UNDERSTANDING WEATHER

The Water cycle

The amount of water vapor in the air is called humidity. As water evaporates and becomes air vapor, the humidity of the air increases. The air’s ability to hold water vapor changes as the temperature of the air changes. As the temperature of the air increases, the air’s ability to hold water vapor also increases.

Relative humidity is the amount of water vapor in the air compared with the maximum amount of water vapor that the air can hold at a certain temperature. Given at a percentage (%) When air holds all of the water that it can at a given temperature, it is said to be saturated. Saturated air has a relative humidity of 100%. To calculate unsaturated air actual water vapor content (g/m 3 ) saturation water vapor content (g/m3) X 100 = relative humidity (%)

2 factors affect relative humidity: 1)Amount of water vapor 2) Temperature The more water vapor there is in the air, the higher the relative humidity. The relative humidity decreases as the temperature rises and increases as the temperature drops. The outside air might have a comfortable level of humidity, but when that air is heated, the relative humidity drops, causing the air to be very dry inside the house. During the dry months, a humidifier can help maintain a comfortable level of humidity. Let's take a look at a simple humidifier.

A psychrometer is an instrument that is used to measure relative humidity. This consists of 2 thermometers, one which is a wet-bulb thermometer and one thermometer is a dry-bulb thermometer. The wet-bulb thermometer is covered with a damp cloth.

The difference in temperature readings between the wet and dry-bulb thermometers indicates the amount of water vapor in the air. The larger the difference is between the two readings, the less water vapor the air contains and thus the lower the humidity is.

A wet-bulb thermometer works differently than a dry-bulb thermometer. Dry-bulb thermometer only measure air temperature. As air passes over the wet bulb thermometer, the water in the cloth evaporates. As the water evaporates, the cloth cools. If the humidity is low, the water will evaporate quickly and the temperature reading on the wet-bulb thermometer will drop. If the humidity is high, only a small amount of water will evaporate from the cloth of the wet-bulb thermometer and the change in temperature will be small.

Condensation is the process by which a gas, such as water vapor, becomes a liquid. Before condensation can occur, the air must be saturated. Condensation occurs when saturated air cools.

Air can become saturated when water vapor is added to the air through evaporation. Air can also become saturated when it cools to its dew point. The dew point is the temperature at which a gas condenses into a liquid. At its dew point, air is saturated. The temperature of the air at a particular location at which saturation occurs and water vapor begins to condense forming Dew. Dew is commonly observed on the leaves of plants in the early morning.

CLOUDS A cloud is a collection of millions of tiny water droplets or ice crystals. Clouds form as warm air rises and cools. As the rising air cools it becomes saturated. When the air is saturated, the water vapor changes to a liquid or solid, depending on the air temperature.

Cumulus Clouds puffy, white clouds that tend to have flat bottoms form when air rises generally indicate fair weather when get larger – produce thunderstorms Thunderstorms come from cumulonimbus clouds If a cloud includes “nimbo-” are likely to produce precipitation

Stratus Clouds Clouds that form in layers Cover large areas of the sky and often block out the sun Can be caused by a gentle lifting of a large body of air into the atmosphere Nimbostratus clouds = dark clouds that usually produce light to heavy, continuous rain. Fog is a stratus cloud that has formed near the ground

Cirrus Clouds thin, feathery, white clouds found at high altitudes form when the wind is strong if they get thicker = a change in the weather is coming

Clouds and Altitudes “ cirro -” = clouds that form at high altitudes Because of the cold temperatures at high altitude, high clouds are made up of ice crystals. “ alto -” = clouds that form at middle altitudes Middle clouds can be made up of both water drops and ice crystals. Low clouds do not have a special prefix to describe them. Low clouds are made up of water drops.

Precipitation When water from the air returns to Earth’s surface, it is returned as precipitation. 4 major forms of precipitation: * rain * sleet * snow * hail

RAIN the most common form of precipitation clouds produce when the water drops become large enough to fall a water drop in a cloud begins as a droplet smaller than a period at the end of a sentence – it must increase its size 100 times before it will be large enough to fall

SLEET AND SNOW Sleet forms when rain falls through a layer of freezing air, which produces falling ice. Snow forms when temperatures are so cold that water vapor changes directly into a solid. Snow can fall as single ice crystals or can join to form snowflakes.

HAIL balls or lumps of ice that fall from clouds form in cumulonimbus clouds when updrafts of air in clouds carry raindrops high in the clouds, the raindrops freeze As hails falls, water drops coat it, another updraft sends the hail up again (over and over) until it becomes too heavy