4 Meteorological Conditions
4.0.1 The design meteorological conditions shall be determined based on the statistical meteorological data along the route and the operating experiences from the existing lines nearby.If the meteorological conditions along the route are similar to the typical conditions as set out in Appendix A hereto，the listed typical data should be used.The return period of reference wind speed and design icing thickness shall comply with the following provisions:
1 750kV and 500kV transmission lines and large crossing:50 years.
2 110kV-330kV transmission lines and large crossing:30 years.
4.0.2 The annual maximum average wind speed with an averaging period of 10minutes from the local meteorological observatory shall be used as the sample to determine the reference wind speed.In addition，the extreme value I-type distribution should be used as the probability model.The elevations for collecting wind speed statistics shall comply with the follows provisions:
1 110kV-750kV transmission lines:10m above ground.
2 Large crossing for all voltage levels:10m above the average minimum water level in gale season over the years.
4.0.3 For transmission lines in mountain areas，methods such as statistical analysis and comparative observation should be used.The reference wind speed can be derived from the meteorological data provided by nearby meteorological observatories in combination with operating experiences.The reference wind speed should be 10% higher than the statistical value from nearby plain areas if reliable data is unavailable.
4.0.4 The reference wind speed for 110kV-330kV transmission lines should not be lower than 23.5m/s and that for 500kV-750kV transmission lines should not be lower than 27m/s.It should be checked with the rare wind speed if necessary.
4.0.5 The power lines should be designed according to ice-free，5mm or 10mm icing criteria in light icing area，15mm or 20mm in medium icing area，and 20mm，30mm，40mm or 50mm in heavy icing area.The design icing thickness should be checked with the rare ice thickness if necessary.
4.0.6 Except for ice-free sections，the design icing thickness for earthwires shall be 5mm more than that for conductors.
4.0.7 In design，the design and operation of built lines along the proposed route shall be thoroughly investigated，and the influences of micro-landscape，micrometeorology and areas where conductors are liable to gallop shall be taken into consideration.
4.0.8 For the reference wind speed for large crossings，if no reliable data is available，the statistical wind speed data from nearby on-land transmission lines should be used after being converted to the elevation 10m above the average minimum water level in gale season over the years，increased by 10% and another 10% accounting for the influence of water surface.The reference wind speed for large crossings shall not be lower than that for on-land transmission lines to which such crossings are connected.
4.0.9 The design icing thickness for large crossings（excluding those in ice-free areas）should be 5mm greater than that for common transmission lines nearby.
4.0.10 The design annual average temperature shall be determined as listed below:
1 Where the annual average temperature is in the range of 3℃-17℃，the multiple of 5 adjacent to the average temperature should be taken.
2 Where the annual average temperature is lower than 3℃or higher than 17℃，the multiple of 5 adjacent to the value obtained by subtracting 3℃or 5℃from the average temperature should be taken respectively.
4.0.11 For installation，the wind speed shall be taken as 10m/s，and the icing thickness shall be icefree，and the atmospheric temperature shall be determined as follows:
1 For areas with minimum atmospheric temperature of-40℃，it should be-15℃.
2 For areas with minimum atmospheric temperature of-20℃，it should be-10℃.
3 For areas with minimum atmospheric temperature of-10℃，it should be-5℃.
4 For areas with minimum atmospheric temperature of-5℃，it should be 0℃.
4.0.12 The atmospheric temperature for lightning overvoltage conditions should be taken as 15℃.When the reference wind speed is greater than or equal to 35m/s after it is converted to the value at the average height of conductor，the wind speed for lightning overvoltage conditions should be 15m/s，or 10m/s otherwise.When checking the clearance between conductor and earthwire，conditions without wind and ice shall be employed.
4.0.13 The atmospheric temperature for switching overvoltage conditions may employ the annual average temperature.The wind speed should be 50% of the value the reference wind speed converted to at the average conductor height，but it should not be less than 15m/s and should be without ice.
4.0.14 The wind speed for live work may employ 10m/s，the atmospheric temperature may employ 15℃，without ice.