Question

Draw and label idealized graphs illustrating (a) how the relative humidity of a mP air mass would change as it traveled from the Pacific Ocean to Idaho and (b) how the temperature and humidity of a cP air mass would change as it traveled from Canada across the Great Lakes to produce lake effect snows.

Short answer

mP air mass: humidity decreases inland; cP air mass: humidity increases across lakes.

Step-by-step solution

Understanding the Scenario

Begin by understanding the characteristics of the maritime polar (mP) and continental polar (cP) air masses. An mP air mass originates over the ocean and carries moisture, while a cP air mass originates over land and is typically cold and dry.

Drawing Relative Humidity for mP Air Mass

For the mP air mass traveling from the Pacific Ocean to Idaho, start by drawing a graph with the x-axis labeled 'Distance from Pacific Ocean' and the y-axis labeled 'Relative Humidity (%)'. At the ocean, the relative humidity is high. As the air mass moves inland, the relative humidity decreases due to less available moisture and increasing temperatures, which increase the air's capacity to hold water vapor.

Drawing Temperature and Humidity for cP Air Mass

For the cP air mass moving from Canada across the Great Lakes, draw two graphs: one with temperature on the y-axis and one with humidity. As the air mass moves southward, the temperature may slightly increase due to warmer underlying surfaces. Over the Great Lakes, increased evaporation due to relatively warmer waters increases humidity, potentially leading to lake effect snows.

Analyzing Graph Patterns

Review the graphs to confirm the patterns. For the mP air mass, relative humidity should decrease consistently to Idaho. For the cP air mass, expect an initial constant or slight increase in temperature and a significant increase in humidity over the Great Lakes, explained by added moisture from evaporation, which is conducive for lake effect snow formation.

Key concepts

Air Mass Movement

Air mass movement refers to the travel of large bodies of air with uniform temperature and humidity characteristics across regions. As these air masses move, they interact with different surfaces. This interaction often leads to exchanges of heat and moisture
with the ground underneath. For students, it's important to remember that air masses are named according to their region of origin and the characteristics they carry.

• Air masses over oceans are typically moist, while those over continents are drier.
• Temperature and humidity characteristics of an air mass can change as it moves across different terrains.

Maritime Polar Air Masses

Maritime polar (mP) air masses originate over the cold oceans, like the North Pacific, and are characterized by being cool and moist. This high moisture content results from the evaporation of ocean water.
When an mP air mass travels inland, such as from the Pacific Ocean to Idaho, its relative humidity decreases. This happens because the air moves away from its moisture source without replenishing, and temperatures may rise.

• Initial high relative humidity when over the ocean.
• Decreasing relative humidity as it moves inland and the air's moisture-holding capacity increases.

Continental Polar Air Masses

Continental polar (cP) air masses originate over large landmasses like northern Canada and are cold and dry. These air masses become even colder during winter months when radiative cooling is strong.
As a cP air mass travels southward across the Great Lakes, there are notable changes. The temperatures may slightly increase due to surface heating.

• Cold and dry at the origin.
• Slight warming and moisture gain over lakes due to increased evaporation.

Temperature Changes

Temperature changes in air masses occur when they move over different surfaces. A maritime air mass might warm up as it travels over land due to the heat from the ground.
Conversely, a continental polar air mass may warm more subtly because of the vast temperature difference between cold air and warmer underlying surfaces.

• Direct heat exchange between air and land leads to temperature changes.
• The degree of change depends on the initial temperature and the nature of the surface.

Evaporation and Humidity

Evaporation adds moisture to the air, which directly affects humidity. When air masses travel over large water bodies, like the Great Lakes, evaporation from the water increases the air's humidity levels.
This principle is particularly important for maritime air masses, which can start humid but become drier over land.

• Evaporation is a key process that adds water vapor to the air.
• Humidity increases as air absorbs moisture, especially over water bodies.

Lake Effect Snow Formation

Lake effect snow happens when cold air moves over warmer lake waters, picking up moisture and heat. This causes the air to rise, cool, and form clouds that lead to snowfall.
For continental polar air masses moving across the Great Lakes, this process is pronounced. The added moisture from evaporation leads to heavy snow.

• Cold, dry air picks up warmth and moisture over lakes.
• Moist air rises, cools down, and results in snowfall downwind.