% 2	2664
	11th to 20th	Night	44% 4 11% 20% 17% 4	2460
	21st to end	Night	46% 4 9% 22% 14% 4	2076
March	1st to 10th	Night	45% 5% 12% 18% 14% 7%	5399
	11th to 20th	Night	42% 7% 16% 21% 10% 4	5276
	21st to end	Night	47% 4 7% 12% 24% 5%	4962
April	1st to 10th	Night	45% 7% 11% 20% 14% 3	2000
	11th to 20th	Night	44% 6% 11% 19% 19% 1	1975
	21st to end	Night	45% 5% 9% 22% 16% 4	1894
May	1st to 10th	Night	58% 7% 13% 11% 9% 1	4060
	11th to 20th	Night	49% 9% 13% 15% 11% 3	3770
	21st to end	Night	49% 6% 13% 21% 9% 1	3927
June	1st to 10th	Night	45% 7% 11% 19% 17% 2	1600
	11th to 20th	Night	40% 9% 15% 24% 11% 1	1600
	21st to end	Night	38% 9% 13% 24% 15% 1	1600
July	1st to 10th	Night	44% 8% 14% 20% 12% 1	3560
	11th to 20th	Night	48% 7% 16% 19% 10% 1	3552
	21st to end	Night	39% 6% 13% 23% 17% 1	3809
August	1st to 10th	Night	52% 9% 13% 16% 9% 1	2700
	11th to 20th	Night	43% 6% 13% 29% 8% 1	3216
	21st to end	Night	27% 6% 15% 33% 17% 1	3586
September	1st to 10th	Night	40% 6% 10% 25% 18% 1	2000
	11th to 20th	Night	33% 5% 10% 24% 25% 2	2134
	21st to end	Night	43% 7% 9% 21% 19% 2	2200
October	1st to 10th	Night	38% 5% 12% 23% 20% 3	3651
	11th to 20th	Night	26% 5% 9% 27% 29% 4	3921
	21st to end	Night	32% 4 13% 29% 20% 3	4610
November	1st to 10th	Night	35% 6% 12% 22% 20% 4	2667
	11th to 20th	Night	40% 4 10% 23% 19% 4	2730
	21st to end	Night	39% 4 13% 29% 12% 1	2767
December	1st to 10th	Night	40% 4 12% 29% 13% 1	4720
	11th to 20th	Night	47% 4 12% 17% 17% 3	5348
	21st to end	Night	45% 5% 10% 19% 16% 5%	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	28% 26% 30% 13% 2	220
	11th to 20th	Day	25% 34% 29% 10% 1	220
	21st to end	Day	25% 33% 29% 8% 4 1	272
February	1st to 10th	Day	26% 29% 33% 11% 1	394
	11th to 20th	Day	26% 27% 36% 10% 1	536
	21st to end	Day	29% 24% 33% 12% 2	446
March	1st to 10th	Day	25% 26% 34% 13% 2	508
	11th to 20th	Day	24% 27% 34% 12% 2 1	500
	21st to end	Day	29% 23% 34% 10% 3 1	550
April	1st to 10th	Day	26% 29% 28% 11% 5% 1	500
	11th to 20th	Day	28% 23% 31% 12% 5% 1	492
	21st to end	Day	27% 19% 35% 14% 4 1	500
May	1st to 10th	Day	30% 21% 30% 13% 5% 1	503
	11th to 20th	Day	22% 18% 37% 17% 5% 1	527
	21st to end	Day	21% 12% 33% 24% 8% 2	573
June	1st to 10th	Day	19% 12% 33% 23% 10% 2	530
	11th to 20th	Day	18% 14% 34% 25% 8% 1	530
	21st to end	Day	14% 13% 34% 27% 10% 2	530
July	1st to 10th	Day	17% 7% 33% 27% 14% 2	525
	11th to 20th	Day	13% 6% 33% 31% 15% 2	524
	21st to end	Day	14% 9% 36% 31% 9% 1	576
August	1st to 10th	Day	13% 8% 28% 36% 10% 5%	526
	11th to 20th	Day	13% 7% 29% 31% 15% 4	519
	21st to end	Day	10% 12% 34% 25% 14% 5%	583
September	1st to 10th	Day	17% 12% 29% 22% 17% 4	530
	11th to 20th	Day	14% 9% 30% 26% 13% 7%	530
	21st to end	Day	22% 12% 33% 21% 9% 3	530
October	1st to 10th	Day	25% 11% 28% 18% 14% 4	530
	11th to 20th	Day	18% 22% 29% 22% 7% 2	522
	21st to end	Day	24% 18% 30% 19% 8% 1	462
November	1st to 10th	Day	18% 20% 30% 22% 9% 2	425
	11th to 20th	Day	22% 28% 27% 19% 4	430
	21st to end	Day	25% 24% 26% 19% 5%	390
December	1st to 10th	Day	27% 22% 32% 15% 4	220
	11th to 20th	Day	34% 27% 26% 11% 2	220
	21st to end	Day	26% 22% 32% 15% 4 1	242
January	1st to 10th	Night	22% 22% 35% 18% 3	840
	11th to 20th	Night	26% 21% 33% 18% 2	840
	21st to end	Night	21% 22% 37% 18% 2	924
February	1st to 10th	Night	21% 21% 36% 20% 2	840
	11th to 20th	Night	25% 22% 37% 15% 1	823
	21st to end	Night	23% 24% 35% 15% 3	692
March	1st to 10th	Night	24% 20% 34% 18% 3	809
	11th to 20th	Night	21% 19% 36% 20% 3 1	800
	21st to end	Night	23% 21% 37% 16% 4	880
April	1st to 10th	Night	24% 21% 34% 18% 3	800
	11th to 20th	Night	24% 18% 34% 21% 3	791
	21st to end	Night	21% 20% 35% 20% 4	694
May	1st to 10th	Night	26% 13% 34% 21% 5%	600
	11th to 20th	Night	21% 15% 38% 21% 5%	484
	21st to end	Night	22% 7% 27% 35% 9%	433
June	1st to 10th	Night	19% 8% 31% 33% 9% 1	400
	11th to 20th	Night	18% 7% 31% 33% 12%	400
	21st to end	Night	15% 7% 30% 40% 8%	400
July	1st to 10th	Night	14% 5% 27% 40% 15% 1	390
	11th to 20th	Night	15% 3 22% 45% 14% 1	388
	21st to end	Night	13% 5% 28% 40% 13% 1	438
August	1st to 10th	Night	13% 4 28% 42% 13% 1	470
	11th to 20th	Night	15% 4 24% 38% 17% 2	779
	21st to end	Night	14% 5% 25% 36% 18% 2	880
September	1st to 10th	Night	16% 6% 28% 33% 15% 2	800
	11th to 20th	Night	16% 6% 25% 31% 18% 3	800
	21st to end	Night	20% 8% 27% 30% 15% 1	800
October	1st to 10th	Night	22% 10% 28% 26% 13% 2	800
	11th to 20th	Night	20% 14% 31% 26% 9% 1	800
	21st to end	Night	25% 13% 35% 20% 7%	880
November	1st to 10th	Night	17% 15% 35% 24% 9% 1	820
	11th to 20th	Night	22% 18% 35% 20% 5% 1	840
	21st to end	Night	21% 19% 31% 24% 4	840
December	1st to 10th	Night	25% 18% 31% 20% 5% 1	840
	11th to 20th	Night	25% 20% 32% 20% 3	840
	21st to end	Night	27% 19% 34% 18% 2	924

Page generated at 2026-02-28 01:40:24. Version ac07

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