NAA Observatory
Clear Sky Chart
History

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

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	13% 9% 3 8% 5% 4 4 4 6% 7% 36%	101328
	Night	14% 10% 4 7% 4 4 4 3 5% 6% 38%	97801
Transparency	Day	44% 6% 12% 22% 14% 2	104895
	Night	45% 5% 11% 22% 15% 2	102375
Seeing	Day	18% 11% 29% 25% 13% 5%	15807
	Night	18% 9% 30% 30% 12% 1	25640

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	15% 13% 7% 11% 5% 5% 5% 3 5% 5% 27%	2527
	11th to 20th	Day	14% 15% 6% 15% 5% 5% 4 7% 7% 3 19%	2530
	21st to end	Day	19% 24% 6% 11% 5% 4 3 2 4 4 17%	2879
February	1st to 10th	Day	16% 14% 4 8% 7% 5% 6% 4 4 6% 26%	1751
	11th to 20th	Day	14% 12% 4 8% 5% 4 4 3 4 6% 34%	1848
	21st to end	Day	15% 14% 5% 10% 5% 3 4 3 5% 5% 30%	1570
March	1st to 10th	Day	15% 15% 3 8% 5% 4 3 2 5% 6% 34%	3120
	11th to 20th	Day	11% 6% 3 8% 5% 4 5% 4 6% 7% 39%	3272
	21st to end	Day	19% 13% 6% 13% 6% 5% 5% 4 5% 6% 19%	4165
April	1st to 10th	Day	17% 13% 3 8% 5% 4 5% 3 5% 6% 31%	2280
	11th to 20th	Day	16% 10% 3 7% 4 4 5% 4 5% 6% 36%	2254
	21st to end	Day	20% 8% 3 8% 5% 4 4 4 5% 6% 32%	2375
May	1st to 10th	Day	17% 7% 2 6% 4 4 5% 5% 6% 8% 34%	4082
	11th to 20th	Day	15% 7% 2 6% 4 4 4 4 6% 7% 42%	4293
	21st to end	Day	11% 6% 2 6% 4 4 4 4 7% 10% 42%	4824
June	1st to 10th	Day	10% 6% 2 5% 4 5% 5% 5% 6% 8% 45%	2660
	11th to 20th	Day	9% 6% 2 6% 5% 4 5% 4 7% 8% 44%	2660
	21st to end	Day	11% 6% 2 6% 5% 5% 6% 5% 7% 9% 36%	2660
July	1st to 10th	Day	10% 8% 3 8% 5% 5% 6% 5% 7% 9% 33%	4440
	11th to 20th	Day	7% 4 2 6% 4 4 4 4 7% 11% 46%	4440
	21st to end	Day	8% 6% 3 8% 5% 5% 5% 4 7% 9% 39%	4813
August	1st to 10th	Day	8% 5% 3 6% 5% 5% 4 5% 7% 9% 44%	4332
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 5% 8% 10% 42%	3730
	21st to end	Day	7% 5% 2 6% 4 4 4 4 7% 10% 47%	4180
September	1st to 10th	Day	11% 5% 2 6% 4 4 4 3 5% 7% 48%	2280
	11th to 20th	Day	11% 4 2 4 4 3 4 4 5% 7% 52%	2161
	21st to end	Day	10% 7% 2 6% 4 4 5% 4 6% 7% 44%	2090
October	1st to 10th	Day	13% 6% 2 6% 4 4 3 3 5% 7% 47%	2038
	11th to 20th	Day	17% 8% 3 8% 4 4 4 4 5% 7% 36%	1894
	21st to end	Day	20% 9% 2 7% 4 4 3 3 5% 7% 35%	1881
November	1st to 10th	Day	13% 9% 3 6% 4 3 4 4 5% 5% 45%	1717
	11th to 20th	Day	16% 11% 4 8% 5% 4 4 3 5% 8% 33%	1710
	21st to end	Day	19% 10% 4 9% 5% 4 3 3 4 5% 35%	1651
December	1st to 10th	Day	15% 13% 4 9% 6% 4 4 4 5% 5% 31%	1520
	11th to 20th	Day	13% 14% 5% 10% 5% 5% 5% 3 5% 6% 29%	1918
	21st to end	Day	21% 22% 9% 13% 6% 4 4 2 2 4 14%	2783
January	1st to 10th	Night	15% 15% 8% 11% 5% 4 4 3 4 4 29%	4428
	11th to 20th	Night	15% 19% 7% 12% 5% 4 4 5% 6% 4 19%	4440
	21st to end	Night	22% 17% 9% 9% 4 4 3 3 3 4 21%	4784
February	1st to 10th	Night	15% 15% 4 8% 6% 4 4 3 4 5% 31%	2428
	11th to 20th	Night	14% 12% 4 7% 4 4 4 4 4 6% 37%	2218
	21st to end	Night	17% 11% 4 9% 4 3 4 3 5% 5% 34%	1884
March	1st to 10th	Night	17% 11% 3 6% 4 4 4 3 4 5% 39%	3733
	11th to 20th	Night	10% 7% 3 7% 5% 4 4 3 5% 6% 47%	3702
	21st to end	Night	20% 11% 4 10% 5% 4 5% 3 4 6% 27%	3466
April	1st to 10th	Night	16% 10% 2 7% 3 4 4 3 5% 8% 38%	1900
	11th to 20th	Night	19% 9% 3 6% 4 3 3 3 5% 6% 39%	1875
	21st to end	Night	18% 7% 3 8% 4 4 4 3 6% 7% 36%	1804
May	1st to 10th	Night	16% 8% 2 7% 5% 5% 5% 4 6% 7% 36%	2813
	11th to 20th	Night	13% 7% 2 5% 4 4 5% 4 7% 9% 40%	2605
	21st to end	Night	11% 8% 2 6% 3 4 5% 4 5% 8% 44%	2743
June	1st to 10th	Night	9% 4 2 6% 3 5% 5% 3 6% 7% 49%	1520
	11th to 20th	Night	8% 5% 2 5% 4 5% 5% 5% 7% 9% 47%	1520
	21st to end	Night	10% 7% 2 6% 5% 5% 5% 4 6% 8% 42%	1520
July	1st to 10th	Night	12% 6% 3 8% 5% 4 5% 4 7% 8% 39%	2530
	11th to 20th	Night	5% 3 2 5% 4 3 5% 4 6% 8% 54%	2530
	21st to end	Night	5% 4 2 6% 5% 6% 6% 5% 7% 9% 45%	2750
August	1st to 10th	Night	5% 4 2 6% 4 5% 5% 4 7% 9% 47%	2604
	11th to 20th	Night	7% 4 2 6% 4 4 4 4 6% 8% 51%	3095
	21st to end	Night	8% 5% 1 5% 4 4 4 4 5% 9% 53%	3476
September	1st to 10th	Night	10% 6% 2 5% 3 3 4 3 5% 7% 53%	1900
	11th to 20th	Night	9% 4 2 5% 3 4 4 3 5% 7% 55%	2000
	21st to end	Night	10% 6% 2 7% 4 4 4 4 6% 7% 46%	2090
October	1st to 10th	Night	10% 5% 2 6% 4 4 3 3 5% 6% 52%	2138
	11th to 20th	Night	17% 7% 2 5% 4 3 3 2 4 6% 46%	2285
	21st to end	Night	20% 10% 2 6% 4 3 4 3 5% 6% 37%	2717
November	1st to 10th	Night	13% 8% 2 6% 4 4 3 3 4 5% 48%	2483
	11th to 20th	Night	15% 10% 3 8% 4 4 4 3 5% 6% 37%	2470
	21st to end	Night	17% 10% 4 8% 5% 4 3 3 5% 6% 37%	2527
December	1st to 10th	Night	17% 11% 4 9% 5% 5% 4 4 4 6% 32%	2660
	11th to 20th	Night	15% 14% 4 10% 5% 3 4 3 4 5% 32%	3279
	21st to end	Night	23% 19% 6% 10% 5% 5% 5% 3 4 5% 15%	4884

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	60% 5% 13% 16% 6%	2528
	11th to 20th	Day	65% 4 10% 12% 8% 1	2530
	21st to end	Day	70% 4 8% 12% 5%	2879
February	1st to 10th	Day	59% 6% 12% 13% 9% 2	1751
	11th to 20th	Day	52% 4 10% 22% 10% 2	1858
	21st to end	Day	55% 6% 10% 20% 7% 2	1570
March	1st to 10th	Day	52% 6% 9% 21% 11% 2	3120
	11th to 20th	Day	41% 6% 12% 28% 11% 1	3272
	21st to end	Day	64% 8% 12% 12% 3	4165
April	1st to 10th	Day	53% 6% 11% 18% 11%	2280
	11th to 20th	Day	47% 7% 10% 20% 12% 3	2254
	21st to end	Day	51% 6% 13% 17% 12% 2	2375
May	1st to 10th	Day	44% 9% 12% 24% 11%	4120
	11th to 20th	Day	41% 6% 12% 27% 13% 1	4297
	21st to end	Day	35% 8% 12% 21% 19% 4	4837
June	1st to 10th	Day	35% 8% 11% 26% 18% 2	2660
	11th to 20th	Day	36% 7% 10% 24% 20% 3	2660
	21st to end	Day	41% 9% 14% 21% 13% 3	2660
July	1st to 10th	Day	46% 6% 13% 22% 11% 2	4371
	11th to 20th	Day	30% 6% 16% 29% 17% 1	4365
	21st to end	Day	39% 7% 17% 16% 19% 3	4662
August	1st to 10th	Day	35% 7% 16% 29% 13% 1	4317
	11th to 20th	Day	34% 7% 12% 31% 15% 1	3766
	21st to end	Day	32% 6% 13% 27% 20% 3	4179
September	1st to 10th	Day	35% 7% 13% 23% 18% 4	2280
	11th to 20th	Day	30% 6% 11% 28% 22% 3	2161
	21st to end	Day	37% 8% 9% 25% 20% 2	2090
October	1st to 10th	Day	26% 4 7% 29% 32% 2	3389
	11th to 20th	Day	33% 4 12% 25% 25% 1	3122
	21st to end	Day	43% 4 10% 26% 15% 3	3081
November	1st to 10th	Day	40% 6% 13% 24% 16% 2	1717
	11th to 20th	Day	51% 5% 12% 21% 11% 1	1710
	21st to end	Day	53% 4 10% 23% 10%	1651
December	1st to 10th	Day	55% 5% 13% 18% 8% 1	1520
	11th to 20th	Day	56% 5% 12% 18% 7% 1	1915
	21st to end	Day	78% 3 7% 9% 3	2783
January	1st to 10th	Night	59% 6% 10% 16% 9% 1	4428
	11th to 20th	Night	67% 4 11% 12% 7%	4440
	21st to end	Night	67% 5% 9% 13% 6%	4784
February	1st to 10th	Night	55% 5% 11% 17% 10% 2	2428
	11th to 20th	Night	49% 5% 10% 21% 14% 1	2219
	21st to end	Night	52% 4 11% 18% 11% 3	1884
March	1st to 10th	Night	48% 5% 11% 23% 10% 3	3733
	11th to 20th	Night	38% 5% 11% 26% 18% 2	3702
	21st to end	Night	58% 7% 11% 17% 6% 1	3466
April	1st to 10th	Night	45% 5% 12% 24% 13% 2	1900
	11th to 20th	Night	45% 5% 8% 21% 16% 4	1875
	21st to end	Night	47% 8% 10% 21% 13% 3	1804
May	1st to 10th	Night	45% 7% 13% 21% 12% 2	2840
	11th to 20th	Night	38% 9% 16% 26% 10% 1	2606
	21st to end	Night	36% 8% 12% 18% 21% 5%	2751
June	1st to 10th	Night	33% 7% 11% 25% 20% 4	1520
	11th to 20th	Night	32% 6% 13% 25% 22% 2	1520
	21st to end	Night	39% 8% 14% 20% 16% 3	1520
July	1st to 10th	Night	43% 5% 15% 20% 12% 5%	2490
	11th to 20th	Night	27% 6% 14% 29% 22% 2	2482
	21st to end	Night	33% 7% 16% 19% 22% 4	2662
August	1st to 10th	Night	32% 7% 17% 27% 17% 1	2595
	11th to 20th	Night	30% 6% 14% 30% 20% 1	3125
	21st to end	Night	30% 5% 12% 32% 17% 4	3476
September	1st to 10th	Night	30% 6% 11% 26% 24% 3	1900
	11th to 20th	Night	28% 5% 8% 30% 27% 2	2000
	21st to end	Night	36% 7% 11% 28% 16% 3	2090
October	1st to 10th	Night	22% 4 7% 37% 28% 2	3567
	11th to 20th	Night	35% 3 6% 24% 29% 2	3796
	21st to end	Night	40% 3 9% 25% 19% 3	4467
November	1st to 10th	Night	39% 5% 9% 25% 19% 3	2483
	11th to 20th	Night	48% 4 12% 21% 13% 2	2470
	21st to end	Night	50% 4 12% 23% 12%	2527
December	1st to 10th	Night	53% 5% 11% 21% 9% 1	2660
	11th to 20th	Night	55% 4 12% 18% 9% 1	3281
	21st to end	Night	71% 5% 8% 11% 4	4884

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	26% 14% 33% 22% 4 1	270
	11th to 20th	Day	26% 16% 38% 16% 4 1	270
	21st to end	Day	24% 19% 31% 21% 4 1	297
February	1st to 10th	Day	24% 19% 39% 14% 3	270
	11th to 20th	Day	26% 18% 35% 19% 3	270
	21st to end	Day	24% 21% 38% 13% 2 1	225
March	1st to 10th	Day	27% 21% 36% 15% 1	270
	11th to 20th	Day	30% 16% 34% 15% 4 1	292
	21st to end	Day	30% 14% 31% 18% 5% 2	506
April	1st to 10th	Day	25% 15% 32% 20% 6% 1	460
	11th to 20th	Day	24% 20% 25% 18% 11% 2	452
	21st to end	Day	25% 13% 30% 23% 7% 1	460
May	1st to 10th	Day	24% 12% 31% 23% 8% 2	463
	11th to 20th	Day	19% 11% 31% 24% 11% 4	608
	21st to end	Day	16% 11% 30% 23% 15% 5%	734
June	1st to 10th	Day	12% 9% 30% 30% 14% 6%	680
	11th to 20th	Day	12% 8% 31% 30% 16% 4	680
	21st to end	Day	15% 8% 33% 29% 12% 4	680
July	1st to 10th	Day	8% 4 23% 31% 21% 13%	671
	11th to 20th	Day	12% 5% 26% 29% 18% 10%	668
	21st to end	Day	7% 5% 29% 33% 20% 7%	739
August	1st to 10th	Day	8% 3 22% 34% 23% 11%	624
	11th to 20th	Day	11% 5% 20% 29% 23% 12%	479
	21st to end	Day	7% 5% 19% 32% 27% 10%	539
September	1st to 10th	Day	13% 6% 21% 33% 21% 6%	490
	11th to 20th	Day	10% 4 18% 32% 26% 9%	490
	21st to end	Day	14% 12% 23% 26% 18% 7%	490
October	1st to 10th	Day	16% 9% 26% 29% 16% 5%	438
	11th to 20th	Day	19% 22% 33% 19% 7% 1	300
	21st to end	Day	31% 15% 32% 16% 6%	330
November	1st to 10th	Day	19% 16% 33% 22% 6% 4	285
	11th to 20th	Day	26% 19% 33% 14% 6% 1	270
	21st to end	Day	25% 14% 36% 20% 6%	270
December	1st to 10th	Day	20% 18% 40% 14% 8%	270
	11th to 20th	Day	22% 17% 39% 16% 5% 1	270
	21st to end	Day	28% 19% 37% 14% 2	297
January	1st to 10th	Night	20% 14% 38% 23% 5%	950
	11th to 20th	Night	24% 14% 35% 22% 5% 1	950
	21st to end	Night	21% 12% 34% 29% 4	1045
February	1st to 10th	Night	23% 13% 34% 25% 4	908
	11th to 20th	Night	21% 13% 35% 25% 5%	743
	21st to end	Night	22% 15% 38% 21% 4	628
March	1st to 10th	Night	21% 15% 35% 23% 6%	760
	11th to 20th	Night	17% 12% 37% 26% 7% 1	737
	21st to end	Night	26% 10% 34% 23% 7% 1	627
April	1st to 10th	Night	20% 13% 35% 25% 6% 1	570
	11th to 20th	Night	23% 12% 31% 27% 7%	563
	21st to end	Night	22% 8% 34% 27% 8%	570
May	1st to 10th	Night	19% 6% 36% 29% 9% 1	570
	11th to 20th	Night	18% 7% 29% 33% 12% 1	561
	21st to end	Night	13% 6% 27% 35% 18% 1	617
June	1st to 10th	Night	11% 4 28% 41% 15% 1	570
	11th to 20th	Night	10% 5% 29% 44% 12% 1	570
	21st to end	Night	11% 5% 29% 38% 16% 1	570
July	1st to 10th	Night	8% 3 16% 44% 26% 3	555
	11th to 20th	Night	7% 3 18% 42% 27% 3	552
	21st to end	Night	5% 2 17% 47% 26% 2	624
August	1st to 10th	Night	6% 3 13% 47% 29% 4	567
	11th to 20th	Night	7% 2 15% 42% 30% 5%	564
	21st to end	Night	8% 2 15% 41% 30% 3	627
September	1st to 10th	Night	12% 3 18% 38% 26% 3	570
	11th to 20th	Night	9% 4 19% 37% 27% 4	670
	21st to end	Night	13% 7% 23% 34% 21% 3	760
October	1st to 10th	Night	12% 6% 26% 35% 18% 2	760
	11th to 20th	Night	22% 10% 31% 26% 11% 1	760
	21st to end	Night	25% 12% 29% 25% 9%	836
November	1st to 10th	Night	18% 10% 37% 24% 10% 1	764
	11th to 20th	Night	20% 15% 33% 25% 7%	760
	21st to end	Night	21% 12% 37% 24% 6%	817
December	1st to 10th	Night	24% 11% 34% 25% 5%	950
	11th to 20th	Night	23% 11% 33% 27% 5%	950
	21st to end	Night	30% 14% 32% 20% 3	1045

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