By: Cathy Jacobs, PH.D, Dept. of Biology
California State University Dominguez Hills
Weather is the daily, even hourly fluctuations in temperature, humidity, precipitation, etc.
Climate is the average condition of weather over many decades.To understand the climate of the Los Angeles area, we need to consider how it fits into a larger picture of climate worldwide, so we will do a quick overview.
To understand belts of climate worldwide, we need to consider that:
- Warm air rises; cold air sinks
- Warm air can hold more moisture than cold air. Because warm air can hold more moisture than cold air, as moist warm air cools the relative humidity increases, and clouds form as water condenses out of the atmosphere; precipitation or dew can result.
% Relative humidity (RH) = Amount of water vapor present in the atmosphere at a particular temperature Amount of water vapor the air can hold at that temperature - As warm air rises, it expands and cools adiabatically; as cold air sinks it compresses and heats adiabatically.
Adiabatic heating (cooling) refers to the change of temperature of a volume of gas that results from compression (heating) or expansion (cooling) of a given volume of gas. As the gas molecules become more compressed, the average kinetic energy per cm cubed because there are fewer molecules of air per cm cubed, and the temperature decreases.
Combining solar input with water, we can see that because the intensity of radiation is highest at the equator, here the earth warms and absorbs visible light, re-radiating it as heat. This hot air picks up water from the evapotranspiration of water from the soil and vegetation, and becomes quite humid. As it rises, it expands and cools adiabatically, and rain falls in abundance, creating tropical rain forests. The rising air creates a permanent low pressure area at the equator, and the climate is hot and wet year-round. The air that rose from the equator, losing its water, continues rising to the top of the troposphere, where it spreads out and travels away from the equator, either north or south, as it is displaced by more air rising at the equator. It ravels to 30 degrees North or South latitude, where it descends, creating a high pressure area. As the very cold, dry air descends it compresses and heats adiabatically. By the time it reaches the earth's surface it is very hot and dry, and the earth's great hot deserts are found at this latitude. The air which reaches the earth's surface at these latitudes then moves out over the earth's surface towards areas of low pressure (the equator and 60 degrees, where air also rise). This air traveling over the surface creates the trade winds and the westerlies. At the equator and at 30 degrees, where air is either rising or sinking, there are no reliable winds. Sailors in sailing ships called these latitudes the doldrums and the horse latitudes, respectively. This circulation pattern, with air rising at the equator, returning to earth at 30 degrees and moving to the equator, is called a Hadley convection cell. At 60 degrees air also rises and cools adiabatically, creating temperate rain forests, particularly at the western margins of continents. At the poles, cold dry air that rose at 60 degrees sinks and heats adiabatically, so the poles are deserts as far as precipitation is concerned.
Cold deserts can be created by the rain shadow effect, where mountains intercept moisture forcing air up and over them, where it cools adiabatically and releases its moisture on the windward side, creating deserts in the leeward side. Towards the center of continents, the climate is also dry due to the fact that any moisture from the sea must travel long distances to reach there. Deserts or grasslands are found in the centers of large continents.
Factors affecting Los Angeles habitats specifically.
I. Los Angeles at about 34 degrees N latitude, has a semiarid climate.As you proceed southward from LA into Baja California, the climate becomes progressively drier, and as you proceed northward toward Oregon, it becomes progressively wetter.
II. On the west coast, we have a Mediterranean climate, where we have two seasons, not four. We have a hot, dry season, starting about mid-Mary and continuing into November, and then a cool, we season starting about mid-November to mid-Mary. This pattern of climate has a profound effect on our local vegetation, since we get most of our rainfall during the season when the plants are less stressed, and little or no rain when they are under great stress due to the high temperature and low humidity. New plant growth often starts with the first rains of the rainy season, and ceases as the soil dries up. Consequently, much of our native vegetation is evergreen.
III. The presence of the ocean has a profound effect due to the thermal capacity of water. Water can absorb much heat without much change in temperature, and can conversely release much heat without much change in temperature. Consequently, areas near the ocean show a maritime climate, with normally much less daily and seasonal fluctuation in temperature than is seen in areas further away from the ocean (compare the climates of Santa Monica, Riverside, and Palm Springs to see the effects of the thermal capacity of water).
IV. The topography of the Los Angeles area is complex, with several east-west tending ranges. These mountain ranges contribute two major factors to producing the huge variety of habitats found here:
- As you go up in elevation, the climate cools and becomes moister (due to the adiabatic lapse rate). Thus, the habitats show changes in belts as you ascend the mountains, paralleling those seen if you travel north in latitude. At the highest elevations in the San Gabriel Mountains, you can find tundra plants.
- The mountains cast a rain shadow, so that on the leeward side of the mountains the climate is far drier than on the windward side. Thus, there are a variety of desert habitats on the far sides of the mountains. Which type of desert habitat occurs in a particular area depends on the amount of freezing weather the plants are exposed to in the winter, which is roughly correlated with altitude, and with the amount of summer rain.
Fragile Habitats is a part of LAEP Learning Exchange.