Moisture in the Atmosphere

The troposphere is also known as the weather sphere. This is due to the water vapor in the air. After the tropopause, water vapor doesn't exist in the atmosphere. Figure shows the temperature, pressure, and water vapor values for each of the layers of the atmosphere.

Figure 1 Cross section of Earth's atmosphere

Measurement of Water in the Air

The amount of water in the air can be measured in different ways. The specific humidity of air is a measure of how much water is in the air. Warmer air can hold more water than colder air. When the air reaches its capacity, it is saturated. This capacity doubles for about every 11°C rise in temperature. The term more often used is relative humidity. This is the measure of how much water is in the air divided by how much it can hold. The relative humidity reading is given as a percent. The relative humidity for saturated air is 100 percent.

Finding Relative Humidity

The relative humidity can be found by two different methods. One involves the use of a hygrometer. This is a pointer attached to a piece of hair. As the humidity increases, the hair stretches out. This is your typical “bad‐hair day.” When the humidity drops, the hair shrinks, causing the needle to point in a different direction. The other method requires the use of two thermometers and a chart. The thermometers and chart all use the Celsius scale. If you are getting the readings from a station model and need to find the relative humidity, you need to convert the temperature readings from Fahrenheit to Celsius. Station models show the surface observations and weather data for a specific city. One of the thermometers measures the air temperature. This is the dry‐bulb reading. The other thermometer has a wet wick on the bottom of the bulb. Water evaporating from the wick into the air takes energy with it, cooling off the thermometer. As the relative humidity increases, less water can evaporate into the air. This makes the temperature readings between the two thermometers closer. If the air is much drier, the thermometers have readings that are much farther apart. The difference between the wet and dry‐bulb temperatures is called the wet‐bulb depression. The wet‐bulb temperature is always lower or the same as the dry‐bulb temperature. If the temperatures are the same, the relative humidity is 100 percent. The wet‐bulb depression is used with the dry‐bulb temperature and a chart to determine the relative humidity. Table shows this chart.

TABLE 1Chart for Determining Relative Humidity (%)

Dew Point

The instrument with the wet and dry‐bulb thermometers is called a psychrometer. A sling psychrometer is a handheld device with the two thermometers that spins around.

The temperature at which water vapor condenses into liquid water is called the dew point. If the dew point is below 0°C, it is called the frost point. These are important numbers in helping to determine where to post frost and freeze warnings. The dew‐point temperature is found in a similar manner to that of relative humidity. The dry‐bulb and wet‐bulb temperatures are determined. The wet‐bulb depression and dry‐bulb temperature are used with the chart in Table to find the dew point.

TABLE 2Chart for Determining Dew-point Temperature (°C)

If the water vapor comes in direct contact with the cooler surface, it can condense onto it. Fog can occur when warm air moves into an area that has a cold surface temperature. An advected fog forms in this situation. This also can be a reverse situation, where cooler air moves over a warmer surface. Ground fog forms by radiational cooling at night. These are common in humid valleys and near rivers and lakes.

Clouds

A cloud is formed when air is cooled to its dew‐point temperature. The air cools as it rises away from the Earth's surface. If that temperature is above 0°C, the cloud is made of water droplets. If the cloud forms below 0°C, the cloud is made from ice and snow crystals and supercooled water.

Cloud formations fall into three categories. Cirrus clouds are very high clouds that are made from ice crystals. They are the thin, feathery clouds you see on a nice day. Stratus clouds are the layered, sheet‐like clouds. They are found at lower altitudes. Cumulus clouds are the puffy, cottonlike clouds formed by vertical rising of air. Other clouds are made from combinations and variations of these clouds. The name of a cloud may also contain a prefix or suffix that tells you more about the cloud. Alto (high) and nimbus (rain) are some examples of these.

Cloud Development

As a parcel of air rises upward, it cools. The air expands and cools because of the decreasing pressure. The rate at which it cools depends on the amount of moisture in the air. If dry air rises, it cools at a rate of 1°C/100 m. This is the dry adiabatic lapse rate. By adding moisture, this rate changes to 0.6°C/100 m. This is the moist adiabatic lapse rate. The high specific heat of the water is the reason for the difference in the rates. When air at the surface is heated, it rises upward. The air is warmer than the air surrounding it and is less dense, which makes it buoyant. This is why clouds appear to “float” in the sky. The clouds can continue to develop vertically. Eventually, a cumulonimbus cloud may form. These are thunderstorm clouds that can be associated with heavy rain, hail, strong winds, and tornadoes. These clouds form in an unstable air mass that has air that is moving due to density differences.

A cloud can form in a stable air mass, but it rises for other reasons. These are layered clouds that form from air that is forced upward by the land (mountains) or by radiational cooling as the air mixes with a cooler layer of air. Some clouds that form have a flat base and billow out on top. The bottom of the cloud is the place where the air temperature is the same as the dew‐point temperature. This is the point known as the condensation level. The height of the cloud base can be found with a simple formula or a chart. To use the formula, take the difference between the temperature and dew point at the surface and divide it by 0.8°C (the amount that the dew‐point temperature gets closer to the air temperature in 100 m). The result is multiplied by 100, which gives you the lifting condensation level or the height that a cloud can form at. Cloud base altitude can also be found by using the air temperature and the dew point temperature. The air temperature is plotted along the solid lines and the dew point follows along the dashed lines in Figure . When the lines meet, read along the side that is labeled “Altitude.” This is the height of the cloud base in kilometers.

Figure 2 Graph for determining cloud base altitude.

Eventually the cloud and air temperatures become equal. The cloud isn't buoyant at this point and begins to spread out. This creates the classic anvil‐shaped tops that are seen at cloud tops.

Precipitation

In order for water vapor to condense, certain conditions are needed. The air must cool down. This can occur in several different ways. It can come in contact with a colder surface; it can radiate heat; it can mix with colder air; or it can expand as it rises upward. The other ingredient that is needed is condensation nuclei. This provides a surface for condensation to occur. These particles can be dust, salt, sulfate, or nitrate particles (these form acid rain) in the air. Scientists have seeded clouds to enhance nucleation and produce needed rain. Silver iodide crystals are put into clouds to provide a surface for condensation to occur. In some instances, water vapor can condense and form water droplets (homogenous nucleation), but this is rare. The type of precipitation that forms depends on the air temperature. If this is above the freezing point, rain forms. If the air temperature is below 0°C, snow forms. Figure shows the air conditions needed for different types of precipitation.

Figure 3 Air and surface temperatures and resulting precipitation.

Updrafts in a cloud move rain droplets around. As they collide, they collect and get larger. When the drop gets too heavy to stay in up the cloud, it falls to the Earth. Small droplets of rain (less than .02 cm in diameter) are called drizzle. Drops larger than these are called rain. When much larger drops of rain fall, they fall apart into smaller drops by vibrations caused by friction with the air.

Hail forms in a tall cloud with strong updrafts. An ice crystal or frozen raindrop moves though the cloud collecting water droplets. As the hailstone rises up in the cloud, the outer layer freezes. When it falls downward, it gathers more water droplets. This circulation process continues until the hailstone falls to the ground. When you cut a hailstone in half, you can see rings. Like rings in a tree, they can tell the hailstone's history of formation. Some hailstones can reach the size of a softball. These can be very damaging to crops, animals, cars, and other property.

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