21. describe each of the four mechanisms that cause air to rise.(4 points)

1. Convective uplift. The hot air is less dense and it rises.
2. Orographic lifting. Air is forced to rise when it meets a mountain.
3. Convergence. Thunderstorms are caused by convergence.
4. Frontal wedging. When a cold air mass meets a warm air mass, the colder air wedges its way beneath the less dense warm air.
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Amath expression is a collection of one or more terms, which are separated by the signs ( + , - , x , / ).example :   8+b8+b  or  a−3a−3in algebra, an equation is a mathematical statement with two expressions where, each expression is separated by an equal (=) sign. variables can be present on both the sides of the equality (=) sign and we use letters to denote such variables.example 1:   x=4+2x=4+2  , [here, we can observe that the variable is present only on one side of the equation].example 2:   2x+3=x+42x+3=x+4  , [here, we can observe that the variables are present on both side of the equation].