<xsd:attribute name="weather-type" use="optional">
    <xsd:simpleType>
        <xsd:restriction base="xsd:string">
            <xsd:enumeration value="freezing drizzle" />
            <xsd:enumeration value="freezing rain" />
            <xsd:enumeration value="snow showers" />
            <xsd:enumeration value="blowing snow" />
            <xsd:enumeration value="blowing dust" />
            <xsd:enumeration value="rain showers" />
            <xsd:enumeration value="ice pellets" />
            <xsd:enumeration value="frosts" />
            <xsd:enumeration value="rain" />
            <xsd:enumeration value="hail" />
            <xsd:enumeration value="snow" />
            <xsd:enumeration value="thunderstorms" />
            <xsd:enumeration value="drizzle" />
            <xsd:enumeration value="fog" />
            <xsd:enumeration value="haze" />
            <xsd:enumeration value="smoke" />
        </xsd:restriction>
    </xsd:simpleType>
</xsd:attribute>