4 - Atmospheric Moisture#
4.2 - What Is Humidity?#
The amount of water vapor in the air is humidity.
Absolute humidity describes the mass of water vapor in the atmosphere.
The amount of atmospheric pressure contributed by water vapor is called vapor pressure.
The maximum amount of water vapor that air can hold is called water-vapor capacity, and it varies by air temperature. If the air is holding as much water vapor as it can for a given temperature, it is saturated.
Relative humidity is a measure saturation as a percentage.
Specific humidity compares the mass of water vapor in a body of air to the total mass of that air.
4.5 - What Happens When Air Rises or Sinks?#
This section concerns rising parcels of air. When there is no energy exchanged between a parcel and its surroundings the process is said to be adiabatic. When such an exchange does occur, it is called diabatic.
The lapse rate is the change in temperature divided by the change in height, and reported as a ratio (\(\text{C}\degree/ \text{km}\)).
If any parcel of air rises adaibatically and does not become saturated during its ascent, it always cools at \(10 \ \text{C}\degree/ \text{km}\). This constant lapse rate, called the unsaturated adiabatic lapse rate, applies everywhere adiabatically rising air remains unsaturated.
The temperature change with height that is not rising or sinking is known as the environmental lapse rate, which can be quite variable.
Unstable air has a steep environmental lapse rate, and any rising air witll remain warmer than its surroundings, and so will continue to rise.
If the environmental lapse rate is between the unsaturated adiabatic lapse rate and the saturated adiabatic lapse rate, it produces conditions that are conditionally unstable. This air will rise if saturated and fall if unsaturated.
When the environmental lapse rate is more gentle than the saturated adiabatic lapse rate, it produces conditions that are stable. This air will sink.
4.7 - What Mechanisms Can Force Air to Rise?#
If an air parcel rises because atmospheric conditions are unstable, free convection occurs. If air is forced to rise due to external factors, it is called forced convection, which can happen from the following effects:
The orographic effect describes how low-altitude air masses can be deflected upwards due to topography. As this air is forced upward, it cools and forms clouds.
As air encounters the base of a mountain, it begins moving up the slope. Typically unsaturated, it rises according to the unsaturated adiabatic lapse rate. As the air cools, it approaches the dew point.
4.8 - What Do Clouds Tell Us About Weather?#
We classify clouds based on their form, altitude, and whether they are associated with precipitation.
Cumuliform clouds are generally taller than they are wide, or at least have a lumpy appearance.
Stratiform clouds have one or more layers and commonly form a continuous cover across the sky.
Cirriform clouds are feathery and wispy.