Answer: front air masses
Explanation: The masses have fairly uniform temperature and moisture content in horizontal direction but its not uniform in vertical.
They are usually characterized by their temperature and humidity properties.
The properties in which air masses are usually determined by the the underlying surface properties where they come from. And immediately formed, air masses migrate within the general circulation. Also Upon movement, air masses displace residual air over locations thus changing temperature and humidity characteristics. Moreover it Contain uniform temperature and humidity characteristics.