diff --git a/metar/Metar.py b/metar/Metar.py
index d5836e3..7cbcaac 100755
--- a/metar/Metar.py
+++ b/metar/Metar.py
@@ -209,15 +209,25 @@ SKY_COVER = { "SKC":"clear",
               "///":"",
               "VV":"indefinite ceiling" }
 
-CLOUD_TYPE = { "TCU":"towering cumulus",
-               "CU":"cumulus",
-               "CB":"cumulonimbus",
-               "SC":"stratocumulus",
-               "CBMAM":"cumulonimbus mammatus",
-               "ACC":"altocumulus castellanus",
-               "SCSL":"standing lenticular stratocumulus",
-               "CCSL":"standing lenticular cirrocumulus",
-               "ACSL":"standing lenticular altocumulus" }
+
+CLOUD_TYPE = {
+    "AC": "altocumulus",
+    "ACC": "altocumulus castellanus",
+    "ACSL": "standing lenticular altocumulus",
+    "AS": "altostratus",
+    "CB": "cumulonimbus",
+    "CBMAM": "cumulonimbus mammatus",
+    "CCSL": "standing lenticular cirrocumulus",
+    "CC": "cirrocumulus",
+    "CI": "cirrus",
+    "CS": "cirrostratus",
+    "CU": "cumulus",
+    "NS": "nimbostratus",
+    "SC": "stratocumulus",
+    "ST": "stratus",
+    "SCSL": "standing lenticular stratocumulus",
+    "TCU": "towering cumulus",
+}
 
 ## translation of the present-weather codes into english
 
@@ -1243,7 +1253,9 @@ class Metar(object):
                 text_list.append(SKY_COVER[cover])
             else:
                 if cloud:
-                    what = CLOUD_TYPE[cloud]
+                    what = CLOUD_TYPE.get(
+                        cloud, "unknown CLOUD_TYPE of %s" % (cloud, )
+                    )
                 elif SKY_COVER[cover].endswith(" "):
                     what = "clouds"
                 else:

AC code should be added to CLOUD_TYPE
Hi,

in this METAR `METAR LOXZ 141420Z 08006KT 20KM VCSH FEW025SC SCT040SC BKN090AC 21/14 Q1015 BECMG SCT090` parsing is ok but when we want the string representation we got an error 
```
line 1161, in sky_conditions
    what = CLOUD_TYPE[cloud]
KeyError: 'AC'
```  

AC for Alto Cumulus should be added.

diff --git a/test/test_metar.py b/test/test_metar.py
index 5630e1b..5482a03 100644
--- a/test/test_metar.py
+++ b/test/test_metar.py
@@ -17,6 +17,19 @@ class MetarTest(unittest.TestCase):
     def raisesParserError(self, code):
         self.assertRaises(Metar.ParserError, Metar.Metar, code )
 
+    def test_issue64_cloudkeyerror(self):
+        """Lookup on CLOUD_TYPE should not keyerror."""
+        report = Metar.Metar(
+          "METAR LOXZ 141420Z 08006KT 20KM VCSH FEW025SC SCT040SC BKN090AC "
+          "21/14 Q1015 BECMG SCT090"
+        )
+        res = report.sky_conditions()
+        ans = (
+            "a few stratocumulus at 2500 feet; scattered stratocumulus at "
+            "4000 feet; broken altocumulus at 9000 feet"
+        )
+        self.assertEqual(res, ans)
+
     def test_issue40_runwayunits(self):
         """Check reported units on runway visual range."""
         report = Metar.Metar(

pip install -e .
parse_log_pytest
pytest
pytest
3.6
pytest --no-header -rA --tb=line --color=no -p no:cacheprovider -W ignore::DeprecationWarning
attrs @ file:///opt/conda/conda-bld/attrs_1642510447205/work
certifi==2021.5.30
importlib-metadata @ file:///tmp/build/80754af9/importlib-metadata_1631916693255/work
iniconfig @ file:///home/linux1/recipes/ci/iniconfig_1610983019677/work
-e git+https://github.com/python-metar/python-metar.git@f092a1bf3a8b35a0366b6c5f42fe8fea6bdba093#egg=metar
more-itertools @ file:///tmp/build/80754af9/more-itertools_1637733554872/work
packaging @ file:///tmp/build/80754af9/packaging_1637314298585/work
pluggy @ file:///tmp/build/80754af9/pluggy_1615976315926/work
py @ file:///opt/conda/conda-bld/py_1644396412707/work
pyparsing @ file:///tmp/build/80754af9/pyparsing_1635766073266/work
pytest==6.2.4
toml @ file:///tmp/build/80754af9/toml_1616166611790/work
typing_extensions @ file:///opt/conda/conda-bld/typing_extensions_1647553014482/work
zipp @ file:///tmp/build/80754af9/zipp_1633618647012/work

name: python-metar
channels:
  - defaults
  - https://repo.anaconda.com/pkgs/main
  - https://repo.anaconda.com/pkgs/r
  - conda-forge
dependencies:
  - _libgcc_mutex=0.1=main
  - _openmp_mutex=5.1=1_gnu
  - attrs=21.4.0=pyhd3eb1b0_0
  - ca-certificates=2025.2.25=h06a4308_0
  - certifi=2021.5.30=py36h06a4308_0
  - importlib-metadata=4.8.1=py36h06a4308_0
  - importlib_metadata=4.8.1=hd3eb1b0_0
  - iniconfig=1.1.1=pyhd3eb1b0_0
  - ld_impl_linux-64=2.40=h12ee557_0
  - libffi=3.3=he6710b0_2
  - libgcc-ng=11.2.0=h1234567_1
  - libgomp=11.2.0=h1234567_1
  - libstdcxx-ng=11.2.0=h1234567_1
  - more-itertools=8.12.0=pyhd3eb1b0_0
  - ncurses=6.4=h6a678d5_0
  - openssl=1.1.1w=h7f8727e_0
  - packaging=21.3=pyhd3eb1b0_0
  - pip=21.2.2=py36h06a4308_0
  - pluggy=0.13.1=py36h06a4308_0
  - py=1.11.0=pyhd3eb1b0_0
  - pyparsing=3.0.4=pyhd3eb1b0_0
  - pytest=6.2.4=py36h06a4308_2
  - python=3.6.13=h12debd9_1
  - readline=8.2=h5eee18b_0
  - setuptools=58.0.4=py36h06a4308_0
  - sqlite=3.45.3=h5eee18b_0
  - tk=8.6.14=h39e8969_0
  - toml=0.10.2=pyhd3eb1b0_0
  - typing_extensions=4.1.1=pyh06a4308_0
  - wheel=0.37.1=pyhd3eb1b0_0
  - xz=5.6.4=h5eee18b_1
  - zipp=3.6.0=pyhd3eb1b0_0
  - zlib=1.2.13=h5eee18b_1
prefix: /opt/conda/envs/python-metar

test/test_metar.py::MetarTest::test_issue64_cloudkeyerror
test/test_metar.py::MetarTest::test_010_parseType_default test/test_metar.py::MetarTest::test_011_parseType_legal test/test_metar.py::MetarTest::test_020_parseStation_legal test/test_metar.py::MetarTest::test_021_parseStation_illegal test/test_metar.py::MetarTest::test_030_parseTime_legal test/test_metar.py::MetarTest::test_031_parseTime_specify_year test/test_metar.py::MetarTest::test_032_parseTime_specify_month test/test_metar.py::MetarTest::test_033_parseTime_auto_month test/test_metar.py::MetarTest::test_034_parseTime_auto_year test/test_metar.py::MetarTest::test_035_parseTime_suppress_auto_month test/test_metar.py::MetarTest::test_040_parseModifier_default test/test_metar.py::MetarTest::test_041_parseModifier test/test_metar.py::MetarTest::test_042_parseModifier_nonstd test/test_metar.py::MetarTest::test_043_parseModifier_illegal test/test_metar.py::MetarTest::test_140_parseWind test/test_metar.py::MetarTest::test_141_parseWind_nonstd test/test_metar.py::MetarTest::test_142_parseWind_illegal test/test_metar.py::MetarTest::test_150_parseVisibility test/test_metar.py::MetarTest::test_151_parseVisibility_direction test/test_metar.py::MetarTest::test_152_parseVisibility_with_following_temperature test/test_metar.py::MetarTest::test_290_ranway_state test/test_metar.py::MetarTest::test_300_parseTrend test/test_metar.py::MetarTest::test_310_parse_sky_conditions test/test_metar.py::MetarTest::test_issue40_runwayunits test/test_metar.py::MetarTest::test_issue51_strict test/test_metar.py::MetarTest::test_not_strict_mode test/test_metar.py::MetarTest::test_snowdepth
metar/Metar.py
metar/Metar.py